 2 L. monocytogenes can also be transmitted at the time of birth.    3 HIV is transmitted primarily at the time of birth but is also transmitted across    1 B, bacterium; V, virus; P, protozoa; H, helminth,  formerly called Norwalklike viruses.   3 AII of the organisms that cause diarrhea by ingestion of drinking water also cause diarrhea by ingestion of water while swimming.   Animals are also an important source of organisms that  infect humans. They can be either the source (reservoir) or  the mode of transmission (vector) of certain organisms.  Diseases for which animals are the reservoirs are called  zoonoses. The important zoonotic diseases caused by bac¬  teria are listed in Table 7-7.   Certain bacteria have specialized structures (e.g., pili) or  produce substances (e.g., capsules or glycocalyces) that  allow them to adhere to the surface of human cells, thereby  enhancing their ability to cause disease. These adherence  mechanisms are essential for organisms that attach to  mucous membranes; mutants that lack these mechanisms    are often nonpathogenic. For example, the pili of Neisseria  gonorrhoeae and E. coli mediate the attachment of the  organisms to the urinary tract epithelium, and the glycoca-  lyx of Staphylococcus epidermidis and certain viridans  streptococci allows the organisms to adhere strongly to the  endothelium of heart valves. The various molecules that  mediate adherence to cell surfaces are called adhesins.   After the bacteria attach, they often form a protective  matrix called a biofilm consisting of various polysaccha¬  rides and proteins. Biofilms form especially on foreign bod¬  ies such as prosthetic joints, prosthetic heart valves, and  intravenous catheters, but they also form on native structures  such as heart valves. Biofilms protect bacteria from both  antibiotics and host immune defenses such as antibodies  and neutrophils. They also retard wound healing resulting  in chronic wound infections, especially in diabetics. Biofilms  play an important role in the persistence of Pseudomonas in  the lungs of cystic fibrosis patients and in the formation of  dental plaque, the precursor of dental caries.   The production of biofilms by bacteria such as  Pseudomonas is controlled by the process of quorum sensing.  In quorum sensing, the bacteria grow in a nonaggressive  manner until a quorum is sensed (i.e., a certain density of  bacteria has been reached), at which point the synthesis of  new bacterial virulence factors (e.g., biofilms) that con¬  tribute to pathogenesis occurs.   Foreign bodies, such as artificial heart valves and artifi¬  cial joints, predispose to infections. Bacteria can adhere to  these surfaces, but phagocytes adhere poorly owing to the  absence of selectins and other binding proteins on the arti¬  ficial surface (see Chapter 8).   Some strains of E. coli and Salmonella have surface pro¬  teins called curb, which mediate binding of the bacteria to  endothelium and to extracellular proteins such as fibronec-  tin. Curb also interact with serum proteins such as factor  XII—a component of the coagulation cascade. Curb, there¬  fore, are thought to play a role in the production of the  thrombi seen in the disseminated intravascular coagulation  (DIC) associated with sepsis caused by these bacteria (see  the discussion of endotoxin on page 44).   One of the two main mechanisms by which bacteria cause  disease is invasion of tissue followed by inflammation.  (The inflammatory response is described in Chapter 8.)  The other main mechanism, toxin production, and a third  mechanism, immunopathogenesis, are described later in  this chapter.   Several enzymes secreted by invasive bacteria play a role  in pathogenesis. Among the most prominent are the  following:   (1) Collagenase and hyaluronidase, which degrade  collagen and hyaluronic acid, respectively, thereby allowing    the bacteria to spread through subcutaneous tissue; they  are especially important in cellulitis caused by Streptococcus  pyogenes.   (2) Coagulase, which is produced by Staphylococcus  aureus and accelerates the formation of a fibrin clot from its  precursor, fibrinogen (this clot may protect the bacteria  from phagocytosis by walling off the infected area and by  coating the organisms with a layer of fibrin). Coagulase is  also produced by Yersinia pestis, the cause of bubonic  plague. See Chapter 20 for the role of coagulase in the  pathogenesis of plague.   (3) Immunoglobulin A (IgA) protease, which degrades  IgA, allowing the organism to adhere to mucous membranes,  and is produced chiefly by N. gonorrhoeae, Haemophilus  influenzae, and Streptococcus pneumoniae.   In addition to these enzymes, several virulence factors  contribute to invasiveness by limiting the ability of the host  defense mechanisms, especially phagocytosis, to operate  effectively.   (1) The most important of these antiphagocytic factors  is the capsule external to the cell wall of several important  pathogens such as S. pneumoniae and Neisseria meningiti¬  dis. The polysaccharide capsule prevents the phagocyte  from adhering to the bacteria; anticapsular antibodies  allow more effective phagocytosis to occur (a process called  opsonization) (see page 55). The vaccines against S. pneu¬  moniae, H. influenzae, and N. meningitidis contain capsular  polysaccharides that induce protective anticapsular  antibodies.   (2) A second group of antiphagocytic factors are the cell  wall proteins of the gram-positive cocci, such as the M pro¬  tein of the group A streptococci (S. pyogenes) and protein  A of S. aureus. The M protein is antiphagocytic, and pro¬  tein A binds to immunoglobulin G (IgG) and prevents the  activation of complement. These virulence factors are  summarized in Table 7-8.   Bacteria can cause two types of inflammation: pyogenic  and granulomatous. In pyogenic (pus-producing) inflam¬  mation, neutrophils are the predominant cells. Some of the  most important pyogenic bacteria are the gram-positive  and gram-negative cocci listed in Table 7-8. In granuloma¬  tous inflammation, macrophages and helper T cells pre¬  dominate. The most important organism in this category is  Mycobacterium tuberculosis. No bacterial enzymes or tox¬  ins that induce granulomas have been identified. Rather, it  appears that bacterial antigens stimulate the cell-mediated  immune system, resulting in sensitized T-lymphocyte and  macrophage activity. Phagocytosis by macrophages kills  most of the bacteria, but some survive and grow within the  macrophages in the granuloma.   Intracellular survival is an important attribute of cer¬  tain bacteria that enhances their ability to cause disease.   Binds to Fc region of IgG, which prevents activation of  complement   ’Do not confuse the serotype with the grouping of streptococci, which is determined by the polysaccharide in the cell wall.    These bacteria are called “intracellular” pathogens and  commonly cause granulomatous lesions. The best-known  of these bacteria belong to the genera Mycobacterium,  Legionella, Brucella, and Listeria. The best-known fungus is  Histoplasma. These organisms are not obligate intracellular  parasites, which distinguishes them from Chlamydia and  Rickettsia. They can be cultured on microbiologic media in  the laboratory and therefore are not obligate intracellular  parasites. Rather, they prefer an intracellular location prob¬  ably because they are protected there from antibody and  neutrophils that function extracellularly.   These bacteria use several different mechanisms to  allow them to survive and grow intracellularly. These  include (1) inhibition of the fusion of the phagosome with  the lysosome, which allows the organisms to avoid the deg-  radative enzymes in the lysosome; (2) inhibition of acidifi¬  cation of the phagosome, which reduces the activity of the  lysosomal degradative enzymes; and (3) escape from the  phagosome into the cytoplasm, where there are no degra¬  dative enzymes. Members of the genera Mycobacterium  and Legionella are known to use the first and second  mechanisms, whereas Listeria species use the third.   The invasion of cells by bacteria is dependent on the  interaction of specific bacterial surface proteins called  invasins and specific cellular receptors belonging to the  integrin family of transmembrane adhesion proteins. The  movement of bacteria into the cell is a function of actin  microfilaments. Once inside the cell, these bacteria typi¬  cally reside within cell vacuoles such as phagosomes. Some  remain there, others migrate into the cytoplasm, and some  move from the cytoplasm into adjacent cells through tun¬  nels formed from actin. Infection of the surrounding cells  in this manner allows the bacteria to evade host defenses.  For example, Listeria monocytogenes aggregates actin   The “Yops” ( Yersinia outer-membrane proteins) pro¬  duced by several Yersinia species are important examples of  bacterial virulence factors that act primarily after invasion  of human cells by the organism. The most important effects  of the Yops proteins are to inhibit phagocytosis by neutro¬  phils and macrophages and to inhibit cytokine production  (e.g., tumor necrosis factor [TNF] production) by macro¬  phages. For example, one of the Yops proteins of Yersinia  pestis (Yop J) is a protease that cleaves signal transduction  proteins required for the induction of TNF synthesis. This  inhibits the activation of our host defenses and contributes  to the ability of the organism to cause bubonic plague.   The genes that encode many virulence factors in bacte¬  ria are clustered in pathogenicity islands on the bacterial  chromosome. For example, in many bacteria, the genes  encoding adhesins, invasins, and exotoxins are adjacent to  each other on these islands. Nonpathogenic variants of  these bacteria do not have these pathogenicity islands. It  appears that these large regions of the bacterial genome  were transferred as a block via conjugation or transduction.  Unlike plasmids and bacteriophage, pathogenicity islands  do not have the ability to replicate independent of the bac¬  terial chromosome. Pathogenicity islands are found in  many gram-negative rods, such as E. coli, Salmonella,  Shigella, Pseudomonas, and Vibrio cholerae, and in gram¬  positive cocci, such as S. pneumoniae. Additional informa¬  tion about pathogenicity islands is given on page 47.   After bacteria have colonized and multiplied at the por¬  tal of entry, they may invade the bloodstream and spread to  other parts of the body. Receptors for the bacteria on the  surface of cells determine, in large part, the organs affected.  For example, certain bacteria or viruses infect the brain    because receptors for these microbes are located on the  surface of brain neurons. The blood-brain barrier, which  limits the ability of certain drugs to penetrate the brain, is  not thought to be a determinant of microbial infection of  the brain. The concept of a blood-brain barrier primarily  refers to the inability of hydrophilic (charged, ionized)  drugs to enter the lipid-rich brain parenchyma, whereas  lipophilic (lipid-soluble) drugs enter well.   Two important diseases, diphtheria and pseudomem¬  branous colitis, are characterized by inflammatory lesions  called pseudomembranes. Pseudomembranes are thick,  adherent, grayish or yellowish exudates on the mucosal  surfaces of the throat in diphtheria and on the colon in  pseudomembranous colitis. The term pseudo refers to the  abnormal nature of these membranes in contrast to the  normal anatomic membranes of the body, such as the tym¬  panic membrane and the placental membranes.   The second major mechanism by which bacteria cause dis¬  ease is the production of toxins. A comparison of the main  features of exotoxins and endotoxins is shown in Table 7-9.   Exotoxins are produced by several gram-positive and  gram-negative bacteria, in contrast to endotoxins, which  are present only in gram-negative bacteria. The essential  characteristic of exotoxins is that they are secreted by the  bacteria, whereas endotoxin is a component of the cell wall.  Exotoxins are polypeptides whose genes are frequently  located on plasmids or lysogenic bacterial viruses (bacte¬  riophages). Some important exotoxins encoded by bacte¬  riophage DNA are diphtheria toxin, cholera toxin, and  botulinum toxin.    Exotoxins are among the most toxic substances known.  For example, the fatal dose of tetanus toxin for a human is  estimated to be less than 1 pg. Because some purified exo¬  toxins can reproduce all aspects of the disease, we can  conclude that certain bacteria play no other role in patho¬  genesis than to synthesize the exotoxin. Exotoxin polypep¬  tides are good antigens and induce the synthesis of  protective antibodies called antitoxins, some of which are  useful in the prevention or treatment of diseases such as  botulism and tetanus. When treated with formaldehyde (or  acid or heat), the exotoxin polypeptides are converted into  toxoids, which are used in protective vaccines because they  retain their antigenicity but have lost their toxicity.   Many exotoxins have an A-B subunit structure; the A  (or active) subunit possesses the toxic activity, and the B (or  binding) subunit is responsible for binding the exotoxin to  specific receptors on the membrane of the human cell. The  binding of the B subunit determines the specific site of the  action of the exotoxin. For example, botulinum toxin acts  at the neuromuscular junction because the B subunit binds  to specific receptors on the surface of the motor neuron at  the junction. Important exotoxins that have an A-B subunit  structure include diphtheria toxin, tetanus toxin, botuli¬  num toxin, cholera toxin, and the enterotoxin of E. coli  (Figure 7-1).   The A subunit of several important exotoxins acts by  ADP-ribosylation (i.e., the A subunit is an enzyme that  catalyzes the addition of adenosine diphosphate ribose  [ADP-ribose] to the target protein in the human cell). The  addition of ADP-ribose to the target protein often inacti¬  vates it but can also hyperactivate it, either of which can  cause the symptoms of disease. For example, diphtheria  toxin and Pseudomonas exotoxin A ADP-ribosylate elonga¬  tion factor-2 (EF-2), thereby inactivating it and resulting in  the inhibition of protein synthesis. On the other hand,    FIGURE 7-1 Mode of action of diphtheria toxin. The toxin  binds to the cell surface via its binding subunit, and the active sub¬  unit enters the cell. The active subunit is an enzyme that catalyzes  the addition of ADP-ribose (ADP-R) to elongation factor-2 (EF-2).This  inactivates EF-2, and protein synthesis is inhibited.    cholera toxin and E. coli toxin ADP-ribosylate G s protein,  thereby activating it. This causes an increase in adenylate  cyclase activity, a consequent increase in the amount of  cyclic adenosine monophosphate (AMP), and the produc¬  tion of watery diarrhea. Pertussis toxin is an interesting  variation on the theme. It ADP-ribosylates G, protein and  inactivates it. Inactivation of the inhibitory G proteins  turns on adenylate cyclase, causing an increase in the  amount of cyclic AMP, which plays a role in causing the  symptoms of whooping cough.    Exotoxins are released from bacteria by specialized  structures called secretion systems. Some secretion sys¬  tems transport the exotoxins into the extracellular space,  but others transport the exotoxins directly into the mam¬  malian cell. Those that transport the exotoxins directly into  the mammalian cell are especially effective because the  exotoxin is not exposed to antibodies in the extracellular  space.   Six types of secretion systems have been identified, but  the type III secretion system (also called an injectosome)  is particularly important in virulence. This secretion sys¬  tem is mediated by a needlelike projection (sometimes  called a “molecular syringe”) and by transport pumps in the  bacterial cell membrane. The importance of the type III  secretion system is illustrated by the finding that the strains  of Pseudomonas aeruginosa that have this secretion system  are significantly more virulent than those that do not.  Other medically important gram-negative rods that utilize  injectosomes include Shigella species, Salmonella species,  E. coli, and Y. pestis.   The mechanisms of action of the important exotoxins  produced by toxigenic bacteria are described in the fol¬  lowing discussion and summarized in Tables 7-10, 7-11,   Blocks release of the inhibitory neurotransmitter glycine by  proteolytic cleavage of releasing proteins   Edema factor is an adenylate cyclase; lethal factor is a protease  that cleaves MAP kinase, which is required for cell division   Is a superantigen; binds to class II MHC protein and T-cell  receptor; induces IL-1 and IL-2   Is a superantigen; action similar to toxic shock syndrome toxin  of S aureus   Labile toxin stimulates adenylate cyclase by ADP-ribosylation;  stable toxin stimulates guanylate cyclase   Shiga toxin inhibits protein synthesis in enterocytes by  removing adenine from 28S ribosomal RNA   Shiga toxin inhibits protein synthesis in enterocytes by  removing adenine from 28S ribosomal RNA   2 The acellular vaccine contains pertussis toxoid and four other proteins.   Diphtheria toxin, cholera toxin, Escherichia  coli heat-labile toxin, and pertussis toxin   Tetanus toxin, botulinum toxin, lethal  factor of anthrax toxin, and scalded skin  toxin    and 7-12. The main location of symptoms of disease  caused by bacterial exotoxins is described in Table 7-13.   The exotoxins produced by gram-positive bacteria have  several different mechanisms of action and produce differ¬  ent clinical effects. Some important exotoxins include diph¬  theria toxin, which inhibits protein synthesis by inactivating  EF-2; tetanus toxin and botulinum toxin, which are neuro¬  toxins that prevent the release of neurotransmitters; and  toxic shock syndrome toxin (TSST), which acts as a supe¬  rantigen causing the release of large amounts of cytokines  from helper T cells and macrophages. The mechanisms of  action and the clinical effects of exotoxins produced by  gram-positive bacteria are described next.   (1) Diphtheria toxin, produced by Corynebacterium  diphtheriae, inhibits protein synthesis by ADP-ribosylation  of EF-2 (Figure 7-1). 1   The consequent death of the cells leads to two promi¬  nent symptoms of diphtheria: pseudomembrane formation  in the throat and myocarditis.   The exotoxin activity depends on two functions medi¬  ated by different domains of the molecule. The toxin is   ADP-ribosylates G s factor,  which activates it, thereby  stimulating adenylate cyclase   ADP-ribosylates Gj factor, which  inactivates it, thereby stimu¬  lating adenylate cyclase    1 Pseudomonas aeruginosa exotoxin A has the same mode of action.    synthesized as a single polypeptide (molecular weight  62,000) that is nontoxic because the active site of the  enzyme is masked (Figure 7-2). A single proteolytic “nick”  plus reduction of the sulfhydryl bonds yield two active  polypeptides. Fragment A, a 22,000-molecular-weight pep¬  tide at the amino-terminal end of the exotoxin, is an  enzyme that catalyzes the transfer of ADP-ribose from  nicotinamide adenine dinucleotide (NAD) to EF-2, thereby  inactivating it. The ADP-ribosylation of EF-2 freezes the  translocation complex, and protein synthesis stops. The  reaction is as follows:   Fragment B, a 40,000-molecular-weight peptide at the  carboxy-terminal end, binds to receptors on the outer  membrane of eukaryotic cells and mediates transport of  fragment A into the cells.   To summarize, the exotoxin binds to cell membrane  receptors via a region near its carboxyl end. The toxin is  transported across the membrane, and the proteolytic nick  and reduction of the disulfide bonds occur. This releases  the active fragment A, which inactivates EF-2. The enzy¬  matic activity is specific for EF-2; no other protein is ADP-  ribosylated. The specificity is due to the presence in EF-2 of  a unique amino acid, a modified histidine called diph-  thamide. The reaction occurs in all eukaryotic cells; there is  no tissue or organ specificity. Prokaryotic and mitochon¬  drial protein synthesis is not affected because a different,  nonsusceptible elongation factor is involved. The enzyme  activity is remarkably potent; a single molecule of frag¬  ment A will kill a cell within a few hours. Other organisms  whose exotoxins act by ADP-ribosylation are E. coli, V.  cholerae, and Bordetella pertussis.   The tox gene, which codes for the exotoxin, is carried by  a lysogenic bacteriophage called beta phage. As a result,  only C. diphtheriae strains lysogenized by this phage cause  diphtheria. (Nonlysogenized C. diphtheriae can be found in  the throat of some healthy people.) This is an important  example of lysogenic conversion, the process by which bac¬  teria acquire new traits when lysogenized by a bacterio¬  phage (see Chapter 4). Regulation of exotoxin synthesis is  controlled by the interaction of iron in the medium with a  tox gene repressor synthesized by the bacterium. As the  concentration of iron increases, the iron-repressor complex  inhibits the transcription of the tox gene.   (2) Tetanus toxin, produced by Clostridium tetani, is a  neurotoxin that prevents release of the inhibitory neu¬  rotransmitter glycine. When the inhibitory neurons are  nonfunctional, the excitatory neurons are unopposed, lead¬  ing to muscle spasms and a spastic paralysis. Tetanus toxin  (tetanospasmin) is composed of two polypeptide subunits  encoded by plasmid DNA. The heavy chain of the polypep¬  tide binds to gangliosides in the membrane of the neuron;  the light chain is a protease that degrades the protein(s)  responsible for the release of the inhibitory neurotransmit¬  ter. The toxin released at the site of the peripheral wound   Edema factor is an adenylate cyclase; lethal factor is a protease    may travel either by retrograde axonal transport or in the  bloodstream to the anterior horn and interstitial neurons of  the spinal cord. Blockage of release of the inhibitory trans¬  mitter leads to convulsive contractions of the voluntary  muscles, best exemplified by spasm of the jaw and neck  muscles (“lockjaw”).   (3) Botulinum toxin, produced by Clostridium botulinum,  is a neurotoxin that blocks the release of acetylcholine at  the synapse, producing a flaccid paralysis. Approximately 1  pg is lethal for humans; it is one of the most toxic com¬  pounds known. The toxin is composed of two polypeptide  subunits held together by disulfide bonds. One of the sub¬  units binds to a receptor on the neuron; the other subunit    is a protease that degrades the protein(s) responsible for the  release of acetylcholine. There are six serotypes of botuli¬  num toxin (A-F). Some serotypes are encoded on a  plasmid, some on a temperate bacteriophage, and some on  the bacterial chromosome.   (4) Two exotoxins are produced by Clostridium difficile,  both of which are involved in the pathogenesis of pseudo¬  membranous colitis. Exotoxin A is an enterotoxin that  causes watery diarrhea. Exotoxin B is a cytotoxin that dam¬  ages the colonic mucosa and causes pseudomembranes to  form. Exotoxins A and B are glucosyltransferases that glu-  cosylate signal transduction proteins called Rho GTPases—  a process that inhibits these GTPases from performing    FIGURE 7~2 Diphtheria exotoxin. Intact extracellular toxin binds to a eukaryotic cell by its B region (dark fragment). After proteolytic  cleavage and reduction of the disulfide bond, the A region (light fragment) containing the ribosylating enzyme is activated. (Adapted from  Pappenheimer, G. Diphtheria: Recent studies have clarified the molecular mechanisms involved in its pathogenesis. Science. 1973;182:354.)    their signal transduction function. Glucosylation by exo¬  toxin B causes disaggregation of actin filaments in the  cytoskeleton, leading to apoptosis and cell death.   (5) Multiple toxins are produced by Clostridium perfrin-  gens and other species of clostridia that cause gas gangrene.  A total of 7 lethal factors and 5 enzymes have been charac¬  terized, but no species of Clostridium makes all 12 prod¬  ucts. The best characterized is the alpha toxin, which is a  lecithinase that hydrolyzes lecithin in the cell membrane,  resulting in destruction of the membrane and widespread  cell death. The other four enzymes are collagenase, prote¬  ase, hyaluronidase, and deoxyribonuclease (DNase). The  seven lethal toxins are a heterogeneous group with hemo¬  lytic and necrotizing activity. Certain strains of C. perfrin-  gens produce an enterotoxin that causes watery diarrhea  This enterotoxin acts as a superantigen similar to the  enterotoxin of S. aureus (see point 8).   (6) Three exotoxins are produced by Bacillus anthracis,  the agent of anthrax: edema factor, lethal factor, and protec¬  tive antigen. The three exotoxins associate with each other,  but each component has a distinct function. Edema factor  is an adenylate cyclase that raises the cyclic AMP concen¬  tration within the cell, resulting in loss of chloride ions and  water and consequent edema formation in the tissue  (Table 7-12). Lethal factor is a protease that cleaves a phos-  phokinase required for the signal transduction pathway  that controls cell growth. Loss of the phosphokinase results  in the failure of cell growth and consequent cell death. Pro¬  tective antigen binds to a cell surface receptor and forms  pores in the human cell membrane that allow edema factor  and lethal factor to enter the cell. The name protective anti¬  gen is based on the finding that antibody against this pro¬  tein protects against disease. The antibody blocks the  binding of protective antigen, thereby preventing edema  factor and lethal factor from entering the cell.   (7) TSST is a superantigen produced primarily by  certain strains of S. aureus but also by certain strains of  S. pyogenes. TSST binds directly to class II major histocom¬  patibility (MHC) proteins on the surface of antigen-  presenting cells (macrophages) without intracellular  processing. This complex interacts with the (3-chain of the  T-cell receptor of many helper T cells (see the discussion of  superantigens in Chapter 58). This causes the release of  large amounts of interleukins, especially interleukin-1,  interleukin-2, and tumor necrosis factor (TNL). These  cytokines produce many of the signs and symptoms of toxic  shock. TSST is also a T-cell “mitogen” (i.e., it induces T cells  to multiply), which contributes to the overproduction of  cytokines.   (8) Staphylococcal enterotoxin is also a superantigen,  but because it is ingested, it acts locally on the lymphoid  cells lining the small intestine. The enterotoxin is produced  by S. aureus in the contaminated food and causes food  poisoning, usually within 1 to 6 hours after ingestion. The  main symptoms are vomiting and watery diarrhea. The  prominent vomiting seen in food poisoning is thought to    be caused by cytokines released from the lymphoid cells  stimulating the enteric nervous system, which activates the  vomiting center in the brain.   (9) Exfoliatin is a protease produced by S. aureus that  causes scalded skin syndrome. Exfoliatin cleaves desmo-  glein, a protein in the desmosomes of the skin, resulting in  the detachment of the superficial layers of the skin. Exfolia¬  tin is also called epidermolytic toxin.   (10) Panton-Valentine (PV) leukocidin is a pore-form¬  ing exotoxin produced by methicillin-resistant strains of  S. aureus (MRSA). It destroys white blood cells, skin, and  subcutaneous tissue. The two subunits of the toxin assem¬  ble in the cell membrane to form a pore through which cell  contents exit into the extracellular space.   (11) Erythrogenic toxin, produced by S. pyogenes,  causes the rash characteristic of scarlet fever. Its mecha¬  nism of action is similar to that of TSST (i.e., it acts as a  superantigen; see point 8 earlier). The DNA that codes for  the toxin resides on a temperate bacteriophage. Nonlyso-  genic bacteria do not cause scarlet fever, although they can  cause pharyngitis.   The exotoxins produced by gram-negative bacteria also  have several different mechanisms of action and produce  different clinical effects. Two very important exotoxins are  the enterotoxins of E. coli and V. cholerae (cholera toxin),  which induce an increase in the amount of cyclic AMP  within the enterocyte, resulting in watery diarrhea  (Table 7-12). The mechanisms of action and the clinical  effects of exotoxins produced by gram-negative bacteria are  described next.   (1) The heat-labile enterotoxin produced by E. coli  causes watery, nonbloody diarrhea by stimulating adenyl¬  ate cyclase activity in cells in the small intestine (Figure 7-3).  The resulting increase in the concentration of cyclic AMP  causes excretion of the chloride ion, inhibition of sodium  ion absorption, and significant fluid and electrolyte loss  into the lumen of the gut. The heat-labile toxin, which is  inactivated at 65°C for 30 minutes, is composed of two  subunits: a B subunit, which binds to a ganglioside receptor  in the cell membrane, and an A subunit, which enters the  cell and mediates the transfer of ADP-ribose from NAD to  a stimulatory coupling protein (G s protein). This locks the  G s protein in the “on” position, thereby continually stimu¬  lating adenylate cyclase to synthesize cyclic AMP. This  activates cyclic AMP-dependent protein kinase, an enzyme  that phosphorylates ion transporters in the cell membrane,  resulting in the loss of water and ions from the cell. The  genes for the heat-labile toxin and for the heat-stable toxin  (described next) are carried on plasmids.   In addition to the labile toxin, there is a heat-stable  toxin, which is a polypeptide that is not inactivated by boil¬  ing for 30 minutes. The heat-stable toxin affects cyclic    FIGURE 7-3 Mode of action of Escherichia coli and Vibrio choierae enterotoxins. The enterotoxin (e.g., cholera toxin) binds to the surface  of the enterocyte via its binding subunit. The active subunit then enters the enterocyte. The active subunit is an enzyme that catalyzes the addi¬  tion of ADP-ribose (ADP-R) to the G s regulatory protein. This activates adenylate cyclase to overproduce cyclic adenosine monophosphate  (AMP). As a consequence, cyclic AMP-dependent protein kinase activity increases, and water and electrolytes leave the enterocyte, causing  watery diarrhea.    guanosine monophosphate (GMP) rather than cyclic AMP.  It stimulates guanylate cyclase and thus increases the con¬  centration of cyclic GMP, which inhibits the reabsorption  of sodium ions and causes diarrhea.   (2) Shiga toxin (also known as verotoxin and Shiga-like  toxin) is an exotoxin produced primarily by strains of E. coli  with the 0157:H7 serotype. These enterohemorrhagic  strains cause bloody diarrhea and are the cause of out¬  breaks associated with eating undercooked meat, especially  hamburger in fast-food restaurants. The toxin is named for  a very similar toxin produced by Shigella dysenteriae. The  toxin inactivates protein synthesis by removing adenine  from a specific site on the 28S rRNA in the large subunit of  the human ribosome. The term verotoxin refers to its cyto-  pathic effect on Vero (monkey) cells in culture.   Shiga toxin is encoded by a temperate (lysogenic) bacte¬  riophage. When Shiga toxin enters the bloodstream, it can  cause hemolytic-uremic syndrome (HUS). Shiga toxin  binds to receptors on the kidney and on the endothelium of  small blood vessels. Inhibition of protein synthesis results  in death of those cells, leading to renal failure and microan¬  giopathic hemolytic anemia. Certain antibiotics, such as  ciprofloxacin, can increase the amounts of Shiga toxin  produced by E. coli 0157, which predisposes to HUS.   (3) The enterotoxins produced by Vi choierae, the agent  of cholera (see Chapter 18), and Bacillus cereus, a cause of  diarrhea, act in a manner similar to that of the heat-labile  toxin of E. coli (Figure 7-3).   (4) Pertussis toxin, produced by B. pertussis, the cause  of whooping cough, is an exotoxin that catalyzes the trans¬  fer of ADP-ribose from NAD to an inhibitory G protein.  Inactivation of this inhibitory regulator has two effects:  one is in the stimulation of adenylate cyclase activity and a  consequent increase in the amount of cyclic AMP within  the affected cells (see Table 7-12). This results in edema  and other changes in the respiratory tract, leading to the  cough of whooping cough. The second effect is the inhibi¬  tion of the signal transduction pathway used by chemokine    receptors. This causes the marked lymphocytosis seen in  patients with pertussis. The toxin inhibits signal transduc¬  tion by all chemokine receptors, resulting in an inability of  lymphocytes to migrate to and enter lymphoid tissue  (spleen, lymph nodes). Because they do not enter tissue,  there is an increase in their number in the blood (see the  discussion of chemokines in Chapter 58).   Endotoxins are integral parts of the cell walls of both gram¬  negative rods and cocci, in contrast to exotoxins, which are  actively released from the cell (see Table 7-9). In addition,  several other features distinguish these substances. Endo¬  toxins are lipopolysaccharides (LPS), whereas exotoxins are  polypeptides; the enzymes that produce the LPS are  encoded by genes on the bacterial chromosome, rather  than by plasmid or bacteriophage DNA, which usually  encodes the exotoxins. The toxicity of endotoxins is low in  comparison with that of exotoxins. All endotoxins produce  the same generalized effects of fever and shock, although  the endotoxins of some organisms are more effective than  those of others (Figure 7-4). Endotoxins are weakly anti¬  genic; they induce protective antibodies so poorly that  multiple episodes of toxicity can occur. No toxoids have  been produced from endotoxins, and endotoxins are not  used as antigens in any available vaccine.   A major site of action of endotoxin is the macrophage.  Endotoxins (LPS) are released from the surface of gram¬  negative bacteria in small pieces of outer membrane that bind  to LPS-binding protein in the plasma. This complex binds to  a receptor on the surface of macrophages called CD 14, which  activates toll-like receptor-4 (TLR-4). A signal cascade within  the macrophage is then activated, resulting in the synthesis of  cytokines such as interleukin-1 (IL-1), tumor necrosis factor  (TNF), and nitric oxide (see later and Figure 7-4).   The findings of fever and hypotension are salient fea¬  tures of septic shock. Additional features include   FIGURE 7-4 Mode of action of endotoxin. Endotoxin is the most important cause of septic shock, which is characterized primarily by  fever, hypotension, and disseminated intravascular coagulation (DIC). Endotoxin causes these effects by activating three critical processes: (1)  activating macrophages to produce interleukin-1 (IL-1), tumor necrosis factor (TNF), and nitric oxide; (2) activating complement to produce C3a  and C5a; and (3) activating tissue factor, an early component of the coagulation cascade.    tachycardia, tachypnea, and leukocytosis (increased white  blood cells, especially neutrophils, in the blood). Septic  shock is one of the leading causes of death in intensive care  units and has an estimated mortality rate of 30% to 50%.  The endotoxins of gram-negative bacteria are the best-  established causes of septic shock, but surface molecules of  gram-positive bacteria (which do not have endotoxins) can  also cause septic shock.   (1) Septic shock is different from toxic shock. In septic  shock, the bacteria are in the bloodstream, whereas in toxic  shock, it is the toxin that is circulating in the blood. The  clinical importance of this observation is that in septic  shock, blood cultures are usually positive, whereas in toxic  shock, they are usually negative.   (2) Septic shock can cause the death of a patient even  though antibiotics have killed the bacteria in the patient’s  blood (i.e., the blood cultures have become negative). This  occurs because septic shock is mediated by cytokines, such  as TNF and interleukin-1, which continue to act even  though the bacteria that induced the cytokines are no lon¬  ger present.   The structure of the LPS is shown in Figure 2-6. The  toxic portion of the molecule is lipid A, which contains  several fatty acids. fl-Hydroxymyristic acid is always one of  the fatty acids and is found only in lipid A. The other fatty  acids differ according to species. The polysaccharide core  in the middle of the molecule protrudes from the surface of  the bacteria and has the same chemical composition within  members of a genus.   The somatic (O) antigen is a polysaccharide on the exte¬  rior that differs in each species and frequently differs  between strains of a single species. It is an important anti¬  gen of some gram-negative bacteria and is composed of 3,  4, or 5 sugars repeated up to 25 times. Because the number  of permutations of this array is very large, many antigenic    types exist. For example, more than 1500 antigenic types  have been identified for Salmonella. Some bacteria, espe¬  cially N. meningitidis and N. gonorrhoeae, have lipooligo-  saccharide (LOS) containing very few repeating sugar  subunits in the O antigen.   (1) Fever due to the release by macrophages of interleu¬  kin-1 (endogenous pyrogen) and interleukin-6, which act  on the hypothalamic temperature-regulatory center.   (2) Hypotension, shock, and impaired perfusion of  essential organs owing to nitric oxide-induced vasodilation,  TNF-induced increased capillary permeability, bradykinin-  induced vasodilation, and increased capillary permeability.   (3) DIC due to activation of the coagulation cascade,  resulting in thrombosis, a petechial or purpuric rash, and  tissue ischemia, leading to failure of vital organs. The  coagulation cascade is activated when tissue factor is  released from the surface of endothelial cells damaged by  infection. Tissue factor interacts with circulating coagula¬  tion factors to cause widespread clotting within capillaries.  A positive D-dimer test provides laboratory evidence for a  diagnosis of DIC.    (4) Activation of the alternative pathway of the comple¬  ment cascade, resulting in inflammation and tissue dam¬  age. C5a is potent chemokine that attracts neutrophils to  the site of infection.   (5) Activation of macrophages, increasing their phago¬  cytic ability, and activation of many clones of B lympho¬  cytes, increasing antibody production. (Endotoxin is a  polyclonal activator of B cells, but not T cells.)   The end result of the above five processes is called the  systemic inflammatory response syndrome, or SIRS. The  most common clinical signs of SIRS are fever, hypotension,  tachycardia, tachypnea, and leukocytosis.   Damage to the vascular endothelium plays a major  role in both the hypotension and DIC seen in septic shock.  Damage to the endothelium allows the leakage of plasma  and red cells into the tissue, resulting in the loss of blood  volume and consequent hypotension. Damaged endothe¬  lium also serves as a site of platelet aggregation and activa¬  tion that leads to the thousands of endovascular clots  manifesting as DIC.   The evidence that endotoxin causes these effects comes  from the following two findings: (1) purified LPS, free of  the organism, reproduces the effects, and (2) antiserum  against endotoxin can mitigate or block these effects.   Clinically, the presence of DIC in the patient can be  assessed by the D-dimer laboratory test. D-dimers are cleav¬  age products of fibrin (fibrin split products) that are  detected in the blood of patients with DIC.   Endotoxins do not cause these effects directly. Rather,  they elicit the production of cytokines such as IL-1 and  TNF from macrophages. 2 TNF is the central mediator  because purified recombinant TNF reproduces the effects  of endotoxin and antibody against TNF blocks the effects  of the endotoxin. Endotoxin also induces macrophage  migration inhibitory factor, which also plays a role in the  induction of septic shock.   Note that TNF in small amounts has beneficial effects  (e.g., causing an inflammatory response to the presence of  a microbe), but in large amounts, it has detrimental effects  (e.g., causing septic shock and DIC). It is interesting that  the activation of platelets, which results in clot formation  and the walling off of infections, is the same process that,  when magnified, causes DIC and the necrosis of tumors. It  is the ability of TNF to activate platelets that causes intra¬  vascular clotting and the consequent infarction and death  of the tumor tissue. The symptoms of certain autoimmune  diseases such as rheumatoid arthritis are also mediated by  TNF; however, these symptoms are not induced by    2 Endotoxin (LPS) induces these factors by first binding to LPS-binding pro¬  tein in the serum. This complex then binds to CD 14, a receptor on the surface  of the macrophage. CD 14 interacts with a transmembrane protein called toll¬  like receptor, which activates an intracellular signaling cascade, leading to the  activation of genes that encode various cytokines such as interleukin-1, TNF,  and other factors.   endotoxin but by other mechanisms, which are described  in Chapter 66. Some of the important beneficial and harm¬  ful effects of TNF are listed in Table 7-15.   Endotoxins can cause fever in the patient if they are  present in intravenous fluids. In the past, intravenous fluids  were sterilized by autoclaving, which killed any organisms  present but resulted in the release of endotoxins that were  not heat inactivated. For this reason, these fluids are now  sterilized by filtration, which physically removes the organ¬  ism without releasing its endotoxin. The contamination of  intravenous fluids by endotoxin is detected by a test based  on the observation that nanogram amounts of endotoxin  can clot extracts of the horseshoe crab, Limulus.   Endotoxin-like pathophysiologic effects can occur in  gram-positive bacteremic infections (e.g., S. aureus and  S. pyogenes infections) as well. Since endotoxin is absent in  these organisms, a different cell wall component—namely,  lipoteichoic acid—causes the release of TNF and IL-1 from  macrophages.   Endotoxin-mediated septic shock is a leading cause of  death, especially in hospitals. Attempts to treat septic shock  with antibodies to lipid A and TNF have been unsuccessful.  Treatment with activated protein C (drotrecogin-alfa,  Xigris) was initially thought to be effective but adverse  effects, such as bleeding, and controversy regarding its  effectiveness, caused Xigris to be withdrawn from the mar¬  ket in 2011. Protein C is a normal human protein that func¬  tions as an anticoagulant by inhibiting thrombin formation.  It also enhances fibrinolysis, which degrades clots once  they are formed. These are the features of Protein C that led  it to be considered as a treatment for endotoxin-mediated  septic shock.   In certain diseases, such as rheumatic fever and acute glo¬  merulonephritis, it is not the organism itself that causes the  symptoms of disease but the immune response to the pres¬  ence of the organism. For example, in rheumatic fever,  antibodies are formed against the M protein of S. pyogenes,  which cross-react with joint, heart, and brain tissue.  Inflammation occurs, resulting in the arthritis, carditis, and  chorea that are the characteristic findings in this disease.   The fact that certain viruses can cause cancer is well estab¬  lished, but the observation that some bacterial infections  are associated with cancers is just emerging. Several docu¬  mented examples include (1) the association of Helico¬  bacter pylori infection with gastric carcinoma and gastric  mucosal-associated lymphoid tissue (MALT) lymphoma,  and (2) the association of Campylobacter jejuni infection  with MALT lymphoma of the small intestine (also known  as alpha-chain disease). Support for the idea that these  cancers are caused by bacteria comes from the observation  that antibiotics can cause these cancers to regress if treated  during an early stage.   Staphylococcus aureus causes inflammatory, pyogenic dis¬  eases such as endocarditis, osteomyelitis, and septic arthri¬  tis, as well as nonpyogenic, exotoxin-mediated diseases  such as toxic shock syndrome, scalded skin syndrome, and  food poisoning. How do bacteria that belong to the same  genus and species cause such widely divergent diseases?  The answer is that individual bacteria produce different    virulence factors that endow those bacteria with the capa¬  bility to cause different diseases.   The different virulence factors are encoded on plasmids,  on transposons, on the genome of temperate (lysogenic)  phages, and on pathogenicity islands. These transferable  genetic elements may or may not be present in any single  bacterium, which accounts for the ability to cause different  diseases. Table 7-16 describes the different virulence fac¬  tors for three of the most important bacterial pathogens:  S. aureus, S. pyogenes, and E. coli. Figure 7-5 describes the  importance of pathogenicity islands in determining the  types of diseases caused by E. coli.   A typical acute infectious disease has four main stages (see  Figure 7-6:   (1) The incubation period, which is the time between  the acquisition of the organism (or toxin) and the begin¬  ning of symptoms (this time varies from hours to days to  weeks, depending on the organism).   (2) The prodrome period, during which nonspecific  symptoms such as fever, malaise, and loss of appetite occur.   (3) The specific-disease period, during which the overt  characteristic signs and symptoms of the disease occur.   Antibody to M protein cross-reacts  with cardiac, joint, and brain tissue    FIGURE 7~5 Pathogenicity islands encode virulence factors that determine the type of infection. The top of the figure depicts a non-  pathogenic strain of Escherichia coli that does not contain a pathogenicity island (PAI) in the genome DNAThe black oval line within the E. coli  cell is the genome DNA PAIs can be transferred, by either conjugation or transduction, from another enteric gram-negative rod into the non-  pathogenic strain of E. coli. Acquisition of a PAI that encodes virulence factors endows the nonpathogenic E. coli with the ability to cause spe¬  cific diseases. In this figure, PAI-A encodes an enterotoxin, PAI-B encodes the pili that bind to urinary tract epithelium, and PAI-C encodes the  enzymes that synthesize the K-1 capsular polysaccharide. This results in three different strains of £. coli capable of causing three different  infections.    (4) The recovery period, also known as the convales¬  cence period, during which the illness abates and the  patient returns to the healthy state. IgG and IgA antibodies  protect the recovered patient from reinfection by the same  organism.   After the recovery period, some individuals become  chronic carriers of the organisms and may shed them  while remaining clinically well. Others may develop a  latent infection, which can recur either in the same form  as the primary infection or manifesting different signs and  symptoms. Although many infections cause symptoms,  many others are subclinical (i.e., the individual remains  asymptomatic although infected with the organism). In  subclinical infections and after the recovery period is over,  the presence of antibodies is often used to determine that  an infection has occurred.    DID THE ORGANISM ISOLATED  FROM THE PATIENT ACTUALLY  CAUSE THE DISEASE?   Because people harbor microorganisms as members of the  permanent normal flora and as transient passengers, this  can be an interesting and sometimes confounding ques¬  tion. The answer depends on the situation. One type of  situation relates to the problems of a disease for which no  agent has been identified and a candidate organism has  been isolated. This is the problem that Robert Koch faced  in 1877 when he was among the first to try to determine the  cause of an infectious disease, namely, anthrax in cattle and  tuberculosis in humans. His approach led to the formula¬  tion of Koch’s postulates, which are criteria that he   FIGURE 7-6 Typical stages of an infectious disease. After infection, the patient progresses through four main stages: incubation period,  prodrome period, specific disease period, and the recovery period. The patient then typically returns to good health and has antibody that pro¬  tects against reinfection and disease.    proposed must be satisfied to confirm the causal role of an  organism. These criteria are as follows:   (1) The organism must be isolated from every patient  with the disease.   (2) The organism must be isolated free from all other  organisms and grown in pure culture in vitro.   (3) The pure organism must cause the disease in a  healthy, susceptible animal.   (4) The organism must be recovered from the inocu¬  lated animal.   The second type of situation pertains to the practical,  everyday problem of a specific diagnosis of a patient’s ill¬  ness. In this instance, the signs and symptoms of the ill¬  ness usually suggest a constellation of possible causative  agents. The recovery of an agent in sufficient numbers    from the appropriate specimen is usually sufficient for an  etiologic diagnosis. This approach can be illustrated with  two examples: (1) in a patient with a sore throat, the pres¬  ence of a few (1-hemolytic streptococci is insufficient for  a microbiologic diagnosis, whereas the presence of many  would be sufficient, and (2) in a patient with fever,  a-hemolytic streptococci in the throat are considered  part of the normal flora, whereas the same organisms in  the blood are likely to be the cause of bacterial  endocarditis.   In some infections, no organism is isolated from the  patient, and the diagnosis is made by detecting a rise in  antibody titer to an organism. For this purpose, the titer  (amount) of antibody in the second or late serum sample  should be at least four times the titer (amount) of antibody  in the first or early serum sample.    The term pathogen refers to those microbes capable of caus¬  ing disease, especially if they cause disease in immunocompe¬  tent people. The term opportunistic pathogen refers to  microbes that are capable of causing disease only in immuno¬  compromised people.   Virulence is a measure of a microbe's ability to cause disease  (i.e., a highly virulent microbe requires fewer organisms to  cause disease than a less virulent one). The ID 50 is the number  of organisms required to cause disease in 50% of the popula¬  tion. A low ID 50 indicates a highly virulent organism.   The virulence of a microbe is determined by virulence factors,  such as capsules, exotoxins, or endotoxins.   Whether a person gets an infectious disease or not is deter¬  mined by the balance between the number and virulence of  the microbes and the competency of that person's host  defenses.   Many infections are asymptomatic or inapparent because  our host defenses have eliminated the microorganism before it  could multiply to sufficient numbers to cause the symptoms of  disease.   The term infection has two meanings: (1) the presence of  microbes in the body and (2) the symptoms of disease. The  presence of microbes in the body does not always result in  symptoms of disease (see the previous bullet).   Bacteria cause the symptoms of disease by two main mecha¬  nisms: production of toxins (both exotoxins and endotoxins)  and induction of inflammation.   Most bacterial infections are communicable (i.e., capable of  spreading from person to person), but some are not (e.g., botu¬  lism and Legionella pneumonia).   Three epidemiologic terms are often used to describe infec¬  tions: endemic infections are those that occur at a persistent,  usually low level in a certain geographic area, epidemics are    those infections that occur at a much higher rate than usual,  and pandemics are those infections that spread rapidly over  large areas of the globe.   • The modes of transmission of microbes include both human-  to-human and nonhuman-to-human processes. Nonhuman  sources include animals, soil, water, and food.   • Human-to-human transmission can occur either by direct con¬  tact or indirectly via a vector such as an insect, notably ticks or  mosquitoes. Animal-to-human transmission can also occur  either by direct contact with the animal or indirectly via a  vector.   • The main "portals of entry" into the body are the respiratory  tract, gastrointestinal tract, skin, and genital tract.   • Human diseases for which animals are the reservoir are called   • Pili are the main mechanism by which bacteria adhere to  human cells. They are fibers that extend from the surface of  bacteria that mediate attachment to specific receptors on  cells.   • Glycocalyx is a polysaccharide "slime layer" secreted by some  strains of bacteria that mediates strong adherence to certain  structures such as heart valves, prosthetic implants, and  catheters.   • Invasion of tissue is enhanced by enzymes secreted by bacteria.  For example, hyaluronidase produced by S. pyogenes degrades    hyaluronic acid in the subcutaneous tissue, allowing the organ¬  ism to spread rapidly.   • IgA protease degrades secretory IgA, allowing bacteria to  attach to mucous membranes.   • The capsule surrounding bacteria is antiphagocytic (i.e., it  retards the phagocyte from ingesting the organism). Mutant  strains of many pathogens that do not produce capsules are  nonpathogenic.   • Inflammation is an important host defense induced by the  presence of bacteria in the body. There are two types of inflam¬  mation, pyogenic and granulomatous, and bacteria typically  elicit one type or the other. Pyogenic inflammation, the host  defense against pyogenic (pus-producing) bacteria such as S.  pyogenes, consists of neutrophils (and antibody and comple¬  ment). Granulomatous inflammation, the host defense  against intracellular, granuloma-producing bacteria, such as M.  tuberculosis, consists of macrophages and CD4-positiveT cells.  The type of inflammatory lesion is an important diagnostic  criterion.   • Bacteria can evade our host defenses by a process called intra¬  cellular survival (i.e., bacteria that can live within cells are  protected from attack by macrophages and neutrophils). Note  that many of these bacteria (e.g., M. tuberculosis) are not obli¬  gate intracellular parasites (which can grow only within cells),  but rather have the ability to enter and survive inside cells.   • Exotoxins are polypeptides secreted by certain bacteria that  alter specific cell functions resulting in the symptoms of dis¬  ease. They are produced by both gram-positive and gram¬  negative bacteria, whereas endotoxin is found only in  gram-negative bacteria.   • Exotoxins are antigenic and induce antibodies called antitox¬  ins. Exotoxins can be modified to form toxoids, which are  antigenic but not toxic. Toxoids, such as tetanus toxoid, are  used to immunize against disease.   • Many exotoxins have an A-B subunit structure in which the A  subunit is the active (toxic) one and the B subunit is the one  that binds to the cell membrane and mediates the entry of the  A subunit into the cell.    • Exotoxins have different mechanisms of action and different  targets within the cell and therefore cause a variety of diseases  with characteristic symptoms (see Tables 7-9 and 7-10). Sev¬  eral exotoxins are enzymes that attach ADP-ribose to a cell  component (ADP-ribosylation). Some exotoxins act by pro¬  teolytic cleavage of a cell component, whereas others act as  superantigens, causing the overproduction of cytokines.   • Endotoxins are lipopolysaccharides (LPS) located in the  outer membrane only of gram-negative bacteria. They are not  secreted by bacteria.   • Lipid A is the toxic component of LPS. It induces the overpro¬  duction of cytokines, such as tumor necrosis factor, interleu-  kin-1, and nitric oxide, from macrophages, which causes the  symptoms of septic shock, such as fever and hypotension. In  addition, LPS activates the complement cascade (alternate  pathway), resulting in increased vascular permeability, and the  coagulation cascade, resulting in increased vascular permea¬  bility and disseminated intravascular coagulation.   • Endotoxins are poorly antigenic, do not induce antitoxins, and  do not form toxoids.   • There are often four discrete stages. The incubation period is  the time between the moment the person is exposed to the  microbe (or toxin) and the appearance of symptoms. The pro¬  drome period is the time during which nonspecific symptoms  occur.The specific-illness period is the time during which the  characteristic features of the disease occur. The recovery  period is the time during which symptoms resolve and health  is restored.   • After the recovery period, some people become chronic carri¬  ers of the organism and in others latent infections develop.   • Some people have subdinical infections during which they  remain asymptomatic. The presence of antibodies reveals that  a prior infection has occurred.   1. Handwashing is an important means of interrupting the chain  of transmission from one person to another. Infection by which  one of the following bacteria is most likely to be interrupted by  handwashing?    2. Vertical transmission is the transmission of organisms from  mother to fetus or newborn child. Infection by which one of the  following bacteria is most likely to be transmitted vertically?    3. The cells involved with pyogenic inflammation are mainly neu¬  trophils, whereas the cells involved with granulomatous inflam¬  mation are mainly macrophages and helper T cells. Infection by  which one of the following bacteria is most likely to elicit granu¬  lomatous inflammation?   4. Which one of the following sets of properties of exotoxins and  endotoxins is correctly matched?   (C) Exotoxins—produced only by gram-negative bacteria; endo¬  toxins—produced only by gram-positive bacteria   5. Which one of the following sets consists of bacteria both of which  produce exotoxins that increase cyclic AMP within human cells?   6. Which one of the following sets of bacteria produces exotoxins  that act by ADP-ribosylation?   7. Which of the following bacteria produce an exotoxin that inhib¬  its the release of acetylcholine at the neuromuscular junction?   8. A 25-year-old man with abdominal pain was diagnosed with  acute appendicitis. He then had a sudden rise in temperature to  39°C and a sudden fall in blood pressure. Which one of the fol¬  lowing is the most likely cause of the fever and hypotension?   (B) An exotoxin that stimulates production of large amounts of  cyclic AMP    9. Several biotech companies have sponsored clinical trials of a  drug consisting of monoclonal antibody to lipid A. Sepsis caused  by which one of the following sets of bacteria is most likely to be  improved following administration of this antibody?   (A) Endotoxin is a polypeptide, the toxic portion of which con¬  sists of two D-alanines.   (B) Endotoxin is produced by both gram-positive cocci as well as  gram-negative cocci.   (C) Endotoxin acts by binding to class II MHC proteins and the  variable portion of the beta chain of the T-cell receptor.   (D) Endotoxin causes fever and hypotension by inducing the  release of interleukins such as interleukin-1 and tumor necro¬  sis factor.   (E) The antigenicity of endotoxin resides in its fatty acid side  chains.   Questions on the topics discussed in this chapter can be found  in the Basic Bacteriology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 709. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   Host defenses are composed of two complementary, fre¬  quently interacting systems: (1) innate (nonspecific)  defenses, which protect against microorganisms in general;  and (2) adaptive (specific) immunity, which protects  against a particular microorganism. Innate defenses can be  classified into three major categories: (1) physical barriers,  such as intact skin and mucous membranes; (2) phagocytic  cells, such as neutrophils, macrophages, and natural killer  cells; and (3) proteins, such as complement, lysozyme, and  interferon. Figure 8-1 shows the role of several compo¬  nents of the nonspecific defenses in the early response to  bacterial infection. Adaptive immunity is mediated by  antibodies and T lymphocytes. Chapter 57 describes these  host defenses in more detail.   There are two main types of host defenses against  bacteria: the pyogenic response and the granuloma¬  tous response. Certain bacteria, such as Staphylococcus  aureus and Streptococcus pyogenes, are defended against  by the pyogenic (pus-producing) response, which con¬  sists of antibody, complement, and neutrophils. These  pyogenic bacteria are often called extracellular patho¬  gens because they do not invade cells. Other bacteria,  such as Mycobacterium tuberculosis and Listeria mono¬  cytogenes, are defended against by the granulomatous  response, which consists of macrophages and CD-I-  positive (helper) T cells. These bacteria are often called  intracellular pathogens because they can invade and  survive within cells.   Intact skin is the first line of defense against many organ¬  isms. In addition to the physical barrier presented by skin,  the fatty acids secreted by sebaceous glands in the skin have  antibacterial and antifungal activity. The increased fatty  acid production that occurs at puberty is thought to explain  the increased resistance to ringworm fungal infections,  which occurs at that time. The low pH of the skin (between 3  and 5), which is due to these fatty acids, also has an antimi¬  crobial effect. Although many organisms live on or in the  skin as members of the normal flora, they are harmless as  long as they do not enter the body.   A second important defense is the mucous membrane of  the respiratory tract, which is lined with cilia and covered  with mucus. The coordinated beating of the cilia drives the  mucus up to the nose and mouth, where the trapped bacte¬  ria can be expelled. This mucociliary apparatus, the ciliary  elevator, can be damaged by alcohol, cigarette smoke, and  viruses; the damage predisposes the host to bacterial infec¬  tions. Other protective mechanisms of the respiratory tract  involve alveolar macrophages, lysozyme in tears and mucus,  hairs in the nose, and the cough reflex, which prevents  aspiration into the lungs.   Loss of the physical barrier provided by the skin and  mucous membranes predisposes to infection. Table 8-1  describes the organisms that commonly cause infections  associated with the loss of these protective barriers.    The nonspecific protection in the gastrointestinal tract  includes hydrolytic enzymes in saliva, acid in the stomach,  and various degradative enzymes and macrophages in the  small intestine. The vagina of adult women is protected by  the low pH generated by lactobacilli that are part of the  normal flora.   Additional protection in the gastrointestinal tract and in  the lower respiratory tract is provided by defensins. These  are highly positively charged (cationic) peptides that create    pores in the membranes of bacteria, which kills them.  Neutrophils and Paneth cells in the intestinal crypts contain  one type of defensin (a-defensins), whereas the respiratory  tract produces different defensins called (3-defensins. The  mechanism by which defensins distinguish between bacte¬  rial membranes and human cell membranes is unknown.   The bacteria of the normal flora of the skin, nasopharynx,  colon, and vagina occupy these ecologic niches, preventing  pathogens from multiplying in these sites. The importance of    TABLE 8-1 Damage to Skin and Mucous Membranes Predisposes to Infection Caused by Certain Bacteria   2 Viridans streptococci do not cause local infection after dental extraction but can enter the bloodstream and cause endocarditis.    the normal flora is appreciated in the occasional case when  antimicrobial therapy suppresses these beneficial organisms,  thereby allowing organisms such as Clostridium difficile and  Candida albicans to cause diseases such as pseudomembra¬  nous colitis and vaginitis, respectively.   The presence of foreign bodies, such as bacteria within  the body, provokes a protective inflammatory response  (Figure 8-2). This response is characterized by the clinical  findings of redness, swelling, warmth, and pain at the site of  infection. These signs are due to increased blood flow,  increased capillary permeability, and the escape of fluid and  cells into the tissue spaces. The increased permeability is due  to several chemical mediators, of which histamine, prosta¬  glandins, and leukotrienes are the most important.  Complement components, C3a and C5a, also contribute to  increased vascular permeability. Bradykinin is an important  mediator of pain.   Neutrophils and macrophages, both of which are  phagocytes, are an important part of the inflammatory  response. Neutrophils predominate in acute pyogenic  infections, whereas macrophages are more prevalent in  chronic or granulomatous infections.   Macrophages perform two functions: they are phagocytic  and they produce two important “proinflammatory” cyto¬  kines: tumor necrosis factor (TNF) and interleukin-1  (IL-1). The synthesis of IL-1 from its inactive precursor is  mediated by proteolytic enzymes (caspases) in a cytoplasmic    structure called an inflammasome. The importance of the  inflammatory response in limiting infection is emphasized  by the ability of anti-inflammatory agents such as corticoste¬  roids to lower resistance to infection.   Certain proteins, known collectively as the acute-phase  response, are also produced early in inflammation, mainly  by the liver. The best known of these are C-reactive protein  and mannose-binding protein, which bind to the surface  of bacteria and enhance the activation of the alternative  pathway of complement (see Chapter 58). C-reactive pro¬  tein was named for its ability to bind with a carbohydrate in  the cell wall of Streptococcus pneumoniae (see page 123).   Lipopolysaccharide (endotoxin)-binding protein is  another important acute-phase protein that is produced in  response to gram-negative bacteria. Interleukin-6 (IL-6) is  the main inducer of the acute-phase response and is also a  proinflammatory cytokine. Macrophages are the principal  source of IL-6, but many other types of cells produce it as  well. Gamma interferon, which activates macrophages and  enhances their microbicidal action, is produced by acti¬  vated helper T cells.   Neutrophils and macrophages are attracted to the site of  infection by small polypeptides called chemokines ( chemo-  tactic cytokines). Chemokines are produced by tissue cells in  the infected area, by local endothelial cells, and by resident  neutrophils and macrophages. Interleukin-8 is a chemokine  that attracts primarily neutrophils, whereas monocyte che-  motactic protein 1 (MCP-1) and macrophage inflammatory  protein (MIP) are attractants for macrophages and mono¬  cytes (see Chapter 58). The C5a component of complement  is another important chemokine (see Chapter 63).   FIGURE 8-2 Inflammation. The inflammatory response can be caused by two different mechanisms. Left: Pyogenic bacteria (e.g.,  Staphylococcus aureus) cause inflammation via antibody- and complement-mediated mechanisms. Right: Intracellular bacteria (e.g.,  Mycobacterium tuberculosis) cause inflammation via cell-mediated mechanisms.    As part of the inflammatory response, bacteria are  engulfed (phagocytized) by polymorphonuclear neutrophils  (PMNs) and macrophages. PMNs make up approximately  60% of the leukocytes in the blood, and their numbers  increase significantly during infection (leukocytosis). It  should be noted, however, that in certain bacterial infections  such as typhoid fever, a decrease in the number of leukocytes  (leukopenia) is found. The increase in PMNs is due to the  production of granulocyte-stimulating factors (granulocyte  colony-stimulating factor [G-CSF] and granulocyte-macro-  phage colony-stimulating factor [GM-CSF]; see Chapter 58)  by macrophages soon after infection.   Note that although both PMNs and macrophages  phagocytose bacteria, PMNs do not present antigen to  helper T lymphocytes, whereas macrophages (and den¬  dritic cells) do (see Chapter 58). Dendritic cells are the  most important antigen-presenting cells. The phagocytic  ability of dendritic cells is enhanced by the presence of  receptors for mannose-binding protein.   The process of phagocytosis can be divided into three  steps: migration, ingestion, and killing. Migration of  PMNs to the infection site is due to the production of che-  mokines, such as interleukin-8 and complement compo¬  nent C5a, at that location. Adhesion of PMNs to the  endothelium at the site of infection is mediated first by the  interaction of the PMNs with selectin proteins on the  endothelium and then by the interaction of integrin pro¬  teins called “LFA proteins,” located on the PMN surface,  with intracellular adhesion molecule (ICAM) proteins on  the endothelial cell surface. 1   ICAM proteins on the endothelium are increased by  inflammatory mediators, such as IL-1 and TNF (see  Chapter 58), which are produced by macrophages in  response to the presence of bacteria. The increase in the  level of ICAM proteins ensures that PMNs selectively  adhere to the site of infection. Increased permeability of  capillaries as a result of histamine, kinins, and prostaglan¬  dins 2 allows PMNs to migrate through the capillary wall to  reach the bacteria. This migration is called diapedesis and  takes several minutes to occur.   The bacteria are ingested by the invagination of the  PMN cell membrane around the bacteria to form a vacuole  (phagosome). This engulfment is enhanced by the binding  of immunoglobulin G (IgG) antibodies (opsonins) to the  surface of the bacteria, a process called opsonization  (Figure 8-3). The C3b component of complement is  another important opsonin. (The outer cell membranes of  both PMNs and macrophages have receptors both for the  Fc portion of IgG and for C3b.) Even in the absence of    1 LFA proteins and ICAM proteins mediate adhesion between many types of  cells. These proteins are described in more detail in Chapter 58.   2 The anti-inflammatory action of aspirin is the result of its ability to inhibit  cyclooxygenase, thus reducing the synthesis of prostaglandins.   FIGURE 8~3 Opsonization. Top: An encapsulated bacterium  poorly phagocytized by a neutrophil in the absence of either immu  noglobulin G (IgG) antibody or C3b. Bottom: In the presence of  either IgG antibody or C3b or both, the bacterium is opsonized  (i.e., it is made more easily phagocytized by the neutrophil).   antibody, the C3b component of complement, which can  be generated by the “alternative” pathway, can opsonize.  This is particularly important for bacterial and fungal  organisms whose polysaccharides activate the alternative  pathway of complement.   At the time of engulfment, a new metabolic pathway,  known as the respiratory burst, is triggered. This results in  the production of two microbicidal agents, the superoxide  radical and hydrogen peroxide. These highly reactive com¬  pounds (often called reactive oxygen intermediates) are  synthesized by the following reactions:   In the first reaction, molecular oxygen is reduced by an  electron by NADPH oxidase (also known as phagocyte  oxidase) to form the superoxide radical, which is weakly  bactericidal. In the next step, the enzyme superoxide dis-  mutase catalyzes the formation of hydrogen peroxide from  two superoxide radicals. Hydrogen peroxide is more toxic  than superoxide but is not effective against catalase-pro¬  ducing organisms such as staphylococci. Hydrogen perox¬  ide also plays a role in the production of hypochlorite, the  most powerful of the microbicidal substances (see later).   Nitric oxide (NO) is another important microbicidal  agent. It is a reactive nitrogen intermediate that is synthe¬  sized by an inducible enzyme called nitric oxide synthase in  response to stimulators such as endotoxin. Overproduction  of NO contributes to the hypotension seen in septic shock  because it causes vasodilation of peripheral blood vessels.   The respiratory burst also results in the production of  the microbicidal agent-NO. NO contains a free radical that  participates in oxidative killing of ingested microbes phago-  cytosed by neutrophils and macrophages. Nitric oxide  synthase, the enzyme that produces NO, is induced in these  cells following phagocytosis.   The killing of the organism within the phagosome is a  two-step process that consists of degranulation followed by  production of hypochlorite, which is probably the most    important microbicidal agent. In degranulation, the two  types of granules in the cytoplasm of the neutrophil fuse  with the phagosome, emptying their contents in the pro¬  cess. These granules are lysosomes that contain a variety of  enzymes essential to the killing and degradation that occur  within the phagolysosome.   (1) The larger lysosomal granules, which constitute  about 15% of the total, contain the important enzyme  myeloperoxidase as well as lysozyme and several other  degradative enzymes. (Myeloperoxidase, which is green,  makes a major contribution to the color of pus.)   (2) The smaller granules, which make up the remaining  85%, contain lactoferrin and additional degradative enzymes  such as proteases, nucleases, and lipases. Lysosomal granules  can empty into the extracellular space as well as into the  phagosome. Outside the cell, the degradative enzymes can  attack structures too large to be phagocytized, such as fungal  mycelia, as well as extracellular bacteria.   The actual killing of the microorganisms occurs by a  variety of mechanisms, which fall into two categories: oxy¬  gen-dependent and oxygen-independent. The most impor¬  tant oxygen-dependent mechanism is the production of the  bactericidal molecule, hypochlorite ion, according to the  following reaction:   Myeloperoxidase catalyzes the reaction between chlo¬  ride ion and hydrogen peroxide, which was produced by  the respiratory burst, to produce hypochlorite. Hypochlo¬  rite by itself damages cell walls but can also react with  hydrogen peroxide to produce singlet oxygen, which dam¬  ages cells by reacting with double bonds in the fatty acids of  membrane lipids.   Rare individuals are genetically deficient in myeloper¬  oxidase, yet their defense systems can kill bacteria, albeit  more slowly. In these individuals, the respiratory burst that  produces hydrogen peroxide and superoxide ion seems to  be sufficient, but with two caveats: if an organism produces  catalase, hydrogen peroxide will be ineffective, and if an  organism produces superoxide dismutase, superoxide ion  will be ineffective.   The oxygen-independent mechanisms are important  under anaerobic conditions. These mechanisms involve    lactoferrin, which chelates iron from the bacteria; lyso¬  zyme, which degrades peptidoglycan in the bacterial cell  wall; cationic proteins, which damage bacterial mem¬  branes; and low pH.   Macrophages also migrate, engulf, and kill bacteria by  using essentially the same processes as PMNs do, but there  are several differences:   (1) Macrophages do not possess myeloperoxidase and  so cannot make hypochlorite ion; however, they do pro¬  duce hydrogen peroxide and superoxide by respiratory  burst.   (2) Certain organisms such as the agents of tuberculosis,  brucellosis, and toxoplasmosis are preferentially ingested  by macrophages rather than PMNs and may remain viable  and multiply within these cells; granulomas formed during  these infections contain many of these macrophages.   (3) Macrophages secrete plasminogen activator, an  enzyme that converts the proenzyme plasminogen to the  active enzyme plasmin, which dissolves the fibrin clot.   The importance of phagocytosis as a host defense mecha¬  nism is emphasized by the observation that reduced num¬  bers or reduced function of phagocytes predisposes to  bacterial infections, especially infections caused by certain  organisms (Table 8-2):   (1) Repeated infections occur in children who have  genetic defects in their phagocytic processes. Two examples  of these defects are chronic granulomatous disease, in  which the phagocyte cannot kill the ingested bacteria  owing to a defect in NADPH oxidase and a resultant failure  to generate H 2 0 2 , and Chediak-Higashi syndrome, in  which abnormal lysosomal granules that cannot fuse with  the phagosome are formed, so that even though bacteria  are ingested, they survive.   (2) Frequent infections occur in neutropenic patients,  especially when the PMN count drops below 500/pL as a  result of immunosuppressive drugs or irradiation. These  infections are frequently caused by opportunistic organ¬  isms (i.e., organisms that rarely cause disease in people with  normal immune systems).    TABLE 8-2 Reduced Phagocytosis Predisposes to Infection Caused by Certain Bacteria    (3) Splenectomy removes an important source of both  phagocytes and immunoglobulins, which predisposes to  sepsis caused by three encapsulated pyogenic bacteria:  S. pneumoniae, Neisseria meningitidis, and Haemophilus  influenzae. S. pneumoniae causes approximately 50% of  episodes of sepsis in splenectomized patients. Patients with  sickle cell anemia and other hereditary anemias can autoin¬  farct their spleen, resulting in a loss of splenic function and  a predisposition to sepsis caused by these bacteria.   (4) People who have diabetes mellitus, especially those  who have poor glucose control or episodes of ketoacidosis,  have an increased number of infections and more severe  infections compared with people who do not have diabetes.  The main host defense defect in these patients is reduced  neutrophil function, especially when acidosis occurs.   Two specific diseases highly associated with diabetes are  malignant otitis externa caused by Pseudomonas aerugi¬  nosa and mucormycosis caused by molds belonging to the  genera Mucor and Rhizopus. In addition, there is an increased  incidence and increased severity of community-acquired  pneumonia caused by bacteria such as S. pneumoniae and  S. aureus and of urinary tract infections caused by organ¬  isms such as Escherichia coli and C. albicans. Candidal  vulvovaginitis is also more common in diabetic patients.  Diabetic patients also have many foot infections because  atherosclerosis compromises the blood supply and necrosis  of tissue occurs. Skin infections, such as ulcers and celluli¬  tis, and soft tissue infections, such as necrotizing fasciitis,  are common and can extend to the underlying bone, caus¬  ing osteomyelitis. S. aureus and mixed facultative anaerobic  bacteria are the most common causes.   Infection causes a rise in the body temperature that is  attributed to endogenous pyrogen (IL-1) released from  macrophages. Fever may be a protective response because a  variety of bacteria and viruses grow more slowly at elevated  temperatures.   Adaptive immunity results either from exposure to the  organism (active immunity) or from receipt of preformed  antibody made in another host (passive immunity).   Passive adaptive immunity is a temporary protection  against an organism and is acquired by receiving serum  containing preformed antibodies from another person or  animal. Passive immunization occurs normally in the form  of immunoglobulins passed through the placenta (IgG) or  breast milk (IgA) from mother to child. This protection is  very important during the early days of life when the child  has a reduced capacity to mount an active response.   Passive immunity has the important advantage that its  protective abilities are present immediately, whereas active    immunity has a delay of a few days to a few weeks, depending  on whether it is a primary or secondary response. However,  passive immunity has the important disadvantage that the  antibody concentration decreases fairly rapidly as the pro¬  teins are degraded, and so the protection usually lasts for  only a month or two. The administration of preformed  antibodies can be lifesaving in certain diseases that are  caused by powerful exotoxins, such as botulism and teta¬  nus. Serum globulins, given intravenously, are a prophylac¬  tic measure in patients with hypogammaglobulinemia or  bone marrow transplants. In addition, they can mitigate the  symptoms of certain diseases such as hepatitis caused by  hepatitis A virus, but they appear to have little effect on  bacterial diseases with an invasive form of pathogenesis.   Active adaptive immunity is protection based on expo¬  sure to the organism in the form of overt disease, subclini-  cal infection (i.e., an infection without symptoms), or a  vaccine. This protection has a slower onset but longer dura¬  tion than passive immunity. The primary response usually  takes 7 to 10 days for the antibody to become detectable.  An important advantage of active immunity is that an  anamnestic (secondary) response occurs (i.e., there is a  rapid response [approximately 3 days] of large amounts of  antibody to an antigen that the immune system has previ¬  ously encountered). Active immunity is mediated by both  antibodies (immunoglobulins) and T cells:   (1) Antibodies protect against organisms by a variety of  mechanisms—neutralization of toxins, lysis of bacteria in  the presence of complement, opsonization of bacteria to  facilitate phagocytosis, and interference with adherence of  bacteria and viruses to cell surfaces. If the level of IgG  drops below 400 mg/dL (normal = 1000-1500 mg/dL), the  risk of pyogenic infections caused by bacteria such as  staphylococci increases.   Because antibodies, especially IgG, are detectable for  days to weeks after infection, they are thought not to play  a major role in combating the primary infection at the  initial site of infection (usually the skin or mucous mem¬  brane), but rather to protect against hematogenous dis¬  semination of the organism to distant sites in the body  and against a second infection by that organism at some  future time.   (2) T cells mediate a variety of reactions, including  cytotoxic destruction of virus-infected cells and bacteria,  activation of macrophages, and delayed hypersensitivity.  T cells, especially Th-1 cells (see Chapter 58) and macro¬  phages, are the main host defense against mycobacteria  such as M. tuberculosis and systemic fungi such as Histoplasma  and Coccidioides. T cells also help B cells to produce anti¬  body against many, but not all, antigens.   Table 8-3 describes the essential host defense mecha¬  nisms against bacteria. These mechanisms include both  humoral immunity against pyogenic bacteria and exotoxins  and cell-mediated immunity against several intracellular  bacteria.   Streptococcus pneumoniae, Streptococcus pyogenes, Staphylococ¬  cus aureus, Neisseria meningitidis, Haemophilus influenzae, Pseu¬  domonas aeruginosa   The frequency or severity of infections is increased when  certain predisposing conditions exist. These predisposing  conditions fall into two main categories: patients are  immunocompromised or patients have foreign bodies such  as indwelling catheters or prosthetic devices. Foreign bod¬  ies predispose because host defenses do not operate effi¬  ciently in their presence. Table 8-4 describes the  predisposing conditions and the most common organisms  causing infections when these predisposing conditions are  present.   Certain diseases and anatomic abnormalities also pre¬  dispose to infections. For example, patients with diabetes  often have S. aureus infections, perhaps for two reasons:  these patients have extensive atherosclerosis, which causes  relative anoxia to tissue, and they appear to have a defect in    neutrophil function. Patients with sickle cell anemia often  have Salmonella osteomyelitis, probably because the abnor¬  mally shaped cells occlude the small capillaries in the bone.  This traps the Salmonella within the bone, increasing the  risk of osteomyelitis.   Patients with certain congenital cardiac defects or rheu¬  matic valvular damage are predisposed to endocarditis  caused by viridans streptococci. Neutrophils have difficulty  in penetrating the vegetations formed on the valves in  endocarditis. Patients with an aortic aneurysm are prone to  vascular infections caused by Salmonella species.   Patients with reduced host defenses often have a muted  response to infection (e.g., a low-grade [or no] fever and a  mild [or absent] inflammatory response). For this reason, a  high index of suspicion regarding the presence of infection  must be present when evaluating patients who are immu¬  nocompromised, especially those who are intentionally  immunosuppressed, such as transplant recipients.    TABLE 8-4 Conditions That Predispose to Infections and the Organisms That Commonly Cause These Infections    AIDS = acquired immunodeficiency syndrome; CGD = chronic granulomatous disease; CMV = cytomegalovirus.    • Host defenses against bacterial infections include both innate  and adaptive (acquired) defenses. Innate defenses are non¬  specific (i.e., they are effective against many different organ¬  isms). These include physical barriers, such as intact skin and  mucous membranes; cells, such as neutrophils and macro¬  phages; and proteins, such as complement and lysozyme.  Adaptive (acquired) defenses are highly specific for the organ¬  ism and include antibodies and cells such as CD4-positive  helperT lymphocytes and CD8-positive cytotoxicT lymphocytes.   • Intact skin and mucous membranes provide a physical barrier  to infection. Loss of skin integrity (e.g., in a burn) predisposes  to infection. The low pH of the skin, stomach, and vagina also  protects against infection.   • The respiratory tract, a very important portal of entry for  microbes, is protected by the ciliary elevator, alveolar macro¬  phages, lysozyme, nose hairs, and the cough reflex.   • The normal flora of the skin and mucous membranes occupy  receptors, which reduce the opportunity for pathogens to  attach-a process called colonization resistance. Suppression  of the normal flora with antibiotics predisposes to infec¬  tion with certain organisms. Two important examples are the  suppression of colon flora predisposing to pseudomembra¬  nous colitis caused by C. difficile and the suppression of vaginal  flora predisposing to vaginitis caused by C. albicans.   • Inflammation (i.e., redness, swelling, warmth, and pain) is an  important host defense. Redness, swelling, and warmth are the  result of increased blood flow and increased vascular perme¬  ability, which has the effect of bringing the cells and proteins of  our host defenses to the site of infection. The increased blood  flow and increased vascular permeability are caused by media¬  tors such as histamine, prostaglandins, and leukotrienes.   • The predominant phagocytic cells in inflammation are neutro¬  phils and macrophages. Neutrophils are seen in the pyogenic  inflammatory response to bacteria such as S. aureus and S. pyo¬  genes, whereas macrophages are seen in the granulomatous  inflammatory response to bacteria such as A/I. tuberculosis.   • The acute-phase response consists of proteins (e.g., C-reactive  protein, mannose-binding protein, and LPS-binding protein)  that enhance the host response to bacteria. Interleukin-6 is the  main inducer of this response.   • Neutrophils and macrophages are attracted to the site of infec¬  tion by chemokines, which are small polypeptides produced  by cells at the infected site. Interleukin-8 and C5a are impor¬  tant chemokines for neutrophils.   • In response to most bacterial infections, there is an increase in  the number of neutrophils in the blood. This increase is   • Both neutrophils and macrophages phagocytose bacteria, but  macrophages (and similar cells called dendritic cells) also pres¬  ent antigen to CD4-positive (helper) T cells, whereas neutro¬  phils do not. Dendritic cells are probably the most important  antigen-presenting cells in the body.   • After neutrophils are attracted to the infected site by chemo¬  kines, they attach to the endothelium first using selectins on  the endothelium, then by the interaction of integrins (LFA  proteins) on the neutrophils with intracellular adhesion mole¬  cule (ICAM) proteins on the endothelium. The concentration of  ICAM proteins is increased by cytokines released by activated  macrophages, which results in neutrophils being attracted to  the infected site.   • Neutrophils then migrate through the endothelium (diapedesis)  and ingest the bacteria. IgG and C3b are opsonins, which  enhance ingestion of the bacteria. There are receptors for the  heavy chain of IgG and for C3b on the surface of the neutrophils.   • Killing of the bacteria within the neutrophil is caused by hypo¬  chlorite, hydrogen peroxide, and superoxides. Lysosomes  contain various degradative enzymes and fuse with the phago¬  some to form a phagolysosome within which the killing occurs.   • Severe, recurrent pyogenic infections occur in those who  have inadequate neutrophils. For example, people with  defective neutrophils, people with fewer than 500 neutrophils/  pL, and those who have had a splenectomy or who have diabe¬  tes mellitus are at increased risk for pyogenic infections.   • Passive immunity refers to protection based on the transfer of  preformed antibody from one person (or animal) to another  person. Passive immunity provides immediate but short¬  lived protection (lasting a few months). Examples of passive  immunity include administration of antitoxin, passage of IgG  from mother to fetus across the placenta, and passage of IgA  from mother to newborn through breast milk.   • Active immunity refers to protection based on the formation  of both antibodies and cell-mediated immunity after expo¬  sure either to the microbe itself (with or without disease) or to  the antigens of the microbe in a vaccine. Active immunity pro¬  vides long-term protection but is not effective for days after  exposure to the microbe. In the primary response, antibody  appears in 7 to 10 days, whereas in the secondary response,  antibody appears in approximately 3 days.   • The main functions of antibodies are to neutralize bacterial  toxins and viruses, opsonize bacteria, activate comple¬  ment to form a membrane attack complex that can kill bacte¬  ria, and interfere with attachment to mucosal surfaces.    IgG is the main opsonizing antibody, IgG and IgM activate  complement, and IgA interferes with attachment to the  mucosa.   The main functions of cell-mediated immunity are to protect  against intracellular bacteria and to kill virus-infected cells.   Helper T cells (and macrophages) protect against intracellular  bacteria, whereas cytotoxicT cells kill virus-infected cells.   • Reduced host defenses result in an increase in the frequency  and severity of infections. The main causes include various  genetic immunodeficiencies, the presence of foreign bodies,  and the presence of certain chronic diseases, such as diabetes  mellitus and renal failure.   1. Which one of the following host defense processes is the MOST  important in preventing the action of exotoxins?   (A) Binding of cytokines to exotoxin-specific receptors inhibits the  attachment of exotoxins.   (B) Degradation of exotoxins by the membrane attack complex of  complement.   (C) Lysis of exotoxins by perforins produced by cytotoxic T cells.   (D) Neutralization of exotoxins by antibody prevents binding to  target cell membrane.   (E) Phagocytosis of exotoxins by neutrophils and subsequent  destruction by hypochlorite.   2. An inflammatory response in the skin is characterized by ery¬  thema (redness). Which one of the following is the most impor¬  tant cause of this erythema?   3. A 1-year-old child with repeated infections was diagnosed with  chronic granulomatous disease (CGD). A defect in which one of  the following is the cause of CGD?   (C) neutrophils migrate from the blood through the endothelium  to reach the site of infection.   Questions on the topics discussed in this chapter can be found  in the Basic Bacteriology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 709. Also see Part XIV:  USMLE (National Board) Practice Examination starting on  page 751.   Blood Cultures  Throat Cultures  Sputum Cultures  Spinal Fluid Cultures  Stool Cultures  Urine Cultures  Genital Tract Cultures  Wound & Abscess Cultures   The laboratory diagnosis of infectious diseases involves two  main approaches: one is the bacteriologic approach, in  which the organism is identified by staining and culturing  the organism, and the other is the immunologic (sero¬  logic) approach, in which the organism is identified by  detection of antibodies against the organism in the patients  serum.   In the bacteriologic approach to the diagnosis of infec¬  tious diseases, several important steps precede the actual  laboratory work, namely, (1) choosing the appropriate  specimen to examine, which requires an understanding of  the pathogenesis of the infection; (2) obtaining the speci¬  men properly to avoid contamination from the normal  flora; (3) transporting the specimen promptly to the labo¬  ratory or storing it correctly; and (4) providing essential  information to guide the laboratory personnel.   In general, there are three approaches to the bacterio¬  logic laboratory work:   (2) Obtaining a pure culture of the organism by inocu¬  lating it onto a bacteriologic medium.   (3) Identifying the organism by using biochemical reac¬  tions, growth on selective media, DNA probes, or specific   Identification of an Organism with Known Antiserum  Identification of Serum Antibodies with Known  Antigens    antibody reactions. Which of these approaches are used  and in what sequence depend on the type of specimen and  organism. After the organism is grown in pure culture, its  sensitivity to various antibiotics is determined by proce¬  dures described in Chapter 11.   A general approach to the diagnosis of a bacterial infec¬  tion is described in Table 9-1. This approach emphasizes  the importance of performing a Gram stain and obtaining  a “pure culture” of the organism. However, sometimes the  organism is not recovered by culturing, and other tech¬  niques must be used. Table 9-2 describes some approaches  to making a diagnosis when the cultures are negative. One  approach that is commonly used is serologic testing, which  determines the presence of antibodies specific for the  organism. In most cases, a fourfold rise in antibody titer  between the acute- and convalescent-phase serum samples  is considered to be significant.   Obtaining a pure culture involves culturing the organ¬  ism on bacteriologic agar. Initially, blood agar is used  because it supports the growth of many bacteria and the  type of hemolysis can be observed.   Blood agar contains red blood cells, but it should be  noted that viruses and obligate intracellular bacteria, such  as Chlamydia and Rickettsia, will not grow on blood agar.  Red blood cells do not have a functioning nucleus and,  therefore, are incapable of supporting the growth of either  viruses or the obligate intracellular bacteria.    TABLE 9-1 General Approach to the Diagnosis of a Bacterial Infection   2. Stain the specimen using the appropriate procedure (e.g., Gram stain or acid-fast stain). If bacteria are seen in the Gram stain specimen, their  shape (e.g., cocci or rods), size, arrangement (e.g., chains or clusters), and whether they are gram-positive or gram-negative should be observed. It  is also important to determine whether only one or more than one type of bacteria is present. The microscopic appearance is not sufficient to  speciate an organism, but it often allows an educated guess to be made regarding the genus of the organism and thereby guides empiric therapy.   3. Culture the specimen on the appropriate media (e.g., blood agar plates). In most instances, the plates should be streaked in such a manner to  obtain isolated colonies (i.e., a "pure culture"). The plates should be incubated in the presence or absence of oxygen as appropriate.   4. Identify the organism using the appropriate tests (e.g., sugar fermentation, DNA probes, antibody-based tests such as agglutination, or  immunofluorescence). Note special features such as hemolysis and pigment formation.    Blood agar contains inhibitors for certain bacteria, such  as members of the Neisseria and Haemophilus genera, and  the blood must be heated to inactivate these inhibitors.  These bacteria, therefore, are grown on cooked blood agar  or chocolate agar (so named because the heated blood  turns a chocolate color). Other media contain either spe¬  cific growth factors required for the bacteria to grow or  antibiotics that inhibit normal flora, which allows the  pathogenic bacteria to obtain sufficient nutrients to grow.   Certain other media, called “selective, differential”  media, are often used. These media are selective because  they contain compounds that selectively allow certain bac¬  teria to grow and differential because they contain other  compounds that allow one type of bacteria to be differenti¬  ated from another based on some biochemical reaction.  Table 9-3 contains a list of various bacteriologic agars com¬  monly used in the diagnostic laboratory and the function  of these agars.   Blood cultures are performed most often when sepsis,  endocarditis, osteomyelitis, meningitis, or pneumonia is  suspected. The organisms most frequently isolated from  blood cultures are two gram-positive cocci, Staphylococcus  aureus and Streptococcus pneumoniae, and three gram¬  negative rods, Escherichia coli, Klebsiella pneumoniae, and  Pseudomonas aeruginosa.    It is important to obtain at least three 10-mL blood  samples in a 24-hour period because the number of organ¬  isms can be small and their presence intermittent. The site  for venipuncture must be cleansed with 2% iodine to pre¬  vent contamination by members of the flora of the skin,  usually Staphylococcus epidermidis. The blood obtained is  added to 100 mL of a rich growth medium such as brain-  heart infusion broth. Whether one or two bottles are inocu¬  lated varies among hospitals. If two bottles are used, one is  kept under anaerobic conditions and the other is not. If one  bottle is used, the low oxygen tension at the bottom of the  bottle permits anaerobes to grow.   Blood cultures are checked for turbidity or for C0 2 pro¬  duction daily for 7 days or longer. If growth occurs, Gram  stain, subculture, and antibiotic sensitivity tests are per¬  formed. If no growth is observed after 1 or 2 days, blind  subculturing onto other media may reveal organisms.  Cultures should be held for 14 days when infective endo¬  carditis, fungemia, or infection by slow-growing bacteria  (e.g., Brucella) is suspected.   Throat cultures are used primarily to detect the presence of  group A (3-hemolytic streptococci (Streptococcus pyogenes),  an important and treatable cause of pharyngitis. They are  also used when diphtheria, gonococcal pharyngitis, or  thrush (Candida) is suspected.   When the specimen is being obtained, the swab should  touch not only the posterior pharynx, but also both tonsils    TABLE 9-2 How to Diagnose a Bacterial Infection When the Culture Is Negative   1. Detect antibody in the patient's serum. Detection of immunoglobulin (Ig) M antibody indicates a current infection. A fourfold or greater rise in  antibody titer between the acute serum sample and the convalescent serum sample also indicates a current infection. (A major drawback with the  use of acute and convalescent serum samples is that the convalescent sample is usually taken 10-14 days after the acute sample. By this time, the  patient has often recovered and the diagnosis becomes a retrospective one.) A single IgG antibody titer is difficult to interpret because it is unclear  whether it represents a current or a previous infection. In certain diseases, a single titer of sufficient magnitude can be used as presumptive evi¬  dence of a current infection.   2. Detect antigen in the patient's specimen. Use known antibody to detect presence of antigens of the organisms (e.g., fluorescent antibody to  detect antigens in tissue, latex agglutination to detect capsular polysaccharide antigens in spinal fluid).   3. Detect nucleic acids in the patient's specimen. Use polymerase chain reaction (PCR) and DNA probes to detect the DNA or RNA of the organism.   Lecithinase produced by the organism degrades egg yolk to pro¬  duce insoluble precipitate   Selects against gram-positive bacteria and differentiates between  lactose fermenters and nonfermenters   Selects against gram-positive bacteria in respiratory tract flora  and contains lipids required for growth   Selects against gram-positive bacteria and differentiates between  lactose fermenters and nonfermenters   Distinguishes lactose fermenters from nonfermenters and H 2 S  producers from nonproducers    or tonsillar fossae as well. The material on the swab is  inoculated onto a blood agar plate and streaked to obtain  single colonies. If colonies of (3-hemolytic streptococci are  found after 24 hours of incubation at 35°C, a bacitracin  disk is used to determine whether the organism is likely to  be a group A streptococcus. If growth is inhibited around  the disk, it is a group A streptococcus; if not, it is a non¬  group A (3-hemolytic streptococcus.   Note that a Gram stain is typically not done on a throat  swab because it is impossible to distinguish between the  appearance of the normal flora streptococci and  S. pyogenes.   Sputum cultures are performed primarily when pneumonia  or tuberculosis is suspected. The most frequent cause of  community-acquired pneumonia is S. pneumoniae, whereas  S. aureus and gram-negative rods, such as K. pneumoniae  and P. aeruginosa, are common causes of hospital-acquired  pneumonias.   It is important that the specimen for culture really be  sputum, not saliva. Examination of a gram-stained smear  of the specimen frequently reveals whether the specimen is  satisfactory. A reliable specimen has more than 25 leuko¬  cytes and fewer than 10 epithelial cells per lOOx field. An  unreliable sample can be misleading and should be rejected  by the laboratory. If the patient cannot cough and the need  for a microbiologic diagnosis is strong, induction of spu¬  tum, transtracheal aspirate, bronchial lavage, or lung biopsy  may be necessary. Because these procedures bypass the    normal flora of the upper airway, they are more likely to  provide an accurate microbiologic diagnosis. A preliminary  assessment of the cause of the pneumonia can be made by  Gram stain if large numbers of typical organisms are seen.   Culture of the sputum on blood agar frequently reveals  characteristic colonies, and identification is made by vari¬  ous serologic or biochemical tests. Cultures of Mycoplasma  are infrequently done; diagnosis is usually confirmed by a  rise in antibody titer. If Legionella pneumonia is suspected,  the organism can be cultured on charcoal-yeast agar, which  contains the high concentrations of iron and sulfur required  for growth.   If tuberculosis is suspected, an acid-fast stain should be  done immediately and the sputum cultured on special  media, which are incubated for at least 6 weeks. In diagnos¬  ing aspiration pneumonia and lung abscesses, anaerobic  cultures are important.   Spinal fluid cultures are performed primarily when meningi¬  tis is suspected. Spinal fluid specimens from cases of enceph¬  alitis, brain abscess, and subdural empyema usually show  negative cultures. The most important causes of acute bacte¬  rial meningitis are three encapsulated organisms: Neisseria  meningitidis, S. pneumoniae, and Haemophilus influenzae.   Because acute meningitis is a medical emergency, the  specimen should be taken immediately to the laboratory. The  gram-stained smear of the sediment of the centrifuged sample  guides the immediate empirical treatment. If organisms  resembling N. meningitidis, H. influenzae, or S. pneumoniae    are seen, the quellung test or immunofluorescence with  specific antisera can identify the organism rapidly. Cultures  are done on blood and on chocolate agar and incubated at  35°C in a 5% C0 2 atmosphere. Hematin and nicotinamide  adenine dinucleotide (NAD) (factors X and V, respectively)  are added to enhance the growth of H. influenzae.   In cases of subacute meningitis, Mycobacterium tubercu¬  losis and the fungus Cryptococcus neoformans are the most  common organisms isolated. Acid-fast stains of the spinal  fluid should be performed, although M. tuberculosis may  not be seen, because it can be present in small numbers.  The fluid should be cultured and the cultures held for a  minimum of 6 weeks. C. neoformans, a budding yeast with  a prominent capsule, can be seen in spinal fluid when India  ink is used.   Immunologic tests to detect the presence of capsular  antigen in the spinal fluid can be used to identify  N. meningitidis, S. pneumoniae, H. influenzae, group B  streptococci, E. coli, and C. neoformans. The two tests most  frequently used are latex particle agglutination and  counterimmunoelectrophoresis.   Stool cultures are performed primarily for cases of entero¬  colitis. The most common bacterial pathogens causing  diarrhea in the United States are Shigella, Salmonella, and  Campylobacter. E. coli 0157 strains are also an important  cause of diarrhea.   A direct microscopic examination of the stool can be  informative from two points of view: (1) a methylene blue  stain that reveals many leukocytes indicates that an invasive  organism rather than a toxigenic one is involved, and (2) a  Gram stain may reveal large numbers of certain organisms,  such as staphylococci, clostridia, or Campylobacters. Gram  stain of the stool is not usually done because the large num¬  bers of bacteria in the normal flora of the colon make the  interpretation difficult.   For culture of Salmonella and Shigella, a selective, dif¬  ferential medium such as MacConkey or Eosin-Methylene  Blue (EMB) agar is used. These media are selective because  they allow gram-negative rods to grow but inhibit many  gram-positive organisms. Their differential properties are  based on the fact that Salmonella and Shigella do not fer¬  ment lactose, whereas many other enteric gram-negative  rods do. On EMB agar, colonies of E. coli, a lactose fermen¬  ter, appear purple and have a green sheen. In contrast,  colonies of non-lactose fermenters, such as Salmonella and  Shigella, appear colorless.   If non-lactose-fermenting colonies are found, a triple  sugar iron (TSI) agar slant is used to distinguish Salmonella  from Shigella. Some species of Proteus resemble Salmonella  on TSI agar but can be distinguished because they produce  the enzyme urease, whereas Salmonella does not. The organ¬  ism is further identified as either a Salmonella or a Shigella  species by using a specific antisera to the organisms cell wall    O antigen in an agglutination test. This is usually done in  hospital laboratories, but precise identification of the species  is performed in public health laboratories.   Campylobacter jejuni is cultured on antibiotic-containing  media (e.g., Skirrow’s agar) at 42°C in an atmosphere con¬  taining 5% 0 2 and 10% C0 2 . It grows well under these  conditions, unlike many other intestinal pathogens.  Although the techniques are available, stool cultures are  infrequently performed for organisms such as Yersinia  enterocolitica, Vibrio parahaemolyticus, and enteropathic or  toxigenic E. coli. Despite the presence of large numbers of  anaerobes in feces, they are rarely pathogens in the intesti¬  nal tract, and anaerobic cultures of stool specimens are,  therefore, unnecessary.   Stool specimens that are grossly bloody are typically  cultured on MacConkey-sorbitol media. E. coli 0157  strains do not ferment sorbitol and appear as colorless colo¬  nies, whereas typical E. coli strains do ferment sorbitol and  appear red.   Urine cultures are performed primarily when pyelonephri¬  tis or cystitis is suspected. By far the most frequent cause of  urinary tract infections is E. coli. Other common agents are  Enterobacter, Proteus, and Enterococcus faecalis.   Urine in the bladder of a healthy person is sterile, but it  acquires organisms of the normal flora as it passes through  the distal portion of the urethra. To avoid these organisms,  a midstream specimen, voided after washing the external  orifice, is used for urine cultures. In special situations,  suprapubic aspiration or catheterization maybe required to  obtain a specimen. Because urine is a good culture medium,  it is essential that the cultures be done within 1 hour after  collection or stored in a refrigerator at 4°C for no more  than 18 hours.   It is commonly accepted that a bacterial count of at least  100,000/mL must be found to conclude that significant  bacteriuria is present (in asymptomatic persons). There is  evidence that as few as 100/mL are significant in symptom¬  atic patients. For this determination to be made, quantita¬  tive or semiquantitative cultures must be performed. There  are several techniques: (1) a calibrated loop that holds 0.001 mL  of urine can be used to streak the culture; (2) serial 10-fold  dilutions can be made and samples from the dilutions  streaked; and (3) a screening procedure suitable for the  physician’s office involves an agar-covered “paddle” that is  dipped into the urine—after the paddle is incubated, the  density of the colonies is compared with standard charts to  obtain an estimate of the concentration of bacteria.   Genital tract cultures are performed primarily on speci¬  mens from individuals with an abnormal discharge or on  specimens from asymptomatic contacts of a person with a  sexually transmitted disease. One of the most important    pathogens in the genital tract is Neisseria gonorrhoeae. The  laboratory diagnosis of gonorrhea is made by microscopic  examination of a gram-stained smear and by culture of the  organism.   Specimens are obtained by swabbing the urethral canal  (for men), the cervix (for women), or the anal canal (for  men and women). A urethral discharge from the penis is  frequently used. Because N. gonorrhoeae is very delicate,  the specimen should be inoculated directly onto a Thayer-  Martin chocolate agar plate or onto a special transport  medium (e.g., Trans-grow).   Gram-negative diplococci found intracellularly within  neutrophils on a smear of a urethral discharge from a man  have over 90% probability of being N. gonorrhoeae. Because  smears are less reliable when made from swabs of the endo-  cervix and anal canal, cultures are necessary. The finding of  only extracellular diplococci suggests that these neisseriae  may be members of the normal flora and that the patient  may have nongonococcal urethritis.   Nongonococcal urethritis and cervicitis are also  extremely common infections. The most frequent cause is  Chlamydia trachomatis, which cannot grow on artificial  medium but must be grown in living cells. For this purpose,  cultures of human cells or the yolk sacs of embryonated  eggs are used. The finding of typical intracytoplasmic  inclusions when using Giemsa stain or fluorescent anti¬  body is diagnostic. Because of the difficulty of culturing  C. trachomatis, nonbacteriologic methods, such as enzyme-  linked immunosorbent assay (ELISA) to detect chlamydial  antigens in exudates or urine or DNA probe assays to  detect chlamydial nucleic acids (nucleic acid amplification  test, NAAT), are now often used to diagnose sexually trans¬  mitted diseases caused by this organism.   Because Treponema pallidum, the agent of syphilis, can¬  not be cultured, diagnosis is made by microscopy and  serology. The presence of motile spirochetes with typical  morphologic features seen by darkfield microscopy of the  fluid from a painless genital lesion is sufficient for the diag¬  nosis. The serologic tests fall into two groups: (1) the non-  treponemal antibody tests such as the Venereal Disease  Research Laboratory (VDRL) or rapid plasma reagin (RPR)  test and (2) the treponemal antibody tests such as the fluo¬  rescent treponemal antibody-absorption (LTA-ABS) test.  These tests are described on pages 66 and 202.   A great variety of organisms are involved in wound and  abscess infections. The bacteria most frequently isolated  differ according to anatomic site and predisposing factors.  Abscesses of the brain, lungs, and abdomen are frequently  caused by anaerobes such as Bacteroides fragilis and gram¬  positive cocci such as S. aureus and S. pyogenes. Traumatic  open-wound infections are caused primarily by members  of the soil flora such as Clostridium perfringens; surgical-  wound infections are usually due to S. aureus. Infections of    dog or cat bites are commonly due to Pasteurella multocida,  whereas human bites primarily involve the mouth  anaerobes.   Because anaerobes are frequently involved in these types  of infection, it is important to place the specimen in anaer¬  obic collection tubes and transport it promptly to the labo¬  ratory. Because many of these infections are due to multiple  organisms, including mixtures of anaerobes and nonanaer¬  obes, it is important to culture the specimen on several  different media under different atmospheric conditions.  The Gram stain can provide valuable information regard¬  ing the range of organisms under consideration.   These methods are described in more detail in Chapter 64.  However, it is of interest here to present information on  how serologic reactions aid the microbiologic diagnosis.  There are essentially two basic approaches: (1) using  known antibody to identify the microorganism, and  (2) using known antigens to detect antibodies in the  patient’s serum.   Several bacteria can be identified directly in clinical speci¬  mens by this reaction, which is based on the microscopic  observation that the capsule swells in the presence of  homologous antiserum. Antisera against the following  organisms are available: all serotypes of S. pneumoniae  (Omniserum), H. influenzae type b, and N. meningitidis  groups A and C.   Antisera can be used to identify Salmonella and Shigella by  causing agglutination (clumping) of the unknown organ¬  ism. Antisera directed against the cell wall O antigens of  Salmonella and Shigella are commonly used in hospital  laboratories. Antisera against the flagellar H antigens and  the capsular Vi antigen of Salmonella are used in public  health laboratories for epidemiologic purposes.   Latex beads coated with specific antibody are agglutinated  in the presence of the homologous bacteria or antigen. This  test is used to determine the presence of the capsular anti¬  gen of H. influenzae, N. meningitidis, several species of  streptococci, and the yeast C. neoformans.   In this test, the unknown bacterial antigen and a known  specific antibody move toward each other in an electrical  field. If they are homologous, a precipitate forms within the    agar matrix. Because antibodies are positively charged at  the pH of the test, only negatively charged antigens, usually  capsular polysaccharides, can be assayed. The test can be  used to detect the presence in the spinal fluid of the capsu¬  lar antigens of H. influenzae, N. meningitidis, S. pneumoniae,  and group B streptococci.   In this test, a specific antibody to which an easily assayed  enzyme has been linked is used to detect the presence of  the homologous antigen. Because several techniques have  been devised to implement this principle, the specific steps  used cannot be detailed here (see Chapter 64). This test is  useful in detecting a wide variety of bacterial, viral, and  fungal infections.   A variety of bacteria can be identified by exposure to  known antibody labeled with fluorescent dye, which is  detected visually in the ultraviolet microscope. Various  methods can be used, such as the direct and indirect tech¬  niques (see Chapter 64).   In this test, serial twofold dilutions of a sample of the  patient’s serum are mixed with standard bacterial suspen¬  sions. The highest dilution of serum capable of agglutinat¬  ing the bacteria is the titer of the antibody. As with most  tests of a patient’s antibody, at least a fourfold rise in titer  between the early and late samples must be demonstrated  for a diagnosis to be made. This test is used primarily to aid  in the diagnosis of typhoid fever, brucellosis, tularemia,  plague, leptospirosis, and rickettsial diseases.   The detection of antibody in the patient’s serum is fre¬  quently used to diagnose syphilis, because T. pallidum does  not grow on laboratory media. There are two kinds of tests.   (1) The nontreponemal tests use a cardiolipin-lecithin-  cholesterol mixture as the antigen, not an antigen of the  organism. Cardiolipin (diphosphatidylglycerol) is a lipid  extracted from normal beef heart. Flocculation (clumping)  of the cardiolipin occurs in the presence of antibody to  T. pallidum. The VDRL and RPR tests are nontreponemal  tests commonly used as screening procedures. They are not  specific for syphilis but are inexpensive and easy to  perform.    (2) The treponemal tests use T. pallidum as the antigen.  The two most widely used treponemal tests are the FTA-  ABS and the MFLA-TP (microhemagglutination- Treponema  pallidum) tests. In the FTA-ABS test, the patient’s serum  sample, which has been absorbed with treponemes other  than T. pallidum to remove nonspecific antibodies, is  reacted with nonviable T. pallidum on a slide. Fluorescein-  labeled antibody against human immunoglobulin G (IgG)  is then used to determine whether IgG antibody against  T. pallidum is bound to the organism. In the MFLA-TP test,  the patient’s serum sample is reacted with sheep erythro¬  cytes coated with antigens of T. pallidum. If antibodies are  present, hemagglutination occurs.   Patients with Mycoplasma pneumoniae infections develop  autoimmune antibodies that agglutinate human red blood  cells in the cold (4°C) but not at 37°C. These antibodies  occur in certain diseases other than Mycoplasma infections;  thus false-positive results can occur.   There are three types of nucleic acid-based tests used in the  diagnosis of bacterial diseases: nucleic acid amplification  tests, nucleic acid probes, and nucleic acid sequence analy¬  sis. Nucleic acid-based tests are highly specific, quite sensi¬  tive (especially the amplification tests), and much faster  than culturing the organism. These tests are especially use¬  ful for those bacteria that are difficult to culture, such as  Chlamydia and Mycobacterium species.   Nucleic acid amplification tests (NAAT) utilize the poly¬  merase chain reaction (PCR) or other amplifying process to  increase the number of bacteria-specific DNA or RNA  molecules so the sensitivity of the test is significantly higher  than that of unamplified tests. Many bacteria can be identi¬  fied using these tests, but they are especially useful in  detecting C. trachomatis and N. gonorrhoeae in urine sam¬  ples in sexually transmitted disease (STD) clinics. These  tests are also used to identify M. tuberculosis in sputum  samples.   Tests that use nucleic acid probes are designed to detect  bacterial DNA or RNA directly (without amplification)  using a labeled DNA or RNA probe that will hybridize spe¬  cifically to the bacterial nucleic acid. These tests are simpler  to perform than the amplification tests but are less sensitive.   Nucleic acid sequence analysis is used to identify bacte¬  ria based on the base sequence of the organism’s ribosomal  RNA. An organism that has never been cultured, Tropheryma  whipplei, was identified using this approach.    • The laboratory diagnosis of infectious diseases includes bac-  teriologic, immunologic (serologic), and molecular (nucleic  acid-based) tests.   • Bacteriologic tests typically begin with staining the patient's  specimen and observing the organism in the microscope.This  is followed by culturing the organism, typically on blood agar,  then performing various tests to identify the causative  organism. Obtaining a pure culture of the bacteria is essential  to accurate diagnosis.   • Blood cultures are useful in cases of sepsis and other diseases  in which the organism is often found in the bloodstream, such  as endocarditis, meningitis, pneumonia, and osteomyelitis.   • Throat cultures are most useful to diagnose pharyngitis  caused by Streptococcus pyogenes ("strep throat"), but they are  also used to diagnose diphtheria, gonococcal pharyngitis, and  thrush caused by the yeast Candida albicans.   • Sputum cultures are used primarily to diagnose the cause of  pneumonia but also are used in suspected cases of  tuberculosis.   • Spinal fluid cultures are most useful in suspected cases of  meningitis. These cultures are often negative in encephalitis,  brain abscess, and subdural empyema.   • Stool cultures are useful primarily when the complaint is  bloody diarrhea (dysentery, enterocolitis) rather than watery  diarrhea, which is often caused by either enterotoxins or viruses.   • Urine cultures are used to determine the cause of either  pyelonephritis or cystitis.   • Genital tract cultures are most often used to diagnose gonor¬  rhea and chancroid. Chlamydia trachomatis is difficult to grow,  so nonbacteriologic methods such as ELISA and DNA probes are  now used more often than are cultures. The agent of syphilis has  not been cultured, so the diagnosis is made serologically.   • Wounds and abscesses can be caused by a large variety of  organisms. Cultures should be incubated both in the presence  and in the absence of oxygen because anaerobes are often  involved.   1. If the venipuncture site is inadequately disinfected, blood cultures  are most often contaminated with which one of the following  bacteria?   • Immunologic (serologic) tests can determine whether anti¬  bodies are present in the patient's serum as well as detect  the antigens of the organism in tissues or body fluids.   • In these tests, the antigens of the causative organism can be  detected by using specific antibody often labeled with a dye  such as fluorescein (fluorescent antibody tests). The presence  of antibody in the patient's serum can be detected using anti¬  gens derived from the organism. In some tests, the patient's  serum contains antibodies that react with an antigen that is  not derived from the causative organism, such as the VDRL test,  in which beef heart cardiolipin reacts with antibodies in the  serum of patients with syphilis.   • In many tests in which antibodies are detected in the patient's  serum, an acute and convalescent serum sample is obtained,  and at least a fourfold increase in titer between the acute and  convalescent samples must be found for a diagnosis to be  made. The reason these criteria are used is that the presence of  antibodies in a single sample could be from a prior infection, so  a significant (fourfold or greater) increase in titer is used to  indicate that this is a current infection. IgM antibody can also  be used as an indicator of current infection.   • Molecular tests can detect the presence of bacterial DNA or  RNA in patient specimens. These tests are both sensitive and  specific, and results are available within a clinically useful time  frame. They have become the diagnostic "gold standard" for  many infections.   • Nucleic acid amplification tests (NAAT) utilize the polymerase  chain reaction (PCR) to detect C. trachomatis and N. gonorrhoeae  in urine samples in sexually transmitted disease (STD) clinics.  These tests are also used to identify M. tuberculosis in sputum  samples.   • The specificity of these tests resides in the ability of the DNA or  RNA probe to bind to DNA or RNA present only in the bacteria  to be identified.    2. The main purpose of performing a throat culture is to detect  the presence of which one of the following bacteria?    3. A sputum culture will be rejected (i.e., it will not be stained or   (A) Salmonella and Shigella produce a blue colony in the presence  of methylene blue.   (B) Salmonella and Shigella produce a colorless colony because  they do not ferment lactose.   (C) Salmonella and Shigella produce a green colony because they  utilize the bile in the media.   (D) Salmonella and Shigella produce a red colony in the presence of  eosin.   (E) Salmonella and Shigella produce a yellow colony because they  ferment glucose.   Questions on the topics discussed in this chapter can be found  in the Basic Bacteriology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 709. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.    ANTIMICROBIAL DRUG STEWARDSHIP  PRINCIPLES OF ANTIMICROBIAL THERAPY  BACTERICIDAL & BACTERIOSTATIC ACTIVITY  MECHANISMS OF ACTION   Inhibition of Cell Wall Synthesis  Inhibition of Protein Synthesis  Inhibition of Nucleic Acid Synthesis    Alteration of Cell Membrane Function  Additional Drug Mechanisms  CHEMOPROPHYLAXIS  PROBIOTICS  Pearls    The discovery of antimicrobials is one of the great advances  in medicine and their use has substantially reduced morbidity  and mortality worldwide. Unfortunately, with widespread  antibiotic use we have witnessed the emergence of multidrug-  resistant pathogens and reduced efficacy of many of our most  powerful antimicrobials. In addition, we have also recognized  many adverse effects of antimicrobials, most notably the ris¬  ing rates of Clostridium difficile colitis. Further, the cost of  medical care is greatly increased due to overuse of antibiotics  and treating infections caused by resistant organisms. It is  critical that health profession learners understand the key  concepts behind antimicrobial stewardship at the time they  learn about microbial pathogens and antimicrobials.   The worldwide problem of antibiotic resistance makes  the need for antimicrobial stewardship evident. The Centers  for Disease Control and Prevention estimates that in the  United States over 2 million infections with multidrug-  resistant pathogens occur each year leading to approximately  20,000 deaths. These pathogens include methicillin-resistant  Staphylococcus aureus and extended-spectrum (3-lactamase  producing gram-negative rods (e.g., Escherichia coli and  Klebsiella pneumoniae). Hospital-associated infections,  many of which are caused by antibiotic-resistant bacteria, are  estimated to cost billions of dollars each year.   The basic principles of good stewardship are threefold:  (1) reduce inappropriate use of antibiotics, (2) encourage  targeted treatment with narrow spectrum drugs, and  (3) limit adverse effects (Table 10-1). Inappropriate    antibiotic use can occur for many reasons, including the  providers’ desire to adhere to patient’s wishes, even when it  is not medically appropriate. The most common example of  inappropriate antibiotic use is the prescribing of antibiotics  for a viral respiratory tract infection.   The concept of targeted treatment refers to making a  microbiologic diagnosis promptly and using the most spe¬  cific antibiotic that has the best safety profile for the  patient. If multiple broad-spectrum antibiotics are used as  empiric therapy early in infection then switching to a narrow-  spectrum antibiotic as soon as possible should be done.  Cultures should be sent prior to starting antibiotics so that  the drugs will not reduce the likelihood of isolating the  causative organism. Further, switching from intravenous  antibiotics to oral dosing reduces the risk of catheter-  associated infections. The net result of targeted treatment  is to reduce the ability of antibiotics to select for resistant  mutants present in the bacterial population.   Limiting the occurrence of adverse effects caused by  antibiotics is another major goal of antimicrobial steward¬  ship. Minimizing the duration of antibiotic use to only as  long as clinically indicated is a key intervention since dura¬  tion of exposure correlates closely with the risk of many  types of adverse effects. Patients with reduced renal func¬  tion should have the dose of some antibiotics adjusted  based on their estimated glomerular filtration rate.   Antibiotic allergies should be identified and explored in  detail. Although reaction to antibiotics may be commonly    TABLE 10-1 Basic Principles of Antimicrobial Drug Stewardship  Current Problems in the Use   of Antibiotics Role of Antimicrobial Drug Stewardship in Mitigating These Problems   Inappropriate use of 1. Use antibiotics only when a microbiologic diagnosis indicates effectiveness   antibiotics 2. Empiric therapy should be tailored to the most likely pathogen(s)    Overuse of broad-spectrum 1. Use narrow-spectrum antibiotics whenever possible  antibiotics 2. Require approval for the use of advanced generation broad-spectrum antibiotics   High rate of adverse effects 1. Stop antibiotics as soon as appropriate to reduce adverse effects, such as antibiotic-associated colitis caused by   2. Be aware of the effect of the patient's renal function on the dose of antibiotic prescribed   4. Determine whether the patient's declared hypersensitivity is correct and clinically significant   5. Warn patients regarding certain idiosyncratic drug reactions, such as photosensitization    reported, they may not always be of significance such as some  alleged hypersensitivity reactions. If optimal treatment requires  a drug such as penicillin to which the patient says they are  allergic, then skin testing can be employed to determine the  accuracy of that claim. In addition, patients should be warned  regarding the possibility that certain drugs may cause adverse  effects. For example, certain photosensitizing antibiotics may  cause a rash when the patient is exposed to sunlight.   The reasons that inappropriate use of antibiotics occurs are  varied. Probably the most important is the lack of knowledge   The most important concept underlying antimicrobial  therapy is selective toxicity (i.e., selective inhibition of the  growth of the microorganism without damage to the host).  Selective toxicity is achieved by exploiting the differences  between the metabolism and structure of the microorgan¬  ism and the corresponding features of human cells. For  example, penicillins and cephalosporins are effective anti¬  bacterial agents because they prevent the synthesis of pep-  tidoglycan, thereby inhibiting the growth of bacterial but  not human cells.   There are four major sites in the bacterial cell that are  sufficiently different from the human cell that they serve as  the basis for the action of clinically effective drugs: cell wall,  ribosomes, nucleic acids, and cell membrane (Table 10-2).   There are far more antibacterial drugs than antiviral  drugs. This is a consequence of the difficulty of designing a    or awareness of the physician. Risk avoidance on the part of  the physician is also common. Inadequate microbiologic  information plays a role as well. Patient expectation and direct  demands for antibiotics contribute to the problem.   In summary, antimicrobial stewardship refers to the effort  to improve the treatment of infectious diseases by the appro¬  priate use of antibiotics. This is critical in this era of rising  rates of multidrug-resistant pathogens. Targeted therapy with  the most appropriate single antibiotic will hopefully improve  clinical outcomes and reduce the cost of care.    drug that will selectively inhibit viral replication. Because  viruses use many of the normal cellular functions of the  host in their growth, it is not easy to develop a drug that  specifically inhibits viral functions and does not damage  the host cell.   Broad-spectrum antibiotics are active against several  types of microorganisms (e.g., tetracyclines are active  against many gram-negative rods, chlamydiae, mycoplas-  mas, and rickettsiae). Narrow-spectrum antibiotics are  active against one or very few types (e.g., vancomycin is  primarily used against certain gram-positive cocci, namely,  staphylococci and enterococci).   Antifungal drugs are included in this chapter because  they have similar unique sites of action such as cell walls,  cell membranes, and nucleic acid synthesis. Additional  information on antifungal drugs is given in Chapter 47.    In some clinical situations, it is essential to use a bacteri¬  cidal drug rather than a bacteriostatic one. A bactericidal  drug kills bacteria, whereas a bacteriostatic drug inhibits  their growth but does not kill them (Figure 10-1). The   salient features of the behavior of bacteriostatic drugs are  that (1) the bacteria can grow again when the drug is with¬  drawn, and (2) host defense mechanisms, such as    phagocytosis, are required to kill the bacteria. Bactericidal  drugs are particularly useful in certain infections (e.g.,  those that are immediately life-threatening; those in  patients whose polymorphonuclear leukocyte count is  below 500/pL; and endocarditis, in which phagocytosis is  limited by the fibrinous network of the vegetations and  bacteriostatic drugs do not effect a cure).    TABLE 10-2 Mechanism of Action of Important Antibacterial and Antifungal Drugs   Penicillins (and cephalosporins) act by inhibiting trans¬  peptidases, the enzymes that catalyze the final cross-linking  step in the synthesis of peptidoglycan (see Figure 2-5). For  example, in S. aureus, transpeptidation occurs between the  amino group on the end of the pentaglycine cross-link and  the terminal carboxyl group of the D-alanine on the tetra-  peptide side chain. Because the stereochemistry of penicil¬  lin is similar to that of a dipeptide, D-alanyl-D-alanine,  penicillin can bind to the active site of the transpeptidase  and inhibit its activity.   (1) The first is that penicillin binds to a variety of recep¬  tors in the bacterial cell membrane and cell wall, called  penicillin-binding proteins (PBPs). Some PBPs are trans¬  peptidases; the others function in the synthesis of peptido¬  glycan. Their specific functions are beyond the scope of  this book. Changes in PBPs are in part responsible for an  organism’s becoming resistant to penicillin.   (2) The second factor is that autolytic enzymes called  murein hydrolases (murein is a synonym for peptidogly¬  can) are activated in penicillin-treated cells and degrade    the peptidoglycan. Some bacteria (e.g., strains of S. aureus)  are tolerant to the action of penicillin, because these auto¬  lytic enzymes are not activated. A tolerant organism is one  that is inhibited but not killed by a drug that is usually bac¬  tericidal, such as penicillin (see page 90).   Penicillin-treated cells die by rupture as a result of the  influx of water into the high-osmotic-pressure interior of   FIGURE 10-1 Bactericidal and bacteriostatic activity of anti¬  microbial drugs. Either a bactericidal or a bacteriostatic drug is added  to the growing bacterial culture at the time indicated by the arrow.  After a brief lag time during which the drug enters the bacteria, the  bactericidal drug kills the bacteria, and a decrease in the number of  viable bacteria occurs. The bacteriostatic drug causes the bacteria to  stop growing, as indicated by the dotted line, but if the bacteriostatic  drug is removed from the culture, the bacteria resume growing.    the bacterial cell. If the osmotic pressure of the medium is  raised about threefold (e.g., by the addition of sufficient  KC1), rupture will not occur and the organism can survive  as a protoplast. Exposure of the bacterial cell to lysozyme,  which is present in human tears, results in degradation of  the peptidoglycan and osmotic rupture similar to that  caused by penicillin.   Penicillin is bactericidal, but it kills cells only when  they are growing. When cells are growing, new peptido¬  glycan is being synthesized, and transpeptidation occurs.  However, in nongrowing cells, no new cross-linkages are  required, and penicillin is inactive. Penicillins are therefore  more active during the log phase of bacterial cell growth  than during the stationary phase (see Chapter 3 for the  bacterial cell growth cycle).   Penicillins (and cephalosporins) are called (3-lactam  drugs because of the importance of the (3-lactam ring  (Figure 10-2). An intact ring structure is essential for anti¬  bacterial activity; cleavage of the ring by penicillinases  ((3-lactamases) inactivates the drug. The most important  naturally occurring compound is benzylpenicillin (penicillin  G), which is composed of the 6-aminopenicillanic acid  nucleus that all penicillins have, plus a benzyl side chain (see  Figure 10-2). Penicillin G is available in three main forms:   (2) Procaine penicillin G, in which penicillin G is con¬  jugated to procaine. This form is metabolized more slowly  and is less painful when injected intramuscularly because  the procaine acts as an anesthetic.    (3) Benzathine penicillin G, in which penicillin G is  conjugated to benzathine. This form is metabolized very  slowly and is often called a “depot” preparation.   Benzylpenicillin is one of the most widely used and  effective antibiotics. However, it has four disadvantages,  three of which have been successfully overcome by chemi¬  cal modification of the side chain. The three disadvantages  are (1) limited effectiveness against many gram-negative  rods, (2) hydrolysis by gastric acids, so that it cannot be  taken orally, and (3) inactivation by (3-lactamases. The lim¬  ited effectiveness of penicillin G against gram-negative  rods is due to the inability of the drug to penetrate the outer  membrane of the organism. The fourth disadvantage com¬  mon to all penicillins that has not been overcome is hyper¬  sensitivity, especially anaphylaxis, in some recipients of the  drug.   The effectiveness of penicillins against gram-negative  rods has been increased by a series of chemical changes in  the side chain (Table 10-3). It can be seen that ampicillin and  amoxicillin have activity against several gram-negative rods  that the earlier penicillins do not have. However, these drugs  are not useful against Pseudomonas aeruginosa and Klebsiella  pneumoniae. Hence other penicillins were introduced. Gen¬  erally speaking, as the activity against gram-negative bacteria  increases, the activity against gram-positive bacteria  decreases.   The second important disadvantage—acid hydrolysis in  the stomach—also has been addressed by modification of  the side chain. The site of acid hydrolysis is the amide  bond between the side chain and penicillanic acid nucleus    FIGURE 10-2 Penicillins. A: The 6-aminopenicillanic acid nucleus is composed of a thiazolidine ring (a), a (3-lactam ring (b), and an amino  group (c).The sites of inactivation by stomach acid and by penicillinase are indicated. B:The benzyl group, which forms benzylpenicillin (peni¬  cillin G) when attached at R. C:The large aromatic ring substituent that forms nafcillin, a |3-lactamase-resistant penicillin, when attached at R.  The large ring blocks the access of |3-lactamase to the (3-lactam ring.   Penicillin G Gram-positive cocci, gram-positive rods, Neisseria, spirochetes such as Treponema pallidum, and many anaerobes   (except Bacteroides fragilis ) but none of the gram-negative rods listed below   Ampicillin or amoxicillin Certain gram-negative rods, such as Haemophilus influenzae, Escherichia coli, Proteus, Salmonella, and Shigella but   Ticarcillin P. aeruginosa, especially when used in synergistic combination with an aminoglycoside   Piperacillin Similar to ticarcillin but with greater activity against P. aeruginosa and Klebsiella pneumoniae   'The spectrum of activity is intentionally incomplete. It is simplified for the beginning student to illustrate the expanded coverage of gram-negative organisms with successive  generations and does not cover all possible clinical uses.    (see Figure 10-2). Minor modifications of the side chain in  that region, such as addition of an oxygen (to produce  penicillin V) or an amino group (to produce ampicillin),  prevent hydrolysis and allow the drug to be taken orally.   The inactivation of penicillin G by (3-lactamases is  another important disadvantage, especially in the treat¬  ment of S. aureus infections. Access of the enzyme to the  (3-lactam ring is blocked by modification of the side chain  with the addition of large aromatic rings containing bulky  methyl or ethyl groups (methicillin, oxacillin, nafcillin, etc.;  see Figure 10-2). Another defense against (3-lactamases is  inhibitors such as clavulanic acid, tazobactam, sulbactam,  and avibactam. These are structural analogues of penicillin  that have little antibacterial activity but bind strongly to  (3-lactamases and thus protect the penicillin. Combina¬  tions, such as amoxicillin and clavulanic acid (Augmentin),  are in clinical use. Some bacteria resistant to these combi¬  nations have been isolated from patient specimens.   Penicillins are usually nontoxic at clinically effective lev¬  els. The major disadvantage of these compounds is hyper¬  sensitivity, which is estimated to occur in 1% to 10% of  patients. The IgE-mediated hypersensitivity reactions  include anaphylactic shock, bronchospasm, and urticarial  rash (see Chapter 65). IgG and cell-mediated hypersensitiv¬  ity reactions include non-urticarial skin rashes, hemolytic  anemia, nephritis, and drug fever. A maculopapular drug-  induced rash is quite common. Anaphylactic shock, the most  serious complication, occurs in 0.5% of patients. Death as a  result of anaphylaxis occurs in 0.002% (1:50,000) of patients.   Patients who say they are allergic to penicillin can be  treated with another equally effective antibiotic, if available.  To determine whether the patient’s allergy is clinically sig¬  nificant, a skin-test using penicilloyl-polylysine as the test  reagent can be performed. A wheal and flare reaction occurs  at the site of injection in allergic individuals. If the patient’s  disease requires treatment with penicillin, the patient can be  desensitized under the supervision of a trained allergist.   Cephalosporins are (3-lactam drugs that act in the same  manner as penicillins (i.e., they are bactericidal agents that    inhibit the cross-linking of peptidoglycan). The structures,  however, are different: Cephalosporins have a six-mem-  bered ring adjacent to the (3-lactam ring and are substituted  in two places on the 7-aminocephalosporanic acid nucleus  (Figure 10-3), whereas penicillins have a five-membered  ring and are substituted in only one place.   The first-generation cephalosporins are active primarily  against gram-positive cocci (Table 10-4). Similar to the  penicillins, new cephalosporins were synthesized with  expansion of activity against gram-negative rods as the  goal. These new cephalosporins have been categorized into  second, third, and fourth generations, with each generation  having expanded coverage against certain gram-negative  rods. The fourth- and fifth-generation cephalosporins have  activity against many gram-positive cocci as well.   Cephalosporins are effective against a broad range of  organisms, are generally well tolerated, and produce fewer  hypersensitivity reactions than do the penicillins. Despite  the structural similarity, a patient allergic to penicillin has  only about a 10% chance of being hypersensitive to cepha¬  losporins also. Most cephalosporins are the products of  molds of the genus Cephalosporium; a few, such as cefoxi¬  tin, are made by the actinomycete Streptomyces.   The inactivation of cephalosporins by (3-lactamases  (cephalosporinases) is an important clinical problem.   FIGURE 10~3 Cephalosporins. A: The 7-aminocephalospo¬  ranic acid nucleus. B: The two R groups in the drug cephalothin.   Gram-positive cocci such as staphylococci and streptococci except enterococci and MRSA   Enteric gram-negative rods such as Escherichia coli, Klebsiella and Proteus. Also Neisseria gonorrhoeae   Enteric gram-negative rods that produce extended-spectrum fi-lactamases; Staphylococcus aureus (not  MRSA) and penicillin-resistant Streptococcus pneumoniae   Gram-positive cocci and gram-negative rods that cause bacterial pneumonia and gram-positive cocci  that cause skin infections including MRSA   Enteric gram-negative rods that produce extended-spectrum (i-lactamases; Pseudomonas aeruginosa;  used in combination with tazobactam   ’The spectrum of activity is intentionally incomplete. It is simplified for the beginning student to illustrate the expanded coverage of gram-negative organisms with successive  generations and does not cover all possible clinical uses.    (3-Lactamase inhibitors such as tazobactam and avibactam  are combined with certain cephalosporins to prevent inac¬  tivation of the cephalosporin. For example, the FDA has  approved the combination of ceftazidime/avibactam (Avy-  caz) and ceftolozane/tazobactam (Zerbaxa) for the treat¬  ment of intra-abdominal infections and complicated  urinary tract infections caused by antibiotic-resistant  gram-negative rods.   Carbapenems are (3-lactam drugs that are structurally dif¬  ferent from penicillins and cephalosporins. For example,  imipenem (N-formimidoylthienamycin), a commonly used  carbapenem, has a methylene group in the ring in place of  the sulfur (Figure 10-4). Imipenem has one of the widest  spectrums of activity of the (3-lactam drugs. It has excellent  bactericidal activity against many gram-positive, gram¬  negative, and anaerobic bacteria. It is effective against most  gram-positive cocci (e.g., streptococci and staphylococci),  most gram-negative cocci (e.g., Neisseria), many gram¬  negative rods (e.g., Pseudomonas, Haemophilus, and mem¬  bers of the family Enterobacteriaceae such as E. coli ), and  various anaerobes (e.g., Bacteroides and Clostridium).    Imipenem is especially useful in treating infections caused  by gram-negative rods that produce extended-spectrum  (3-lactamases that make them resistant to all penicillins and  cephalosporins. Carbapenems are often the “drugs of last  resort” against bacteria resistant to multiple antibiotics.   Imipenem is prescribed in combination with cilastatin,  which is an inhibitor of dehydropeptidase, a kidney enzyme  that inactivates imipenem. Imipenem is not inactivated by  most (3-lactamases; however, carbapenemases produced by  K. pneumoniae that degrade imipenem and other car¬  bapenemases have emerged. Other carbapenems, such as  ertapenem and meropenem, are not inactivated by dehy¬  dropeptidase and are not prescribed in combination with  cilastatin.   Monobactams are also (3-lactam drugs that are structurally  different from penicillins and cephalosporins. Mono¬  bactams are characterized by a (3-lactam ring without an  adjacent sulfur-containing ring structure (i.e., they are  monocyclic) (see Figure 10-4). Aztreonam, currently the  most useful monobactam, has excellent activity against  many gram-negative rods, such as Enterobacteriaceae and    Pseudomonas, but is inactive against gram-positive and  anaerobic bacteria. It is resistant to most [3-lactamases. It is  very useful in patients who are hypersensitive to penicillin,  because there is no cross-reactivity.   Vancomycin is a glycopeptide that inhibits cell wall pepti-  doglycan synthesis by blocking transpeptidation but by a   mechanism different from that of the (3-lactam drugs.  Vancomycin binds directly to the D-alanyl-D-alanine por¬  tion of the pentapeptide, which blocks the transpeptidase  from binding, whereas the (3-lactam drugs bind to the  transpeptidase itself. Vancomycin also inhibits a second  enzyme, the bacterial transglycosylase, which also func¬  tions in synthesizing the peptidoglycan, but this appears to  be less important than inhibition of the transpeptidase.   Vancomycin is a bactericidal agent effective against  certain gram-positive bacteria. Its most important use is  in the treatment of infections caused by S. aureus strains  that are resistant to the penicillinase-resistant penicillins  such as nafcillin and methicillin (e.g., methicillin-resistant  S. aureus [MRSA]). Note that vancomycin is not a (3-lactam  drug and, therefore, is not degraded by (3-lactamase.   Vancomycin is also used in the treatment of infections  caused by Staphylococcus epidermidis, penicillin-resistant  Streptococcus pneumoniae, and enterococci. Strains of S.  aureus, S. epidermidis, and enterococci with partial or com¬  plete resistance to vancomycin have been recovered from  patients.   A well-known adverse effect of vancomycin is “red man”  syndrome. “Red” refers to the flushing caused by vasodila¬  tion induced by histamine release from mast cells and  basophils. This is a direct effect of vancomycin on these  cells and is not an IgE-mediated response.   Telavancin is a synthetic derivative of vancomycin that  both inhibits peptidoglycan synthesis and disrupts bacte¬  rial cell membranes. It is used for the treatment of skin and  soft tissue infections, especially those caused by MRSA.  Oritavancin and dalbavancin, are lipoglycopeptide deriva¬  tives of vancomycin and teicoplanin, respectively. These  drugs inhibit the transpeptidases and transglycosylases  required to synthesize the peptiodglycan of gram-positive  bacteria. They are effective in the treatment of infections  caused by S. aureus, including MRSA and Enterococcus,  including vancomycin-resistant enterococci (VRE).   Cycloserine is a structural analogue of D-alanine that  inhibits the synthesis of the cell wall dipeptide D-alanyl-D-  alanine. It is used as a second-line drug in the treatment of  tuberculosis. Bacitracin is a cyclic polypeptide antibiotic  that prevents the dephosphorylation of the phospholipid  that carries the peptidoglycan subunit across the cell mem¬  brane. This blocks the regeneration of the lipid carrier and    inhibits cell wall synthesis. Bacitracin is a bactericidal drug  useful in the treatment of superficial skin infections but too  toxic for systemic use.   Echinocandins, such as caspofungin (Cancidas) and mica-  fungin (Mycamine), are lipopeptides that block fungal cell  wall synthesis by inhibiting the enzyme that synthesizes  (3-glucan. (3-Glucan is a polysaccharide composed of long  chains of d- glucose, which is an essential component of  certain medically important fungal pathogens.   Caspofungin inhibits the growth of Aspergillus and  Candida but not Cryptococcus or Mucor. Caspofungin is  used for the treatment of disseminated candidiasis and for  the treatment of invasive aspergillosis that does not respond  to amphotericin B. Micafungin is approved for the treat¬  ment of esophageal candidiasis and the prophylaxis of inva¬  sive Candida infections in bone marrow transplant patients.  Anidulafungin is approved for the treatment of esophageal  candidiasis and other serious Candida infections.   Several drugs inhibit protein synthesis in bacteria without  significantly interfering with protein synthesis in human  cells. This selectivity is due to the differences between bac¬  terial and human ribosomal proteins, RNAs, and associated  enzymes. Bacteria have 70S 1 ribosomes with 50S and 30S  subunits, whereas human cells have 80S ribosomes with  60S and 40S subunits.   Chloramphenicol, macrolides such as azithromycin and  erythromycin, clindamycin, and linezolid act on the 50S  subunit, whereas tetracyclines such as doxycycline and  aminoglycosides such as gentamicin act on the 30S subunit.  A summary of the modes of action of these drugs is pre¬  sented in Table 10-5, and a summary of their clinically  useful activity is presented in Table 10-6.   Aminoglycosides are bactericidal drugs especially useful  against many gram-negative rods. Certain aminoglycosides  are used against other organisms (e.g., streptomycin is used  in the multidrug therapy of tuberculosis, and gentamicin is  used in combination with penicillin G against enterococci).  Aminoglycosides are named for the amino sugar compo¬  nent of the molecule, which is connected by a glycosidic  linkage to other sugar derivatives (Figure 10-5).    1 “S” stands for Svedberg units, a measure of sedimentation rate in a density  gradient. The rate of sedimentation is proportionate to the mass of the particle.    TABLE 10-5 Mode of Action of Antibiotics That Inhibit Protein Synthesis    The two important modes of action of aminoglycosides  have been documented best for streptomycin; other amino¬  glycosides probably act similarly. Both inhibition of the  initiation complex and misreading of messenger RNA    subunit, and mRNA will not function—that is, no peptide  bonds are formed, no polysomes are made, and a frozen  “streptomycin monosome” results. Misreading of the triplet  codon of mRNA so that the wrong amino acid is inserted    (mRNA) occur; the former is probably more important for into the protein also occurs in streptomycin-treated bacteria,  the bactericidal activity of the drug. An initiation complex The site of action on the 30S subunit includes both a ribo-  composed of a streptomycin-treated 30S subunit, a 50S somal protein and the ribosomal RNA (rRNA). As a result   TABLE 10-6 Spectrum of Activity of Antibiotics That Inhibit Protein Synthesis 1   Too toxic to be used systemically; use orally  since not absorbed   Skin infections caused by various gram-positive cocci and  intra-abdominal infections caused by various facultative and  anaerobic bacteria (see text)   Haemophilus influenzae meningitis, typhoid fever, anaerobic infections  (especially Bacteroides fragilis )   Pneumonia caused by Mycoplasma and Legionella, infections by  gram-positive cocci in penicillin-allergic patients   Vancomycin-resistant enterococci, methicillin-resistant Staphylococcus  aureus and Staphylococcus epidermidis, and penicillin-resistant  pneumococci   Many bacteria that are resistant to other macro¬  lides are susceptible to telithromycin   No cross-resistance between streptogramins  and other drugs that inhibit protein synthesis    ’The spectrum of activity is intentionally incomplete. It is simplified for the beginning student to illustrate the expanded coverage of gram-negative organisms with successive  generations and does not cover all possible clinical uses.   FIGURE 10~5 Aminoglycosides. Aminoglycosides consist of  amino sugars joined by a glycosidic linkage. The structure of genta¬  micin is shown.   of inhibition of initiation and misreading, membrane dam¬  age occurs and the bacterium dies. (In 1993, another possible  mode of action was described, namely, that aminoglycosides  inhibit ribozyme-mediated self-splicing of rRNA.)   Aminoglycosides have certain limitations in their use:  (1) They have a toxic effect both on the kidneys and on the  auditory and vestibular portions of the eighth cranial nerve.  To avoid toxicity, serum levels of the drug, blood urea nitro¬  gen, and creatinine should be measured. (2) They are poorly  absorbed from the gastrointestinal tract and cannot be  given orally. (3) They penetrate the spinal fluid poorly and  must be given intrathecally in the treatment of meningitis.  (4) They are ineffective against anaerobes, because their  transport into the bacterial cell requires oxygen.   Tetracyclines are a family of antibiotics with bacteriostatic  activity against a variety of gram-positive and gram¬  negative bacteria, mycoplasmas, chlamydiae, and rickett-  siae. They inhibit protein synthesis by binding to the 30S  ribosomal subunit and by blocking the aminoacyl transfer  RNA (tRNA) from entering the acceptor site on the ribo¬  some. However, the selective action of tetracycline on bac¬  teria is not at the level of the ribosome, because tetracycline  in vitro will inhibit protein synthesis equally well in puri¬  fied ribosomes from both bacterial and human cells. Its  selectivity is based on its greatly increased uptake into  susceptible bacterial cells compared with human cells.   Tetracyclines, as the name indicates, have four cyclic  rings with different substituents at the three R groups  (Figure 10-6). The various tetracyclines (e.g., doxycycline,  minocycline, oxytetracycline) have similar antimicrobial  activity but different pharmacologic properties. In general,  tetracyclines have low toxicity but are associated with some  important side effects. One is suppression of the normal  flora of the intestinal tract, which can lead to diarrhea and  overgrowth by drug-resistant bacteria and fungi. Second is    FIGURE 10~6 Tetracycline structure. The four-ring struc¬  ture is depicted with its three R sites. Chlortetracycline, for  example, has R = Cl, R t = CH 3 , and R 2 = H.   that suppression of Lactobacillus in the vaginal normal  flora results in a rise in pH, which allows Candida albicans  to grow and cause vaginitis. Third is brown staining of the  teeth of fetuses and young children as a result of deposition  of the drug in developing teeth; tetracyclines are avid cal¬  cium chelators. For this reason, tetracyclines are contrain¬  dicated for use in pregnant women and in children younger  than 8 years of age. Tetracyclines also chelate iron, and so  products containing iron, such as iron-containing vita¬  mins, should not be taken during therapy with tetracy¬  clines. Photosensitivity (rash upon exposure to sunlight)  can also occur during tetracycline therapy.   Tigecycline (Tygacil) is the first clinically available mem¬  ber of the glycylcycline class of antibiotics. They have a  structure similar to tetracyclines and have the same mecha¬  nism of action as tetracyclines; namely, they bind to the 30S  ribosomal subunit and inhibit bacterial protein synthesis.  They have a similar range of adverse effects. Tigecycline is  used to treat skin and skin structure infections caused by  methicillin-sensitive and methicillin-resistant S. aureus,  group A and group B streptococci, vancomycin-resistant  enterococci, E. coli, and Bacteroides fragilis. It is also used to  treat complicated intra-abdominal infections caused by a  variety of facultative and anaerobic bacteria.   Chloramphenicol is active against a broad range of organ¬  isms, including gram-positive and gram-negative bacteria  (including anaerobes). It is bacteriostatic against certain  organisms, such as Salmonella typhi, but has bactericidal  activity against the three important encapsulated organisms  that cause meningitis: Haemophilus influenzae, Streptococcus  pneumoniae, and Neisseria meningitidis.   Chloramphenicol inhibits protein synthesis by binding  to the 50S ribosomal subunit and blocking the action of  peptidyltransferase; this prevents the synthesis of new  peptide bonds. It inhibits bacterial protein synthesis selec¬  tively, because it binds to the catalytic site of the transferase  in the 50S bacterial ribosomal subunit but not to the trans¬  ferase in the 60S human ribosomal subunit. Chloramphen¬  icol inhibits protein synthesis in the mitochondria of  human cells to some extent, since mitochondria have a 50S   subunit (mitochondria are thought to have evolved from  bacteria). This inhibition may be the cause of the dose-  dependent toxicity of chloramphenicol to bone marrow  (see next paragraph).   Chloramphenicol is a comparatively simple molecule  with a nitrobenzene nucleus (Figure 10-7). Nitrobenzene is  a bone marrow depressant and is likely to be involved in the  hematologic problems reported with this drug. The most  important side effect of chloramphenicol is bone marrow  toxicity, of which there are two types. One is a dose-dependent  suppression, which is more likely to occur in patients  receiving high doses for long periods and which is revers¬  ible when administration of the drug is stopped. The other  is aplastic anemia, which is caused by an idiosyncratic reac¬  tion to the drug. This reaction is not dose-dependent, can  occur weeks after administration of the drug has been  stopped, and is not reversible. Fortunately, this reaction is  rare, occurring in about 1:30,000 patients.   One specific toxic manifestation of chloramphenicol is  “gray baby” syndrome, in which the infant’s skin appears gray  and vomiting and shock occur. This is due to reduced gluc-  uronyl transferase activity in infants, resulting in a toxic con¬  centration of chloramphenicol. Glucuronyl transferase is the  enzyme responsible for detoxification of chloramphenicol.   Macrolides are a group of bacteriostatic drugs with a wide  spectrum of activity The name macrolide refers to their  large (13-16 carbon) ring structure (Figure 10-8). Azithro¬  mycin, erythromycin, and clarithromycin are the main  macrolides in clinical use. Azithromycin is used to treat  genital tract infections caused by Chlamydia trachomatis  and respiratory tract infections caused by Legionella, Myco¬  plasma, Chlamydia pneumoniae, and S. pneumoniae.    H OH OH H O CH 3 CH 3 H CH 3 H CH 3    Erythromycin has a similar spectrum of activity but has a  shorter half-life and so must be taken more frequently and  has more adverse effects, especially on the gastrointestinal  tract. Clarithromycin is used primarily in the treatment of  Helicobacter infections and in the treatment and prevention  of Mycobacterium avium-intracellulare infections. An impor¬  tant adverse effect of clarithromycin is prolongation of the  Q-T interval that may increase the risk of cardiac death.   Macrolides inhibit bacterial protein synthesis by bind¬  ing to the 50S ribosomal subunit and blocking transloca¬  tion. They prevent the release of the uncharged tRNA after  it has transferred its amino acid to the growing peptide  chain. The donor site remains occupied, a new tRNA can¬  not attach, and protein synthesis stops.   The most useful clinical activity of this bacteriostatic drug  is against anaerobes, both gram-positive bacteria such as  Clostridium perfringens and gram-negative bacteria such as  B. fragilis.   Clindamycin binds to the 50S subunit and blocks pep¬  tide bond formation by an undetermined mechanism. Its  specificity for bacteria arises from its inability to bind to the  60S subunit of human ribosomes.   The most important side effect of clindamycin is pseu¬  domembranous colitis, which, in fact, can occur with virtu¬  ally any antibiotic, whether taken orally or parenterally.  The pathogenesis of this potentially severe complication is  suppression of the normal flora of the bowel by the drug  and overgrowth of a drug-resistant strain of C. difficile. The  organism secretes an exotoxin that produces the pseudo¬  membrane in the colon and severe, often bloody diarrhea.   Linezolid is useful for the treatment of vancomycin-resistant  enterococci, methicillin-resistant S. aureus and S. epidermidis,  and penicillin-resistant pneumococci. It is bacteriostatic  against enterococci and staphylococci but bactericidal  against pneumococci.   Linezolid binds to the 23S ribosomal RNA in the 50S  subunit and inhibits protein synthesis, but the precise  mechanism is unknown. It appears to block some early step  (initiation) in ribosome formation. Tedizolid is a second-  generation drug in the same class as linezolid but is approxi¬  mately 10 times more effective. It is used for the treatment  of skin and soft tissue infections caused by a similar range of  bacteria as linezolid and has a similar mechanism of action.   Telithromycin (Ketek) is the first clinically useful member  of the ketolide group of antibiotics. It is similar to the mac¬  rolides in general structure and mode of action but is suf¬  ficiently different chemically such that organisms resistant  to macrolides may be sensitive to telithromycin. It has a  wide spectrum of activity against a variety of gram-positive   Either alone or in combination with trimethoprim, sulfon¬  amides are useful in a variety of bacterial diseases such as  urinary tract infections caused by E. coli, otitis media  caused by S. pneumoniae or H. influenzae in children, shig¬  ellosis, nocardiosis, and chancroid. In combination, they  are also the drugs of choice for two additional diseases,  toxoplasmosis and Pneumocystis pneumonia. The sulfon¬  amides are a large family of bacteriostatic drugs that are  produced by chemical synthesis. In 1935, the parent com¬  pound, sulfanilamide, became the first clinically effective  antimicrobial agent.   The mode of action of sulfonamides is to block the syn¬  thesis of tetrahydrofolic acid, which is required as a methyl  donor in the synthesis of the nucleic acid precursors ade¬  nine, guanine, and thymine. Sulfonamides are structural  analogues of p-aminobenzoic acid (PABA). PABA con¬  denses with a pteridine compound to form dihydropteroic  acid, a precursor of tetrahydrofolic acid (Figure 10-9).  Sulfonamides compete with PABA for the active site of the  enzyme dihydropteroate synthetase. This competitive inhi¬  bition can be overcome by an excess of PABA.   The basis of the selective action of sulfonamides on  bacteria is that many bacteria synthesize their folic acid  from PABA-containing precursors, whereas human cells  require preformed folic acid as an exogenous nutrient  because they lack the enzymes to synthesize it. Human  cells therefore bypass the step at which sulfonamides act.  Bacteria that can use preformed folic acid are similarly  resistant to sulfonamides.   The p-amino group on the sulfonamide is essential for  its activity. Modifications are therefore made on the sul¬  fonic acid side chain. Sulfonamides are inexpensive and  infrequently cause side effects. However, drug-related fever,    TABLE 10-7 Mode of Action and Activity of Selected Nucleic Acid Inhibitors 1   Inhibit folic acid synthesis; act as  a competitive inhibitor of PABA   Used in combination with trimethoprim for UTI caused by Escherichia coli ; otitis  media and sinusitis caused by Haemophilus influenzae; MRSA; Pneumocystis  pneumonia   Ciprofloxacin is used to treat Gl tract infections caused by Shigella and   Salmonella, and UTI caused by enteric gram-negative rods. Levofloxacin  is used to treat respiratory tract infections, especially those caused by  penicillin-resistant Streptococcus pneumoniae   Used in combination with isoniazid and other drugs to treat tuberculosis    DHFR = dihydrofolate reductase; Gl = gastrointestinal; MRSA = methicillin-resistant Staphylococcus aureus; PABA = para-aminobenzoic acid; UTI = urinary tract infection.  ’The spectrum of activity is intentionally incomplete. It is simplified for the beginning student to emphasize the most common uses.    and gram-negative bacteria (including macrolide-resistant  pneumococci) and is used in the treatment of community-  acquired pneumonia, bronchitis, and sinusitis.   A combination of two streptogramins, quinupristin, and  dalfopristin (Synercid), is used for the treatment of  bloodstream infections caused by vancomycin-resistant  Enterococcus faecium (but not vancomycin-resistant  Enterococcus faecalis). It is also approved for use in infec¬  tions caused by Streptococcus pyogenes, penicillin-resistant  S. pneumoniae, methicillin-resistant S. aureus, and methicillin-  resistant S. epidermidis.   Streptogramins cause premature release of the growing  peptide chain from the 50S ribosomal subunit. The struc¬  ture and mode of action of streptogramins is different from  all other drugs that inhibit protein synthesis, and there is  no cross-resistance between streptogramins and these  other drugs.   Retapamulin (Altabax) is the first clinically available mem¬  ber of a new class of antibiotics called pleuromutilins.  These drugs inhibit bacterial protein synthesis by binding  to the 23S RNA of the 50S subunit and blocking attachment  of the donor tRNA. Retapamulin is a topical antibiotic used  in the treatment of skin infections, such as impetigo, caused  by S. pyogenes and methicillin-sensitive S. aureus.   The mode of action and clinically useful activity of the  important drugs that act by inhibiting nucleic acid synthe¬  sis are summarized in Table 10-7.    FIGURE 10-9 Mechanism of action of sulfonamides and trimethoprim. A: Comparison of the structures of p-aminobenzoic acid (PABA)  and sulfanilamide. Note that the only difference is that PABA has a carboxyl (COOH) group, whereas sulfanilamide has sulfonamide (S0 2 NH 2 )  group. B: Structure of trimethoprim. C: Inhibition of the folic acid pathway by sulfonamide and trimethoprim. Sulfonamides inhibit the syn¬  thesis of dihydrofolic acid (DHF) from its precursor PABA.Trimethoprim inhibits the synthesis of tetrahydrofolic acid (THF) from its precursor  DHF. Loss of THF inhibits DNA synthesis because THF is required to transfer a methyl group onto uracil to produce thymidine, an essential  component of DN A. {Adapted from Corcoran JW, Hahn FE, eds. Mechanism of Action of Antimicrobial Agents. Vol. 3 of Antibiotics. Springer-Verlag; 1975.)    rashes, photosensitivity (rash upon exposure to sunlight),  and bone marrow suppression can occur. They are the most  common group of drugs that cause erythema multiforme  and its more severe forms, Stevens-Johnson syndrome and  toxic epidermal necrolysis.   Trimethoprim also inhibits the production of tetrahydrofo¬  lic acid but by a mechanism different from that of the sul¬  fonamides (i.e., it inhibits the enzyme dihydrofolate  reductase) (see Figure 10-9). Its specificity for bacteria is  based on its much greater affinity for bacterial reductase  than for the human enzyme.   Trimethoprim is used most frequently together with  sulfamethoxazole. Note that both drugs act on the same  pathway—but at different sites—to inhibit the synthesis of  tetrahydrofolate. The advantages of the combination are  that (1) bacterial mutants resistant to one drug will be  inhibited by the other and that (2) the two drugs can act  synergistically (i.e., when used together, they cause signifi¬  cantly greater inhibition than the sum of the inhibition  caused by each drug separately).   Trimethoprim-sulfamethoxazole is clinically useful in  the treatment of urinary tract infections, Pneumocystis  pneumonia, and shigellosis. It also is used for prophylaxis  in granulopenic patients to prevent opportunistic  infections.   Fluoroquinolones are bactericidal drugs that block bacterial  DNA synthesis by inhibiting DNA gyrase (topoisomerase).  Fluoroquinolones, such as ciprofloxacin (Figure 10-10),  levofloxacin, norfloxacin, ofloxacin, and others, are active  against a broad range of organisms that cause infections of  the lower respiratory tract, intestinal tract, urinary tract,  and skeletal and soft tissues. Nalidixic acid, which is a qui-  nolone but not a fluoroquinolone, is much less active and is  used only for the treatment of urinary tract infections.  Fluoroquinolones should not be given to pregnant women  and children under the age of 18 years because they damage  growing bone and cartilage. The Food and Drug Adminis¬  tration has issued a warning regarding the possibility of    Achilles tendonitis and tendon rupture associated with  fluoroquinolone use, especially in those over 60 years of age  and in patients receiving corticosteroids, such as predni¬  sone. Another important adverse effect of fluoroquino¬  lones is peripheral neuropathy, the symptoms of which  include pain, burning, numbness, or tingling in the arms or  legs.   Flucytosine (5-fluorocytosine, 5-FC) is an antifungal  drug that inhibits DNA synthesis. It is a nucleoside ana¬  logue that is metabolized to fluorouracil, which inhibits  thymidylate synthetase, thereby limiting the supply of  thymidine. It is used in combination with amphotericin  B in the treatment of disseminated cryptococcal or can¬  didal infections, especially cryptococcal meningitis. It is  not used alone because resistant mutants emerge very  rapidly.   Rifampin is used primarily for the treatment of tubercu¬  losis in combination with other drugs and for prophylaxis  in close contacts of patients with meningitis caused by  either N. meningitidis or H. influenzae. It is also used in  combination with other drugs in the treatment of pros¬  thetic-valve endocarditis caused by S. epidermidis. With  the exception of the short-term prophylaxis of meningitis,  rifampin is given in combination with other drugs because  resistant mutants appear at a high rate when it is used  alone.   The selective mode of action of rifampin is based on  blocking mRNA synthesis by bacterial RNA polymerase  without affecting the RNA polymerase of human cells.  Rifampin is red, and the urine, saliva, and sweat of patients  taking rifampin often turn orange; this is disturbing but  harmless. Rifampin is excreted in high concentration in  saliva, which accounts for its success in the prophylaxis of  bacterial meningitis since the organisms are carried in the  throat.   Rifabutin, a rifampin derivative with the same mode of  action as rifampin, is useful in the prevention of disease  caused by Mycobacterium avium-intracellulare in patients  with severely reduced numbers of helper T cells (e.g.,  acquired immunodeficiency syndrome [AIDS] patients).  Note that rifabutin does not increase cytochrome P-450 as  much as rifampin, so rifabutin is used in HIV/AIDS  patients taking protease inhibitors or non-reverse tran¬  scriptase inhibitors (NRTI).   Fidaxomicin (Dificid) inhibits the RNA polymerase of  C. difficile. It is used in the treatment of pseudomembra¬  nous colitis and in preventing relapses of this disease. It  specifically inhibits C. difficile and does not affect the  gram-negative normal flora of the colon.   There are few antimicrobial compounds that act on the cell  membrane because the structural and chemical similarities  of bacterial and human cell membranes make it difficult to  provide sufficient selective toxicity.   Polymyxins are a family of polypeptide antibiotics of  which the clinically most useful compound is polymyxin E  (colistin). It is active against gram-negative rods, especially  P. aeruginosa, Acinetobacter baumannii, and carbapene-  mase-producing Enterobacteriaceae. Polymyxins are cyclic  peptides composed of 10 amino acids, 6 of which are  diaminobutyric acid. The positively charged free amino  groups act like a cationic detergent to disrupt the phospho¬  lipid structure of the cell membrane.   Daptomycin is a cyclic lipopeptide that disrupts the cell  membranes of gram-positive cocci. It is bactericidal for  organisms such as S. aureus, S. epidermidis, S. pyogenes,  Enterococcus faecalis, and E. faecium, including methicillin-  resistant strains of S. aureus and S. epidermidis, vancomy¬  cin-resistant strains of S. aureus, and vancomycin-resistant  strains of E. faecalis and E. faecium. It is approved for use in  complicated skin and soft tissue infections caused by these  bacteria.   Amphotericin B, the most important antifungal drug, is  used in the treatment of a variety of disseminated fungal  diseases. It is a polyene with a series of seven unsaturated  double bonds in its macrolide ring structure (poly means  many, and -ene is a suffix indicating the presence of double  bonds; Figure 10-11). It disrupts the cell membrane of  fungi because of its affinity for ergosterol, a component of  fungal membranes but not of bacterial or human cell mem¬  branes. Fungi resistant to amphotericin B have rarely been  recovered from patient specimens.   Amphotericin B has significant renal toxicity; measure¬  ment of serum creatinine levels is used to monitor the dose.  Nephrotoxicity is significantly reduced when the drug is  administered in liposomes, but liposomal amphotericin B  is expensive. Fever, chills, nausea, and vomiting are com¬  mon side effects.   Nystatin is another polyene antifungal agent, which,  because of its toxicity, is used topically for infections caused  by the yeast Candida.   Terbinafine blocks ergosterol synthesis by inhibiting  squalene epoxidase. It is used in the treatment of dermato¬  phyte infections of the skin, fingernails, and toenails.   Azoles are antifungal drugs that act by inhibiting ergos¬  terol synthesis. They block cytochrome P-450-dependent  demethylation of lanosterol, the precursor of ergosterol.   Fluconazole, ketoconazole, voriconazole, posaconazole,  and itraconazole are used to treat systemic fungal diseases;  clotrimazole and miconazole are used only topically  because they are too toxic to be given systemically. The two  nitrogen-containing azole rings of fluconazole can be seen  in Figure 10-12.   Ketoconazole is useful in the treatment of blastomyco¬  sis, chronic mucocutaneous candidiasis, coccidioidomyco¬  sis, and skin infections caused by dermatophytes.  Fluconazole is useful in the treatment of candidal and  cryptococcal infections. Itraconazole is used to treat histo¬  plasmosis and blastomycosis. Posaconazole is used for the  treatment of oropharyngeal candidiasis and the prevention  of Candida and Aspergillus infections in immunocompro¬  mised individuals. Miconazole and clotrimazole, two other  imidazoles, are useful for topical therapy of Candida infec¬  tions and dermatophytoses. Fungi resistant to the azole  drugs have rarely been recovered from patient specimens.   Isoniazid, or isonicotinic acid hydrazide (INH), is a bacte¬  ricidal drug highly specific for Mycobacterium tuberculosis  and other mycobacteria. It is used in combination with  other drugs to treat tuberculosis and by itself to prevent  tuberculosis in exposed persons. Because it penetrates    human cells well, it is effective against the organisms resid¬  ing within macrophages. The structure of isoniazid is  shown in Figure 10-13.   INF! inhibits mycolic acid synthesis, which explains  why it is specific for mycobacteria and relatively nontoxic  for humans. The drug inhibits a reductase required for the  synthesis of the long-chain fatty acids called mycolic acids  that are an essential constituent of mycobacterial cell walls.  The active drug is probably a metabolite of INH formed by  the action of catalase peroxidase because deletion of the  gene for these enzymes results in resistance to the drug. Its  main side effect is liver toxicity. It is given with pyridoxine  to prevent neurologic complications.   Metronidazole (Flagyl) is bactericidal against anaerobic  bacteria such as Bacteroides fragilis and Clostridium difficile  and it is used to treat bacterial vaginosis. (It is also effective  against certain protozoa such as Giardia and Trichomonas.)  Metronidazole is a prodrug that is activated to the active  compound within anaerobic bacteria by ferredoxin-medi-  ated reduction of its nitro group.   This drug has two possible mechanisms of action, and it  is unclear which is the more important. The first, which  explains its specificity for anaerobes, is its ability to act as  an electron sink. By accepting electrons, the drug deprives  the organism of required reducing power. In addition,  when electrons are acquired, the drug ring is cleaved and a  toxic intermediate is formed that damages DNA. The pre¬  cise nature of the intermediate and its action is unknown.  The structure of metronidazole is shown in Figure 10-13.   The second mode of action of metronidazole relates to  its ability to inhibit DNA synthesis. The drug binds to DNA    and causes strand breakage, which prevents its proper  functioning as a template for DNA polymerase.   Ethambutol is a bacteriostatic drug used in the treat¬  ment of infections caused by M. tuberculosis and many of  the atypical mycobacteria. It acts by inhibiting the synthesis  of arabinogalactan, which functions as a link between the  mycolic acids and the peptidoglycan of the organism.   Pyrazinamide (PZA) is a bactericidal drug used in the  treatment of tuberculosis but not in the treatment of most  atypical mycobacterial infections. PZA is particularly  effective against semidormant organisms in the lesion,  which are not affected by INH or rifampin. The mecha¬  nism of action of PZA is uncertain but there is evidence  that it acts by inhibiting a fatty acid synthetase that    prevents the synthesis of mycolic acid. It is converted to  the active intermediate, pyrazinoic acid, by an amidase in  the mycobacteria.   Griseofulvin is an antifungal drug that is useful in the  treatment of hair and nail infections caused by dermato¬  phytes. It binds to tubulin in microtubules and may act by  preventing formation of the mitotic spindle.   Pentamidine is active against fungi and protozoa. It is  widely used to prevent or treat pneumonia caused by  Pneumocystis jiroveci. It inhibits DNA synthesis by an  unknown mechanism.   In most instances, the antimicrobial agents described in this  chapter are used for the treatment of infectious diseases.  However, there are times when they are used to prevent dis¬  eases from occurring—a process called chemoprophylaxis.    Chemoprophylaxis is used in three circumstances: prior  to surgery, in immunocompromised patients, and in people  with normal immunity who have been exposed to certain  pathogens. Table 10-8 describes the drugs and the   Penicillin 1. Prevent recurrent pharyngitis in high-risk patients who have had rheumatic fever 15   2. Prevent syphilis in high-risk patients exposed to Treponema pallidum 24   Ampicillin Prevent neonatal sepsis and meningitis in children born of mothers carrying group B 15   Amoxicillin Prevent endocarditis caused by viridans streptococci in high-risk patients with damaged heart 15   Ceftriaxone Prevent gonorrhea in high-risk patients exposed to Neisseria gonorrhoeae 16   Ciprofloxacin 1. Prevent meningitis in high-risk patients exposed to Neisseria meningitidis 16   2. Prevent anthrax in high-risk patients exposed to Bacillus anthracis 17   Rifampin Prevent meningitis in high-risk patients exposed to N. meningitidis and Haemophilus influenzae 16, 19   Isoniazid Prevent progression of Mycobacterium tuberculosis in high-risk patients recently infected who 21   Erythromycin 1. Prevent pertussis in high-risk patients exposed to Bordetellapertussis 19   Clotrimazole Prevent thrush in AIDS patients and in others with reduced cell-mediated immunity 50   Trimethoprim- 1. Prevent Pneumocystis pneumonia and Toxoplasma encephalitis in AIDS patients 52   'Chemoprophylaxis with isoniazid is also viewed as treatment of asymptomatic individuals (see Chapter 21).    situations in which they are used. For more information,  see the chapters on the individual organisms.   Of particular importance is the prevention of endocar¬  ditis in high-risk patients undergoing dental surgery by  using amoxicillin perioperatively. High-risk patients  include those who have a damaged heart valve or a pros¬  thetic heart valve. Prophylaxis to prevent endocarditis in  patients undergoing gastrointestinal or genitourinary tract  surgery is not recommended.    Cefazolin is often used to prevent staphylococcal infec¬  tions in patients undergoing orthopedic surgery, including  prosthetic joint implants, and in vascular graft surgery.  Chemoprophylaxis is unnecessary in those with an implanted  dialysis catheter, a cardiac pacemaker, or a ventriculoperito¬  neal shunt.   In contrast to the chemical antibiotics previously described  in this chapter, probiotics are live, nonpathogenic bacteria  (or yeasts) that may be effective in the treatment or preven¬  tion of certain human diseases. The suggested basis for the  possible beneficial effect lies either in providing coloniza¬  tion resistance by which the nonpathogen excludes the  pathogen from binding sites on the mucosa, in enhancing  the immune response against the pathogen, or in reducing  the inflammatory response against the pathogen.    For example, the oral administration of live Lactobacillus  rhamnosus strain GG significantly reduces the number of  cases of nosocomial diarrhea in young children. Also, the  yeast Saccharomyces boulardii reduces the risk of antibiotic-  associated diarrhea caused by C. difficile. Adverse effects  are few; however, serious complications have arisen in  highly immunosuppressed patients and in patients with  indwelling vascular catheters.    • For an antibiotic to be clinically useful, it must exhibit selective  toxicity (i.e., it must inhibit bacterial processes significantly  more than it inhibits human cell processes).   • There are four main targets of antibacterial drugs: cell wall,  ribosomes, cell membrane, and nucleic acids. Fluman cells  are not affected by these drugs because our cells do not have  a cell wall, and our cells have different ribosomes, nucleic acid  enzymes, and sterols in the membranes.   • Bactericidal drugs kill bacteria, whereas bacteriostatic drugs  inhibit the growth of the bacteria but do not kill. Bacteriostatic  drugs depend on the phagocytes of the patient to kill the  organism. If a patient has too few neutrophils, then bactericidal  drugs should be used.   • Penicillins and cephalosporins act by inhibiting transpepti¬  dases, the enzymes that cross-link peptidoglycan. Transpepti¬  dases are also referred to as penicillin-binding proteins.   Several medically important bacteria (e.g., S. pneumoniae)  manifest resistance to penicillins based on mutations in the  genesencoding penicillin-binding proteins.   • Exposure to penicillins activates autolytic enzymes that  degrade the bacteria. If these autolytic enzymes are not acti¬  vated (e.g., in certain strains of S. aureus), the bacteria are not  killed and the strain is said to be tolerant.   • Penicillins kill bacteria when they are growing (i.e., when they  are synthesizing new peptidoglycan). Penicillins are therefore    more active during the log phase of bacterial growth than  during the lag phase or the stationary phase.   Penicillins and cephalosporins are p-lactam drugs (i.e., an  intact p-lactam ring is required for activity). p~Lactamases  (e.g., penicillinases and cephalosporinases) cleave the P-lactam  ring and inactivate the drug.   Modification of the side chain adjacent to the p-lactam ring  endows these drugs with new properties, such as expanded  activity against gram-negative rods, ability to be taken orally,  and protection against degradation by P-lactamases. For  example, the original penicillin (benzyl penicillin, penicillin G)  cannot be taken orally because stomach acid hydrolyzes the  bond between the P-lactam ring and the side chain. But ampi-  cillin and amoxicillin can be taken orally because they have a  different side chain.   Flypersensitivity to penicillins, especially IgE-mediated ana¬  phylaxis, remains a significant problem.   Cephalosporins are structurally similar to penicillins: both  have a P-lactam ring. The first-generation cephalosporins are  active primarily against gram-positive cocci, and the second,  third, and fourth generations have expanded coverage against  gram-negative rods.   Carbapenems, such as imipenem, and monobactams, such as  aztreonam, are also P-lactam drugs but are structurally differ¬  ent from penicillins and cephalosporins.   Vancomycin is a glycopeptide (i.e., it is not a P-lactam drug),  but its mode of action is very similar to that of penicillins and  cephalosporins (i.e., it inhibits transpeptidases).    • Caspofungin is a lipopeptide that inhibits fungal cell wall  synthesis by blocking the synthesis of P-glucan, a polysac¬  charide component of the cell wall.   • Aminoglycosides and tetracyclines act at the level of the  30S ribosomal subunit, whereas chloramphenicol, erythro¬  mycins, and clindamycin act at the level of the 50S ribo¬  somal subunit.   • Aminoglycosides inhibit bacterial protein synthesis by bind¬  ing to the 30S subunit, which blocks the initiation complex.  No peptide bonds are formed, and no polysomes are made.  Aminoglycosides are a family of drugs that includes gentami¬  cin, tobramycin, and streptomycin.   • Tetracyclines inhibit bacterial protein synthesis by blocking  the binding of aminoacyl tRNA to the 30S ribosomal sub¬  unit. The tetracyclines are a family of drugs; doxycycline is  used most often.   • Chloramphenicol inhibits bacterial protein synthesis by  blocking peptidyl transferase, the enzyme that adds the  new amino acid to the growing polypeptide. Chlorampheni¬  col can cause bone marrow suppression.   • Erythromycin inhibits bacterial protein synthesis by block¬  ing the release of the tRNA after it has delivered its amino  acid to the growing polypeptide. Erythromycin is a member  of the macrolide family of drugs that includes azithromycin  and clarithromycin.   • Clindamycin binds to the same site on the ribosome as does  erythromycin and is thought to act in the same manner. It is  effective against many anaerobic bacteria. Clindamycin is  one of the antibiotics that predisposes to pseudomembra¬  nous colitis caused by C. difficile and is infrequently used.   • Sulfonamides and trimethoprim inhibit nucleotide synthe¬  sis, quinolones inhibit DNA synthesis, and rifampin inhibits   • Sulfonamides and trimethoprim inhibit the synthesis of  tetrahydrofolic acid —the main donor of the methyl groups  that are required to synthesize adenine, guanine, and thymine.  Sulfonamides are structural analogues of p-aminobenzoic  acid, which is a component of folic acid. Trimethoprim inhibits  dihydrofolate reductase —the enzyme that reduces dihydro-  folic acid to tetrahydrofolic acid. A combination of sulfa¬  methoxazole and trimethoprim is often used because bacteria  resistant to one drug will be inhibited by the other.    • Quinolones inhibit DNA synthesis in bacteria by blocking DNA  gyrase (topoisomerase)—the enzyme that unwinds DNA  strands so that they can be replicated. Quinolones are a family of  drugs that includes ciprofloxacin, ofloxacin, and levofloxacin.   • Rifampin inhibits RNA synthesis in bacteria by blocking the  RNA polymerase that synthesizes mRNA. Rifampin is typically  used in combination with other drugs because there is a high  rate of mutation of the RNA polymerase gene, which results  in rapid resistance to the drug.   • Antifungal drugs predominate in this category. These drugs  have selective toxicity because fungal cell membranes con¬  tain ergosterol, whereas human cell membranes have choles¬  terol. Bacteria, with the exception of Mycoplasma, do not  have sterols in their membranes and therefore are resistant to  these drugs.   • Amphotericin B disrupts fungal cell membranes by binding  at the site of ergosterol in the membrane. It is used to treat  the most serious systemic fungal diseases but has significant  side effects, especially on the kidney.   • Azoles are antifungal drugs that inhibit ergosterol synthesis.  The azole family includes drugs such as ketoconazole, flucon¬  azole, itraconazole, and clotrimazole. They are useful in the  treatment of systemic as well as skin and mucous membrane  infections.   • Isoniazid inhibits the synthesis of mycolic acid —a long-  chain fatty acid found in the cell wall of mycobacteria. Isoniazid  is a prodrug that requires a bacterial peroxidase (catalase) to  activate isoniazid to the metabolite that inhibits mycolic acid  synthesis. Isoniazid is the most important drug used in the  treatment of tuberculosis and other mycobacterial diseases.   • Metronidazole is effective against anaerobic bacteria and  certain protozoa because it acts as an electron sink, taking  away the electrons that the organisms need to survive. It also  forms toxic intermediates that damage DNA.   • Antimicrobial drugs are used to prevent infectious diseases  as well as to treat them. Chemoprophylactic drugs are given  primarily in three circumstances: to prevent surgical wound  infections, to prevent opportunistic infections in immunocom¬  promised patients, and to prevent infections in those known to  be exposed to pathogens that cause serious infectious  diseases.   1. Cefazolin is often given prior to surgery to prevent postsurgical  wound infections. Which one of the following best describes the  mode of action of cefazolin?   (A) It acts as an electron sink depriving the bacteria of reducing  power.   (B) It binds to the 30S ribosome and inhibits bacterial protein  synthesis.   (E) It inhibits folic acid synthesis needed to act as a methyl donor.   2. Which one of the following drugs inhibits bacterial nucleic acid  synthesis by blocking the production of tetrahydrofolic acid?   3. Regarding both penicillins and aminoglycosides, which one of  the following is the most accurate?   (D) Both should not be given to children under the age of 8 years  because damage to cartilage can occur.   (E) They should not be given together because they are  antagonistic.   4. Listed below are drug combinations that are used to treat certain  infections. Which one of the following is a combination in which  both drugs act to inhibit the same metabolic pathway?   5. Regarding penicillins and cephalosporins, which one of the fol¬  lowing is the most accurate?   (A) Cleavage of the P-lactam ring will inactivate penicillins but  not cephalosporins.   (B) Penicillins act by inhibiting transpeptidases but cephalospo¬  rins do not.   (D) Penicillins and cephalosporins are active against gram-positive  cocci but not against gram-negative rods.   (E) Renal tubule damage is an important adverse effect caused by  both penicillins and cephalosporins.   6. Regarding antimicrobial drugs that act by inhibiting nucleic acid  synthesis in bacteria, which one of the following is the most accurate?   (A) Ciprofloxacin inhibits RNA polymerase by acting as a nucleic  acid analogue.   (C) Sulfonamides inhibit DNA synthesis by chain termination of  the elongating strand.   (D) Trimethoprim inhibits DNA polymerase by preventing the  unwinding of double-stranded DNA.   7. Regarding aminoglycosides and tetracyclines, which one of the  following is the most accurate?   (B) Both classes of drugs inhibit protein synthesis by binding to  the 30S ribosomal subunit.    (C) Both classes of drugs inhibit peptidyl transferase, the enzyme  that synthesizes the peptide bond.   (D) Both classes of drugs must be acetylated within human cells  to form the active antibacterial compound.   (E) Both classes of drugs cause brown staining of teeth when  administered to young children.   8. The selective toxicity of antifungal drugs, such as amphotericin B  and itraconazole, is based on the presence in fungi of which one  of the following?   9. The next three questions ask about the adverse effects of anti¬  biotics, which are an important consideration when deciding  which antibiotic to prescribe. Which antibiotic causes significant  neurotoxicity and must be taken in conjunction with pyridoxine  (vitamin B g ) to prevent these neurologic complications?   10. Of the following antibiotics, which one causes the most photo¬  toxicity (rash when exposed to sunlight)?   Questions on the topics discussed in this chapter can be found  in the Basic Bacteriology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 709. Also see Part XIV:  USMLE (National Board) Practice Examination starting on  page 751.   Chromosome-Mediated Resistance  Plasmid-Mediated Resistance  Transposon-Mediated Resistance  Specific Mechanisms of Resistance  Nongenetic Basis of Resistance  Selection of Resistant Bacteria by Overuse & Misuse  of Antibiotics   Minimal Inhibitory Concentration  Minimal Bactericidal Concentration  Serum Bactericidal Activity  (3-Lactamase Production   There are four major mechanisms that mediate bacterial  resistance to drugs (Table 11-1). (1) Bacteria produce  enzymes that inactivate the drug (e.g., (3-lactamases can  inactivate penicillins and cephalosporins by cleaving the  (3-lactam ring of the drug). (2) Bacteria synthesize modi¬  fied targets against which the drug has a reduced effect  (e.g., a mutant protein in the 30S ribosomal subunit can  result in resistance to streptomycin, and a methylated 23S  rRNA can result in resistance to erythromycin). (3) Bacteria    reduce permeability to the drug such that an effective  intracellular concentration of the drug is not achieved (e.g.,  changes in porins can reduce the amount of penicillin enter¬  ing the bacterium). (4) Bacteria actively export drugs using  a “multidrug-resistance pump” (MDR pump, or “efflux”  pump). The MDR pump imports protons and, in an  exchange-type reaction, exports a variety of foreign mole¬  cules including certain antibiotics, such as tetracyclines.   Most drug resistance is due to a genetic change in the  organism, either a chromosomal mutation or the acquisition  of a plasmid or transposon. Nongenetic changes, which are  of lesser importance, are discussed on page 91.    The term high-level resistance refers to resistance that  cannot be overcome by increasing the dose of the antibi¬  otic. A different antibiotic, usually from another class of  drugs, is used. Resistance mediated by enzymes such as  (3-lactamases often result in high-level resistance, as all the  drug is destroyed. Low-level resistance refers to resistance  that can be overcome by increasing the dose of the antibi¬  otic. Resistance mediated by mutations in the gene encod¬  ing a drug target is often low level, as the altered target can  still bind some of the drug but with reduced strength.   To illustrate the use of these terms, strains of Neisseria  gonorrhoeae that produce penicillinase cannot be treated  successfully with penicillin G. They exhibit high-level resis¬  tance, and a different drug such as ceftriaxone must be used.  However, strains of N. gonorrhoeae that synthesize altered  penicillin-binding proteins exhibit low-level resistance and  can be treated successfully with high-dose penicillin G.   Hospital-acquired infections are significantly more likely  to be caused by antibiotic-resistant organisms than are  community-acquired infections. This is especially true for  hospital infections caused by Staphylococcus aureus and enteric  gram-negative rods such as Escherichia coli and Pseudomonas  aeruginosa. Antibiotic-resistant organisms are common in the  hospital setting because widespread antibiotic use in hospitals  selects for these organisms. Furthermore, hospital strains are  often resistant to multiple antibiotics. This resistance is usually  due to the acquisition of plasmids carrying several genes that  encode the enzymes that mediate resistance.   Table 11-2 describes certain medically important  bacteria and the main drugs to which they are resistant.   The family Enterobacteriaceae includes bacteria such as Escherichia coli, Enterobac-  ter cloacae, Klebsiella pneumoniae, and Serratia marcescens.   3 Some strains of M. tuberculosis are resistant to more than two drugs.    Note that although these bacteria are resistant to other  drugs as well, for simplicity, only the most characteristic  drugs are listed. Some strains of the bacteria listed in  Table 11-2 are highly resistant to multiple antibiotics,  namely methicillin-resistant S. aureus (MRSA; see  Chapter 15), vancomycin-resistant Enterococcus faecium  (VRE; see Chapter 15), multidrug-resistant Streptococcus  pneumoniae (MDR-SP; see Chapter 15), P. aeruginosa (see  Chapter 18), and multidrug-resistant Mycobacterium  tuberculosis (MDR-MTB; see Chapter 21).   Chromosomal resistance is due to a mutation in the gene  that codes for either the target of the drug or the transport  system in the membrane that controls the uptake of the  drug. The frequency of spontaneous mutations usually  ranges from 10~ 7 to 1(T 9 , which is much lower than the  frequency of acquisition of resistance plasmids. Therefore,  chromosomal resistance is less of a clinical problem than is  plasmid-mediated resistance.   The treatment of certain infections with two or more  drugs is based on the following principle. If the frequency  that a bacterium mutates to become resistant to antibiotic  A is 1(T 7 (1 in 10 million) and the frequency that the same  bacterium mutates to become resistant to antibiotic B  is 10~ 8 (1 in 100 million), then the chance that the bacte¬  rium will become resistant to both antibiotics (assuming  that the antibiotics act by different mechanisms) is the  product of the two probabilities, or 1(T 15 . It is therefore  highly unlikely that the bacterium will become resistant to  both antibiotics. Stated another way, although an organism  may be resistant to one antibiotic, it is likely that it will be  effectively treated by the other antibiotic.   Plasmid-mediated resistance is very important from a  clinical point of view for three reasons:   (1) It occurs in many different species, especially gram¬  negative rods.   (3) Plasmids have a high rate of transfer from one cell to  another, usually by conjugation.   extrachromosomal, circular, double-stranded DNA mole¬  cules that carry the genes for a variety of enzymes that can  degrade antibiotics and modify membrane transport sys¬  tems (Figure 11-1). Table 11-3 describes the most impor¬  tant mechanisms of resistance for several important drugs.   R factors may carry one antibiotic resistance gene or may  carry two or more of these genes. The medical implications   FIGURE 11-1 Resistance plasmid (R plasmid, R factor). Most  resistance plasmids have two sets of genes: (1) resistance transfer  genes that encode the sex pilus and other proteins that mediate  transfer of the plasmid DNA during conjugation, and (2) drug resis¬  tance genes that encode the proteins that mediate drug resistance.  The bottom half of the figure depicts (from left to right) the genes  that encode resistance to tetracycline, streptomycin, penicillin  (|3-lactamase), chloramphenicol, erythromycin, and gentamicin.    of a plasmid carrying more than one resistance gene is  twofold: first and most obvious is that a bacterium contain¬  ing that plasmid can be resistant to more than one class of  antibiotics (e.g., penicillins and aminoglycosides) and sec¬  ond, that the use of an antibiotic that selects for an organ¬  ism resistant to one antibiotic will select for an organism  that is resistant to all the antibiotics whose resistance genes  are carried by the plasmid. For example, if an organism has  the R plasmid depicted in Figure 11-1, then the use of  penicillin will select for an organism resistant not only to  penicillin, but also to tetracyclines, aminoglycosides (e.g.,  streptomycin and gentamicin), chloramphenicol, and  erythromycin.   In addition to producing drug resistance, R factors have  two very important properties: (1) they can replicate   Active export out of the cell and reduced  affinity of enzyme    independently of the bacterial chromosome; therefore, a  cell can contain many copies; and (2) they can be trans¬  ferred not only to cells of the same species, but also to other  species and genera. Note that this conjugal transfer is under  the control of the genes of the R plasmid and not of the  F (fertility) plasmid, which governs the transfer of the bac¬  terial chromosome (see Chapter 4).   R factors exist in two broad size categories: large plasmids,  with molecular weights of about 60 million, and small ones,  with molecular weights of about 10 million. The large plas¬  mids are conjugative R factors, which contain the extra DNA  to code for the conjugation process. The small R factors are  not conjugative and contain only the resistance genes.   In addition to conveying antibiotic resistance, R factors  impart two other traits: (1) resistance to metal ions (e.g.,  they code for an enzyme that reduces mercuric ions to  elemental mercury) and (2) resistance to certain bacterial  viruses by coding for restriction endonucleases that degrade  the DNA of the infecting bacteriophages.   Transposons are genes that are transferred either within or  between larger pieces of DNA such as the bacterial chro¬  mosome and plasmids. A typical drug resistance transpo-  son is composed of three genes flanked on both sides by  shorter DNA sequences, usually a series of inverted  repeated bases that mediate the interaction of the transpo-  son with the larger DNA (see Figure 2-7). The three genes  code for (1) transposase, the enzyme that catalyzes excision  and reintegration of the transposon; (2) a repressor that  regulates synthesis of the transposase; and (3) the drug  resistance gene.   Penicillins & Cephalosporins—There are several mecha¬  nisms of resistance to these drugs. Cleavage by P-lactamases  (penicillinases and cephalosporinases) is by far the most  important (see Figure 10-2). (3-Lactamases produced by  various organisms have different properties. For example,  staphylococcal penicillinase is inducible by penicillin and is  secreted into the medium. In contrast, some P-lactamases  produced by several gram-negative rods are constitutively  produced, are located in the periplasmic space near the  peptidoglycan, and are not secreted into the medium. The  P-lactamases produced by various gram-negative rods have  different specificities: some are more active against cepha¬  losporins, others against penicillins. Clavulanic acid, tazo-  bactam, sulbactam, and avibactam are penicillin analogues  that bind strongly to P-lactamases and inactivate them.  Combinations of these inhibitors and penicillins (e.g., cla¬  vulanic acid and amoxicillin [Augmenting can overcome  resistance mediated by many but not all p-lactamases.    Extended-spectrum (3-lactamases (ESBLs) are pro¬  duced by several enteric bacteria, notably E. coli, Klebsiella,  Enterobacter, and Proteus. ESBLs endow the bacteria with  resistance to all penicillins, cephalosporins, and mono-  bactams. However, these bacteria remain sensitive to com¬  binations such as piperacillin/tazobactam. In 2009, a new  strain of highly resistant Klebsiella was isolated in India  carrying a plasmid that encoded New Delhi metallo-(3-  lactamase (NDM-1). This plasmid confers high-level resis¬  tance to many antibiotics and has spread from Klebsiella to  other member of the Enterobacteriaceae. Resistant Entero-  bacteriaceae carrying NDM-1 have emerged in many coun¬  tries, including the United States.   Resistance to penicillins can also be due to changes in  the penicillin-binding proteins (PBPs) in the bacterial cell  membrane. These changes account for both the low-level  and high-level resistance exhibited by S. pneumoniae to  penicillin G and for the resistance of S. aureus to nafcillin  and other (3-lactamase-resistant penicillins. The resistance  of MRSA to almost all (3-lactams is attributed to the pres¬  ence of PBP2a, which is found particularly in MRSA. The  relative resistance of Enterococcus faecalis to penicillins may  be due to altered penicillin-binding proteins. Low-level  resistance of N. gonorrhoeae to penicillin is attributed to  poor permeability to the drug. High-level resistance is due  to the presence of a plasmid coding for penicillinase.   Some isolates of S. aureus demonstrate yet another form  of resistance, called tolerance, in which growth of the  organism is inhibited by penicillin but the organism is not  killed. This is attributed to a failure of activation of the  autolytic enzymes, murein hydrolases, which degrade the  peptidoglycan.   Carbapenems —Resistance to carbapenems, such as  imipenem, is caused by carbapenemases that degrade the  (3-lactam ring. This enzyme endows the organism with  resistance to penicillins and cephalosporins as well.  Carbapenemases are produced by many enteric gram-negative  rods, especially Klebsiella, Escherichia, and Pseudomonas.  Carbapenem-resistant strains of Klebsiella pneumoniae are  an important cause of hospital-acquired infections and are  resistant to almost all known antibiotics.   Vancomycin— Resistance to vancomycin is caused by a  change in the peptide component of peptidoglycan from  D-alanyl-D-alanine, which is the normal binding site for  vancomycin, to D-alanine- D-lactate, to which the drug  does not bind. Of the four gene loci mediating vancomycin  resistance, VanA is the most important. It is carried by a  transposon on a plasmid and provides high-level resistance  to both vancomycin and teicoplanin. (Teicoplanin is used  in Europe but is not approved in the United States.) The  VanA locus encodes those enzymes that synthesize D-ala-  nine-D-lactate as well as several regulatory proteins.   Vancomycin-resistant strains of enterococci (VRE) have  been recovered from clinical specimens. Rare isolates of  S. aureus that exhibit resistance to vancomycin have also    been recovered from patient specimens. Rare isolates of  S. pneumoniae that exhibit tolerance to vancomycin have  been recovered as well.   Aminoglycosides —Resistance to aminoglycosides  occurs by three mechanisms: (1) modification of the drugs  by plasmid-encoded phosphorylating, adenylylating, and  acetylating enzymes (the most important mechanism);  (2) chromosomal mutation (e.g., a mutation in the gene  that codes for the target protein in the 30S subunit of the  bacterial ribosome); and (3) decreased permeability of  the bacterium to the drug.   Tetracyclines —Resistance to tetracyclines is the result  of failure of the drug to reach an inhibitory concentration  inside the bacteria. This is due to plasmid-encoded pro¬  cesses that either reduce the uptake of the drug or enhance  its transport out of the cell.   Chloramphenicol —Resistance to chloramphenicol is  due to a plasmid-encoded acetyltransferase that acetylates  the drug, thus inactivating it.   Erythromycin —Resistance to erythromycin is due pri¬  marily to a plasmid-encoded enzyme that methylates the  23S rRNA, thereby blocking binding of the drug. An efflux  pump that reduces the concentration of erythromycin  within the bacterium causes low-level resistance to the  drug. An esterase produced primarily by enteric gram¬  negative rods cleaves the macrolide ring, which inactivates  the drug.   Sulfonamides —Resistance to sulfonamides is mediated  primarily by two mechanisms: (1) a plasmid-encoded  transport system that actively exports the drug out of the  cell, and (2) a chromosomal mutation in the gene coding  for the target enzyme dihydropteroate synthetase, which  reduces the binding affinity of the drug.   Trimethoprim —Resistance to trimethoprim is due pri¬  marily to mutations in the chromosomal gene that encodes  dihydrofolate reductase, the enzyme that reduces dihydro¬  folate to tetrahydrofolate.   Quinolones —Resistance to quinolones is due primarily  to chromosomal mutations that modify the bacterial DNA  gyrase.   Rifampin —Resistance to rifampin is due to a chromo¬  somal mutation in the gene encoding the bacterial RNA  polymerase, resulting in ineffective binding of the drug.  Because resistance occurs at high frequency (10~ 5 ),  rifampin is not prescribed alone for the treatment of infec¬  tions. It is used alone for the prevention of certain infec¬  tions because it is administered for only a short time (see  Table 10-8).   Isoniazid —Resistance of M. tuberculosis to isoniazid is  due to mutations in the organism’s catalase-peroxidase  gene. Catalase or peroxidase enzyme activity is required to  synthesize the metabolite of isoniazid that actually inhibits  the growth of M. tuberculosis.   Ethambutol —Resistance of M. tuberculosis to ethambu-  tol is due to mutations in the gene that encodes arabinosyl    transferase, the enzyme that synthesizes the arabinogalac-  tan in the organism’s cell wall.   Pyrazinamide—Resistance of M. tuberculosis to pyra-  zinamide (PZA) is due to mutations in the gene that  encodes bacterial amidase, the enzyme that converts PZA  to the active form of the drug, pyrazinoic acid.   There are several nongenetic reasons for the failure of  drugs to inhibit the growth of bacteria:   (1) Bacteria can be walled off within an abscess cavity  that the drug cannot penetrate effectively. Surgical drainage  is therefore a necessary adjunct to chemotherapy.   (2) Bacteria can be in a resting state (i.e., not growing);  they are therefore insensitive to cell wall inhibitors such as  penicillins and cephalosporins. Similarly, M. tuberculosis  can remain dormant in tissues for many years, during  which time it is insensitive to drugs. If host defenses are  lowered and the bacteria begin to multiply, they are again  susceptible to the drugs, indicating that a genetic change  did not occur.   (3) Under certain circumstances, organisms that would  ordinarily be killed by penicillin can lose their cell walls,  survive as protoplasts, and be insensitive to cell wall-active  drugs. Later, if such organisms resynthesize their cell walls,  they are fully susceptible to these drugs.   (4) The presence of foreign bodies makes successful  antibiotic treatment more difficult. This applies to foreign  bodies such as surgical implants and catheters as well as  materials that enter the body at the time of penetrating  injuries, such as splinters and shrapnel.   (5) Several artifacts can make it appear that the organ¬  isms are resistant (e.g., administration of the wrong drug  or the wrong dose or failure of the drug to reach the appro¬  priate site in the body). (A good example of the latter is the  poor penetration into spinal fluid by several early-genera-  tion cephalosporins.) Failure of the patient to take the drug  (noncompliance, nonadherence) is another artifact.   Serious outbreaks of diseases caused by gram-negative rods  resistant to multiple antibiotics have occurred in many  developing countries. In North America, many hospital-  acquired infections are caused by multidrug-resistant  organisms. Three main points of overuse and misuse of  antibiotics increase the likelihood of these problems by  enhancing the selection of resistant mutants:    (1) Some physicians use multiple antibiotics when one  would be sufficient, prescribe unnecessarily long courses of  antibiotic therapy, use antibiotics in self-limited infections  for which they are not needed, and overuse antibiotics for  prophylaxis before and after surgery.   (2) In many countries, antibiotics are sold over the  counter to the general public; this practice encourages  inappropriate and indiscriminate use of the drugs.   (3) Antibiotics are used in animal feed to prevent infec¬  tions and promote growth. This selects for resistant organ¬  isms in the animals and may contribute to the pool of  resistant organisms in humans.   An antibiogram is the term used to describe the results of  antibiotic susceptibility tests performed on the bacteria  isolated from the patient. These results are the most impor¬  tant factor in determining the choice of antibiotic with  which to treat the patient. Other factors such as the  patient’s renal function and hypersensitivity profile must  also be considered in choosing the antibiotic.   There are two types of tests used to determine the anti¬  biogram: (1) the tube dilution test that determines the  minimal inhibitory concentration and (2) the disk diffusion  (Kirby-Bauer) test that determines the diameter of the zone  of inhibition (see following discussion and Figures 11-2  and 11-3).   For many infections, the results of sensitivity testing are  important in the choice of antibiotic. These results are  commonly reported as the minimal inhibitory concentra¬  tion (MIC), which is defined as the lowest concentration of  drug that inhibits the growth of the organism. The MIC is  determined by inoculating the organism isolated from the  patient into a series of tubes or cups containing twofold  dilutions of the drug (Figure 11-2). After incubation at  35°C for 18 hours, the lowest concentration of drug that  prevents visible growth of the organism is the MIC. This  provides the physician with a precise concentration of drug  to guide the choice of both the drug and the dose.   A second method of determining antibiotic sensitivity is  the disk diffusion method, in which disks impregnated  with various antibiotics are placed on the surface of an agar  plate that has been inoculated with the organism isolated  from the patient (Figure 11-3). After incubation at 35°C  for 18 hours, during which time the antibiotic diffuses out¬  ward from the disk, the diameter of the zone of inhibition  is determined. The size of the zone of inhibition is com¬  pared with standards to determine the sensitivity of the  organism to the drug.   FIGURE 11-2 Determination of minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC). Top: The  patient's organism is added to tubes containing decreasing amounts of the antibiotic. After incubation at 37°C overnight, growth of the  bacteria is observed visually. The lowest concentration of drug that inhibits growth (i.e., 3.1 pg/mL) is the MIC. However, at this point, it  is not known whether the bacteria have been killed or whether the drug has only inhibited their growth. Bottom: To determine  whether that concentration of drug is bactericidal (i.e., to determine its MBC), an aliquot (0.1 mL) from the tubes is plated on an agar  plate that does not contain any drug. The concentration of drug that inhibits at least 99.9% of the bacterial colonies (i.e., 6.2 pg/mL) is  the MBC.   FIGURE 11-3 Antibiotic sensitivity testing. A zone of inhibi¬  tion surrounds several antibiotic-containing disks. A zone of certain  diameter or greater indicates that the organism is sensitive. Some  resistant organisms will grow all the way up to the disk (e.g., disk N).  (Wistreich GA. Laboratory Exercises in Microbiology. 5th edition © 1984. Reprinted  by permission of Pearson Education Inc, New York, New York.)   For certain infections, such as endocarditis, it is important to  know the concentration of drug that actually kills the organ¬  ism rather than the concentration that merely inhibits growth.  This concentration, called the minimal bactericidal concen¬  tration (MBC), is determined by taking a small sample  (0.01 or 0.1 mL) from the tubes used for the MIC assay and  spreading it over the surface of a drug-free blood agar plate  (Figure 11-2). Any organisms that were inhibited but not  killed now have a chance to grow because the drug has been  diluted significantly After incubation at 35°C for 48 hours,  the lowest concentration that has reduced the number of  colonies by 99.9%, compared with the drug-free control, is  the MBC. Bactericidal drugs usually have an MBC equal or  very similar to the MIC, whereas bacteriostatic drugs usually  have an MBC significantly higher than the MIC.   In the treatment of endocarditis, it can be useful to deter¬  mine whether the drug is effective by assaying the ability of  the drug in the patients serum to kill the organism. This test,  called the serum bactericidal activity, is performed in a  manner similar to that of the MBC determination, except  that it is a serum sample from the patient, rather than a stan¬  dard drug solution, that is used. After a standard inoculum  of the organism has been added and the mixture has been  incubated at 35°C for 18 hours, a small sample is subcultured  onto blood agar plates, and the serum dilution that kills  99.9% of the organisms is determined. Clinical experience   FIGURE 11-4 Drug interaction. The solid lines represent the response of bacteria to drug A alone, drug B alone, or no drug. The dotted  lines represent the response to drug A and drug B together.    has shown that a peak 1 serum bactericidal activity of 1:8 or  1:16 is adequate for successful therapy of endocarditis.   For severe infections caused by certain organisms, such as  S. aureus and Haemophilus influenzae, it is important to  know as soon as possible whether the organism isolated  from the patient is producing (3-lactamase. For this pur¬  pose, rapid assays for the enzyme can be used that yield an  answer in a few minutes, as opposed to an MIC test or a  disk diffusion test, both of which take 18 hours.   A commonly used procedure is the chromogenic  (3-lactam method, in which a colored (3-lactam drug is  added to a suspension of the organisms. If (3-lactamase is  made, hydrolysis of the (3-lactam ring causes the drug to  turn a different color in 2 to 10 minutes. Disks impregnated  with a chromogenic (3-lactam can also be used.   In most cases, the single best antimicrobial agent should be  selected for use because this minimizes side effects.  However, there are several instances in which two or more  drugs are commonly given:   (1) To treat serious infections before the identity of the  organism is known.    1 One variable in this test is whether the serum is drawn shortly after the  drug has been administered (at the “peak concentration”) or shortly before  the next dose is due (at the “trough”). Another variable is the inoculum size.    (3) To prevent the emergence of resistant organisms. (If  bacteria become resistant to one drug, the second drug will  kill them, thereby preventing the emergence of resistant  strains.)   Two drugs can interact in one of several ways  (Figure 11-4). They are usually indifferent to each other  (i.e., additive only). Sometimes there is a synergistic inter¬  action, in which the effect of the two drugs together is sig¬  nificantly greater than the sum of the effects of the two  drugs acting separately. Rarely, the effect of the two drugs  together is antagonistic, in which the result is significantly  lower activity than the sum of the activities of the two drugs  alone.   A synergistic effect can result from a variety of mecha¬  nisms. For example, the combination of a penicillin and an  aminoglycoside such as gentamicin has a synergistic action  against enterococci ( E. faecalis), because penicillin dam¬  ages the cell wall sufficiently to enhance the entry of ami¬  noglycoside. When given alone, neither drug is effective. A  second example is the combination of a sulfonamide with  trimethoprim. In this instance, the two drugs act on the  same metabolic pathway, such that if one drug does not  inhibit folic acid synthesis sufficiently, the second drug  provides effective inhibition by blocking a subsequent step  in the pathway.   Although antagonism between two antibiotics is  unusual, one example is clinically important. This involves  the use of penicillin G combined with the bacteriostatic  drug tetracycline in the treatment of meningitis caused by  S. pneumoniae. Antagonism occurs because the tetracycline  inhibits the growth of the organism, thereby preventing the  bactericidal effect of penicillin G, which kills only growing  organisms.    • The four main mechanisms of antibiotic resistance are (1) enzy¬  matic degradation of the drug, (2) modification of the drug's  target, (3) reduced permeability of the drug, and (4) active  export of the drug.   • Most drug resistance is the result of a genetic change in the  organism, caused either by a chromosomal mutation or the  acquisition of a plasmid or transposon.   • Chromosomal mutations typically either change the target  of the drug so that the drug does not bind or change the  membrane so that the drug does not penetrate well into the  cell. Chromosomal mutations occur at a low frequency (per¬  haps 1 in 10 million organisms) and often affect only one drug  or one family of drugs.   • Plasmids cause drug resistance by encoding enzymes that  degrade or modify drugs. Plasmid-mediated resistance occurs  at a higher frequency than chromosomal mutations, often  affecting multiple drugs or families of drugs.   • Resistance plasmids (R plasmids, R factors) usually carry two  sets of genes. One set encodes the enzymes that degrade or  modify drugs, and the other encodes the proteins that medi¬  ate conjugation, the main process by which resistance genes  are transferred from one bacterium to another.   • Transposons are small pieces of DN A that move from one site  on the bacterial chromosome to another or from the bacte¬  rial chromosome to plasmid DNA. Transposons often carry  drug resistance genes. Many R plasmids carry one or more  transposons.   • Resistance to penicillins and cephalosporins is mediated by  three main mechanisms: (1) degradation by (3-lactamases,   • Resistance to vancomycin is caused by a change in the D-alanyl-  D-alanine part of the peptide in peptidoglycan to D-alanine-D  lactate, resulting in an inability of vancomycin to bind.   • Resistance to aminoglycosides is mediated by three main  mechanisms: (1) modification of the drug by phosphorylat-  ing, adenylylating, and acetylating enzymes; (2) mutations  in the genes encoding one of the 30S ribosomal proteins; and  (3) reduced permeability.   • Resistance to tetracyclines is often caused by either reduced  permeability or active export of the drug from the bacterium.    • Resistance to erythromycins is primarily caused by a plasmid-  encoded enzyme that methylates the 23S ribosomal RNA,   • Resistance to sulfonamides is due primarily to plasmid-  encoded enzymes that actively export the drug from the  bacterium.   • Resistance to quinolones is primarily caused by mutations in  the gene encoding the bacterial DNA gyrase.   • Resistance to rifampin is primarily caused by mutations in the  gene encoding the bacterial RNA polymerase.   • Resistance to isoniazid is due primarily to the loss of the  bacterial peroxidase (catalase) that activates isoniazid to the  metabolite that inhibits mycolic acid synthesis.   • Nongenetic reasons why bacteria may not be inhibited by anti¬  biotics are that drugs may not reach bacteria located in the  center of an abscess and that certain drugs, such as penicillins,  will not affect bacteria that are not growing. Also, the presence  of foreign bodies makes successful antibiotic treatment more  difficult.   • The minimal inhibitory concentration (MIC) is the lowest  concentration of drug that inhibits the growth of the bacteria  isolated from the patient. In this test, it is not known whether  the inhibited bacteria have been killed or just have stopped  growing.   • The minimal bactericidal concentration (MBC) is the lowest  concentration of drug that kills the bacteria isolated from the  patient. In certain diseases, such as endocarditis, it is often  necessary to use a concentration of drug that is bactericidal.   • Two or more antibiotics are used under certain circumstances,  such as to treat life-threatening infections before the cause has  been identified, to prevent the emergence of resistant bacteria  during prolonged treatment regimens, and to achieve a syner¬  gistic (augmented) effect.   • A synergistic effect is one in which the effect of two drugs  given together is much greater than the sum of the effect of  the two drugs given individually. The best example of synergy  is the marked killing effect of the combination of a penicillin  and an aminoglycoside on enterococci compared with the  minor effect of either drug given alone.   1. The spread of antibiotic resistance from one bacterium to  another is a well-recognized and clinically important phe¬  nomenon. Which one of the following mechanisms is most  likely to be involved with the spread of resistance?   2. Regarding the specific mechanisms by which bacteria  become resistant to antimicrobial drugs, which one of the  following is the most accurate?   (A) Some bacteria contain an enzyme that cleaves the ring of  aminoglycosides.   (B) Some bacteria contain clavulanic acid, which binds to  penicillin G and inactivates it.   (C) Some bacteria contain a mutated gene encoding an  altered transpeptidase, which makes it resistant to  doxycycline.   (D) Some bacteria contain a mutated gene that encodes an  altered RNA polymerase, which makes it resistant to  rifampin.   (E) Some bacteria contain an altered ribosomal protein,  which makes it resistant to isoniazid.   3. The susceptibility of bacteria to an antibiotic is often deter¬  mined by using the minimal inhibitory concentration (MIC)  assay. Regarding the MIC assay, which one of the following is  the most accurate?   (A) MIC is the lowest concentration of the bacteria isolated  from the patient that inhibits the activity of a standard  dose of antibiotic.   (B) MIC is the lowest concentration of antibiotic that inhibits  the growth of the bacteria isolated from the patient.   (C) MIC is the lowest concentration of antibiotic that kills the  bacteria isolated from the patient.   (D) MIC is the lowest concentration of antibiotic in the  patient’s serum that inhibits the activity of a standard dose  of antibiotic.    4. The minimal inhibitory concentration (MIC) of the patients  organism to penicillin is 1 pg/mL and the MIC to gentamicin  is 8 pg/mL. However, the MIC to a combination of penicillin  and gentamicin is 0.01 pg/mL. Which one of the following  terms is the most accurate to describe this effect?   5. Regarding the mechanisms of resistance to specific drugs,  which one of the following is most accurate?   (A) Certain strains of Enterococcus faecalis produce D-lactate  rather than D-alanine, which causes them to be resistant  to vancomycin.   (B) Certain strains of Escherichia coli produce ergosterol,  which causes them to be resistant to gentamicin.   (C) Certain strains of Neisseria gonorrhoeae produce a mutant  peptidyl transferase, which causes them to be resistant to  tetracycline.   (D) Certain strains of Streptococcus pyogenes produce a |3-lactamase,  which causes them to be resistant to erythromycin.   Questions on the topics discussed in this chapter can be  found in the Basic Bacteriology section of Part XIII:  USMLE (National Board) Practice Questions starting on  page 709. Also see Part XIV: USMLE (National Board)  Practice Examination starting on page 751.   Bacterial diseases can be prevented by using immuniza¬  tions that induce either active or passive immunity. Active  immunity is induced by vaccines prepared from bacteria or  their products. This chapter presents a summary of the  types of vaccines (Table 12-1); detailed information regard¬  ing each vaccine is located in the chapters on the specific  organisms. Passive immunity is provided by the adminis¬  tration of preformed antibody in preparations called  immune globulins. The immune globulins useful against  bacterial diseases are described later. Passive-active immu¬  nity involves giving both immune globulins to provide    immediate protection and a vaccine to provide long-term  protection. This approach is described later in the section  on tetanus antitoxin.   Bacterial vaccines are composed of capsular polysaccha¬  rides, inactivated protein exotoxins (toxoids), killed bacte¬  ria, or live, attenuated bacteria. The available bacterial  vaccines and their indications are described next.  Table 12-2 lists the bacterial (and viral) vaccines recom¬  mended for children from 0 to 6 years of age as of 2011.  Advice regarding vaccines for travelers can be found at the  Web site for the Centers for Disease Control and Preven¬  tion: www.cdc.gov/travel.    vaccines are listed in alphabetical order. A complete description of the vaccine  schedule is available on the Centers for Disease Control and Prevention Web site,  www.cdc.gov.   (1) Both versions of the vaccine against Streptococcus  pneumoniae contain the capsular polysaccharide of the bac¬  teria as the immunogen. One version contains the capsular  polysaccharide of the 23 most prevalent serotypes. It is rec¬  ommended for persons older than 60 years of age and adult  patients of any age with such chronic diseases as diabetes and  cirrhosis or with compromised spleen function or splenec¬  tomy. A second version containing the capsular polysaccha¬  ride of 13 pneumococcal serotypes coupled to a carrier  protein (diphtheria toxoid) is available for the protection of  young children who do not respond well to the unconjugated  vaccine. The function of the carrier protein is explained in  Figure 57-5.   A potential problem regarding the use of the pneumococ¬  cal vaccine (or a vaccine against any organism with multiple  serotypes) is that of serotype replacement. Will the vaccine  reduce the incidence of disease caused by the serotypes in  the vaccine but not the overall incidence of disease because  other serotypes that are not in the vaccine will now cause  disease? In fact, this occurred. An increase in invasive pneu¬  mococcal disease caused by serotype 19A, a serotype not in  the previous vaccine, was observed. In view of this, serotype  19A is included in the current 13 serotype vaccine.   (2) Neisseria meningitidis vaccine contains capsular  polysaccharide of four important types (A, C, W-135,  and Y). Two forms of the vaccine are available: one contains  the polysaccharides conjugated to a carrier protein (diphthe¬  ria toxoid), and the other contains only the polysaccharides.  It is given when there is a high risk of meningitis (e.g., during  an outbreak, when students enter college and are living in a  dormitory, when military recruits enter boot camp, or for  travelers to areas where meningitis is hyperendemic).   (3) Haemophilus influenzae vaccine contains the type  b polysaccharide conjugated to diphtheria toxoid or other  carrier protein. It is given to children between the ages of  2 and 15 months to prevent meningitis. The capsular  polysaccharide alone is a poor immunogen in young chil¬  dren, but coupling it to a carrier protein greatly enhances    its immunogenicity. A combined vaccine consisting of this  vaccine plus the diphtheria, tetanus, and pertussis (DTP)  vaccines is available.   (4) One of the vaccines against typhoid fever contains  the capsular polysaccharide of Salmonella typhi. It is indi¬  cated for persons living or traveling in areas where there is  a high risk of typhoid fever and for persons in close contact  with either infected patients or chronic carriers.   (1) Corynebacterium diphtheriae vaccine contains the  toxoid (formaldehyde-treated exotoxin). Immunization  against diphtheria is indicated for every child and is given  in three doses at 2, 4, and 6 months of age, with boosters  given 1 year later and at intervals thereafter.   (2) Clostridium tetani vaccine contains tetanus toxoid  and is given to everyone both early in life and later as  boosters for protection against tetanus.   (3) Bordetella pertussis vaccine contains pertussis toxoid  but includes other proteins as well. It is, therefore,  described in the next section.   (1) There are two types of B. pertussis vaccines: an acellu¬  lar vaccine containing purified proteins and a vaccine con¬  taining whole killed bacteria. The acellular vaccine is now  recommended in the United States. The principal antigen in  the acellular vaccine is inactivated pertussis toxin (pertussis  toxoid), but other proteins, such as filamentous hemaggluti¬  nin and pertactin, are also required for full protection.  Pertussis toxin for the vaccine is inactivated genetically by  introducing two amino acid changes that eliminate its toxic  (ADP-ribosylating) activity but retain its antigenicity. It is the  first vaccine to contain a genetically inactivated toxoid. The  vaccine is indicated for every child as a protection against  whooping cough. It is usually given in combination with  diphtheria and tetanus toxoids (DTP or DTaP vaccine).   (2) Bacillus anthracis vaccine contains “protective antigen”  purified from the organism. It is given to persons whose  occupations place them at risk of exposure to the organism.   (1) The vaccine against tuberculosis contains a live,  attenuated strain of Mycobacterium bovis called BCG and,  in some countries, is recommended for children at high  risk for exposure to active tuberculosis.   (2) One of the vaccines against typhoid fever contains  live, attenuated S. typhi. It is indicated for persons living or  traveling in areas where there is a high risk of typhoid fever  and for persons in close contact with either infected  patients or chronic carriers.   (3) The vaccine against tularemia contains live, attenu¬  ated Francisella tularensis organisms and is used primarily  in people who are exposed in their occupation, such as  laboratory personnel, veterinarians, and hunters.   (1) Vibrio cholerae vaccine contains killed organisms  and is given to persons traveling to areas where cholera is  endemic.   (2) Yersinia pestis vaccine contains killed organisms and  is indicated for persons at high risk for contracting plague.   (3) The vaccine against typhus contains killed Rickettsia  rickettsiae organisms and is used primarily to immunize  members of the armed forces.   The vaccine against Q fever contains killed Coxiella burnetii  organisms and is used to immunize those who are at high risk  for being exposed to animals infected with the organism.   Antitoxins (immune globulins) can be used for either the  treatment or prevention of certain bacterial diseases. The  following preparations are available:   (1) Tetanus antitoxin is used in the treatment of tetanus  and in its prevention (prophylaxis). In treatment, because    the goal is to neutralize any unbound toxin to prevent the  disease from getting worse, the antitoxin should be given  promptly. In prevention, the antitoxin is given to inade¬  quately immunized persons with contaminated (“dirty”)  wounds. The antitoxin is made in humans to avoid hyper¬  sensitivity reactions. In addition to the antitoxin, these  people should receive tetanus toxoid. This is an example of  passive-active immunity. The toxoid and the antitoxin  should be given at different sites in the body to prevent the  antitoxin from neutralizing the toxoid.   (2) Botulinum antitoxin is used in the treatment of botu¬  lism. Because the antitoxin can neutralize unbound toxin to  prevent the disease from progressing, it should be given  promptly. It contains antibodies against botulinum toxins A,  B, and E, the most commonly occurring types. The antitoxin  is made in horses, so hypersensitivity may be a problem.   (3) Diphtheria antitoxin is used in the treatment of  diphtheria. The antitoxin can neutralize unbound toxin to  prevent the disease from progressing; therefore, the anti¬  toxin should be given promptly. The antitoxin is made in  horses, so hypersensitivity may be a problem.    • Immunity to certain bacterial diseases can be induced either by  immunization with bacterial antigens (active immunity) or by  administration of preformed antibodies (passive immunity).   (either killed or live, attenuated) or (2) purified proteins isolated  from bacteria.   • Vaccines containing capsular polysaccharide as the immu¬  nogen are directed against S. pneumoniae, H. influenzae, N.  meningitidis, and 5. typhi. The capsular polysaccharide in the  pneumococcal vaccine, the meningococcal vaccine, and the H.  influenzae vaccine is conjugated to a carrier protein to enhance  the antibody response.   • Two vaccines contain toxoids as the immunogen, the vaccines  against diphtheria and tetanus. A toxoid is an inactivated  toxin that has lost its ability to cause disease but has retained  its immunogenicity. (The pertussis vaccine also contains toxoid  but contains other bacterial proteins as well.)   • Two vaccines contain purified bacterial proteins as the immuno¬  gen. The most commonly used is the acellular pertussis vac¬  cine, which in combination with diphtheria and tetanus toxoids  is recommended for all children. The vaccine against anthrax  also contains purified proteins but is recommended only for  individuals who are likely to be exposed to the organism.    • The BCG vaccine against tuberculosis contains live, attenu¬  ated M. bovis and is used in countries where the disease is  endemic. One of the vaccines against typhoid fever contains  live, attenuated S. typhi.   • The vaccines against cholera, plague, typhus, and Q fever con¬  tain whole killed bacteria. These vaccines are used only to  protect those likely to be exposed.   • Passive immunity in the form of antitoxins is available for the  prevention and treatment of tetanus, botulism, and diphthe¬  ria. These three diseases are caused by exotoxins. Antitoxins  (antibodies against the exotoxins) bind to exotoxins and pre¬  vent their toxic effects (i.e., they neutralize the toxins).   • This involves providing both immediate (but short-term)  protection in the form of antibodies and long-term protec¬  tion in the form of active immunization. An excellent exam¬  ple of the use of passive-active immunity is the prevention  of tetanus in an unimmunized person who has sustained a  contaminated wound. Both tetanus antitoxin and tetanus  toxoid should be given. They should be given at different  sites so that the antibodies in the antitoxin do not neutralize  the toxoid.   1. Which one of the following is the immunogen in the vaccine  against Streptococcus pneumoniae ?   2. Disease caused by which one of the following bacteria is pre¬  vented by a toxoid vaccine?   3. Disease caused by which one of the following bacteria is pre¬  vented by a vaccine in which the immunogen is covalently  bound to a carrier protein (conjugate vaccine)?    4. Passive immunity is used to prevent or to treat disease caused  by which one of the following sets of bacteria?   Questions on the topics discussed in this chapter can be  found in the Basic Bacteriology section of Part XIII:  USMLE (National Board) Practice Questions starting on  page 709. Also see Part XIV: USMLE (National Board)  Practice Examination starting on page 751.   PRINCIPLES OF STERILIZATION & DISINFECTION  RATE OF KILLING OF MICROORGANISMS  CHEMICAL AGENTS   Disruption of Cell Membranes  Modification of Proteins  Modification of Nucleic Acids    The purpose of sterilization and disinfection procedures is  to prevent transmission of microbes to patients. In addition  to sterilization and disinfection, other important measures  to prevent transmission are included in the protocol of  “standard precautions” (previously known as Universal  Precautions).These standard precautions should be used in  interaction with all patients because it is unknown whether  any particular patient may be the reservoir of transmissible  bacteria, viruses, or other microbes.   Standard precautions include (1) hand hygiene, (2) respira¬  tory hygiene and cough etiquette, (3) safe injection practices,  and (4) proper disposal of needles and scalpels. Further, if  exposure to body fluids or aerosols is likely, personal protective  equipment (PPE) such as masks or face shields, gloves, gowns,    and protective eyewear should be used. The precautions taken  should be specific for the task rather than for the particular  patient.   In addition, there are transmission-based precautions  that supplement the standard precautions and should be  employed when the patient is infected (or suspected to be  infected) with a highly transmissible organism. The three  categories of transmission-based precautions are contact,  droplet, and airborne. Table 13-1 describes some of the  specific information referable to these categories. For addi¬  tional information, please consult the CDC Web site  <http://www.cdc.gov/hai/>, where health care associated  infections (HAI) are discussed.    Sterilization is the killing or removal of all microorgan¬  isms, including bacterial spores, which are highly resistant.  Sterilization is usually carried out by autoclaving, which  consists of exposure to steam at 121°C under a pressure of  15 lb/in 2 for 15 minutes. Surgical instruments that can be  damaged by moist heat are usually sterilized by exposure to  ethylene oxide gas, and most intravenous solutions are  sterilized by filtration.    Disinfection is the killing of many, but not all, micro¬  organisms. For adequate disinfection, pathogens must be  killed, but some organisms and bacterial spores may  survive. Disinfectants vary in their tissue-damaging prop¬  erties from the corrosive phenol-containing compounds,  which should be used only on inanimate objects, to less  toxic materials such as ethanol and iodine, which can  be used on skin surfaces. Chemicals used to kill   If exposure to blood,  secretions, or body flu¬  ids is likely to occur   Personal protective equip¬  ment (PPE) such as  mask, face shield, gog¬  gles, gloves, or gown   2. Face mask or face  shield for patient and  provider. N-95 respira¬  tor, if available.   Tincture of iodine followed by  70% ethanol, or iodophor,  or chlorhexidine   Cleanup of blood spill from a patient  with hepatitis B or C (disinfect area)   Sterilize surgical instruments and  heat-sensitive materials (e.g.,  endoscopes, respiratory therapy  equipment)    Death of microorganisms occurs at a certain rate depen¬  dent primarily on two variables: the concentration of the  killing agent and the length of time the agent is applied.  The rate of killing is defined by the relationship   which shows that the number of survivors, N, is inversely  proportionate to the concentration of the agent, C, and to  the time of application of the agent, T. Collectively, CT is  often referred to as the dose. Stated alternatively, the num¬  ber of microorganisms killed is directly proportionate to  CT. The relationship is usually stated in terms of survivors  because they are easily measured by colony formation.  Death is defined as the inability to reproduce. In certain    circumstances, the physical remains of dead bacteria can  still cause problems (see page 46).   Chemicals vary greatly in their ability to kill microorgan¬  isms. A quantitative measure of this variation is expressed  as the phenol coefficient, which is the ratio of the concen¬  tration of phenol to the concentration of the agent required  to cause the same amount of killing under the standard  conditions of the test.   Chemical agents act primarily by one of the three  mechanisms: (1) disruption of the lipid-containing cell  membrane, (2) modification of proteins, or (3) modifica¬  tion of DNA. Each of the following chemical agents has    been classified into one of the three categories, but some of  the chemicals act by more than one mechanism.   Ethanol is widely used to clean the skin before immuniza¬  tion or venipuncture. It acts mainly by disorganizing the  lipid structure in membranes, but it denatures proteins as  well. Ethanol requires the presence of water for maximal  activity (i.e., it is far more effective at 70% than at 100%).  Seventy percent ethanol is often used as an antiseptic to  clean the skin prior to venipuncture. However, because it  is not as effective as iodine-containing compounds, the  latter should be used prior to obtaining a blood culture  and installing intravenous catheters. Ethanol will not kill  bacterial spores and therefore cannot be used for  sterilization.   Detergents are “surface-active” agents composed of a long-  chain, lipid-soluble, hydrophobic portion and a polar  hydrophilic group, which can be a cation, an anion, or a  nonionic group. These surfactants interact with the lipid in  the cell membrane through their hydrophobic chain and  with the surrounding water through their polar group and  thus disrupt the membrane. Quaternary ammonium com¬  pounds (e.g., benzalkonium chloride) are cationic deter¬  gents widely used for skin antisepsis. Benzalkonium  chloride is the active ingredient in Lysol, a commonly used  disinfectant for floors and other surfaces.   Phenol was the first disinfectant used in the operating  room (by Lister in the 1860s), but it is rarely used as a dis¬  infectant today because it is too caustic. Chlorhexidine is a  chlorinated phenol that is widely used as a hand disinfec¬  tant prior to surgery (“surgical scrub”) and in the cleansing  of wounds. Hexachlorophene, which is a biphenol with six  chlorine atoms, is used in germicidal soaps, but concern  over possible neurotoxicity has limited its use. Phenols not  only damage membranes, but also denature proteins.   Chlorine is used as a disinfectant to purify the water supply  and to treat swimming pools. It is also the active compo¬  nent of hypochlorite (bleach, Clorox), which is used as a  disinfectant in the home and in hospitals. Chlorine is a  powerful oxidizing agent that kills by cross-linking essen¬  tial sulfhydryl groups in enzymes to form the inactive  disulfide.   Iodine is the most effective skin antiseptic used in medical  practice and should be used prior to obtaining a blood  culture and installing intravenous catheters because con¬  tamination with skin flora such as Staphylococcus epidermi-  dis can be a problem. Iodine, like chlorine, is an oxidant  that inactivates sulfhydryl-containing enzymes. It also  binds specifically to tyrosine residues in proteins.   (1) Tincture of iodine (2% solution of iodine and  potassium iodide in ethanol) is used to prepare the skin  prior to blood culture. Because tincture of iodine can be  irritating to the skin, it should be removed with alcohol.   (2) Iodophors are complexes of iodine with detergents that  are frequently used to prepare the skin prior to surgery because  they are less irritating than tincture of iodine. Povidone-  iodine is an iodophor commonly used as an antiseptic.   Mercury and silver have the greatest antibacterial activity  of the heavy metals and are the most widely used in medi¬  cine. They act by binding to sulfhydryl groups, thereby  blocking enzymatic activity. Thimerosal (Merthiolate) and  merbromin (Mercurochrome), which contain mercury, are  used as skin antiseptics. Silver nitrate drops are effective in  preventing gonococcal neonatal conjunctivitis (ophthalmia  neonatorum). Silver sulfadiazine is used to prevent infec¬  tion of burn wounds.   Hydrogen peroxide is used as an antiseptic to clean  wounds and to disinfect contact lenses. Its effectiveness is  limited by the organism’s ability to produce catalase, an  enzyme that degrades H 2 0 2 . (The bubbles produced when  peroxide is used on wounds which are formed by oxygen  arising from the breakdown of H 2 0 2 by tissue catalase.)  Hydrogen peroxide is an oxidizing agent that attacks sulf¬  hydryl groups, thereby inhibiting enzymatic activity.   Formaldehyde, which is available as a 37% solution in  water (formalin), denatures proteins and nucleic acids.  Both proteins and nucleic acids contain essential -NH 2 and  -OH groups, which are the main sites of alkylation by the  hydroxymethyl group of formaldehyde. Glutaraldehyde,  which has two reactive aldehyde groups, is 10 times more  effective than formaldehyde and is less toxic. In hospitals, it  is used to sterilize respiratory therapy equipment, endo¬  scopes, and hemodialysis equipment.   Ethylene oxide gas is used extensively in hospitals for the  sterilization of heat-sensitive materials such as surgical    instruments and plastics. It kills by alkylating both proteins  and nucleic acids (i.e., the hydroxyethyl group attacks the  reactive hydrogen atoms on essential amino and hydroxyl  groups). It is classified as a mutagen and a carcinogen.   Strong acids and alkalis kill by denaturing proteins.  Although most bacteria are susceptible, it is important to  note that Mycobacterium tuberculosis and other mycobacte¬  ria are relatively resistant to 2% NaOH, which is used in the  clinical laboratory to liquefy sputum prior to culturing the  organism. Weak acids, such as benzoic, propionic, and  citric acids, are frequently used as food preservatives  because they are bacteriostatic. The action of these acids is    partially a function of the organic moiety (e.g., benzoate),  as well as the low pH.   A variety of dyes not only stain microorganisms, but also  inhibit their growth. One of these is crystal violet (gentian  violet), which is used as a skin antiseptic. Its action is based  on binding of the positively charged dye molecule to the  negatively charged phosphate groups of the nucleic acids.  Malachite green, a triphenylamine dyelike crystal violet, is  a component of Lowenstein-Jenseris medium, which is  used to grow M. tuberculosis. The dye inhibits the growth  of unwanted organisms in the sputum during the 6-week  incubation period.   The physical agents act either by imparting energy in the  form of heat or radiation or by removing organisms  through filtration.   Heat energy can be applied in three ways: in the form of  moist heat (either boiling or autoclaving) or dry heat or by  pasteurization. In general, heat kills by denaturing proteins,  but membrane damage and enzymatic cleavage of DNA  may also be involved. Moist heat sterilizes at a lower tem¬  perature than dry heat, because water aids in the disruption  of noncovalent bonds (e.g., hydrogen bonds), which hold  protein chains together in their secondary and tertiary  structures.   Moist heat sterilization, usually autoclaving, is the most  frequently used method of sterilization. Because bacterial  spores are resistant to boiling (100°C at sea level), they  must be exposed to a higher temperature; this cannot be  achieved unless the pressure is increased. For this purpose,  an autoclave chamber is used in which steam, at a pressure  of 15 lb/in 2 , reaches a temperature of 121°C and is held at  that temperature for 15 to 20 minutes. This kills even the  highly heat-resistant spores of Clostridium botulinum, the  cause of botulism, with a margin of safety. To test the effec¬  tiveness of the autoclaving process, spore-forming organ¬  isms, such as members of the genus Clostridium, are used.   Sterilization by dry heat, on the other hand, requires  temperatures in the range of 180°C for 2 hours. This pro¬  cess is used primarily for glassware and is used less fre¬  quently than autoclaving.   Pasteurization, which is used primarily for milk, con¬  sists of heating the milk to 62°C for 30 minutes followed by  rapid cooling. (“Flash” pasteurization at 72°C for 15 sec¬  onds is often used.) This is sufficient to kill the vegetative  cells of the milk-borne pathogens (e.g., Mycobacterium    bovis. Salmonella, Streptococcus, Listeria, and Brucella), but  not to sterilize the milk.   The two types of radiation used to kill microorganisms are  ultraviolet (UV) light and X-rays. The greatest antimicro¬  bial activity of UV light occurs at 250 to 260 nm, which is  the wavelength region of maximum absorption by the  purine and pyrimidine bases of DNA. The most significant  lesion caused by UV irradiation is the formation of thy¬  mine dimers, but addition of hydroxyl groups to the bases  also occurs. As a result, DNA replication is inhibited and  the organism cannot grow. Cells have repair mechanisms  against UV-induced damage that involve either cleavage of  dimers in the presence of visible light (photoreactivation)  or excision of damaged bases, which is not dependent on  visible light (dark repair). Because UV radiation can dam¬  age the cornea and skin, the use of UV irradiation in medi¬  cine is limited. However, it is used in hospitals to kill  airborne organisms, especially in operating rooms when  they are not in use. Bacterial spores are quite resistant and  require a dose up to 10 times greater than do the vegetative  bacteria.   X-rays have higher energy and penetrating power than  UV radiation and kill mainly by the production of free  radicals (e.g., production of hydroxyl radicals by the hydro¬  lysis of water). These highly reactive radicals can break  covalent bonds in DNA, thereby killing the organism.  Sulfhydryl-containing compounds, such as the amino acid  cysteine, can protect DNA from free-radical attack. Another  mechanism is a direct hit on a covalent bond in DNA,  resulting in chain breakage, but this is probably less impor¬  tant than the mechanism involving free radicals.   X-rays kill vegetative cells readily, but spores are remark¬  ably resistant, probably because of their lower water    content. X-rays are used in medicine for sterilization of  heat-sensitive items, such as sutures and surgical gloves,  and plastic items, such as syringes.   Filtration is the preferred method of sterilizing certain  solutions (e.g., those with heat-sensitive components). In  the past, solutions for intravenous use were autoclaved, but    heat-resistant endotoxin in the cell walls of the dead gram¬  negative bacteria caused fever in recipients of the solutions.  Therefore, solutions are now filtered to make them pyro-  gen-free prior to autoclaving.   The most commonly used filter is composed of nitrocel¬  lulose and has a pore size of 0.22 pm. This size will retain  all bacteria and spores. Filters work by physically trapping  particles larger than the pore size and by retaining some¬  what smaller particles via electrostatic attraction of the  particles to the filters.    Sterilization is the killing of all forms of microbial life, includ¬  ing bacterial spores. Spores are resistant to boiling, so steril¬  ization of medical equipment is typically achieved at 121 °C for  15 minutes in an autoclave. Sterilization of heat-sensitive mate¬  rials is achieved by exposure to ethylene oxide, and liquids can  be sterilized by filtration.   Disinfection is reducing the number of bacteria to a level  low enough that disease is unlikely to occur. Spores and some  bacteria will survive. For example, disinfection of the water  supply is achieved by treatment with chlorine. Disinfection of  the skin prior to venipuncture is achieved by treatment with  70% ethanol. Disinfectants that are mild enough to use on skin  and other tissues, such as 70% ethanol, are called antiseptics.  The killing of microbes by either chemicals or radiation is pro¬  portional to the dose, which is defined as the product of the  concentration multiplied by the time of exposure.    Chemical agents kill bacteria by one of three actions: disrup¬  tion of lipid in cell membranes, modification of proteins, or  modification of DNA.   Physical agents kill (or remove) bacteria by one of three pro¬  cesses: heat, radiation, or filtration.   Pleat is usually applied at temperatures above boiling (121 °C)  to kill spores, but heat-sensitive materials such as milk are  exposed to temperatures below boiling (pasteurization) that  kill the pathogens in milk but do not sterilize it.   Radiation, such as ultraviolet light and X-radiation, is often  used to sterilize heat-sensitive items. Ultraviolet light and  X-radiation kill by damaging DNA.   Filtration can sterilize liquids if the pore size of the filter is small  enough to retain all bacteria and spores. Pleat-sensitive liquids  (e.g., intravenous fluids) are often sterilized by filtration.   (A) Seventy percent alcohol is a better antiseptic than iodine,  so 70% alcohol should be used to disinfect the skin prior  to drawing a blood culture rather than iodine.   (C) During sterilization by autoclaving, the temperature must  be raised above boiling in order to kill bacterial spores.   (D) Transmission of milk-borne diseases can be prevented by  pasteurization, which kills both bacterial cells and spores.   (E) Ultraviolet light used in the operating room to disinfect  the room kills bacteria primarily by causing oxidation of  lipids in the cell membrane.   2. Which one of the following chemicals is used to sterilize   Questions on the topics discussed in this chapter can be  found in the Basic Bacteriology section of Part XIII:  USMLE (National Board) Practice Questions starting on  page 709. Also see Part XIV: USMLE (National Board)  Practice Examination starting on page 751.   The major bacterial pathogens are presented in Table 14-1  and described in Chapters 15 through 26. So that the reader  may concentrate on the important pathogens, the bacteria  that are less medically important are described in a separate  chapter (see Chapter 27).   Table 14-1 is divided into organisms that are readily  Gram stained and those that are not. The readily stained  organisms fall into four categories: gram-positive cocci, gram¬  negative cocci, gram-positive rods, and gram-negative rods.  Because there are so many kinds of gram-negative rods, they  have been divided into three groups:   For ease of understanding, the organisms associated  with the enteric tract are further subdivided into three  groups: (1) pathogens both inside and outside the enteric  tract, (2) pathogens inside the enteric tract, and (3) patho¬  gens outside the enteric tract.    As is true of any classification dealing with biologic enti¬  ties, this one is not entirely precise. For example, Campylo¬  bacter causes enteric tract disease but frequently has an  animal source. Nevertheless, despite some uncertainties,  subdivision of the large number of gram-negative rods into  these functional categories should be helpful to the reader.   The organisms that are not readily Gram stained fall  into six major categories: Mycobacterium species, which are  acid-fast rods; Mycoplasma species, which have no cell wall  and so do not stain with Gram stain; Treponema and Lepto¬  spira species, which are spirochetes too thin to be seen  when stained with Gram stain; and Chlamydia and Rickett¬  sia species, which are very small, intracellular bacteria and  are difficult to visualize within the cytoplasm of the cell.   Table 14-2 presents the 10 most common “notifiable”  bacterial diseases in the United States for 2012 as compiled  by the Centers for Disease Control and Prevention. Note  that only notifiable diseases are included and that certain  common conditions such as streptococcal pharyngitis and  impetigo are not included. Two sexually transmitted dis¬  eases, chlamydial infection and gonorrhea, are by far the  most common diseases listed, followed by salmonellosis,  syphilis, and Lyme disease in the top five.   Anaerobes are characterized by their ability to grow only in  an atmosphere containing less than 20% oxygen (i.e., they  grow poorly if at all in room air). They are a heterogeneous  group composed of a variety of bacteria, from those that  can barely grow in 20% oxygen to those that can grow only  in less than 0.02% oxygen. Table 14-3 describes the optimal  oxygen requirements for several representative groups of  organisms. The obligate aerobes, such as Pseudomonas  aeruginosa, grow best in the 20% oxygen of room air and    TABLE 14-2 The 10 Most Common Notifiable  Bacterial Diseases in the United States in 2012 1    not at all under anaerobic conditions. Facultative anaerobes  such as Escherichia coli can grow well under either circum¬  stance. Aerotolerant organisms such as Clostridium histo-  lyticum can grow to some extent in air but multiply much  more rapidly in a lower oxygen concentration. Microaero-  philic organisms such as Campylobacter jejuni require a  reduced oxygen concentration (approximately 5%) to grow  optimally. The obligate anaerobes such as Bacteroides fragi¬  le and Clostridium perfringens require an almost total  absence of oxygen. Many anaerobes use nitrogen rather  than oxygen as the terminal electron acceptor.   The main reason why the growth of anaerobes is inhib¬  ited by oxygen is the reduced amount (or absence) of cata¬  lase and superoxide dismutase (SOD) in anaerobes.  Catalase and SOD eliminate the toxic compounds hydro¬  gen peroxide and superoxide, which are formed during  production of energy by the organism (see Chapter 3).  Another reason is the oxidation of essential sulfhydryl  groups in enzymes without sufficient reducing power to  regenerate them.   In addition to oxygen concentration, the oxidation-  reduction potential (£ h ) of a tissue is an important determi¬  nant of the growth of anaerobes. Areas with low E*, such as  the periodontal pocket, dental plaque, and colon, support  the growth of anaerobes well. Crushing injuries that result in  devitalized tissue caused by impaired blood supply produce  a low E h , allowing anaerobes to grow and cause disease.   The anaerobes of medical interest are presented in Table 14-4.  It can be seen that they include both rods and cocci and  both gram-positive and gram-negative organisms. The rods    'C. jejuni grows best (3+) in 5% 0 2 plus 10% C0 2 . It is also called capnophilic in view of its need for C0 2 for optimal growth.    are divided into the spore formers (e.g., Clostridium) and  the nonspore formers (e.g., Bacteroides). In this book,  three genera of anaerobes are described as major bacterial  pathogens, namely, Clostridium, Actinomyces, and Bacte¬  roides. Streptococcus is a genus of major pathogens consist¬  ing of both anaerobic and facultative organisms. The  remaining anaerobes are less important and are discussed  in Chapter 27.   Many of the medically important anaerobes are part of the  normal human flora. As such, they are nonpathogens in  their normal habitat and cause disease only when they leave  those sites. The two prominent exceptions to this are Clos¬  tridium botulinum and Clostridium tetani, the agents of  botulism and tetanus, respectively, which are soil organ¬  isms. Clostridium perfringens, another important human  pathogen, is found in the colon and in the soil.   Diseases caused by members of the anaerobic normal  flora are characterized by abscesses, which are most fre¬  quently located in the brain, lungs, female genital tract, bili¬  ary tract, and other intra-abdominal sites. Most abscesses    contain more than one organism, either multiple anaerobes  or a mixture of anaerobes plus facultative anaerobes. It is  thought that the facultative anaerobes consume sufficient  oxygen to allow the anaerobes to flourish.   Three important findings on physical examination that  arouse suspicion of an anaerobic infection are a foul-smell¬  ing discharge, gas in the tissue, and necrotic tissue. In addi¬  tion, infections in the setting of pulmonary aspiration,  bowel surgery, abortion, cancer, or human and animal bites  frequently involve anaerobes.   Two aspects of microbiologic diagnosis of an anaerobic  infection are important even before the specimen is cul¬  tured: (1) obtaining the appropriate specimen and (2) rap¬  idly transporting the specimen under anaerobic conditions  to the laboratory. An appropriate specimen is one that does  not contain members of the normal flora to confuse the  interpretation. For example, such specimens as blood, pleu¬  ral fluid, pus, and transtracheal aspirates are appropriate,  but sputum and feces are not.   In the laboratory, the cultures are handled and incu¬  bated under anaerobic conditions. In addition to the usual  diagnostic criteria of Gram stain, morphology, and bio¬  chemical reactions, the special technique of gas chromatog¬  raphy is important. In this procedure, organic acids such as  formic, acetic, and propionic acids are measured.   In general, surgical drainage of the abscess plus administra¬  tion of antimicrobial drugs are indicated. Drugs commonly  used to treat anaerobic infections are penicillin G, cefoxitin,  chloramphenicol, clindamycin, and metronidazole. Note,  however, that many isolates of the important pathogen B.  fragilis produce cc-lactamase and are thus resistant to peni¬  cillin. Note also, that aminoglycosides such as gentamicin  are not effective against anaerobes because they require an  oxygen-dependent process for uptake into the bacterial cell.   1. The main reason why some bacteria are anaerobes (i.e., they can¬  not grow in the presence of oxygen) is because:   (B) they have too much ferrous ion that is oxidized to ferric ion in  the presence of oxygen.   (C) they have unusual mitochondria that cannot function in the  presence of oxygen.   (D) transcription of the gene for the pilus protein is repressed in the  presence of oxygen.   2. Which one of the following sets consists of bacteria that are both  anaerobes?   Questions on the topics discussed in this chapter can be found  in the Clinical Bacteriology section of Part XIII: USMLE  (National Board) Practice Questions starting on page 713. Also  see Part XIV: USMLE (National Board) Practice Examination  starting on page 751.   There are two medically important genera of gram-positive  cocci: Staphylococcus and Streptococcus. Two of the most  important human pathogens, Staphylococcus aureus and  Streptococcus pyogenes, are described in this chapter. Staph¬  ylococci and streptococci are nonmotile and do not form  spores.   Both staphylococci and streptococci are gram-positive  cocci, but they are distinguished by two main criteria:   (1) Microscopically, staphylococci appear in grapelike  clusters, whereas streptococci are in chains.   (2) Biochemically, staphylococci produce catalase (i.e.,  they degrade hydrogen peroxide), whereas streptococci  do not.   Additional information regarding the clinical aspects of  infections caused by the organisms in this chapter is pro¬  vided in Part IX entitled Infectious Diseases beginning on  page 593.   Staphylococcus aureus causes abscesses (Figure 15-1), vari¬  ous pyogenic infections (e.g., endocarditis, septic arthritis,  and osteomyelitis), food poisoning, scalded skin syndrome  (Figure 15-2), and toxic shock syndrome. It is one of the  most common causes of hospital-acquired pneumonia,  septicemia, and surgical-wound infections. It is an impor¬  tant cause of skin and soft tissue infections, such as follicu¬  litis (Figure 15-3), cellulitis, and impetigo (Figure 15-4). It  is the most common cause of bacterial conjunctivitis.   Methicillin-resistant Staphylococcus aureus (MRSA) is  the most common cause of skin abscesses in the United   FIGURE 15-1 Abscess on foot. Note central raised area of  whitish pus surrounded by erythema. An abscess is the classic lesion  caused by Staphylococcus aureus. (Reproduced with permission from Wolff K,  Johnson R (eds): Fitzpatrick's Color Atlas & Synopsis of Clinical Dermatology. 6th ed.  New York: McGraw-Hill, 2009. Copyright © 2009 by The McGraw-Hill Companies, Inc.)    States. It is also an important cause of pneumonia, necrotiz¬  ing fasciitis, and sepsis in immunocompetent patients.   Staphylococcus epidermidis causes prosthetic valve  endocarditis and prosthetic joint infections. It is the most  common cause of central nervous system shunt infections  and an important cause of sepsis in newborns. Staphylococ¬  cus saprophyticus causes urinary tract infections, especially  cystitis. Kawasaki syndrome is a disease of unknown etiol¬  ogy that may be caused by certain strains of S. aureus.   Staphylococci are spherical gram-positive cocci arranged in  irregular grapelike clusters (Figure 15-5). All staphylococci  produce catalase, whereas no streptococci do (catalase   FIGURE 15-2 Scalded skin syndrome. Note widespread areas  of "rolled up" desquamated skin in infant. Caused by an exotoxin pro¬  duced by Staphylococcus aureus. (Reproduced with permission from Wolff  K, Johnson R (eds): Fitzpatrick's Color Atlas & Synopsis of Clinical Dermatology. 6th ed.  New York: McGraw-Hill, 2009. Copyright © 2009 by The McGraw-Hill Companies, Inc.)   FIGURE 15-3 Folliculitis. Note the multiple, small pustules on  the chin and neck. Staphylococcus aureus is the most common cause  of folliculitis. (Reproduced with permission from Wolff K, Goldsmith LA, Katz SI et  al (eds): Fitzpatrick's Dermatology in General Medicine. 7th ed. New York: McGraw-Hill,  2008, pg 1699. Copyright © 2008 by The McGraw-Hill Companies, Inc.)   FIGURE 15-4 Impetigo. Lesions of impetigo are crops of vesi¬  cles with a "honey-colored" crust. Impetigo is caused by either Staph¬  ylococcus aureus or Streptococcus pyogenes. (Reproduced with permission  from Wolff K, Johnson R (eds): Fitzpatrick's Color Atlas & Synopsis of Clinical Derma¬  tology. 6th ed. New York: McGraw-Hill, 2009. Copyright © 2009 by The McGraw-Hill  Companies, Inc.)    degrades H 2 0 2 into 0 2 and H 2 0). Catalase is an important  virulence factor. Bacteria that make catalase can survive the  killing effect of H 2 0 2 within neutrophils.   Three species of staphylococci are important human  pathogens: S. aureus, S. epidermidis, and S. saprophyticus  (Table 15-1). Of these three, S. aureus is by far the most   point to two "grapelike" clusters of gram-positive cocci. Arrowhead  points to neutrophil with pink segmented nuclei. (Used with permission  from Professor Shirley Lowe, University of California, San Francisco School of  Medicine.)   Infection of prosthetic heart valves and hips;  common member of skin flora    common and causes the most serious infections. Staphylo¬  coccus aureus is distinguished from the others primarily by  coagulase production (Figure 15-6). Coagulase is an  enzyme that causes plasma to clot by activating prothrom¬  bin to form thrombin. Thrombin then catalyzes the activa¬  tion of fibrinogen to form the fibrin clot. Staphylococcus  epidermidis and S. saprophyticus are often referred to as  coagulase-negative staphylococci.   Staphylococcus aureus produces a carotenoid pigment  called staphyloxanthin, which imparts a golden color to its  colonies. This pigment enhances the pathogenicity of the  organism by inactivating the microbicidal effect of super¬  oxides and other reactive oxygen species within neutro¬  phils. Staphylococcus epidermidis does not synthesize this  pigment and produces white colonies. The virulence of  S. epidermidis is significantly less than that of S. aureus.   Two other characteristics further distinguish these species,  namely, S. aureus usually ferments mannitol and hemolyzes  red blood cells, whereas S. epidermidis and S. saprophyticus   FIGURE 15~6 Coagulase test—Upper tube inoculated with  Staphylococcus aureus; lower tube inoculated with Staphylococcus  epidermidis. Arrow points to clotted plasma formed by coagulase  produced by 5. aureus. (Used with permission from Professor Shirley Lowe, Uni¬  versity of California, San Francisco School of Medicine.)    do not. Hemolysis of red cells by hemolysins produced by  S. aureus is the source of iron required for growth of the  organism. The iron in hemoglobin is recovered by the bacte¬  ria and utilized in the synthesis of cytochrome enzymes used  to produce energy.   More than 90% of S. aureus strains contain plasmids  that encode P-lactamase, the enzyme that degrades many,  but not all, penicillins. Some strains of S. aureus are resis¬  tant to the (3-lactamase-resistant penicillins, such as methi-  cillin and nafcillin, by virtue of changes in the  penicillin-binding proteins (PBP) in their cell membrane.  Genes on the bacterial chromosome called mecA genes  encode these altered PBPs.   These strains are commonly known as methicillin-  resistant S. aureus (MRSA) or nafcillin-resistant S. aureus  (NRSA). MRSA causes both health care-acquired (HCA-  MRSA) and community-acquired (CA-MRSA) infections.  MRSA currently accounts for more than 50% of S. aureus  strains isolated from hospital patients in the United States.  CA-MRSA is a very common cause of community-acquired  staphylococcal infections. Almost all strains of CA-MRSA  produce P-V leukocidin (see later) whereas relatively few  strains of HCA-MRSA do so. The most common strain of  MRSA in the United States is the “USA300” strain.   Strains of S. aureus with intermediate resistance to van¬  comycin (VISA) and with full resistance to vancomycin  (VRSA) have also been detected. The cassette of genes that  encodes vancomycin resistance in S. aureus is the same as  the cassette that provides vancomycin resistance in entero¬  cocci. These genes are located in a transposon on a plasmid  and encode the enzymes that substitute D-lactate for  D-alanine in the peptidoglycan.   (1) Protein A is the major protein in the cell wall. It is  an important virulence factor because it binds to the Fc  portion of IgG at the complement-binding site, thereby  preventing the activation of complement. As a conse¬  quence, no C3b is produced, and the opsonization and  phagocytosis of the organisms are greatly reduced. Protein  A is used in certain tests in the clinical laboratory because  it binds to IgG and forms a “coagglutinate” with antigen-  antibody complexes. The coagulase-negative staphylococci  do not produce protein A.    (2) Teichoic acids are polymers of ribitol phosphate.  They mediate adherence of the staphylococci to mucosal  cells. Lipoteichoic acids play a role in the induction of  septic shock by inducing cytokines such as interleukin-1  (IL-1) and tumor necrosis factor (TNF) from macrophages  (See the discussion of septic shock in the Endotoxin section  of Chapter 7).   (3) Polysaccharide capsule is also an important viru¬  lence factor. There are 11 serotypes based on the antigenic¬  ity of the capsular polysaccharide, but types 5 and 8 cause  85% of infections. Some strains of S. aureus are coated with  a small amount of polysaccharide capsule, called a micro¬  capsule. The capsule is poorly immunogenic, which has  made producing an effective vaccine difficult.   (5) The peptidoglycan of S. aureus has endotoxin-like  properties (i.e., it can stimulate macrophages to produce  cytokines and can activate the complement and coagulation  cascades). This explains the ability of S. aureus to cause the  clinical findings of septic shock yet not possess endotoxin.   Humans are the reservoir for staphylococci. The nose is the  main site of colonization of S. aureus, and approximately  30% of people are colonized at any one time. People who  are chronic carriers of S. aureus in their nose have an  increased risk of skin infections caused by S. aureus.   The skin, especially of hospital personnel and patients,  is also a common site of S. aureus colonization. Hand con¬  tact is an important mode of transmission, and handwash¬  ing decreases transmission.   Staphylococcus aureus is also found in the vagina of  approximately 5% of women, which predisposes them to  toxic shock syndrome. Additional sources of staphylococ¬  cal infection are shedding from human lesions and fomites  such as towels and clothing contaminated by these lesions.   Disease caused by S. aureus is favored by a heavily con¬  taminated environment (e.g., family members with boils)  and a compromised immune system. Reduced humoral  immunity, including low levels of antibody, complement, or  neutrophils, especially predisposes to staphylococcal infec¬  tions. Diabetes and intravenous drug use predispose to  infections by S. aureus. Patients with chronic granuloma¬  tous disease (CGD), a disease characterized by a defect in  the ability of neutrophils to kill bacteria, are especially  prone to S. aureus infections (see Chapter 68).   Staphylococcus epidermidis is found primarily on the  human skin and can enter the bloodstream at the site of  intravenous catheters that penetrate through the skin.  Staphylococcus saprophyticus is found primarily on the  mucosa of the genital tract in young women and from that    site can ascend into the urinary bladder to cause urinary  tract infections.   Staphylococcus aureus causes disease both by producing  toxins and by inducing pyogenic inflammation. The typical  lesion of S. aureus infection is an abscess. Abscesses  undergo central necrosis and usually drain to the outside  (e.g., furuncles and boils), but organisms may disseminate  via the bloodstream as well. Foreign bodies, such as  sutures and intravenous catheters, are important predis¬  posing factors to infection by S. aureus.   Several important toxins and enzymes are produced by  S. aureus. The three clinically important exotoxins are  enterotoxin, toxic shock syndrome toxin, and exfoliatin.   (1) Enterotoxin causes food poisoning characterized by  prominent vomiting and watery, nonbloody diarrhea. It  acts as a superantigen within the gastrointestinal tract to  stimulate the release of large amounts of IL-1 and IL-2 from  macrophages and helper T cells, respectively. The promi¬  nent vomiting appears to be caused by cytokines released  from the lymphoid cells, which stimulate the enteric ner¬  vous system to activate the vomiting center in the brain.  Enterotoxin is fairly heat-resistant and is therefore usually  not inactivated by brief cooking. It is resistant to stomach  acid and to enzymes in the stomach and jejunum. There are  six immunologic types of enterotoxin, types A-F.   (2) Toxic shock syndrome toxin (TSST) causes toxic  shock, especially in tampon-using menstruating women or  in individuals with wound infections. Toxic shock also  occurs in patients with nasal packing used to stop bleeding  from the nose. TSST is produced locally by S. aureus in the  vagina, nose, or other infected site. The toxin enters the  bloodstream, causing a toxemia. Blood cultures typically do  not grow S. aureus.   TSST is a superantigen and causes toxic shock by  stimulating the release of large amounts of IL-1, IL-2, and  TNF (see the discussions of exotoxins in Chapter 7 and  superantigens in Chapter 58). Approximately 5% to 25%  of isolates of S. aureus carry the gene for TSST. Toxic  shock occurs in people who do not have antibody against  TSST.   (3) Exfoliatin causes “scalded skin” syndrome in young  children. It is “epidermolytic” and acts as a protease that  cleaves desmoglein in desmosomes, leading to the separa¬  tion of the epidermis at the granular cell layer.   (4) Several exotoxins can kill leukocytes (leukocidins)  and cause necrosis of tissues in vivo. Of these, the two most  important are alpha toxin and P- V leukocidin. Alpha toxin  causes marked necrosis of the skin and hemolysis. The  cytotoxic effect of alpha toxin is attributed to the formation  of holes in the cell membrane and the consequent loss of  low-molecular-weight substances from the damaged cell.    P-V leukocidin is a pore-forming toxin that kills cells,  especially white blood cells, by damaging cell membranes.  The two subunits of the toxin assemble in the cell mem¬  brane to form a pore through which cell contents leak out.  The gene encoding P-V leukocidin is located on a lysogenic  phage. P-V leukocidin is an important virulence factor for  CA-MRSA and plays a role in the severe skin and soft tissue  infection caused by this organism. A severe necrotizing  pneumonia is also caused by strains of S. aureus that pro¬  duce P-V leukocidin. Approximately 2% of clinical isolates  of S. aureus produce P-V leukocidin.   (5) The enzymes include coagulase, fibrinolysin, hyal-  uronidase, proteases, nucleases, and lipases. Coagulase, by  clotting plasma, serves to wall off the infected site, thereby  retarding the migration of neutrophils into the site. Staph-  ylokinase is a fibrinolysin that can lyse thrombi.   Unlike S. aureus, these two coagulase-negative staphylo¬  cocci do not produce exotoxins. Thus, they do not cause  food poisoning or toxic shock syndrome. They do, how¬  ever, cause pyogenic infections. For example, S. epidermi¬  dis is a prominent cause of pyogenic infections on  prosthetic implants such as heart valves and hip joints, and  S. saprophyticus causes urinary tract infections, especially  cystitis.   The important clinical manifestations caused by S. aureus  can be divided into two groups: pyogenic (pus-producing)  and toxin-mediated (Table 15-2). Staphylococcus aureus is a  major cause of skin, soft tissue, bone, joint, lung, heart, and  kidney infections. Pyogenic diseases are the first group  described, and toxin-mediated diseases are the second  group.   (1) Skin and soft tissue infections are very common.  These include abscess (see Figure 15-1), impetigo (see Fig¬  ure 15-4), furuncles, carbuncles (Figure 15-7), paronychia,  cellulitis, folliculitis (see Figure 15-3), hidradenitis suppu¬  rativa, conjunctivitis, eyelid infections (blepharitis and  hordeolum), and postpartum breast infections (mastitis).  Lymphangitis can occur, especially on the forearm associ¬  ated with an infection on the hand.   Severe necrotizing skin and soft tissue infections are  caused by MRSA strains that produce P-V leukocidin.  These infections are typically community-acquired rather  than hospital-acquired. In the United States, community-  acquired MRSA (CA-MRSA) strains are the most common  cause of skin and soft tissue infections. These CA-MRSA  strains are an especially common cause of infection among  the homeless and intravenous drug users. Athletes who  engage in close personal contact such as wrestlers and foot¬  ball players are also at risk. Note that hospital-acquired  MRSA (F1A-MRSA) causes approximately 50% of all noso¬  comial S. aureus infections. Molecular analysis reveals that  the CA-MRSA strains are different from the HA-MRSA strains.   (2) Septicemia (sepsis) can originate from any localized  lesion, especially wound infection, or as a result of intrave¬  nous drug abuse. Sepsis caused by S. aureus has clinical fea¬  tures similar to those of sepsis caused by certain gram-negative  bacteria, such as Neisseria meningitidis (see Chapter 16).   (3) Endocarditis may occur on normal or prosthetic  heart valves, especially right-sided endocarditis (tricuspid  valve) in intravenous drug users. (Prosthetic valve endocar¬  ditis is often caused by S. epidermidis.)   (4) Osteomyelitis and septic arthritis may arise either by  hematogenous spread from a distant infected focus or be  introduced locally at a wound site. Staphylococcus aureus is  a very common cause of these diseases, especially in  children.   ’For simplicity, many forms of disseminated diseases caused by 5. aureus (e.g., osteomyelitis, arthritis) were not included in the table.   FIGURE 15-7 Carbuncle. A carbuncle is a multiheaded  abscess often located on the back of the neck. Note drop of yellow¬  ish pus near the center of the lesion. Carbuncles are caused by Staph¬  ylococcus auteus. (Reproduced with permission from Wolff K, Johnson R (eds):  Fitzpatrick's Color Atlas & Synopsis of Clinical Dermatology. 6th ed. New York: McGraw-  Hill, 2009. Copyright © 2009 by The McGraw-Hill Companies, Inc.)    (5) Staphylococcus aureus is the most common cause of  postsurgical wound infections which are an important  cause of morbidity and mortality in hospitals. For example,  S. aureus and S. epidermidis are the most common causes of  infections at the site where cardiac pacemakers are installed.   (6) Pneumonia can occur in postoperative patients or  following viral respiratory infection, especially influenza.  Staphylococcal pneumonia often leads to empyema or lung  abscess. In many hospitals, it is the most common cause of  nosocomial pneumonia in general and especially of ventilator-  associated pneumonia in intensive care units. CA-MRSA causes  a severe necrotizing pneumonia.   (7) Conjunctivitis typically presents with unilateral  burning eye pain, hyperemia of the conjunctiva, and a  purulent discharge. The organism is transmitted to the eye  by contaminated fingers. Staphylococcus aureus is the most  common cause overall, but S. pneumoniae and Haemophi¬  lus influenzae are more common in children. Gonococcal  and nongonococcal (caused by Chlamydia trachomatis )  conjunctivitis is acquired by infants during passage through  the birth canal.   (8) Abscesses can occur in any organ when S. aureus  circulates in the bloodstream (bacteremia). These abscesses    are often called “metastatic abscesses” because they occur  by the spread of bacteria from the original site of infection,  often in the skin.   (1) Food poisoning (gastroenteritis) is caused by inges¬  tion of enterotoxin, which is preformed in foods and hence  has a short incubation period (1-8 hours). In staphylococ¬  cal food poisoning, vomiting is typically more prominent  than diarrhea.   (2) Toxic shock syndrome is characterized by fever;  hypotension; a diffuse, macular, sunburn-like rash that  goes on to desquamate; and involvement of three or more  of the following organs: liver, kidney, gastrointestinal tract,  central nervous system, muscle, or blood.   (3) Scalded-skin syndrome is characterized by fever,  large bullae, and an erythematous macular rash. Large  areas of skin slough, serous fluid exudes, and electrolyte  imbalance can occur. Hair and nails can be lost. Recovery  usually occurs within 7-10 days. This syndrome occurs  most often in young children.   Kawasaki disease (KD) is a disease of unknown etiology  that is discussed here because several of its features resem¬  ble toxic shock syndrome caused by the superantigens of  S. aureus (and S. pyogenes). KD is a vasculitis involving  small and medium-size arteries, especially the coronary  arteries. It is the most common cause of acquired heart  disease in children in the United States.   Clinically, KD is characterized by a high fever of at least  5 days’ duration; bilateral nonpurulent conjunctivitis;  lesions of the lips and oral mucosa (e.g., strawberry tongue,  edema of the lips, and erythema of the oropharynx); cervi¬  cal lymphadenopathy; a diffuse erythematous, maculo-  papular rash; and erythema and edema of the hands and  feet that often ends with desquamation.   The most characteristic clinical finding of KD is cardiac  involvement, especially myocarditis, arrhythmias, and  regurgitation involving the mitral or aortic valves. The  main cause of morbidity and mortality in KD is aneurysm  of the coronary arteries.   KD is much more common in children of Asian ances¬  try, leading to speculation that certain major histocompat¬  ibility complex (MHC) alleles may predispose to the  disease. It is a disease of children younger than 5 years of  age, often occurring in mini-outbreaks. It occurs world¬  wide but is much more common in Japan.   There is no definitive diagnostic laboratory test for KD.  Effective therapy consists of high-dose immune globulins  (IVIG) plus high-dose aspirin, which promptly reduce the  fever and other symptoms and, most importantly, signifi¬  cantly reduce the occurrence of aneurysms.   Two coagulase-negative staphylococci are common human  pathogens: S. epidermidis and S. saprophyticus. Staphylococ¬  cus epidermidis infections are almost always hospital-  acquired, whereas S. saprophyticus infections are almost  always community-acquired.   Staphylococcus epidermidis is part of the normal human  flora on the skin and mucous membranes but can enter the  bloodstream (bacteremia) and cause metastatic infections,  especially at the site of implants. It commonly infects intra¬  venous catheters and prosthetic implants (e.g., prosthetic  heart valves [endocarditis], vascular grafts, and prosthetic  joints [arthritis or osteomyelitis]) (see Table 15-2). Staphy¬  lococcus epidermidis is also a major cause of sepsis in neo¬  nates and of peritonitis in patients with renal failure who  are undergoing peritoneal dialysis through an indwelling  catheter. It is the most common bacterium to cause cere¬  brospinal fluid shunt infections.   Strains of S. epidermidis that produce a glycocalyx are  more likely to adhere to prosthetic implant materials and  therefore are more likely to infect these implants than  strains that do not produce a glycocalyx. Hospital person¬  nel are a major reservoir for antibiotic-resistant strains of  S. epidermidis.   Staphylococcus saprophyticus causes urinary tract infec¬  tions, particularly in sexually active young women. Most  women with this infection have had sexual intercourse  within the previous 24 hours. This organism is second to  Escherichia coli as a cause of community-acquired urinary  tract infections in young women.   Staphylococcus lugdenensis is a relatively uncommon  coagulase-negative staphylococcus that causes prosthetic  valve endocarditis and skin infections.   Smears from staphylococcal lesions reveal gram-positive  cocci in grapelike clusters (see Figure 15-5). Cultures of  S. aureus typically yield golden-yellow colonies that are  usually p-hemolytic. Staphylococcus aureus is coagulase-  positive (see Figure 15-6). Mannitol-salt agar is a com¬  monly used screening device for S. aureus. Staphylococcus  aureus ferments mannitol, which lowers the pH causing the  agar to turn yellow whereas S. epidermidis does not ferment  mannitol and the agar remains pink.   Cultures of coagulase-negative staphylococci typically yield  white colonies that are nonhemolytic. The two coagulase-  negative staphylococci are distinguished by their reaction  to the antibiotic novobiocin: S. epidermidis is sensitive,  whereas S. saprophyticus is resistant. There are no serologic  or skin tests used for the diagnosis of any acute staphylo¬  coccal infection.   In toxic shock syndrome, isolation of S. aureus is not  required to make a diagnosis as long as the clinical criteria  are met. Laboratory findings that support a diagnosis of    toxic shock syndrome include the isolation of a TSST-  producing strain of S. aureus and development of antibod¬  ies to the toxin during convalescence, although the latter is  not useful for diagnosis during the acute disease.   For epidemiologic purposes, S. aureus can be subdi¬  vided into subgroups based on the susceptibility of the  clinical isolate to lysis by a variety of bacteriophages. A  person carrying S. aureus of the same phage group as that  which caused the outbreak may be the source of the  infections.   In the United States, 90% or more of S. aureus strains are  resistant to penicillin G. Most of these strains produce  P-lactamase. Such organisms can be treated with  P-lactamase-resistant penicillins (e.g., nafcillin or cloxacil-  lin), some cephalosporins, or vancomycin. Treatment with  a combination of a P-lactamase-sensitive penicillin (e.g.,  amoxicillin) and a p-lactamase inhibitor (e.g., clavulanic  acid) is also useful.   Approximately 20% of S. aureus strains are methicillin-  resistant or nafcillin-resistant by virtue of altered penicillin¬  binding proteins. These resistant strains of S. aureus are  often abbreviated MRSA or NRSA, respectively. Such  organisms can produce sizable outbreaks of disease,  especially in hospitals. The drug of choice for these staphy¬  lococci is vancomycin, to which gentamicin is sometimes  added. Daptomycin is also useful. Trimethoprim-  sulfamethoxazole or clindamycin can be used to treat  non-life-threatening infections caused by these organisms.  Note that MRSA strains are resistant to almost all P-lactam  drugs, including both penicillins and cephalosporins. Cef-  taroline fosamil is the first p-lactam drug useful for the  treatment of MRSA infections.   Strains of S. aureus with intermediate resistance to van¬  comycin (VISA strains) and with complete resistance to  vancomycin (VRSA strains) have been isolated from  patients. These strains are typically methicillin-/nafcillin-  resistant as well, which makes them very difficult to treat.  Daptomycin (Cubicin) can be used to treat infections by  these organisms. Quinupristin-dalfopristin (Synercid) is  another useful choice.   The treatment of toxic shock syndrome involves correc¬  tion of the shock by using fluids, pressor drugs, and inotro¬  pic drugs; administration of a P-lactamase-resistant  penicillin such as nafcillin; and removal of the tampon or  debridement of the infected site as needed. Pooled serum  globulins, which contain antibodies against TSST, may be  useful.   Mupirocin is very effective as a topical antibiotic in skin  infections caused by S. aureus. It has also been used to  reduce nasal carriage of the organism in hospital personnel  and in patients with recurrent staphylococcal infections. A  topical skin antiseptic, such as chlorhexidine, can be added  to mupirocin.    Some strains of staphylococci exhibit tolerance (i.e.,  they can be inhibited by antibiotics but are not killed).  (That is, the ratio of minimum bactericidal concentration  [MBC] to minimum inhibitory concentration [MIC] is  very high.) Tolerance may result from failure of the drugs  to inactivate inhibitors of autolytic enzymes that degrade  the organism. Tolerant organisms should be treated with  drug combinations (see Chapter 10).   Drainage (spontaneous or surgical) is the cornerstone of  abscess treatment. Incision and drainage (I&D) is often  sufficient treatment for a skin abscess (e.g., furuncle [boil]);  antibiotics are not necessary in most cases. Previous infec¬  tion provides only partial immunity to reinfection.   Staphylococcus epidermidis is highly antibiotic resistant.  Most strains produce (3-lactamase but are sensitive to  (3-lactamase-resistant drugs such as nafcillin. These are  called methicillin-sensitive strains (MSSE). Some strains  are methicillin/nafcillin resistant (MRSE) due to altered  penicillin-binding proteins. The drug of choice is vanco¬  mycin, to which either rifampin or an aminoglycoside can  be added. Removal of the catheter or other device is often  necessary. Staphylococcus saprophyticus urinary tract infec¬  tions can be treated with trimethoprim-sulfamethoxazole  or a quinolone, such as ciprofloxacin.   There is no vaccine against staphylococci. Cleanliness, fre¬  quent handwashing, and aseptic management of lesions  help to control spread of S. aureus. Persistent colonization  of the nose by S. aureus can be reduced by intranasal mupi-  rocin or by oral antibiotics, such as ciprofloxacin or  trimethoprim-sulfamethoxazole, but is difficult to elimi¬  nate completely. Shedders may have to be removed from  high-risk areas (e.g., operating rooms and newborn nurser¬  ies). Cefazolin is often used perioperatively to prevent  staphylococcal surgical-wound infections.   Streptococci of medical importance are listed in Table  15-3. All but one of these streptococci are discussed in this  section; S. pneumoniae is discussed separately at the end of  this chapter because it is so important.   Streptococci cause a wide variety of infections. Streptococcus  pyogenes (group A streptococcus) is the leading bacterial cause  of pharyngitis (Figure 15-8) and cellulitis (Figure 15-9). It is  an important cause of impetigo (see Figure 15-3), necrotizing  fasciitis, and streptococcal toxic shock syndrome. It is also the  inciting factor of two important immunologic diseases,  namely, rheumatic fever and acute glomerulonephritis. Strep¬  tococcus agalactiae (group B streptococcus) is the leading  cause of neonatal sepsis and meningitis. Enterococcus faecalis  is an important cause of hospital-acquired urinary tract infec¬  tions and endocarditis. Viridans group streptococci are the  most common cause of endocarditis (Figure 15-10). Strepto¬  coccus bovis (also known as Streptococcus gallolyticus) is an  uncommon cause of endocarditis.   Streptococci are spherical gram-positive cocci arranged in  chains or pairs (Figure 15-11). All streptococci are catalase¬  negative, whereas staphylococci are catalase-positive (see  Table 15-3).   One of the most important characteristics for identifica¬  tion of streptococci is the type of hemolysis (Figure 15-12).   (1) a-Hemolytic streptococci form a green zone around  their colonies as a result of incomplete lysis of red blood  cells in the agar. The green color is formed when hydrogen  peroxide produced by the bacteria oxidizes hemoglobin  (red color) to biliverdin (green color).   2 Both E. faecalis and S. bovis grow on bile-esculin agar, whereas other streptococci do not. They hydrolyze the esculin, and this results in a characteristic black discoloration of  the agar.   Viridans group streptococci include several species, such as S. sanguinis, S. mutans, S. mitis, S. gordonii , 5. salivarius, S. anginosus, S. milled, and 5. intermedius.   FIGURE 15~8 Pharyngitis. Note erythema of soft palate, uvula,  and posterior pharynx and swelling of the uvula. The most common  bacterial cause of pharyngitis is Streptococcus pyogenes. Note: The  curved white lines on the uvula and the palate are artifacts of pho¬  tography. (Source: Centers for Disease Control and Prevention. CDC #6323.)   FIGURE 15-10 Endocarditis. Note vegetations (black arrows)  on mitral valve. Viridans streptococci are the most common cause of  subacute bacterial endocarditis. (Reproduced with permission from Longo DL  et al (eds): Harrison's Principles of Internal Medicine. 18th ed. New York: McGraw-Hill,  2012, pg 1052. Copyright © 2012 by The McGraw-Hill Companies, Inc.)    (2) P-Hemolytic streptococci form a clear zone around  their colonies because complete lysis of the red cells occurs.  P-Hemolysis is due to the production of enzymes (hemoly¬  sins) called streptolysin O and streptolysin S (see “Patho¬  genesis” later).   (1) C carbohydrate determines the group of P-hemolytic  streptococci. It is located in the cell wall, and its specificity    is determined by an amino sugar. For example, Group A  P-hemolytic streptococci (S. pyogenes) is distinguished  from Group B P-hemolytic streptococci (S. agalactiae )  because it has a different C carbohydrate.   (2) M protein is the most important virulence factor of  S. pyogenes. It protrudes from the outer surface of the cell  and blocks phagocytosis (i.e., it is antiphagocytic). It inac¬  tivates C3b, a component of complement that opsonizes  the bacteria prior to phagocytosis (see Chapter 63). Strains  of S. pyogenes that do not produce M protein are  nonpathogenic.   FIGURE 15-9 Cellulitis. Note erythema and swelling of the dor¬  sum of the foot. Streptococcus pyogenes is the most common cause of  Cellulitis. (Reproduced with permission from Usatine, RP et al: The Color Atlas of Family  Medicine, New York: McGraw-Hill, 2009. Courtesy of Richard P. Usatine, MD.)   points to a long chain of gram-positive cocci. (Used with permission from  Professor Shirley Lowe, University of California, San Francisco School of Medicine.)   FIGURE 15-12 a-Hemolysis and p-hemolysis on blood agar—  Short arrow points to an a-hemolytic colony, probably a viridans  group streptococcus. Long arrow points to a p-hemolytic colony,  probably Streptococcus pyogenes. The specimen was a throat swab  taken from a person with a sore throat. (Used with permission from Profes-  sor Shirley Lowe, University of California, San Francisco School of Medicine.)    M protein also determines the type of group A  p-hemolytic streptococci. There are approximately 100  serotypes based on the M protein, which explains why mul¬  tiple infections with S. pyogenes can occur. Antibody to M  protein provides type-specific immunity.   Strains of S. pyogenes that produce certain M protein  types are rheumatogenic (i.e., cause primarily rheumatic  fever), whereas strains of S. pyogenes that produce other M  protein types are nephritogenic (i.e., cause primarily acute  glomerulonephritis). Although M protein is the main anti¬  phagocytic component of S. pyogenes, the organism also  has a polysaccharide capsule that plays a role in retarding  phagocytosis.   These are arranged into groups A-U (known as Lancefield  groups) on the basis of antigenic differences in C carbohy¬  drate. In the clinical laboratory, the group is determined by  precipitin tests with specific antisera or by  immunofluorescence.   Group A streptococci (S. pyogenes) are one of the most  important human pathogens. They are the most frequent  bacterial cause of pharyngitis and a very common cause of  skin infections. They adhere to pharyngeal epithelium via  pili composed of lipoteichoic acid and M protein. Many  strains have a hyaluronic acid capsule that is antiphago¬  cytic. The growth of S. pyogenes on agar plates in the labo¬  ratory is inhibited by the antibiotic bacitracin, an important  diagnostic criterion (Figure 15-13).    Group B streptococci (S. agalactiae) colonize the genital  tract of some women and can cause neonatal meningitis  and sepsis. They are usually bacitracin-resistant. They  hydrolyze (break down) hippurate, an important diagnostic  criterion.   Group D streptococci include enterococci (e.g., E.faeca-  lis and Enterococcus faecium) and nonenterococci (e.g.,  S. bovis). Enterococci are members of the normal flora of  the colon and are noted for their ability to cause urinary,  biliary, and cardiovascular infections. They are very hardy  organisms; they can grow in hypertonic (6.5%) saline or in  bile and are not killed by penicillin G. As a result, a syner¬  gistic combination of penicillin and an aminoglycoside  (e.g., gentamicin) is required to kill enterococci. Vancomy¬  cin can also be used, but vancomycin-resistant enterococci  (VRE) have emerged and become an important and much  feared cause of life-threatening nosocomial infections.  More strains of E. faecium are vancomycin-resistant than  are strains of E. faecalis.   Nonenterococcal group D streptococci, such as S. bovis,  can cause similar infections but are much less hardy organ¬  isms (e.g., they are inhibited by 6.5% NaCl and killed by  penicillin G). Note that the hemolytic reaction of group D  streptococci is variable: most are a-hemolytic, but some are  p-hemolytic, and others are nonhemolytic.   Groups C, E, F, G, H, and K-U streptococci infrequently  cause human disease.   bition of growth of group A streptococci ( Streptococcus pyogenes)  caused by bacitracin that has diffused from the disk labeled A. Upper  half of blood agar plate shows |3-hemolysis caused by group A strep¬  tococci, except in the region around the bacitracin disk. Lower half of  blood agar plate shows p-hemolysis caused by group B streptococci  (.Streptococcus agalactiae), and there is no zone of inhibition around  the bacitracin disk. (Used with permission from Professor Shirley Lowe, Univer-  sity of California, San Francisco School of Medicine.)   Some streptococci produce no hemolysis; others produce  a-hemolysis. The principal a-hemolytic organisms are  S. pneumoniae (pneumococci) and the viridans group of  streptococci (e.g., Streptococcus mitis, Streptococcus sangui¬  nis, and Streptococcus mutans). Pneumococci and viridans  streptococci are distinguished in the clinical laboratory by  two main criteria: (1) the growth of pneumococci is inhib¬  ited by optochin, whereas the growth of viridans strepto¬  cocci is not inhibited; and (2) colonies of pneumococci  dissolve when exposed to bile (bile-soluble), whereas colo¬  nies of viridans streptococci do not dissolve.   Viridans streptococci are part of the normal flora of the  human pharynx and intermittently reach the bloodstream  to cause infective endocarditis. Streptococcus mutans syn¬  thesizes polysaccharides (dextrans) that are found in dental  plaque and lead to dental caries. Streptococcus intermedins  and Streptococcus anginosus (also known as the S. anginosus-  milleri group) are usually a-hemolytic or nonhemolytic,  but some isolates are (3-hemolytic. They are found primar¬  ily in the mouth and colon.   These grow under anaerobic or microaerophilic conditions  and produce variable hemolysis. Peptostreptococci are  members of the normal flora of the gut, mouth, and female  genital tract and participate in mixed anaerobic infections.  The term mixed anaerobic infections refers to the fact that  these infections are caused by multiple bacteria, some of  which are anaerobes and others are facultatives. For exam¬  ple, peptostreptococci and viridans streptococci, both    members of the oral flora, are often found in brain  abscesses following dental surgery. Peptostreptococcus mag-  nus and Peptostreptococcus anaerobius are the species fre¬  quently isolated from clinical specimens.   Most streptococci are part of the normal flora of the human  throat, skin, and intestines but produce disease when they  gain access to tissues or blood. Viridans streptococci and  S. pneumoniae are found chiefly in the oropharynx; S. pyo¬  genes is found on the skin and in the oropharynx in small  numbers; S. agalactiae occurs in the vagina and colon; and  both the enterococci and anaerobic streptococci are located  in the colon.   Group A streptococci (S. pyogenes) cause disease by three  mechanisms: (1) pyogenic inflammation, which is induced  locally at the site of the organisms in tissue; (2) exotoxin  production, which can cause widespread systemic symp¬  toms in areas of the body where there are no organisms;  and (3) immunologic, which occurs when antibody against  a component of the organism cross-reacts with normal tis¬  sue or forms immune complexes that damage normal tissue  (see the section on poststreptococcal diseases later in the  chapter). The immunologic reactions cause inflammation  (e.g., the inflamed joints of rheumatic fever), but there are  no organisms in the lesions (Table 15-4).   The M protein of S. pyogenes is its most important anti¬  phagocytic factor, but its capsule, composed of hyaluronic   Main Site of Disease (D),  Colonization (C), or Normal  Flora (NF)    acid, is also antiphagocytic. Antibodies are not formed  against the capsule because hyaluronic acid is a normal  component of the body and humans are tolerant to it.   (1) Hyaluronidase degrades hyaluronic acid, which is  the ground substance of subcutaneous tissue. Hyaluroni¬  dase is known as spreading factor because it facilitates the  rapid spread of S. pyogenes in skin infections (cellulitis).   (2) Streptokinase (fibrinolysin) activates plasminogen  to form plasmin, which dissolves fibrin in clots, thrombi,  and emboli. It can be used to lyse thrombi in the coronary  arteries of heart attack patients.   (3) DNase (streptodornase) degrades DNA in exudates  or necrotic tissue. Antibody to DNase B develops during  pyoderma; this can be used for diagnostic purposes.  Streptokinase-streptodornase mixtures applied as a skin  test give a positive reaction in most adults, indicating nor¬  mal cell-mediated immunity.   In addition, group A streptococci produce five impor¬  tant toxins and hemolysins:   (1) Erythrogenic toxin causes the rash of scarlet fever.  Its mechanism of action is similar to that of the TSST of S.  aureus (i.e., it acts as a superantigen; see S. aureus, earlier,  and Chapter 58). It is produced only by certain strains of S.  pyogenes lysogenized by a bacteriophage carrying the gene  for the toxin. The injection of a skin test dose of erythro¬  genic toxin (Dick test) gives a positive result in persons  lacking antitoxin (i.e., susceptible persons).   (2) Streptolysin O is a hemolysin that is inactivated by  oxidation (oxygen-labile). It causes (3-hemolysis only when  colonies grow under the surface of a blood agar plate. It is  antigenic, and antibody to it (ASO) develops after group A  streptococcal infections. The titer of ASO antibody can be  important in the diagnosis of rheumatic fever.   (3) Streptolysin S is a hemolysin that is not inactivated  by oxygen (oxygen-stable). It is not antigenic but is respon¬  sible for (3-hemolysis when colonies grow on the surface of  a blood agar plate.   (4) Pyrogenic exotoxin A is the toxin responsible for  most cases of streptococcal toxic shock syndrome. It has  the same mode of action as does staphylococcal TSST (i.e.,  it is a superantigen that causes the release of large amounts  of cytokines from helper T cells and macrophages; see  pages 43 and 510).   (5) Exotoxin B is a protease that rapidly destroys tis¬  sue and is produced in large amounts by the strains of  S. pyogenes, the so-called “flesh-eating” streptococci that  cause necrotizing fasciitis.   Pathogenesis by group B streptococci (S. agalactiae ) is  based on the ability of the organism to induce an inflam¬  matory response. However, unlike S. pyogenes, no cytotoxic    enzymes or exotoxins have been described, and there is no  evidence for any immunologically induced disease. Group  B streptococci have a polysaccharide capsule that is anti¬  phagocytic, and anticapsular antibody is protective.   Pathogenesis by S. pneumoniae and the viridans strepto¬  cocci is uncertain, as no exotoxins or tissue-destructive  enzymes have been demonstrated. The main virulence fac¬  tor of S. pneumoniae is its antiphagocytic polysaccharide  capsule. Many of the strains of viridans streptococci that  cause endocarditis produce a glycocalyx that enables the  organism to adhere to the heart valve.   Streptococcus pyogenes causes three types of diseases: (1)  pyogenic diseases such as pharyngitis and cellulitis, (2)  toxigenic diseases such as scarlet fever and toxic shock  syndrome, and (3) immunologic diseases such as rheu¬  matic fever and acute glomerulonephritis (AGN). (See next  section on poststreptococcal diseases.)   Streptococcus pyogenes (group A streptococcus) is the  most common bacterial cause of pharyngitis (sore throat).  Streptococcal pharyngitis (strep throat) is characterized by  throat pain and fever. On examination, an inflamed throat  and tonsils, often with a yellowish exudate, are found,  accompanied by tender cervical lymph nodes. If untreated,  spontaneous recovery often occurs in 10 days, but rheu¬  matic fever may occur (see next section on poststreptococ¬  cal diseases). Untreated pharyngitis may extend to the  middle ear (otitis media), the sinuses (sinusitis), the mas-  toids (mastoiditis), or the meninges (meningitis). Continu¬  ing inability to swallow may indicate a peritonsillar or  retropharyngeal abscess.   If the infecting streptococci produce erythrogenic toxin  and the host lacks antitoxin, scarlet fever may result. A  “strawberry” tongue is a characteristic lesion seen in scarlet  fever. Streptococcus pyogenes also causes another toxin-  mediated disease, streptococcal toxic shock syndrome,  which has clinical findings similar to those of staphylococ¬  cal toxic shock syndrome (see page 114). However, strepto¬  coccal toxic shock syndrome typically has a recognizable  site of pyogenic inflammation and blood cultures are often  positive, whereas staphylococcal toxic shock syndrome  typically has neither a site of pyogenic inflammation nor  positive blood cultures.   Group A streptococci cause skin and soft tissue  infections, such as cellulitis, erysipelas (Figure 15-14),  necrotizing fasciitis (streptococcal gangrene), and impe¬  tigo (see Figure 15-3). Impetigo, a form of pyoderma, is  a superficial skin infection characterized by “honey-  colored” crusted lesions. Lymphangitis can occur, espe¬  cially on the forearm associated with an infection on the  hand.   Group A streptococci also cause endometritis (puer¬  peral fever), a serious infection of pregnant women, and    sepsis. Immune-mediated poststreptococcal AGN can also  occur, especially following skin infections caused by certain  M protein types of S. pyogenes.   Group B streptococci cause neonatal sepsis and menin¬  gitis. The main predisposing factor is prolonged (longer  than 18 hours) rupture of the membranes in women who  are colonized with the organism. Children born prior to  37 weeks’ gestation have a greatly increased risk of disease.  Also, children whose mothers lack antibody to group B  streptococci and who consequently are born without trans-  placentally acquired IgG have a high rate of neonatal sepsis  caused by this organism. Group B streptococci are an  important cause of neonatal pneumonia as well.   Although most group B streptococcal infections are in  neonates, this organism also causes such infections as  pneumonia, endocarditis, arthritis, cellulitis, and osteomy¬  elitis in adults. Postpartum endometritis also occurs. Dia¬  betes is the main predisposing factor for adult group B  streptococcal infections.   Viridans streptococci (e.g., S. mutans, S. sanguinis, S.  salivarius, and S. mitis) are the most common cause of  infective endocarditis. They enter the bloodstream (bacte¬  remia) from the oropharynx, typically after dental surgery.  Signs of endocarditis are fever, heart murmur, anemia, and  embolic events such as splinter hemorrhages, subconjunc¬  tival petechial hemorrhages, and Janeway lesions. The  heart murmur is caused by vegetations on the heart valve   the inflamed area. Streptococcus pyogenes is the most common cause  of erysipelas. (Reproduced with permission from Longo DL et al (eds): Harrison's  Principles of Internal Medicine. 18th ed. New York: McGraw-Hill, 2012. Copyright ©  2012 by The McGraw-Hill Companies, Inc.)    (see Figure 15-10). It is 100% fatal unless effectively treated  with antimicrobial agents. About 10% of endocarditis cases  are caused by enterococci, but any organism causing bacte¬  remia may settle on deformed valves. At least three blood  cultures are necessary to ensure recovery of the organism in  more than 90% of cases.   Viridans streptococci, especially 5. anginosus, S. milleri,  and S. intermedins, also cause brain abscesses, often in  combination with mouth anaerobes (a mixed aerobic-  anaerobic infection). Dental surgery is an important pre¬  disposing factor to brain abscess because it provides a  portal for the viridans streptococci and the anaerobes in  the mouth to enter the bloodstream (bacteremia) and  spread to the brain. Viridans streptococci are involved in  mixed aerobic-anaerobic infections in other areas of the  body as well (e.g., lung abscesses and abdominal abscesses,  including liver abscesses).   Enterococci cause urinary tract infections, especially  in hospitalized patients. Indwelling urinary catheters and  urinary tract instrumentation are important predisposing  factors. Enterococci also cause endocarditis, particularly in  patients who have undergone gastrointestinal or urinary  tract surgery or instrumentation. They also cause intra¬  abdominal and pelvic infections, typically in combination  with anaerobes. Streptococuss bovis, a nonenterococcal  group D streptococcus, causes endocarditis, especially in  patients with carcinoma of the colon. This association is so  strong that patients with S. bovis, bacteremia, or endocardi¬  tis should be investigated for the presence of colonic  carcinoma.   Peptostreptococci are one of the most common bacteria  found in brain, lung, abdominal, and pelvic abscesses.   These are disorders in which a local infection with group A  streptococci is followed weeks later by inflammation in an  organ that was not infected by the streptococci. The inflam¬  mation is caused by an immunologic (antibody) response  to streptococcal M proteins that cross-react with human  tissues. Some strains of S. pyogenes bearing certain M pro¬  teins are nephritogenic and cause AGN, and other strains  bearing different M proteins are rheumatogenic and cause  acute rheumatic fever. Note that these diseases appear sev¬  eral weeks after the actual infection because that is the  length of time it takes to produce sufficient antibodies.   AGN typically occurs 2 to 3 weeks after skin infection by  certain group A streptococcal types in children (e.g., M  protein type 49 causes AGN most frequently). AGN is more  frequent after skin infections than after pharyngitis. The  most striking clinical features are hypertension, edema of  the face (especially periorbital edema) and ankles, and    “smoky” urine (due to red cells in the urine). Most patients  recover completely. Reinfection with streptococci rarely  leads to recurrence of glomerulonephritis.   The disease is initiated by antigen-antibody complexes  on the glomerular basement membrane. Complement is  activated and C5a attracts neutrophils that secrete enzymes  that damage the endothelium of the glomerular capillaries.  It can be prevented by early eradication of nephritogenic  streptococci from skin colonization sites but not by admin¬  istration of penicillin after the onset of symptoms.   Approximately 2 weeks after a group A streptococcal  infection—usually pharyngitis—rheumatic fever, charac¬  terized by fever, migratory polyarthritis, and carditis, may  develop. The carditis damages myocardial and endocardial  tissue, especially the mitral and aortic valves, resulting in  vegetations on the valves. Uncontrollable, spasmodic move¬  ments of the limbs or face (chorea) may also occur. ASO  titers and the erythrocyte sedimentation rate are elevated.  Note that group A streptococcal skin infections do not  cause rheumatic fever. Most cases of pharyngitis caused by  group A streptococci occur in children age 5 to 15 years,  and hence rheumatic fever occurs in that age group.   Rheumatic fever is due to an immunologic cross¬  reaction between antibodies formed against M proteins of  S. pyogenes and proteins on the surface of joint, heart and  brain tissue. It is an autoimmune disease, greatly exacer¬  bated by recurrence of streptococcal infections. If strepto¬  coccal infections are treated within 8 days of onset, rheumatic  fever is usually prevented. After a heart-damaging attack of  rheumatic fever, reinfection must be prevented by long¬  term prophylaxis. In the United States, fewer than 0.5% of  group A streptococcal infections lead to rheumatic fever,  but in developing tropical countries, the rate is higher  than 5%.   Gram-stained smears are useless in streptococcal pharyngi¬  tis because viridans streptococci are members of the nor¬  mal flora and cannot be visually distinguished from the  pathogenic S. pyogenes. However, stained smears from skin  lesions or wounds that reveal streptococci are diagnostic.  Cultures of swabs from the pharynx or lesion on blood agar  plates show small, translucent (3-hemolytic colonies in 18 to  48 hours. If inhibited by bacitracin disk, they are likely to  be group A streptococci (see Figure 15-13).   Group B streptococci are characterized by their ability  to hydrolyze hippurate and by the production of a protein  that causes enhanced hemolysis on sheep blood agar when  combined with fl-hemolysin of S. aureus (CAMP test).  Group D streptococci hydrolyze esculin in the presence of  bile (i.e., they produce a black pigment on bile-esculin  agar). The group D organisms are further subdivided: the    enterococci grow in hypertonic (6.5%) NaCl, whereas the  nonenterococci do not.   Although cultures remain the gold standard for the  diagnosis of streptococcal pharyngitis, a problem exists  because the results of culturing are not available for at least  18 hours, and it is beneficial to know while the patient is in  the office whether antibiotics should be prescribed. For this  reason, rapid tests that provide a diagnosis in approxi¬  mately 10 minutes were developed.   The rapid test detects bacterial antigens in a throat swab  specimen. In the test, specific antigens from the group A  streptococci are extracted from the throat swab with cer¬  tain enzymes and are reacted with antibody to these anti¬  gens bound to latex particles. Agglutination of the colored  latex particles occurs if group A streptococci are present in  the throat swab. The specificity of these tests is high, but the  sensitivity is low (i.e., false-negative results can occur). If  the test result is negative but the clinical suspicion of strep¬  tococcal pharyngitis is high, a culture should be done.   A rapid test is also available for the detection of group B  streptococci in vaginal and rectal samples. It detects the  DNA of the organism, and results can be obtained in  approximately 1 hour.   Viridans group streptococci form a-hemolytic colonies  on blood agar and must be distinguished from S. pneumoniae  (pneumococci), which is also a-hemolytic. Viridans group  streptococci are resistant to lysis by bile and will grow in the  presence of optochin, whereas pneumococci will not. The  various viridans group streptococci are classified into spe¬  cies by using a variety of biochemical tests.   ASO titers are high soon after group A streptococcal  infections. In patients suspected of having rheumatic  fever, an elevated ASO titer is typically used as evidence  of previous infection because throat culture results are  often negative at the time the patient presents with rheu¬  matic fever. Titers of anti-DNase B are high in group A  streptococcal skin infections and serve as an indicator of  previous streptococcal infection in patients suspected of  having AGN.   Group A streptococcal infections can be treated with either  penicillin G or amoxicillin, but neither rheumatic fever nor  AGN patients benefit from penicillin treatment after the  onset of the two diseases. In mild group A streptococcal  infections, oral penicillin V can be used. In penicillin-  allergic patients, erythromycin or one of its long-acting  derivatives (e.g., azithromycin) can be used. However,  erythromycin-resistant strains of S. pyogenes have emerged  that may limit the effectiveness of the macrolide class of  drugs in the treatment of streptococcal pharyngitis.  Clindamycin can also be used in penicillin-allergic patients.  Streptococcal pyogenes is not resistant to penicillins.    Endocarditis caused by most viridans streptococci is  curable by prolonged penicillin treatment. However,  enterococcal endocarditis can be eradicated only by a peni¬  cillin or vancomycin combined with an aminoglycoside.   Enterococci resistant to multiple drugs (e.g., penicillins,  aminoglycosides, and vancomycin) have emerged. Resis¬  tance to vancomycin in enterococci is mediated by a cas¬  sette of genes that encode the enzymes that substitute  D-lactate for D-alanine in the peptidoglycan. The same set  of genes encodes vancomycin resistance in S. aureus.   VREs are now an important cause of nosocomial infec¬  tions; there is no reliable antibiotic therapy for these organ¬  isms. At present, two drugs are being used to treat infections  caused by VRE: linezolid (Zyvox) and daptomycin  (Cubicin).   Nonenterococcal group D streptococci (e.g., S. bovis)  are not highly resistant and can be treated with penicillin G.   The drug of choice for group B streptococcal infections  is either penicillin G or ampicillin. Some strains may  require higher doses of penicillin G or a combination of  penicillin G and an aminoglycoside to eradicate the organ¬  ism. Peptostreptococci can be treated with penicillin G.   Rheumatic fever can be prevented by prompt treatment of  group A streptococcal pharyngitis with penicillin G or oral  penicillin V. Prevention of streptococcal infections (usually  with benzathine penicillin once each month for several  years) in persons who have had rheumatic fever is impor¬  tant to prevent recurrence of the disease. There is no evi¬  dence that patients who have had AGN require similar  penicillin prophylaxis.   In patients with damaged heart valves who undergo  invasive dental procedures, endocarditis caused by viridans  streptococci can be prevented by using amoxicillin periop-  eratively. To avoid unnecessary use of antibiotics, it is rec¬  ommended to give amoxicillin prophylaxis only to those  patients who have the highest risk of severe consequences  from endocarditis (e.g., those with prosthetic heart valves  or with previous infective endocarditis) and who are  undergoing high-risk dental procedures, such as manipula¬  tion of gingival tissue. It is no longer recommended that  patients undergoing gastrointestinal or genitourinary tract  procedures receive prophylaxis.   The incidence of neonatal sepsis caused by group B  streptococci can be reduced by a two-pronged approach:  (1) All pregnant women at 35 to 37 weeks’ gestation should  be screened by doing vaginal and rectal cultures. If cultures  are positive, then penicillin G (or ampicillin) should be  administered intravenously at the time of delivery. (2) If the  patient has not had cultures done, then penicillin G (or  ampicillin) should be administered intravenously at the  time of delivery to women who experience prolonged (lon¬  ger than 18 hours) rupture of membranes, whose labor  begins before 37 weeks’ gestation, or who have a fever at the    time of labor. If the patient is allergic to penicillin, either  cefazolin or vancomycin can be used.   Oral ampicillin given to women who are vaginal carriers  of group B streptococci does not eradicate the organism.  Rapid screening tests for group B streptococcal antigens in  vaginal specimens can be insensitive, and neonates born of  antigen-negative women have, nevertheless, had neonatal  sepsis. Note, however, that as group B streptococcal infec¬  tions have declined as a result of these prophylactic mea¬  sures, neonatal infections caused by E. coli have increased.   There are no vaccines available against any of the strep¬  tococci except S. pneumoniae (see following section).   Streptococcus pneumoniae causes pneumonia, bacteremia,  meningitis, and infections of the upper respiratory tract  such as otitis media, mastoiditis, and sinusitis. Pneumo¬  cocci are the most common cause of community-acquired  pneumonia, meningitis, sepsis in splenectomized indi¬  viduals, otitis media, and sinusitis. They are a common  cause of conjunctivitis, especially in children. Note that  S. pneumoniae is also known as the pneumococcus (plural,  pneumococci).   Pneumococci are gram-positive lancet-shaped cocci  arranged in pairs (diplococci) or short chains (Figure  15-15). (The term lancet-shaped means that the diplococci  are oval with somewhat pointed ends rather than being  round.) On blood agar, they produce a-hemolysis. In con¬  trast to viridans streptococci, they are lysed by bile or  deoxycholate, and their growth is inhibited by optochin  (Figure 15-16).   Pneumococci possess polysaccharide capsules of more  than 85 antigenically distinct types. With type-specific  antiserum, capsules swell (quellung reaction), and this can  be used to identify the type. Capsules are virulence factors  (i.e., they interfere with phagocytosis and favor invasive¬  ness). Specific antibody to the capsule opsonizes the organ¬  ism, facilitates phagocytosis, and promotes resistance. Such  antibody develops in humans as a result either of infection  (asymptomatic or clinical) or of administration of polysac¬  charide vaccine. Capsular polysaccharide elicits primarily a  (3-cell (i.e., T-independent) response.   Another important surface component of S. pneumoniae  is a teichoic acid in the cell wall called C-substance (also  known as C-polysaccharide). It is medically important not  for itself, but because it reacts with a normal serum protein  made by the liver called C-reactive protein (CRP). CRP is  an “acute-phase” protein that is elevated as much as 1000-  fold in acute inflammation. CRP is not an antibody (which  are y-globulins) but rather a (3-globulin. (Plasma contains   Arrows point to typical gram-positive diplococci. Note that the clear  area around the organism is the capsule. (Used with permission from Pro-  fessor Shirley Lowe, University of California, San Francisco School of Medicine.)    a-, (3-, and y-globulins.) Note that CRP is a nonspecific  indicator of inflammation and is elevated in response to  the presence of many organisms, not just S. pneumoniae.  Clinically, CRP in human serum is measured in the laboratory  by its reaction with the carbohydrate of S. pneumoniae. The  medical importance of CRP is that an elevated CRP appears   FIGURE 15-16 Optochin test—Arrow points to zone of inhi¬  bition of growth of Streptococcus pneumoniae caused by optochin  that has diffused from the disk labeled P. In the lower half of the  blood agar plate, there is a-hemolysis caused by S. pneumoniae,  except in the region around the optochin disk.The arrow points to  the outer limit of the zone of inhibition. Upper half of blood agar  plate shows a-hemolysis caused by a viridans streptococcus, and  there is no zone of inhibition around the optochin disk. (Used with per-  mission from Professor Shirley Lowe, University of California, San Francisco School of  Medicine.)    to be a better predictor of heart attack risk than an elevated  cholesterol level.   Humans are the natural hosts for pneumococci; there is no  animal reservoir. Because a proportion (5%—50%) of the  healthy population harbors virulent organisms in the oro¬  pharynx, pneumococcal infections are not considered to be  communicable. Resistance is high in healthy young people,  and disease results most often when predisposing factors  (see following discussion) are present.   The most important virulence factor is the capsular poly¬  saccharide, and anticapsular antibody is protective. Lipo-  teichoic acid, which activates complement and induces  inflammatory cytokine production, contributes to the  inflammatory response and to the septic shock syndrome  that occurs in some immunocompromised patients. Pneu-  molysin, the hemolysin that causes a-hemolysis, may also  contribute to pathogenesis.   Pneumococci produce IgA protease that enhances the  organism’s ability to colonize the mucosa of the upper  respiratory tract by cleaving IgA. Pneumococci multiply in  tissues and cause inflammation. When they reach alveoli,  there is outpouring of fluid and red and white blood cells,  resulting in consolidation of the lung. During recovery  pneumococci are phagocytized, mononuclear cells ingest  debris, and the consolidation resolves.   Factors that lower resistance and predispose persons to  pneumococcal infection include (1) alcohol or drug intoxi¬  cation or other cerebral impairment that can depress the  cough reflex and increase aspiration of secretions; (2)  abnormality of the respiratory tract (e.g., viral infections),  pooling of mucus, bronchial obstruction, and respiratory  tract injury caused by irritants (which disturb the integrity  and movement of the mucociliary blanket); (3) abnormal  circulatory dynamics (e.g., pulmonary congestion and heart  failure); (4) splenectomy; and (5) certain chronic diseases  such as sickle cell anemia and nephrosis. Patients with sickle  cell anemia auto-infarct their spleen, become functionally  asplenic, and are predisposed to pneumococcal sepsis.  Trauma to the head that causes leakage of spinal fluid  through the nose predisposes to pneumococcal meningitis.   Pneumonia often begins with a sudden chill, fever, cough,  and pleuritic pain. Sputum is a red or brown “rusty” color.  Bacteremia occurs in 15% to 25% of cases. Spontaneous  recovery may begin in 5 to 10 days and is accompanied by  development of anticapsular antibodies. Pneumococci are a  prominent cause of otitis media, sinusitis, mastoiditis, con¬  junctivitis, purulent bronchitis, pericarditis, bacterial men¬  ingitis, and sepsis. Pneumococci are the leading cause of  sepsis in patients without a functional spleen.   In sputum, pneumococci are seen as lancet-shaped gram¬  positive diplococci in Gram-stained smears (see Figure  15-15). They can also be detected by using the quellung  reaction with multitype antiserum. On blood agar, pneu¬  mococci form small a-hemolytic colonies. The colonies  are bile-soluble (i.e., are lysed by bile), and growth is  inhibited by optochin (see Figure 15-16).   Blood cultures are positive in 15% to 25% of pneumo¬  coccal infections. Culture of cerebrospinal fluid is usually  positive in meningitis. Rapid diagnosis of pneumococcal  meningitis can be made by detecting its capsular polysac¬  charide in spinal fluid using the latex agglutination test. A  rapid test that detects urinary antigen is also available for  the diagnosis of pneumococcal pneumonia and bactere¬  mia. The urinary antigen is the C polysaccharide (also  known as the C substance), not the capsular polysaccha¬  ride. Because of the increasing numbers of strains resistant  to penicillin, antibiotic sensitivity tests must be done on  organisms isolated from serious infections.   Most pneumococci are susceptible to penicillins and eryth¬  romycin, although significant resistance to penicillins has  emerged (see next paragraph). In severe pneumococcal  infections, penicillin G is the drug of choice, whereas in  mild pneumococcal infections, oral penicillin V can be  used. A fluoroquinolone with good antipneumococcal  activity, such as levofloxacin, can also be used. In penicillin-  allergic patients, erythromycin or one of its long-acting  derivatives (e.g., azithromycin) can be used.   In the United States, about 25% of isolates exhibit low-  level resistance to penicillin, primarily as a result of changes  in penicillin-binding proteins. An increasing percentage of  isolates, ranging from 15% to 35% depending on location,  show high-level resistance, which is attributed to multiple  changes in penicillin-binding proteins. They do not pro¬  duce (3-lactamase. Vancomycin is the drug of choice for the  penicillin-resistant pneumococci, especially for severely ill  patients. Ceftriaxone or levofloxacin can be used for less  severely ill patients. However, strains of pneumococci toler¬  ant to vancomycin have emerged. (Tolerance to antibiotics  is described on pages 71 and 90.) Strains of pneumococci  resistant to multiple drugs have also emerged.   Despite the efficacy of antimicrobial drug treatment, the  mortality rate of pneumococcal infections is high in immu¬  nocompromised (especially splenectomized) patients and  children under the age of 5 years. Such persons should be  immunized with the 13-valent pneumococcal conjugate  vaccine (Prevnar 13). The immunogen in this vaccine is the  pneumococcal polysaccharide of the 13 most prevalent  serotypes conjugated (coupled) to a carrier    protein (diphtheria toxoid). The unconjugated 23-valent  pneumococcal vaccine (Pneumovax 23) should be given to  healthy individuals age 50 years or older.   These vaccines are safe and effective and provide long-  lasting (at least 5 years) protection. Immunization of chil¬  dren reduces the incidence of pneumococcal disease in  adults because children are the main source of the organ¬  ism for adults and immunization reduces the carrier rate in  children.   A booster dose is recommended for (1) people older  than 65 years who received the vaccine more than 5 years  ago and who were younger than 65 years when they  received the vaccine, and (2) people between the ages of 2  and 64 years who are asplenic, infected with human immu¬  nodeficiency virus (HIV), receiving cancer chemotherapy,  or receiving immunosuppressive drugs to prevent trans¬  plant rejection.   A potential problem regarding the use of the pneumo¬  coccal vaccine is that of serotype replacement. Will the  vaccine reduce the incidence of disease caused by the sero¬  types in the vaccine but not the overall incidence of pneu¬  mococcal disease because other serotypes that are not in  the vaccine will now cause disease? In fact, an increase in  invasive pneumococcal disease caused by serotype 19A,  which was not in the previously used 7-valent vaccine,  occurred. This led to the production of the current conju¬  gate vaccine containing 13 serotypes, including 19A.   1 . You’re in the clinical laboratory looking at a Gram stain when the  laboratory technician comes up to you and says, “I think your  patient has Staph epi [short for Staphylococcus epidermidis] bac¬  teremia.” Which one of the following sets of results did the tech  find with the organism recovered from the blood culture?   2. Superantigen production by Staphylococcus aureus is involved in  the pathogenesis of which one of the following diseases?   3. Which one of the following is the virulence factor produced by  Staphylococcus aureus that prevents the activation of complement  and thereby reduces opsonization by C3b?   4. The main reason why methicillin-resistant Staphylococcus aureus  (MRSA) strains are resistant to methicillin and nafcillin is:   (B) they have altered penicillin-binding proteins that have  reduced binding of the antibiotics.   (C) they have mutant porin proteins that prevent the antibiotics  from entering the bacteria.   (D) they have plasmid-encoded export proteins that remove the  drug from the bacteria.   5. A pore-forming exotoxin produced by Staphylococcus aureus  that kills cells and is important in the severe, rapidly spreading  necrotizing lesions caused by MRSA strains is:   6. Of the following antibiotics, which one is the most appropriate  to treat a severe necrotizing skin infection caused by an MRSA  strain of Staphylococcus aureus ?   7. An outbreak of serious pneumococcal pneumonia and sepsis  among inmates in an overcrowded prison has occurred. Labora¬  tory analysis determined that one serotype was involved. The prison  physician said that the pneumococcal vaccine might have limited  the outbreak. Which one of the following structures of the pneumo¬  coccus is responsible for determining the serotype and is also the  immunogen in the vaccine?   8. Which one of the following best describes the pathogenesis of  rheumatic fever?   (A) An exotoxin produced by Streptococcus pyogenes that acts as a  superantigen damages cardiac muscle.   (B) An exotoxin produced by Streptococcus pyogenes that ADP-  ribosylates a G protein damages joint tissue.   (C) Antibody to the capsular polysaccharide of Streptococcus pyo¬  genes cross-reacts with joint tissue and damages it.   (D) Antibody to the M protein of Streptococcus pyogenes cross-  reacts with cardiac muscle and damages it.   (E) Endotoxin produced by Streptococcus pyogenes activates mac¬  rophages to release cytokines that damage cardiac muscle.   9. Which one of following laboratory tests is the most appropriate  to distinguish Streptococcus pyogenes from other |3-hemolytic  streptococci?    10. Infections by which one of the following bacteria are typically  treated with penicillins such as amoxicillin, because they exhibit  neither low-level resistance nor high-level resistance and synergy  with an aminoglycoside is not required in order for penicillins to  be effective?   11. Your patient in the emergency room has a 5-cm ulcer on her leg  that is surrounded by a red, warm, and tender area of inflam¬  mation. You do a Gram stain on pus from the ulcer and see  gram-positive cocci in chains. Culture of the pus grows small  (3-hemolytic colonies that are catalase-negative and are inhibited  by bacitracin. These results indicate that the organism causing  her lesion is most likely:   12. The Jones family of four had a delicious picnic lunch last Sunday.  It was a warm day, and the food sat in the sun for several hours.  Alas, 3 hours later, everyone came down with vomiting and non-  bloody diarrhea. In the emergency room, it was found that Mrs.  Jones, who prepared the food, had a paronychia on her thumb.  Which one of the following is the most likely causative organism?   13. A 20-year-old sexually active woman reports dysuria and other  symptoms of a urinary tract infection. Gram stain of the urine  reveals gram-positive cocci. Which one of the following sets of  bacteria is most likely to cause this infection?   14. Your patient is a 2-week-old infant who was well until 2 days ago,  when she stopped feeding and became irritable. She now has a  fever to 38°C, developed a petechial rash all over her body, and  is very difficult to arouse. In the emergency room, a blood cul¬  ture and a spinal tap were done. Gram stain of the spinal fluid  showed gram-positive cocci in chains. Culture of the spinal fluid  on blood agar revealed |3-hemolytic colonies that grew in the  presence of bacitracin and hydrolyzed hippurate. Which one of  the following is the most likely causative organism?   15. Your patient is a 50-year-old woman who has a community-  acquired pneumonia caused by Streptococcus pneumoniae. Anti¬  biotic susceptibility tests reveal an MIC of less than 0.1 mg/mL    to penicillin G. Which one of the following is the best antibiotic  to treat the infection?   16. Your patient is a 70-year-old man with endocarditis caused by  Enterococcusfaecalis. Which one of the following is the best com¬  bination of antibiotics to treat the infection?   Brief summaries of the organisms described in this chapter  begin on page 653. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Clinical Bacteriology section of Part XIII: USMLE  (National Board) Practice Questions starting on page 713. Also  see Part XIV: USMLE (National Board) Practice Examination  starting on page 751.   The genus Neisseria contains two important human patho¬  gens: Neisseria meningitidis and Neisseria gonorrhoeae.  Neisseria meningitidis mainly causes meningitis and menin-  gococcemia (Figure 16-1). In the United States, it is the  leading cause of death from infection in children. Neisseria  gonorrhoeae causes gonorrhea (Figure 16-2), the second  most common notifiable bacterial disease in the United  States (Tables 16-1 and 16-2). It also causes neonatal con¬  junctivitis (ophthalmia neonatorum) (Figure 16-3) and  pelvic inflammatory disease (PID). Note that N. meningiti¬  dis is also known as the meningococcus (plural, meningo¬  cocci), and N. gonorrhoeae is also known as the gonococcus  (plural, gonococci).   Additional information regarding the clinical aspects of  infections caused by the organisms in this chapter is pro¬  vided in Part IX entitled Infectious Diseases beginning on  page 593.   Neisseriae are gram-negative cocci that resemble paired  kidney beans (Figure 16-4).   (1) Neisseria meningitidis (meningococcus) has a prom¬  inent polysaccharide capsule that enhances virulence by  its antiphagocytic action. The capsule also is the immuno¬  gen in the vaccine that induces protective antibodies (Table  16-3). Meningococci are divided into at least 13 serologic  groups on the basis of the antigenicity of their capsular  polysaccharides. Five serotypes cause most cases of menin¬  gitis and meningococcemia: A, B, C, Y, and W-135. Sero¬  type A is the leading cause of epidemic meningitis  worldwide. Serotype B accounts for most disease in the    United States. This is because the group B polysaccharide is  not immunogenic in humans and therefore is not part of  the vaccines that contain the capsular polysaccharide of the  other four groups. In 2014, a vaccine against group B  meningococci containing factor H binding protein as the  immunogen was approved.   (2) Neisseria gonorrhoeae (gonococcus) has no polysac¬  charide capsule but has multiple serotypes based on the  antigenicity of its pilus protein. There is marked antigenic  variation in the gonococcal pili as a result of chromosomal  rearrangement; more than 100 serotypes are known.  Gonococci have three outer membrane proteins (proteins  I, II, and III). Protein II plays a role in attachment of the  organism to cells and varies antigenically as well.   Neisseriae are gram-negative bacteria and contain endo¬  toxin in their outer membrane. Note that the endotoxin of  Neisseriae consist of lipooligosaccharide (LOS), in contrast  to the lipopolysaccharide (LPS) found in enteric gram¬  negative rods. Both LPS and LOS contain lipid A, but the  oligosaccharide part of LOS contains few sugars, whereas  the polysaccharide part of LPS contains a long repeating  sugar side chain.   The growth of both organisms is inhibited by toxic  trace metals and fatty acids found in certain culture media  (e.g., blood agar plates). They are therefore cultured on  “chocolate” agar containing blood heated to 80°C, which  inactivates the inhibitors. Neisseriae are oxidase-positive  (Figure 16-5) (i.e., they possess the enzyme cytochrome c ).  This is an important laboratory diagnostic test in which  colonies exposed to phenylenediamine turn purple or  black as a result of oxidation of the reagent by the enzyme  (see Figure 16-2).   The genus Neisseria is one of several in the family  Neisseriaceae. A separate genus contains the organism  Moraxella catarrhalis, which is part of the normal throat   FIGURE 16~1 Meningococcemia. Note purpuric lesions on leg  caused by endotoxin-mediated disseminated intravascular coagula¬  tion (DIC). (Reproduced with permission from Wolff K, Johnson R (eds): Fitzpatrick's  Color Atlas & Synopsis of Clinical Dermatology. 6th ed. New York: McGraw-Hill, 2009.  Copyright © 2009 by The McGraw-Hill Companies, Inc.)   FIGURE 16-2 Gonorrhea. Note purulent urethral discharge  caused by Neisseria gonorrhoeae. (Reproduced with permission from Wolff K,  Johnson R (eds): Fitzpatrick's Color Atlas & Synopsis of Clinical Dermatology. 6th ed.  New York: McGraw-Hill, 2009. Copyright © 2009 by The McGraw-Hill Companies, Inc.)    1 0ther drugs can also be used. See treatment guidelines published by the Centers for Disease Control and Prevention.  2 Add doxycycline for possible coinfection with Chlamydia trachomatis.   FIGURE 16-3 Neonatal conjunctivitis (ophthalmia neonato¬  rum) caused by Neisseria gonorrhoeae. Note purulent exudate, espe¬  cially on lower right eyelid. The other common cause of neonatal  conjunctivitis is Chlamydia trachomatis.   FIGURE 16-4 Neisseria gonorrhoeae —Gram stain. Arrow  points to typical "kidney bean"-shaped gram-negative diplococci  within a neutrophil. (Used with permission from Professor Shirley Lowe, Univer-  sity of California, San Francisco School of Medicine.)    TABLE 16-3 Properties of the Polysaccharide  Capsule of the Meningococcus 1   (3) Is the antigen detected in the spinal fluid of patients with  meningitis   'The same four features apply to the capsule of the pneumococcus and Haemophi¬  lus influenzae.   FIGURE 16-5 Oxidase test—A drop of the oxidase reagent  was placed on the left and right side of the filter paper. Bacteria from  a colony of Neisseria gonorrhoeae were rubbed on the drop on the  left, and the purple color indicates a positive test (i.e., the organism is  oxidase-positive). Bacteria from a colony of Escherichia coli were  rubbed on the drop on the right, and the absence of a purple color  indicates a negative test (i.e., the organism is oxidase-negative). (Used  with permission from Professor Shirley Lowe, University of California, San Francisco  School of Medicine.)    flora but can cause such respiratory tract infections as  sinusitis, otitis media, bronchitis, and pneumonia.  Moraxella catarrhalis and members of other genera, such as  Branhamella, Kingella, and Acinetobacter, are described in  Chapter 27. ( Moraxella catarrhalis is the new name for  Branhamella catarrhalis.)   Humans are the only natural hosts for meningococci. The  organisms are transmitted by airborne droplets; they colo¬  nize the membranes of the nasopharynx and become part  of the transient flora of the upper respiratory tract. Carriers  are usually asymptomatic. From the nasopharynx, the  organism can enter the bloodstream and spread to specific  sites, such as the meninges or joints, or be disseminated  throughout the body (meningococcemia).   About 5% of people become chronic carriers and serve as  a source of infection for others. The carriage rate can be as  high as 35% in people who live in close quarters (e.g., mili¬  tary recruits); this explains the high frequency of outbreaks  of meningitis in the armed forces prior to the use of the  vaccine. The carriage rate is also high in close (family) con¬  tacts of patients. Outbreaks of meningococcal disease also  have occurred in college students living in dormitories.   Two organisms cause more than 80% of cases of bacte¬  rial meningitis in infants older than 2 months of age: S.  pneumoniae and N. meningitidis. Of these organisms,    meningococci, especially those in group A, are most likely  to cause epidemics of meningitis. Group B meningococci  cause many cases of meningitis in developed countries  because it is not present in the capsular polysaccharide vac¬  cine (see “Prevention,” later). Overall, N. meningitidis ranks  second to S. pneumoniae as a cause of meningitis but is the  most common cause in persons between the ages of 2 and  18 years.   (1) A polysaccharide capsule that enables the organism  to resist phagocytosis by polymorphonuclear leukocytes  (PMNs). The capsule is the immunogen in several com¬  monly used vaccines against meningococci.   (2) Endotoxin, which causes fever, shock, and other  pathophysiologic changes (in purified form, endotoxin can  reproduce many of the clinical manifestations of  meningococcemia).   (3) An immunoglobulin A (IgA) protease that helps  the bacteria attach to the membranes of the upper respira¬  tory tract by cleaving secretory IgA.   (4) Factor H binding protein (FHBP) on meningo¬  cocci binds Factor H, an inhibitor of complement factor  C3b. The presence of Factor H on the surface of meningo¬  cocci reduces the opsonizing activity of C3b and reduces  the amount of membrane attack complex produced (see  complement action in Chapter 63). FHBP is the immuno¬  gen in the vaccine against group B meningococci.   Resistance to disease correlates with the presence of  antibody to the capsular polysaccharide. Most carriers  develop protective antibody titers within 2 weeks of coloni¬  zation. Because immunity is group-specific, it is possible to  have protective antibodies to one group of organisms yet be  susceptible to infection by organisms of the other groups.  Complement is an important feature of the host defenses,  because people with complement deficiencies, particularly  in the late-acting complement components (C6-C9),  have an increased incidence of meningococcal bacteremia.   The two most important manifestations of disease are  meningococcemia (see Figure 16-1) and meningitis. The  most severe form of meningococcemia is the life-threatening  Waterhouse-Friderichsen syndrome, which is character¬  ized by high fever, shock, widespread purpura, dissemi¬  nated intravascular coagulation, thrombocytopenia, and  adrenal insufficiency. Bacteremia can result in the seeding  of many organs, especially the meninges. The symptoms of  meningococcal meningitis are those of typical bacterial  meningitis, namely, fever, headache, stiff neck, and an  increased level of PMNs in spinal fluid.   The principal laboratory procedures are smear and culture  of blood and spinal fluid samples. A presumptive diagnosis    of meningococcal meningitis can be made if gram-negative  cocci are seen in a smear of spinal fluid (see Figure 16-4).  The organism grows best on chocolate agar incubated at  37°C in a 5% C0 2 atmosphere. A presumptive diagnosis of  Neisseria can be made if oxidase-positive colonies of gram¬  negative diplococci are found (see Figure 16-5). The dif¬  ferentiation between N. meningitidis and N. gonorrhoeae is  made on the basis of sugar fermentation: meningococci  ferment maltose, whereas gonococci do not (both organ¬  isms ferment glucose). Immunofluorescence can also be  used to identify these species. Tests for serum antibodies  are not useful for clinical diagnosis. However, a procedure  that can assist in the rapid diagnosis of meningococcal  meningitis is the latex agglutination test, which detects  capsular polysaccharide in the spinal fluid.   Penicillin G is the treatment of choice for meningococcal  infections. A third-generation cephalosporin such as ceftri¬  axone can also be used. Strains resistant to penicillin have  rarely emerged, but sulfonamide resistance is common. In  2007-2008, strains of N. meningitidis resistant to cipro¬  floxacin emerged.   Chemoprophylaxis and immunization are both used to  prevent meningococcal disease. Either rifampin or cipro¬  floxacin can be used for prophylaxis in people who have  had close contact with the index case. These drugs are  preferred because they are efficiently secreted into the  saliva, in contrast to penicillin G.   The vaccines against groups A, C, Y, and W-135 menin¬  gococci contain the polysaccharide capsule as the immuno¬  gen. The vaccine against group B meningococci contains  Factor H binding protein as the immunogen.   There are three forms of the polysaccharide vaccine for  use in the United States, all of which contain the capsular  polysaccharide of groups A, C, Y, and W-135 as the immu¬  nogen. There are two forms of the conjugate vaccine: Men-  actra contains the four polysaccharides conjugated to  diphtheria toxoid as the carrier protein, whereas Menveo  contains the four polysaccharides conjugated to a nontoxic  mutant of diphtheria toxin as the carrier protein. Meno-  mune, the unconjugated vaccine, contains only the four  polysaccharides (not conjugated to a carrier protein). The  conjugate vaccines induce higher titers of antibodies in chil¬  dren than does the unconjugated vaccine. The vaccines  induce similar antibody titers in adults. Note that none of the  vaccines contain the group B polysaccharide because it is not  immunogenic in humans. A fourth vaccine created for use in  the meningitis belt of Africa called MenAfriVac is a conju¬  gate vaccine that contains only the group A polysaccharide.   In general, the conjugate vaccines are preferred over the  unconjugated version. The unconjugated vaccine is effec¬  tive in preventing epidemics of meningitis and in reducing    the carrier rate, especially in military personnel. Travelers  to areas where epidemics are occurring should receive the  vaccine. College students living in dormitories are encour¬  aged to receive the vaccine. No booster dose is recom¬  mended for either form of the vaccine if given after the age  of 16 years. The conjugate vaccine is recommended for  children at the age of 11 to 12 years, which will reduce the  incidence of meningococcal disease in teenagers and young  adults. A booster dose is recommended for those who  received the conjugate vaccine prior to the age of 16. Vac¬  cine Adverse Events Reports describe several cases of  Guillain-Barre syndrome following immunization with  Menactra. A causal relationship between the immunization  and Guillain-Barre syndrome has not been established.   The vaccine against group B meningococci containing  Factor H binding protein as the immunogen was approved  in 2014. It induces antibody against the binding protein  thereby inhibiting the ability of the bacteria to bind factor  H. This enhances the action of complement, an important  host defense, because factor H inhibits complement com¬  ponent C3b. Factor H binding protein for use in the vaccine  is made by recombinant DNA techniques in Escherichia  coli. The vaccine is approved for use in people 10 to 25 years  of age. In 2015, a second vaccine against group B meningo¬  cocci containing four surface proteins (fHbp, NadA,  NHBA, and PorA) was approved.   Gonococci, like meningococci, cause disease only in  humans. The organism is usually transmitted sexually;  newborns can be infected during birth. Because gonococ¬  cus is quite sensitive to dehydration and cool conditions,  sexual transmission favors its survival. Gonorrhea is usu¬  ally symptomatic in men but often asymptomatic in  women. Genital tract infections are the most common  source of the organism, but anorectal and pharyngeal infec¬  tions are important sources as well.   Pili constitute one of the most important virulence fac¬  tors, because they mediate attachment to mucosal cell sur¬  faces and are antiphagocytic. Piliated gonococci are usually  virulent, whereas nonpiliated strains are avirulent. Two  virulence factors in the cell wall are endotoxin (lipooligo-  saccharide, LOS) and the outer membrane proteins. The  organism’s IgA protease can hydrolyze secretory IgA,  which could otherwise block attachment to the mucosa.  Gonococci have no capsules.   The main host defenses against gonococci are antibod¬  ies (IgA and IgG), complement, and neutrophils. Antibody-  mediated opsonization and killing within phagocytes  occur, but repeated gonococcal infections are common,  primarily as a result of antigenic changes of pili and the  outer membrane proteins.   Gonococci infect primarily the mucosal surfaces (e.g.,  the urethra and vagina), but dissemination occurs. Certain    strains of gonococci cause disseminated infections more  frequently than others. The most important feature of these  strains is their resistance to being killed by antibodies and  complement. The mechanism of this “serum resistance” is  uncertain, but the presence of a porin protein (porin A) in  the cell wall, which inactivates the C3b component of  complement, appears to play an important role.   The occurrence of a disseminated infection is a function  not only of the strain of gonococcus but also of the effec¬  tiveness of the host defenses. Persons with a deficiency of  the late-acting complement components (C6-C9) are at  risk for disseminated infections, as are women during men¬  ses and pregnancy. Disseminated infections usually arise  from asymptomatic infections, indicating that local inflam¬  mation may deter dissemination.   Gonococci cause both localized infections, usually in the  genital tract, and disseminated infections with seeding of  various organs. Gonococci reach these organs via the  bloodstream (gonococcal bacteremia).   Gonorrhea in men is characterized primarily by urethri¬  tis accompanied by dysuria and a purulent discharge (see  Figure 16-2). Epididymitis can occur.   In women, infection is located primarily in the endocer-  vix, causing a purulent vaginal discharge and intermen-  strual bleeding (cervicitis). The most frequent complication  in women is an ascending infection of the uterine tubes  (salpingitis, PID), which can result in sterility or ectopic  pregnancy as a result of scarring of the tubes.   Disseminated gonococcal infections (DGI) commonly  manifest as arthritis, tenosynovitis, or pustules in the skin.  Disseminated infection is the most common cause of septic  arthritis in sexually active adults. The clinical diagnosis of  DGI is often difficult to confirm using laboratory tests  because the organism is not cultured in more than 50% of  cases.   Other infected sites include the anorectal area, throat,  and eyes. Anorectal infections occur chiefly in women and  homosexual men. They are frequently asymptomatic, but a  bloody or purulent discharge (proctitis) can occur. In the  throat, pharyngitis occurs, but many patients are asymp¬  tomatic. In newborn infants, purulent conjunctivitis (oph¬  thalmia neonatorum) (see Figure 16-3) is the result of  gonococcal infection acquired from the mother during  passage through the birth canal. The incidence of gonococ¬  cal ophthalmia has declined greatly in recent years because  of the widespread use of prophylactic erythromycin eye  ointment (or silver nitrate) applied shortly after birth.  Gonococcal conjunctivitis also occurs in adults as a result  of the transfer of organisms from the genitals to the eye.   Other sexually transmitted infections (e.g., syphilis and  nongonococcal urethritis caused by Chlamydia trachoma¬  tis) can coexist with gonorrhea; therefore, appropriate  diagnostic and therapeutic measures must be taken.   The diagnosis of urogenital infections depends on Gram  staining and culture of the discharge (see Figure 16-4).  However, nucleic acid amplification tests are widely used as  screening tests (see later).   In men, the finding of gram-negative diplococci within  PMNs in a urethral discharge specimen is sufficient for  diagnosis (see Figure 16-4). In women, the use of the  Gram stain alone can be difficult to interpret; therefore,  cultures should be done. Gram stains on cervical speci¬  mens can be falsely positive because of the presence of  gram-negative diplococci in the normal flora and can be  falsely negative because of the inability to see small num¬  bers of gonococci when using the oil immersion lens. Cul¬  tures must also be used in diagnosing suspected pharyngitis  or anorectal infections.   Specimens from mucosal sites, such as the urethra and  cervix, are cultured on Thayer-Martin medium, which is a  chocolate agar containing antibiotics (vancomycin, colistin,  trimethoprim, and nystatin) to suppress the normal flora.  The finding of an oxidase-positive colony (see Figure 16-5)  composed of gram-negative diplococci is sufficient to iden¬  tify the isolate as a member of the genus Neisseria. Specific  identification of the gonococcus can be made either by its  fermentation of glucose (but not maltose) or by fluorescent-  antibody staining. Note that specimens from sterile sites,  such as blood or joint fluid, can be cultured on chocolate  agar without antibiotics because there is no competing  normal flora.   Nucleic acid amplification tests, often abbreviated  NAAT, detect the presence of gonococcal nucleic acids in  patient specimens. These tests are widely used for screen¬  ing purposes, produce results rapidly, and are highly sensi¬  tive and specific. They can be used on urine samples,  obviating the need for more invasive collection techniques.  Note that serologic tests to determine the presence of anti¬  body to gonococci in the patient’s serum are not useful for  diagnosis.   Ceftriaxone is the treatment of choice in uncomplicated  gonococcal infections. Azithromycin or ciprofloxacin  should be used if the patient is allergic to penicillins or  cephalosporins. Because mixed infections with C. trachomatis  are common, azithromycin or doxycycline should be pre¬  scribed also. A follow-up culture should be performed  1 week after completion of treatment to determine whether  gonococci are still present. Treatment of complicated gono¬  coccal infections, such as PID, typically requires hospital¬  ization. Treatment regimens are complex and beyond the  scope of this book.   Prior to the mid-1950s, all gonococci were highly sensi¬  tive to penicillin. Subsequently, isolates emerged with low-  level resistance to penicillin and to other antibiotics such as  tetracycline and chloramphenicol. This type of resistance is    encoded by the bacterial chromosome and is due to  reduced uptake of the drug or to altered binding sites rather  than to enzymatic degradation of the drug.   Then, in 1976, penicillinase-producing (PPNG) strains  that exhibited high-level resistance were isolated from  patients. Penicillinase is plasmid-encoded. PPNG strains  are now common in many areas of the world, including  several urban areas in the United States, where approxi¬  mately 10% of isolates are resistant. Isolates resistant to  fluoroquinolones, such as ciprofloxacin, have become a  significant problem, and fluoroquinolones are not recom¬  mended as treatment.   The prevention of gonorrhea involves the use of condoms  and the prompt treatment of symptomatic patients and  their contacts. Cases of gonorrhea must be reported to the  public health department to ensure proper follow-up. A  major problem is the detection of asymptomatic carriers.  Gonococcal conjunctivitis in newborns is prevented most  often by the use of erythromycin ointment. Silver nitrate  drops are used less frequently. No vaccine is available.   1. Regarding the differences between N. meningitidis (meningo¬  cocci) and N. gonorrhoeae (gonococci), which one of the following  is the most accurate statement?   (B) Meningococci have a thick polysaccharide capsule, whereas  gonococci do not.   2. Your patient is a 14-year-old girl who was sent home from school  because she had a fever of 102°C, a severe headache, and was fall¬  ing asleep in class. When her fever rose to 104°C, her mother took  her to the emergency room, where a blood pressure of 60/20 and  several petechial hemorrhages were found. Gram-negative diplo¬  cocci were seen in a Gram stain of the spinal fluid. Which one of  the following is most likely to cause the fever, hypotension, and  petechial hemorrhages?   3. Regarding the patient in Question 2, which one of the following is  the best antibiotic to treat the infection?    4. Regarding the differences between N. meningitidis (meningo¬  cocci) and N. gonorrhoeae (gonococci), which one of the following  is the most accurate statement?   (B) Many clinical isolates of meningococci produce |3-lactamase,  but clinical isolates of gonococci do not.   (C) Meningococci have multiple antigenic types, but gonococci  have only one antigenic type.   (D) The conjugate vaccine against gonorrhea contains seven types  of the pilus protein as the immunogen.   (E) The main mode of transmission for both organisms is respira¬  tory droplets.   5. Your patient is a 20-year-old man with a urethral exudate. You do  a Gram stain of the pus and see gram-negative diplococci with  neutrophils. Which one of the following is the best antibiotic to  treat the infection?   Brief summaries of the organisms described in this chapter  begin on page 655. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Clinical Bacteriology section of Part XIII: USMLE  (National Board) Practice Questions starting on page 713. Also  see Part XIV: USMLE (National Board) Practice Examination  starting on page 751.   There are five medically important genera of gram-positive  rods: Bacillus, Clostridium, Corynebacterium, Listeria, and  Gardnerella. Bacillus and Clostridium form spores, whereas  Corynebacterium, Listeria, and Gardnerella do not. Members  of the genus Bacillus are aerobic, whereas those of the genus  Clostridium are anaerobic (Table 17-1).   These gram-positive rods can also be distinguished  based on their appearance on Gram stain. Bacillus and    Clostridium species are longer and more deeply staining than  Corynebacterium and Listeria species. Corynebacterium  species are club-shaped (i.e., they are thinner on one end  than the other). Corynebacterium and Listeria species char¬  acteristically appear as V- or L-shaped rods. Gardnerella  vaginalis is a short gram-variable rod.   Additional information regarding the clinical aspects of  infections caused by the organisms in this chapter is provided  in Part IX entitled Infectious Diseases beginning on page 593.   There are two medically important Bacillus species: Bacillus  anthracis and Bacillus cereus. Important features of pathogen¬  esis by these two Bacillus species are described in Table 17-2.   Bacillus anthracis causes anthrax (Figure 17-1), which is  common in animals but rare in humans. Human disease  occurs in three main forms: cutaneous, pulmonary (inhala¬  tion), and gastrointestinal. In 2001, an outbreak of both  inhalation and cutaneous anthrax occurred in the United  States. The outbreak was caused by sending spores of the  organism through the mail. There were 18 cases, causing 5  deaths in this outbreak.   Bacillus anthracis is a large gram-positive rod with  square ends, frequently found in chains (Figure 17-2).  Its antiphagocytic capsule is composed of D-glutamate.   Exotoxin has three components: protective antigen  binds to cells; edema factor is an adenylate cyclase;  lethal factor is a protease that inhibits cell growth  resulting in cell death (necrosis)   Spores germinate in  reheated rice, then  bacteria produce exo¬  toxins, which are  ingested    (This is unique—capsules of other bacteria are polysac¬  charides.) It is nonmotile, whereas other members of the  genus are motile. Anthrax toxin is encoded on one plas¬  mid, and the polyglutamate capsule is encoded on a dif¬  ferent plasmid.   Spores of the organism persist in soil for years. Humans are  most often infected cutaneously at the time of trauma to  the skin, which allows the spores on animal products,  such as hides, bristles, and wool, to enter. Spores can also be  inhaled into the respiratory tract. Pulmonary (inhalation)  anthrax occurs when spores are inhaled into the lungs.  Gastrointestinal anthrax occurs when contaminated meat  is ingested.   Inhalation anthrax is not communicable from person to  person, despite the severity of the infection. After being  inhaled into the lung, the organism moves rapidly to the    mediastinal lymph nodes, where it causes hemorrhagic  mediastinitis. Because it leaves the lung so rapidly, it is not  transmitted by the respiratory route to others.   Pathogenesis is based primarily on the production of  two exotoxins, collectively known as anthrax toxin. The  two exotoxins, edema factor and lethal factor, each  consist of two proteins in an A-B subunit configuration.  The B, or binding, subunit in each of the two exotoxins  is protective antigen. The A, or active, subunit has enzy¬  matic activity.   Edema factor, an exotoxin, is an adenylate cyclase that  causes an increase in the intracellular concentration of  cyclic adenosine monophosphate (AMP). This causes an  outpouring of fluid from the cell into the extracellular  space, which manifests as edema. (Note the similarity of  action to that of cholera toxin.)   FIGURE 17-1 Skin lesion of anthrax. Note the black eschar, a  necrotic lesion covered by a crust, caused by lethal factor, an exo¬  toxin produced by Bacillus anthracis. Note the area of edema sur¬  rounding the eschar, which is caused by another exotoxin called  edema factor. (Source: Dr. James H. Steele, Centers for Disease Control and  Prevention. CDC # 2033.)   to one large "box car-like" gram-positive rod within a long chain.  (Source: Public Health Image Library, Centers for Disease Control and  Prevention.)    Lethal factor is a protease that cleaves the phosphoki-  nase that activates the mitogen-activated protein kinase  (MAPK) signal transduction pathway. This pathway con¬  trols the growth of human cells, and cleavage of the phos-  phokinase inhibits cell growth. Protective antigen forms  pores in the human cell membrane that allows edema fac¬  tor and lethal factor to enter the cell. The name protective  antigen refers to the fact that antibody against this protein  protects against disease.   The typical lesion of cutaneous anthrax is a painless ulcer  with a black eschar (crust, scab) (see Figure 17-1). Local  edema is striking. The lesion is called a malignant pustule.  Untreated cases progress to bacteremia and death.   Pulmonary (inhalation) anthrax, also known as  “wool-sorter’s disease,” begins with nonspecific respira¬  tory tract symptoms resembling influenza, especially a  dry cough and substernal pressure. This rapidly pro¬  gresses to hemorrhagic mediastinitis, bloody pleural  effusions, septic shock, and death. Although the lungs  are infected, the classic features and X-ray picture of  pneumonia are not present. Mediastinal widening seen  on chest X-ray is an important diagnostic criterion.  Hemorrhagic mediastinitis and hemorrhagic meningitis  are severe life-threatening complications. The symptoms  of gastrointestinal anthrax include vomiting, abdominal  pain, and bloody diarrhea.   Smears show large, gram-positive rods in chains (see  Figure 17-2). Spores are usually not seen in smears of  exudate because spores form when nutrients are insufficient,  and nutrients are plentiful in infected tissue. Nonhemolytic  colonies form on blood agar aerobically. In case of a bio¬  terror attack, rapid diagnosis can be performed in special  laboratories using polymerase chain reaction (PCR)-  based assays. Another rapid diagnostic procedure is the  direct fluorescent antibody test that detects antigens of  the organism in the lesion. Serologic tests, such as an  enzyme-linked immunosorbent assay (ELISA) test for  antibodies, require acute and convalescent serum samples  and can only be used to make a diagnosis retrospectively.   Ciprofloxacin is the drug of choice. Doxycycline is an alter¬  native drug. No resistant strains have been isolated  clinically.   Ciprofloxacin or doxycycline was used as prophylaxis in  those exposed during the outbreak in the United States  in 2001. People at high risk can be immunized with cell-  free vaccine containing purified protective antigen as    immunogen. The vaccine is weakly immunogenic, and  six doses of vaccine over an 18-month period are given.  Annual boosters are also given to maintain protection.  Incinerating animals that die of anthrax, rather than  burying them, will prevent the soil from becoming con¬  taminated with spores.   Spores on grains such as rice survive steaming and rapid  frying. The spores germinate when rice is kept warm for  many hours (e.g., reheated fried rice). The portal of entry  is the gastrointestinal tract.   Bacillus cereus produces two enterotoxins. The mode of  action of one of the enterotoxins is the same as that of chol¬  era toxin (i.e., it adds adenosine diphosphate ribose, a  process called ADP-ribosylation, to a G protein, which  stimulates adenylate cyclase and leads to an increased con¬  centration of cyclic AMP within the enterocyte). The mode  of action of the other enterotoxin resembles that of staphy¬  lococcal enterotoxin (i.e., it is a superantigen).   There are two syndromes. (1) One syndrome has a short  incubation period (4 hours) and consists primarily of nau¬  sea and vomiting, similar to staphylococcal food poisoning.  (2) The other has a long incubation period (18 hours) and  features watery, nonbloody diarrhea, resembling clostridial  gastroenteritis.   There is no specific means of prevention. Rice should not  be kept warm for long periods.   There are four medically important species: Clostridium  tetani, Clostridium botulinum, Clostridium perfringens  (which causes either gas gangrene or food poisoning),  and Clostridium difficile. All clostridia are anaerobic,   points to a large gram-positive rod. (Used with permission from Professor  Shirley Lowe, University of California, San Francisco School of Medicine.)    spore-forming, gram-positive rods (Figure 17-3). Impor¬  tant features of pathogenesis and prevention are described  in Table 17-3.   Spores are widespread in soil. The portal of entry is usually  a wound site (e.g., where a nail penetrates the foot), but the  spores can also be introduced during “skin-popping,” a  technique used by drug addicts to inject drugs into the  skin. Germination of spores is favored by necrotic tissue  and poor blood supply in the wound. Neonatal tetanus, in  which the organism enters through a contaminated umbi¬  licus or circumcision wound, is a major problem in some  developing countries.   Tetanus toxin (tetanospasmin) is an exotoxin produced by  vegetative cells at the wound site. This polypeptide toxin is    carried intra-axonally (retrograde) to the central nervous  system, where it binds to ganglioside receptors and blocks  release of inhibitory mediators (e.g., glycine and  y-aminobutyric acid [GABA]) at spinal synapses.   Tetanus toxin and botulinum toxin (see later) are among  the most toxic substances known. They are both proteases  that cleave the proteins involved in mediator release from  the neurons.   Tetanus toxin has one antigenic type, unlike botuli¬  num toxin, which has eight. There is therefore only one  antigenic type of tetanus toxoid in the vaccine against  tetanus.   Tetanus is characterized by strong muscle spasms (spastic  paralysis, tetany). Specific clinical features include lockjaw  (trismus) due to rigid contraction of the jaw muscles,  which prevents the mouth from opening; a characteristic  grimace known as risus sardonicus; and exaggerated  reflexes. Opisthotonos, a pronounced arching of the back  due to spasm of the strong extensor muscles of the back, is  often seen (see Figure 17-4). Respiratory failure ensues. A  high mortality rate is associated with this disease. Note that  in tetanus, spastic paralysis (strong muscle contractions)  occurs, whereas in botulism, flaccid paralysis (weak or  absent muscle contractions) occurs.   There is no microbiologic or serologic diagnosis. Organ¬  isms are rarely isolated from the wound site. Clostridium  tetani produces a terminal spore (i.e., a spore at the end of  the rod). This gives the organism the characteristic appear¬  ance of a “tennis racket.”   Tetanus immune globulin (tetanus antitoxin) is used to  neutralize the toxin. The role of antibiotics is uncertain. If  antibiotics are used, either metronidazole or penicillin G  can be given. An adequate airway must be maintained and  respiratory support given. Benzodiazepines (e.g., diazepam  [Valium]) should be given to prevent spasms.   FIGURE 17-4 Tetanus. Note the marked hyperextension of  the back, a position called opisthotonos, caused by tetanus toxin, an  exotoxin that inhibits the release of mediators of the inhibitory neu¬  rons in the spinal cord. (Source: Centers for Disease Control and Prevention.  CDC # 6373.)   Tetanus is prevented by immunization with tetanus toxoid  (formaldehyde-treated toxin) in childhood and every 10 years  thereafter. Tetanus toxoid is usually given to children in com¬  bination with diphtheria toxoid and the acellular pertussis  vaccine (DTaP).   When trauma occurs, the wound should be cleaned  and debrided, and tetanus toxoid booster should be  given. If the wound is grossly contaminated, tetanus  immune globulin, as well as the toxoid booster, should  be given and penicillin administered. Half of the immune  globulins should be infiltrated into the wound and the  other half given IM at a site separate from the tetanus  toxoid.   Tetanus immune globulin (tetanus antitoxin) is made  in humans to avoid serum sickness reactions that occur  when antitoxin made in horses is used. The administra¬  tion of both immune globulins and tetanus toxoid (at  different sites in the body) is an example of passive-  active immunity.   Spores, widespread in soil, contaminate vegetables and  meats. When these foods are canned or vacuum-packed  without adequate sterilization, spores survive and germi¬  nate in the anaerobic environment. Toxin is produced  within the canned food and ingested preformed. The  highest-risk foods are (1) alkaline vegetables such as green  beans, peppers, and mushrooms and (2) smoked fish. The    toxin is relatively heat-labile; it is inactivated by boiling for  several minutes. Thus, disease can be prevented by suffi¬  cient cooking.   Botulinum toxin is absorbed from the gut and carried via  the blood to peripheral nerve synapses, where it blocks  release of acetylcholine. It is a protease that cleaves the  proteins involved in acetylcholine release. The toxin is a  polypeptide encoded by a lysogenic phage. Along with  tetanus toxin, it is among the most toxic substances  known. There are eight immunologic types of toxin; types   A, B, and E are the most common in human illness. Botox  is a commercial preparation of exotoxin A used to  remove wrinkles on the face. Minute amounts of the  toxin are effective in the treatment of certain spasmodic  muscle disorders such as torticollis, “writer’s cramp,” and  blepharospasm.   Descending weakness and paralysis, including diplopia,  dysphagia, and respiratory muscle failure, are seen. No  fever is present. In contrast, Guillain-Barre syndrome is an  ascending paralysis (see Chapter 66).   Two special clinical forms occur: (1) wound botulism,  in which spores contaminate a wound, germinate, and pro¬  duce toxin at the site; and (2) infant botulism, in which the  organisms grow in the gut and produce the toxin there.  Ingestion of honey containing the organism is implicated in  transmission of infant botulism. Affected infants develop  weakness or paralysis and may need respiratory support  but usually recover spontaneously. In the United States,  infant botulism accounts for about half of the cases of botu¬  lism, and wound botulism is associated with drug abuse,  especially skin-popping with black tar heroin.   The organism is usually not cultured. Botulinum toxin  is demonstrable in uneaten food and the patient’s  serum by mouse protection tests. Mice are inoculated  with a sample of the clinical specimen and will die  unless protected by antitoxin. Enzyme-linked immuno¬  assay (EIA) tests are also used to detect the toxin and  polymerase-chain reaction (PCR) tests are being  developed.   The heptavalent antitoxin containing all seven types (A to  G) is preferred to the trivalent antitoxin containing types A,   B, and E. Respiratory support is provided as well. The anti¬  toxin is made in horses and serum sickness may occur. A  bivalent antitoxin (types A and B) purified from the plasma  of humans immunized with botulinum toxoid is available  for the treatment of infant botulism.   Proper sterilization of all canned and vacuum-packed  foods is essential. Food must be adequately cooked to inac¬  tivate the toxin. Swollen cans must be discarded (clostridial  proteolytic enzymes form gas, which swells cans).   Clostridium perfringens causes two distinct diseases, gas  gangrene and food poisoning, depending on the route of  entry into the body.   of the two diseases caused by C. perfringens (Figure 17-5).  Gas gangrene is also caused by other histotoxic clostridia  such as Clostridium histolyticum, Clostridium septicum,  Clostridium novyi, and Clostridium sordellii. (C. sordellii  also causes toxic shock syndrome in postpartum and posta¬  bortion women.)   FIGURE 17-5 Gas gangrene. Note large area of necrosis on  lateral aspect of foot. Necrosis is mainly caused by lecithinase pro¬  duced by Clostridium perfringens. Gas in tissue is a feature of gangrene  produced by these anaerobic bacteria. A large gas- and fluid-filled  bulla is seen near the ankle. (Used with permission from David Kaplan, MD.)   Spores are located in the soil; vegetative cells are members  of the normal flora of the colon and vagina. Gas gangrene  is associated with war wounds, automobile and motorcycle  accidents, and septic abortions (endometritis).   Organisms grow in traumatized tissue (especially muscle)  and produce a variety of toxins. The most important is  alpha toxin (lecithinase), which damages cell membranes,  including those of erythrocytes, resulting in hemolysis.  Degradative enzymes produce gas in tissues.   Pain, edema, cellulitis, and gangrene (necrosis) occur in the  wound area (see Figure 17-5). If crepitus is palpated in the  affected tissue, it indicates gas in the tissue. This gas is typi¬  cally hydrogen produced by the anaerobic bacteria. Hemoly¬  sis and jaundice are common, as are blood-tinged exudates.  Shock and death can ensue. Mortality rates are high.   Smears of tissue and exudate samples show large gram¬  positive rods. Spores are not usually seen because they are  formed primarily under nutritionally deficient conditions.  The organisms are cultured anaerobically and then identi¬  fied by sugar fermentation reactions and organic acid pro¬  duction. Clostridium perfringens colonies exhibit a double  zone of hemolysis on blood agar. The colonies also produce  a precipitate in egg yolk agar caused by the action of its leci¬  thinase. Serologic tests are not useful.   Penicillin G is the antibiotic of choice. Wounds should be  debrided.   Wounds should be cleansed and debrided. Penicillin may  be given for prophylaxis. There is no vaccine.   Spores are located in soil and can contaminate food. The  heat-resistant spores survive cooking and germinate. The  organisms grow to large numbers in reheated foods, espe¬  cially meat dishes.   Clostridium perfringens is a member of the normal flora in  the colon but not in the small bowel, where the enterotoxin    acts to cause diarrhea. The mode of action of the entero-  toxin is the same as that of the enterotoxin of Staphylococ¬  cus aureus (i.e., it acts as a superantigen).   The disease has an 8- to 16-hour incubation period and is  characterized by watery diarrhea with cramps and little  vomiting. It resolves in 24 hours.   This is not usually done. There is no assay for the toxin.  Large numbers of the organisms can be isolated from  uneaten food.   There are no specific preventive measures. Food should be  adequately cooked to kill the organism.   Clostridium difficile causes antibiotic-associated pseudo¬  membranous colitis (Figure 17-6). Clostridium difficile  is the most common nosocomial (hospital-acquired)  cause of diarrhea. It is the leading infectious cause of  gastrointestinal-associated deaths in the United States.   The organism colonizes the large intestine of approxi¬  mately 3% of the general population and up to 30% of  hospitalized patients. Note that most people are not colo¬  nized, which explains why most people who take antibiot¬  ics do not get pseudomembranous colitis. It is transmitted  by the fecal-oral route. Either the spores or the bacterial  organism itself can be transmitted.   The majority of cases occur in hospitalized patients but  about one-third of cases are community-acquired. The  hands of hospital personnel are important intermediaries.   Antibiotics suppress drug-sensitive members of the nor¬  mal flora of the colon, allowing C. difficile to multiply and  produce exotoxins A and B. Both exotoxin A and exotoxin  B are glucosyltransferases (i.e., enzymes that glucosylate  [add glucose to] a G protein called Rho GTPase). The main  effect of exotoxin B in particular is to cause depolymeriza¬  tion of actin, resulting in a loss of cytoskeletal integrity,  apoptosis, and death of the enterocytes.    Clindamycin was the first antibiotic to be recognized  as a cause of pseudomembranous colitis, but many antibi¬  otics are known to cause this disease. At present, third-  generation cephalosporins are the most common cause  because they are so frequently used. Ampicillin and fluoro¬  quinolones are also commonly implicated. In addition to  antibiotics, cancer chemotherapy also predisposes to pseu¬  domembranous colitis. Clostridium difficile rarely invades  the intestinal mucosa.   Clostridium difficile causes diarrhea associated with pseu¬  domembranes (yellow-white plaques) on the colonic  mucosa (see Figure 17-6). (The term pseudomembrane is  defined in Chapter 7 on page 39). The diarrhea is usually  not bloody, and neutrophils are found in the stool in about  half of the cases. Fever and abdominal pain often occur.  The organism rarely enters the blood stream and rarely  causes metastatic infection.   The pseudomembranes are visualized by sigmoidos¬  copy. Toxic megacolon can occur, and surgical resection  of the colon maybe necessary. Pseudomembranous colitis  can be distinguished from the transient diarrhea that  occurs as a side effect of many oral antibiotics by testing  for the presence of the toxin in the stool. Even with ade¬  quate treatment, the organism may not be eradicated  from the colon and recurrences occur at a rate of approxi¬  mately 15% to 20%.   In 2005, a new, more virulent strain of C. difficile  emerged. This hypervirulent strain causes more severe dis¬  ease, is more likely to cause recurrences, and responds less   FIGURE 17-6 Pseudomembranous colitis. Note yellowish  plaquelike lesions in colon. Caused by an exotoxin produced by Clos¬  tridium difficile that inhibits a signal transduction protein, leading to  death of enterocytes. (Reproduced with permission from Fauci AS et al (eds):  Harrison's Principles of Internal Medicine. 17th ed. New York: McGraw-Hill, 2008, pg  1837. Copyright © 2008 by The McGraw-Hill Companies, Inc.)    well to metronidazole than the previous strain. The strain  is also characterized by resistance to quinolones. It is  thought that the widespread use of quinolones for diarrheal  disease may have selected for this new strain.   The presence of exotoxins in the filtrate of a patient’s stool  specimen is the basis of the laboratory diagnosis. It is insuf¬  ficient to culture the stool for the presence of C. difficile  because people can be colonized by the organism and not  have disease.   There are two types of tests used to make the labora¬  tory diagnosis. One detects the exotoxin itself and the  other detects the genes that encode the exotoxin. To  detect the exotoxin itself an ELISA test employing anti¬  body to the exotoxin is used. To detect the genes that  encode the exotoxin, a PCR assay to determine the pres¬  ence of the toxin gene DNA is used. The DNA-based test  has greater sensitivity and specificity than the ELISA test.  However, these NAAT tests (nucleic acid amplification  tests) should be interpreted with caution because a person  may only be colonized by C. difficile and be recorded as  positive when in fact, C. difficile is not the cause of the  patient’s disease.    replaced. Metronidazole is preferred because using van¬  comycin may select for vancomycin-resistant entero¬  cocci. However, in life-threatening cases, vancomycin  should be used because it is more effective than metro¬  nidazole. Also in life-threatening cases, surgical removal  of the colon may be required.   In many patients, treatment does not eradicate the  carrier state, and recurrent episodes of colitis can occur.  Fidaxomicin (Dificid) is used both in the treatment of  pseudomembranous colitis and in preventing relapses of  this disease. It is effective in life-threatening cases.   Fecal transplantation is another possible therapeutic  approach. It involves administering bowel flora from a  normal individual either by enema or by nasoduodenal  tube to the patient with pseudomembranous colitis. This  approach is based on the concept of bacterial interference  (i.e., to replace the C. difficile with normal bowel flora).  Very high cure rates are claimed for this technique, but  aesthetic considerations have limited its acceptance.   There are no preventive vaccines or drugs. Because antibi¬  otics are an important predisposing factor for pseudomem¬  branous colitis, they should be prescribed only when  necessary. In the hospital, strict infection control proce¬  dures, including rigorous handwashing, are important.  Probiotics, such as the yeast Saccharomyces, may be useful  to prevent pseudomembranous colitis.    There are three important pathogens in this group: Coryne-  bacterium diphtheriae. Listeria monocytogenes, and Gard-  nerella vaginalis. Important features of pathogenesis and  prevention of C. diphtheriae and L. monocytogenes are  described in Table 17-4.   Corynebacterium diphtheriae causes diphtheria (Figure 17-7).  Other Corynebacterium species (diphtheroids) are implicated  in opportunistic infections.   Corynebacteria are gram-positive rods that appear club-  shaped (wider at one end) and are arranged in palisades or  in V- or L-shaped formations (Figure 17-8). The rods have  a beaded appearance. The beads consist of granules of  highly polymerized polyphosphate—a storage mechanism  for high-energy phosphate bonds. The granules stain meta-  chromatically (i.e., a dye that stains the rest of the cell blue  will stain the granules red).   Humans are the only natural host of C. diphtheriae. Both  toxigenic and nontoxigenic organisms reside in the upper    TABLE 17-4 Important Features of Pathogenesis by Corynebacterium diphtheriae and Listeria  monocytogenes   FIGURE 17-7 Diphtheria. Note whitish-gray pseudomem¬  brane covering posterior pharynx and marked inflammation of pal¬  ate and pharynx. Caused by diphtheria toxin, an exotoxin that  inhibits protein synthesis by inhibiting elongation factor-2. (Used with  permission from Dr. Peter Strebel.)   respiratory tract and are transmitted by airborne droplets.  The organism can also infect the skin at the site of a preex¬  isting skin lesion. This occurs primarily in the tropics but  can occur worldwide in indigent persons with poor skin  hygiene.   Although exotoxin production is essential for pathogenesis,  invasiveness is also necessary because the organism must  first establish and maintain itself in the throat. Diphtheria    toxin inhibits protein synthesis by ADP-ribosylation of  elongation factor-2 (EF-2). The toxin affects all eukaryotic  cells regardless of tissue type but has no effect on the analo¬  gous factor in prokaryotic cells.   The toxin is a single polypeptide with two functional  domains. The binding (B) domain mediates binding of  the toxin to glycoprotein receptors on the cell membrane.  The active (A) domain possesses enzymatic activity that  cleaves nicotinamide from nicotinamide adenine dinucle¬  otide (NAD) and transfers the remaining ADP-ribose to  EF-2, thereby inactivating it. Other organisms whose  exotoxins act by ADP-ribosylation are described in Tables  7-10 and 7-11.   The DNA that codes for diphtheria toxin is part of the  DNA of a temperate bacteriophage called beta phage.  During the lysogenic phase of viral growth, the DNA of  this virus integrates into the bacterial chromosome and the  toxin is synthesized. Corynebacterium diphtheriae cells that  are not lysogenized by this phage do not produce exotoxin  and are nonpathogenic.   (1) A local inflammation in the throat, with a fibrinous  exudate that forms the tough, adherent, gray pseudomem¬  brane characteristic of the disease.   (2) Antibody that can neutralize exotoxin activity by  blocking the interaction of the binding domain with the  receptors, thereby preventing entry into the cell. The  immune status of a person can be assessed by Schick’s  test. The test is performed by intradermal injection of  0.1 mL of purified standardized toxin. If the patient has  no antitoxin, the toxin will cause inflammation at the site  4 to 7 days later. If no inflammation occurs, antitoxin is  present and the patient is immune. The test is rarely per¬  formed in the United States except under special epide¬  miologic circumstances.   Arrow points to a "club-shaped" gram-positive rod. Arrowhead points  to typical V- or L-shaped corynebacteria. (Source: Public Health Image  Library, Centers for Disease Control and Prevention.)   Although diphtheria is rare in the United States, physicians  should be aware of its most prominent sign, the thick, gray,  adherent pseudomembrane over the tonsils and throat  (see Figure 17-7). (The term pseudomembrane is defined in  Chapter 7 on page 39.) The other aspects are nonspecific:  fever, sore throat, and cervical adenopathy. There are three  prominent complications:   (1) Extension of the membrane into the larynx and tra¬  chea, causing airway obstruction.   (3) Nerve weakness or paralysis, especially of the cra¬  nial nerves. Paralysis of the muscles of the soft palate and  pharynx can lead to regurgitation of fluids through the  nose. Peripheral neuritis affecting the muscles of the  extremities also occurs.    Cutaneous diphtheria causes ulcerating skin lesions  covered by a gray membrane. These lesions are often indo¬  lent and often do not invade surrounding tissue. Systemic  symptoms rarely occur. In the United States, cutaneous  diphtheria occurs primarily in the indigent.   Laboratory diagnosis involves both isolating the organism  and demonstrating toxin production. It should be empha¬  sized that the decision to treat with antitoxin is a clinical  one and cannot wait for the laboratory results. A throat  swab should be cultured on Loeffler’s medium, a tellurite  plate, and a blood agar plate. The tellurite plate contains a  tellurium salt that is reduced to elemental tellurium within  the organism. The typical gray-black color of tellurium in  the colony is a telltale diagnostic criterion. If C. diphtheriae  is recovered from the cultures, either animal inoculation or  an antibody-based gel diffusion precipitin test is performed  to document toxin production. A PCR assay for the pres¬  ence of the toxin gene in the organism isolated from the  patient can also be used.   Smears of the throat swab should be stained with both  Gram stain and methylene blue. Although the diagnosis of  diphtheria cannot be made by examination of the smear,  the finding of many tapered, pleomorphic gram-positive  rods can be suggestive. The methylene blue stain is excel¬  lent for revealing the typical metachromatic granules.   The treatment of choice is antitoxin, which should be  given immediately on the basis of clinical impression  because there is a delay in laboratory diagnostic proce¬  dures. The toxin binds rapidly and irreversibly to cells and,  once bound, cannot be neutralized by antitoxin. The func¬  tion of antitoxin is therefore to neutralize unbound toxin in  the blood. Because the antiserum is made in horses, the  patient must be tested for hypersensitivity, and medications  for the treatment of anaphylaxis must be available. Serum  sickness (see Chapter 65) may occur after administration of  antiserum made in horses.   Treatment with penicillin G or erythromycin is also rec¬  ommended, but neither is a substitute for antitoxin. Antibi¬  otics inhibit growth of the organism, reduce toxin production,  and decrease the incidence of chronic carriers.   Diphtheria is very rare in the United States because chil¬  dren are immunized with diphtheria toxoid (usually  given as a combination of diphtheria toxoid, tetanus  toxoid, and acellular pertussis vaccine, often abbreviated  as DTaP). Diphtheria toxoid is prepared by treating the  exotoxin with formaldehyde. This treatment inactivates  the toxic effect but leaves the antigenicity intact.    Immunization consists of three doses given at 2, 4, and  6 months of age, with boosters at 1 and 6 years of age.  Because immunity wanes, a booster every 10 years is  recommended. Immunization does not prevent naso¬  pharyngeal carriage of the organism.   Listeria monocytogenes causes meningitis and sepsis in  newborns, pregnant women, and immunosuppressed  adults. It also causes outbreaks of febrile gastroenteritis. It  is a major cause of concern for the food industry.   Listeria monocytogenes is a small gram-positive rod  arranged in V- or L-shaped formations similar to coryne-  bacteria. The organism exhibits an unusual tumbling  movement that distinguishes it from the corynebacteria,  which are nonmotile. Colonies on a blood agar plate pro¬  duce a narrow zone of (3-hemolysis that resembles the  hemolysis of some streptococci.   Listeria grows well at cold temperatures, so storage of  contaminated food in the refrigerator can increase the risk  of gastroenteritis. This paradoxical growth in the cold is  called “cold enhancement.”   Listeria infections occur primarily in two clinical settings: (1)  in the fetus or in a newborn as a result of transmission across  the placenta or during delivery; and (2) in pregnant women  and immunosuppressed adults, especially renal transplant  patients. (Note that pregnant women have reduced cell-  mediated immunity during the third trimester.)   The organism is distributed worldwide in animals,  plants, and soil. From these reservoirs, it is transmitted to  humans primarily by ingestion of unpasteurized milk prod¬  ucts, undercooked meat, and raw vegetables. Contact with  domestic farm animals and their feces is also an important  source. In the United States, listeriosis is primarily a food-  borne disease associated with eating unpasteurized cheese  and delicatessen meats. Following ingestion, the bacteria  appear in the colon and then can colonize the female geni¬  tal tract. From this location, they can infect the fetus if  membranes rupture or infect the neonate during passage  through the birth canal.   The pathogenesis of Listeria depends on the organism’s  ability to invade and survive within cells. Invasion of cells  is mediated by internalin made by Listeria and E-cadherin  on the surface of human cells. The ability of Listeria to pass  the placenta, enter the meninges, and invade the gastroin¬  testinal tract depends on the interaction of internalin and  E-cadherin on those tissues.    Upon entering the cell, the organism produces listerio-  lysin, which allows it to escape from the phagosome into  the cytoplasm, thereby escaping destruction in the phago¬  some. Because Listeria preferentially grows intracellularly,  cell-mediated immunity is a more important host defense  than humoral immunity. Suppression of cell-mediated  immunity predisposes to Listeria infections.   Listeria monocytogenes can move from cell to cell by  means of actin rockets—filaments of actin polymerize and  propel the bacteria through the membrane of one human  cell and into another.   Infection during pregnancy can cause abortion, premature  delivery, or sepsis during the peripartum period. Newborns  infected at the time of delivery can have acute meningitis 1  to 4 weeks later. The bacteria reach the meninges via the  bloodstream (bacteremia). The infected mother either is  asymptomatic or has an influenzalike illness. Listeria  monocytogenes infections in immunocompromised adults  can be either sepsis or meningitis.   Gastroenteritis caused by L. monocytogenes is character¬  ized by watery diarrhea, fever, headache, myalgias, and  abdominal cramps but little vomiting. Outbreaks are usu¬  ally caused by contaminated dairy products, but under¬  cooked meats such as chicken and hot dogs and ready-to-eat  foods such as coleslaw have also been involved.   Laboratory diagnosis is made primarily by Gram stain and  culture. The appearance of gram-positive rods resembling  diphtheroids and the formation of small, gray colonies  with a narrow zone of (3-hemolysis on a blood agar plate  suggest the presence of Listeria. The isolation of Listeria is  confirmed by the presence of motile organisms, which dif¬  ferentiate them from the nonmotile corynebacteria. Identi¬  fication of the organism as L. monocytogenes is made by  sugar fermentation tests.   Treatment of invasive disease, such as meningitis and  sepsis, consists of trimethoprim-sulfamethoxazole. Com¬  binations, such as ampicillin and gentamicin or ampicillin  and trimethoprim-sulfamethoxazole, can also be used.  Resistant strains are rare. Listeria gastroenteritis typically  does not require treatment.   Prevention is difficult because there is no immunization.  Limiting the exposure of pregnant women and immuno-  suppressed patients to potential sources such as farm ani¬  mals, unpasteurized milk products, and raw vegetables is  recommended. Trimethoprim-sulfamethoxazole given to   Gardnerella vaginalis is the main organism associated with  bacterial vaginosis. This disease is the most common vagi¬  nal infection of sexually active women.   Gardnerella vaginalis is a small, facultative gram-variable  rod. The term “gram-variable” refers to the observation  that some organisms are purple while others are pink in a  Gram-stained specimen. Structurally, it has a gram-positive  cell wall but the wall is thin and older organisms tend to  lose the purple color.   The pathogenesis of bacterial vaginosis is uncertain.  Gardnerella vaginalis is often found in association with  anaerobes such as Mobiluncus (see Chapter 27) and together  they cause the symptoms of this disease. It is not considered  to be a sexually transmitted infection.   Bacterial vaginosis is characterized by a malodorous,  white or gray-colored vaginal discharge. The discharge  has a characteristic “fishy” odor. Inflammatory changes  are typically absent which is why it is called a “vaginosis”  rather than a “vaginitis.” Mild itching may occur. Women  with bacterial vaginosis have a higher incidence of pre¬  term deliveries and, consequently, a higher incidence of  morbidity and mortality occurs in their newborn  children.   Clue cells, which are vaginal epithelial cells covered with  bacteria, are an important laboratory finding seen in a micro¬  scopic examination of the vaginal discharge (Figure 17-9). In  addition, the “whiff” test, which consists of treating the vagi¬  nal discharge with 10% KOH and smelling a pungent, “fishy”  odor, is often positive. However, trichomoniasis, which can  also cause a positive whiff test, must be ruled out before a  diagnosis of bacterial vaginosis can be made. A pH of greater  than 4.5 of the vaginal discharge supports the diagnosis of  bacterial vaginosis.   The drug of choice is metronidazole. Treatment of sexual  partners is not recommended. There is no vaccine.   FIGURE 17-9 Clue cells in bacterial vaginosis. Note that the  lower epithelial cell is a "clue cell" because its surface is covered with  bacteria. The upper epithelial cell is not a "clue cell" because its surface  has few bacteria. (Reproduced with permission from Usatine RP et al: The Color Atlas  of Family Medicine, New York: McGraw-Hill, 2009. Courtesy of EJ. Mayeaux, Jr., MD.)   1. Which one of the following is a club-shaped, gram-positive rod  that causes disease by producing an exotoxin that kills cells by  inhibiting elongation factor-2, resulting in the inhibition of pro¬  tein synthesis?   2. Which one of the following is a large gram-positive rod that  causes necrosis of tissue by producing an exotoxin that degrades  lecithin, resulting in the lysis of cell membranes?   3. Which one of the following sets of bacteria causes disease charac¬  terized by a pseudomembrane?    4. Disease caused by which one of the following sets of bacteria can  be prevented by a toxoid vaccine?   5. Your patient in the pediatric intensive care unit is a 2-week-old  boy with a high fever and the signs of meningitis. Gram stain  of the spinal fluid reveals small gram-positive rods. Colonies on  blood agar show a narrow zone of (1-hemolysis. Which one of the  following is the most likely cause of his neonatal meningitis?   6. Regarding the patient in Question 5, which one of the following  is the best antibiotic to treat the infection?   7. Your patient is a 40-year-old woman with diplopia and other signs of  cranial nerve weakness. History reveals she grows her own vegetables  and likes to preserve them in jars that she prepares at home. She is  fond of her preserved string beans, which is what she ate uncooked  in a salad for dinner last night. Which one of the following is the most  likely cause of this clinical picture?   8. Your patient is a 30-year-old man with a 2-cm lesion on his arm.  It began as a painless papule that enlarged and, within a few days,  ulcerated and formed a black crust (eschar). He works in an abat¬  toir where his job is removing the hide from the cattle. A Gram  stain of fluid from the lesion reveals large gram-positive rods.  Which one of the following bacteria is likely to be the cause?   9. Your patient is a 30-year-old man who was brought to the emer¬  gency room following a motorcycle accident in which he sus¬  tained a compound fracture of his leg. He now has a high fever  and a rapidly spreading cellulitis with crepitus in the area of  the fracture. Large gram-positive rods are seen on the exudate.  Necrotic tissue was debrided. Which one of the following is the  best antibiotic to treat the infection?   10. Your patient is a 65-year-old woman who is several days post-op  following removal of her carcinoma of the colon. She now spikes    a fever and has a cough, and chest X-ray shows pneumonia.  While being treated with the appropriate antibiotics, she devel¬  ops severe diarrhea. You suspect she may have pseudomembra¬  nous colitis. Which one of the following is the best antibiotic to  treat the infection?   Brief summaries of the organisms described in this chapter  begin on page 656. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Clinical Bacteriology section of Part XIII: USMLE  (National Board) Practice Questions starting on page 713. Also  see Part XIV: USMLE (National Board) Practice Examination  starting on page 751.   Gram-negative rods are a large group of diverse organisms  (Figures 18-1, 18-2, and 19-1). In this book, these bacteria  are subdivided into three clinically relevant categories, each   FIGURE 18-1 Escherichia coli —Gram stain. Arrow points to a  gram-negative rod. (Used with permission from Professor Shirley Lowe, Univer¬  sity of California, San Francisco School of Medicine.)   FIGURE 18-2 Vibrio cholerae —Gram stain. Long arrow points  to a curved gram-negative rod. Arrowhead points to a flagellum at  one end of a curved gram-negative rod. (Source: Public Health Image  Library, Centers for Disease Control and Prevention.)    in a separate chapter, according to whether the organism is  related primarily to the enteric or the respiratory tract or to  animal sources (Table 18-1). Although this approach leads  to some overlap, it should be helpful because it allows gen¬  eral concepts to be emphasized.   Gram-negative rods related to the enteric tract include a  large number of genera. These genera have therefore been  divided into three groups depending on the major ana¬  tomic location of disease, namely, (1) pathogens both  within and outside the enteric tract, (2) pathogens primar¬  ily within the enteric tract, and (3) pathogens outside the  enteric tract (see Table 18-1).   The frequency with which the organisms related to the  enteric tract cause disease in the United States is shown  in Table 18-2. Salmonella, Shigella, and Campylobacter  are frequent pathogens in the gastrointestinal tract,  whereas Escherichia, Vibrio, and Yersinia are less so.  Enterotoxigenic strains of Escherichia coli are a common  cause of diarrhea in developing countries but are less  common in the United States. The medically important  gram-negative rods that cause diarrhea are described in  Table 18-3. Urinary tract infections are caused primarily  by E. coli; the other organisms occur less commonly. The  medically important gram-negative rods that cause urinary  tract infections are described in Table 18-4.   Additional information regarding the clinical aspects  of infections caused by the organisms in this chapter is  provided in Part IX entitled Infectious Diseases begin¬  ning on page 593.    TABLE 18-2 Frequency of Diseases Caused in the  United States by Gram-Negative Rods Related to the  Enteric Tract    Patients infected with such enteric pathogens as Shigella,  Salmonella, Campylobacter, and Yersinia have a high inci¬  dence of certain autoimmune diseases such as reactive  arthritis and Reiter’s syndrome (see Chapter 66). In addi¬  tion, infection with Campylobacter jejuni predisposes to  Guillain-Barre syndrome.   Before describing the specific organisms, it is appropri¬  ate to describe the family Enterobacteriaceae, to which  many of these gram-negative rods belong.   The Enterobacteriaceae is a large family of gram-negative  rods found primarily in the colon of humans and other ani¬  mals, many as part of the normal flora. These organisms are  the major facultative anaerobes in the large intestine but are  present in relatively small numbers compared with anaer¬  obes such as Bacteroides. Although the members of the  Enterobacteriaceae are classified together taxonomically,  they cause a variety of diseases with different pathogenetic  mechanisms. The organisms and some of the diseases they  cause are listed in Table 18-5.   Features common to all members of this heterogeneous  family are their anatomic location and the following four  metabolic processes: (1) they are all facultative anaerobes;  (2) they all ferment glucose (fermentation of other sugars  varies); (3) none have cytochrome oxidase (i.e., they are  oxidase-negative); and (4) they reduce nitrates to nitrites as  part of their energy-generating processes.   These four reactions can be used to distinguish the  Enterobacteriaceae from another medically significant group  of organisms—the nonfermenting gram-negative rods, the  most important of which is Pseudomonas aeruginosa. 1   Pseudomonas aeruginosa, a significant cause of urinary  tract infection and sepsis in hospitalized patients, does not  ferment glucose or reduce nitrates and is oxidase-positive.    lr The other less frequently isolated organisms in this group are members  of the following genera: Achromobacter, Acinetobacter, Alcaligenes,  Eikenella, Flavobacterium, Kingella, and Moraxella; see Chapter 27.    'Some strains produce enterotoxin, but its pathogenic role is not clear.    TABLE 18-4 Gram-Negative Rods Causing Urinary Tract Infection 1 or Sepsis 2   Blue-green pigment and fruity odor produced; causes nosocomial infections and  often drug-resistant    EMB = eosin-methylene blue,  diagnosed by quantitative culture of urine.  2 Diagnosed by culture of blood or pus.    In contrast to the Enterobacteriaceae, it is a strict aerobe  and derives its energy from oxidation, not fermentation.   All members of the Enterobacteriaceae, being gram¬  negative, contain endotoxin in their cell walls. In addition,  several exotoxins are produced (e.g., E. coli and Vibrio  cholerae secrete exotoxins, called enterotoxins, that activate  adenylate cyclase within the cells of the small intestine,  causing diarrhea) (see Chapter 7). In addition, E. coli 0157  produces Shiga toxin that causes hemolytic-uremic  syndrome (HUS).   The antigens of several members of the Enterobacteriaceae,  especially Salmonella and Shigella, are important; they are  used for identification purposes both in the clinical labora¬  tory and in epidemiologic investigations. The three surface  antigens are as follows:   (1) The cell wall antigen (also known as the somatic, or  O, antigen) is the outer polysaccharide portion of the lipo-  polysaccharide (see Figure 2-6). The O antigen, which is  composed of repeating oligosaccharides consisting of three  or four sugars repeated 15 or 20 times, is the basis for the  serologic typing of many enteric rods. The number of dif¬  ferent O antigens is very large (e.g., there are approximately  1500 types of Salmonella and 150 types of E. coli).   (2) The H antigen is on the flagellar protein. Only flagel¬  lated organisms, such as Escherichia and Salmonella, have  H antigens, whereas the nonmotile ones, such as Klebsiella  and Shigella, do not. The H antigens of certain Salmonella  species are unusual because the organisms can reversibly  alternate between two types of H antigens called phase 1  and phase 2. The organisms may use this change in antige¬  nicity to evade the immune response.   (3) The capsular or K polysaccharide antigen is particu¬  larly prominent in heavily encapsulated organisms such as  Klebsiella. The K antigen is identified by the quellung    TABLE 18-6 Lactose Fermentation by Members of  the Enterobacteriaceae and Related Organisms    (capsular swelling) reaction in the presence of specific  antisera and is used to serotype E. coli and Salmonella typhi  for epidemiologic purposes. In S. typhi, the cause of  typhoid fever, it is called the Vi (or virulence) antigen.   Specimens suspected of containing members of the  Enterobacteriaceae and related organisms are usually  inoculated onto two media, a blood agar plate and a selec¬  tive differential medium such as MacConkey’s agar or  eosin-methylene blue (EMB) agar. The differential ability  of these latter media is based on lactose fermentation,  which is the most important metabolic criterion used in  the identification of these organisms (Table 18-6). On  these media, the non-lactose fermenters (e.g., Salmonella  and Shigella) form colorless colonies, whereas the lactose  fermenters (e.g., E. coli) form colored colonies. On EMB  agar, E. coli colonies have a characteristic green sheen. The  selective effect of the media in suppressing unwanted  gram-positive organisms is exerted by bile salts or bacte¬  riostatic dyes in the agar.   An additional set of screening tests, consisting of tri¬  ple sugar iron (TSI) agar and urea agar, is performed  prior to the definitive identification procedures. The  rationale for the use of these media and the reactions of  several important organisms are presented in the box  titled “Agar Media for Enteric Gram-Negative Rods” and  in Table 18-7. The results of the screening process are    1 Acid production causes the phenol red indicator to turn yellow; the indicator is red under alkaline conditions. The presence of black FeS in the butt indicates H 2 S production.  Not every species within the various genera will give the above appearance on TSI agar. For example, some Serratia strains can ferment lactose slowly and give an acid reac¬  tion on the slant.   2 Pseudomonas, although not a member of the Enterobacteriaceae, is included in this table because its reaction on TSI agar is a useful diagnostic criterion.   The important components of this medium are ferrous sulfate  and the three sugars glucose, lactose, and sucrose. Glucose is  present in one-tenth the concentration of the other two sugars.  The medium in the tube has a solid, poorly oxygenated area on  the bottom, called the butt, and an angled, well-oxygenated  area on top, called the slant. The organism is inoculated into  the butt and across the surface of the slant.   The interpretation of the test results is as follows: (1) If  lactose (or sucrose) is fermented, a large amount of acid is  produced, which turns the phenol red indicator yellow both  in the butt and on the slant. Some organisms generate gases,  which produce bubbles in the butt. (2) If lactose is not fer¬  mented but the small amount of glucose is, the oxygen-  deficient butt will be yellow, but on the slant, the acid will be  oxidized to C0 2 and H 2 0 by the organism and the slant will  be red (neutral or alkaline). (3) If neither lactose nor glucose    is fermented, both the butt and the slant will be red. The  slant can become a deeper red-purple (more alkaline) as a  result of the production of ammonia from the oxidative  deamination of amino acids. (4) If H 2 S is produced, the  black color of ferrous sulfide is seen.   The reactions of some of the important organisms are  presented in Table 18-7. Because several organisms can give  the same reaction, TSI agar is only a screening device.   The import ant components of this medium are urea and  the pH indicator phenol red. If the organism produces ure¬  ase, the urea is hydrolyzed to NH 3 and C0 2 . Ammonia  turns the medium alkaline, and the color of the phenol red  changes from light orange to reddish purple. The important  organisms that are urease-positive are Proteus species and  K. pneumoniae.    often sufficient to identify the genus of an organism;  however, an array of 20 or more biochemical tests is  required to identify the species.   Another valuable piece of information used to identify  some of these organisms is their motility, which is depen¬  dent on the presence of flagella. Proteus species are very  motile and characteristically swarm over the blood agar  plate, obscuring the colonies of other organisms. Motility is  also an important diagnostic criterion in the differentiation  of Enterobacter cloacae, which is motile, from K. pneu¬  moniae, which is nonmotile.   If the results of the screening tests suggest the presence  of a Salmonella or Shigella strain, an agglutination test can  be used to identify the genus of the organism and to deter¬  mine whether it is a member of group A, B, C, or D.   Contamination of the public water supply system by sew¬  age is detected by the presence of coliforms in the water. In  a general sense, the term coliform includes not only E. coli,  but also other inhabitants of the colon such as Enterobacter  and Klebsiella. However, because only E. coli is exclusively  a large intestine organism, whereas the others are found in  the environment also, it is used as the indicator of fecal  contamination. In water quality testing, E. coli is identified  by its ability to ferment lactose with the production of acid  and gas, its ability to grow at 44.5°C, and its characteristic    colony type on EMB agar. An E. coli colony count above  4/dL in municipal drinking water is indicative of unac¬  ceptable fecal contamination. Because E. coli and the  enteric pathogens are killed by chlorination of the drink¬  ing water, there is rarely a problem with meeting this  standard. Disinfection of the public water supply is one of  the most important advances of public health in the twen¬  tieth century.   The appropriate treatment for infections caused by mem¬  bers of the Enterobacteriaceae and related organisms must  be individually tailored to the antibiotic sensitivity of the  organism. Generally speaking, a wide range of antimicrobial  agents are potentially effective (e.g., some penicillins and  cephalosporins, aminoglycosides, chloramphenicol, tetra¬  cyclines, quinolones, and sulfonamides). The specific choice  usually depends on the results of antibiotic sensitivity tests.   Note that many isolates of these enteric gram¬  negative rods are highly antibiotic resistant because of  the production of (3-lactamases and other drug-modifying  enzymes. These organisms undergo conjugation fre¬  quently, at which time they acquire plasmids (R factors)  that mediate multiple drug resistance. For example,  plasmid-encoded New Delhi metallo-(3-lactamase causes  resistance to penicillins, cephalosporins, monobactams,  and carbapenems.   Escherichia coli is the most common cause of urinary tract  infection and gram-negative rod sepsis. It is one of the two  important causes of neonatal meningitis and the agent  most frequently associated with “traveler’s diarrhea,” a  watery diarrhea. Some strains of E. coli are enterohemor-  rhagic and cause bloody diarrhea.   Escherichia coli is a straight gram-negative rod (see  Figure 18-1), in contrast to the curved gram-negative rods  of the genera Vibrio, Campylobacter, and Helicobacter.   Escherichia coli is the most abundant facultative anaer¬  obe in the colon and feces. It is, however, greatly outnum¬  bered by the obligate anaerobes such as Bacteroides.   Escherichia coli ferments lactose, a property that distin¬  guishes it from the two major intestinal pathogens, Shigella  and Salmonella. It has three antigens that are used to iden¬  tify the organism in epidemiologic investigations: the O, or  cell wall, antigen; the H, or flagellar, antigen; and the K, or  capsular, antigen. Because there are more than 150 O, 50 H,  and 90 K antigens, the various combinations result in  more than 1000 antigenic types of E. coli. Specific sero¬  types are associated with certain diseases (e.g., 055 and  0111 cause outbreaks of neonatal diarrhea).   The reservoir of E. coli includes both humans and animals.  The source of the E. coli that causes urinary tract infec¬  tions is the patient’s own colonic flora that colonizes the  urogenital area. The source of the E. coli that causes neo¬  natal meningitis is the mother’s birth canal; the infection is  acquired during birth. In contrast, the E. coli that causes  traveler’s diarrhea is acquired by ingestion of food or water  contaminated with human feces. Note that the main reser¬  voir of enterohemorrhagic E. coli 0157 is cattle and the  organism is acquired in undercooked beef, for example,  hamburgers.   Escherichia coli has several clearly identified compo¬  nents that contribute to its ability to cause disease: pili, a  capsule, endotoxin, and three exotoxins (enterotoxins), two  that cause watery diarrhea and one that causes bloody diar¬  rhea and hemolytic-uremic syndrome.   The first step is the adherence of the organism to the cells  of the jejunum and ileum by means of pili that protrude  from the bacterial surface. Once attached, the bacteria  synthesize enterotoxins (exotoxins that act in the enteric  tract), which act on the cells of the jejunum and ileum to    cause diarrhea. The toxins are strikingly cell-specific; the  cells of the colon are not susceptible, probably because  they lack receptors for the toxin. Enterotoxigenic strains  of E. coli (ETEC) can produce either or both of two  enterotoxins.   (1) The heat-labile toxin (LT) acts by stimulating ade¬  nylate cyclase. Both LT and cholera toxin act by catalyzing  the addition of adenosine diphosphate-ribose (a process  called ADP-ribosylation) to the G protein that stimulates  the cyclase. This irreversibly activates the cyclase. The  resultant increase in intracellular cyclic adenosine mono¬  phosphate (AMP) concentration stimulates cyclic AMP-  dependent protein kinase, which phosphorylates ion  transporters in the membrane. The transporters export  ions, which cause an outpouring of fluid, potassium, and  chloride from the enterocytes into the lumen of the gut,  resulting in watery diarrhea. Note that cholera toxin has the  same mode of action.   (2) The other enterotoxin is a low-molecular-weight,  heat-stable toxin (ST), which stimulates guanylate cyclase.   The enterotoxin-producing strains do not cause inflam¬  mation, do not invade the intestinal mucosa, and cause a  watery, nonbloody diarrhea. However, certain strains of E.  coli are enteropathic (enteroinvasive) and cause disease not  by enterotoxin formation but by invasion of the epithelium  of the large intestine, causing bloody diarrhea (dysentery)  accompanied by inflammatory cells (neutrophils) in the  stool.   Certain enterohemorrhagic strains of E. coli (i.e., those  withthe0157:H7 serotype) (STEC) also cause bloody diar¬  rhea by producing an exotoxin called Shiga toxin, so called  because it is very similar to that produced by Shigella spe¬  cies. Shiga toxin acts by removing an adenine from the  large (28S) ribosomal RNA, thereby stopping protein syn¬  thesis. Shiga toxin is encoded by temperate (lysogenic)  bacteriophages. Shiga toxin is also called verotoxin because  it has a cytopathic effect on Vero (monkey) cells in  culture.   These 0157:H7 strains are associated with outbreaks of  bloody diarrhea following ingestion of undercooked ham¬  burger, often at fast-food restaurants. The bacteria on the  surface of the hamburger are killed by the cooking, but  those in the interior, which is undercooked, survive. Also,  direct contact with animals (e.g., visits to farms and petting  zoos) has resulted in bloody diarrhea caused by 0157:H7  strains. E. coli 0157 has a low ID 50 of approximately 100  organisms.   Some patients with bloody diarrhea caused by 0157:H7  strains also have a life-threatening complication called  hemolytic-uremic syndrome (HUS), which occurs when  Shiga toxin enters the bloodstream. This syndrome consists  of hemolytic anemia, thrombocytopenia, and acute renal    AMP = adenosine monophosphate; CSF = cerebrospinal fluid; DIC = disseminated intravascular coagulation; RBC = red blood cell; WBC = white blood cell.    failure. The hemolytic anemia and renal failure occur  because there are receptors for Shiga toxin on the surface of  the endothelium of small blood vessels and on the surface  of kidney epithelium. Death of the endothelial cells of small  blood vessels results in a microangiopathic hemolytic ane¬  mia in which the red cells passing through the damaged  area become grossly distorted (schistocytes) and then lyse.  Thrombocytopenia occurs because platelets adhere to the  damaged endothelial surface. Death of the kidney epithelial  cells leads to renal failure. Treatment of diarrhea caused by  0157:H7 strains with antibiotics, such as ciprofloxacin,  increases the risk of developing HUS by increasing the  amount of Shiga toxin released by the dying bacteria.   Certain O serotypes of E. coli preferentially cause urinary  tract infections. These uropathic strains are characterized  by pili with adhesin proteins that bind to specific receptors  on the urinary tract epithelium. The binding site on these  receptors consists of dimers of galactose (Gal-Gal dimers).  These pili are also called P fimbria or pyelonephritis-  associated pili (PAP).   Cranberry juice contains flavonoids that inhibit the  binding of pili to receptors and may be useful in the pre¬  vention of recurrent urinary tract infections. The motility  of E. coli may aid its ability to ascend the urethra into the  bladder and ascend the ureter into the kidney.   The other two structural components, the capsule and the  endotoxin, play a more prominent role in the pathogenesis  of systemic, rather than intestinal tract, disease. The capsu¬  lar polysaccharide interferes with phagocytosis, thereby  enhancing the organism’s ability to cause infections in vari¬  ous organs. For example, E. coli strains that cause neonatal  meningitis usually have a specific capsular type called the  K1 antigen. The endotoxin of E. coli is the cell wall lipo-  polysaccharide, which causes several features of gram¬  negative sepsis such as fever, hypotension, and disseminated  intravascular coagulation.    Th-17 helper T cells that produce interleukin-17 are an  important host defense against sepsis caused by enteric  bacteria such as E. coli and Klebsiella. Patients infected with  human immunodeficiency virus (HIV) experience a loss of  Th-17 cells and are predisposed to sepsis caused by E. coli  and Klebsiella.   Escherichia coli causes a variety of diseases both within and  outside the intestinal tract. The main clinical findings, the  major pathogenetic factors, and the main laboratory results  are described in Table 18-8.   Diarrhea caused by enterotoxigenic E. coli (ETEC) is  usually watery, nonbloody, self-limited, and of short dura¬  tion (1-3 days). It is frequently associated with travel (trav¬  eler’s diarrhea, or “turista”). 2   Infection with enterohemorrhagic E. coli (EHEC), on  the other hand, results in a dysentery-like syndrome char¬  acterized by bloody diarrhea, abdominal cramping, and  fever similar to that caused by Shigella.   The 0157:H7 strains of E. coli (STEC) also cause bloody  diarrhea, which can be complicated by HUS. This syn¬  drome is characterized by kidney failure, hemolytic ane¬  mia, and thrombocytopenia. The hemolytic anemia is  caused by exotoxin-induced capillary damage, which  results in damage to the red cells as they pass through the  capillaries. These distorted, fragmented red cells called  schistocytes can be seen on blood smear and are character¬  istic of a microangiopathic hemolytic anemia.   In 2011, an outbreak of diarrhea and HUS in Germany  was caused by a Shiga toxin-producing strain of E. coli that  was typed as O104:H4, not 0157:H7. This indicates that  strains of E. coli other than 0157:H7 can also cause HUS.    ^Enterotoxigenic E. coli is the most common cause of traveler s diarrhea,  but other bacteria (e.g., Salmonella, Shigella, Campylobacter, and Vibrio  species), viruses such as Norwalk virus, and protozoa such as Giardia  and Cryptosporidium species are also involved.    HUS occurs particularly in children who have been  treated with fluoroquinolones or other antibiotics for their  diarrhea. For this reason, antibiotics should not be used to  treat diarrhea caused by EHEC.   Escherichia coli is the leading cause of community-  acquired urinary tract infections. These infections occur  primarily in women; this finding is attributed to three  features that facilitate ascending infection into the blad¬  der, namely, a short urethra, the proximity of the urethra  to the anus, and colonization of the vagina by members of  the fecal flora. It is also the most frequent cause of noso¬  comial (hospital-acquired) urinary tract infections, which  occur equally frequently in both men and women and are  associated with the use of indwelling urinary catheters.  Urinary tract infections can be limited to the bladder or  extend up the collecting system to the kidneys. If only the  bladder is involved, the disease is called cystitis, whereas  infection of the kidney is called pyelonephritis. The most  prominent symptoms of cystitis are pain (dysuria) and  frequency of urination; patients are usually afebrile.  Pyelonephritis is characterized by fever, flank pain, and  costovertebral angle tenderness; dysuria and frequency  may or may not occur.   Escherichia coli is also a major cause, along with the  group B streptococci, of meningitis and sepsis in neonates.  Exposure of the newborn to E. coli and group B strepto¬  cocci occurs during birth as a result of colonization of the  vagina by these organisms in approximately 25% of preg¬  nant women. Escherichia coli is the organism isolated most  frequently from patients with hospital-acquired sepsis,  which arises primarily from urinary, biliary, or peritoneal  infections. Peritonitis is usually a mixed infection caused by  E. coli or other facultative enteric gram-negative rod plus  anaerobic members of the colonic flora such as Bacteroides  and Fusobacterium.   Specimens suspected of containing enteric gram-negative  rods, such as E. coli, are grown initially on a blood agar  plate and on a differential medium, such as EMB agar or  MacConkey’s agar. Escherichia coli, which ferments lac¬  tose, forms pink colonies, whereas lactose-negative  organisms are colorless. On EMB agar, E. coli colonies  have a characteristic green sheen. Some of the important  features that help distinguish E. coli from other lactose-  fermenting gram-negative rods are as follows: (1) it pro¬  duces indole from tryptophan, (2) it decarboxylates  lysine, (3) it uses acetate as its only source of carbon, and  (4) it is motile. Escherichia coli 0157:H7 does not fer¬  ment sorbitol, which serves as an important criterion that  distinguishes it from other strains of E. coli. The isolation  of enterotoxigenic or enteropathogenic E. coli from  patients with diarrhea is not a routine diagnostic  procedure.   Treatment of E. coli infections depends on the site of dis¬  ease and the resistance pattern of the specific isolate. For  example, an uncomplicated lower urinary tract infection  (cystitis) can be treated using oral trimethoprim-  sulfamethoxazole or nitrofurantoin. Pyelonephritis can be  treated with ciprofloxacin or ceftriaxone. However, E. coli  sepsis requires treatment with parenteral antibiotics (e.g., a  third-generation cephalosporin, such as cefotaxime, with  or without an aminoglycoside, such as gentamicin). For the  treatment of neonatal meningitis, a combination of ampi-  cillin and cefotaxime is usually given. Antibiotic therapy is  usually not indicated in E. coli diarrheal diseases. However,  administration of trimethoprim-sulfamethoxazole or lop¬  eramide (Imodium) may shorten the duration of symp¬  toms. Rehydration is typically all that is necessary in this  self-limited disease.   There is no specific prevention for E. coli infections, such  as active or passive immunization. However, various gen¬  eral measures can be taken to prevent certain infections  caused by E. coli and other organisms. For example, the  incidence of urinary tract infections can be lowered by the  judicious use and prompt withdrawal of catheters and, in  recurrent infections, by prolonged prophylaxis with uri¬  nary antiseptic drugs (e.g., nitrofurantoin or trimethoprim-  sulfamethoxazole). The use of cranberry juice to prevent  recurrent urinary tract infections appears to be based on  the ability of flavonoids in the juice to inhibit the binding  of the pili of the uropathic strains of E. coli to the bladder  epithelium rather than to acidification of the urine, which  was the previous explanation.   Some cases of sepsis can be prevented by prompt removal  of or switching the site of intravenous lines. Travelers diar¬  rhea can sometimes be prevented by the prophylactic use of  doxycycline, ciprofloxacin, trimethoprim-sulfamethoxazole,  or Pepto-Bismol. Ingestion of uncooked foods and unpurified  water should be avoided while traveling in certain countries.   Salmonella species cause enterocolitis, enteric fevers such  as typhoid fever, and septicemia with metastatic infections  such as osteomyelitis. They are one of the most common  causes of bacterial enterocolitis in the United States.   Salmonellae are gram-negative rods that do not ferment  lactose but do produce H 2 S—features that are used in their  laboratory identification. Their antigens—cell wall O, fla¬  gellar H, and capsular Vi (virulence)—are important for  taxonomic and epidemiologic purposes. The O antigens,    ’An organism with a low ID 50 requires very few bacteria to cause disease.    which are the outer polysaccharides of the cell wall, are  used to subdivide the salmonellae into groups A-I. There  are two forms of the H antigens, phases 1 and 2. Only one  of the two H proteins is synthesized at any one time,  depending on which gene sequence is in the correct align¬  ment for transcription into mRNA. The Vi antigens (capsu¬  lar polysaccharides) are antiphagocytic and are an  important virulence factor for S. typhi, the agent of typhoid  fever. The Vi antigens are also used for the serotyping of S.  typhi in the clinical laboratory.   There are three methods for naming the salmonellae.  Ewing divides the genus into three species: S. typhi, Salmo¬  nella choleraesuis, and Salmonella enteritidis. In this  scheme there is one serotype in each of the first two spe¬  cies and 1500 serotypes in the third. Kaufman and White  assign different species names to each serotype; there are  roughly 1500 different species, usually named for the city  in which they were isolated. Salmonella dublin according  to Kaufman and White would be S. enteritidis serotype  dublin according to Ewing. The third approach to naming  the salmonellae is based on relatedness determined by  DNA hybridization analysis. In this scheme, S. typhi is not  a distinct species but is classified as Salmonella enterica  serotype (or serovar) typhi. All three of these naming sys¬  tems are in current use.   Clinically, the Salmonella species are often thought of in  two distinct categories, namely, the typhoidal species (i.e.,  those that cause typhoid fever) and the nontyphoidal spe¬  cies (i.e., those that cause diarrhea [enterocolitis] and meta¬  static infections, such as osteomyelitis). The typhoidal  species are S. typhi and S. paratyphi. The nontyphoidal spe¬  cies are the many serotypes of S. enterica. Of the serotypes,  S. enterica serotype choleraesuis is the species most often  involved in metastatic infections.   The three types of Salmonella infections (enterocolitis,  enteric fevers, and septicemia) have different pathogenic  features.    (1) Enterocolitis is characterized by an invasion of the  epithelial and subepithelial tissue of the small and large  intestines. Strains that do not invade do not cause disease.  The organisms penetrate both through and between the  mucosal cells into the lamina propria, with resulting  inflammation and diarrhea. Neutrophils limit the infection  to the gut and the adjacent mesenteric lymph nodes; bacte¬  remia is infrequent in enterocolitis. In contrast to Shigella  enterocolitis, in which the infectious dose is very small (on  the order of 100 organisms), the dose of Salmonella  required is much higher, at least 100,000 organisms.  Various properties of salmonellae and shigellae are com¬  pared in Table 18-9. Gastric acid is an important host  defense; gastrectomy or use of antacids lowers the infec¬  tious dose significantly.   (2) In typhoid and other enteric fevers, infection  begins in the small intestine, but few gastrointestinal  symptoms occur. The organisms enter, multiply in the  mononuclear phagocytes of Peyer’s patches, and then  spread to the phagocytes of the liver, gallbladder, and  spleen. This leads to bacteremia, which is associated with  the onset of fever and other symptoms, probably caused  by endotoxin. Survival and growth of the organism  within phagosomes in phagocytic cells are a striking fea¬  ture of this disease, as is the predilection for invasion of  the gallbladder, which can result in establishment of the  carrier state and excretion of the bacteria in the feces for  long periods.   (3) Septicemia accounts for only about 5% to 10% of  Salmonella infections and occurs in one of two settings: a  patient with an underlying chronic disease, such as sickle  cell anemia or cancer, or a child with enterocolitis. The  septic course is more indolent than that seen with many  other gram-negative rods. Bacteremia results in the seed¬  ing of many organs, with osteomyelitis, pneumonia, and  meningitis as the most common sequelae. Osteomyelitis  in a child with sickle cell anemia is an important example  of this type of salmonella infection. Previously damaged  tissues, such as infarcts and aneurysms, especially aortic  aneurysms, are the most frequent sites of metastatic    abscesses. Salmonella are also an important cause of vascu¬  lar graft infections.   The epidemiology of Salmonella infections is related to  the ingestion of food and water contaminated by human  and animal wastes. Salmonella typhi, the cause of typhoid  fever, is transmitted only by humans, but all other spe¬  cies have a significant animal as well as human reservoir.  Human sources are either persons who temporarily  excrete the organism during or shortly after an attack of  enterocolitis or chronic carriers who excrete the organism  for years. The most frequent animal source is poultry  and eggs, but meat products that are inadequately cooked  have been implicated as well. Dogs and other pets, includ¬  ing turtles, snakes, lizards, and iguanas, are additional  sources.   After an incubation period of 12 to 48 hours, enterocolitis  begins with nausea and vomiting and then progresses to  abdominal pain and diarrhea, which can vary from mild  to severe, with or without blood. Usually the disease lasts  a few days, is self-limited, causes nonbloody diarrhea, and  does not require medical care except in the very young  and very old. HIV-infected individuals, especially those  with a low CD4 count, have a much greater number of  Salmonella infections, including more severe diarrhea and  more serious metastatic infections than those who are not  infected with HIV. Salmonella typhimurium is the most  common species of Salmonella to cause enterocolitis in  the United States, but almost every species has been  involved.   In typhoid fever, caused by S. typhi, and in enteric fever,  caused by organisms such as S. paratyphi A, B, and C  (S. paratyphi B and C are also known as Salmonella schott-  muelleri and Salmonella hirschfeldii, respectively), the onset  of illness is slow, with fever and constipation rather than  vomiting and diarrhea predominating. Diarrhea may occur  early but usually disappears by the time the fever and bac¬  teremia occur. After the first week, as the bacteremia  becomes sustained, high fever, delirium, tender abdomen,  and enlarged spleen occur. Rose spots (i.e., rose-colored  macules on the abdomen) are associated with typhoid fever  but occur only rarely. Leukopenia and anemia are often  seen. Liver function tests are often abnormal, indicating  hepatic involvement.   The disease begins to resolve by the third week, but  severe complications such as intestinal hemorrhage or per¬  foration can occur. About 3% of typhoid fever patients  become chronic carriers. The carrier rate is higher among  women, especially those with previous gallbladder disease  and gallstones.   Septicemia is most often caused by S. choleraesuis. The  symptoms begin with fever but little or no enterocolitis and  then proceed to focal symptoms associated with the affected  organ, frequently bone, lung, or meninges.   In enterocolitis, the organism is most easily isolated from a  stool sample. However, in the enteric fevers, a blood culture  is the procedure most likely to reveal the organism during  the first 2 weeks of illness. Bone marrow cultures are often  positive. Stool cultures may also be positive, especially in  chronic carriers in whom the organism is secreted in the  bile into the intestinal tract.   Salmonellae form non-lactose-fermenting (colorless)  colonies on MacConkey’s or EMB agar. On TSI agar, an  alkaline slant and an acid butt, frequently with both gas and  H 2 S (black color in the butt), are produced. S. typhi is the  major exception; it does not form gas and produces only a  small amount of H 2 S. If the organism is urease-negative  (Proteus organisms, which can produce a similar reaction  on TSI agar, are urease-positive), the Salmonella isolate can  be identified and grouped by the slide agglutination test  into serogroup A, B, C, D, or E based on its O antigen.  Definitive serotyping of the O, H, and Vi antigens is per¬  formed by special public health laboratories for epidemio¬  logic purposes.   Salmonellosis is a notifiable disease, and an investiga¬  tion to determine its source should be undertaken. In cer¬  tain cases of enteric fever and sepsis, when the organism is  difficult to recover, the diagnosis can be made serologically  by detecting a rise in antibody titer in the patient’s serum  (Widal test).   Enterocolitis caused by Salmonella is usually a self-limited  disease that resolves without treatment. Fluid and electro¬  lyte replacement may be required. Antibiotic treatment  does not shorten the illness or reduce the symptoms; in  fact, it may prolong excretion of the organisms, increase the  frequency of the carrier state, and select mutants resistant  to the antibiotic. Antimicrobial agents are indicated only  for neonates or persons with chronic diseases who are at  risk for septicemia and disseminated abscesses. Plasmid-  mediated antibiotic resistance is common, and antibiotic  sensitivity tests should be done. Drugs that retard intestinal  motility (i.e., that reduce diarrhea) appear to prolong the  duration of symptoms and the fecal excretion of the  organisms.   The treatment of choice for enteric fevers such as  typhoid fever and septicemia with metastatic infection is  either ceftriaxone or ciprofloxacin. Ampicillin or cipro¬  floxacin should be used in patients who are chronic carriers  of S. typhi. Cholecystectomy may be necessary to abolish  the chronic carrier state. Focal abscesses should be drained  surgically when feasible.   Salmonella infections are prevented mainly by public health  and personal hygiene measures. Proper sewage treatment,    a chlorinated water supply that is monitored for contami¬  nation by coliform bacteria, cultures of stool samples from  food handlers to detect carriers, handwashing prior to food  handling, pasteurization of milk, and proper cooking of  poultry, eggs, and meat are all important.   Two vaccines are available, but they confer limited  (50%-80%) protection against S. typhi. One contains the Vi  capsular polysaccharide of S. typhi (given intramuscularly),  and the other contains a live, attenuated strain (Ty21a) of S.    typhi (given orally). The two vaccines are equally effective.  The vaccine is recommended for those who will travel or  reside in high-risk areas and for those whose occupation  brings them in contact with the organism. A new conjugate  vaccine against typhoid fever containing the capsular poly¬  saccharide (Vi) antigen coupled to a carrier protein is safe  and immunogenic in young children but is not available in  the United States at this time.   Shigella species cause enterocolitis. Enterocolitis caused by  Shigella is often called bacillary dysentery. The term dysen¬  tery refers to bloody diarrhea.   Shigellae are non-lactose-fermenting, gram-negative rods  that can be distinguished from salmonellae by three crite¬  ria: they produce no gas from the fermentation of glucose,  they do not produce H 2 S, and they are nonmotile. All  shigellae have O antigens (polysaccharide) in their cell  walls, and these antigens are used to divide the genus into  four groups: A, B, C, and D.   Shigellae are the most effective pathogens among the  enteric bacteria. They have a very low ID 50 (see page 31).  Ingestion of as few as 100 organisms causes disease,  whereas at least 10 5 V. cholerae or Salmonella organisms are  required to produce symptoms. Various properties of shi¬  gellae and salmonellae are compared in Table 18-9.   Shigellosis is only a human disease (i.e., there is no  animal reservoir). The organism is transmitted by the  fecal-oral route. The four Fs—fingers, flies, food, and  feces—are the principal factors in transmission. Foodborne  outbreaks outnumber waterborne outbreaks by 2 to 1. Out¬  breaks occur in day care nurseries and in mental hospitals,  where fecal-oral transmission is likely to occur. Children  younger than 10 years account for approximately half of  Shigella- positive stool cultures. There is no prolonged car¬  rier state with Shigella infections, unlike that seen with S.  typhi infections.   Shigellae, which cause disease almost exclusively in the  gastrointestinal tract, produce bloody diarrhea (dysentery)  by invading the cells of the mucosa of the distal ileum and  colon. Local inflammation accompanied by ulceration  occurs, but the organisms rarely penetrate through the  wall or enter the bloodstream, unlike salmonellae.  Although some strains produce an enterotoxin (called    Shiga toxin), invasion is the critical factor in pathogenesis.  The evidence for this is that mutants that fail to produce  enterotoxin but are invasive can still cause disease, whereas  noninvasive mutants are nonpathogenic. Shiga toxins are  encoded by lysogenic bacteriophages. Shiga toxins very  similar to those produced by Shigella are produced by  enterohemorrhagic E. coli 0157:H7 strains that cause  enterocolitis and F1US.   After an incubation period of 1 to 4 days, symptoms  begin with fever and abdominal cramps, followed by  diarrhea, which may be watery at first but later contains  blood and mucus. The disease varies from mild to severe  depending on two major factors: the species of Shigella  and the age of the patient, with young children and  elderly people being the most severely affected. Shigella  dysenteriae, which causes the most severe disease, is usu¬  ally seen in the United States only in travelers returning  from abroad. Shigella sonnei, which causes mild disease,  is isolated from approximately 75% of all individuals with  shigellosis in the United States. The diarrhea frequently  resolves in 2 or 3 days; in severe cases, antibiotics can  shorten the course. Serum agglutinins appear after recov¬  ery but are not protective because the organism does not  enter the blood. The role of intestinal IgA in protection is  uncertain.   Shigellae form non-lactose-fermenting (colorless) colonies  on MacConkey’s or EMB agar. On TSI agar, they cause an  alkaline slant and an acid butt, with no gas and no H 2 S.  Confirmation of the organism as Shigella and determina¬  tion of its group are done by slide agglutination.   One important adjunct to laboratory diagnosis is a  methylene blue stain of a fecal sample to determine  whether neutrophils are present. If they are found, an inva¬  sive organism such as Shigella, Salmonella, or Campylo¬  bacter is involved rather than a toxin-producing organism  such as V. cholerae, E. coli, or Clostridium perfringens.  (Certain viruses also cause diarrhea without neutrophils in  the stool.)   The main treatment for shigellosis is fluid and electrolyte  replacement. In mild cases, no antibiotics are indicated. In  severe cases, a fluoroquinolone (e.g., ciprofloxacin) is the  drug of choice, but the incidence of plasmids conveying  multiple drug resistance is high enough that antibiotic  sensitivity tests must be performed. Trimethoprim-  sulfamethoxazole is an alternative choice. Antiperistaltic  drugs are contraindicated in shigellosis, because they pro¬  long the fever, diarrhea, and excretion of the organism.   Prevention of shigellosis is dependent on interruption of  fecal-oral transmission by proper sewage disposal, chlori¬  nation of water, and personal hygiene (handwashing by  food handlers). There is no vaccine, and prophylactic anti¬  biotics are not recommended.   Vibrio cholerae, the major pathogen in this genus, is the  cause of cholera. Vibrio parahaemolyticus causes diarrhea  associated with eating raw or improperly cooked seafood.  Vibrio vulnificus causes cellulitis and sepsis. Important fea¬  tures of pathogenesis by V. cholerae, C. jejuni, and Helico¬  bacter pylori are described in Table 18-10.   Vibrios are curved, comma-shaped, gram-negative rods  (see Figure 18-2). V cholerae is divided into two groups  according to the nature of its O cell wall antigen. Members  of the 01 group cause epidemic disease, whereas non-01  organisms either cause sporadic disease or are nonpatho¬  gens. The 01 organisms have two biotypes, called classic  and El Tor, and three serotypes, called Ogawa, Inaba, and  Hikojima. (Biotypes are based on differences in biochemi¬  cal reactions, whereas serotypes are based on antigenic  differences.) These features are used to characterize isolates  in epidemiologic investigations. Serogroup 0139 organ¬  isms, which caused a major epidemic in 1992, are identified  by their reaction to antisera to the 0139 polysaccharide  antigens (O antigen).    Vibrio parahaemolyticus and V vulnificus are marine  organisms; they live primarily in the ocean, especially in  warm salt water. They are halophilic (i.e., they require a  high NaCl concentration to grow).   Vibrio cholerae is transmitted by fecal contamination of  water and food, primarily from human sources. Human  carriers are frequently asymptomatic and include individu¬  als who are either in the incubation period or convalescing.  The main animal reservoirs are marine shellfish, such as  shrimp and oysters. Ingestion of these without adequate  cooking can transmit the disease.   A major epidemic of cholera, which spanned the 1960s  and 1970s, began in Southeast Asia and spread over three  continents to areas of Africa, Europe, and the rest of Asia.  Another epidemic of cholera began in Peru in 1991 and has  spread to many countries in Central and South America.  The organism isolated most frequently was the El Tor bio-  type of 01 V cholerae, usually of the Ogawa serotype. The  factors that predispose to epidemics are poor sanitation,  malnutrition, overcrowding, and inadequate medical ser¬  vices. Quarantine measures failed to prevent the spread of  the disease because there were many asymptomatic carri¬  ers. In 1992, V. cholerae serogroup 0139 emerged and  caused a widespread epidemic of cholera in India and  Bangladesh.   The pathogenesis of cholera is dependent on coloni¬  zation of the small intestine by the organism and secre¬  tion of enterotoxin. For colonization to occur, large  numbers of bacteria must be ingested because the organ¬  ism is particularly sensitive to stomach acid. Persons  with little or no stomach acid, such as those taking ant¬  acids or those who have had gastrectomy, are much more  susceptible. Adherence to the cells of the brush border of  the gut, which is a requirement for colonization, is  related to secretion of the bacterial enzyme mucinase,  which dissolves the protective glycoprotein coating over  the intestinal cells.   After adhering, the organism multiplies and secretes an  enterotoxin called choleragen (cholera toxin). This exo¬  toxin can reproduce the symptoms of cholera even in the  absence of the Vibrio organisms. The mode of action of    TABLE 18-10 Important Features of Pathogenesis by Curved Gram-Negative Rods Affecting the  Gastrointestinal Tract    cholera toxin is described in the next paragraph and in  Figure 7-3 in the chapter on bacterial pathogenesis.   Choleragen consists of an A (active) subunit and a B  (binding) subunit. The B subunit, which is a pentamer  composed of five identical proteins, binds to a ganglioside  receptor on the surface of the enterocyte. The A subunit is  inserted into the cytosol, where it catalyzes the addition of  ADP-ribose to the G s protein (G s is the stimulatory G pro¬  tein). This locks the G s protein in the “on” position, which  causes the persistent stimulation of adenylate cyclase. The  resulting overproduction of cyclic AMP activates cyclic  AMP-dependent protein kinase, an enzyme that phos-  phorylates ion transporters in the cell membrane, resulting  in the loss of water and ions from the cell. The watery  efflux enters the lumen of the gut, resulting in a massive  watery diarrhea that contains neither neutrophils nor red  blood cells. Morbidity and death are due to dehydration  and electrolyte imbalance. However, if treatment is insti¬  tuted promptly, the disease runs a self-limited course in up  to 7 days.   The genes for cholera toxin and other virulence factors  are carried on a single-stranded DNA bacteriophage called  CTX. Lysogenic conversion of non-toxin-producing strains  to toxin-producing ones can occur when the CTX phage  transduces these genes. The pili that attach the organism to  the gut mucosa are the receptors for the phage.   Non-Ol V. cholerae is an occasional cause of diarrhea  associated with eating shellfish obtained from the coastal  waters of the United States.   Watery diarrhea in large volumes is the hallmark of chol¬  era. There are no red blood cells or white blood cells in the  stool. Rice-water stool is the term often applied to the  nonbloody effluent. There is no abdominal pain, and sub¬  sequent symptoms are referable to the marked dehydration.  The loss of fluid and electrolytes leads to cardiac and renal  failure. Acidosis and hypokalemia also occur as a result of  loss of bicarbonate and potassium in the stool. The mortal¬  ity rate without treatment is 40%.   The approach to laboratory diagnosis depends on the situ¬  ation. During an epidemic, a clinical judgment is made and  there is little need for the laboratory. In an area where the  disease is endemic or for the detection of carriers, a variety  of selective media 3 that are not in common use in the  United States are used in the laboratory.   For diagnosis of sporadic cases in this country, a culture  of the diarrhea stool containing V. cholerae will show    3 Media such as thiosulfate-citrate-bile salts agar or tellurite-taurocholate-  gelatin are used.    colorless colonies on MacConkey’s agar because lactose is  fermented slowly. The organism is oxidase-positive, which  distinguishes it from members of the Enterobacteriaceae.  On TSI agar, an acid slant and an acid butt without gas or  H 2 S are seen because the organism ferments sucrose. A  presumptive diagnosis of V. cholerae can be confirmed by  agglutination of the organism by polyvalent 01 or non-01  antiserum. A retrospective diagnosis can be made serologi¬  cally by detecting a rise in antibody titer in acute- and  convalescent-phase sera.   Treatment consists of prompt, adequate replacement of  water and electrolytes, either orally or intravenously.  Glucose is added to the solution to enhance the uptake of  water and electrolytes. Antibiotics such as tetracycline are  not necessary, but they do shorten the duration of symp¬  toms and reduce the time of excretion of the organisms.   Prevention is achieved mainly by public health measures  that ensure a clean water and food supply. There are two  oral vaccines. One, called Dukoral, contains killed whole  cells of the 0-1 strain plus recombinant cholera toxin sub¬  unit B. Antibodies induced by the vaccine prevent ingested  V. cholerae from attaching to the intestinal mucosa and  neutralize any cholera toxin that is produced. The second  vaccine is a killed whole cell vaccine called Shanchol. It  contains both 0-1 and 0-159 strains and was reported to  be very effective in field trials in 2014. As of this writing,  neither vaccine is available in the United States. The inject¬  able killed vaccine is no longer in use.   The use of tetracycline for prevention is effective in  close contacts but does not prevent the spread of a major  epidemic. Prompt detection of carriers is important in lim¬  iting outbreaks.   Vibrio parahaemolyticus is a marine organism transmitted  by ingestion of raw or undercooked seafood, especially  shellfish such as oysters. It is a major cause of diarrhea in  Japan, where raw fish is eaten in large quantities, but is an  infrequent pathogen in the United States, although several  outbreaks have occurred aboard cruise ships in the Carib¬  bean. Little is known about its pathogenesis, except that an  enterotoxin similar to choleragen is secreted and limited  invasion sometimes occurs.   The clinical picture caused by V. parahaemolyticus var¬  ies from mild to quite severe watery diarrhea, nausea and  vomiting, abdominal cramps, and fever. The illness is self¬  limited, lasting about 3 days. Vibrio parahaemolyticus is  distinguished from V. cholerae mainly on the basis of  growth in NaCl: V. parahaemolyticus grows in 8% NaCl  solution (as befits a marine organism), whereas V. cholerae    does not. No specific treatment is indicated, because the  disease is relatively mild and self-limited. Disease can be  prevented by proper refrigeration and cooking of seafood.   Vibrio vulnificus is also a marine organism (i.e., it is found  in warm salt waters such as the Caribbean Sea). It causes  severe skin and soft tissue infections (cellulitis), especially  in shellfish handlers, who often sustain skin wounds. It  can also cause a rapidly fatal septicemia in immunocom¬  promised people who have eaten raw shellfish containing  the organism. Hemorrhagic bullae in the skin often occur  in patients with sepsis caused by V. vulnificus. Chronic liver  disease (e.g., cirrhosis) predisposes to severe infections.  The recommended treatment is doxycycline.   Campylobacter jejuni is a frequent cause of enterocolitis,  especially in children. C. jejuni infection is a common ante¬  cedent to Guillain-Barre syndrome. Other Campylobacter  species are rare causes of systemic infection, particularly  bacteremia.   Campylobacters are curved, gram-negative rods that  appear either comma- or S-shaped. They are microaero-  philic, growing best in 5% oxygen rather than in the 20%  present in the atmosphere. C. jejuni grows well at 42°C,  whereas Campylobacter intestinalis 4 does not—an observa¬  tion that is useful in microbiologic diagnosis.   Domestic animals such as cattle, chickens, and dogs serve  as a source of the organisms for humans. Transmission is  usually fecal-oral. Food and water contaminated with ani¬  mal feces are the major sources of human infection. Foods,  such as poultry, meat, and unpasteurized milk, are com¬  monly involved. Puppies with diarrhea are a common  source for children. Human-to-human transmission occurs  but is less frequent than animal-to-human transmission.  Campylobacter jejuni is a major cause of diarrhea in the  United States; it was recovered in 4.6% of patients with  diarrhea, compared with 2.3% and 1% for Salmonella and  Shigella, respectively. Campylobacter jejuni is the leading  cause of diarrhea associated with consumption of unpas¬  teurized milk.   Features of pathogenesis by Campylobacter are described  in Table 18-10. Inflammation of the intestinal mucosa    often occurs, accompanied by blood in stools. Systemic  infections (e.g., bacteremia) occur most often in neonates  or debilitated adults.   Enterocolitis, caused primarily by C. jejuni, begins as  watery, foul-smelling diarrhea followed by bloody stools  accompanied by fever and severe abdominal pain. Systemic  infections, most commonly bacteremia, are caused more  often by C. intestinalis. The symptoms of bacteremia (e.g.,  fever and malaise) are associated with no specific physical  findings.   Gastrointestinal infection with C. jejuni is associated  with Guillain-Barre syndrome, the most common cause of  acute neuromuscular paralysis. Guillain-Barre syndrome is  an autoimmune disease attributed to the formation of anti¬  bodies against C. jejuni that cross-react with antigens on  neurons (see Chapter 66). Infection with Campylobacter is  also associated with two other autoimmune diseases: reac¬  tive arthritis and Reiter’s syndrome. These are also described  in Chapter 66.   If the patient has diarrhea, a stool specimen is cultured on  a blood agar plate containing antibiotics 5 that inhibit most  other fecal flora.   The plate is incubated at 42°C in a microaerophilic  atmosphere containing 5% oxygen and 10% carbon diox¬  ide, which favors the growth of C. jejuni. It is identified by  failure to grow at 25°C, oxidase positivity, and sensitivity to  nalidixic acid. Unlike Shigella and Salmonella, lactose fer¬  mentation is not used as a distinguishing feature. If bacte¬  remia is suspected, a blood culture incubated under  standard temperature and atmospheric conditions will  reveal the growth of the characteristically comma- or  S-shaped, motile, gram-negative rods. Identification of the  organism as C. intestinalis is confirmed by its failure to  grow at 42°C, its ability to grow at 25°C, and its resistance  to nalidixic acid.   Erythromycin or ciprofloxacin is used successfully in C.  jejuni enterocolitis. The treatment of choice for C. intestina¬  lis bacteremia is an aminoglycoside.   There is no vaccine or other specific preventive measure.  Proper sewage disposal and personal hygiene (handwash¬  ing) are important.    5 For example, Skirrow s medium contains vancomycin, trimethoprim,  cephalothin, polymyxin, and amphotericin B.   Helicobacter pylori causes gastritis and peptic ulcers. Infection  with H. pylori is a risk factor for gastric carcinoma and is  linked to mucosal-associated lymphoid tissue (MALT)  lymphomas.   Helicobacters are curved gram-negative rods similar in  appearance to Campylobacters, but because they differ suf¬  ficiently in certain biochemical and flagellar characteristics,  they are classified as a separate genus. In particular, helico-  bacters are strongly urease-positive, whereas Campylobac¬  ters are urease-negative.   Helicobacter pylori attaches to the mucus-secreting cells of  the gastric mucosa. The production of large amounts of  ammonia from urea by the organism’s urease, coupled with  an inflammatory response, leads to damage to the mucosa.  Loss of the protective mucus coating predisposes to gastri¬  tis and peptic ulcer (see Table 18-10). The ammonia also  neutralizes stomach acid, allowing the organism to survive.  Epidemiologically, most patients with these diseases show  H. pylori in biopsy specimens of the gastric epithelium.   The natural habitat of H. pylori is the human stomach,  and it is probably acquired by ingestion. However, it has not  been isolated from stool, food, water, or animals. Person-  to-person transmission probably occurs because there is  clustering of infection within families. The rate of infection  with H. pylori in developing countries is very high—a find¬  ing that is in accord with the high rate of gastric carcinoma  in those countries.   MALT lymphomas are B-cell tumors located typically in  the stomach, but they occur elsewhere in the gastrointesti¬  nal tract as well. Helicobacter pylori is often found in the  MALT lesion, and the chronic inflammation induced by    the organism is thought to stimulate B-cell proliferation  and eventually a B-cell lymphoma. Antibiotic treatment  directed against the organism often causes the tumor to  regress.   Gastritis and peptic ulcer are characterized by recurrent  pain in the upper abdomen, frequently accompanied by  bleeding into the gastrointestinal tract. No bacteremia or  disseminated disease occurs.   The organism can be seen on Gram-stained smears of  biopsy specimens of the gastric mucosa. It can be cultured  on the same media as Campylobacters. In contrast to C.  jejuni, H. pylori is urease-positive. Urease production is the  basis for a noninvasive diagnostic test called the “urea  breath” test. In this test, radiolabeled urea is ingested. If the  organism is present, urease will cleave the ingested urea,  radiolabeled C0 2 is evolved, and the radioactivity is  detected in the breath.   A test for Helicobacter antigen in the stool can be used  for diagnosis and for confirmation that treatment has  eliminated the organism. The presence of IgG antibodies in  the patient’s serum can also be used as evidence of  infection.   The concept that underlies the choice of drugs is to use anti¬  biotics to eliminate Helicobacter plus a drug to reduce gastric  acidity. A combination of two antibiotics is used because  resistance, especially to metronidazole, has emerged. Treat¬  ment of duodenal ulcers with antibiotics (e.g., amoxicillin  and metronidazole) and bismuth salts (Pepto-Bismol) results  in a greatly decreased recurrence rate. Tetracycline can be  used instead of amoxicillin. There is no vaccine or other  specific preventive measure.   These organisms are usually opportunistic pathogens that  cause nosocomial infections, especially pneumonia and  urinary tract infections. Klebsiella pneumoniae is an impor¬  tant respiratory tract pathogen outside hospitals as well.   Klebsiella pneumoniae, Enterobacter cloacae, and Serratia  marcescens are the species most often involved in human    infections. They are frequently found in the large intes¬  tine but are also present in soil and water. These organisms  have very similar properties and are usually distinguished  on the basis of several biochemical reactions and motility.  Klebsiella pneumoniae has a very large polysaccharide  capsule, which gives its colonies a striking mucoid appear¬  ance. Serratia marcescens produces red-pigmented colo¬  nies (Figure 18-3).   Of the three organisms, K. pneumoniae is most likely to be  a primary, nonopportunistic pathogen; this property is   Arrow points to a red-pigmented colony of S. marcescens. (Used with  permission from Professor Shirley Lowe, University of California, San Francisco  School of Medicine.)   related to its antiphagocytic capsule. Although this organ¬  ism is a primary pathogen, patients with K. pneumoniae  infections frequently have predisposing conditions such as  advanced age, chronic respiratory disease, diabetes, or alco¬  holism. The organism is carried in the respiratory tract of  about 10% of healthy people, who are prone to pneumonia  if host defenses are lowered.   Enterobacter and Serratia infections are clearly related to  hospitalization, especially to invasive procedures such as  intravenous catheterization, respiratory intubation, and  urinary tract manipulations. In addition, outbreaks of Ser¬  ratia pneumonia have been associated with contamination  of the water in respiratory therapy devices. Prior to the  extensive use of these procedures, S. marcescens was a  harmless organism most frequently isolated from environ¬  mental sources such as water.   Serratia also causes endocarditis in users of injection  drugs. As with many other gram-negative rods, the patho¬  genesis of septic shock caused by these organisms is related  to the endotoxins in their cell walls.   Urinary tract infections and pneumonia are the usual clini¬  cal entities associated with these three bacteria, but bacte¬  remia and secondary spread to other areas such as the  meninges and liver occur. It is difficult to distinguish infec¬  tions caused by these organisms on clinical grounds, with    the exception of pneumonia caused by Klebsiella, which  produces a thick, mucoid, bloody sputum (“currant-jelly”  sputum) and can progress to necrosis and abscess  formation.   There are two other species of Klebsiella that cause  unusual human infections rarely seen in the United States.  Klebsiella ozaenae is associated with atrophic rhinitis, and  Klebsiella rhinoscleromatis causes a destructive granuloma  of the nose and pharynx.   Organisms of this group produce lactose-fermenting (col¬  ored) colonies on differential agar such as MacConkey’s or  EMB, although Serratia, which is a late lactose fermenter,  can produce a negative reaction. These organisms are dif¬  ferentiated by the use of biochemical tests.   Because the antibiotic resistance of these organisms can  vary greatly, the choice of drug depends on the results of  sensitivity testing. Isolates from hospital-acquired infec¬  tions are frequently resistant to multiple antibiotics.  Carbapenem-resistant strains are an important cause of  hospital-acquired infections and are resistant to almost all  known antibiotics. An aminoglycoside (e.g., gentamicin)  and a cephalosporin (e.g., cefotaxime) are used empirically  until the results of testing are known. In severe Enterobacter  infections, a combination of imipenem and gentamicin is  often used.   Some hospital-acquired infections caused by gram¬  negative rods can be prevented by such general measures  as changing the site of intravenous catheters, removing  urinary catheters when they are no longer needed, and  taking proper care of respiratory therapy devices. There is  no vaccine.   These organisms primarily cause urinary tract infections,  both community- and hospital-acquired.   These gram-negative rods are distinguished from other  members of the Enterobacteriaceae by their ability to pro¬  duce the enzyme phenylalanine deaminase. In addition,  they produce the enzyme urease, which cleaves urea to  form NH 3 and C0 2 . Certain species are very motile and  produce a striking swarming effect on blood agar,   FIGURE 18-4 Proteus species—swarming motility on blood  agar. Arrowhead points to the site where Proteus bacteria were  placed on the blood agar. Short arrow points to the edge of the first  ring of swarming motility. Long arrow points to the edge of the sec¬  ond ring of swarming motility. (Used with permission from Professor Shirley  Lowe, University of California, San Francisco School of Medicine.)    characterized by expanding rings (waves) of organisms  over the surface of the agar (Figure 18-4).   The cell wall O antigens of certain strains of Proteus,  such as OX-2, OX-19, and OX-K, cross-react with antigens  of several species of rickettsiae. These Proteus antigens can  be used in laboratory tests to detect the presence of anti¬  bodies against certain rickettsiae in patients’ serum. This  test, called the Weil-Felix reaction after its originators, is  being used less frequently as more specific procedures are  developed.   In the past, there were four medically important species  of Proteus. Flowever, molecular studies of DNA relatedness  showed that two of the four were significantly different.  These species have been renamed: Proteus morganii is now  Morganella morganii, and Proteus rettgeri is now Providen-  cia rettgeri. In the clinical laboratory, these organisms are  distinguished from Proteus vulgaris and Proteus mirabilis  on the basis of several biochemical tests.   The organisms are present in the human colon as well as in  soil and water. Their tendency to cause urinary tract infec¬  tions is probably due to their presence in the colon and to  colonization of the urethra, especially in women. The vig¬  orous motility of Proteus organisms may contribute to their  ability to invade the urinary tract.   Production of the enzyme urease is an important feature  of the pathogenesis of urinary tract infections by this group.  Urease hydrolyzes the urea in urine to form ammonia, which  raises the pH, producing an alkaline urine. This encourages    the formation of stones (calculi) called “struvite” composed  of magnesium ammonium phosphate. Struvite stones often  manifest, as staghorn calculi in the renal pelvis. They  obstruct urine flow, damage urinary epithelium, and serve as  a nidus for recurrent infection by trapping bacteria within  the stone. Because alkaline urine also favors growth of the  organisms and more extensive renal damage, treatment  involves keeping the urine at a low pH.   The signs and symptoms of urinary tract infections caused  by these organisms cannot be distinguished from those  caused by E. coli or other members of the Enterobacteria-  ceae. Proteus species can also cause pneumonia, wound  infections, and septicemia. Proteus mirabilis is the species  of Proteus that causes most community- and hospital-  acquired infections, but P. rettgeri is emerging as an impor¬  tant agent of nosocomial infections.   These organisms usually are highly motile and produce a  “swarming” overgrowth on blood agar, which can frustrate  efforts to recover pure cultures of other organisms. Growth  on blood agar containing phenylethyl alcohol inhibits  swarming, thus allowing isolated colonies of Proteus  and other organisms to be obtained. They produce non¬  lactose-fermenting (colorless) colonies on MacConkey’s or  EMB agar. Proteus vulgaris and P. mirabilis produce H 2 S,  which blackens the butt of TSI agar, whereas neither M.  morganii nor P. rettgeri does. Proteus mirabilis is indole¬  negative, whereas the other three species are indole¬  positive—a distinction that can be used clinically to guide  the choice of antibiotics. These four medically important  species are urease-positive. Identification of these organ¬  isms in the clinical laboratory is based on a variety of bio¬  chemical reactions.   Most strains are sensitive to aminoglycosides and  trimethoprim-sulfamethoxazole, but because individual  isolates can vary, antibiotic sensitivity tests should be per¬  formed. Proteus mirabilis is the species most frequently sen¬  sitive to ampicillin. The indole-positive species ( P. vulgaris,  M. morganii, and P. rettgeri) are more resistant to antibiotics  than is P. mirabilis, which is indole-negative. The treatment  of choice for the indole-positive species is a cephalosporin  (e.g., cefotaxime). Proteus rettgeri is frequently resistant to  multiple antibiotics.   There are no specific preventive measures, but many hospital-  acquired urinary tract infections can be prevented by  prompt removal of urinary catheters.   FIGURE 18-5 Cellulitis caused by Pseudomonas aeruginosa.  Note the blue-green color of the pus in the burn wound infection.  (Used with permission from Dr. Robert L. Sheridan.)   Pseudomonas aeruginosa causes infections (e.g., sepsis,  pneumonia, and urinary tract infections) primarily in  patients with lowered host defenses. It also causes chronic  lower respiratory tract infections in patients with cystic  fibrosis, wound infections (cellulitis) in burn patients  (Figure 18-5), and malignant otitis externa in diabetic  patients. It is the most common cause of ventilator-  associated pneumonia. ( P. aeruginosa is also known as  Burkholderia aeruginosa.) Pseudomonas cepacia (renamed  Burkholderia cepacia) and Pseudomonas maltophilia  (renamed Xanthomonas maltophilia and now called Ste-  notrophomonas maltophilia) also cause these infections,  but much less frequently. Pseudomonas pseudomallei (also  known as Burkholderia pseudomallei), the cause of melioi¬  dosis, is described in Chapter 27.   Pseudomonads are gram-negative rods that resemble the  members of the Enterobacteriaceae but differ in that they  are strict aerobes (i.e., they derive their energy only by oxi¬  dation of sugars rather than by fermentation). Because they  do not ferment glucose, they are called nonfermenters, in  contrast to the members of the Enterobacteriaceae, which  do ferment glucose. Oxidation involves electron transport  by cytochrome c (i.e., they are oxidase-positive).   Pseudomonads are able to grow in water containing  only traces of nutrients (e.g., tap water), and this favors  their persistence in the hospital environment. Pseudomonas  aeruginosa and B. cepacia have a remarkable ability to with¬  stand disinfectants; this accounts in part for their role in  hospital-acquired infections. They have been found grow¬  ing in hexachlorophene-containing soap solutions, in anti¬  septics, and in detergents.   FIGURE 18-6 Pseudomonas aeruginosa —blue-green pigment.  Blue-green pigment (pyocyanin) produced by P. aeruginosa diffuses  into the agar. (Used with permission from Professor Shirley Lowe, University of  California, San Francisco School of Medicine.)    Pseudomonas aeruginosa produces two pigments useful  in clinical and laboratory diagnosis: (1) pyocyanin, which  can color the pus in a wound blue, and (2) pyoverdin  (fluorescein), a yellow-green pigment that fluoresces under  ultraviolet light, a property that can be used in the early  detection of skin infection in burn patients. In the labora¬  tory, these pigments diffuse into the agar, imparting a  blue-green color that is useful in identification. Pseudomo¬  nas aeruginosa is the only species of Pseudomonas that  synthesizes pyocyanin (Figure 18-6).   Strains of P. aeruginosa isolated from cystic fibrosis  patients have a prominent slime layer (glycocalyx), which  gives their colonies a very mucoid appearance. The slime  layer mediates adherence of the organism to mucous mem¬  branes of the respiratory tract and prevents antibody from  binding to the organism.   Pseudomonas aeruginosa is found chiefly in soil and water,  although approximately 10% of people carry it in the nor¬  mal flora of the colon. It is found on the skin in moist areas  and can colonize the upper respiratory tract of hospital¬  ized patients. Its ability to grow in simple aqueous solu¬  tions has resulted in contamination of respiratory therapy  and anesthesia equipment, intravenous fluids, and even  distilled water.   Pseudomonas aeruginosa is primarily an opportunistic  pathogen that causes infections in hospitalized patients  (e.g., those with extensive burns), in whom the skin host  defenses are destroyed; in those with chronic respiratory  disease (e.g., cystic fibrosis), in whom the normal clearance  mechanisms are impaired; in those who are immunosup-  pressed; in those with neutrophil counts of less than 500/pL;    and in those with indwelling catheters. It causes 10% to  20% of hospital-acquired infections and, in many hospitals,  is the most common cause of gram-negative nosocomial  pneumonia, especially ventilator-associated pneumonia.   Pathogenesis is based on multiple virulence factors:  endotoxin, exotoxins, and enzymes. Its endotoxin, like that  of other gram-negative bacteria, causes the symptoms of  sepsis and septic shock. The best known of the exotoxins is  exotoxin A, which causes tissue necrosis. It inhibits eukary¬  otic protein synthesis by the same mechanism as diphtheria  exotoxin, namely, ADP-ribosylation of elongation factor-2.  It also produces enzymes, such as elastase and proteases  that are histotoxic and facilitate invasion of the organism  into the bloodstream. Pyocyanin damages the cilia and  mucosal cells of the respiratory tract.   Strains of P. aeruginosa that have a “type III secretion  system” are significantly more virulent than those that do  not. This secretion system transfers the exotoxin from the  bacterium directly into the adjacent human cell, which  allows the toxin to avoid neutralizing antibody. Type III  secretion systems are mediated by transport pumps in the  bacterial cell membrane. Of the four exoenzymes known to  be transported by this secretion system, Exo S is the one  most clearly associated with virulence. Exo S has several  modes of action, the most important of which is ADP-  ribosylation of a Ras protein, leading to damage to the  cytoskeleton.   Pseudomonas aeruginosa can cause infections virtually  anywhere in the body, but urinary tract infections, pneu¬  monia (especially in cystic fibrosis patients), and wound  infections (especially burns) (see Figure 18-5) predomi¬  nate. It is an important cause of hospital-acquired pneu¬  monia, especially in those undergoing mechanical  ventilation (ventilator-associated pneumonia). From these  sites, the organism can enter the blood, causing sepsis. The  bacteria can spread to the skin, where they cause black,  necrotic lesions called ecthyma gangrenosum (Figure  18-7). Patients with P. aeruginosa sepsis have a mortality  rate of greater than 50%. It is an important cause of endo¬  carditis in intravenous drug users.   Severe external otitis (malignant otitis externa) and  other skin lesions (e.g., folliculitis) occur in users of swim¬  ming pools and hot tubs (hot tub folliculitis) in which the  chlorination is inadequate. Pseudomonas aeruginosa is the  most common cause of osteomyelitisof the foot in those  who sustain puncture wounds through the soles of gym  shoes. Corneal infections caused by P. aeruginosa are seen  in contact lens users.   Pseudomonas aeruginosa grows as non-lactose-fermenting  (colorless) colonies on MacConkey’s or EMB agar. It is   FIGURE 18~7 Ecthyma gangrenosum. Necrotic skin lesion  caused by Pseudomonas aeruginosa. (Reproduced with permission from  Wolff K, Johnson R ( Saavedra A (eds): Fitzpatrick's Color Atlas & Synopsis of Clinical  Dermatology. 7th ed. New York: McGraw-Hill, 2013. Copyright © 2013 by The  McGraw-Hill Companies, Inc.)    oxidase-positive. A typical metallic sheen of the growth on  TSI agar, coupled with the blue-green pigment on ordinary  nutrient agar (see Figure 18-6), and a fruity aroma are suf¬  ficient to make a presumptive diagnosis. The diagnosis is  confirmed by biochemical reactions. Identification for  epidemiologic purposes is done by bacteriophage or pyo-  cin 6 typing.   Because P. aeruginosa is resistant to many antibiotics,  treatment must be tailored to the sensitivity of each iso¬  late and monitored frequently; resistant strains can  emerge during therapy. The treatment of choice is an  antipseudomonal penicillin (e.g., piperacillin/tazobac-  tam or ticarcillin/clavulanate) plus an aminoglycoside  (e.g., gentamicin or amikacin).Ceftazidime is also effec¬  tive.For infections caused by highly resistant strains,  colistin (polymyxin E) is useful. The drug of choice for  urinary tract infections is ciprofloxacin. The drug of  choice for infections caused by B. cepacia and S. malto-  philia is trimethoprim-sulfamethoxazole.   Prevention of P. aeruginosa infections involves keeping  neutrophil counts above 500/pL, removing indwelling cath¬  eters promptly, taking special care of burned skin, and tak¬  ing other similar measures to limit infection in patients  with reduced host defenses.    6 A pyocin is a type of bacteriocin produced by P. aeruginosa. Different  strains produce various pyocins, which can serve to distinguish the  organisms.   Members of the genus Bacteroides are the most common  cause of serious anaerobic infections (e.g., sepsis, perito¬  nitis, and abscesses). Bacteroides fragilis is the most fre¬  quent pathogen. Prevotella melaninogenica is also an  important pathogen. It was formerly known as Bacteroi¬  des melaninogenicus, and both names are still  encountered.   Bacteroides and Prevotella organisms are anaerobic, non¬  spore-forming, gram-negative rods. Of the many species of  Bacteroides, two are human pathogens: B. fragilis 7 and Bac¬  teroides corrodens.   Members of the B. fragilis group are the predominant  organisms in the human colon, numbering approximately  10 u /g of feces, and are found in the vagina of approxi¬  mately 60% of women. Prevotella melaninogenica and  B. corrodens occur primarily in the oral cavity.   Because Bacteroides and Prevotella species are part of the  normal flora, infections are endogenous, usually arising  from a break in a mucosal surface, and are not communi¬  cable. These organisms cause a variety of infections, such as  local abscesses at the site of a mucosal break, metastatic  abscesses by hematogenous spread to distant organs, or  lung abscesses by aspiration of oral flora.   Predisposing factors such as surgery, trauma, and  chronic disease play an important role in pathogenesis.  Local tissue necrosis, impaired blood supply, and growth of  facultative anaerobes at the site contribute to anaerobic  infections. The facultative anaerobes, such as E. coli, utilize  the oxygen, thereby reducing it to a level that allows the  anaerobic Bacteroides and Prevotella strains to grow. As a  result, many anaerobic infections contain a mixed faculta¬  tive and anaerobic flora. This has important implications  for therapy; both the facultative anaerobes and the anaer¬  obes should be treated.   The polysaccharide capsule of B. fragilis is an important  virulence factor. The host response to the capsule plays a  major role in abscess formation. Note also that the endo¬  toxin of B. fragilis contains a variant lipid A that is missing  one of the fatty acids and consequently is 1000-fold less  active than the typical endotoxin of bacteria such as Neis¬  seria meningitidis.    7 B. fragilis is divided into five subspecies, the most important of  which is B. fragilis subsp. fragilis. The other four subspecies are B.  fragilis subsp. distasonis, ovatus, thetaiotamicron, and vulgatus. It is  proper, therefore, to speak of the B. fragilis group rather than simply  B. fragilis.    Enzymes such as hyaluronidase, collagenase, and phos¬  pholipase are produced and contribute to tissue damage.  Enterotoxin-producing strain of B. fragilis can cause diar¬  rhea in both children and adults.   The B. fragilis group of organisms is most frequently associ¬  ated with intra-abdominal infections, either peritonitis or  localized abscesses. Pelvic abscesses, necrotizing fasciitis,  and bacteremia occur as well. Abscesses of the mouth,  pharynx, brain, and lung are more commonly caused by  P. melaninogenica, a member of the normal oral flora, but  B. fragilis is found in about 25% of lung abscesses. In gen¬  eral, B. fragilis causes disease below the diaphragm, whereas  P. melaninogenica causes disease above the diaphragm.  Prevotella intermedia is an important cause of gingivitis,  periodontitis, and dental abscess.   Bacteroides species can be isolated anaerobically on blood  agar plates containing kanamycin and vancomycin to inhibit  unwanted organisms. They are identified by biochemical  reactions (e.g., sugar fermentations) and by production of  certain organic acids (e.g., formic, acetic, and propionic  acids), which are detected by gas chromatography. Prevotella  melaninogenica produces characteristic black colonies  (Figure 18-8).   colonies. Arrow points to a black pigmented colony of P. melanino¬  genica. (Used with permission from Professor Shirley Lowe, University of California,  San Francisco School of Medicine.)   Members of the B. fragilis group are resistant to penicillins,  first-generation cephalosporins, and aminoglycosides,  making them among the most antibiotic-resistant of the  anaerobic bacteria. Penicillin resistance is the result of  |3-lactamase production. Metronidazole is the drug of  choice, with cefoxitin, clindamycin, and chloramphenicol  as alternatives. Aminoglycosides are frequently combined  to treat the facultative gram-negative rods in mixed infec¬  tions. The drug of choice for P. melaninogenica infections  is either metronidazole or clindamycin. |3-Lactamase-  producing strains of P. melaninogenica have been isolated  from patients. Surgical drainage of abscesses usually  accompanies antibiotic therapy, but lung abscesses often  heal without drainage.   Prevention of Bacteroides and Prevotella infections centers  on perioperative administration of a cephalosporin, fre¬  quently cefoxitin, for abdominal or pelvic surgery. There is  no vaccine.   Fusobacterium species are long, anaerobic gram-negative  rods with pointed ends (Figure 18-9). They are part of the  human normal flora of the mouth, colon, and female geni¬  tal tract and are isolated from brain, pulmonary, intra¬  abdominal, and pelvic abscesses. They are frequently found  in mixed infections with other anaerobes and facultative  anaerobes.   Fusobacterium nucleatum occurs, along with various  spirochetes, in cases of Vincent’s angina (trench mouth),   the long, thin gram-negative rods with pointed ends. (Source: Dr. V.R.  Dowell, Jr. Public Health Image Library, Centers for Disease Control and  Prevention.)    which is characterized by a necrotizing ulcerative  gingivitis. Fusobacterium necrophorum causes Lemierre’s  disease, which is an anaerobic infection of the posterior  pharyngeal space accompanied by thrombophlebitis of  the internal jugular vein and metastatic infectious  emboli to the lung.   The laboratory diagnosis is made by culturing the  organism anaerobically. The drug of choice for Fusobacte¬  rium infections is either penicillin G, clindamycin, or met¬  ronidazole. There is no vaccine.   1. Your patient is a 75-year-old man with an indwelling urinary  catheter following prostatectomy for prostate cancer. He now has  the sudden onset of fever to 40°C, blood pressure of 70/40, and a  pulse of 140. You draw several blood cultures, and the laboratory  reports that all are positive for a gram-negative rod that forms  red pigmented colonies. Which one of the following bacteria is  the most likely cause of this infection?   2. You’re a public health epidemiologist who is called to investigate  an outbreak of bloody diarrhea in 16 people. You find that it is  associated with eating rare hamburgers in a particular fast-food  restaurant. A culture of the remaining uncooked hamburger  grows a gram-negative rod that produces a dark purple colony on  EMB agar, which is evidence that it ferments lactose. Which one  of the following bacteria is the most likely cause of this outbreak?   3. Your patient has third-degree burns over most of his body. He  was doing well until 2 days ago, when he spiked a fever, and his  dressings revealed pus that had a blue-green color. Gram stain  of the pus revealed a gram-negative rod that formed colorless  colonies on EMB agar. Which one of the following bacteria is the  most likely cause of this infection?   4. Regarding the patient in Question 3, which one of the follow¬  ing is the best combination of antibiotics to treat the infection?    5. Regarding the members of the family Enterobacteriaceae, which  one of the following is the most accurate?   (A) All members of the family are anaerobic, which means they  must be cultured in the absence of oxygen.   (B) All members of the family ferment lactose, which is an impor¬  tant diagnostic criterion in the clinical laboratory.   (C) All members of the family have endotoxin, an important  pathogenetic factor.   (D) All members of the family produce an enterotoxin, which  ADP-ribosylates a G protein in human enterocytes.   6. You’re on a summer program working in a clinic in a small village  in Ecuador. There is an outbreak of cholera, and your patient has  massive diarrhea and a blood pressure of 70/40. Which one of the  following would be the most appropriate action to take?   (D) Perform stool cultures and fecal leukocyte tests to make an  accurate diagnosis.   7. Your patient is a 20-year-old woman with diarrhea. She has  just returned to the United States from a 3-week trip to Peru,  where she ate some raw shellfish at the farewell party. She now  has severe watery diarrhea, perhaps 20 bowel movements a  day, and is feeling quite weak and dizzy. Her stool is guaiac-  negative, a test that determines whether there is blood in the  stool. A Gram stain of the stool reveals curved gram-negative  rods. Culture of the stool on MacConkey s agar shows colorless  colonies. Which one of the following bacteria is the most likely  cause of this infection?   8. Your patient is a 6-year-old boy with bloody diarrhea for the  past 2 days accompanied by fever to 40°C and vomiting. He has  a pet corn snake. Blood culture and stool culture from the boy  and stool culture from the snake (taken very carefully!) revealed  the same organism. The cultures grew a gram-negative rod that  formed colorless colonies on EMB agar. Which one of the follow¬  ing bacteria is the most likely cause of this infection?   9. Your patient is a 25-year-old woman with pain on urination and  cloudy urine but no fever or flank pain. She has not been hospi¬  talized. You think she probably has cystitis, an infection of the  urinary bladder. A Gram stain of the urine reveals gram-negative  rods. Culture of the urine on EMB agar shows colorless colonies,  and a urease test was positive. Swarming motility was noted on  the blood agar plate. Which one of the following bacteria is the  most likely cause of this infection?    10. Your patient has abdominal pain, and a mass is discovered in  the left lower quadrant. Upon laparotomy (surgical opening of  the abdomen), an abscess is found. Culture of the pus revealed  Bacteroides fragilis. Regarding this organism, which one of the  following is the most accurate?   (A) A stage in the life cycle of Bacteroides fragilis involves forming  spores in the soil.   (B) Bacteroides fragilis is an anaerobic gram-negative rod whose  natural habitat is the human colon.   (C) Bacteroides fragilis produces black colonies when grown on  blood agar.   (D) Pathogenesis by Bacteroides fragilis involves an exotoxin that  increases cyclic AMP by ADP-ribosylation of a G protein.   (E) The toxoid vaccine should be administered to prevent disease  caused by Bacteroides fragilis.   11. Regarding the patient in Question 10, which one of the following  is the best antibiotic to treat the infection?   12. Your patient in the gastrointestinal clinic is a 50-year-old insur¬  ance salesman with what he describes as a “sour stomach” for  several months. Antacids relieve the symptoms. After taking a  complete history and doing a physical examination, you discuss  the case with your resident, who suggests doing a urea breath  test, which tests for the presence of urease. Which one of the  following bacteria does the resident think is the most likely  cause of the patients disease?   13. Your patient is a 35-year-old woman with epilepsy who had a  grand-mal seizure about 2 months ago. She comes to see you  now because she has been coughing up foul-smelling sputum  for the past week. Chest X-ray reveals a cavity with an air-fluid  level. Gram stain of the sputum reveals gram-negative rods, and  culture reveals black colonies that grow on blood agar only in  the absence of air. Which one of the following bacteria is the  most likely cause of this infection?   Questions on the topics discussed in this chapter can be found  in the Clinical Bacteriology section of Part XIII: USMLE  (National Board) Practice Questions starting on page 713. Also  see Part XIV: USMLE (National Board) Practice Examination  starting on page 751.    Brief summaries of the organisms described in this chapter  begin on page 658. Please consult these summaries for a rapid  review of the essential material.   There are four medically important gram-negative rods  typically associated with the respiratory tract, namely, Hae¬  mophilus influenzae, Bordetella pertussis, Legionella pneu¬  mophila, and Acinetobacter baumannii (Table 19-1).  Hemophilus influenzae and B. pertussis are found only in  humans, whereas L. pneumophila is found primarily in  environmental water sources. Acinetobacter baumannii is  found in environmental water sources but also colonizes  the skin and upper respiratory tract.   Additional information regarding the clinical aspects of  infections caused by the organisms in this chapter is provided  in Part EX entitled Infectious Diseases beginning on page 593.   Hemophilus influenzae used to be the leading cause of men¬  ingitis in young children, but the use of the highly effective    “conjugate” vaccine has greatly reduced the incidence of  meningitis caused by this organism. It is still an important  cause of upper respiratory tract infections (otitis media,  sinusitis, conjunctivitis, and epiglottitis) and sepsis in chil¬  dren. It also causes pneumonia in adults, particularly in  those with chronic obstructive lung disease. Haemophilus  ducreyi, the agent of chancroid, is discussed in Chapter 27.   Hemophilus influenzae is a small gram-negative rod (cocco-  bacillary rod)) with a polysaccharide capsule (Figure 19-1).  It is one of the three important encapsulated pyogens,  along with the pneumococcus and the meningococcus.  Serologic typing is based on the antigenicity of the capsular  polysaccharide. Of the six serotypes (a-f), type b is the  most important. Type b used to cause most of the severe,  invasive diseases, such as meningitis and sepsis, but the  widespread use of the vaccine containing the type b capsu¬  lar polysaccharide as the immunogen, has greatly reduced    1 In countries where the H. influenzae b conjugate vaccine has been deployed, the vaccine has greatly reduced the incidence of meningitis caused by this organism.   point to two small "coccobacillary" gram-negative rods. (Used with  permission from Professor Shirley Lowe, University of California, San Francisco  School of Medicine.)    the incidence of invasive disease caused by this type. The  type b capsule is composed of polyribitol phosphate.   Unencapsulated strains can also cause disease, espe¬  cially mucosal diseases of the upper respiratory tract such  as sinusitis and otitis media, but are usually noninvasive.  Growth of the organism on laboratory media requires the  addition of two components, heme (factor X) and NAD  (factor V), for adequate energy production.   Hemophilus influenzae infects only humans; there is no ani¬  mal reservoir. It enters the body by the inhalation of airborne  droplets into the respiratory tract, resulting in either asymp¬  tomatic colonization or infections such as otitis media,  sinusitis, or pneumonia. The organism produces an IgA  protease that degrades secretory IgA, thus facilitating attach¬  ment to the respiratory mucosa. After becoming established  in the upper respiratory tract, the organism can enter the  bloodstream (bacteremia) and spread to the meninges. Men¬  ingitis is caused primarily by the encapsulated strains, but  nonencapsulated strains are frequently involved in otitis  media, sinusitis, and pneumonia. Note that the incidence of  meningitis caused by capsular type b has been greatly  reduced because the vaccine contains the type b polysaccha¬  ride as the immunogen. Pathogenesis of H. influenzae  involves its antiphagocytic capsule and endotoxin; no exo¬  toxin is produced.   Most infections occur in children between the ages of  6 months and 6 years, with a peak in the age group from  6 months to 1 year. This age distribution is attributed to a  decline in maternal IgG in the child coupled with the  inability of the child to generate sufficient antibody against  the polysaccharide capsular antigen until the age of approx¬  imately 2 years.   Meningitis caused by H. influenzae cannot be distinguished  on clinical grounds from that caused by other bacterial  pathogens (e.g., pneumococci or meningococci). The rapid  onset of fever, headache, and stiff neck, along with drowsi¬  ness, is typical. Sinusitis and otitis media cause pain in the  affected area, opacification of the infected sinus, and red¬  ness with bulging of the tympanic membrane. Hemophilus  influenzae is second only to the pneumococcus as a cause  of these two infections.   Other serious infections caused by this organism include  septic arthritis, cellulitis, and sepsis, the latter occurring  especially in splenectomized patients. Rarely, epiglottitis,  which can obstruct the airway, occurs. A swollen “cherry-  red” epiglottis is seen. This life-threatening disease of  young children is caused almost exclusively by H. influen¬  zae. Pneumonia in elderly adults, especially those with  chronic respiratory disease, can be caused by untypeable  strains of H. influenzae.   Laboratory diagnosis depends on isolation of the organism  on heated-blood (“chocolate”) agar enriched with two  growth factors required for bacterial respiration, namely,  factor X (a heme compound) and factor V (NAD). The  blood used in chocolate agar is heated to inactivate nonspe¬  cific inhibitors of H. influenzae growth.   An organism that grows only in the presence of both  growth factors is presumptively identified as H. influen¬  zae; other species of Haemophilus, such as Haemophilus  parainfluenzae, do not require both factors. Definitive  identification can be made with either biochemical tests  or the capsular swelling (quellung) reaction. Additional  means of identifying encapsulated strains include  fluorescent-antibody staining of the organism and coun-  terimmunoelectrophoresis or latex agglutination tests,  which detect the capsular polysaccharide.   The treatment of choice for meningitis or other serious  systemic infections caused by H. influenzae is ceftriaxone.  From 20% to 30% of H. influenzae type b isolates produce  a (3-lactamase that degrades penicillinase-sensitive  (3-lactams such as ampicillin but not ceftriaxone. It is  important to institute antibiotic treatment promptly,  because the incidence of neurologic sequelae (e.g., subdural  empyema) is high. Untreated H. influenzae meningitis has  a fatality rate of approximately 90%. H. influenzae upper  respiratory tract infections, such as otitis media and sinus¬  itis, are treated with either amoxicillin-clavulanate or  trimethoprim-sulfamethoxazole.   The vaccine contains the capsular polysaccharide of H.  influenzae type b conjugated to diphtheria toxoid or other  carrier protein. Depending on the carrier protein, it is given  some time between the ages of 2 and 15 months. This vac¬  cine is much more effective in young children than the  unconjugated vaccine and has reduced the incidence of  meningitis caused by this organism by approximately 90%  in immunized children. Meningitis in close contacts of the  patient can be prevented by rifampin. Rifampin is used  because it is secreted in the saliva to a greater extent than  ampicillin. Rifampin decreases respiratory carriage of the  organism, thereby reducing transmission.   Bordetella pertussis, a pathogen only for humans, is trans¬  mitted by airborne droplets produced during the severe  coughing episodes. The organisms attach to the ciliated  epithelium of the upper respiratory tract but do not invade  the underlying tissue. Decreased cilia activity and subse¬  quent death of the ciliated epithelial cells are important  aspects of pathogenesis.   Pertussis is a highly contagious disease that occurs pri¬  marily in infants and young children and has a worldwide  distribution. The number of cases has declined in the  United States because use of the vaccine is widespread.  However, outbreaks of pertussis during the years 2005,  2010, and 2012 has led to concern about waning immunity  to the vaccine and to the recommendation that an addi¬  tional booster immunization be given (see “Prevention”).  Several factors play a role in the pathogenesis:   (1) Attachment of the organism to the cilia of the epi¬  thelial cells is mediated by a protein on the pili called fila¬  mentous hemagglutinin. Antibody against the filamentous  hemagglutinin inhibits attachment and protects against  disease.   (2) Pertussis toxin stimulates adenylate cyclase by cata¬  lyzing the addition of adenosine diphosphate ribose—a  process called ADP-ribosylation—to the inhibitory subunit  of the G protein complex (G ; protein). This results in pro¬  longed stimulation of adenylate cyclase and a consequent  rise in cyclic adenosine monophosphate (AMP) and in    cyclic AMP-dependent protein kinase activity. This results  in edema of the respiratory mucosa that contributes to the  severe cough of pertussis. The toxin also has a domain that  mediates its binding to receptors on the surface of respira¬  tory tract epithelial cells. It is an A-B subunit toxin.   Pertussis toxin also causes a striking lymphocytosis in  the blood of patients with pertussis. The toxin inhibits sig¬  nal transduction by chemokine receptors, resulting in a  failure of lymphocytes to enter lymphoid tissue such as the  spleen and lymph nodes. Because the lymphocytes do not  enter lymphoid tissue, there is an increase in their number  in the blood (see the discussion of chemokines in Chapter  58). The inhibition of signal transduction by chemokine  receptors is also caused by ADP-ribosylation of the G,  protein.   (3) The organisms also synthesize and export adenylate  cyclase. This enzyme, when taken up by phagocytic cells  (e.g., neutrophils), can inhibit their bactericidal activity.  Bacterial mutants that lack cyclase activity are avirulent.   (4) Tracheal cytotoxin is a fragment of the bacterial  peptidoglycan that damages ciliated cells of the respiratory  tract. Tracheal cytotoxin appears to act in concert with  endotoxin to induce nitric oxide, which kills the ciliated  epithelial cells.   Whooping cough is an acute tracheobronchitis that begins  with mild upper respiratory tract symptoms followed by a  severe paroxysmal cough, which lasts from 1 to 4 weeks.  The paroxysmal pattern is characterized by a series of hack¬  ing coughs, accompanied by production of copious amounts  of mucus, that end with an inspiratory “whoop” as air rushes  past the narrowed glottis. Despite the severity of the symp¬  toms, the organism is restricted to the respiratory tract and  blood cultures are negative. A pronounced leukocytosis  with up to 70% lymphocytes is seen. Although central ner¬  vous system anoxia and exhaustion can occur as a result of  the severe coughing, death is due mainly to pneumonia.   The classic picture of whooping cough described above  occurs primarily in young children. In adults, B. pertussis  infection often manifests as a paroxysmal cough of varying  severity lasting weeks. The characteristic whoop is often  absent, leading to difficulty in recognizing the cough as  caused by this organism. In the correct clinical setting,  adults with a cough lasting several weeks (often called the  100-day cough) should be evaluated for infection with B.  pertussis.   The organism can be isolated from nasopharyngeal swabs  taken during the paroxysmal stage. Bordet-Gengou 1  medium used for this purpose contains a high percentage  of blood (20%-30%) to inactivate inhibitors in the agar.    Identification of the isolated organism can be made by  agglutination with specific antiserum or by fluorescent-  antibody staining. However, the organism grows very  slowly in culture, so direct fluorescent-antibody staining of  the nasopharyngeal specimens can be used for diagnosis.  Polymerase chain reaction-based tests are highly specific  and sensitive and should be used if available.   Isolation of the organism in patients with a prolonged  cough is often difficult. Serologic tests that detect antibody  in the patients serum can be used for diagnosis in those  patients.   Azithromycin is the drug of choice. Note that azithromycin  reduces the number of organisms in the throat and  decreases the risk of secondary complications but has little  effect on the course of the disease at the “prolonged cough”  stage because the toxins have already damaged the respira¬  tory mucosa. Supportive care (e.g., oxygen therapy and  suction of mucus) during the paroxysmal stage is impor¬  tant, especially in infants.   There are two types of vaccines: an acellular vaccine con¬  taining purified proteins from the organism and a killed  vaccine containing inactivated B. pertussis organisms. The  acellular vaccine contains five antigens purified from the  organism. It is the vaccine currently used in the United  States. The main immunogen in this vaccine is inactivated  pertussis toxin (pertussis toxoid). The toxoid in the vaccine  is pertussis toxin that has been inactivated genetically by  introducing two amino acid changes, which eliminates its  ADP-ribosylating activity but retains its antigenicity. It is  the first vaccine to contain a genetically inactivated toxoid.  The other pertussis antigens in the vaccine are filamentous  hemagglutinin, pertactin, and fimbriae types 2 and 3. The  acellular vaccine has fewer side effects than the killed vac¬  cine but has a shorter duration of immunity.   The pertussis vaccine is usually given combined with  diphtheria and tetanus toxoids (DTaP) in three doses  beginning at 2 months of age. A booster at 12 to 15 months  of age and another at the time of entering school are recom¬  mended. Because outbreaks of pertussis have occurred  among teenagers, a booster for those between 10 and  18 years old is recommended. This vaccine, called Boostrix,  contains diphtheria and tetanus toxoids also. Another vac¬  cine called Adacel also contains diphtheria and tetanus  toxoids. A pertussis booster dose is recommended for  adults as well. To protect newborns, pregnant women  should receive pertussis vaccine. Antipertussis IgG will  pass the placenta and protect the newborn.    1 The French scientists who first isolated the organism in 1906.    The killed vaccine is no longer used in the United States  because it is suspected of causing various side effects,  including postvaccine encephalopathy at a rate of about  one case per million doses administered. The killed vaccine  is in use in many countries other than the United States.   Azithromycin is useful in prevention of disease in  exposed, unimmunized individuals. It should also be given  to immunized children younger than 4 years who have  been exposed because vaccine-induced immunity is not  completely protective.   Legionella pneumophila (and other legionellae) causes  pneumonia, both in the community and in hospitalized  immunocompromised patients. The genus is named after  the famous outbreak of pneumonia among people attend¬  ing the American Legion convention in Philadelphia in  1976 (Legionnaires’ disease).   Legionellae are gram-negative rods that stain faintly with the  standard Gram stain. They do, however, have a gram-negative  type of cell wall, and increasing the time of the safranin  counterstain enhances visibility. Legionellae in lung biopsy  sections do not stain by the standard hematoxylin-and-eosin  (H&E) procedure; therefore, special methods, such as the  Dieterle silver impregnation stain, are used to visualize the  organisms.   During the 1976 outbreak, initial attempts to grow the  organisms on ordinary culture media failed. This is because  the organism requires a high concentration of iron and  cysteine. Culture media supplemented with these nutrients  will support growth.   Legionella pneumophila causes approximately 90% of  pneumonia attributed to legionellae. There are 16 sero-  groups of L. pneumophila, with most cases caused by sero-  group 1 organisms. There are about 30 other Legionella  species that cause pneumonia, but most of the remaining  10% of cases are caused by two species, Legionella micdadei  and Legionella bozemanii.   Legionellae are associated chiefly with environmental  water sources such as air conditioners and water-cooling  towers. Outbreaks of pneumonia in hospitals have been  attributed to the presence of the organism in water taps,  sinks, and showers. Legionellae can replicate to large num¬  bers in free-living amebas in these water sources. The  amebas also enhance the survival of Legionellae. Under  adverse environmental conditions, the amebas encyst,  ensuring both their own survival and the survival of the  intracellular Legionellae as well.    The portal of entry is the respiratory tract, and patho¬  logic changes occur primarily in the lung. However, in  severe cases, bacteremia occurs, accompanied by damage  to the vascular endothelium in multiple organs, especially  the brain and kidneys. The major virulence factor of the  organism is lipopolysaccharide (endotoxin). No exotoxins  are produced.   The typical candidate for Legionnaires’ disease is an older  man who smokes and consumes substantial amounts of alco¬  hol. Patients with acquired immunodeficiency syndrome  (AIDS), cancer, or transplants (especially renal transplants)  or patients being treated with corticosteroids are predisposed  to Legionella pneumonia, which indicates that cell-mediated  immunity is the most important defense mechanism.  Despite airborne transmission of the organism, person-to-  person spread does not occur, as shown by the failure of  secondary cases to occur in close contacts of patients.   The clinical picture can vary from a mild influenzalike ill¬  ness to a severe pneumonia accompanied by mental confu¬  sion, nonbloody diarrhea, proteinuria, and microscopic  hematuria. Although cough is a prominent symptom, spu¬  tum is frequently scanty and nonpurulent. Hyponatremia  (serum sodium <130 mEq/L) is an important laboratory  finding that occurs more often in Legionella pneumonia  than in pneumonia caused by other bacteria. Most cases  resolve spontaneously in 7 to 10 days, but in older or  immunocompromised patients, the infection can be fatal.   Legionellosis is an atypical pneumonia 2 and must be  distinguished from other similar pneumonias such as  Mycoplasma pneumonia, viral pneumonia, psittacosis, and  Q fever.   Pontiac fever is a mild, flulike form of Legionella infec¬  tion that does not result in pneumonia. The name “Pon¬  tiac” is derived from the city in Michigan that was the site  of an outbreak in 1968.   Sputum Gram stains reveal many neutrophils but no bacte¬  ria. The organism fails to grow on ordinary media in a  culture of sputum or blood, but it will grow on charcoal-  yeast agar, a special medium supplemented with iron and  cysteine. Diagnosis usually depends on a significant  increase in antibody titer in convalescent-phase serum by  the indirect immunofluorescence assay. Detection of  L. pneumophila antigens in the urine is a rapid means of  making a diagnosis. The urinary antigen test is available  only for serogroup 1 organisms. If tissue is available, it is  possible to demonstrate Legionella antigens in infected  lung tissue by using fluorescent-antibody staining. The    2 A pneumonia is atypical when its causative agent cannot be isolated on  ordinary laboratory media or when its clinical picture does not resemble  that of typical pneumococcal pneumonia.    cold-agglutinin titer does not rise in Legionella pneumonia,  in contrast to pneumonia caused by Mycoplasma.   Azithromycin or erythromycin (with or without rifampin)  is the treatment of choice. Certain fluoroquinolones, such as  levofloxacin and trovafloxacin, are also drugs of choice.  These drugs are effective not only against L. pneumophila,  but also against Mycoplasma pneumoniae and Streptococcus  pneumoniae. The organism frequently produces (3-lactamase,  and so penicillins and cephalosporins are less effective.   Prevention involves reducing cigarette and alcohol con¬  sumption, eliminating aerosols from water sources, and  reducing the incidence of Legionella in hospital water sup¬  plies by using high temperatures and hyperchlorination.  There is no vaccine.   Acinetobacter species are small coccobacillary gram-negative  rods found commonly in soil and water, but they also colo¬  nize the skin and upper respiratory tract. They are oppor¬  tunistic pathogens that readily colonize patients with  compromised host defenses. Acinetobacter baumannii, the  species usually involved in human infection, causes disease  chiefly in a hospital setting usually associated with respira¬  tory therapy equipment (ventilator-associated pneumonia)  and indwelling catheters. Pneumonia and urinary tract  infections are the most frequent manifestations. Laboratory  diagnosis is made by culturing the organism. Acinetobacter  baumannii is remarkably antibiotic resistant, and some  isolates are resistant to all known antibiotics. Imipenem is  the drug of choice for infections caused by susceptible  strains. Colistin is useful in carbapenem-resistant strains.  There is no vaccine. Previous genus names for this organ¬  ism include Herellea and Mima.   1. Your patient is a 75-year-old man who has smoked cigarettes  (two packs a day for more than 50 years) and consumed alcoholic  drinks (a six pack of beer each day) for most of his adult life. He  now has the signs and symptoms of pneumonia. Gram stain of the  sputum reveals neutrophils but no bacteria. Colonies appear on  buffered charcoal yeast (BYCE) agar but not on blood agar. Which  one of the following bacteria is most likely to be the cause of his  pneumonia?    2. Regarding the patient in Question 1, which one of the following is  the best antibiotic to treat the infection?   3. Your patient is a 6-year-old boy who is complaining that his ear  hurts. His mother says this began yesterday and that he has a fever  of 103°E On physical exam, you see a perforated ear drum that is  exuding a small amount of pus. Using a swab, you obtain a sam¬  ple of the pus and do a Gram stain and culture. The Gram stain  reveals small coccobacillary rods. There is no growth on a blood  agar plate, but a chocolate agar plate supplemented with X and  V factors grows small grey colonies. Which one of the following  bacteria is the most likely cause of his otitis media?   4. It’s time to play “What’s my name?” I am a small gram-negative  rod that causes an important respiratory tract disease. I produce  an exotoxin that ADP-ribosylates a G protein. One remarkable  feature of my disease is a great increase in lymphocytes. I don’t  cause disease commonly in the United States now because of the  widespread use of the vaccine that induces antibodies against five  of my proteins, one of which is the exotoxin. The identity of the  mystery organism is mostly likely which of the following?   5. Your patient is a 75-year-old woman with a 110 pack-year history  of cigarette smoking who now has a fever of 39°C and a cough  productive of yellowish sputum. Gram stain of the sputum shows  small gram-negative rods. There is no growth on blood agar, but  colonies do grow on chocolate agar supplemented with hemin and  NAD. Which one of the following bacteria is the most likely cause  of her pneumonia?   6. Your patient is a 5-year-old boy with a high fever and signs of  respiratory tract obstruction. Visualization of the epiglottis shows  inflammation characterized by marked swelling and “cherry-red”  appearance. Which one of the following is the best antibiotic to  treat the infection?   Brief summaries of the organisms described in this chapter  begin on page 662. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Clinical Bacteriology section of Part XIII: USMLE  (National Board) Practice Questions starting on page 713. Also  see Part XIV: USMLE (National Board) Practice Examination  starting on page 751.   Zoonoses are human diseases caused by organisms that  are acquired from animals. There are bacterial, viral,  fungal, and parasitic zoonoses. Some zoonotic organisms  are acquired directly from the animal reservoir, whereas  others are transmitted by vectors, such as mosquitoes,  fleas, or ticks.   There are four medically important gram-negative rods  that have significant animal reservoirs: Brucella species,  Francisella tularensis, Yersinia pestis, and Pasteurella multo-  cida (Table 20-1).   Additional information regarding the clinical aspects of  infections caused by the organisms in this chapter is pro¬  vided in Part IX entitled Infectious Diseases beginning on  page 593.   Brucellae are small gram-negative rods without a capsule.  The three major human pathogens and their animal reser¬  voirs are Brucella melitensis (goats and sheep), Brucella  abortus (cattle), and Brucella suis (pigs).   The organisms enter the body either by ingestion of con¬  taminated milk products or through the skin by direct    contact in an occupational setting such as an abattoir. They  localize in the reticuloendothelial system, namely, the  lymph nodes, liver, spleen, and bone marrow. Many organ¬  isms are killed by macrophages, but some survive within  these cells, where they are protected from antibody. The  host response is granulomatous, with lymphocytes and  epithelioid giant cells, which can progress to form focal  abscesses. The mechanism of pathogenesis of these organ¬  isms is not well defined, except that endotoxin is involved.  No exotoxins are produced.   Imported cheese made from unpasteurized goats’ milk  produced in either Mexico or the Mediterranean region has  been a source of B. melitensis infection in the United States.  The disease occurs worldwide but is rare in the United  States because pasteurization of milk kills the organism.   After an incubation period of 1 to 3 weeks, nonspecific  symptoms such as fever, chills, fatigue, malaise, anorexia,  and weight loss occur. The onset can be acute or gradual.  The undulating (rising-and-falling) fever pattern that gives  the disease its name occurs in a minority of patients.  Enlarged lymph nodes, liver, and spleen are frequently  found. Pancytopenia occurs. Brucella melitensis infections  tend to be more severe and prolonged, whereas those  caused by B. abortus are more self-limited. Osteomyelitis is  the most frequent complication. Secondary spread from  person to person is rare.   Recovery of the organism requires the use of enriched cul¬  ture media and incubation in 10% C0 2 . The organisms can  be presumptively identified by using a slide agglutination  test with Brucella antiserum, and the species can be identi¬  fied by biochemical tests. If organisms are not isolated,  analysis of a serum sample from the patient for a rise in  antibody titer to Brucella can be used to make a diagnosis.  In the absence of an acute-phase serum specimen, a titer of  at least 1:160 in the convalescent-phase serum sample is  diagnostic.   The treatment of choice is tetracycline plus rifampin. There  is no significant resistance to these drugs.   Prevention of brucellosis involves pasteurization of milk,  immunization of animals, and slaughtering of infected ani¬  mals. There is no human vaccine.   Francisella tularensis is a small, pleomorphic gram-negative  rod. It has a single serologic type. There are two biotypes, A  and B, which are distinguished primarily on their virulence  and epidemiology. Type A is more virulent and found pri¬  marily in the United States, whereas type B is less virulent  and found primarily in Europe.   Francisella tularensis is remarkable in the wide variety of  animals that it infects and in the breadth of its distribution  in the United States. It is enzootic (endemic in animals) in  every state, but most human cases occur in the rural areas  of Arkansas and Missouri. It has been isolated from more  than 100 different species of wild animals, the most impor¬  tant of which are rabbits, deer, and a variety of rodents. The  bacteria are transmitted among these animals by vectors  such as ticks, mites, and lice, especially the Dermacentor  ticks that feed on the blood of wild rabbits. The tick main¬  tains the chain of transmission by passing the bacteria to its  offspring by the transovarian route. In this process, the  bacteria are passed through ovum, larva, and nymph stages  to adult ticks capable of transmitting the infection.   Humans are accidental “dead-end” hosts who acquire  the infection most often by being bitten by the vector or by  having skin contact with the animal during removal of the  hide. Rarely, the organism is ingested in infected meat,  causing gastrointestinal tularemia, or is inhaled, causing  pneumonia. There is no person-to-person spread. The  main type of tularemia in the United States is tick-borne  tularemia from a rabbit reservoir.   The organism enters through the skin, forming an ulcer  at the site in most cases. It then localizes to the cells of the  reticuloendothelial system, and granulomas are formed.  Caseation necrosis and abscesses can also occur. Symptoms  are caused primarily by endotoxin. No exotoxins have been  identified.   Presentation can vary from sudden onset of an influenza-  like syndrome to prolonged onset of a low-grade fever and  adenopathy. Approximately 75% of cases are the “ulcero-  glandular” type, in which the site of entry ulcerates and the  regional lymph nodes are swollen and painful. Other, less  frequent forms of tularemia include glandular, oculoglan-  dular, typhoidal, gastrointestinal, and pulmonary. Disease  usually confers lifelong immunity.   Attempts to culture the organism in the laboratory are  rarely undertaken, because there is a high risk to laboratory  workers of infection by inhalation, and the special cysteine-  containing medium required for growth is not usually  available. The most frequently used diagnostic method is    the agglutination test with acute- and convalescent-phase  serum samples. Fluorescent-antibody staining of infected  tissue can be used if available.   Streptomycin is the drug of choice. There is no significant  antibiotic resistance.   Prevention involves avoiding both being bitten by ticks and  handling wild animals. There is a live, attenuated bacterial  vaccine that is given only to persons, such as fur trappers,  whose occupation brings them into close contact with wild  animals. The vaccine is experimental and not available  commercially but can be obtained from the U.S. Army  Medical Research Command, Fort Detrick, Maryland. This  and the bacillus of Calmette-Guerin (BCG) vaccine for  tuberculosis are the only two live bacterial vaccines for  human use.   Yersinia pestis is the cause of plague, also known as the  black death, the scourge of the Middle Ages. It is also a  contemporary disease, occurring in the western United  States and in many other countries around the world. Two  less important species, Yersinia enterocolitica and Yersinia  pseudotuberculosis, are described in Chapter 27.   Yersinia pestis is a small gram-negative rod that exhibits  bipolar staining (i.e., it resembles a safety pin, with a cen¬  tral clear area). Freshly isolated organisms possess a capsule  composed of a polysaccharide-protein complex. The cap¬  sule can be lost with passage in the laboratory; loss of the  capsule is accompanied by a loss of virulence. It is one of  the most virulent bacteria known and has a strikingly low  ID 50 (i.e., 1-10 organisms are capable of causing disease).   The plague bacillus has been endemic in the wild rodents  of Europe and Asia for thousands of years but entered  North America in the early 1900s, probably carried by a rat  that jumped ship at a California port. It is now endemic in  the wild rodents in the western United States, although  99% of cases of plague occur in Southeast Asia.   The enzootic (sylvatic) cycle consists of transmission  among wild rodents by fleas. In the United States, prairie  dogs are the main reservoir. Rodents are relatively resistant  to disease; most are asymptomatic. Humans are accidental  hosts, and cases of plague in this country occur as a result  of being bitten by a flea that is part of the sylvatic cycle.    The urban cycle, which does not occur in the United  States, consists of transmission of the bacteria among  urban rats (the reservoir), with the rat flea as vector. This  cycle predominates during times of poor sanitation (e.g.,  wartime), when rats proliferate and come in contact with  the fleas in the sylvatic cycle.   The events within the flea are fascinating as well as  essential. The flea ingests the bacteria while taking a blood  meal from a bacteremic rodent. A thick biofilm containing  many organisms forms in the upper gastrointestinal tract  that prevents any food from proceeding down the gastroin¬  testinal tract of the flea. This “blocked flea” then regurgi¬  tates the organisms into the bloodstream of the next animal  or human it bites.   The organisms inoculated at the time of the bite spread  to the regional lymph nodes, which become swollen and  tender. These swollen lymph nodes are the buboes that  have led to the name bubonic plague. The organisms can  reach high concentrations in the blood (bacteremia) and  disseminate to form abscesses in many organs. The  endotoxin-related symptoms, including disseminated  intravascular coagulation and cutaneous hemorrhages,  probably were the genesis of the term black death.   In addition to the sylvatic and urban cycles of transmis¬  sion, respiratory droplet transmission of the organism from  patients with pneumonic plague can occur.   The organism has several factors that contribute to its  virulence; (1) the envelope capsular antigen, called F-l,  which protects against phagocytosis; (2) endotoxin; (3) an  exotoxin; and (4) two proteins known as V antigen and W  antigen. The V and W antigens allow the organism to sur¬  vive and grow intracellularly, but their mode of action is  unknown. The action of the exotoxin is unknown.   Other factors that contribute to the extraordinary patho¬  genicity of Y. pestis are a group of virulence factors collec¬  tively called Yops ( Yersinia outer proteins). These are  injected into the human cell via type III secretion systems  and inhibit phagocytosis and cytokine production by macro¬  phages and neutrophils. For example, one of the Yops pro¬  teins (YopJ) is a protease that cleaves two signal transduction  pathway proteins required for the induction of tumor necro¬  sis factor synthesis. This inhibits the activation of our host  defenses and contributes to the ability of the organism to  replicate rapidly within the infected individual.   Bubonic plague, which is the most frequent form, begins  with pain and swelling of the lymph nodes draining the site  of the flea bite and systemic symptoms such as high fever,  myalgias, and prostration. The affected nodes enlarge and  become exquisitely tender. These buboes are an early char¬  acteristic finding. Septic shock and pneumonia are the  main life-threatening subsequent events. Pneumonic  plague can arise either from inhalation of an aerosol or  from septic emboli that reach the lungs. Untreated bubonic    plague is fatal in approximately half of the cases, and  untreated pneumonic plague is invariably fatal.   Smear and culture of blood or pus from the bubo is the best  diagnostic procedure. Great care must be taken by the phy¬  sician during aspiration of the pus and by laboratory work¬  ers doing the culture not to create an aerosol that might  transmit the infection. Giemsa or Wayson stain reveals the  typical safety-pin appearance of the organism better than  does Gram stain. Fluorescent-antibody staining can be  used to identify the organism in tissues. A rise in antibody  titer to the envelope antigen can be useful retrospectively.   The treatment of choice is a combination of streptomycin  and a tetracycline such as doxycycline, although streptomy¬  cin alone can be used. Levofloxacin can also be used. There  is no significant antibiotic resistance. In view of the rapid  progression of the disease, treatment should not wait for  the results of the bacteriologic culture. Incision and drain¬  age of the buboes are not usually necessary.   Prevention of plague involves controlling the spread of rats  in urban areas, preventing rats from entering the country  by ship or airplane, and avoiding both flea bites and contact  with dead wild rodents. A patient with plague must be  placed in strict isolation (quarantine) for 72 hours after  antibiotic therapy is started. Only close contacts need to  receive prophylactic tetracycline, but all contacts should be  observed for fever. Reporting a case of plague to the public  health authorities is mandatory.   A vaccine consisting of formalin-killed organisms pro¬  vides partial protection against bubonic but not pneumonic  plague. This vaccine was used in the armed forces during  the Vietnam War but is not recommended for tourists trav¬  eling to Southeast Asia.   Pasteurella multocida causes wound infections associated  with cat and dog bites.   Pasteurella multocida is a short, encapsulated gram¬  negative rod that exhibits bipolar staining.   The organism is part of the normal flora in the mouths of  many animals, particularly domestic cats and dogs, and is    transmitted by biting. About 25% of animal bites become  infected with the organism, with sutures acting as a predis¬  posing factor to infection. Most bite infections are polymi¬  crobial, with a variety of facultative anaerobes, especially  Streptococcus species, and anaerobic organisms present in  addition to P. multocida. Pathogenesis is not well under¬  stood, except that the capsule is a virulence factor and endo¬  toxin is present in the cell wall. No exotoxins are made.   A rapidly spreading cellulitis at the site of an animal bite is  indicative of P. multocida infection. The incubation period  is brief, usually less than 24 hours. Osteomyelitis can com¬  plicate cat bites in particular, because cats’ sharp, pointed  teeth can implant the organism under the periosteum.   The diagnosis is made by finding the organism in a culture  of a sample from the wound site.   Penicillin G is the treatment of choice. There is no signifi¬  cant antibiotic resistance.   People who have been bitten by a cat should be given ampi-  cillin to prevent P. multocida infection. Animal bites, espe¬  cially cat bites, should not be sutured.   Bartonella henselae is the cause of cat-scratch disease and  bacillary angiomatosis. Cat-scratch disease is one of the  most common zoonotic diseases in the United States.   Bartonella henselae is a small, pleomorphic gram-negative  rod. It is a fastidious organism and will not grow on routine  blood agar. It can be cultured on specialized media in the  clinical laboratory.   Cat scratches or bites, especially from kittens, are the main  mode of transmission of B. henselae to humans. The organ¬  ism is a member of the oral flora of many cats. There is  evidence that it is transmitted from cats to humans by the  bite of cat fleas. Exposure to cat urine or feces does not pose  a risk of transmission. Person-to-person transmission of  B. henselae does not play a significant role in infection.  Bartonella henselae is a low virulence organism, and disease  is self-limited in immunocompetent individuals.    The pathogenesis of angiomas that occur in Bartonella  infections in immunocompromised individuals is uncer¬  tain. One current explanation is that infection of endothe¬  lial cells by Bartonella induces the synthesis of angiogenesis  factor that causes endothelial cells to proliferate.   In immunocompetent people, B. henselae causes cat-  scratch disease (CSD). This disease is characterized by  fever and tender, enlarged lymph nodes, typically on the  same side as the scratch (Figure 20-1). A papule at the site  of the scratch may precede the lymphadenopathy. CSD  has a prolonged course but eventually resolves, even with¬  out antibiotics. A small percentage of those infected  develop systemic disease, such as endocarditis or  encephalitis.   In immunocompromised individuals, especially patients  with acquired immunodeficiency syndrome (AIDS), B.  henselae causes bacillary angiomatosis (BA). BA is charac¬  terized by raised, cherry-red vascular lesions in the skin  and visceral organs (Figure 20-2). The lesions appear pap¬  ular or nodular. Bacillary peliosis (peliosis hepatis) is simi¬  lar to bacillary angiomatosis except that in peliosis, the  lesions occur primarily in the liver and spleen.   FIGURE 20-1 Cat-scratch disease. Note the two enlarged,  inflamed axillary lymph nodes in a patient with cat-scratch disease.  (Reproduced with permission from Wolff K, Johnson R. Fitzpatrick's Color Atlas & Syn¬  opsis of Clinical Dermatology. 6th ed. New York: McGraw-Hill, 2009. Copyright © 2009  by The McGraw-Hill Companies, Inc.)   FIGURE 20~2 Bacillary angiomatosis. Note the cherry-red  hemangioma-like skin lesion. (Reproduced with permission from Wolff K,  Johnson R. Fitzpatrick's Color Atlas & Synopsis of Clinical Dermatology. 6th ed. New  York: McGraw-Hill, 2009. Copyright © 2009 by The McGraw-Hill Companies, Inc.)   The diagnosis of CSD is usually made serologically. Anti¬  bodies against B. henselae antigens can be detected in a  patient’s serum by a variety of immunologic tests. The  organism can be cultured on artificial media but takes 5 days  or longer to grow and so is not usually done. The diagnosis  of BA is often made by finding pleomorphic rods in biopsy  tissue using the Warthin-Starry silver stain. Pathologic  examination of tissue from the lesion will distinguish bacil¬  lary angiomatosis from Kaposi’s sarcoma.   No antibiotic therapy is typically recommended for CSD. If  the patient has severe lymphadenitis, azithromycin is the  drug of choice. Treatment of BA with doxycycline or eryth¬  romycin is effective. There is no significant antibiotic  resistance.   Antibiotics are not recommended for people who have  sustained a cat scratch. There is no vaccine.   1. Your patient is a 10-year-old boy who has a high fever and swollen,  painful axillary lymph nodes on the left side. His mother says that  he brought home a dead rat a few days ago. You suspect he may  have bubonic plague. Regarding the causative organism, which  one of the following is most accurate?   (C) It is endemic primarily in the states along the East Coast of the  United States.   (D) Its main virulence factor is an exotoxin that induces interleu-  kin-2 (IL-2) production by CD4-positive helper T cells.   (E) Infection should be treated with high doses of penicillin G  intravenously.   2. Your patient is a 20-year-old man who was bitten on the hand  when he tried to break up a fight between two cats yesterday. He  now has a red, hot, tender, swollen lesion at the bite site that has  spread rapidly across his hand. Which one of the following bacte¬  ria is the most likely cause of his cellulitis?   3. Your patient is a 30-year-old woman who reports that she has  had intermittent fever of 102°F, sweating, and fatigue for the  past month or so. She has lost her appetite and has lost about  10 pounds in that period. She enjoys eating unpasteurized goat  cheese. On examination, hepatosplenomegaly is detected. A blood  count reveals pancytopenia. Which one of the following bacteria  is the most likely cause of this infection?    4. Regarding B. henselae, which one of the following is most accurate?   (B) The natural habitat of B. henselae is the cats mouth.   (C) Bartonella henselae causes cellulitis in immunocompromised  patients such as AIDS patients.   (D) Diagnosis in the clinical laboratory depends on detecting anti¬  bodies in the patients serum that will agglutinate cardiolipin.   (E) The drug of choice for B. henselae infections is metronidazole.   Brief summaries of the organisms described in this chapter  begin on page 663. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Clinical Bacteriology section of Part XIII: USMLE  (National Board) Practice Questions starting on page 713. Also  see Part XIV: USMLE (National Board) Practice Examination  starting on page 751.   Mycobacteria are aerobic, acid-fast bacilli (rods) (Figure 21-1).  They are neither gram-positive nor gram-negative (i.e.,  they are stained poorly by the dyes used in Gram stain).  They are virtually the only bacteria that are acid-fast. (One  exception is Nocardia asteroides, the major cause of nocar¬  diosis, which is also acid-fast.) The term acid-fast refers to  an organism’s ability to retain the carbolfuchsin stain  despite subsequent treatment with an ethanol-hydrochloric  acid mixture. The high lipid content (approximately 60%)  of their cell wall makes mycobacteria acid-fast.   The major pathogens are Mycobacterium tuberculosis,  the cause of tuberculosis, and Mycobacterium leprae, the   FIGURE 21~1 Mycobacterium tuberculosis —acid-fast stain.  Long red rods of M. tuberculosis are seen on a blue background.  (Source: Dr. George Kubica, Public Health Image Library, Centers for Disease  Control and Prevention.)    cause of leprosy. Atypical mycobacteria, such as Mycobacte¬  rium avium-intracellulare complex and Mycobacterium  kansasii, can cause tuberculosis-like disease but are less  frequent pathogens. Rapidly growing mycobacteria, such as  Mycobacterium chelonae, occasionally cause human disease  in immunocompromised patients or those in whom pros¬  thetic devices have been implanted (Table 21-1). The clini¬  cal features of three important mycobacteria are described  in Table 21-2.   Additional information regarding the clinical aspects of  infections caused by the organisms in this chapter is pro¬  vided in Part IX entitled Infectious Diseases beginning on  page 593.   This organism causes tuberculosis. Worldwide, M. tubercu¬  losis causes more deaths than any other single microbial  agent. Approximately one-third of the world’s population is  infected with this organism. Each year, it is estimated that  1.7 million people die of tuberculosis and that 9 million  new cases occur. An estimated 500,000 people are infected  with a multidrug-resistant strain of M. tuberculosis.   Mycobacterium tuberculosis grows slowly (i.e., it has a dou¬  bling time of 18 hours, in contrast to most bacteria, which  can double in number in 1 hour or less). Because growth is  so slow, cultures of clinical specimens must be held for 6 to  8 weeks before being recorded as negative. Mycobacterium  tuberculosis can be cultured on bacteriologic media,    whereas M. leprae cannot. Media used for its growth (e.g.,  Lowenstein-Jensen medium) contain complex nutrients  (e.g., egg yolk) and dyes (e.g., malachite green). The dyes  inhibit the unwanted normal flora present in sputum  samples.   Mycobacterium tuberculosis is an obligate aerobe; this  explains its predilection for causing disease in highly oxy¬  genated tissues such as the upper lobe of the lung and the  kidney. The acid-fast property of M. tuberculosis (and other  mycobacteria) is attributed to long-chain (C78-C90) fatty  acids called mycolic acids in the cell wall.   Cord factor (trehalose dimycolate) is correlated with  virulence of the organism. Virulent strains grow in a char¬  acteristic “serpentine” cordlike pattern, whereas avirulent  strains do not. The organism also contains several proteins,  which, when combined with waxes, elicit delayed hyper¬  sensitivity. These proteins are the antigens in the purified  protein derivative (PPD) skin test (also known as the  tuberculin skin test). A lipid located in the bacterial cell  wall called phthiocerol dimycocerosate is required for  pathogenesis in the lung.   Mycobacterium tuberculosis is relatively resistant to  acids and alkalis. NaOH is used to concentrate clinical  specimens; it destroys unwanted bacteria, human cells, and  mucus but not the organism. M. tuberculosis is resistant to  dehydration and therefore survives in dried expectorated  sputum; this property may be important in its transmission  by aerosol.   Strains of M. tuberculosis resistant to the main antimy-  cobacterial drug, isoniazid (isonicotinic acid hydrazide,    INH), as well as strains resistant to multiple antibiotics  (called multidrug-resistant or MDR strains), have become  a worldwide problem. This resistance is attributed to one or  more chromosomal mutations, because no plasmids have  been found in this organism. One of these mutations is in a  gene for mycolic acid synthesis, and another is in a gene for  catalase-peroxidase, an enzyme required to activate INH  within the bacterium.   Mycobacterium tuberculosis is transmitted from person to  person by respiratory aerosols produced by coughing. The  source of the organism is a cavity in the lung that has eroded  into a bronchus. The portal of entry is the respiratory tract  and the initial site of infection is the lung. In tissue, it resides  chiefly within reticuloendothelial cells (e.g., macrophages).  Macrophages kill most, but not all, of the infecting organ¬  isms. The ones that survive can continue to infect other  adjacent cells or can disseminate to other organs.   Humans are the natural reservoir of M. tuberculosis.  Although some animals, such as cattle, can be infected,  they are not the main reservoir for human infection. Most  transmission occurs by aerosols generated by the coughing  of “smear-positive” people (i.e., those whose sputum con¬  tains detectable bacilli in the acid-fast stain). However,  about 20% of people are infected by aerosols produced by  the coughing of “smear-negative” people.   In the United States, tuberculosis is almost exclusively a  human disease. In developing countries, Mycobacterium    bovis also causes tuberculosis in humans. Mycobacterium  bovis is found in cow’s milk, which, unless pasteurized, can  cause gastrointestinal tuberculosis in humans. The disease  tuberculosis occurs in only a small number of infected  individuals. In the United States, most cases of tuberculosis  are associated with reactivation in elderly, malnourished  men. The risk of infection and disease is highest among  socioeconomically disadvantaged people, who have poor  housing and poor nutrition. These factors, rather than  genetic ones, probably account for the high rate of infection   An overall scheme of pathogenesis by M. tuberculosis is  shown in Figure 21-2. It describes primary tuberculosis  which typically results in a Ghon focus in the lower lung.  Primary tuberculosis can heal by fibrosis, can lead to pro¬  gressive lung disease, can cause bacteremia and miliary    FIGURE 21 ~2 Pathogenesis by Mycobacterium tuberculosis. (Reproduced with permission from LeT, Bhushan V, and Sochat, M. First Aid for the USMLE  Step 1. 25th ed. New York: McGraw-Hill, 2015.)    tuberculosis, or cause hematogenous dissemination result¬  ing in no immediate disease but with the risk of reactiva¬  tion in later life.   Figure 21-2 also describes secondary tuberculosis with  a cavity in the upper lobes. This can cause disease directly  or result in reactivation disease in later life with central  nervous system lesions, vertebral osteomyelitis (Pott’s dis¬  ease), or involvement of other organs.   Mycobacterium tuberculosis produces no exotoxins and  does not contain endotoxin in its cell wall. In fact, no  mycobacteria produce toxins. The organism preferentially  infects macrophages and other reticuloendothelial cells.  Mycobacterium tuberculosis survives and multiplies within  a cellular vacuole called a phagosome. The organism pro¬  duces a protein called “exported repetitive protein” that  prevents the phagosome from fusing with the lysosome,  thereby allowing the organism to escape the degradative  enzymes in the lysosome.   Lesions are dependent on the presence of the organism  and the host response. There are two types of lesions:   (1) Exudative lesions, which consist of an acute inflam¬  matory response and occur chiefly in the lungs at the initial  site of infection.   (2) Granulomatous lesions, which consist of a central  area of giant cells containing tubercle bacilli surrounded by  a zone of epithelioid cells. These giant cells, called Langhans’  giant cells, are an important pathologic finding in tuber¬  culous lesions. A tubercle is a granuloma surrounded by  fibrous tissue that has undergone central caseation necro¬  sis. Tubercles heal by fibrosis and calcification.   The primary lesion of tuberculosis usually occurs in the  lungs. The parenchymal exudative lesion and the draining  lymph nodes together are called a Ghon complex. Primary  lesions usually occur in the lower lobes, whereas reactiva¬  tion lesions usually occur in the apices. Reactivation lesions  also occur in other well-oxygenated sites such as the kid¬  neys, brain, and bone. Reactivation is seen primarily in  immunocompromised or debilitated patients.   Spread of the organism within the body occurs by two  mechanisms:   (1) A tubercle can erode into a bronchus, empty its case¬  ous contents, and thereby spread the organism to other  parts of the lungs, to the gastrointestinal tract if swallowed,  and to other persons if expectorated.   (2) It can disseminate via the bloodstream to many inter¬  nal organs. Dissemination can occur at an early stage if cell-  mediated immunity fails to contain the initial infection or at  a late stage if a person becomes immunocompromised.   After recovery from the primary infection, resistance to the  organism is mediated by cellular immunity (i.e., by CD4-  positive T cells and macrophages). The CD4-positive T  cells are Th-1 helper T cells (see Chapter 58).    Circulating antibodies also form, but they play no role  in resistance and are not used for diagnostic purposes.  Patients deficient in cellular immunity, such as patients  with acquired immunodeficiency syndrome (AIDS), are at  much higher risk for disseminated, life-threatening tuber¬  culosis. Mutations in the interferon-y receptor gene are  another cause of defective cellular immunity that predis¬  poses to severe tuberculosis. This emphasizes the impor¬  tance of activation of macrophages by interferon-y in the  host defense against M. tuberculosis.   Prior infection can be detected by a positive tuberculin  skin test result, which is due to a delayed hypersensitivity  reaction. PPD is used as the antigen in the tuberculin skin  test. The intermediate-strength preparation of PPD, which  contains five tuberculin units, is usually used. The skin test  is evaluated by measuring the diameter of the induration  surrounding the skin test site (Figure 21-3). Note that  induration (thickening), not simply erythema (reddening),  must be observed.   The diameter required to judge the test as positive varies  depending on the status of the individual being tested.  Induration of 15 mm or more is positive in a person who  has no known risk factors. Induration of 10 mm or more is  positive in a person with high-risk factors, such as a home¬  less person, intravenous drug users, or nursing home resi¬  dents. Induration of 5 mm or more is positive in a person  who has deficient cell-mediated immunity (e.g., AIDS  patients) or has been in close contact with a person with  active tuberculosis.   by the organism but not necessarily active disease. The  tuberculin test becomes positive 4 to 6 weeks after infection.  Immunization with bacillus Calmette-Guerin (BCG)  vaccine (see page 189) can cause a positive test, but the reac¬  tions are usually only 5 to 10 mm and tend to decrease with   FIGURE 21-3 Tuberculin skin test. Purified protein derivative  (PPD) was injected intradermally, and 48 hours later, the diameter of  induration was measured with a caliper. (Reproduced with permission from  Talaro KP. Foundations in Microbiology. 8th ed. New York: McGraw-Hill, 2011.}    time. People with PPD reactions of 15 mm or more are  assumed to be infected with M. tuberculosis even if they  have received the BCG vaccine. A positive skin test reverts  to negative in about 5% to 10% of people. Reversion to  negative is more common in the United States now than  many years ago because now a person is less likely to be  exposed to the organism and therefore less likely to receive  a boost to the immune system.   The skin test itself does not induce a positive response in  a person who has not been exposed to the organism. It can,  however, “boost” a weak or negative response in a person  who has been exposed to produce a positive reaction. The  clinical implications of this “booster effect” are beyond the  scope of this book.   Tuberculin reactivity is mediated by the cellular arm of  the immune system; it can be transferred by CD4-positive  T cells but not by serum. Infection with measles virus can  suppress cell-mediated immunity, resulting in a loss of  tuberculin skin test reactivity and, in some instances, reac¬  tivation of dormant organisms and clinical disease.   A gene called Nramp determines natural resistance to  tuberculosis. People who have mutations in the Nramp  gene have a much higher rate of clinical tuberculosis than  those with a normal allele. The NRAMP protein is located  in the membrane of the phagosome in macrophages and  plays an important role in killing the organism within the  phagosome.   Clinical findings are protean; many organs can be involved.  Fever, fatigue, night sweats, and weight loss are common.  The main findings in pulmonary tuberculosis are cough  and hemoptysis.   Scrofula is mycobacterial cervical lymphadenitis that  presents as swollen, nontender lymph nodes, usually uni¬  laterally. Mycobacterium tuberculosis causes most cases of  scrofula, but nontuberculous Mycobacteria, such as  Mycobacterium scrofulaceum, can also cause scrofula.  Lymphadenitis is the most common extrapulmonary mani¬  festation of tuberculosis. Patients infected with human  immunodeficiency virus (HIV) are more likely to have  multifocal lymphadenitis than those not infected with HIV.   Erythema nodosum, characterized by tender nodules  along the extensor surfaces of the tibia and ulna, is a mani¬  festation of primary infection seen in patients who are  controlling the infection with a potent cell-mediated  response (Figure 21-4). Miliary tuberculosis is character¬  ized by multiple disseminated lesions that resemble millet  seeds. Tuberculous meningitis and tuberculous osteomy¬  elitis, especially vertebral osteomyelitis (Potts disease), are  important disseminated forms.   Gastrointestinal tuberculosis is characterized by  abdominal pain and diarrhea accompanied by more gener¬  alized symptoms of fever and weight loss. Intestinal  obstruction or hemorrhage may occur. The ileocecal region   FIGURE 21-4 Erythema nodosum. Note erythematous  nodules over the anterior surface of the tibia bilaterally. (Used with  permission from Dr. Hanus Rozsypal.)    is the site most often involved. Tuberculosis of the gastro¬  intestinal tract can be caused by either M. tuberculosis  when it is swallowed after being coughed up from a lung  lesion or by M. bovis when it is ingested in unpasteurized  milk products. Oropharyngeal tuberculosis typically pres¬  ents as a painless ulcer accompanied by local adenopathy.   In renal tuberculosis, dysuria, hematuria, and flank  pain occur. “Sterile pyuria” is a characteristic finding. The  urine contains white blood cells, but cultures for the com¬  mon urinary tract bacterial pathogens show no growth.  However, mycobacterial cultures are often positive.   Note that most (approximately 90%) infections with M.  tuberculosis are asymptomatic. Asymptomatic infections,  also known as latent infections, can reactivate and cause  symptomatic tuberculosis. The most important determi¬  nant of whether overt disease occurs is the adequacy of  the host’s cell-mediated immune (CMI) response. For  example, AIDS patients have a very high rate of reactiva¬  tion of prior asymptomatic infection and of rapid pro¬  gression of the disease. In these patients, untreated  disease caused by M. tuberculosis has a 50% mortality  rate. Furthermore, administration of infliximab (Remi-  cade), a monoclonal antibody that neutralizes tumor  necrosis factor (TNF), has activated latent tuberculosis in  some patients. Remicade is used in the treatment of rheu¬  matoid arthritis (see Chapter 66). Diabetics also are predis¬  posed to reactivation and progression of disease.    In some patients with AIDS who are infected with M.  tuberculosis, treating the patient with highly active antiret¬  roviral therapy (HAART) causes an exacerbation of symp¬  toms. This phenomenon is called immune reconstitution  inflammatory syndrome (IRIS). The explanation of the  exacerbation of symptoms is that HAART increases the  number of CD4 cells, which increases the inflammatory  response. To prevent this, patients should be treated for the  underlying infection before starting HAART.   Acid-fast staining of sputum or other specimens is the  usual initial test (see Figure 21-1). Either the Kinyoun  version of the acid-fast stain or the older Ziehl-Neelsen ver¬  sion can be used. For rapid screening purposes, auramine  stain, which can be visualized by fluorescence microscopy,  is used.   After digestion of the specimen by treatment with  NaOH and concentration by centrifugation, the material is  cultured on special media, such as Lowenstein-Jensen agar,  for up to 8 weeks. It will not grow on a blood agar plate. In  liquid BACTEC medium, radioactive metabolites are pres¬  ent, and growth can be detected by the production of radio¬  active carbon dioxide in about 2 weeks. A liquid medium is  preferred for isolation because the organism grows more  rapidly and reliably than it does on agar. If growth in the  culture occurs, the organism can be identified by biochemi¬  cal tests. For example, M. tuberculosis produces niacin,  whereas almost no other mycobacteria do. It also produces  catalase.   Nucleic acid amplification tests can be used to detect the  presence of M. tuberculosis directly in clinical specimens  such as sputum. Tests are available that detect either the  ribosomal RNA or the DNA of the organism. These tests  are highly specific, but their sensitivity varies. In sputum  specimens that are acid-fast stain positive, the sensitivity is  high, but in “smear-negative” sputums, the sensitivity is  significantly lower. These tests are quite useful in deciding  whether to initiate therapy prior to obtaining the culture  results.   Because drug resistance, especially to isoniazid (see  later), is a problem, susceptibility tests should be per¬  formed. However, the organism grows very slowly, and  susceptibility tests usually take several weeks, which is too  long to guide the initial choice of drugs. To address this  problem, molecular tests are available, which detect muta¬  tions in the chromosomal genes that encode either the  catalase gene that mediates resistance to isoniazid or the  RNA polymerase gene that mediates resistance to rifampin.  The luciferase assay, which can detect drug-resistant  organisms in a few days, is also used. Luciferase is an  enzyme isolated from fireflies that produces flashes of light  in the presence of adenosine triphosphate (ATP). If the  organism isolated from the patient is resistant, it will not be  damaged by the drug (i.e., it will make a normal amount of    ATP), and the luciferase will produce the normal amount  of light. If the organism is sensitive to the drug, less ATP  will be made and less light produced.   There are two approaches to the diagnosis of latent  infections. One is the PPD skin test as described in the  “Immunity & Hypersensitivity” section earlier in this chap¬  ter. Because there are problems both in the interpretation of  the PPD test and with the person returning for the skin test  to be read, a quantifiable laboratory-based test is valuable.  This laboratory test is an interferon-y release assay  (IGRA), and there are two versions available: Quantiferon-  TB and T-spot-TB. In this assay, blood cells from the  patient are exposed to antigens from M. tuberculosis, and  the amount of interferon-y released from the cells is mea¬  sured. The sensitivity and specificity of the IGRA are as  good as the PPD skin test. Because the antigens used in the  test are specific for M. tuberculosis and are not present in  BCG, the test is not influenced by whether a person has  been previously immunized with the BCG vaccine.   Multidrug therapy is used to prevent the emergence of  drug-resistant mutants during the long (6- to 9-month)  duration of treatment. (Organisms that become resistant to  one drug will be inhibited by the other.) Isoniazid (INH),  a bactericidal drug, is the mainstay of treatment. Treatment  for most patients with pulmonary tuberculosis is with three  drugs: INH, rifampin, and pyrazinamide. INH and rifampin  are given for 6 months, but pyrazinamide treatment is  stopped after 2 months. A somewhat different regimen can  also be used. A convenient way to remember that regimen  is to give four drugs (isoniazid, rifampin, pyrazinamide,  and ethambutol) for 2 months and two drugs (isoniazid  and rifampin) for 4 months. In patients who are immuno¬  compromised (e.g., AIDS patients), who have disseminated  disease, or who are likely to have INH-resistant organisms,  a fourth drug, ethambutol, is added, and all four drugs are  given for 9 to 12 months.   Although therapy is usually given for months, the  patient’s sputum becomes noninfectious within 2 to 3 weeks.  The necessity for protracted therapy is attributed to (1) the  intracellular location of the organism; (2) caseous material,  which blocks penetration by the drug; (3) the slow growth  of the organism; and (4) metabolically inactive “persisters”  within the lesion. Because metabolically inactive organisms  may not be killed by antitubercular drugs, treatment may  not eradicate the infection, and reactivation of the disease  may occur in the future.   Lymphadenitis, including cervical lymphadenitis (scrof¬  ula) caused by M. tuberculosis, should be treated with the  drug regimens described earlier for disseminated disease.  Scrofula caused by M. scrofulaceum can be treated by surgi¬  cal excision of the single cervical lymph node, but alterna¬  tive approaches exist. A complete discussion of these is  beyond the scope of this book.    Treatment of latent (asymptomatic) infections consists  of INH taken for 6 to 9 months or INH plus rifapentine for  3 months. This approach is most often used in asymptom¬  atic patients whose PPD skin test recently converted to  positive. The risk of symptomatic infection is greatest  within the first 2 years after infection, so INH is particu¬  larly indicated for these “recent converters.” INH is also  used in children exposed to patients with symptomatic  tuberculosis. Patients who receive INH should be evaluated  for drug-induced hepatitis, especially those over the age of  35 years. Rifampin can be used in those exposed to INH-  resistant strains. A combination of rifampin and pyrazin-  amide should not be used because it causes a high rate of  severe liver injury.   Resistance to INH and other antituberculosis drugs is  being seen with increasing frequency in the United States,  especially in immigrants from Southeast Asia and Latin  America. Strains of M. tuberculosis resistant to multiple  drugs (MDR strains) have emerged, primarily in AIDS  patients. The most common pattern is resistance to both  INH and rifampin, but some isolates are resistant to three  or more drugs. The treatment of MDR organisms usually  involves the use of four or five drugs, including ciprofloxa¬  cin, amikacin, ethionamide, and cycloserine. The precise  recommendations depend on the resistance pattern of the  isolate and are beyond the scope of this book.   In 2013, a new drug, bedaquiline, was approved for the  treatment of MDR strains. It should be used in combina¬  tion with other drugs, not as monotherapy. It is a diaryl-  quinoline that inhibits an ATP synthase unique to M.  tuberculosis.   Previous treatment for tuberculosis predisposes to the  selection of these MDR organisms. Noncompliance (i.e.,  the failure of patients to complete the full course of ther¬  apy) is a major factor in allowing the resistant organisms to  survive. One approach to the problem of noncompliance is  directly observed therapy (DOT), in which health care  workers observe the patient taking the medication.   The strains of M. tuberculosis resistant to INH, rifampin, a  fluoroquinolone, and at least one additional drug are called  extensively drug-resistant (XDR) strains. XDR strains emerged  in 2005 among HIV-infected patients in South Africa.   The incidence of tuberculosis began to decrease markedly  even before the advent of drug therapy in the 1940s. This is  attributed to better housing and nutrition, which have  improved host resistance. At present, prevention of the  spread of the organism depends largely on the prompt  identification and adequate treatment of patients who are  coughing up the organism. The use of masks and other  respiratory isolation procedures to prevent spread to medi¬  cal personnel is also important. Contact tracing of indi¬  viduals exposed to patients with active pulmonary disease  who are coughing should be done.    An important component of prevention is the use of the  PPD skin test to detect recent converters and to institute  treatment for latent infections as described earlier. Groups  that should be screened with the PPD skin test include  people with HIV infection, close contacts of patients with  active tuberculosis, low-income populations, alcoholics  and intravenous drug users, prison inmates, and foreign-  born individuals from countries with a high incidence of  tuberculosis.   Because there are some problems associated with PPD  skin tests, such as the measurement and the interpretation  of results and the inconvenience of the patient having to  return for the skin test to be read, a laboratory test to  detect latent infections was developed. This test, called  Quantiferon-TB (QFT), measures the amount of  interferon-y released from the patient’s lymphocytes after  exposure to PPD in cell culture. QFT requires only a single  blood specimen and determines the amount of interferon-y  by an enzyme-linked immunosorbent assay (ELISA) test.   BCG vaccine can be used to induce partial resistance to  tuberculosis. The vaccine contains a strain of live, attenu¬  ated M. bovis called bacillus Calmette-Guerin. The vaccine  is effective in preventing the appearance of tuberculosis as  a clinical disease, especially in children, although it does  not prevent infection by M. tuberculosis. However, a major  problem with the vaccine is its variable effectiveness, which  can range from 0% to 70%. It is used primarily in areas of  the world where the incidence of the disease is high. It is  not usually used in the United States because of its variable  effectiveness and because the incidence of the disease is low  enough that it is not cost-effective.   The skin test reactivity induced by the vaccine given to  children wanes with time, and the interpretation of the skin  test reaction in adults is not altered by the vaccine. For  example, skin test reactions of 10 mm or more should not  be attributed to the vaccine unless it was administered  recently. In the United States, use of the vaccine is limited  to young children who are in close contact with individuals  with active tuberculosis and to military personnel. BCG  vaccine should not be given to immunocompromised peo¬  ple because the live BCG organisms can cause disseminated  disease.   BCG vaccine is also used to treat bladder cancer. The  vaccine is instilled into the bladder and serves to nonspe-  cifically stimulate cell-mediated immunity, which can  inhibit the growth of the carcinoma cells.   Pasteurization of milk and destruction of infected cattle  are important in preventing intestinal tuberculosis.   Several species of mycobacteria are characterized as atypi¬  cal, because they differ in certain respects from the proto¬  type, M. tuberculosis. For example, atypical mycobacteria  are widespread in the environment and are not pathogenic    for guinea pigs, whereas M. tuberculosis is found only in  humans and is highly pathogenic for guinea pigs. The  atypical mycobacteria are sometimes called mycobacteria  other than tuberculosis (MOTTs) or non-tuberculous  mycobacteria (NTM).   The atypical mycobacteria are classified into four groups  according to their rate of growth and whether they produce  pigment under certain conditions (Table 21-3). The atypical  mycobacteria in groups I, II, and III grow slowly, at a rate  similar to that of M. tuberculosis, whereas those in group IV  are “rapid growers,” producing colonies in fewer than 7 days.  Group I organisms produce a yellow-orange-pigmented  colony only when exposed to light (photochromogens),  whereas group II organisms produce the pigment chiefly in  the dark (scotochromogens). Group III mycobacteria pro¬  duce little or no yellow-orange pigment, irrespective of the  presence or absence of light (nonchromogens).   Mycobacterium kansasii causes lung disease clinically  resembling tuberculosis. Because it is antigenically similar  to M. tuberculosis, patients are frequently tuberculin skin  test-positive. Its habitat in the environment is unknown,  but infections by this organism are localized to the mid-  western states and Texas. It is susceptible to the standard  antituberculosis drugs.   Mycobacterium marinum causes “swimming pool gran¬  uloma,” also known as “fish tank granuloma.” These granu¬  lomatous, ulcerating lesions occur in the skin at the site of  abrasions incurred at swimming pools and aquariums. The  natural habitat of the organism is both fresh and salt water.  Treatment with a tetracycline such as minocycline is  effective.   Mycobacterium scrofulaceum causes scrofula, a granuloma¬  tous cervical adenitis, usually in children. (M. tuberculosis  also causes scrofula.) The organism enters through the  oropharynx and infects the draining lymph nodes. Its natu¬  ral habitat is environmental water sources, but it has also  been isolated as a saprophyte from the human respiratory  tract. Scrofula can often be cured by surgical excision of the  affected lymph nodes.   Mycobacterium avium-intracellulare complex (MAI, MAC)  is composed of two species, M. avium and M. intracellulare,  that are very difficult to distinguish from each other by  standard laboratory tests. They cause pulmonary disease  clinically indistinguishable from tuberculosis, primarily in  immunocompromised patients such as those with AIDS  who have CD4 cell counts of less than 200/pL. MAI is the  most common bacterial cause of disease in AIDS patients.  The organisms are widespread in the environment, includ¬  ing water and soil, particularly in the southeastern United  States. They are highly resistant to antituberculosis drugs,  and as many as six drugs in combination are frequently  required for adequate treatment. Current drugs of choice  are azithromycin or clarithromycin plus one or more of  the following: ethambutol, rifabutin, or ciprofloxacin.  Azithromycin is currently recommended for preventing  disease in AIDS patients.   Mycobacterium fortuitum-chelonae complex is composed  of two similar species, M. fortuitum and M. chelonae. They  are saprophytes, found chiefly in soil and water, and rarely  cause human disease. Infections occur chiefly in two popu¬  lations: (1) immunocompromised patients and (2) indi¬  viduals with prosthetic hip joints and indwelling catheters.  Skin and soft tissue infections occur at the site of puncture  wounds (e.g., at tattoo sites). They are often resistant to  antituberculosis therapy, and therapy with multiple drugs  in combination plus surgical excision may be required for  effective treatment. Current drugs of choice are amikacin  plus doxycycline.   Mycobacterium abscessus is another rapidly growing  mycobacteria acquired from the environment. It causes  chronic lung infections, as well as infections of the skin,  bone, and joints. It is highly antibiotic-resistant. Current  drugs of choice are amikacin plus imipenem or cefoxitin  plus clarithromycin.   Mycobacterium smegmatis is a rapidly growing myco¬  bacterium that is not associated with human disease. It is  part of the normal flora of smegma, the material that col¬  lects under the foreskin of the penis.   Mycobacterium leprae has not been grown in the laboratory,  either on artificial media or in cell culture. It can be grown  in experimental animals, such as mice and armadillos. Humans  are the natural hosts, although the armadillo appears to be a  reservoir for human infection in the Mississippi Delta  region where these animals are common. In view of this,  leprosy can be thought of as a zoonotic disease, at least in  certain southern states, such as Louisiana and Texas.   The optimal temperature for growth (30°C) is lower  than body temperature; therefore, M. leprae grows prefer¬  entially in the skin and superficial nerves. It grows very  slowly, with a doubling time of 14 days. This makes it the  slowest-growing human bacterial pathogen. One conse¬  quence of this is that antibiotic therapy must be continued  for a long time, usually several years.   with lepromatous leprosy, who discharge M. leprae in large  numbers in nasal secretions and from skin lesions. In the  United States, leprosy occurs primarily in Texas, Louisiana,  California, and Hawaii. Most cases are found in immigrants  from Mexico, the Philippines, Southeast Asia, and India.  The disease occurs worldwide, with most cases in the tropi¬  cal areas of Asia and Africa. The armadillo is unlikely to be  an important reservoir because it is not found in many  areas of the world where leprosy is endemic.   The organism replicates intracellularly, typically within  skin histiocytes, endothelial cells, and the Schwann cells of  nerves. The nerve damage in leprosy is the result of two  processes: damage caused by direct contact with the bacte¬  rium and damage caused by CMI attack on the nerves.   There are two distinct forms of leprosy—tuberculoid  and lepromatous—with several intermediate forms  between the two extremes (Table 21-4).    (1) In tuberculoid (also known as paucibacillary) lep¬  rosy, the CMI response to the organism limits its growth,  very few acid-fast bacilli are seen, and granulomas contain¬  ing giant cells form. The nerve damage seems likely to be  caused by cell-mediated immunity as there are few organ¬  isms and the CMI response is strong.   The CMI response consists primarily of CD4-positive  cells and a Th-1 profile of cytokines, namely, interferon-y,  interleukin-2, and interleukin-12. It is the CMI response  that causes the nerve damage seen in tuberculoid leprosy.   The lepromin skin test result is positive. The lepromin  skin test is similar to the tuberculin test (see earlier). An  extract of M. leprae is injected intradermally, and indura¬  tion is observed 48 hours later in those in whom a CMI  response against the organism exists.   (2) In lepromatous (also known as multibacillary) lep¬  rosy, the cell-mediated response to the organism is poor,  the skin and mucous membrane lesions contain large num¬  bers of organisms, foamy histiocytes rather than granulo¬  mas are found, and the lepromin skin test result is negative.  The nerve damage seems likely to be caused by direct con¬  tact as there are many organisms and the CMI response is  poor.   There is evidence that people with lepromatous leprosy  produce interferon-(3 (antiviral interferon) in response to  M. leprae infection, whereas people with tuberculoid lep¬  rosy produce interferon-y. Interferon-13 inhibits the synthe¬  sis of interferon-y thereby reducing the CMI response  needed to contain the infection.   Note that in lepromatous leprosy, only the cell-mediated  response to M. leprae is defective (i.e., the patient is anergic  to M. leprae). The cell-mediated response to other organ¬  isms is unaffected, and the humoral response to M. leprae  is intact. However, these antibodies are not protective. The  T-cell response consists primarily of Th-2 cells.   The incubation period averages several years, and the onset  of the disease is gradual. In tuberculoid leprosy, hypopig-  mented macular or plaquelike skin lesions, thickened  superficial nerves, and significant anesthesia of the skin  lesions occur (Figure 21-5). In lepromatous leprosy, mul¬  tiple nodular skin lesions occur, resulting in the typical   FIGURE 21-5 Tuberculoid leprosy. The tuberculoid form is  characterized by a single, flat, hypopigmented lesion that has lost  sensation. (Reproduced with permission from Longo DL et al [eds]. Harrison's Prin¬  ciples of Internal Medicine. 18th ed. New York: McGraw-Hill, 2012. Copyright © 2012  by The McGraw-Hill Companies, Inc.)   leonine (lionlike) facies (Figure 21-6). After the onset of  therapy, patients with lepromatous leprosy often develop  erythema nodosum leprosum (ENL), which is interpreted  as a sign that cell-mediated immunity is being restored.  ENL is characterized by painful nodules, especially along  the extensor surfaces of the tibia and ulna, neuritis, and  uveitis.   FIGURE 21 ~6 Lepromatous leprosy. The lepromatous form is  characterized by multiple, raised lesions, often with the appearance  of leonine facies (the face resembles a lion with a prominent brow).  (Used with permission from Robert H. Gelber, MD.)    The disfiguring appearance of the disease results from  several factors: (1) the skin anesthesia results in burns and  other traumas, which often become infected; (2) resorption  of bone leads to loss of features such as the nose and finger¬  tips; and (3) infiltration of the skin and nerves leads to  thickening and folding of the skin. In most patients with a  single skin lesion, the disease resolves spontaneously.  Patients with forms of the disease intermediate between  tuberculoid and lepromatous can progress to either extreme.   In lepromatous leprosy, the bacilli are easily demonstrated  by performing an acid-fast stain of skin lesions or nasal  scrapings. Lipid-laden macrophages called “foam cells”  containing many acid-fast bacilli are seen in the skin. In the  tuberculoid form, very few organisms are seen, and the  appearance of typical granulomas is sufficient for diagno¬  sis. Cultures are negative because the organism does not  grow on artificial media.   A serologic test for IgM against phenolic glycolipid-1 is  useful in the diagnosis of lepromatous leprosy but is not  useful in the diagnosis of tuberculoid leprosy. The diagno¬  sis of lepromatous leprosy can be confirmed by using the  polymerase chain reaction (PCR) test on a skin sample.  False-positive results in the nonspecific serologic tests for  syphilis, such as the Venereal Disease Research Laboratory  (VDRL) and rapid plasma reagin (RPR) tests, occur fre¬  quently in patients with lepromatous leprosy.   The mainstay of therapy is dapsone (diaminodiphenylsul-  fone), but because sufficient resistance to the drug has  emerged, combination therapy is now recommended. For  tuberculoid (paucibacillary) leprosy, dapsone and rifampin  are given for 6 to 12 months whereas for lepromatous (mul-  tibacillary) leprosy, a combination of dapsone, rifampin,  and clofazimine is given for 12 to 24 months. A combina¬  tion of ofloxacin plus clarithromycin is an alternative regi¬  men. Thalidomide is the treatment of choice for severe  ENL reactions.   Isolation of all lepromatous patients, coupled with chemo¬  prophylaxis with dapsone for exposed children, is required.  There is no vaccine.   1. Your patient is a 25-year-old homeless man who complains of a  cough for the past month. The cough is now productive of sev¬  eral tablespoons of blood-streaked sputum per day. The sputum is  not foul-smelling. He has lost 10 pounds but says that he doesn’t  eat regularly. On physical exam, temperature is 38°C, and coarse  rales were heard in the apex of the left lung. An acid-fast stain of    the sputum reveals acid-fast rods. Culture of the sputum shows no  growth at 7 days, but buff-colored colonies are visible at 21 days.  Of the following organisms, which one is most likely to be the  cause of this infection?   2. Which one of the following regimens is optimal initial treatment  for the patient in Question 1?   3. Your patient is a 70-year-old man with progressive weakness in  both legs that began about a week ago. He reports back pain and  fever for the past month. Magnetic resonance imaging (MRI) of  the spine reveals destruction of the seventh thoracic vertebra and a  paravertebral mass. Surgical decompression and debridement was  performed. Histologic examination of the mass revealed caseating  granulomas, and Langhans’ giant cells were observed in the granu¬  lomas. Gram stain revealed no organisms, but an acid-fast stain  showed red rods. Culture shows no growth at 7 days, but growth  is seen at 28 days. Of the following, which one is the most likely  cause?   4. Your patient is a 30-year-old woman who is infected with HIV  and has a low CD4 count. She now has the findings of pulmo¬  nary tuberculosis, but you are concerned that she may be infected  with Mycobacterium avium-intracellulare (MAI). Regarding MAI,  which one of the following is most accurate?   (A) Disseminated disease caused by MAI is typically the result of  decreased antibody production, whereas disseminated disease  caused by M. tuberculosis is typically caused by reduced cell-  mediated immunity.   (B) Immigrants from Southeast Asia are more likely to be infected  with MAI than with M. tuberculosis.   (C) In the clinical laboratory, MAI forms colonies in 7 days,  whereas M. tuberculosis colonies typically require at least 21 days  of incubation for colonies to appear.   (D) MAI is typically susceptible to a drug regimen of isoniazid and  rifampin, whereas M. tuberculosis is often resistant.   (E) The natural habitat of MAI is the environment, whereas the  natural habitat of M. tuberculosis is humans.    5. Regarding the patient in Question 4, if MAI was shown to be the  cause of her symptoms, which one of the following is the best  choice of antibiotics to prescribe?   6. Your patient is a 20-year-old man with a single, slowly expanding,  nonpainful scaly lesion on his chest for the past 2 months. The  lesion is nonpruritic, and he has lost sensation at the site of the  lesion. He is otherwise well. He is a recent immigrant from Central  America. An acid-fast stain of a scraping of the lesion is positive.  Which one of the following diseases is he most likely to have?   Brief summaries of the organisms described in this chapter  begin on page 664. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Clinical Bacteriology section of Part XIII: USMLE  (National Board) Practice Questions starting on page 713. Also  see Part XIV: USMLE (National Board) Practice Examination  starting on page 751.   Actinomycetes are a family of bacteria that form long,  branching filaments that resemble the hyphae of fungi  (Figure 22-1). They are gram-positive, but some (such  as Nocardia asteroides) are also weakly acid-fast rods  (Table 22-1).   Additional information regarding the clinical aspects of  infections caused by the organisms in this chapter is pro¬  vided in Part IX entitled Infectious Diseases beginning on  page 593.   Actinomyces israelii is an anaerobe that forms part of the  normal flora of the oral cavity. After local trauma such as  a broken jaw or dental extraction, it may invade tissues,  forming filaments surrounded by areas of inflammation.   FIGURE 22-1 Nocardia asteroides —Gram stain. Arrow points  to area of filaments of gram-positive rods. (Source: Dr. Thomas F. Sellers,  Public Health Image Library, Centers for Disease Control and Prevention.)   The typical lesion of actinomycosis appears as a hard, non¬  tender swelling that develops slowly and eventually drains  pus through sinus tracts (Figure 22-2). Hard, yellow gran¬  ules (sulfur granules) composed of a mass of filaments are  formed in pus.   In about 50% of cases, the initial lesion involves the face  and neck; in the rest, the chest or abdomen is the site. Pelvic  actinomycosis can occur in women who have retained an  intrauterine device for a long period of time. Actinomyces  israelii and Arachnia species are the most common causes  of actinomycosis in humans. The disease is not  communicable.   Diagnosis in the laboratory is made by (1) seeing gram¬  positive branching rods, especially in the presence of sulfur  granules; and (2) seeing growth when pus or tissue speci¬  mens are cultured under anaerobic conditions. Organisms  can be identified by immunofluorescence. Note that in  contrast to N. asteroides (see later), Actinomyces is not  acid-fast. There are no serologic tests.   Actinomycosis (abscess with  draining sinus tract and "sulfur  granules" in pus)   Treatment consists of prolonged administration of penicil¬  lin G, coupled with surgical drainage. There is no signifi¬  cant resistance to penicillin G. No vaccine or prophylactic  drug is available.   Nocardia species are aerobes and are found in the environ¬  ment, particularly in the soil. In immunocompromised  individuals, they can produce lung infection and may dis¬  seminate. In tissues, Nocardia species are thin, branching  filaments that are gram-positive on Gram stain. Many iso¬  lates of N. asteroides are weakly acid-fast (i.e., the staining  process uses a weaker solution of hydrochloric acid to  decolorize than that used in the stain for mycobacteria). If  the regular-strength acid is used, N. asteroides will  decolorize.   FIGURE 22-2 Actinomycosis. Note inflamed lesion with small  sinus tract opening anterior to right ear. Yellowish "sulfur granule" can  be seen at the opening. (Source: Dr. Thomas F. Sellers, Public Health Image  Library, Centers for Disease Control and Prevention.)   Nocardia asteroides typically causes either pneumonia, lung  abscess with cavity formation, lung nodules, or empyema.  From the lung, the organism can spread to various organs,  notably the brain, where it causes brain abscess. Disease  occurs most often in immunocompromised individuals,  especially those with reduced cell-mediated immunity.  Nocardia brasiliensis, a different species of Nocardia, causes  skin infections in the southern regions of the United States  and mycetoma, usually in tropical regions.   Diagnosis in the laboratory involves (1) seeing branching  rods or filaments that are gram-positive (see Figure 22-1)  or weakly acid-fast in an acid-fast stain and (2) seeing aero¬  bic growth on bacteriologic media in a few days.   Treatment is with trimethoprim-sulfamethoxazole. Surgical  drainage may also be needed. Occasional drug resistance  occurs. No vaccine or prophylactic drug is available.   1. Your patient is a 75-year-old woman with fever and a painful nod¬  ule on her forearm. She also has a nonproductive cough that she  says is worse than her usual smoking-related cough. She is taking  high-dose corticosteroids (prednisone) for an autoimmune dis¬  ease. Chest X-ray reveals a nodular lesion in the right upper lobe.  A biopsy of the nodule on her arm was obtained. Gram stain of the    specimen showed filaments of gram-positive rods. The rods were  also weakly acid-fast. Regarding the causative organism, which  one of the following is most accurate?   (A) Culture of the organism should be done under anaerobic  conditions.   (C) It produces an exotoxin that inhibits protein synthesis by  ADP-ribosylation.   (E) The vaccine against this organism contains the capsular poly¬  saccharide as the immunogen.   2. Your patient is a 20-year-old man who was in a fist fight in a bar  about 3 weeks ago. He took a punch that broke his left second  molar. He now has a 3-cm inflamed area on the skin overlying the  broken tooth that is draining pus. A Gram stain of the pus reveals  gram-positive filamentous rods. The rods did not appear red in  the acid-fast stain. Regarding the causative organism, which one  of the following is most accurate?   (A) Infections caused by this organism occur primarily in the Ohio  and Mississippi River Valley area.   (D) Sulfur granules are often seen in the pus located at the orifice  of the sinus tract in the skin lesion.   (E) The vaccine against this organism contains a toxoid as the  immunogen.   Brief summaries of the organisms described in this chapter  begin on page 665. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Clinical Bacteriology section of Part XIII: USMLE  (National Board) Practice Questions starting on page 713. Also  see Part XIV: USMLE (National Board) Practice Examination  starting on page 751.   Mycoplasmas are a group of very small, wall-less organisms,  of which Mycoplasma pneumoniae is the major pathogen.   Additional information regarding the clinical aspects of  infections caused by the organisms in this chapter is pro¬  vided in Part IX entitled Infectious Diseases beginning on  page 593.   Mycoplasmas are the smallest free-living organisms;  many are as small as 0.3 pm in diameter. Their most strik¬  ing feature is the absence of a cell wall. 1   Consequently, mycoplasmas stain poorly with Gram  stain, and antibiotics that inhibit cell wall (peptidoglycan)  synthesis (e.g., penicillins and cephalosporins) are ineffec¬  tive. Their outer surface is a flexible cell membrane; hence  these organisms can assume a variety of shapes. It is the only  bacterial membrane that contains cholesterol, a sterol usu¬  ally found in eukaryotic cell membranes.   Mycoplasmas can be grown in the laboratory on artifi¬  cial media, but they have complex nutritional require¬  ments, including several lipids. They grow slowly and  require at least 1 week to form a visible colony. The colony    1 Other types of bacteria, in the presence of penicillin, can exist in a wall¬  less state called an “L form” but can resynthesize their cell walls when  penicillin is removed.    frequently has a characteristic “fried-egg” shape, with a  raised center and a thinner outer edge.   Mycoplasma pneumoniae, a pathogen only for humans, is  transmitted by respiratory droplets. In the lungs, the  organism is rod-shaped, with a tapered tip that contains  specific proteins that serve as the point of attachment to the  respiratory epithelium. The respiratory mucosa is not  invaded, but ciliary motion is inhibited and necrosis of the  epithelium occurs. The mechanism by which M. pneu¬  moniae causes inflammation is uncertain. It does produce  hydrogen peroxide, which contributes to the damage to the  respiratory tract cells.   Mycoplasma pneumoniae has only one serotype and is  antigenically distinct from other species of Mycoplasma.  Immunity is incomplete, and second episodes of disease can  occur. During M. pneumoniae infection, autoantibodies are  produced against red cells (cold agglutinins) and brain,  lung, and liver cells. These antibodies may be involved in  some of the extrapulmonary manifestations of infection.   Mycoplasma pneumoniae infections occur worldwide,  with an increased incidence in the winter. This organism is  the most common cause of pneumonia in young adults  and is responsible for outbreaks in groups with close contacts  such as families, military personnel, and college students. It  is estimated that only 10% of infected individuals actually get  pneumonia. Mycoplasma pneumonia accounts for about 5%  to 10% of all community-acquired pneumonia.   Mycoplasma pneumonia is the most common type of atypi¬  cal pneumonia. It was formerly called primary atypical  pneumonia. (Atypical pneumonia is also caused by    Legionella pneumophila [Legionnaires’ disease], Chlamydia  pneumoniae. Chlamydia psittaci [psittacosis], Coxiella bur¬  netii [Q fever], and viruses such as such as influenza virus  and adenovirus. The term atypical means that a causative  bacterium cannot be isolated on routine media in the diag¬  nostic laboratory or that the disease does not resemble  pneumococcal pneumonia.) The onset of Mycoplasma  pneumonia is gradual, usually beginning with a nonpro¬  ductive cough, sore throat, or earache. Small amounts of  whitish, nonbloody sputum are produced. Constitutional  symptoms of fever, headache, malaise, and myalgias are  pronounced. The paucity of findings on chest examination  is in marked contrast to the prominence of the infiltrates  seen on the patient’s chest X-ray. The disease resolves spon¬  taneously in 10 to 14 days. In addition to pneumonia, M.  pneumoniae also causes bronchitis.   The extrapulmonary manifestations include Stevens-  Johnson syndrome, erythema multiforme, Raynaud’s phe¬  nomenon, cardiac arrhythmias, arthralgias, hemolytic  anemia, and neurologic manifestations such as Guillain-  Barre syndrome.   Diagnosis is usually not made by culturing sputum sam¬  ples; it takes at least 1 week for colonies to appear on special  media. Culture on regular media reveals only normal flora.   Currently, a polymerase chain reaction (PCR) assay that  detects M. pneumoniae specific nucleic acids in sputum or  in respiratory secretions is the best diagnostic procedure.  Serologic testing for the presence of antibodies in the  patient’s serum may also be useful. A cold-agglutinin titer  of 1:128 or higher is indicative of recent infection. Cold  agglutinins are IgM autoantibodies against type O red  blood cells that agglutinate these cells at 4°C but not at  37°C. However, only half of patients with Mycoplasma  pneumonia will be positive for cold agglutinins. The test is  nonspecific; false-positive results occur in influenza virus  and adenovirus infections. The diagnosis of M. pneu¬  moniae infection can be confirmed by a fourfold or greater  rise in specific antibody titer in either a complement fixa¬  tion or an ELISA test.   The treatment of choice is either a macrolide, such as  erythromycin or azithromycin, or a tetracycline, such as  doxycycline. The fluoroquinolone levofloxacin is also effec¬  tive. These drugs can shorten the duration of symptoms,  although, as mentioned earlier, the disease resolves sponta¬  neously. Penicillins and cephalosporins are inactive  because the organism has no cell wall.   Mycoplasma hominis has been implicated as an infrequent  cause of pelvic inflammatory disease. Ureaplasma urealyti-  cum may cause approximately 20% of cases of nongonococ¬  cal urethritis. Ureaplasmas can be distinguished from  mycoplasmas by their ability to produce the enzyme urease,  which degrades urea to ammonia and carbon dioxide.   1. Mycoplasma pneumoniae is an important cause of atypical pneu¬  monia. Regarding this organism, which one of the following is the  most accurate?   (A) Amoxicillin is the drug of choice for pneumonia caused by this  organism.   (B) Antibody in a patient’s serum will agglutinate human red blood  cells at 4°C, but not at 37°C.   (D) It is an obligate intracellular parasite that can only grow within  human cells in the clinical laboratory.   (E) People with cystic fibrosis are predisposed to pneumonia  caused by this organism.   2. Which one of the following is the drug of choice for atypical pneu¬  monia caused by M. pneumoniae ?   Brief summaries of the organisms described in this chapter  begin on page 665. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Clinical Bacteriology section of Part XIII: USMLE  (National Board) Practice Questions starting on page 713. Also  see Part XIV: USMLE (National Board) Practice Examination  starting on page 751.   Three genera of spirochetes cause human infection: (1)  Treponema, which causes syphilis and the nonvenereal  treponematoses; (2) Borrelia, which causes Lyme disease  and relapsing fever; and (3) Leptospira, which causes lepto¬  spirosis (Table 24-1).    Spirochetes are thin-walled, flexible, spiral rods  (Figure 24-1). They are motile through the undulation of  axial filaments that lie under the outer sheath. Treponemes  and leptospirae are so thin that they are seen only by dark  field microscopy, silver impregnation, or immunofluores¬  cence. Borreliae are larger, accept Giemsa and other blood  stains, and can be seen in the standard light microscope.   Thin, tight, spirals, seen by  dark field illumination,  silver impregnation, or  immunofluorescent  stain   Food or drink con¬  taminated by  urine of infected  animals (rats,  dogs, pigs, cows)   The coiled form of this spirochete is in the center of the field. (Source:  Dr. Schwartz, Centers for Disease Control and Prevention.)    Additional information regarding the clinical aspects of  infections caused by the organisms in this chapter is pro¬  vided in Part IX entitled Infectious Diseases beginning on  page 593.   Treponema pallidum has not been grown on bacteriologic  media or in cell culture. Nonpathogenic treponemes, which  are part of the normal flora of human mucous membranes,  can be cultured.   Treponema pallidum grows very slowly. The medical  importance of that fact is that antibiotics must be present at  an effective level for several weeks to kill the organisms and  cure the disease (see “Treatment” section later). For exam¬  ple, benzathine penicillin is the form of penicillin used to  treat primary and secondary syphilis because the penicillin  is released very slowly from this depot preparation, and  bactericidal concentrations are present for weeks after  administration of the antibiotic.   The antigens of T. pallidum induce specific antibodies,  which can be detected by immunofluorescence or hemag¬  glutination tests in the clinical laboratory. They also induce  nonspecific antibodies (reagin), 1 which can be detected by    Syphilitic reagin (IgM and IgG) should not be confused with the reagin  (IgE) antibody involved in allergy.    the flocculation of lipids (cardiolipin) extracted from nor¬  mal mammalian tissues (e.g., beef heart).   Both specific antitreponemal antibody and nonspecific  reagin are used in the serologic diagnosis of syphilis.   Treponema pallidum is transmitted from spirochete-  containing lesions of skin or mucous membranes (e.g.,  genitalia, mouth, and anus) of an infected person to other  persons by intimate contact. It can also be transmitted  from pregnant women to their fetuses. Rarely, blood for  transfusions collected during early syphilis is also infec¬  tious. Treponema pallidum is a human organism only.  There is no animal reservoir.   Syphilis occurs worldwide, and its incidence is increas¬  ing. It is one of the leading notifiable diseases in the United  States. Many cases are believed to go unreported, which  limits public health efforts. There has been a marked  increase in incidence of the disease in men who have sex  with men in recent years.   Treponema pallidum produces no important toxins or  enzymes. The organism often infects the endothelium of  small blood vessels, causing endarteritis. This occurs dur¬  ing all stages of syphilis but is particularly important in the  pathogenesis of the brain and cardiovascular lesions seen in  tertiary syphilis.   In primary syphilis, the spirochetes multiply at the site  of inoculation, and a local, nontender ulcer (chancre) usu¬  ally forms in 2 to 10 weeks (Figure 24-2). The ulcer heals   FIGURE 24-2 Chancre of primary syphilis. Note the shallow  ulcer with a rolled edge (red arrow) that is typical of a syphilitic chan-  ere. (Reproduced with permission from Wolff K, Johnson R. Fitzpatrick's Color Atlas &  Synopsis of Clinical Dermatology. 6th ed. New York: McGraw-Hill, 2009. Copyright ©  2009 by The McGraw-Hill Companies, Inc.)   FIGURE 24-3 Palmar lesions of secondary syphilis. Note the  papulosquamous lesions on the right palm. Palmar lesions are typi¬  cally bilateral. (Reproduced with permission from Wolff K, Johnson R. Fitzpatrick's  Color Atlas & Synopsis of Clinical Dermatology. 6th ed. New York: McGraw-Hill, 2009.  Copyright © 2009 by The McGraw-Hill Companies, Inc.)    spontaneously, but spirochetes spread widely via the blood¬  stream (bacteremia) to many organs.   One to 3 months later, the lesions of secondary syphilis  may occur. These often appear as a maculopapular rash,  notably on the palms and soles (Figure 24-3), or as moist  papules on skin and mucous membranes (mucous patches).  Moist lesions on the genitals are called condylomata lata  (Figure 24-4). These lesions are rich in spirochetes and are  highly infectious, but they also heal spontaneously. Patchy  alopecia also occurs. Constitutional symptoms of second¬  ary syphilis include low-grade fever, malaise, anorexia,  weight loss, headache, myalgias, and generalized lymph-  adenopathy. Pharyngitis, meningitis, nephritis, and hepati¬  tis may also occur. In some individuals, the symptoms of  the primary and secondary stages may not occur, and yet  the disease may progress.   About one-third of these early (primary and secondary)  syphilis cases will “cure” themselves, without treatment.  Another third remain latent (i.e., no lesions appear, but  positive serologic tests indicate continuing infection). The  latent period can be divided into early and late stages. In  the early latent period, which can last for 1 or 2 years after  the secondary stage, the symptoms of secondary syphilis  can reappear and patients can infect others. In the late  latent period, which can last for many years, no symptoms   FIGURE 24~4 Condylomata lata of secondary syphilis. Note the  flat, moist perianal lesions (black arrow). (Reproduced with permission from  Wolff K, Johnson R. Fitzpatrick's Color Atlas & Synopsis of Clinical Dermatology. 6th ed.  New York: McGraw-Hill, 2009. Copyright © 2009 by The McGraw-Hill Companies, Inc.)   occur and patients are not infectious. In the remaining one-  third of people, the disease progresses to the tertiary stage.  Tertiary syphilis may show granulomas (gummas), espe¬  cially of skin and bones; central nervous system involve¬  ment, also known as neurosyphilis (e.g., tabes, paresis); or  cardiovascular lesions (e.g., aortitis, aneurysm of the  ascending aorta). In tertiary lesions, treponemes are rarely  seen.   The organism is transmitted across the placenta, typically  after the third month of pregnancy, and fetal infection can  occur. In the infected neonates, skin and bone lesions, such  as Hutchinson’s teeth, mulberry molars, saber shins, saddle  nose, rhagades, snuffles, and frontal bossing, are common.  Other findings, such as hepatosplenomegaly, interstitial  keratitis, and eighth nerve deafness, also occur. Fetal infec¬  tion can also result in stillbirth.   Immunity to syphilis is incomplete. Antibodies to the  organism are produced but do not stop the progression of  the disease. Patients with early syphilis who have been  treated can contract syphilis again. Patients with late syphi¬  lis are relatively resistant to reinfection.   Spirochetes are demonstrated in the lesions of primary or  secondary syphilis, such as chancres or condylomata lata,  by dark field microscopy or by direct fluorescent antibody  (DFA) test. They are not seen on a Gram-stained smear. In  biopsy specimens, such as those obtained from the gum¬  mas seen in tertiary syphilis, histologic stains such as silver  stain or fluorescent antibody can be used.   These tests involve the use of nontreponemal antigens.  Extracts of normal mammalian tissues (e.g., cardiolipin  from beef heart) react with antibodies in serum samples  from patients with syphilis. These antibodies, which are a  mixture of IgG and IgM, are called “reagin” antibodies (see  earlier). Flocculation tests (e.g., Venereal Disease Research  Laboratory [VDRL] and rapid plasma reagin [RPR] tests)  detect the presence of these antibodies. These tests are  positive in most cases of primary syphilis and are almost  always positive in secondary syphilis. The titer of these  nonspecific antibodies decreases with effective treatment,  in contrast to the specific antibodies, which are positive for  life (see later).   False-positive reactions occur in infections such as lep¬  rosy, hepatitis B, and infectious mononucleosis and in vari¬  ous autoimmune diseases. Therefore, positive results have  to be confirmed by specific tests (see later). Results of  nonspecific tests usually become negative after treatment  and should be used to determine the response to treatment.  These tests can also be falsely negative as a result of the  prozone phenomenon. In the prozone phenomenon, the  titer of antibody is too high (antibody excess), and no floc¬  culation will occur. On dilution of the serum, however, the  test result becomes positive (see Chapter 64). These tests  are inexpensive and easy to perform and therefore are used  as a method of screening the population for infection. The  nonspecific tests and the specific tests (see later) are  described in more detail in Chapter 9.   The laboratory diagnosis of congenital syphilis is based on  the finding that the infant has a higher titer of antibody in the  VDRL test than has the mother. Furthermore, if a positive  VDRL test result in the infant is a false-positive one because  maternal antibody has crossed the placenta, the titer will  decline with time. If the infant is truly infected, the titer will  remain high. However, irrespective of the VDRL test results,  any infant whose mother has syphilis should be treated.   These tests involve the use of treponemal antigens and there¬  fore are more specific than those described earlier. In these  tests, T. pallidum reacts in immunofluorescence (FTA-ABS) 2    2 FTA-ABS is the fluorescent treponemal antibody-absorbed test. The  patient’s serum is absorbed with nonpathogenic treponemes to remove  cross-reacting antibodies prior to reacting with T. pallidum.    or hemagglutination (TPLLA, MHA-TP) 3 assays with specific  treponemal antibodies in the patients serum.   These antibodies arise within 2 to 3 weeks of infection;  therefore, the test results are positive in most patients with  primary syphilis. These tests remain positive for life after  effective treatment and cannot be used to determine the  response to treatment or reinfection. They are more expen¬  sive and more difficult to perform than the nonspecific  tests and therefore are not used as screening procedures.   Penicillin G is effective in the treatment of all stages of  syphilis. A single injection of benzathine penicillin G  (2.4 million units) can eradicate T. pallidum and cure early  (primary and secondary) syphilis. Note that benzathine  penicillin is used because the penicillin is released very  slowly from this depot preparation. Treponema pallidum  grows very slowly, which requires that the penicillin be pres¬  ent in bactericidal concentration for weeks. If the patient is  allergic to penicillin, doxycycline can be used but must be  given for prolonged periods to effect a cure. In neurosyphilis,  high doses of aqueous penicillin G are administered because  benzathine penicillin penetrates poorly into the central ner¬  vous system. No resistance to penicillin has been observed.  However, strains resistant to azithromycin have emerged.   Pregnant women with syphilis should be treated  promptly with the type of penicillin used for the stage of  their disease. Neonates with a positive serological test  should also be treated. Although it is possible that the posi¬  tive test is caused by maternal antibody rather than infec¬  tion of the neonate, it is prudent to treat without waiting  several months to determine whether the titer of antibody  declines.   More than half of patients with secondary syphilis who  are treated with penicillin experience fever, chills, myalgias,  and other influenzalike symptoms a few hours after  receiving the antibiotic. This response, called the Jarisch-  Herxheimer reaction, is attributed to the lysis of the trepo¬  nemes and the release of endotoxin-like substances. Patients  should be alerted to this possibility, advised that it may last  for up to 24 hours, and told that symptomatic relief can be  obtained with aspirin. The Jarisch-Herxheimer reaction  also occurs after treatment of other spirochetal diseases  such as Lyme disease, leptospirosis, and relapsing fever.  Tumor necrosis factor (TNF) is an important mediator of  this reaction because passive immunization with antibody  against TNF can prevent its symptoms.   Prevention depends on early diagnosis and adequate treat¬  ment, use of condoms, administration of antibiotic after    3 TPHA is the T. pallidum hemagglutination assay. MHA-TP is a hemag¬  glutination assay done in a microtiter plate.    suspected exposure, and serologic follow-up of infected  individuals and their contacts. The presence of any sexually  transmitted disease makes testing for syphilis mandatory,  because several different infections are often transmitted  simultaneously. There is no vaccine against syphilis.   These are infections caused by spirochetes that are virtually  indistinguishable from those caused by T. pallidum. They  are endemic in populations and are transmitted by direct  contact. All these infections result in positive (nontrepone-  mal and treponemal) results on serologic tests for syphilis.  None of these spirochetes have been grown on bacterio-  logic media. The diseases include bejel in Africa, yaws  (caused by T. pallidum subspecies pertenue) in many humid  tropical countries, and pinta (caused by Treponema cara-  teum ) in Central and South America. All can be cured by  penicillin.   Borrelia species are irregular, loosely coiled spirochetes that  stain readily with Giemsa and other stains. They can be  cultured in bacteriologic media containing serum or tissue  extracts. They are transmitted by arthropods. They cause  two major diseases, Lyme disease and relapsing fever.   Borrelia burgdorferi causes Lyme disease (named after a  town in Connecticut). Lyme disease is also known as Lyme  borreliosis. Lyme disease is the most common tick-borne  disease in the United States. It is also the most common  vector-borne disease in the United States. Approximately  20,000 cases each year are reported to the Centers for Dis¬  ease Control and Prevention, and that number is thought to  be significantly less than the actual number.   Borrelia burgdorferi is a flexible, motile spirochete that can  be visualized by dark field microscopy and by Giemsa and  silver stains. It can be grown in certain bacteriologic media,  but routine cultures obtained from patients (e.g., blood,  spinal fluid) are typically negative. In contrast, culture of  the organism from the tick vector is usually positive.   Borrelia burgdorferi is transmitted by tick bite (Figures  24-5 through 24-7). The tick Ixodes scapularis is the vector  on the East Coast and in the Midwest; Ixodes pacificus is  involved on the West Coast. The organism is found in a  much higher percentage of I. scapularis (35%-50%) than I.  pacificus (approximately 2%) ticks. This explains the lower   FIGURE 24-5 Ixodes tick. Nymph form of tick with head bur¬  ied in skin surrounded by an erythematous macular rash. (Reproduced  with permission from Wolff K, Johnson R. Fitzpatrick's Color Atlas & Synopsis of Clinical  Dermatology. 6th ed. New York: McGraw-Hill, 2009. Copyright © 2009 by The  McGraw-Hill Companies, Inc.)   incidence of disease on the West Coast. The main reservoir  of the organism consists of small mammals, especially the  white-footed mouse, upon which the nymphs feed. 4   Large mammals, especially deer, are an obligatory host  in the tick’s life cycle but are not an important reservoir of  the organism.   The nymphal stage of the tick transmits the disease more  often than the adult and larval stages do. Nymphs feed pri¬  marily in the summer, which accounts for the high incidence  of disease during the months of May to September.   The tick must feed for 24 to 48 hours to transmit an  infectious dose. This implies that inspecting the skin after  being exposed can prevent the disease. However, the  nymphs are quite small and can easily be missed. There is  no human-to-human spread.   The disease occurs worldwide. In the United States, three  regions are primarily affected: the states along the North  Atlantic seaboard, the northern midwestern states (e.g.,  Wisconsin), and the West Coast, especially California.  Approximately 80% of the reported cases occurred in four  states: New York, Connecticut, Pennsylvania, and New Jersey.    4 In California, the wood rat is the main reservoir, and a second tick,  Ixodes neotomae, perpetuates the infection in the wood rat but does not  transmit the infection to humans.   FIGURE 24~6 Ixodesscapularis —" blacklegged" tick. Engorged  female tick after feeding. (Source: Dr. Gary Alpert, Centers for Disease Control  and Prevention.)   FIGURE 24-7 Ixodes tick on a blade of grass questing for a  host, such as a deer or human. (Source: Drs. Amanda Loftis, Will Reeves, and  Chris Paddock, Centers for Disease Control and Prevention.)    Lyme disease is the most common vector-borne disease  in the United States. The major bacterial diseases transmit¬  ted by ticks in the United States are Lyme disease, Rocky  Mountain spotted fever, ehrlichiosis, anaplasmosis, relaps¬  ing fever, and tularemia. Ixodes scapularis ticks transmit  three diseases: two bacterial diseases, Lyme disease and  human granulocytic ehrlichiosis, and the protozoan dis¬  ease, babesiosis. Coinfection with B. burgdorferi and Babe¬  sia occurs, especially in endemic areas such as Massachusetts  and other northeastern states.   Pathogenesis is associated with spread of the organism  from the bite site through the surrounding skin followed by  dissemination via the blood (bacteremia) to various organs,  especially the heart, joints, and central nervous system. No  exotoxins, enzymes, or other important virulence factors  have been identified.   Note that the organism must adapt to two markedly dif¬  ferent hosts, the tick and the mammal (either mice or  humans). It does so by changing its outer surface protein  (OSP). These OSPs vary antigenically within humans.   Multiple episodes of Lyme disease are due to reinfection,  rather than relapse caused by reactivation of the organism.  There is no evidence for a latent stage of B. burgdorferi.   The clinical findings have been divided into three stages;  however, this is a progressive disease, and the stages are not  discrete. In stage 1 (early localized stage), the most com¬  mon finding is erythema chronicum migrans (also called  erythema migrans), an expanding, erythematous, macular  rash that often has a “target” or “bull’s eye” appearance  (Figure 24-8).   The rash appears between 3 and 30 days after the tick  bite. Both the tick bite and the rash are painless and  nonpruritic.   FIGURE 24~8 Erythema chronicum migrans rash of Lyme dis¬  ease. Note oval-shaped expanding erythematous macular"bull's eye"  rash of primary Lyme disease. (Used with permission from Vijay K. Sikand, MD.)    The rash may sometimes be accompanied by nonspe¬  cific “flulike” symptoms such as fever, chills, fatigue, and  headache. Secondary skin lesions frequently occur. Arthral¬  gias, but not arthritis, are another common finding in this  early stage. In approximately 25% of cases of Lyme disease,  no rash is seen.   In stage 2 (early disseminated stage), which occurs  weeks to months later, cardiac and neurologic involvement  predominates. Myocarditis, accompanied by various forms  of heart block, occurs. Acute (aseptic) meningitis and cra¬  nial neuropathies, such as facial nerve palsy (Bell’s palsy),  are prominent during this stage. Bilateral facial nerve palsy  is highly suggestive of Lyme disease. Peripheral neuropa¬  thies also occur.   A latent phase lasting weeks to months typically ensues.  In stage 3 (late disseminated stage), arthritis, usually of the  large joints (e.g., knees), is a characteristic finding. Lyme  arthritis is thought to be autoimmune in origin. Encepha¬  lopathy also occurs in stage 3.   Some patients treated for Lyme infection continue to have  prolonged subjective symptoms of fatigue, joint pains, or  mental status changes after objective findings have disap¬  peared. No confirmed microbiologic evidence for B. burgdor¬  feri infection has been detected in those patients, and  prolonged antibiotic therapy has not relieved the symptoms.   Although the organism can be grown in the laboratory,  cultures are rarely positive and hence are usually not per¬  formed. The diagnosis is typically made serologically by  detecting either IgM antibody or a rising titer of IgG anti¬  body with an enzyme-linked immunosorbent assay  (ELISA) or an indirect immunofluorescence test. IgM is  typically detectable 2 weeks after infection and peaks at 3 to  6 weeks. Serologic tests done before 2 weeks are likely to  yield negative results. Thirty days after infection, tests for  IgG are more reliable.   Unfortunately, there are problems with the specificity  and sensitivity of these tests because of the presence of  cross-reacting antibodies against spirochetes in the normal  flora. A positive test result should be confirmed with a  Western blot (immunoblot) analysis. In addition, patients  treated early in the disease may not develop detectable  antibodies. A polymerase chain reaction (PCR) test that  detects the organism’s DNA is also available.   The treatment of choice for stage 1 disease or other mild  manifestations is either doxycycline or amoxicillin. Amoxi¬  cillin should be used in pregnant women and young chil¬  dren, as doxycycline is contraindicated. For more severe  forms or late-stage disease, ceftriaxone is recommended.  There is no significant antibiotic resistance.   Prevention involves wearing protective clothing and  using insect repellents. Examining the skin carefully for    ticks is also very important, because the tick must feed for  24 to 48 hours to transmit an infective dose.   Should prophylactic antibiotics be given to people who  have been bitten by a tick? The decision depends on two  main factors: the percentage of infected ticks in the area  and the length of time the tick has fed on the person. If the  percentage of infected ticks is high and the length of time is  more than 48 hours, it may be cost-effective to prescribe  doxycycline prophylactically. Any person bitten by a tick  should be advised to watch carefully for a rash or flulike  symptoms for the next 3 weeks.   A vaccine containing a recombinant outer surface pro¬  tein (OspA) of B. burgdorferi as the immunogen was avail¬  able but has been withdrawn.   Borrelia recurrentis, Borrelia hermsii, and several other bor-  reliae cause relapsing fever. During infection, the antigens  of these organisms undergo variation. As antibodies  develop against one antigen, variants emerge and produce  relapses of the illness. This can be repeated 3 to 10 times.   Borrelia recurrentis is transmitted from person to person  by the human body louse. Humans are the only hosts. Bor¬  relia hermsii and many other Borrelia species are transmit¬  ted to humans by soft ticks ( Ornithodoros ). Rodents and  other small animals are the main reservoirs. These species  of Borrelia are passed transovarially in the ticks, a phenom¬  enon that plays an important role in maintaining the  organism in nature.   During infection, the arthropod bite introduces spiro¬  chetes, which then multiply in many tissues, producing  fever, chills, headaches, and multiple-organ dysfunction.  Each attack is terminated as antibodies arise.   Diagnosis is usually made by seeing the large spiro¬  chetes in stained smears of peripheral blood. They can be  cultured in special media. Serologic tests are rarely useful.  Tetracycline may be beneficial early in the illness and may  prevent relapses. Avoidance of arthropod vectors is the best  means of prevention.   Borrelia miyamotoi causes a relapsing feverlike syndrome.  It was discovered in 1995 in Japan but causes disease world¬  wide, including the United States. It is transmitted by  Ixodes ticks. Clinically, the disease begins with an influ¬  enzalike syndrome (fever, headache, and myalgia) accom¬  panied by hepatitis and thrombocytopenia. Relapsing  episodes occur. The manifestations can resemble anaplas-  mosis (see Chapter 26) that is also transmitted by Ixodes  ticks. There is no rash, unlike Lyme disease.   The diagnosis is typically made serologically by detecting  IgM antibody or by PCR assay testing for the gene encoding  the Glp Q protein that is specific for B. miyamotoi.    Doxycycline and ceftriaxone are effective treatment choices.  There is no vaccine. Wearing clothing impregnated with  Permethrin can reduce the risk of tick bites.   Leptospiras are tightly coiled spirochetes with hooked  ends. They stain poorly with dyes and so are not seen by  light microscopy, but they are seen by dark field micros¬  copy. They grow in bacteriologic media containing serum.   Leptospira interrogans is the cause of leptospirosis. Lepto¬  spirosis is common in tropical countries, especially in the  rainy season, but is rare in the United States. Leptospira inter¬  rogans is divided into serogroups that occur in different  animals and geographic locations. Each serogroup is subdi¬  vided into serovars by the response to agglutination tests.   Leptospiras infect various animals, including rats and  other rodents, domestic livestock, and household pets. In  the United States, dogs are the most important reservoir.  Animals excrete leptospiras in urine, which contaminates  water and soil.   Swimming in contaminated water or consuming con¬  taminated food or drink can result in human infection.  Outbreaks have occurred among participants in triathlons  and adventure tours involving swimming in contaminated  waters. Miners, farmers, and people who work in sewers  are at high risk. In the United States, the urban poor have a  high rate of infection as determined by the presence of  antibodies. Person-to-person transmission is rare.   Human infection results when leptospiras are ingested  or pass through mucous membranes or skin. They circulate  in the blood and multiply in various organs, producing  fever and dysfunction of the liver (jaundice), kidneys (ure¬  mia), lungs (hemorrhage), and central nervous system  (aseptic meningitis). The illness is typically biphasic, with  fever, chills, intense headache, and conjunctival suffusion  (diffuse reddening of the conjunctivae) appearing early in  the disease, followed by a short period of resolution of these  symptoms as the organisms are cleared from the blood. The  second, “immune,” phase is most often characterized by the  findings of aseptic meningitis and, in severe cases, liver  damage (jaundice) and impaired kidney function. Serovar-  specific immunity develops with infection.   Diagnosis is based on history of possible exposure, sug¬  gestive clinical signs, and a marked rise in IgM antibody  titers. Occasionally, leptospiras are isolated from blood and  urine cultures.   The treatment of choice is penicillin G. There is no signifi¬  cant antibiotic resistance. Prevention primarily involves avoid¬  ing contact with the contaminated environment. Doxycycline  is effective in preventing the disease in exposed persons.   Anaerobic saprophytic spirochetes are prominent in the  normal flora of the human mouth. Such spirochetes    participate in mixed anaerobic infections, infected human  bites, stasis ulcers, etc.   1. Your patient is a 65-year-old man with gradually increasing confu¬  sion and unsteadiness while walking. A lumbar puncture revealed  clear spinal fluid, a normal glucose, and an elevated protein. There  were 96 cells/mm 3 , of which 86% were lymphocytes. Gram stain  of the cerebrospinal fluid (CSF) was negative. Magnetic resonance  imaging (MRI) of the brain was normal. A sample of CSF reacted  with beef heart cardiolipin at a titer of 1/1024. Regarding the caus¬  ative organism of his infection, which one of the following is most  accurate?   (B) Resistance to penicillin G is common, so ceftriaxone should be  used.   (C) It has never been grown on bacteriologic media in the clinical  laboratory.   (D) It is unlikely to be eradicated because beef cattle are a major  reservoir for the organism.   (E) A confirmatory test for this organism utilizes an agglutination  reaction with the capsular polysaccharide of the organism.   2. Your patient is a 20-year-old man with an erythematous, macular,  nonpainful rash on the right arm for the past 4 days. The rash is  approximately 10 cm in diameter. He also has a fever to 100°F and  a mild headache. He reports hiking on several weekends recently  in New York State. You suspect the rash is erythema migrans and  that he has Lyme disease. Which one of the following is the best  approach to confirm your clinical diagnosis?   (E) Grow on human cells in cell culture and identify with fluores¬  cent antibody   3. Assume the patient in Question 2 does have Lyme disease. Which  one of the following antibiotics is the most appropriate to treat his  infection?   4. Regarding syphilis, which one of the following is most accurate?   (A) The characteristic lesion of primary syphilis is a painful vesicle  on the genitals.   (B) In secondary syphilis, the number of organisms is low, so the  chance of transmitting the disease to others is low.   (C) In secondary syphilis, both the rapid plasma reagin (RPR) and  the fluorescent treponemal antibody-absorbed (FTA-ABS)  tests are usually positive.   (D) The antibody titer in the FTA-ABS test typically declines when  the patient has been treated adequately.   (E) In congenital syphilis, no antibody is formed against T. palli¬  dum because the fetus is tolerant to the organism.   (A) Borrelia burgdorferi infects a larger percentage of the rodent  reservoir in western states, such as California, than in north¬  eastern states, such as New York.   (B) Pathogenesis of Lyme disease is based on the production of an  exotoxin that induces interleukin-2 production by T-helper  cells.   (C) The vaccine against Lyme disease contains the capsular poly¬  saccharide of all four serotypes as the immunogen.   (D) Close family members of those infected with B. burgdorferi  should be given ciprofloxacin.   (E) Borrelia burgdorferi is transmitted to humans by the bite of  ticks of the genus Ixodes.   6. Benzathine penicillin G is used to treat primary and secondary   syphilis rather than procaine penicillin G. Which one of the fol¬  lowing is the best reason for this choice?   (A) Patients allergic to procaine penicillin G are not allergic to  benzathine penicillin G.   (B) Benzathine penicillin G has a higher minimal inhibitory con¬  centration than procaine penicillin G.   (C) Benzathine penicillin G penetrates the central nervous system  to a greater degree than procaine penicillin G.   (D) Benzathine penicillin G is a depot preparation that provides a  long-lasting, high level of drug that kills the slow-growing  T. pallidum.   Brief summaries of the organisms described in this chapter  begin on page 666. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Clinical Bacteriology section of Part XIII: USMLE  (National Board) Practice Questions starting on page 713. Also  see Part XIV: USMLE (National Board) Practice Examination  starting on page 751.   Chlamydiae are obligate intracellular bacteria (i.e., they can  grow only within cells). They are the agents of common  sexually transmitted diseases, such as urethritis and cervi¬  citis, as well as other infections, such as pneumonia, psit¬  tacosis, trachoma, and lymphogranuloma venereum.   Additional information regarding the clinical aspects of  infections caused by the organisms in this chapter is pro¬  vided in Part IX entitled Infectious Diseases beginning on  page 593.   Chlamydia trachomatis causes eye (conjunctivitis, tra¬  choma), respiratory (pneumonia), and genital tract (ure¬  thritis, lymphogranuloma venereum) infections. Chlamydia  trachomatis is the most common bacterial cause of sexu¬  ally transmitted disease in the United States. Infection    with C. trachomatis is also associated with Reiter’s syn¬  drome, an autoimmune disease.   Chlamydia pneumoniae causes atypical pneumonia.  Chlamydia psittaci causes psittacosis, also a disease charac¬  terized mainly by pneumonia (Table 25-1).   Chlamydia pneumoniae and C. psittaci are sufficiently  different molecularly from C. trachomatis that they have  been reclassified into a new genus called Chlamydophila.  Taxonomically, they are now Chlamydophila pneumoniae  and Chlamydophila psittaci. However, from a medical per¬  spective, they are still known as Chlamydia pneumoniae  and Chlamydia psittaci, and those are the names that are  used in this book.   Chlamydiae are obligate intracellular bacteria. They lack the  ability to produce sufficient energy to grow independently  and therefore can grow only inside host cells. They have a   Inclusions in epithelial  cells seen with Giemsa  stain or by immunofluo¬  rescence; also cell  culture    FIGURE 25-1 Life cycle of Chlamydia. The extracellular, inert elementary body enters an epithelial cell and changes into a reticulate body  that divides many times by binary fission. The daughter reticulate bodies change into elementary bodies and are released from the epithelial  cell. The cytoplasmic inclusion body, which is characteristic of chlamydial infections, consists of many daughter reticulate and elementary  bodies. (Reproduced with permission from Ryan K et at. Sherris Medical Microbiology. 3rd ed. Originally published by Appleton & Lange. Copyright 1994 by McGraw-Hill.)    rigid cell wall but do not have a typical peptidoglycan layer.  Their cell walls resemble those of gram-negative bacteria  but lack muramic acid.   Chlamydiae have a replicative cycle different from that  of all other bacteria. The cycle begins when the extracellu¬  lar, metabolically inert, “sporelike” elementary body enters  the cell and reorganizes into a larger, metabolically active  reticulate body (Figure 25-1). The latter undergoes  repeated cycles of binary fission to form daughter reticulate  bodies, which then develop into elementary bodies, which  are released from the cell. Within cells, the site of replication  appears as an inclusion body in the cytoplasm, which can  be stained and visualized microscopically (Figure 25-2).  These inclusions are useful in the identification of these  organisms in the clinical laboratory.   All chlamydiae share a group-specific lipopolysaccha-  ride antigen, which is detected by complement fixation  tests. They also possess species-specific and immunotype-  specific antigens (proteins), which are detected by immu¬  nofluorescence. Chlamydia psittaci and C. pneumoniae  each have one immunotype, whereas C. trachomatis has at  least 15 immunotypes.   Chlamydia trachomatis infects only humans and is usually  transmitted by close personal contact (e.g., sexually or by  passage through the birth canal). Individuals with    asymptomatic genital tract infections are an important  reservoir of infection for others. In trachoma, C. trachomatis  is transmitted by finger-to-eye or fomite-to-eye contact.   Chlamydia pneumoniae infects only humans and is  transmitted from person to person by aerosol. Chlamydia  psittaci infects birds (e.g., parrots, pigeons, and poultry,  and many mammals including humans). Humans are   culture. Long arrow points to cytoplasmic inclusion body of C. tracho¬  matis; short arrow points to nucleus of cell. (Source: Dr. E. Arum and  Dr. N. Jacobs, Public Health Image Library, Centers for Disease Control and  Prevention.)   Sexually transmitted disease caused by C. trachomatis  occurs worldwide, but trachoma is most frequently found  in developing countries in dry, hot regions such as north¬  ern Africa. Trachoma is a leading cause of blindness in  those countries.   Patients with a sexually transmitted disease are coin¬  fected with both C. trachomatis and Neisseria gonorrhoeae  in approximately 10% to 30% of cases.   Chlamydiae infect primarily epithelial cells of the mucous  membranes or the lungs. They rarely cause invasive, dis¬  seminated infections.    pneumonia 2 to 12 weeks after birth. Chlamydial conjunc¬  tivitis also occurs in adults as a result of the transfer of  organisms from the genitals to the eye.   Patients with genital tract infections caused by C. tra¬  chomatis have a high incidence of reactive arthritis and  Reiter’s syndrome, which is characterized by urethritis,  arthritis, and uveitis. These are autoimmune diseases  caused by antibodies formed against C. trachomatis cross¬  reacting with antigens on the cells of the urethra, joints,  and uveal tract (see Chapter 66).   Chlamydia trachomatis L1-L3 immunotypes cause lym¬  phogranuloma venereum, a sexually transmitted disease  with lesions on genitalia and in lymph nodes.   Infection by C. trachomatis leads to formation of anti¬  bodies and cell-mediated reactions but not to resistance to  reinfection or elimination of organisms.    Chlamydia trachomatis has more than 15 immunotypes  (A-L). Types A, B, and C cause trachoma, a chronic con¬  junctivitis endemic in Africa and Asia. Trachoma may  recur over many years and may lead to blindness but causes  no systemic illness.   Types D-K cause genital tract infections. In men, it is  a common cause of nongonococcal urethritis (often abbre¬  viated NGU), which is characterized by dysuria and a  watery, nonpurulent urethral discharge (Figure 25-3). The  discharge may be slight, detectable only by staining of  underwear overnight. This infection may progress to epi¬  didymitis, prostatitis, or proctitis. In women, cervicitis  develops and may progress to salpingitis and pelvic inflam¬  matory disease (PID). Repeated episodes of salpingitis or  PID can result in infertility or ectopic pregnancy.   Infants born to infected mothers often develop mucopu¬  rulent conjunctivitis (neonatal inclusion conjunctivitis) 7  to 12 days after delivery, and some develop chlamydial   FIGURE 25~3 Nongonococcal urethritis. Note watery, nonpu¬  rulent discharge caused by Chlamydia trachomatis. The urethral  discharge caused by Neisseria gonorrhoeae is more mucoid and purulent.  (Used with permission from Seattle STD/HIV Prevention Training Center.)    Chlamydia pneumoniae causes upper and lower respiratory  tract infections, especially bronchitis and pneumonia, in  young adults. Most infections are mild or asymptomatic.  The clinical picture resembles other atypical pneumonias,  especially that caused by Mycoplasma pneumoniae. It is  unclear whether C. pneumoniae causes upper respiratory  infections such as sinusitis and otitis media.   Chlamydia psittaci infects the lungs primarily. The infec¬  tion may be asymptomatic (detected only by a rising anti¬  body titer) or may produce high fever and pneumonia.  Human psittacosis is not generally communicable from  human to human.   Although most infections are mild, some are quite severe  and involve organs other than the lung. The respiratory  infection typically manifests with fever, cough, dyspnea,  myalgias, and headache. The most common extrapulmonary  organs involved are the liver (hepatomegaly, jaundice), heart  (myocarditis, pericarditis), and nervous system (hearing  loss, transverse myelitis, and encephalitis).   Chlamydiae form cytoplasmic inclusions, which can be  seen with special stains (e.g., Giemsa stain) or by immuno¬  fluorescence (see Figure 25-2). In general, the Gram stain  is not useful as the organisms are too small to visualize  within the cytoplasm. However, a gram stain of a urethral  discharge that shows neutrophils but no gram-negative  diplococci resembling Neisseria gonorrhoeae is presumptive  evidence for infection by C. trachomatis.   Nucleic acid amplification tests (NAATs) using the  patient’s urine are widely used to diagnose chlamydial  sexually transmitted disease. Tests not involving culture,  such as NAAT, are now more commonly used than culture-  based tests (see later).    In exudates, the organism can be identified within epi¬  thelial cells by fluorescent-antibody staining or hybridiza¬  tion with a DNA probe. Chlamydial antigens can also be  detected in exudates or urine by enzyme-linked immuno¬  sorbent assay (ELISA).   Chlamydiae can be grown in cell cultures treated with  cycloheximide, which inhibits host cell but not chlamydial  protein synthesis, thereby enhancing chlamydial replica¬  tion. In culture, C. trachomatis forms inclusions containing  glycogen, whereas C. psittaci and C. pneumoniae form  inclusions that do not contain glycogen. The glycogen-  filled inclusions are visualized by staining with iodine.  Exudates from the eyes, respiratory tract, or genital tract  yield positive cultures in about half of cases.   Serologic tests are used to diagnose infections by C. psittaci  and C. pneumoniae but are rarely helpful in diagnosing disease  caused by C. trachomatis because the frequency of infection is  so high that many people already have antibodies.   All chlamydiae are susceptible to tetracyclines, such as  doxycycline, and macrolides, such as erythromycin and  azithromycin. The drug of choice for C. trachomatis sexu¬  ally transmitted diseases is azithromycin. Because the rate  of coinfection with gonococci and C. trachomatis is high,  any patient with a diagnosis of gonorrhea should also be  treated for C. trachomatis with azithromycin.   The drug of choice for neonatal inclusion conjunctivitis  and pneumonia is oral erythromycin. The drug of choice for  C. psittaci and C. pneumoniae infections and for lympho¬  granuloma venereum is a tetracycline, such as doxycycline.   There is no vaccine against any chlamydial disease. The  best preventive measure against C. trachomatis sexually  transmitted diseases is to limit transmission by safe sex  practices and prompt treatment of both the patient and the  sexual partners, including persons who are asymptomatic.  Sexual contacts should be traced, and those who had con¬  tact within 60 days should be treated. Several types of sexu¬  ally transmitted diseases are often present simultaneously.  Thus, diagnosis of one requires a search for other causative  agents.   Oral erythromycin given to newborn infants of infected  mothers can prevent inclusion conjunctivitis and pneumo¬  nitis caused by C. trachomatis. Note that erythromycin  ointment used to prevent neonatal gonococcal conjunctivi¬  tis is much less effective against neonatal chlamydial con¬  junctivitis. Oral erythromycin should be used.   Psittacosis in humans is controlled by restricting the  importation of psittacine birds, treating or destroying sick  birds, and adding tetracycline to bird feed. Domestic flocks  of turkeys and ducks are tested for the presence of  C. psittaci.   1. Your patient is a 20-year-old man with a urethral discharge. Gram  stain of the pus reveals many neutrophils but no bacteria. You sus¬  pect this infection may be caused by C. trachomatis. Which one of  the following is the laboratory result that best supports your clini¬  cal diagnosis?   (B) The organism produces beta-hemolytic colonies on blood agar  plates when incubated aerobically.   (C) The organism produces alpha-hemolytic colonies on blood  agar plates when incubated anaerobically.   (D) Fluorescent-antibody staining of cytoplasmic inclusions in  epithelial cells in the exudate.   (E) Fourfold or greater rise in antibody titer against C. trachomatis.   2. Regarding chlamydiae, which one of the following is the most  accurate?   (A) Lifelong immunity usually follows an episode of disease caused  by these organisms.   (B) The reservoir host for the three species of chlamydiae that  cause human infection is humans.   (C) Their life cycle consists of elementary bodies outside of cells  and reticulate bodies within cells.   (D) They can only replicate within cells because they lack the ribo¬  somes to synthesize their proteins.   (E) The vaccine against C. pneumoniae contains the capsular poly¬  saccharide as the immunogen conjugated to a carrier protein.   3. Which one of the following is the drug of choice for sexually trans¬  mitted disease (urethritis, cervicitis) caused by C. trachomatis ?   Brief summaries of the organisms described in this chapter  begin on page 666. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Clinical Bacteriology section of Part XIII: USMLE  (National Board) Practice Questions starting on page 713. Also  see Part XIV: USMLE (National Board) Practice Examination  starting on page 751.   Rickettsiae are obligate intracellular bacteria; that is, they  can grow only within cells. They are the agents of several  important diseases, namely typhus, spotted fevers such as  Rocky Mountain spotted fever, Q fever, anaplasmosis, and  ehrlichiosis. Other less important rickettsial diseases such  as endemic and scrub typhus occur primarily in developing  countries. Rickettsial pox, caused by Rickettsia akari, is a  rare disease found in certain densely populated cities in the  United States.   Additional information regarding the clinical aspects of  infections caused by the organisms in this chapter is pro¬  vided in Part IX entitled Infectious Diseases beginning on  page 593.   Rickettsia rickettsii causes Rocky Mountain spotted fever, a  life-threatening disease that occurs primarily in the South¬  eastern states, for example, North Carolina, of the United  States. Rickettsia prowazekii causes epidemic typhus, also a  life-threatening disease that occurs mainly in crowded,  unsanitary living conditions during wartime.   Rickettsiae are very short rods that are barely visible in the  light microscope. Structurally, their cell wall resembles that  of gram-negative rods, but they stain poorly with the stan¬  dard Gram stain.    Rickettsiae are obligate intracellular parasites, because  they are unable to produce sufficient energy to replicate  extracellularly. Therefore, rickettsiae must be grown in cell  culture, embryonated eggs, or experimental animals. Rick¬  ettsiae divide by binary fission within the host cell, in con¬  trast to chlamydiae, which are also obligate intracellular  parasites but replicate by a distinctive intracellular cycle  (see Chapter 25).   Several rickettsiae, such as R. rickettsii, R. prowazekii,  and Rickettsia tsutsugamushi (renamed Orientia tsutsuga-  mushi), possess antigens that cross-react with antigens of  the OX strains of Proteus vulgaris. The Weil-Felix test,  which detects antirickettsial antibodies in a patient’s serum  by agglutination of the Proteus organisms, is based on this  cross-reaction.   The most striking aspect of the life cycle of the rickettsiae  is that they are maintained in nature in certain arthropods  such as ticks, lice, fleas, and mites and, with one exception,  are transmitted to humans by the bite of the arthropod.  The rickettsiae circulate widely in the bloodstream (bacte¬  remia), infecting primarily the endothelium of the blood  vessel walls.   The exception to arthropod transmission is Coxiella  burnetii, the cause of Q fever, which is transmitted by aero¬  sol and inhaled into the lungs (see later). Virtually all rick¬  ettsial diseases are zoonoses (i.e., they have an animal  reservoir), with the prominent exception of epidemic  typhus, which occurs only in humans. It occurs only in  humans because the causative organism, R. prowazekii, is  transmitted by the human body louse. A summary of the    vectors and reservoirs for selected rickettsial diseases is  presented in Table 26-1.   The incidence of the disease depends on the geographic  distribution of the arthropod vector and on the risk of  exposure, which is enhanced by such things as poor  hygienic conditions and camping in wooded areas. These  factors are discussed later with the individual diseases.   The typical lesion caused by these rickettsiae is a vasculitis,  particularly in the endothelial lining of the vessel wall where  the organism is found. Damage to the vessels of the skin  results in the characteristic rash and in edema and hemor¬  rhage caused by increased capillary permeability. Vasculitis  of the vessels in the brain leads to the prominent headache.   The basis for pathogenesis by these organisms is unclear.  There is some evidence that endotoxin is involved, which is  in accord with the nature of some of the lesions such as  fever, petechiae and thrombocytopenia, but its role has not  been confirmed. No exotoxins or cytolytic enzymes have  been found.   This disease is characterized by the acute onset of nonspe¬  cific symptoms (e.g., fever, severe headache, myalgias, and  prostration). The typical rash, which appears 2 to 6 days  later, begins with macules that frequently progress to pete¬  chiae (Figure 26-1). The rash usually appears first on the  hands and feet and then moves inward to the trunk. In  addition to headache, other profound central nervous  system changes such as delirium and coma can occur. Dis¬  seminated intravascular coagulation, edema, and circula¬  tory collapse may ensue in severe cases. The diagnosis must  be made on clinical grounds and therapy started promptly,  because the laboratory diagnosis is delayed until a rise in  antibody titer can be observed.    The name of the disease is misleading, because it occurs  primarily along the East Coast of the United States (in the  southeastern states of Virginia, North Carolina, and  Georgia), where the dog tick, Dermacentor variabilis, is  located. The name “Rocky Mountain spotted fever” is  derived from the region in which the disease was first  found. In the western United States, it is transmitted by the  wood tick, Dermacentor andersoni.   The tick is an important reservoir of R. rickettsii as well  as the vector; the organism is passed by the transovarian  route from tick to tick, and a lifetime infection results. Cer¬  tain mammals, such as dogs and rodents, are also reservoirs  of the organism. Humans are accidental hosts and are not  required for the perpetuation of the organism in nature;  there is no person-to-person transmission. Most cases  occur in children during spring and early summer, when  the ticks are active. Rocky Mountain spotted fever accounts  for 95% of the rickettsial disease in the United States; there  are about 1000 cases per year. It can be fatal if untreated,  but if it is diagnosed and treated, a prompt cure results.   petechial rash. (Reproduced from MMWR, Diagnosis and Management ofTickborne  Rickettsial Diseases: Rocky Mountain Spotted Fever, Ehrlichiosis, and Anaplasmosis-  United States. March 13,2006/55(RR04);1-27, http://www.cdc.gov/mmwr.)   There are several forms of typhus, namely, louse-borne epi¬  demic typhus caused by R. prowazekii, flea-borne endemic  typhus caused by Rickettsia typhi, chigger-borne scrub  typhus caused by 0 . tsutsugamushi, and several other quite  rare forms. Cases of flea-borne endemic typhus, also called  murine typhus, occur in small numbers in the southern  regions of California and Texas. The following description  is limited to epidemic typhus, the most important of the  typhus group of diseases.   Typhus begins with the sudden onset of chills, fever,  headache, and other influenzalike symptoms approxi¬  mately 1 to 3 weeks after the louse bite occurs. Between the  fifth and ninth days after the onset of symptoms, a maculo-  papular rash begins on the trunk and spreads peripherally.  The rash becomes petechial and spreads over the entire  body but spares the face, palms, and soles. Signs of severe  meningoencephalitis, including delirium and coma, begin  with the rash and continue into the second and third weeks.  In untreated cases, death occurs from peripheral vascular  collapse or from bacterial pneumonia.   Epidemic typhus is transmitted from person to person by  the human body louse, Pediculus. When a bacteremic patient  is bitten, the organism is ingested by the louse and multiplies  in the gut epithelium. It is excreted in the feces of the louse  during the act of biting the next person and autoinoculated by  the person while scratching the bite. The infected louse dies  after a few weeks, and there is no louse-to-louse transmission;  therefore, human infection is an obligatory stage in the cycle.  Epidemic typhus is associated with wars and poverty; at pres¬  ent it is found in developing countries in Africa and South  America but not in the United States.   A recurrent form of epidemic typhus is called Brill-  Zinsser disease. The signs and symptoms are similar to  those of epidemic typhus but are less severe, of shorter  duration, and rarely fatal. Recurrences can appear as long  as 50 years later and can be precipitated by another inter¬  current disease. In the United States, the disease is seen in  older people who had epidemic typhus during World War II  in Europe. Brill-Zinsser disease is epidemiologically inter¬  esting; persistently infected patients can serve as a source of  the organism should a louse bite occur.   Laboratory diagnosis of rickettsial diseases is based on  serologic analysis rather than isolation of the organism.  Although rickettsiae can be grown in cell culture or embry-  onated eggs, this is a hazardous procedure that is not avail¬  able in the standard clinical laboratory.   Of the serologic tests, the indirect immunofluorescence  and enzyme-linked immunosorbent assay (ELISA) tests are  most often used. The Weil-Felix test is of historic interest  but is no longer performed because its specificity and sen¬  sitivity are too low. The basis of the Weil-Felix test is  described later.    A fourfold or greater rise in titer between the acute and  convalescent serum samples is the most common way the  laboratory diagnosis is made. This is usually a retrospective  diagnosis, because the convalescent sample is obtained  2 weeks after the acute sample. If the clinical picture is typi¬  cal, a single acute-phase titer of 1:128 or greater is accepted  as presumptive evidence. If the test is available, a diagnosis  can be made during the acute phase of the disease by  immunofluorescence assay on tissue obtained from the site  of the petechial rash.   The Weil-Felix test is based on the cross-reaction of an  antigen present in many rickettsiae with the O antigen poly¬  saccharide found in P. vulgaris OX-2, OX-19, and OX-K.  The test measures the presence of antirickettsial antibodies  in the patient’s serum by their ability to agglutinate Proteus  bacteria. The specific rickettsial organism can be identified  by the agglutination observed with one or another of these  three different strains of P. vulgaris. However, as mentioned,  this test is no longer used in the United States.   Prevention of many of these diseases is based on reducing  exposure to the arthropod vector by wearing protective  clothing and using insect repellent. Frequent examination  of the skin for ticks is important in preventing Rocky  Mountain spotted fever; the tick must be attached for sev¬  eral hours to transmit the disease. There is no vaccine  against Rocky Mountain spotted fever. Prophylactic antibi¬  otics are not recommended in the asymptomatic person  bitten by a tick.   Prevention of typhus is based on personal hygiene and  “delousing” with DDT. A typhus vaccine containing formalin -  killed R. prowazekii organisms is effective and useful in the  military during wartime but is not available to civilians in  the United States.   Coxiella burnetii causes Q fever. Q stands for “Query”; the  cause of this disease was a question mark (i.e., was  unknown) when the disease was first described in Australia  in 1937.   Coxiella burnetii has a sporelike stage that is highly resistant  to drying, which enhances its ability to cause infection. It  also has a very low ID 50 estimated to be approximately one  organism.    Coxiella burnetii exists in two phases that differ in their  antigenicity and their virulence: phase I organisms are iso¬  lated from the patient, are virulent, and synthesize certain  surface antigens, whereas phase II organisms are produced  by repeated passage in culture, are nonvirulent, and have  lost the ability to synthesize certain surface antigens. The  clinical importance of phase variation is that patients with  chronic Q fever have a much higher antibody titer to phase I  antigens than those with acute Q fever.   Coxiella burnetii, the cause of Q fever, is transmitted by  aerosol and inhaled into the lungs. Q fever is the one rick¬  ettsial disease that is not transmitted to humans by the bite  of an arthropod. The important reservoirs for human infec¬  tion are cattle, sheep, and goats. Coxiella burnetii causes an  inapparent infection in these reservoir hosts and is found  in high concentrations in the urine, feces, placental tissue,  and amniotic fluid of the animals. It is transmitted to  humans by inhalation of aerosols of these materials.   Unlike other rickettsial diseases, the main organ involved  in Q fever is the lungs. It begins suddenly with fever, severe  headache, cough, and other influenzalike symptoms. This  is all that occurs in many patients, but pneumonia ensues  in about half. Hepatitis is frequent enough that the combi¬  nation of pneumonia and hepatitis should suggest Q fever.  A rash is rare, unlike in most of the other rickettsial dis¬  eases. In general, Q fever is an acute disease, and recovery  is expected even in the absence of antibiotic therapy. Rarely,  chronic Q fever characterized by life-threatening endocar¬  ditis occurs.   The disease occurs worldwide, chiefly in individuals  whose occupations expose them to livestock, such as shep¬  herds, abattoir employees, and farm workers. Ingestion of  cow’s milk is usually responsible for subclinical infections  rather than disease in humans. Pasteurization of milk kills  the organism.   Serologic tests, such as the indirect immunofluorescence  assay, are used rather than isolation of the organism. Coxi¬  ella burnetii can be grown in cell culture or embryonated  eggs but this is a hazardous procedure that is not available  in the standard clinical laboratory.   Persons at high risk of contracting Q fever, such as veterinar¬  ians, shepherds, abattoir workers, and laboratory personnel    exposed to C. burnetii, should receive the vaccine that  consists of the killed organism. Pasteurization of milk  kills C. burnetii.   Anaplasma phagocytophilum is a member of the Rickettsia  family that causes human granulocytic anaplasmosis  (HGA). Disease is endemic in northeastern and north cen¬  tral states (e.g., Connecticut and Wisconsin). Distribution  is similar to that of Lyme disease. Ixodes ticks are the main  vectors. Rodents and dogs are important reservoirs. In  HGA, granulocytes rather than mononuclear cells are  infected, but the disease is clinically indistinguishable from  that caused by Ehrlichia chaffeensis (see later). The organ¬  ism forms an inclusion body called a morula in the cyto¬  plasm of infected cells. The morula, which is shaped like a  mulberry, is indistinguishable from that formed by  Ehrlichia. The diagnosis is made serologically by detecting  a rise in antibody titer. Doxycycline is the treatment of  choice. This organism was formerly known as Ehrlichia  equi, and the disease it caused was formerly known as  human granulocytic ehrlichiosis (HGE).   Ehrlichia chaffeensis is a member of the Rickettsia family  and causes human monocytic ehrlichiosis (HME). This  disease resembles Rocky Mountain spotted fever, except  that the typical rash usually does not occur. High fever,  severe headache, and myalgias are prominent symptoms.  The organism is endemic in dogs and is transmitted to  humans by ticks, especially the dog tick, Dermacentor, and  the Lone Star tick, Amblyomma. Ticks of the genus Ixodes  are also vectors. Ehrlichia chaffeensis primarily infects  mononuclear leukocytes and forms characteristic morulae  in the cytoplasm. (A morula is an inclusion body that  resembles a mulberry. It consists of many E. chaffeensis  cells.) Lymphopenia, thrombocytopenia, and elevated  liver enzyme values are seen. In the United States, the  disease occurs primarily in the southern states, especially  Arkansas. The diagnosis is usually made serologically by  detecting a rise in antibody titer. Doxycycline is the treat¬  ment of choice.   1. Your patient is a 40-year-old woman with the sudden onset of  fever to 40°C, severe headache, and petechial rash over most of her  body including the palms. Blood cultures are negative. Unfortu¬  nately, despite antibiotics and other support, she died the follow¬  ing day. An autopsy was performed, and immunohistochemical  tests on her brain tissue revealed an infection by Rickettsia rickettsii.  Of the following, which one is the most accurate?    (A) It is likely she lives in Colorado and was bitten by a tick.   (B) It is likely she lives in Colorado and was bitten by a mosquito.   (C) It is likely she lives in Virginia and was bitten by a tick.   (D) It is likely she lives in Virginia and was bitten by a flea.   (E) It is likely she lives in Connecticut and was bitten by a  mosquito.   2. Regarding Q fever, which one of the following is most accurate?   (B) The natural habitat of the causative agent is the white-footed  mouse.   (C) The diagnosis is made primarily by Gram stain and culture on  chocolate agar.   (D) Occupations that predispose people to Q fever include veteri¬  narians and abattoir workers.   (E) Patients with Q fever often have a petechial rash involving the  palms.   Brief summaries of the organisms described in this chapter  begin on page 667. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Clinical Bacteriology section of Part XIII: USMLE  (National Board) Practice Questions starting on page 713. Also  see Part XIV: USMLE (National Board) Practice Examination  starting on page 751.   The bacterial pathogens of lesser medical importance are  briefly described in this chapter. Experts may differ on  their choice of which organisms to put in this category  Nevertheless, separating the minor from the major patho¬  gens should allow the reader to focus on the more    important pathogens while providing at least some infor¬  mation about the less important ones.   These organisms are presented in alphabetical order.  Table 27-1 lists the organisms according to their appear¬  ance on Gram stain.   Additional information regarding the clinical aspects of  infections caused by the organisms in this chapter is provided in  Part IX entitled Infectious Diseases beginning on page 593.   Achromobacter, Acinetobacter, Actinobacillus (Aggrigatibacter), Aeromonas, Alcaligenes, Arizona, Bartonella quintana  and B. bacilliformis, Bradyrhizobium, Calymmatobacterium, Capnocytophaga, Cardiobacterium, Chromobacterium,  Chryseobacterium, Citrobacter, Corynebacterium jeikeium, Corynebacterium minutissimum, Edwardsiella, Eikenella,  Erwinia, HACEK group, Haemophilus ducreyi, Hafnia, Kingella, Moraxella, Plesiomonas, Porphyromonas, Pseudomo¬  nas pseudomallei (also known as Burkholderia pseudomallei), Spirillum, Streptobacillus, Yersinia enterocolitica, Yer¬  sinia pseudotuberculosis    Abiotrophia species were formerly known as nutritionally  deficient streptococci. They are members of the normal  flora of the mouth and can cause subacute bacterial  endocarditis.   Achromobacter species are gram-negative coccobacillary  rods found chiefly in water supplies. They are opportunis¬  tic pathogens and are involved in sepsis, pneumonia, and  urinary tract infections.   Actinobacillus species are gram-negative coccobacillary  rods. Actinobacillus actinomycetemcomitans is found as  part of the normal flora in the upper respiratory tract. It is  a rare opportunistic pathogen, causing endocarditis on  damaged heart valves and sepsis. Actinobacillus actinomy¬  cetemcomitans has been renamed Aggrigatibacter actinomy¬  cetemcomitans but the former genus name Actinobacillus is  often used.    Arachnia species are anaerobic gram-positive rods that form  long, branching filaments similar to those of Actinomyces.  They are found primarily in the mouth (associated with  dental plaque) and in the tonsillar crypts. Arachnia propionica,  the major species, causes abscesses similar to those of  Actinomyces israelii, including the presence of “sulfur  granules” in the lesions.   Arcanobacterium haemolyticum is a club-shaped gram¬  positive rod that closely resembles corynebacteria. It is a  rare cause of pharyngitis and chronic skin ulcers. The phar¬  yngitis can be accompanied by a rash resembling the rash  of scarlet fever.   Arizona species are gram-negative rods in the family  Enterobacteriaceae; they ferment lactose slowly. Arizona  hinshawii is found in the feces of chickens and other  domestic animals and causes diseases similar to those  caused by Salmonella, such as enterocolitis and enteric  fevers. The organism is usually transmitted by contami¬  nated food (e.g., dried eggs).    Aeromonas species are gram-negative rods found in water,  soil, food, and animal and human feces. Aeromonas  hydrophila causes wound infections, diarrhea, and sepsis,  especially in immunocompromised patients.   Alcaligenes species are gram-negative coccobacillary rods  found in soil and water and are associated with water-  containing materials such as respirators in hospitals.  Alcaligenes faecalis is an opportunistic pathogen, causing  sepsis and pneumonia.   Bartonella quintana is the cause of trench fever and also is  implicated as the cause of some cases of bacillary angioma¬  tosis. Trench fever is transmitted by body lice, and humans  are the reservoir for the organism. Bartonella bacilliformis  causes two rare diseases: Oroya fever and verruga peruana,  both of which are stages of Carrion’s disease. The disease  occurs only in certain areas of the Andes Mountains, and  an animal reservoir is suspected.   Bifidobacterium eriksonii is a gram-positive, filamentous,  anaerobic rod found as part of the normal flora in the  mouth and gastrointestinal tract. It occurs in mixed anaer¬  obic infections.   Bradyrhizobium enterica is a gram-negative rod that is  thought to be the cause of cord colitis. Cord colitis mani¬  fests as nonbloody diarrhea in patients who have received  an allogeneic hematopoietic stem-cell transplant of umbili¬  cal cord cells. It was identified by using DNA sequencing  and polymerase chain reaction (PCR) assays on infected  tissue from biopsies of the colon.   Bradyrhizobium species are common soil bacteria that  fix nitrogen in leguminous plants. Bradyrhizobium enterica  is the first member of the genus to be identified as a human  opportunistic pathogen.   Calymmatobacterium granulomatis is a gram-negative rod  that causes granuloma inguinale (also known as donovano-  sis), a sexually transmitted disease characterized by genital  ulceration and soft tissue and bone destruction. The diag¬  nosis is made by visualizing the stained organisms (Dono¬  van bodies) within large macrophages from the lesion.  Tetracycline is the treatment of choice for this disease,  which is rare in the United States but endemic in many  developing countries. Calymmatobacterium granulomatis is  also known as Klebsiella granulomatis.   Capnocytophaga gingivalis is a gram-negative fusiform rod  that is associated with periodontal disease, but it can also  be an opportunistic pathogen, causing sepsis and mucositis  in immunocompromised patients. Capnocytophaga cani-  morsus is a member of the oral flora of dogs and causes  infections following dog bites. It also can cause sepsis in  immunocompromised patients, especially those without a  spleen and those who abuse alcohol.   Cardiobacterium hominis is a gram-negative pleomorphic  rod. It is a member of the normal flora of the human colon,  but it can be an opportunistic pathogen, causing mainly  endocarditis.   Chromobacterium violaceum is a gram-negative rod that  produces a violet pigment. It is found in soil and water and  can cause wound infections, especially in subtropical parts  of the world.   Chryseobacterium species are gram-negative rods found in  soil and water. Chryseobacterium meningosepticum, the  major pathogen in this genus, is an opportunistic pathogen,  causing meningitis and sepsis, especially in premature  infants. In adults, it causes outbreaks of nosocomial pneu¬  monia, especially in intubated patients. It is resistant to most  antibiotics but is noteworthy as the only gram-negative  bacterium that is susceptible to vancomycin. The genus  Chryseobacterium was formerly called Flavobacterium.   Citrobacter species are gram-negative rods (members of the  Enterobacteriaceae) related to Salmonella and Arizona.  They occur in the environment and in the human colon  and can cause sepsis in immunocompromised patients.   Cory neb acterium jeikeium is a small gram-positive rod  primarily found on the skin of hospitalized patients. It  causes sepsis in immunocompromised patients, most often  those who are neutropenic. Infections are often associated  with indwelling catheters and prosthetic heart valves. The  drug of choice is vancomycin. Hospital-acquired strains are  resistant to many other antibiotics.   Corynebacterium minutissimum is a small gram-positive  rod that causes erythrasma. Erythrasma is characterized by  pruritic, scaly, brownish macules on the skin of the genital  region. The diagnosis is usually made by visualizing a  coral-red fluorescence with a Wood’s lamp rather than by  culturing the organism. The drug of choice is oral  erythromycin.   Edwardsiella species are gram-negative rods (members of  the Enterobacteriaceae) resembling Salmonella. They can  cause enterocolitis, sepsis, and wound infections.   Eikenella corrodens is a gram-negative rod that is a member  of the normal flora in the human mouth. It causes skin and  bone infections associated with human bites and “clenched  fist” injuries. It also causes sepsis and soft tissue infections  of the head and neck, especially in immunocompromised    patients and in drug abusers who lick needles prior to  injection. Eikenella corrodens is also called Bacteroides  ureolyticus.   Erwinia species are gram-negative rods (members of the  Enterobacteriaceae) found in soil and water and are rarely  involved in human disease.   Erysipelothrix rhusiopathiae is a gram-positive rod that  causes erysipeloid, a skin infection that resembles erysipe¬  las (caused by streptococci). Erysipeloid usually occurs on  the hands of persons who handle meat and fish.   Eubacterium species are gram-positive, anaerobic, non¬  spore-forming rods that are present in large numbers as  part of the normal flora of the human colon. They rarely  cause human disease.   This is a group of small gram-negative rods that have in  common the following: slow growth in culture, the require¬  ment for high C0 2 levels to grow in culture, and the ability  to cause endocarditis. They are members of the human  oropharyngeal flora and can enter the bloodstream from  that site. The name “HACEK” is an acronym of the first  letters of the genera of the following bacteria: Haemophilus  aphrophilus and Haemophilus paraphrophilus, Actinobacillus  (. Aggrigatibacter) actinomycetemcomitans, Cardiobacterium  hominis, Eikenella corrodens, and Kingella kingae.   Haemophilus aegyptius (Koch-Weeks bacillus) is a small  gram-negative rod that is an important cause of conjuncti¬  vitis in children. Certain strains of H. aegyptius cause  Brazilian purpuric fever, a life-threatening childhood infec¬  tion characterized by purpura and shock. This organism is  also known as Haemophilus influenzae biogroup aegyptius.   This small gram-negative rod causes the sexually transmit¬  ted disease chancroid (soft chancre), which is common in  tropical countries but uncommon in the United States. The  disease begins with penile lesions, which are painful; non-  indurated (soft) ulcers; and local lymphadenitis (bubo).  The diagnosis is made by isolating H. ducreyi from the  ulcer or from pus aspirated from a lymph node. The organ¬  ism requires heated (chocolate) blood agar supplemented  with X factor (heme) but, unlike H. influenzae, does not  require V factor (NAD). Chancroid can be treated with  erythromycin, azithromycin, or ceftriaxone. Because many    strains of H. ducreyi produce a plasmid-encoded penicil¬  linase, penicillins cannot be used.   Hafnia species are gram-negative rods (members of the  Enterobacteriaceae) found in soil and water and are rare  opportunistic pathogens.   Kingella kingae is a gram-negative rod in the normal flora  of the human oropharynx. It is a rare cause of opportunistic  infection and endocarditis.   Lactobacilli are gram-positive non-spore-forming rods  found as members of the normal flora in the mouth, colon,  and female genital tract. In the mouth, they may play a role  in the production of dental caries. In the vagina, they are  the main source of lactic acid, which keeps the pH low.  Lactobacilli are rare causes of opportunistic infection.   Micrococci are gram-positive cocci that are part of the  normal flora of the skin. They are rare human pathogens.   Mobiluncus species are anaerobic gram-positive, curved rods  that often stain gram-variable. They are associated with bac¬  terial vaginosis in women. Gardnerella (see above), a facul¬  tative rod, is often found in this disease as well.   Moraxella species are gram-negative coccobacillary rods  that resemble the diplococcal appearance of the Neisseriae.  Moraxella catarrhalis is the major pathogen in this  genus. It causes otitis media and sinusitis, primarily in  children, as well as bronchitis and pneumonia in older  people with chronic obstructive pulmonary disease. It is  found only in humans and is transmitted by respiratory  aerosol. Trimethoprim-sulfamethoxazole or amoxicillin-  clavulanate can be used to treat these infections. Most clini¬  cal isolates produce (3-lactamase. Moraxella nonliquefaciens  is one of the two common causes of blepharitis (infection  of the eyelid); Staphylococcus aureus is the other. The usual  treatment is local application of antibiotic ointment, such  as erythromycin.   Peptococci are anaerobic gram-positive cocci, resembling  staphylococci, found as members of the normal flora of the  mouth and colon. They are also isolated from abscesses of  various organs, usually from mixed anaerobic infections.   Peptostreptococci are anaerobic gram-positive cocci found  as members of the normal flora of the mouth and colon.  They are also isolated from abscesses of various organs,  usually from mixed anaerobic infections.   Plesiomonas shigelloides is a gram-negative rod associated  with water sources. It causes self-limited gastroenteritis,  primarily in tropical areas, and can cause invasive disease  in immunocompromised individuals.   Porphyromonas gingivalis and Porphyromonas endodontalis  are anaerobic gram-negative rods found in the mouth.  They cause periodontal infections, such as gingivitis and  dental abscesses.   Propionibacteria are pleomorphic, anaerobic gram-positive  rods found on the skin and in the gastrointestinal tract.  Propionibacterium acnes is part of the normal flora of the  skin and can cause catheter and shunt infections. It is  involved in mixed infections associated with cat and dog  bites and in head and neck abscesses.   Propionibacteria acnes is also involved in the pathogen¬  esis of acne, a condition that affects more than 85% of  teenagers. The pathogenesis of acne involves impaction of  the sebaceous gland followed by inflammation caused by  the presence of P. acnes. The pustules of acne are composed  of sebum, inflammatory cells such as neutrophils and lym¬  phocytes, and the organism. Antibiotics, such as erythro¬  mycin, administered either topically or orally, are effective  especially when coupled with other agents such as benzoyl  peroxide or retinoids.   Pseudomonas pseudomallei (also known as Burkholderia  pseudomallei) is a gram-negative rod that causes melioido¬  sis, a rare disease found primarily in Southeast Asia. The  organism is found in soil and is transmitted most often  when soil contaminates skin abrasions. This disease has  been seen in the United States, because infections acquired  by members of the armed forces during the Vietnam War  have reactivated many years later. The acute disease is char¬  acterized by high fever and bloody, purulent sputum.  Untreated cases can proceed to sepsis and death. In the  chronic form, the disease can appear as pneumonia or lung  abscess or may resemble tuberculosis. Diagnosis is made by  culturing the organism from blood or sputum. The treat¬  ment of choice is ceftazidime, which is administered for  several weeks.   Rhodococcus equi is a gram-positive bacterium whose shape  varies from a coccus to a club-shaped rod. It is a rare cause  of pneumonia and cavitary lung disease in patients whose  cell-mediated immunity is compromised. The diagnosis is  made by isolating the organism on laboratory agar and  observing salmon-pink colonies that do not ferment most  carbohydrates. It may appear acid-fast and, if so, can be  confused with Mycobacterium tuberculosis. The treatment of  choice is a combination of rifampin and erythromycin.  (.Rhodococcus equi used to be called Corynebacterium equi.)   Sarcina species are anaerobic gram-positive cocci grouped  in clusters of four or eight. They are minor members of the  normal flora of the colon and are rarely pathogens.   Spirillum minor is a gram-negative, spiral-shaped rod that  causes rat-bite fever (“sodoku”). The disease is character¬  ized by a reddish brown rash spreading from the bite,  accompanied by fever and local lymphadenopathy. The  diagnosis is made by a combination of microscopy and  animal inoculation.   Streptobacillus moniliformis is a gram-negative rod that  causes another type of rat-bite fever (see Spirillum, preced¬  ing paragraph).   In August 2005, it was reported that Streptococcus suis  caused the death of 37 farmers in China. The illness is char¬  acterized by the sudden onset of hemorrhagic shock. This  species is known to cause disease in pigs but only rarely in  people prior to this outbreak. Spread of the bacteria from  the index case to others has not occurred.   Tropheryma whipplei is the cause of Whipple’s disease, a  rare disease characterized by prolonged weight loss, diar¬  rhea, and polyarthritis. Without antibiotic treatment, it is  ultimately fatal. Infiltrates of “foamy” macrophages in  affected tissue, especially in the small intestine, are com¬  monly seen. The reservoir of the organism, its mode of  transmission, and pathogenesis are unknown.   The nature of this organism was unknown for many  years. In 1992, it was identified as an actinomycete when  ribosomal RNA taken from bacilli seen in duodenal lesions  was compared with ribosomal RNA of other bacteria.  Tropheryma is an intracellular organism that has been  grown in human cell culture, but that procedure is not used  to diagnose the disease. Laboratory diagnosis is typically    made by periodic acid-Schiff (PAS) staining of biopsy  specimens of the small bowel in which inclusions are seen  in the macrophages. PAS staining, however, is nonspecific,  and PCR assays, which are more specific, are used to con¬  firm the diagnosis. First-line treatment typically involves  2 weeks of ceftriaxone, followed by at least 1 year of  trimethoprim-sulfamethoxazole.   Veillonella parvula is an anaerobic gram-negative diplococ-  cus that is part of the normal flora of the mouth, colon, and  vagina. It is a rare opportunistic pathogen that causes  abscesses of the sinuses, tonsils, and brain, usually in mixed  anaerobic infections.   Wolbachia species are Rickettsia-like bacteria found intra-  cellularly within filarial nematodes such as Wuchereria and  Onchocerca (see Chapter 56). Wolbachia release endotoxin¬  like molecules that are thought to play a role in the patho¬  genesis of Wuchereria and Onchocerca infections. Treatment  of patients with Wuchereria and Onchocerca infections with  doxycycline to kill Wolbachia results in a significant  decrease in the number of filarial worms in the patient.  Wolbachia themselves are not known to cause human dis¬  ease but do infect many species of insects worldwide.   Yersinia enterocolitica and Yersinia pseudotuberculosis are  gram-negative, oval rods that are larger than Yersinia pestis.  The virulence factors produced by Y. pestis are not made by  these species. These organisms are transmitted to humans  by contamination of food with the excreta of domestic  animals such as dogs, cats, and cattle. Yersinia infections are  relatively infrequent in the United States, but the number of  documented cases has increased during the past few years,  perhaps as a result of improved laboratory procedures.   Yersinia enterocolitica causes enterocolitis that is clini¬  cally indistinguishable from that caused by Salmonella or  Shigella. Both Y. enterocolitica and Y. pseudotuberculosis can  cause mesenteric adenitis that clinically resembles acute  appendicitis. Mesenteric adenitis is the main finding in  appendectomies in which a normal appendix is found.  Rarely, these organisms are involved in bacteremia or  abscesses of the liver or spleen, mainly in persons with  underlying disease.   Yersinia infection is associated with two autoimmune  diseases: reactive arthritis and Reiters syndrome. Other enteric  pathogens such as Salmonella, Shigella, and Campylobacter  also trigger these diseases. Reactive arthritis and Reiters  syndrome are described further in Chapter 66.   Yersinia enterocolitica is usually isolated from stool  specimens and forms a lactose-negative colony on    MacConkey’s agar. It grows better at 25°C than at 37°C;  most biochemical test results are positive at 25°C and nega¬  tive at 37°C. Incubation of a stool sample at 4°C for 1 week,  a technique called cold enrichment, increases the frequency  of recovery of the organism. Yersinia enterocolitica can be  distinguished from Y. pseudotuberculosis by biochemical  reactions.   The laboratory is usually not involved in the diagnosis  of Y. pseudotuberculosis; cultures are rarely performed in  cases of mesenteric adenitis, and the organism is rarely  recovered from stool specimens. Serologic tests are not  available in most hospital clinical laboratories.   Enterocolitis and mesenteric adenitis caused by the  organisms do not require treatment. In cases of bacteremia  or abscess, either trimethoprim-sulfamethoxazole or cipro¬  floxacin is usually effective. There are no preventive mea¬  sures except to guard against contamination of food by the  excreta of domestic animals.   1. Regarding Fusobacterium nucleatum, which one of the following is  most accurate?   (C) The drug of choice for infections caused by F. nucleatum is  azithromycin.   (D) Laboratory diagnosis is based on detecting the ability of the  exotoxin to kill cells in tissue culture.   2. Regarding Haemophilus ducreyi, which one of the following is  most accurate?   (A) It requires both X and V factors to grow on MacConkey s agar.   (B) Gram stain of exudate from the lesion shows large gram¬  positive rods.   (C) Penicillin G is the drug of choice to treat infections caused by  H. ducreyi.   (D) It causes chancroid, which is characterized by a painful ulcer  on the genitals.   3. Regarding Yersinia enterocolitica, which one of the following is  most accurate?   (C) It is the most common cause of enterocolitis in the United States.   (D) Its natural habitat is the human oropharynx, and there is no  animal reservoir.   4. Regarding Ehrlichia chaffeensis, which one of the following is most  accurate?   (D) It is endemic on the islands off the coast of Massachusetts (e.g.,  Nantucket).   (E) It forms an inclusion body called a morula in the cytoplasm of  infected cells.   Questions on the topics discussed in this chapter can be found  in the Clinical Bacteriology section of Part XIII: USMLE  (National Board) Practice Questions starting on page 713. Also  see Part XIV: USMLE (National Board) Practice Examination  starting on page 751.    Brief summaries of the organisms described in this chapter  begin on page 668. Please consult these summaries for a rapid  review of the essential material.    The other infectious agents described in this book, namely,  bacteria, fungi, protozoa, and worms, are either single cells  or composed of many cells. Cells are capable of indepen¬  dent replication, can synthesize their own energy and pro¬  teins, and can be seen in the light microscope. In contrast,  viruses are not cells; they are not capable of independent  replication, can synthesize neither their own energy nor  their own proteins, and are too small to be seen in the light  microscope.   (1) Viruses are particles composed of an internal core  containing either DNA or RNA (but not both) covered by a  protective protein coat. Some viruses have an outer lipo¬  protein membrane, called an envelope, external to the coat.  Viruses do not have a nucleus, cytoplasm, mitochondria, or  ribosomes. Cells, both prokaryotic and eukaryotic, have    mitochondria, and ribosomes. Prokaryotic cells, such as  bacteria, are not divided into nucleus and cytoplasm and  do not have mitochondria but do have ribosomes; there¬  fore, they can synthesize their own proteins.   (2) Viruses must reproduce (replicate) within cells,  because they cannot generate energy or synthesize proteins.  Because they can reproduce only within cells, viruses are  obligate intracellular parasites. (The only bacteria that are  obligate intracellular parasites are chlamydiae and rickett-  siae. They cannot synthesize sufficient energy to replicate  independently.)   (3) Viruses replicate in a manner different from that of  cells (i.e., viruses do not undergo binary fission or mitosis).  One virus can replicate to produce hundreds of progeny  viruses, whereas one cell divides to produce only two  daughter cells.    both DNA and RNA. Eukaryotic cells, such as fungal, pro¬  tozoal, and human cells, have a nucleus, cytoplasm,   Table III 1 compares some of the attributes of viruses  and cells.   Envelope present in some viruses Cell membrane present in all cells    Size & Shape of Viruses  Viral Nucleic Acids  Viral Capsid & Symmetry  Viral Proteins  Viral Envelope   Viruses range from 20 to 300 nm in diameter; this corre¬  sponds roughly to a range of sizes from that of the largest  protein to that of the smallest cell (see Figure 2-2). Their  shapes are frequently referred to in colloquial terms (e.g.,  spheres, rods, bullets, or bricks), but in reality they are  complex structures of precise geometric symmetry (see  later). The shape of virus particles is determined by the  arrangement of the repeating subunits that form the pro¬  tein coat (capsid) of the virus. The shapes and sizes of some  important viruses are depicted in Figure 28-1.   The anatomy of two representative types of virus particles  is shown in Figure 28-2. The viral nucleic acid (genome) is  located internally and can be either single- or double-  stranded DNA or single- or double-stranded RNA. 1   Only viruses have genetic material composed of single-  stranded DNA or of single-stranded or double-stranded  RNA. The nucleic acid can be either linear or circular. The  DNA is always a single molecule; the RNA can exist either  as a single molecule or in several pieces. For example, both  influenza virus and rotavirus have a segmented RNA  genome. Almost all viruses contain only a single copy of  their genome (i.e., they are haploid). The exception is the  retrovirus family, whose members have two copies of their  RNA genome (i.e., they are diploid).    'The nature of the nucleic acid of each virus is listed in Tables 31-1 and  31-2.   The nucleic acid is surrounded by a protein coat called a  capsid made up of subunits called capsomers. Each cap-  somer, consisting of one or several proteins, can be seen in  the electron microscope as a spherical particle, sometimes  with a central hole.   The structure composed of the nucleic acid genome and  the capsid proteins is called the nucleocapsid. The arrange¬  ment of capsomers gives the virus structure its geometric  symmetry. Viral nucleocapsids have two forms of symme¬  try: (1) icosahedral, in which the capsomers are arranged  in 20 triangles that form a symmetric figure (an icosahe¬  dron) with the approximate outline of a sphere; and (2)  helical, in which the capsomers are arranged in a hollow  coil that appears rod-shaped. The helix can be either rigid  or flexible. All human viruses that have a helical nucleocap¬  sid are enclosed by an outer membrane called an envelope  (i.e., there are no naked helical viruses). Viruses that have  an icosahedral nucleocapsid can be either enveloped or  naked (see Figure 28-2).   The advantage of building the virus particle from identi¬  cal protein subunits is twofold: (1) it reduces the need for  genetic information, and (2) it promotes self-assembly (i.e.,  no enzyme or energy is required). In fact, functional virus  particles have been assembled in the test tube by combining  the purified nucleic acid with the purified proteins in the  absence of cells, energy source, and enzymes.   Viral proteins serve several important functions. The cap¬  sid proteins protect the genome DNA or RNA from degra¬  dation by nucleases. The proteins on the surface of the    FIGURE 28~1 Shapes and sizes of medically important viruses. (Reproduced with permission from Fenner F, White DO. Medical Virology. 4th ed.  Academic Press. Copyright 1994 Elsevier.)    virus mediate the attachment of the virus to specific  receptors on the host cell surface. This interaction of the  viral proteins with the cell receptor is the major determi¬  nant of species and organ specificity. Outer viral proteins  are also important antigens that induce neutralizing anti¬  body and activate cytotoxic T cells to kill virus-infected  cells. These outer viral proteins not only induce antibodies,  but are also the target of antibodies (i.e., antibodies bind to    these viral proteins and prevent [“neutralize”] the virus  from entering the cell and replicating). The outer proteins  induce these immune responses following both the natural  infection and immunization (see later).   The term “serotype” is used to describe a subcategory of  a virus based on its surface antigens. For example, measles  virus has one serotype, polioviruses have three serotypes,  and rhinoviruses have over 100 serotypes. This is because   A: Nonenveloped virus with an icosahedral nucleocapsid. B: Enveloped  virus with a helical nucleocapsid. (Reproduced with permission from Brooks  GF et al. Medical Microbiology. 20th ed. Originally published by Appleton & Lange.  Copyright 1995 by McGraw-Hill.)    all measles viruses have only one antigenic determinant on  their surface protein that induces neutralizing antibody  capable of preventing infection. In contrast, polioviruses  have three different antigenic determinants on their surface  proteins (i.e., poliovirus type 1 has one kind of antigenic  determinant, poliovirus type 2 has a different antigenic deter¬  minant, and poliovirus type 3 has a different antigenic  determinant from types 1 and 2); hence polioviruses have  three serotypes. There are two important medical implica¬  tions of this. First is that a person can be immune (have  antibodies) to poliovirus type 1 and still get the disease,  poliomyelitis, caused by poliovirus types 2 or 3. The other  implication is that the polio vaccine must contain all three  serotypes in order to be completely protective.   Some of the internal viral proteins are structural (e.g., the  capsid proteins of the enveloped viruses), whereas others are  enzymes (e.g., the polymerases that synthesize the viral  mRNA). The internal viral proteins vary depending on the  virus. Some viruses have a DNA or RNA polymerase attached  to the genome; others do not. If a virus has an envelope, then  a matrix protein that mediates the interaction between the  capsid proteins and the envelope proteins is present.   Some viruses produce proteins that act as “superanti¬  gens,” similar in their action to the superantigens produced  by bacteria, such as the toxic shock syndrome toxin of  Staphylococcus aureus (see Chapters 15 and 58). Viruses  known to produce superantigens include two members of  the herpesvirus family, namely, Epstein-Barr virus and  cytomegalovirus, and the retrovirus mouse mammary  tumor virus. The current hypothesis offered to explain why  these viruses produce a superantigen is that activation of  CD4-positive T cells is required for replication of these  viruses to occur.    Some viruses contain regulatory proteins in the virion  in a structure called the tegument, which is located  between the nucleocapsid and the envelope. These regula¬  tory proteins include transcription and translation factors  that control either viral or cellular processes. Members of  the herpesvirus family, such as herpes simplex virus and  cytomegalovirus, have a prominent, well-characterized  tegument.   In addition to the capsid and internal proteins, there are  two other types of proteins, both of which are associated  with the envelope. The envelope is a lipoprotein mem¬  brane composed of lipid derived from the host cell mem¬  brane and protein that is virus-specific. Furthermore, there  are frequently glycoproteins in the form of spikelike projec¬  tions on the surface, which attach to host cell receptors  during the entry of the virus into the cell. Another protein,  the matrix protein, mediates the interaction between the  capsid proteins and the envelope.   The viral envelope is acquired as the virus exits from the  cell in a process called “budding” (see Chapter 29). The enve¬  lope of most viruses is derived from the cell’s outer mem¬  brane, with the notable exception of herpesviruses that  derive their envelope from the cells nuclear membrane.   In general, the presence of an envelope confers instability  on the virus. Enveloped viruses are more sensitive to heat,  drying, detergents, and lipid solvents such as alcohol and  ether than are nonenveloped (nucleocapsid) viruses, which  are composed only of nucleic acid and capsid proteins.   An interesting clinical correlate of this observation is  that virtually all viruses that are transmitted by the fecal-  oral route (those that have to survive in the environment)  do not have an envelope; that is, they are naked nucleocap¬  sid viruses. These include viruses such as hepatitis A virus,  poliovirus, Coxsackie virus, echovirus, Norwalk virus, and  rotavirus. In contrast, enveloped viruses are most often  transmitted by direct contact, such as by blood or by sexual  transmission. Examples of these include human immuno¬  deficiency virus, herpes simplex virus type 2, and hepatitis  B and C viruses. Other enveloped viruses are transmitted  directly by insect bite (e.g., yellow fever virus and West Nile  virus) or by animal bite (e.g., rabies virus).   Many other enveloped viruses are transmitted from  person to person in respiratory aerosol droplets, such as  influenza virus, measles virus, rubella virus, respiratory  syncytial virus, and varicella-zoster virus. If the droplets do  not infect directly, they can dry out in the environment,  and these enveloped viruses are rapidly inactivated. Note  that rhinoviruses, which are transmitted by respiratory  droplets, are naked nucleocapsid viruses and can survive in  the environment for significant periods. Therefore, they  can also be transmitted by hands that make contact with  the virus on contaminated surfaces.    As described earlier in this chapter, the surface proteins  of the virus, whether they are the capsid proteins or the  envelope glycoproteins, are the principal antigens against  which the host mounts its immune response to viruses.  They are also the determinants of type specificity (often  called the serotype). There is often little cross-protection  between different serotypes. Viruses that have multiple  serotypes (i.e., have antigenic variants) have an enhanced  ability to evade our host defenses because antibody against  one serotype will not protect against another serotype.   There are four exceptions to the typical virus as described  earlier:   (1) Defective viruses are composed of viral nucleic acid  and proteins but cannot replicate without a “helper” virus,  which provides the missing function. Defective viruses  usually have a mutation or a deletion of part of their genetic  material. During the growth of most human viruses, many  more defective than infectious virus particles are produced.  The ratio of defective to infectious particles can be as high  as 100:1. Because these defective particles can interfere  with the growth of the infectious particles, it has been  hypothesized that the defective viruses may aid in recovery  from an infection by limiting the ability of the infectious  particles to grow.   (2) Pseudovirions contain host cell DNA instead of  viral DNA within the capsid. They are formed during infec¬  tion with certain viruses when the host cell DNA is frag¬  mented and pieces of it are incorporated within the capsid  protein. Pseudovirions can infect cells, but they do not  replicate.   (3) Viroids consist solely of a single molecule of circular  RNA without a protein coat or envelope. There is extensive  homology between bases in the viroid RNA, leading to  large double-stranded regions. The RNA is quite small  (molecular weight 1 x 10 * 1 2 3 4 5 ) and apparently does not code  for any protein. Nevertheless, viroids replicate, but the  mechanism is unclear. They cause several plant diseases but  are not implicated in any human disease.    acid). They are implicated as the cause of certain “slow” dis¬  eases called transmissible spongiform encephalopathies,  which include such diseases as Creutzfeldt-Jakob disease in  humans and scrapie in sheep (see Chapter 44). Because nei¬  ther DNA nor RNA has been detected in prions, they are  clearly different from viruses (Table 28-1). Furthermore,  electron microscopy reveals filaments rather than virus par¬  ticles. Prions are much more resistant to inactivation by  ultraviolet light and heat than are viruses. They are remark¬  ably resistant to formaldehyde and nucleases. However,  they are inactivated by hypochlorite, NaOH, and autoclav¬  ing. Hypochlorite is used to sterilize surgical instruments  and other medical supplies that cannot be autoclaved.   Prions are composed of a single glycoprotein with a  molecular weight of 27,000 to 30,000. With scrapie prions  as the model, it was found that this protein is encoded by a  single cellular gene. This gene is found in equal numbers in  the cells of both infected and uninfected animals. Further¬  more, the amount of prion protein mRNA is the same in  uninfected as in infected cells. In view of these findings,  posttranslational modifications of the prion protein are  hypothesized to be the important distinction between the  protein found in infected and uninfected cells.   There is evidence that a change in the conformation  from the normal alpha-helical form (known as PrP sc , or  prion protein cellular) to the abnormal beta-pleated sheet  form (known as PrP sc , or prion protein scrapie) is the  important modification. The abnormal form then recruits  additional normal forms to change their configuration, and  the number of abnormal pathogenic particles increases.  Although prions are composed only of proteins, specific  cellular RNAs enhance the conversion of the normal alpha-  helical form to the pathologic beta-pleated sheet form.   Evidence that recruitment is an essential step comes  from “knockout” mice in which the gene for the prion pro¬  tein is nonfunctional and no prion protein is made. These  mice do not get scrapie despite the injection of the patho¬  genic scrapie prion protein.   The function of the normal prion protein is unclear.  There is some evidence that it is one of the signal transduc¬  tion proteins in neurons and that it is a copper-binding  protein. Knockout mice in which the gene encoding the  prion protein is inactive appear normal. The prion protein    in normal cells is protease-sensitive, whereas the prion  protein in infected cells is protease-resistant, probably  because of the change in conformation.   The observation that the prion protein is the product of  a normal cellular gene may explain why no immune  response is formed against this protein (i.e., tolerance  occurs). Similarly, there is no inflammatory response in    infected brain tissue. A vacuolated (spongiform) appear¬  ance is found, without inflammatory cells. Prion proteins  in infected brain tissue form rod-shaped particles that are  morphologically and histochemically indistinguishable  from amyloid, a substance found in the brain tissue of  individuals with various central nervous system diseases  (as well as diseases of other organs).   • Viruses range in size from that of large proteins (—20 nm) to  that of the smallest cells (-300 nm). Most viruses appear as  spheres or rods in the electron microscope.   • All viruses have a protein coat called a capsid that covers the  genome. The capsid is composed of repeating subunits called  capsomers. In some viruses, the capsid is the outer surface, but  in other viruses, the capsid is covered with a lipoprotein enve¬  lope that becomes the outer surface. The structure composed  of the nucleic acid genome and the capsid proteins is called  the nudeocapsid.   • The repeating subunits of the capsid give the virus a symmetric  appearance that is useful for classification purposes. Some viral  nucleocapsids have spherical (icosahedral) symmetry,  whereas others have helical symmetry.   • All human viruses that have a helical nudeocapsid are envel¬  oped (i.e., there are no naked helical viruses that infect humans).  Viruses that have an icosahedral nudeocapsid can be either  enveloped or naked.   • The genome of some viruses is DNA, whereas the genome of  others is RNA. These DNA and RNA genomes can be either  single-stranded or double-stranded.   • Some RNA viruses, such as influenza virus and rotavirus, have a  segmented genome (i.e., the genome is in several pieces).   • All viruses have one copy of their genome (haploid) except  retroviruses, which have two copies (diploid).   • Viral surface proteins mediate attachment to host cell recep¬  tors. This interaction determines the host specificity and  organ specificity of the virus.   • The surface proteins are the targets of antibody (i.e., antibody  bound to these surface proteins prevents the virus from attaching  to the cell receptor). This "neutralizes" (inhibits) viral replication.   • Viruses also have internal proteins, some of which are DNA or   • The matrix protein mediates the interaction between the viral  nudeocapsid proteins and the envelope proteins.    • Some viruses produce antigenic variants of their surface proteins  that allow the viruses to evade our host defenses. Antibody  against one antigenic variant (serotype) will not neutralize a dif¬  ferent serotype. Some viruses have one serotype; others have  multiple serotypes.   • The viral envelope consists of a membrane that contains lipid  derived from the host cell and proteins encoded by the virus.  Typically, the envelope is acquired as the virus exits from the  cell in a process called budding.   • Viruses with an envelope are less stable (i.e., they are more eas¬  ily inactivated) than naked viruses (those without an envelope).  In general, enveloped viruses are transmitted by direct contact  via blood and body fluids, whereas naked viruses can survive  longer in the environment and can be transmitted by indirect  means such as the fecal-oral route.   • Prions are infectious particles composed entirely of protein.  They have no DNA or RNA.   • They cause diseases such as Creutzfeldt-Jakob disease and  kuru in humans and mad cow disease and scrapie in animals.  These diseases are called transmissible spongiform enceph¬  alopathies. The term spongiform refers to the spongelike  appearance of the brain seen in these diseases. The holes of the  sponge are vacuoles resulting from dead neurons. These dis¬  eases are described in Chapter 44.   • Prion proteins are encoded by a cellular gene. When these  proteins are in the normal, alpha-helix configuration, they  are nonpathogenic, but when their configuration changes to  a beta-pleated sheet, they aggregate into filaments, which  disrupts neuronal function and results in the symptoms of  disease.   • Prions are highly resistant to inactivation by ultraviolet  light, heat, and other inactivating agents. As a result, they  have been inadvertently transmitted by human growth hor¬  mone and neurosurgical instruments.   • Because they are normal human proteins, they do not elicit  an inflammatory response or an antibody response in   1. The proteins on the external surface of viruses serve several  important functions. Regarding these proteins, which one of the  following statements is most accurate?   (A) They are the antigens against which neutralizing antibodies are  formed.   (C) They are the proteases that degrade cellular proteins leading to  cell death.   (E) Change in conformation of these proteins can result in prion-  mediated diseases such as Creutzfeldt-Jakob disease.   2. If a virus has an envelope, it is more easily inactivated by lipid  solvents and detergents than viruses that do not have an envelope.  Which one of the following viruses is the most sensitive to inacti¬  vation by lipid solvents and detergents?   3. Regarding the tegument, which one of the following is most  accurate?   (B) It mediates the binding of the virion to the cell surface.   (C) It guides the viral core from the cytoplasm to the nucleus.   (D) It is the site at which new virions bud from the surface of the  infected cell.   (E) It is the location of proteins in the virion that act as viral tran¬  scription factors.   Questions on the topics discussed in this chapter can be found  in the Basic Virology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 720. Also see Part XIV:  USMLE (National Board) Practice Examination starting on  page 751.   The viral replication cycle is described in this chapter in  two different ways. The first approach is a growth curve,  which shows the amount of virus produced at different  times after infection. The second is a stepwise description  of the specific events within the cell during virus growth.   The growth curve depicted in Figure 29-1 shows that when  one virion (one virus particle) infects a cell, it can replicate  in approximately 10 hours to produce hundreds of virions  within that cell. This remarkable amplification explains  how viruses spread rapidly from cell to cell. Note that the  time required for the growth cycle varies; it is minutes for  some bacterial viruses and hours for some human viruses.   The first event shown in Figure 29-1 is quite striking:  the virus disappears, as represented by the solid line drop¬  ping to the x axis. Although the virus particle, as such, is no  longer present, the viral nucleic acid continues to function  and begins to accumulate within the cell, as indicated by  the dotted line. The time during which no virus is found  inside the cell is known as the eclipse period. The eclipse  period ends with the appearance of virus (solid line). The  latent period, in contrast, is defined as the time from the  onset of infection to the appearance of virus extracellularly.  Note that infection begins with one virus particle and ends  with several hundred virus particles having been produced;  this type of reproduction is unique to viruses.   Alterations of cell morphology accompanied by marked  derangement of cell function begin toward the end of the    latent period. This cytopathic effect (CPE) culminates in  the lysis and death of cells. CPE can be seen in the light  microscope and, when observed, is an important initial  step in the laboratory diagnosis of viral infection. Not all   FIGURE 29-1 Viral growth curve. The figure shows that one  infectious virus particle (virion) entering a cell at the time of infection  results in more than 100 infectious virions 10 hours later, a remark¬  able increase. Note the eclipse period during which no infectious  virus is detectable within the infected cells. In this growth curve, the  amount of infecting virus is 1 virion/cell (i.e., 1 infectious unit/cell).  (Reproduced with permission from Brooks GF et al. Medical Microbiology. 20th ed.  Originally published by Appleton & Lange. Copyright 1995 by McGraw-Hill.)    viruses cause CPE; some can replicate while causing little  morphologic or functional change in the cell.   An overview of the events is described in Table 29-1 and  presented in diagrammatic fashion in Figure 29-2. The  infecting parental virus particle attaches to the cell mem¬  brane and then penetrates the host cell. The viral genome is  “uncoated” by removing the capsid proteins, and the  genome is free to function. Early mRNA and proteins are  synthesized; the early proteins are enzymes used to repli¬  cate the viral genome. Late mRNA and proteins are then  synthesized. These late proteins are the structural, capsid  proteins. The progeny virions are assembled from the rep¬  licated genetic material, and newly made capsid proteins  and are then released from the cell.   Another, more general way to describe the growth cycle  is as follows: (1) early events (i.e., attachment, penetration,  and uncoating); (2) middle events (i.e., gene expression  and genome replication); and (3) late events (i.e., assem¬  bly and release). With this sequence in mind, each stage  will be described in more detail.   1 Early is defined as the period before genome replication. Not all viruses exhibit a  distinction between early and late functions. In general, early proteins are enzymes,  whereas late proteins are structural components of the virus.   2 ln some cases, the viral genome is functionally equivalent to mRNA; thus early  mRNA need not be synthesized.   The proteins on the surface of the virion attach to specific  receptor proteins on the cell surface through weak, nonco-  valent bonding. The specificity of attachment determines  the host range of the virus. Some viruses have a narrow  range, whereas others have quite a broad range. For exam¬  ple, poliovirus can enter the cells of only humans and other  primates, whereas rabies virus can enter all mammalian  cells. The organ specificity of viruses is governed by recep¬  tor interaction as well. Those cellular receptors that have  been identified are surface proteins that serve various other  functions (see later).   Enveloped viruses undergo another process called  fusion in which the envelope of the virion fuses with the  outer membrane of the cell. The clinical importance of  fusion is illustrated by the antiviral drug, enfuvertide,  which blocks E1IV from entering the cell by inhibiting the  fusion process.   The virus particle penetrates by being engulfed in a  pinocytotic vesicle, within which the process of uncoating  begins. A low pEl within the vesicle favors uncoating. Rup¬  ture of the vesicle or fusion of the outer layer of virus with  the vesicle membrane deposits the inner core of the virus  into the cytoplasm.   The receptors for viruses on the cell surface are pro¬  teins that have other functions in the life of the cell. Prob¬  ably the best known is the CD4 protein that serves as one of  the receptors for HIV but whose normal function is the  binding of class 2 major histocompatibility complex (MHC)  proteins involved in the activation of helper T cells. A few  other examples will serve to illustrate the point: rabies  virus binds to the acetylcholine receptor, Epstein-Barr virus  binds to a complement receptor, herpes simplex virus type 1  binds to the fibroblast growth factor receptor, and vaccinia  virus binds to the receptor for epidermal growth factor.   It is appropriate at this point to describe the phenomenon  of infectious nucleic acid, because it provides a transition  between the concepts of host specificity described earlier and  early genome functioning, which is discussed later. Note that  we are discussing whether the purified genome is infectious.  All viruses are “infectious” in a person or in cell culture, but  not all purified genomes are infectious.   Infectious nucleic acid is purified viral DNA or RNA  (without any protein) that can carry out the entire viral  growth cycle and result in the production of complete virus  particles. This is interesting from three points of view:   (1) The observation that purified nucleic acid is infec¬  tious is the definitive proof that nucleic acid, not protein, is  the genetic material.   (2) Infectious nucleic acid can bypass the host range  specificity provided by the viral protein-cell receptor inter¬  action. For example, although intact poliovirus can grow  only in primate cells, purified poliovirus RNA can enter  nonprimate cells, go through its usual growth cycle, and  produce normal poliovirus. The poliovirus produced in the   FIGURE 29-2 Viral growth cycle. The growth cycle of adenovirus, a nonenveloped DNA virus, is shown. (Reproduced with permission from  Jawetz E, Melnick JL, Adelberg EA. Review of Medical Microbiology. 16th ed. Originally published by Appleton & Lange. Copyright 1984 by McGraw-Hill.)    nonprimate cells can infect only primate cells because it  now has its capsid proteins. These observations indicate  that the internal functions of the nonprimate cells are  capable of supporting viral growth once entry has occurred.   (3) Only certain viruses yield infectious nucleic acid.  The reason for this is discussed later. Note that all viruses  are infectious, but not all purified viral DNAs or RNAs  (genomes) are infectious.   The first step in viral gene expression is mRNA synthesis.  It is at this point that viruses follow different pathways  depending on the nature of their nucleic acid and the part  of the cell in which they replicate (Figure 29-3).   DNA viruses, with one exception, replicate in the  nucleus and use the host cell DNA-dependent RNA poly¬  merase to synthesize their mRNA. The poxviruses are the  exception because they replicate in the cytoplasm, where    they do not have access to the host cell RNA polymerase.  They therefore carry their own polymerase within the virus  particle. The genome of all DNA viruses consists of double-  stranded DNA, except for the parvoviruses, which have a  single-stranded DNA genome (Table 29-2).   Most RNA viruses undergo their entire replicative  cycle in the cytoplasm. The two principal exceptions are  retroviruses and influenza viruses, both of which have an  important replicative step in the nucleus. Retroviruses inte¬  grate a DNA copy of their genome into the host cell DNA,  and influenza viruses synthesize their progeny genomes in  the nucleus. In addition, the mRNA of hepatitis delta virus  is also synthesized in the nucleus of hepatocytes.   The genome of all RNA viruses consists of single-  stranded RNA, except for members of the reovirus fam¬  ily, which have a double-stranded RNA genome. Rotavirus  is the important human pathogen in the reovirus family.   RNA viruses fall into four groups with quite different  strategies for synthesizing mRNA (Table 29-3).    Legend:  = Strand with same polarity as mRNA (±) = Doubie-stranded    = Strand complementary to mRNA * = These viruses contain a polymerase in the virion   FIGURE 29~3 Synthesis of viral mRNA by medically important viruses. The following information starts at the top of the figure and  moves clockwise: Viruses with a double-stranded DNA genome (e.g., papovaviruses such as human papillomavirus) use host cell RNA poly¬  merase to synthesize viral mRNA. Note that hepadnaviruses (e.g., hepatitis B virus) contain a virion DNA polymerase that synthesizes the miss¬  ing portion of the DNA genome, but the viral mRNA is synthesized by host cell RNA polymerase. Parvoviruses use host cell DNA polymerase to  synthesize viral double-stranded DNA and host cell RNA polymerase to synthesize viral mRNA. Viruses with a single-stranded, negative-polarity  RNA genome (e.g., orthomyxoviruses such as influenza virus) use a virion RNA polymerase to synthesize viral mRNA. Viruses with a double-  stranded RNA genome (e.g., reoviruses) use a virion RNA polymerase to synthesize viral mRNA. Some viruses with a single-stranded, positive-  polarity RNA genome (e.g., retroviruses) use a virion DNA polymerase to synthesize a DNA copy of the RNA genome but a host cell RNA  polymerase to synthesize the viral mRNA. Some viruses with a single-stranded, positive-polarity RNA genome (e.g., picornaviruses) use the  virion genome RNA itself as their mRNA. {Reproduced with permission from Ryan K et al. Sherris Medical Microbiology. 3rd ed. Originally published by Appleton &  Lange. Copyright 1994 by McGraw-Hill.)    (1) The simplest strategy is illustrated by poliovirus,  which has single-stranded RNA of positive polarity 1 as its  genetic material. These viruses use their RNA genome  directly as mRNA.   (2) The second group has single-stranded RNA of  negative polarity as its genetic material. An mRNA must  be transcribed by using the negative strand as a template.  Because the cell does not have an RNA polymerase capable  of using RNA as a template, the virus carries its own RNA-  dependent RNA polymerase. There are two subcategories  of negative-polarity RNA viruses: those that have a single  piece of RNA (e.g., measles virus [a paramyxovirus] or  rabies virus [a rhabdovirus]) and those that have multiple  pieces of RNA (e.g., influenza virus [a myxovirus]).   Certain viruses, such as arenaviruses and some bunyavi-  ruses, have a segmented RNA genome, most of which is  negative stranded, but there are some positive strand regions  as well. RNA segments that contain both positive polarity  and negative polarity regions are called “ambisense.”    1 Positive polarity is defined as an RNA with the same base sequence as  the mRNA. RNA with negative polarity has a base sequence that is com¬  plementary to the mRNA. For example, if the mRNA sequence is an  A-C-U-G, an RNA with negative polarity would be U-G-A-C and an  RNA with positive polarity would be A-C-U-G.    (3) The third group has double-stranded RNA as its  genetic material. Because the cell has no enzyme capable of  transcribing this RNA into mRNA, the virus carries its own  polymerase. Note that plus strand in double-stranded RNA  cannot be used as mRNA because it is hydrogen-bonded to  the negative strand. Rotavirus, an important cause of diar¬  rhea in children, has 11 segments of double-stranded RNA.   (4) The fourth group, exemplified by retroviruses, has  single-stranded RNA of positive polarity that is transcribed  into double-stranded DNA by the RNA-dependent DNA  polymerase (reverse transcriptase) carried by the virus.  This DNA copy is then transcribed into viral mRNA by the  regular host cell RNA polymerase (polymerase II). Retro¬  viruses are the only family of viruses that are diploid (i.e.,  that have two copies of their genome RNA).   These differences explain why some viruses yield infec¬  tious nucleic acid and others do not. Viruses that do not  require a polymerase in the virion can produce infec¬  tious DNA or RNA. By contrast, viruses such as the  poxviruses, the negative-stranded RNA viruses, the double-  stranded RNA viruses, and the retroviruses, which require  a virion polymerase, cannot yield infectious nucleic acid.  Several additional features of viral mRNA are described in  the “Viral mRNA” box.   Note that two families of viruses utilize a reverse tran¬  scriptase (an RNA-dependent DNA polymerase) during    ’mRNA is synthesized by host cell RNA polymerase in the nucleus.   2 Single-stranded genome DNA is converted to double-stranded DNA by host cell polymerase. A virus-encoded DNA polymerase then synthesizes progeny DNA.   3 Hepatitis B virus uses a virion-encoded RNA-dependent DNA polymerase to synthesize its progeny DNA with full-length mRNA as the template. This enzyme is a type of  "reverse transcriptase" but functions at a different stage in the replicative cycle than does the reverse transcriptase of retroviruses.   Note: All DNA viruses encode their own DNA polymerase that replicates the genome. They do not use the host cell DNA polymerase (with the minor exception of the parvovi¬  ruses as mentioned earlier).    their replicative cycle, but the purpose of the enzyme dur¬  ing the cycle is different. As described in Table 29-4, retro¬  viruses, such as HIV, use their genome RNA as the  template to synthesize a DNA intermediate early in the  replicative cycle. However, hepadnaviruses, such as  hepatitis B virus (HBV), use an RNA intermediate as the  template to produce their DNA genome late in the repli¬  cative cycle. The clinical importance of this is that some  antiviral drugs such as lamivudine, are effective against  infections caused by both HIV and HBV because they  inhibit the reverse transcriptase of both viruses.   Note that the DNA polymerase of HBV has both DNA-  dependent and RNA-dependent activity that function at  different stages of the replicative cycle. The DNA-  dependent DNA polymerase activity in the virion synthe¬  sizes the missing section of the genome and produces a  complete covalent circular DNA shortly after entering the  cell. The RNA-dependent DNA polymerase activity uses a  full length RNA copy of the DNA genome as the template  to synthesize the progeny DNA genomes late in the replica¬  tive cycle. See Chapter 41 for additional information on  HBV replication.   Once the viral mRNA of either DNA or RNA viruses is  synthesized, it is translated by host cell ribosomes into viral    proteins, some of which are early proteins (i.e., enzymes  required for replication of the viral genome) and others of  which are late proteins (i.e., structural proteins) of the  progeny viruses. (The term early is defined as occurring  before the replication of the genome, and late is defined as  occurring after genome replication.) The most important  of the early proteins for many RNA viruses is the poly¬  merase that will synthesize many copies of viral genetic  material for the progeny virus particles. No matter how a  virus makes its mRNA, most viruses make a virus-encoded  polymerase (a replicase) that replicates the genome (i.e.,  that makes many copies of the parental genome that will  become the genome of the progeny virions). Table 29-5  describes which viruses encode their own replicase and  which viruses use host cell polymerases to replicate their  genome.   Some viral mRNAs are translated into precursor  polypeptides that must be cleaved by proteases to pro¬  duce the functional structural proteins (Figure 29-4 and  Table 29-6), whereas other viral mRNAs are translated  directly into structural proteins. A striking example of the  former occurs during the replication of picornaviruses  (e.g., poliovirus, rhinovirus, and hepatitis A virus), in  which the genome RNA, acting as mRNA, is translated into    1 Retroviruses contain an RNA-dependent DNA polymerase.  z mRNA transcribed from DNA intermediate.   ^Although the retroviral genome RNA is not infectious, the DNA intermediate is.  4 HTLV = human T-cell leukemia virus; HIV = human immunodeficiency virus.    TABLE 29-4 Comparison of Reverse Transcriptase Activity of HIV (Retroviruses) and HBV (Hepadnaviruses)    a single polypeptide, which is then cleaved by a virus-  coded protease into various proteins. This protease is one  of the proteins in the single polypeptide, an interesting  example of a protease acting on its own polypeptide.   Another important family of viruses in which precursor  polypeptides are synthesized is the retrovirus family. For  example, the gag and pol genes of HIV are translated into  precursor polypeptides, which are then cleaved by a virus-  encoded protease. It is this protease that is inhibited by the  drugs classified as protease inhibitors. Flaviviruses, such as  hepatitis C virus and yellow fever virus, also synthesize pre¬  cursor polypeptides that must be cleaved to form functional  proteins by a virus-encoded protease. In contrast, other  viruses, such as influenza virus and rotavirus, have seg¬  mented genomes, and each segment encodes a specific func¬  tional polypeptide rather than a precursor polypeptide.   Replication of the viral genome is governed by the prin¬  ciple of complementarity, which requires that a strand  with a complementary base sequence be synthesized; this  strand then serves as the template for the synthesis of  the actual viral genome. The following examples from  Table 29-7 should make this clear: (1) poliovirus makes a  negative-strand intermediate, which is the template for the  positive-strand genome; (2) influenza, measles, and rabies  viruses make a positive-strand intermediate, which is the  template for the negative-strand genome; (3) rotavirus  makes a positive strand that acts both as mRNA and as the  template for the negative strand in the double-stranded  genome RNA; (4) retroviruses use the negative strand of  the DNA intermediate to make positive-strand progeny  RNA; (5) hepatitis B virus uses its mRNA as a template to  make progeny double-stranded DNA; and (6) the other  double-stranded DNA viruses replicate their DNA by the  same semiconservative process by which cell DNA is  synthesized.    As the replication of the viral genome proceeds, the  structural capsid proteins to be used in the progeny virus  particles are synthesized. In some cases, the newly repli¬  cated viral genomes can serve as templates for the late  mRNA to make these capsid proteins.   The progeny particles are assembled by packaging the viral  nucleic acid within the capsid proteins. Little is known  about the precise steps in the assembly process. Surpris¬  ingly, certain viruses can be assembled in the test tube by  using only purified RNA and purified protein. This indi¬  cates that the specificity of the interaction resides within  the RNA and protein and that the action of enzymes and  expenditure of energy are not required.   In the assembly of cytomegalovirus in the nucleus of the  cell, the viral DNA enters the interior of the intact capsid  after the capsid has formed. In this process, the capsomers  first aggregate to form a hollow capsid shell. The genome  DNA is then threaded into the interior of the capsid  through a “portal protein” located at an apex of the capsid.   Virus particles are released from the cell by either of two  processes. One is rupture of the cell membrane and release  of the mature particles; this usually occurs with nonenvel-  oped viruses. The other, which occurs with enveloped  viruses, is release of viruses by budding through the outer  cell membrane (Figure 29-5). (An exception is the herpes¬  virus family, whose members acquire their envelopes from  the nuclear membrane rather than from the outer cell  membrane.) The budding process begins when virus-spe¬  cific proteins enter the cell membrane at specific sites. The  viral nucleocapsid then interacts with the specific mem¬  brane site mediated by the matrix protein. The cell mem¬  brane evaginates at that site, and an enveloped particle buds    TABLE 29-5 Origin of the Genes That Encode the Polymerases That Synthesize the Viral Genome   Herpesviruses (HSV, VZV, CMV, EBV), adenovirus, hepatitis B virus, smallpox virus   Poliovirus, HAV, HCV, influenza virus, measles virus, respiratory syncytial virus, rabies virus,  rubella virus, rotavirus, Ebola virus, arenavirus, hantavirus    CMV = cytomegalovirus; EBV = Epstein-Barr virus; HAV = hepatitis A virus; HCV = hepatitis C virus; HDV= hepatitis D virus; HIV = human immunodeficiency virus; HSV = herpes  simplex virus; HTLV = human T-cell leukemia virus; VZV = varicella-zoster virus.    There are four interesting aspects of viral mRNA and its  expression in eukaryotic cells. (1) Viral mRNAs have three  attributes in common with cellular mRNAs: on the 5' end  there is a methylated GTP “cap,” which is linked by an  “inverted” (3'-to-5') bond instead of the usual 5'-to-3'  bond; on the 3' end there is a tail of 100-200 adenosine resi¬  dues [poly(A)]; and the mRNA is generated by splicing from  a larger transcript of the genome. In fact, these three modifi¬  cations were first observed in studies on viral mRNAs and  then extended to cellular mRNAs. (2) Some viruses use their  genetic material to the fullest extent by making more than  one type of mRNA from the same piece of DNA by “shifting  the reading frame.” This is done by starting transcription one  or two bases downstream from the original initiation site.  (3) With some DNA viruses, there is temporal control over  the region of the genome that is transcribed into mRNA.  During the beginning stages of the growth cycle, before DNA    replication begins, only the early region of the genome is  transcribed, and therefore, only certain early proteins are  made. One of the early proteins is a repressor of the late  genes; this prevents transcription until the appropriate time.  (4) Three different processes are used to generate the mono-  cistronic mRNAs that will code for a single protein from the  polycistronic viral genome:   (1) Individual mRNAs are transcribed by starting at many  specific initiation points along the genome, which is the  same mechanism used by eukaryotic cells and by herpesvi¬  ruses, adenoviruses, and the DNA and RNA tumor viruses.   (2) In the reoviruses and influenza viruses, the genome is  segmented into multiple pieces, each of which codes for a  single mRNA.   (3) In polioviruses, the entire RNA genome is translated  into one long polypeptide, which is then cleaved into specific  proteins by a protease.   FIGURE 29-4 Synthesis of viral precursor polypeptides. A: Poliovirus mRNA is translated into a full-length precursor polypeptide, which is  cleaved by the virus-encoded protease into the functional viral proteins. B: Retroviral mRNAs are translated into precursor polypeptides, which  are then cleaved by the virus-encoded protease into the functional viral proteins. The cleavage of the Gag-Pol precursor polyprotein by the virion  protease occurs in the immature virion after it has budded out from the cell membrane. The cleavage produces the capsid protein (p24), the  matrix protein (pi 7), and enzymes such as the reverse transcriptase and the integrase.The cleavage of the Env polyprotein is carried out by a cel¬  lular protease, not by the virion protease. Inhibitors of the virion protease are effective drugs against human immunodeficiency virus.    off from the membrane. Budding frequently does not dam¬  age the cell, and in certain instances the cell survives while  producing large numbers of budding virus particles.   The typical replicative cycle described above occurs most of  the time when viruses infect cells. However, some viruses  can use an alternative pathway, called the lysogenic cycle,  in which the viral DNA becomes integrated into the host  cell chromosome and no progeny virus particles are pro¬  duced at that time (Figure 29-6). The viral nucleic acid  continues to function in the integrated state in a variety of  ways.   One of the most important functions of lysogeny from a  medical point of view is the synthesis of several exotoxins  in bacteria, such as diphtheria, botulinum, cholera, and  erythrogenic toxins, encoded by the genes of the inte¬  grated bacteriophage (prophage). Lysogenic conversion is  the term applied to the new properties that a bacterium  acquires as a result of expression of the integrated pro¬  phage genes (Figure 29-7). Lysogenic conversion is  mediated by the transduction of bacterial genes from the  donor bacterium to the recipient bacterium by bacterio¬  phages. Transduction is the term used to describe the    transfer of genes from one bacterium to another by viruses  (Figures 29-7 and 29-8 and see page 20).   The lysogenic or “temperate” cycle is described for  lambda bacteriophage, because it is the best-understood  model system (see Figure 29-8). Several aspects of infec¬  tions by tumor viruses and herpesviruses are similar to the  events in the lysogenic cycle of lambda phage.   Infection by lambda phage in E. coli begins with injec¬  tion of the linear, double-stranded DNA genome through  the phage tail into the cell. The linear DNA becomes a cir¬  cle as the single-stranded regions on the ends pair their  complementary bases. A ligating enzyme makes a covalent  bond in each strand to close the circle. Circularization is  important because it is the circular form that integrates into  host cell DNA.   The choice between the pathway leading to lysogeny  and that leading to full replication is made as early protein  synthesis begins. Simply put, the choice depends on the  balance between two proteins, the repressor produced by  the c-I gene and the antagonizer of the repressor pro¬  duced by the cro gene (Figure 29-9). If the repressor pre¬  dominates, transcription of other early genes is shut off and  lysogeny ensues. Transcription is inhibited by binding of  the repressor to the two operator sites that control early  protein synthesis. If the cro gene product prevents the    'Code: ss = single-stranded; ds = double-stranded; + = positive polarity; - = negative polarity.   FIGURE 29~5 Budding. Most enveloped viruses derive their  lipoprotein envelope from the cell membrane. The matrix protein  mediates the interaction between the viral nucleocapsid and the  viral envelope. (Reproduced with permission from Mims CA. The Pathogenesis of  infectious Disease. 3rd ed, Academic Press. Copyright 1987 Elsevier.)    synthesis of sufficient repressor, replication and lysis of the  cell result. One correlate of the lysogenic state is that the  repressor can also prevent the replication of additional  lambda phages that infect subsequently. This is called  “immunity” and is specifically directed against lambda  phage because the repressor binds only to the operator sites  in lambda DNA; other phages are not affected.   The next important step in the lysogenic cycle is the  integration of the viral DNA into the cell DNA. This  occurs by the matching of a specific attachment site on the  lambda DNA to a homologous site on the E. coli DNA and  the integration (breakage and rejoining) of the two DNAs  mediated by a phage-encoded recombination enzyme. The  integrated viral DNA is called a prophage. Most lysogenic  phages integrate at one or a few specific sites, but some,  such as the Mu (or mutator) phage, can integrate their    FIGURE 29~6 Comparison of the lytic and lysogenic cycles of bacteriophage (phage) replication. In the lytic cycle, replication of the  phage is completed without interruption. In the lysogenic cycle, replication of the phage is interrupted, and the phage DNA integrates into the  bacterial DNA. The integrated DNA is called a prophage and can remain in the integrated state for long periods. If the bacteria are exposed to  certain activators such as ultraviolet (UV) light, the prophage DNA is excised from the bacterial DNA and the phage enters the lytic cycle, which  ends with the production of progeny phage.   P phage carrying  diphtheria toxin gene  infects C. diphtheriae  not lysogenized by  P phage; bacterium is  nonpathogenic prior  to infection by p phage.    Diphtheria toxin  genes integrated  into chromosome  of C. diphtheriae',  bacterium becomes  pathogenic.    FIGURE 29~7 Lysogenic conversion. In the left-hand panel,  transduction of the diphtheria toxin gene by beta bacteriophage  results in lysogenic conversion of the nonlysogenized, nonpatho¬  genic Corynebacterium diphtheriae. In the right-hand panel, the recip¬  ient lysogenized bacterium can now produce diphtheria toxin and  can cause the disease diphtheria. Note that no progeny phages are  made within the lysogenized bacterium because the diphtheria toxin  gene has replaced some of the beta-phage genes required for repli¬  cation. The beta phage therefore cannot replicate. The lysogenized  bacterium is not killed by the phage and can multiply, produce diph¬  theria toxin, and cause disease.    DNA at many sites, and other phages, such as the P1 phage,  never actually integrate but remain in a “temperate” state  extrachromosomally, similar to a plasmid.   Because the integrated viral DNA is replicated along  with the cell DNA, each daughter cell inherits a copy.    However, the prophage is not permanently integrated. It  can be induced to resume its replicative cycle by the action  of ultraviolet (UV) light and certain chemicals that damage  DNA. UV light induces the synthesis of a protease, which  cleaves the repressor. Early genes then function, including  the genes coding for the enzymes that excise the prophage  from the cell DNA. The virus then completes its replicative  cycle, leading to the production of progeny virus and lysis  of the cell.   Members of the herpesvirus family, such as herpes simplex  virus (HSV), varicella-zoster virus, cytomegalovirus  (CMV), and Epstein-Barr virus, exhibit latency—the phe¬  nomenon in which no or very little virus is produced after  the initial infection but, at some later time, reactivation and  full virus replication occur. The parallel to lysogeny with  bacteriophage is clear.   What is known about how the herpesviruses initiate and  maintain the latent state? Shortly after HSV infects neu¬  rons, a set of “latency-associated transcripts” (LATS) are  synthesized. These are noncoding, regulatory RNAs that  suppress viral replication. The precise mechanism by which  they do so is unclear. Reactivation of viral replication at a  later time occurs when the genes encoding LATS are  excised.   CMV employs different mechanisms. The CMV genome  encodes microRNAs that inhibit the translation of mRNAs  required for viral replication. Also, the CMV genome  encodes both a protein and an RNA that inhibit apoptosis  in infected cells. This allows the infected cell to survive.   FIGURE 29~8 Lysogeny. The linear lambda (X) phage DNA is injected into the bacterium, circularizes, and then integrates into the bacte¬  rial DNA. When integrated, the phage DNA is called a prophage. When the prophage is induced to enter the replicative cycle, aberrant excision  of the phage DNA can occur (i.e., part of the phage DNA and part of the bacterial DNA including the adjacent gal gene are excised). The gal  gene can now be transduced to another bacterium. Transduction is also described in Figure 4-4. (Reproduced with permission from Jawetz E et al. Review  of Medical Microbiology. 17th ed. Originally published by Appleton & Lange. Copyright 1986 McGraw-Hill.)    FIGURE 29-9 Control of lysogeny. Shortly after infection, transcription of the A/and cro genes begins. The N protein is an antiterminator  that allows transcription of c-ll and c-lll and the genes to the right of c-ll and to the left of c-lll. The c-ll protein enhances the production of the  c-l repressor protein, c-l has two important functions: (1) It inhibits transcription at P R 0 R and P L 0 L , thereby preventing phage replication, and  (2) It is a positive regulator of its own synthesis by binding to P RM . The crucial decision point in lysogeny is the binding of either c-l repressor or  the cro protein to the 0 R site. If c-l repressor occupies 0 R , lysogeny ensues; if era protein occupies 0 R , viral replication occurs. N, antiterminator  gene; c-l, repressor gene; c-ll and c-lll, genes that influence the production of c-l; P L 0 L , left promoter and operator; P R 0 R , right promoter and  operator; P RM , promoter for repressor maintenance; cro, gene that antagonizes the c-l repressor.   • One virion infects a cell and hundreds of progeny virions are  produced within hours. This is a remarkable amplification and  explains the rapid spread of virus from cell to cell.   • The eclipse period is the time when no virus particles are  detected within the infected cell. It occurs soon after the cell is  infected.   • Cytopathic effect (CPE) is the term used to describe the dam¬  age, both morphologic and functional, inflicted on the cell by  the virus. In the clinical laboratory, the presence of a virus in the  patient's specimen is often detected by seeing a CPE in cell  culture.   • Attachment: The interaction of proteins on the surface of the  virus with specific receptor proteins on the surface of the cell is  one of the main determinants of both the species specificity  and the organ specificity of the virus.   • Infectious nucleic acid is viral genome DNA or RNA, purified  free of all proteins, that can undergo the entire replicative cycle  within a cell and produce infectious progeny viruses. Infectious  nucleic acid, because it has no associated protein, can enter  and replicate within cells that the intact virion cannot.   • Polarity of viral genome RNA: Genome RNA that has the  same base sequence as the mRNA is, by definition, positive-  polarity RNA. Most positive-polarity genomes are translated  into viral proteins without the need for a polymerase in the  virion. The exception is the retroviruses, which use reverse  transcriptase in the virion to transcribe the genome RNA into  DNA. Genome RNA that has a base sequence complementary  to mRNA has, by definition, negative polarity. A virus with a  negative-polarity RNA genome must have an RNA polymerase  in the virion to synthesize its mRNA.   • Viral gene expression: All viruses require virus-specific mes¬  senger RNA to synthesize virus-specific proteins.   • RNA viruses: Some RNA viruses, such as poliovirus, have a  positive-polarity RNA genome that serves as the mRNA (i.e., the    genome is the mRNA). Other viruses, such as influenza virus,  have a negative-polarity RNA genome and have an RNA poly¬  merase in the virion that synthesizes the viral mRNA. Rotavirus  has a double-stranded RNA genome and has an RNA poly¬  merase in the virion that synthesizes the viral mRNA. Retrovi¬  ruses, such as HIV, have a positive-polarity RNA genome and  have a DNA polymerase in the virion that synthesizes a DNA  copy of the RNA genome. This DNA is the template used by the  host cell RNA polymerase to synthesize the viral mRNA.   DNA viruses: Most DNA viruses, such as herpesviruses, adeno¬  viruses, and papillomaviruses, have a double-stranded DNA  genome and use the host cell RNA polymerase to synthesize  the viral mRNA. Poxviruses have a double-stranded DNA  genome but have an RNA polymerase in the virion that syn¬  thesizes the viral mRNA. Poxviruses have an RNA polymerase  in the virion because they replicate in the cytoplasm and  do not have access to the host cell RNA polymerase in the  nucleus.   Viral replication: All DNA viruses replicate in the nucleus,  except poxviruses, which replicate in the cytoplasm. All RNA  viruses replicate in the cytoplasm, except retroviruses, influ¬  enza virus, and hepatitis D virus, which require an intranuclear  step in their replication. Many viruses encode a replicase, which  is a DNA or RNA polymerase that synthesizes the many copies  of the progeny viral genomes.   Viral genome: The genome of all DNA viruses is double-stranded  except for that of parvoviruses, which is single-stranded. The  genome of all RNA viruses is single-stranded except for that of  reoviruses (e.g., rotavirus), which is double-stranded.   Viral proteins: Early proteins are typically enzymes used in the  synthesis of viral nucleic acids, whereas late proteins are typi¬  cally structural proteins of the progeny viruses. Some viruses,  such as poliovirus and retroviruses, translate their mRNA into  precursor polyproteins, which must be cleaved by proteases to  produce functional proteins.   Assembly and release: All enveloped viruses acquire their  envelope by budding through the external cell membrane as  they exit the cell, except herpesviruses, which acquire their    envelope by budding through the nuclear membrane. The  matrix protein mediates the interaction of the nucleocapsid  with the envelope.   Lysogeny is the process by which viral DNA becomes inte¬  grated into host cell DNA, replication stops, and no progeny  virus is made. Later, if DNA is damaged by, for example, UV  light, viral DNA is excised from the host cell DNA, and progeny    viruses are made. The integrated viral DNA is called a pro¬  phage. Bacterial cells carrying a prophage can acquire new  traits, such as the ability to produce exotoxins such as diphthe¬  ria toxin. Transduction is the process by which viruses carry  genes from one cell to another. Lysogenic conversion is the  term used to indicate that the cell has acquired a new trait as a  result of the integrated prophage.   1. Many viruses are highly specific regarding the type of cells they  infect. Of the following, which one is the most important determi¬  nant of this specificity?   2. Your summer research project is to study the viruses that cause  upper respiratory tract infections. You have isolated a virus from  a patient’s throat and find that its genome is RNA. Furthermore  you find that the genome is the complement of viral mRNA within  the infected cell. Of the following, which is the most appropriate  conclusion you could draw?   3. The purified genome of certain viruses can enter a cell and elicit  the production of progeny viruses (i.e., the genome is infectious).  Regarding these viruses, which one of the following statements is  most accurate?   4. Regarding viral replication, which one of the following is most  accurate?   (B) The early proteins are typically enzymes, whereas the late pro¬  teins are typically capsid proteins.   (C) The assembly of a nonenveloped virus typically occurs as the  virion buds from the cell membrane.   (D) Influenza viruses synthesize their mRNA using host cell-  encoded RNA-dependent RNA polymerase.   (E) Retroviruses (e.g., HIV) synthesize their mRNA using an  enzyme in the virion called reverse transcriptase.    5. Regarding the viral growth cycle, which one of the following is  most accurate?   (A) During the lysogenic phase, the typical result is the production  of hundreds of progeny virions.   (B) Hepatitis B virus has an RNA polymerase in the virion that is  required to synthesize messenger RNA from the positive strand  of the viral DNA.   (D) Lysogenic conversion is the process by which bacteria acquire  new genes due to transduction by a lysogenic bacteriophage.   (E) Smallpox virus translates its genome into a single polypeptide,  which is then cleaved into structural and nonstructural  proteins.   6. Which one of the following choices names two viruses that both  translate their messenger RNA into precursor polypeptides that  must be cleaved by virion-encoded proteases?   Questions on the topics discussed in this chapter can be found  in the Basic Virology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 720. Also see Part XIV:  USMLE (National Board) Practice Examination starting on  page 751.   The study of viral genetics falls into two general areas:  (1) mutations and their effect on replication and pathogen¬  esis; and (2) the interaction of two genetically distinct  viruses that infect the same cell. In addition, viruses serve  as vectors in gene therapy and in recombinant vaccines,  two areas that hold great promise for the treatment of  genetic diseases and the prevention of infectious diseases.   Mutations in viral DNA and RNA occur by the same pro¬  cesses of base substitution, deletion, and frameshift as those  described for bacteria in Chapter 4. Probably the most  important practical use of mutations is in the production of  vaccines containing live, attenuated virus. These attenuated  mutants have lost their pathogenicity but have retained  their antigenicity; therefore, they induce immunity without  causing disease.   There are two other kinds of mutants of interest. The first  are antigenic variants such as those that occur frequently  with influenza viruses, which have an altered surface protein  and are therefore no longer inhibited by a person’s preexist¬  ing antibody. The variant can thus cause disease, whereas the  original strain cannot. Human immunodeficiency virus and  hepatitis C virus also produce many antigenic variants.  These viruses have an “error-prone” polymerase that causes  the mutations. The second are drug-resistant mutants,  which are insensitive to an antiviral drug because the target  of the drug, usually a viral enzyme, has been modified.    Conditional lethal mutations are extremely valuable in  determining the function of viral genes. These mutations  function normally under permissive conditions but fail to  replicate or to express the mutant gene under restrictive  conditions. For example, temperature-sensitive condi¬  tional lethal mutants express their phenotype normally at a  low (permissive) temperature, but at a higher (restrictive)  temperature, the mutant gene product is inactive. To give a  specific example, temperature-sensitive mutants of Rous  sarcoma virus can transform cells to malignancy at the  permissive temperature of 37°C. When the transformed  cells are grown at the restrictive temperature of 41°C, their  phenotype reverts to normal appearance and behavior. The  malignant phenotype is regained when the permissive tem¬  perature is restored.   Note that temperature-sensitive mutants have now  entered clinical practice. Temperature-sensitive mutants of  influenza virus are now being used to make a vaccine,  because this virus will grow in the cooler, upper airways  where it causes few symptoms and induces antibodies, but  it will not grow in the warmer, lower airways where it can  cause pneumonia.   Some deletion mutants have the unusual property of  being defective interfering particles. They are defective  because they cannot replicate unless the deleted function is  supplied by a “helper” virus. They also interfere with the  growth of normal virus if they infect first and preempt the  required cellular functions. Defective interfering particles  may play a role in recovery from viral infection; they inter¬  fere with the production of progeny virus, thereby limiting  the spread of the virus to other cells.   When two genetically distinct viruses infect a cell, three  different phenomena can ensue.   (1) Recombination is the exchange of genes between  two chromosomes that is based on crossing over within  regions of significant base sequence homology. Recombi¬  nation can be readily demonstrated for viruses with double-  stranded DNA as the genetic material and has been used to  determine their genetic map. However, recombination by  RNA viruses occurs at a very low frequency, if at all. Reas¬  sortment is the term used when viruses with segmented  genomes, such as influenza virus, exchange segments. This  usually results in a much higher frequency of gene exchange  than does recombination. Reassortment of influenza virus  RNA segments is involved in the major antigenic changes  in the virus that are the basis for recurrent influenza  epidemics.   (2) Complementation can occur when either one or  both of the two viruses that infect the cell have a mutation  that results in a nonfunctional protein (Figure 30-1). The  nonmutated virus “complements” the mutated one by mak¬  ing a functional protein that serves for both viruses.  Complementation is an important method by which a helper  virus permits replication of a defective virus. One clinically  important example of complementation is hepatitis B virus  providing its surface antigen to hepatitis delta virus, which is  defective in its ability to produce its own outer protein.   This phenomenon is the basis for the complementation  test, which can be used to determine how many genes exist  in a viral genome. It is performed by determining whether  mutant virus A can complement mutant virus B. If it can,  the two mutations are in separate genes because they make   infects a cell, no virus is produced because each has a mutated gene.  If both virus A and virus B infect a cell, the protein product of gene Y  of virus A will complement virus B, the protein product of gene Z of  virus B will complement virus A, and progeny of both virus A and  virus B will be produced. Note that no recombination has occurred  and that the virus A progeny will contain the mutated z gene and the  virus B progeny will contain the mutant y gene. Y, Z, functional  genes; y, z, mutated, nonfunctional genes.    different, complementary proteins. If it cannot, the two  mutations are in the same gene, and both proteins are non¬  functional. By performing many of these paired tests with  different mutants, it is possible to determine functional  domains of complementation groups that correspond to  genes. Appropriate controls are needed to obviate the  effects of recombination.   (3) In phenotypic mixing, the genome of virus type A  can be coated with the surface proteins of virus type B  (Figure 30-2). This phenotypically mixed virus can infect  cells as determined by its type B protein coat. However, the  progeny virus from this infection has a type A coat; it is  encoded solely by its type A genetic material. An interest¬  ing example of phenotypic mixing is that of pseudotypes,  which consist of the nucleocapsid of one virus and the  envelope of another. Pseudotypes composed of the nucleo¬  capsid of vesicular stomatitis virus (a rhabdovirus) and the  envelope of human immunodeficiency virus (HIV; a retro¬  virus) are currently being used to study the immune  response to HIV.   Viruses are being used as genetic vectors in two novel ways:  (1) to deliver new, functional genes to patients with genetic  diseases (gene therapy); and (2) to produce new viral vac¬  cines that contain recombinant viruses carrying the genes  of several different viruses, thereby inducing immunity to  several diseases with one immunization.   Retroviruses are currently being used as vectors of the gene  encoding adenine deaminase (ADA) in patients with  immunodeficiencies resulting from a defective ADA gene.  Retroviruses are excellent vectors because a DNA copy of  their RNA genome is stably integrated into the host cell  DNA and the integrated genes are expressed efficiently.  Retroviral vectors are constructed by removing the genes  encoding several viral proteins from the virus and replac¬  ing them with the human gene of interest (e.g., the ADA  gene). Virus particles containing the human gene are pro¬  duced within “helper cells” that contain the deleted viral  genes and therefore can supply, by complementation, the  missing viral proteins necessary for the virus to replicate.  The retroviruses produced by the helper cells can infect the  patient’s cells and introduce the human gene into the cells,  but the viruses cannot replicate because they lack several  viral genes. This inability of these viruses to replicate is an  important advantage in human gene therapy.   Recombinant viral vaccines contain viruses that have been  genetically engineered to carry the genes of other viruses.    Both types of  virus genomes  and proteins 0  are synthesized  within infected  mouse cell    FIGURE 30~2 Phenotypic mixing. Initially, Virus 1 (Blue capsid proteins and vertical genome) and Virus 2 (Yellow capsid proteins and  horizontal genome) infect the same mouse cell. Assume that Virus 1 can infect human cells but not chicken cells (a property determined by the  blue surface proteins) and that Virus 2 can infect chicken cells but not human cells (a property determined by the yellow surface proteins).  However, both Virus 1 and Virus 2 can infect a mouse cell. Within the mouse cell, both genomes are replicated and both blue and yellow capsid  proteins are synthesized. As shown, some of the progeny virus (Viruses 3 and 4) exhibit phenotypic mixing because they have both the blue and  the yellow surface proteins and therefore can infect both chicken cells and human cells. Note that in the next round of infection, when progeny  Virus 3 infects either human cells or chicken cells, the progeny of that infection (Viruses 5 and 6) is determined by the vertical genome and will be  identical to Virus 1 with only blue capsid proteins and a vertical genome. Similarly (but not shown), when progeny Virus 4 infects either human  cells or chicken cells, the progeny of that infection is determined by the horizontal genome and will be identical to Virus 2. (Reproduced with  permission from Joklik W et al. Zinsser Microbiology, 20th ed. Appleton & Lange, Norwalk, CT, 1992.)    Viruses with large genomes (e.g., vaccinia virus) are excel¬  lent candidates for this purpose. To construct the recombi¬  nant virus, any vaccinia virus gene that is not essential for  viral replication is deleted, and the gene from the other  virus that encodes the antigen that elicits neutralizing anti¬  body is introduced. For example, the gene for the surface  antigen of hepatitis B virus has been introduced into vac¬  cinia virus and is expressed in infected cells. Recombinant  vaccines are not yet clinically available, but vaccines of this  type promise to greatly improve the efficiency of our  immunization programs.    Mutations in the viral genome can produce antigenic vari¬  ants and drug-resistant variants. Mutations can also produce  attenuated (weakened) variants that cannot cause disease  but retain their antigenicity and are useful in vaccines.  Temperature-sensitive mutants can replicate at a low (per¬  missive) temperature but not at a high (restrictive) tem¬  perature. Temperature-sensitive mutants of influenza virus  are used in one of the vaccines against this disease.  Reassortment (exchange) of segments of the genome  RNA of influenza virus is important in the pathogenesis of  the worldwide epidemics caused by this virus.  Complementation occurs when one virus produces a pro¬  tein that can be used by another virus. A medically impor¬  tant example is hepatitis D virus, which uses the surface  antigen of hepatitis B virus as its outer coat protein.  Phenotypic mixing occurs when two different viruses  infect the same cell and progeny viruses contain proteins of  both parental viruses. This can endow the progeny viruses  with the ability to infect cells of species that ordinarily  parental virus could not.   1. In the lab, a virologist was studying the properties of HIV. She  infected the same cell with both HIV and rabies virus. (HIV can  infect only human CD4-positive cells, whereas rabies virus can infect  both human cells and dog cells.) Some of the progeny virions were  able to infect dog cells, within which she found HIV-specific RNA.  Which one of the following is the term used to describe these results?   2. You have isolated two mutants of poliovirus, one mutated at gene  X and the other mutated at gene Y. If you infect cells with each one  alone, no virus is produced. If you infect a single cell with both  mutants, which one of the following statements is most accurate?   (A) If complementation between the mutant gene products occurs,  both X and Y progeny viruses will be made.   (B) If phenotypic mixing occurs, then both X and Y progeny  viruses will be made.   (C) If the genome is transcribed into DNA, then both X and Y  viruses will be made.   (D) Because reassortment of the genome segments occurs at high  frequency, both X and Y progeny viruses will be made.   Questions on the topics discussed in this chapter can be found  in the Basic Virology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 720. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   The classification of viruses is based on chemical and mor¬  phologic criteria. The two major components of the virus  used in classification are (1) the nucleic acid (its molecular    weight and structure) and (2) the capsid (its size and sym¬  metry and whether it is enveloped). A classification scheme  based on these factors is presented in Tables 31-1 and 31-2  for DNA and RNA viruses, respectively. This scheme was  simplified from the complete classification to emphasize   2 The herpesvirus nucleocapsid is 100 nm, but the envelope varies in size. The entire virus can be as large as 200 nm in diameter.   Yellow fever virus, dengue  virus, West Nile virus,  hepatitis C virus   2 Retrovirus genome RNA contains two identical molecules, each with a molecular weight (MW) of 3.5 x 10 6 .  3 Particles are 80 nm wide but can be thousands of nanometers long.    organisms of medical importance. Only the virus families  are listed; subfamilies are described in the chapter on the  specific virus. Figures 31-1 and 31-2 show a classification  outline for DNA viruses and RNA viruses, respectively,  based on the type of genome, the nature of the nucleocap¬  sid, and whether an envelope is present.   The families of DNA viruses are described in Table 31-1.  The four naked (i.e., nonenveloped) icosahedral virus  families—the parvoviruses, polyomaviruses, papillomavi¬  ruses, and adenoviruses—are presented in order of increas¬  ing particle size, as are the three enveloped families. The  hepadnavirus family, which includes hepatitis B virus, and    the herpesviruses are enveloped icosahedral viruses. The  largest viruses, the poxviruses, have a complex internal  symmetry   These are very small (22 nm in diameter), naked icosahe¬  dral viruses with single-stranded linear DNA. There are  two types of parvoviruses: defective and nondefective. The  defective parvoviruses (e.g., adeno-associated virus)  require a helper virus for replication. The DNA of defec¬  tive parvoviruses is unusual because plus-strand DNA and  minus-strand DNA are carried in separate particles. The  nondefective parvoviruses are best illustrated by Parvovirus  B19 virus which has a minus-strand DNA genome. This  virus causes hydrops fetalis, aplastic anemia in sickle cell   anemia patients and erythema infectiosum (fifth disease,  slapped cheeks syndrome)—an innocuous childhood dis¬  ease characterized by an erythematous “slapped-cheeks”  rash.   These are naked icosahedral viruses (45 nm in diameter)  with double-stranded circular supercoiled DNA. Two  human polyomaviruses are JC virus, isolated from patients  with progressive multifocal leukoencephalopathy, and BK  virus, isolated from the urine of immunosuppressed kidney  transplant patients. Polyomavirus and simian vacuolating  virus 40 (SV40 virus) are polyomaviruses of mice and mon¬  keys, respectively, which induce malignant tumors in a  variety of species.   Papillomaviruses are naked icosahedral viruses (55 nm in  diameter) with double-stranded supercoiled DNA. The  human pathogen in the family is human papillomavirus  (HPV). It causes papillomas (warts) of many body sites,  and certain strains cause carcinoma of the cervix. Many    animal species are infected by papillomaviruses, but those  viruses are species-specific and do not infect humans.   These are naked icosahedral viruses (75 nm in diameter)  with double-stranded linear DNA. They cause pharyngitis,  upper and lower respiratory tract disease, and a variety of  other less common infections. There are at least 40 anti¬  genic types, some of which cause sarcomas in animals but  no tumors in humans.   These are double-shelled viruses (42 nm in diameter) with  an icosahedral capsid covered by an envelope. The DNA is  a double-stranded circle that is unusual because the com¬  plete strand is not a covalently closed circle and the other  strand is missing approximately 25% of its length. Hepatitis  B virus is the human pathogen in this family.   These are enveloped viruses (100 nm in diameter) with an  icosahedral nucleocapsid and double-stranded linear DNA.  They are noted for causing latent infections. The five impor¬  tant human pathogens are herpes simplex virus types 1 and  2, varicella-zoster virus, cytomegalovirus, and Epstein-Barr  virus (the cause of infectious mononucleosis).   These are the largest viruses, with a bricklike shape, an  envelope with an unusual appearance, and a complex cap¬  sid symmetry. They are named for the skin lesions, or  “pocks,” that they cause. Smallpox virus and molluscum  contagiosum virus are the two important members.    * Viruses labeled with asterisk  have a polymerase in the virion   The 14 families of RNA viruses are described in Table 31-2.  The three naked icosahedral virus families are listed first  and are followed by the three enveloped icosahedral  viruses. The remaining eight families are enveloped helical  viruses; the first five have single-stranded linear RNA as  their genome, whereas the last three have single-stranded  circular RNA.   These are the smallest (28 nm in diameter) RNA viruses.  They have single-stranded, linear, nonsegmented, positive-  polarity RNA within a naked icosahedral capsid. The name  “picorna” is derived from pico (small), RAfA-containing.  There are two groups of human pathogens: (1) enterovi¬  ruses, such as poliovirus, Coxsackie virus, echovirus, and  hepatitis A virus; and (2) rhinoviruses.   These are naked viruses (30 nm in diameter) with an icosa¬  hedral nucleocapsid. They have single-stranded, linear,  nonsegmented, positive-polarity RNA. The main human  pathogen is hepatitis E virus.   These are naked viruses (38 nm in diameter) with an icosa¬  hedral capsid. They have single-stranded, linear, nonseg¬  mented, positive-polarity RNA. The main human pathogen  is norovirus.   These are naked viruses (75 nm in diameter) with two ico¬  sahedral capsid coats. They have 10 or 11 segments of  double-stranded linear RNA. The name is an acronym of  respiratory enteric orphan, because they were originally  found in the respiratory and enteric tracts and were not  associated with any human disease. The main human  pathogen is rotavirus, which causes diarrhea, mainly in  infants. The rotavirus genome has 11 segments of double-  stranded RNA.   These are enveloped viruses with an icosahedral capsid and  single-stranded, linear, nonsegmented, positive-polarity  RNA. The flaviviruses include hepatitis C virus, yellow  fever virus, dengue virus, West Nile virus, and St. Louis and  Japanese encephalitis viruses.   These are enveloped viruses with an icosahedral capsid and  single-stranded, linear, nonsegmented, positive-polarity  RNA. There are two major groups of human pathogens: the    alphaviruses and rubiviruses. The alphavirus group  includes eastern and western encephalitis viruses; the rubi-  virus group consists only of rubella virus.   These are enveloped viruses with an icosahedral capsid and  two identical strands (said to be “diploid”) of single-  stranded, linear, positive-polarity RNA. The term retro  pertains to the reverse transcription of the RNA genome  into DNA. There are two medically important groups: (1)  the oncovirus group, which contains the sarcoma and leu¬  kemia viruses (e.g., human T-cell leukemia virus [HTLV]);  and (2) the lentivirus (“slow virus”) group, which includes  human immunodeficiency virus (HIV) and certain animal  pathogens (e.g., visna virus). A third group, spumaviruses,  is described in Chapter 46.   These viruses (myxoviruses) are enveloped, with a helical  nucleocapsid and eight segments of linear, single-stranded,  negative-polarity RNA. The term myxo refers to the affinity  of these viruses for mucins, and ortho is added to distin¬  guish them from the paramyxoviruses. Influenza virus is  the main human pathogen.   These are enveloped viruses with a helical nucleocapsid  and single-stranded, linear, nonsegmented, negative-polar¬  ity RNA. The important human pathogens are measles,  mumps, parainfluenza, and respiratory syncytial viruses.   These are bullet-shaped enveloped viruses with a helical  nucleocapsid and a single-stranded, linear, nonsegmented,  negative-polarity RNA. The term rhabdo refers to the bullet  shape. Rabies virus is the only important human pathogen.   These are enveloped viruses with a helical nucleocapsid  and single-stranded, linear, nonsegmented, negative-polar¬  ity RNA. They are highly pleomorphic, long filaments that  are 80 nm in diameter but can be thousands of nanometers  long. The term filo means “thread” and refers to the long  filaments. The two human pathogens are Ebola virus and  Marburg virus.   These are enveloped viruses with a helical nucleocapsid and  a single-stranded, linear, nonsegmented, positive-polarity  RNA. The term corona refers to the prominent halo of spikes  protruding from the envelope. Coronaviruses cause respira¬  tory tract infections, such as the common cold and severe  acute respiratory syndrome (SARS), in humans.   These are enveloped viruses with a helical nucleocapsid  and a single-stranded, circular, negative-polarity RNA in  two segments. (A part of both segments is positive-polarity  RNA, and the term ambisense RNA is used to describe this  unusual genome.) The term arena means “sand” and refers  to granules on the virion surface that are nonfunctional  ribosomes. Two human pathogens are lymphocytic chorio¬  meningitis virus and Lassa fever virus.   These are enveloped viruses with a helical nucleocapsid  and a single-stranded, circular, negative-polarity RNA in  three segments. Some bunyaviruses contain ambisense  RNA in their genome (see Arenaviruses). The term bunya  refers to the prototype, Bunyamwera virus, which is named  after the place in Africa where it was isolated. These viruses  cause encephalitis and various fevers such as Korean hem¬  orrhagic fever. Hantaviruses, such as Sin Nombre virus (see  Chapter 46), are members of this family.   Hepatitis delta virus (HDV) is the only member of this  genus. It is an enveloped virus with an RNA genome that is  a single-stranded, negative-polarity, covalently closed cir¬  cle. The symmetry of the nucleocapsid is uncertain. It is a  defective virus because it cannot replicate unless hepatitis B  virus (HBV) is present within the same cell. HBV is  required because it encodes hepatitis B surface antigen  (HBsAg), which serves as the outer protein coat of HDV.  The RNA genome of HDV encodes only one protein, the  internal core protein called delta antigen.    The classification of viruses is based primarily on the nature  of the genome and whether the virus has an envelope.  Poxviruses, herpesviruses, and hepadnaviruses are DNA  viruses with an envelope, whereas adenoviruses, polyoma-  viruses, papillomaviruses, and parvoviruses are DNA viruses  without an envelope (i.e., they are naked nucleocapsid  viruses). Parvoviruses have single-stranded DNA, whereas  all the other families of DNA viruses have double-stranded  DNA. The DNA of hepadnaviruses (hepatitis B virus) is  mostly double-stranded but has a single-stranded region.  Picornaviruses, hepeviruses, caliciviruses, and reoviruses  are RNA viruses without an envelope, whereas all the other  families of RNA viruses have an envelope. Reoviruses have  double-stranded RNA; all the other families of RNA  viruses have single-stranded RNA. Reoviruses and influ¬  enza viruses have segmented RNA; all the other families of  RNA viruses have nonsegmented RNA. Picornaviruses, hep¬  eviruses, caliciviruses, flaviviruses, togaviruses, retroviruses,  and coronaviruses have positive-polarity RNA, whereas all  the other families have negative-polarity RNA.   Questions on the topics discussed in this chapter can be found  in the Basic Virology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 720. Also see Part  XIV; USMLE (National Board) Practice Examination starting  on page 751.   The ability of viruses to cause disease can be viewed on two  distinct levels: (1) the changes that occur within individual  cells and (2) the process that takes place in the infected  patient.   There are four main effects of virus infection on the cell:  (1) death, (2) fusion of cells to form multinucleated cells,  (3) malignant transformation, and (4) no apparent mor¬  phologic or functional change.   Death of the cell is probably due to inhibition of macro-  molecular synthesis. Inhibition of host cell protein synthe¬  sis frequently occurs first and is probably the most  important effect. Inhibition of DNA and RNA synthesis  may be a secondary effect. It is important to note that syn¬  thesis of cellular proteins is inhibited but viral protein  synthesis still occurs. For example, poliovirus inactivates an  initiation factor (IF) required for cellular mRNA to be  translated into cellular proteins, but poliovirus mRNA has  a special ribosome-initiating site that allows it to bypass the  IF so that viral proteins can be synthesized.   Infected cells frequently contain inclusion bodies,  which are discrete areas containing viral proteins or viral  particles. They have a characteristic intranuclear or intra-  cytoplasmic location and appearance depending on the  virus. One of the best examples of inclusion bodies that can  assist in clinical diagnosis is that of Negri bodies, which are    eosinophilic cytoplasmic inclusions found in rabies virus-  infected brain neurons. Another important example is the  owl’s eye inclusion seen in the nucleus of cytomegalovirus-  infected cells. Electron micrographs of inclusion bodies can  also aid in the diagnosis when virus particles of typical  morphology are visualized.   Fusion of virus-infected cells produces multinucleated  giant cells, which characteristically form after infection  with herpesviruses and paramyxoviruses. Fusion occurs  as a result of cell membrane changes, which are probably  caused by the insertion of viral proteins into the mem¬  brane. The clinical diagnosis of herpesvirus skin infections  is aided by the finding of multinucleated giant cells with  eosinophilic intranuclear inclusions in skin scrapings.   A hallmark of viral infection of the cell is the cytopathic  effect (CPE). This change in the appearance of the infected  cell usually begins with a rounding and darkening of the cell  and culminates in either lysis (disintegration) or giant cell  formation. Detection of virus in a clinical specimen fre¬  quently is based on the appearance of CPE in cell culture.  In addition, CPE is the basis for the plaque assay, an impor¬  tant method for quantifying the amount of virus in a  sample.   Infection with certain viruses causes malignant trans¬  formation, which is characterized by unrestrained growth,  prolonged survival, and morphologic changes such as focal  areas of rounded, piled-up cells. These changes are  described in more detail in Chapter 43.   Infection of the cell accompanied by virus production  can occur without morphologic or gross functional    changes. This observation highlights the wide variations in  the nature of the interaction between the virus and the cell,  ranging from rapid destruction of the cell to a symbiotic  relationship in which the cell survives and multiplies  despite the replication of the virus.   Pathogenesis in the infected patient involves (1) transmis¬  sion of the virus and its entry into the host; (2) replication  of the virus and damage to cells; (3) spread of the virus to  other cells and organs; (4) the immune response, both as a  host defense and as a contributing cause of certain diseases;  and (5) persistence of the virus in some instances.    The stages of a typical viral infection are the same as  those described for a bacterial infection in Chapter 7,  namely, an incubation period during which the patient is  asymptomatic, a prodromal period during which nonspe¬  cific symptoms occur, a specific-illness period during  which the characteristic symptoms and signs occur, and a  recovery period during which the illness wanes and the  patient regains good health. In some patients, the infection  persists and a chronic carrier state or a latent infection  occurs (see later).   Viruses are transmitted to the individual by many different  routes, and their portals of entry are varied (Table 32-1).   transmission of these viruses is typically by the fecal-oral route in contaminated food or water.   transmission of these viruses is typically by the bite of an infected animal.   ’Note that there are important bacteria, namely, Treponema pallidum and Listeria  monocytogenes, and an important protozoan, namely, Toxoplasma gondii, that are  also transmitted transplacentally.   2 Note that there are important bacteria, namely, Neisseria gonorrhoeae, Chlamydia  trachomatis, and group B Streptococcus that are also transmitted at the time of birth.  3 Human immunodeficiency virus is also transmitted transplacentally and in breast milk.   For example, person-to-person spread occurs by transfer of  respiratory secretions, saliva, blood, or semen and by fecal  contamination of water or food. The transfer of blood,  either by transfusion or by sharing needles during intrave¬  nous drug use, can transmit various viruses (and bacteria).  The screening of donated blood for human immunodefi¬  ciency virus, human T-cell lymphotropic virus, hepatitis B  virus, hepatitis C virus, and West Nile virus (as well as  Treponema pallidum) has greatly reduced the risk of infec¬  tion by these pathogens.   Transmission can occur also between mother and off¬  spring in utero across the placenta, at the time of delivery,  or during breast feeding (Table 32-2). Transmission  between mother and offspring is called vertical transmis¬  sion. Person-to-person transmission that is not from  mother to offspring is called horizontal transmission.   Animal-to-human transmission can take place either  directly from the bite of a reservoir host as in rabies or    indirectly through the bite of an insect vector, such as a  mosquito, which transfers the virus from an animal reser¬  voir to the person. The zoonotic diseases caused by viruses  are described in Table 32-3. In addition, activation of a  latent, nonreplicating virus to form an active, replicating  virus can occur within the individual, with no transmission  from an external source.   Most viral infections are either localized to the portal of  entry or spread systemically through the body. The best  example of the localized infection is the common cold  caused by rhinoviruses, which involves only the upper respi¬  ratory tract. Influenza is localized primarily to the upper and  lower respiratory tracts. Respiratory viruses have a short  incubation period because they replicate directly in the  mucosa, but systemic infections such as poliomyelitis and  measles have a long incubation period because viremia and  secondary sites of replication are required.   One of the best-understood systemic viral infections is  paralytic poliomyelitis (Figure 32-1). After poliovirus is  ingested, it infects and multiplies within the cells of the  small intestine and then spreads to the mesenteric lymph  nodes, where it multiplies again. It then enters the blood¬  stream and is transmitted to certain internal organs, where  it multiplies again. The virus reenters the bloodstream and  is transmitted to the central nervous system, where damage  to the anterior horn cells occurs, resulting in the character¬  istic muscle paralysis. It is during this obligatory viremia  that circulating IgG antibodies induced by the polio vac¬  cine can prevent the virus from infecting the central ner¬  vous system. Viral replication in the gastrointestinal tract  results in the presence of poliovirus in the feces, thus per¬  petuating its transmission to others.   Some viral infections spread systemically, not via the  bloodstream, but rather by retrograde axonal flow within  neurons. Four important human pathogens do this: rabies  virus, herpes simplex type 1, herpes simplex type 2, and   In United States, skunks, raccoons, and  bats; in developing countries, dogs    ’Sin Nombre virus is the most important hantavirus in the United States.   important encephalitis viruses in the United States include eastern and western equine encephalitis viruses, West Nile virus, and St. Louis encephalitis virus.  3 SARS = severe acute respiratory syndrome.   FIGURE 32~1 Systemic viral infection by poliovirus, resulting  in paralytic poliomyelitis. CNS, central nervous system. (Reproduced  with permission from Brooks GF et al. Medical Microbiology. 20th ed.  Originally published by Appleton & Lange. Copyright 1995 by  McGraw-Hill.)   varicella-zoster virus. As an example, rabies virus is intro¬  duced into the body at the site of an animal bite. The virus  infects a local sensory neuron and ascends into the central  nervous system by retrograde axonal flow, where it causes  encephalitis.   The signs and symptoms of most viral diseases undoubt¬  edly are the result of cell killing by virus-induced inhibition  of macromolecular synthesis. Death of the virus-infected  cells results in a loss of function and in the symptoms of  disease. For example, when poliovirus kills motor neurons,  paralysis of the muscles innervated by those neurons  results. Also, the hemorrhages caused by Ebola virus are  due to the damage to the vascular endothelial cells caused  by the envelope glycoprotein of the virus.    However, there are some diseases that are not caused by  the virus damaging or killing the infected cell. For example,  rotavirus-induced diarrhea is caused primarily by stimula¬  tion of the enteric nervous system. It is thought that the  rotavirus-infected enterocytes produce cytokines that stim¬  ulate the enteric neurons, resulting in excess fluid and  electrolyte secretion into the bowel lumen.   There are other diseases in which cell killing by immu¬  nologic attack plays an important role in pathogenesis.  Both cytotoxic T cells and antibodies play a role in  immunopathogenesis.   (1) The best-studied system is lymphocytic choriomenin¬  gitis (LCM) in mice; LCM occurs in humans also but is quite  rare. When LCM virus is inoculated into the brain of an  adult mouse, virus replication occurs and death follows.  However, when LCM virus is inoculated into the brain of an  immunosuppressed adult mouse or a newborn mouse, the  animal remains well despite extensive virus replication.  When immune lymphocytes are inoculated into these  infected, but otherwise healthy mice, death ensues. It appears  that death of the cells is caused by immune attack by cyto¬  toxic T cells on the new viral antigens in the cell membrane  rather than by virus-induced inhibition of cell functions.   (2) Cytotoxic T cells are involved in the pathogenesis  of hepatitis caused by hepatitis A, B, and C viruses. These  viruses do not cause a CPE, and the damage to the hepato-  cytes is the result of the recognition of viral antigens on the  hepatocyte surface by cytotoxic T cells. The rash of measles  is similarly caused by these cells attacking the infected vas¬  cular endothelium in the skin.   (3) Immune-mediated pathogenesis also occurs when  virus-antibody-complement complexes form and are  deposited in various tissues. This occurs in hepatitis B  virus infection, in which immune complexes play a role in  producing the arthritis characteristic of the early stage of  hepatitis B. Immune complexes also cause the arthritis  seen in parvovirus B19 and rubella virus infections. The  pathogenesis of pneumonia caused by respiratory syncytial  virus in infants is attributed to immune complexes formed  by maternal IgG and viral antigens.   Strains of viruses differ greatly in their ability to cause disease.  For example, there are strains of poliovirus that have mutated  sufficiently such that they have lost the ability to cause polio  in immunocompetent individuals (i.e., they are attenuated).  These strains are used in vaccines. The viral genes that con¬  trol the virulence of the virus are poorly characterized, and  the process of virulence is poorly understood.   Viruses have several ways by which they evade our host  defenses (Table 32-4). These processes are often called  immune evasion. Two very important processes are   Some viruses encode the receptors for various media¬  tors of immunity such as interleukin-1 (IL-1) and tumor  necrosis factor (TNF). For example, vaccinia virus encodes  a protein that binds to IL-1, and fibroma virus encodes a  protein that binds to TNF. Cytomegalovirus (CMV)  encodes a chemokine receptor that binds to several chemo-  kines. When released from virus-infected cells, these pro¬  teins bind to the immune mediators and block their ability  to interact with receptors on their intended targets, our  immune cells that mediate host defenses against the viral  infection. By reducing our host defenses, the virulence of  the virus is enhanced. These virus-encoded proteins that  block host immune mediators are often called cytokine  decoys.   In addition, some viruses (e.g., human immunodefi¬  ciency virus [HIV] and herpesviruses, such as herpes sim¬  plex virus and cytomegalovirus [CMV]) can reduce the  expression of class I MHC (major histocompatibility com¬  plex) proteins, thereby reducing the ability of cytotoxic T  cells to kill the virus-infected cells, and others (e.g., herpes  simplex virus) inhibit complement.   Several viruses (HIV, Epstein-Barr virus, and adenovi¬  rus) synthesize RNAs that block the phosphorylation of an  initiation factor (eIF-2), which reduces the ability of inter¬  feron to block viral replication (see Chapter 33). CMV  encodes a microRNA that binds to the mRNA of a cell  surface ligand for natural killer cells. Binding of the  microRNA prevents synthesis of the ligand, which prevents  killing of the CMV-infected cells by the natural killer cells.  Measles virus blocks synthesis of IL-12, thereby reducing  an effective Th-1 response. Ebola virus synthesizes two  proteins, one of which blocks the induction of interferon,  whereas the other blocks its action. Collectively, these viral  virulence factors are called virokines.    Another important way by which viruses evade our  host defenses is by having multiple antigenic types (also  known as multiple serotypes). The clinical importance of  a virus having multiple serotypes is that a patient can be  infected with one serotype, recover, and have antibodies  that protect from infection by that serotype in the future;  however, that person can be infected by another serotype  of that virus.   The classic example of a virus with multiple serotypes is  rhinovirus, which has more than 100 serotypes. This is the  reason why the “common cold” caused by rhinoviruses is so  common. Influenza virus also has multiple serotypes, and  the severe worldwide epidemics of influenza are attributed  to the emergence of new antigenic types. HIV and hepatitis  C virus have multiple serotypes, which contribute to the  difficulty in obtaining a vaccine against these viruses. Note  that only some viruses have multiple serotypes. Many  important human pathogens (e.g., measles virus, rubella  virus, varicella-zoster virus, and rabies virus) have only one  serotype, and some have only a few serotypes (e.g., poliovi¬  rus has three serotypes).   In most viral infections, the virus does not remain in the  body for a significant period after clinical recovery. How¬  ever, in certain instances, the virus persists for long periods  either intact or in the form of a subviral component (e.g.,  the genome). The mechanisms that may play a role in the  persistence of viruses include (1) integration of a DNA  provirus into host cell DNA, as occurs with retroviruses;  (2) immune tolerance, because neutralizing antibodies are  not formed; (3) formation of virus-antibody complexes,  which remain infectious; (4) location within an immuno-  logically sheltered “sanctuary” (e.g., the brain); (5) rapid  antigenic variation; (6) spread from cell to cell without an  extracellular phase, so that virus is not exposed to antibody;   Reduces MHC class 1 proteins, thereby decreasing killing by cytotoxic T cells   Blocks IL-12, which reduces formation of Th-1 cells, thereby decreasing  cell-mediated immunity   Encodes receptors for immune mediators; receptors are secreted by infected  cells, binds mediators, and inactivates them   Encodes chemokine receptor; this blocks action of chemokine, thereby  inhibiting migration of inflammatory cells to site of infection   Encodes protein that binds to complement protein C3b; this blocks  opsonizing action of C3b as well as its ability to participate in forming the  membrane attack complex    CMV = cytomegalovirus; EBV = Epstein-Barr virus; HIV = human immunodeficiency virus; HSV = herpes simplex virus; IL = interleukin.   There are three types of persistent viral infections of  clinical importance. They are distinguished primarily by  whether virus is usually produced by the infected cells and  by the timing of the appearance both of the virus and of the  symptoms of disease.   Some patients who have been infected with certain viruses  continue to produce significant amounts of the virus for  long periods. This carrier state can follow an asymptom¬  atic infection as well as the actual disease and can itself  either be asymptomatic or result in chronic illness. Impor¬  tant clinical examples are chronic hepatitis, which occurs in  hepatitis B and hepatitis C virus carriers, and neonatal  rubella virus and CMV infections, in which carriers can  produce virus for years.   In these infections, best illustrated by the herpesvirus  group, the patient recovers from the initial infection and  virus production stops. Subsequently, the symptoms may  recur, accompanied by the production of virus. In herpes  simplex virus infections, the virus enters the latent state in  the cells of the sensory ganglia. The molecular nature of the  latent state is unknown. Herpes simplex virus type 1, which    causes infections primarily of the eyes and face, is latent in  the trigeminal ganglion, whereas herpes simplex virus type  2, which causes infections primarily of the genitals, is latent  in the lumbar and sacral ganglia. Varicella-zoster virus,  another member of the herpesvirus family, causes varicella  (chickenpox) as its initial manifestation and then remains  latent, primarily in the trigeminal or thoracic ganglion  cells. It can recur in the form of the painful vesicles of zos¬  ter (shingles), usually on the face or trunk.   The term slow refers to the prolonged period between the  initial infection and the onset of disease, which is usually  measured in years. In instances in which the cause has been  identified, the virus has been shown to have a normal, not  prolonged, growth cycle. It is not, therefore, that virus  growth is slow; rather, the incubation period and the pro¬  gression of the disease are prolonged. Two of these diseases  are caused by conventional viruses, namely, subacute scleros¬  ing panencephalitis, which follows several years after measles  virus infections, and progressive multifocal leukoencepha-  lopathy (PML), which is caused by JC virus, a papovavirus.  PML occurs primarily in patients who have lymphomas or  are immunosuppressed. Other slow infections in humans  (e.g., Creutzfeldt-Jakob disease and kuru) may be caused by  unconventional agents called prions (see Chapter 28). Slow  virus infections are described in Chapter 44.   • Death of infected cells is probably caused by inhibition of cel¬  lular protein synthesis. Translation of viral mRNA into viral pro¬  teins preempts the ribosomes preventing synthesis of cellular  proteins.   • Inclusion bodies are aggregates of virions in specific locations  in the cell that are useful for laboratory diagnosis. Two impor¬  tant examples are Negri bodies in the cytoplasm of rabies  virus-infected cells and owl's eye inclusions in the nucleus of  cytomegalovirus-infected cells.   • Multinucleated giant cells form when cells are infected with  certain viruses, notably herpesviruses and paramyxoviruses  such as respiratory syncytial virus.   • Cytopathic effect (CPE) is a visual or functional change in  infected cells typically associated with the death of cells.   • Malignant transformation occurs when cells are infected with  oncogenic viruses. Transformed cells are capable of unre¬  strained growth.   • Some virus-infected cells appear visually and functionally nor¬  mal, yet are producing large numbers of progeny viruses.   • Viral infection in the person typically has four stages: incuba¬  tion period, prodromal period, specific-illness period, and  recovery period.   • The main portals of entry are the respiratory, gastrointestinal,  and genital tracts, but through the skin, across the placenta,  and via blood are important as well.   • Transmission from mother to offspring is called vertical transmis¬  sion; all other modes of transmission (e.g., fecal-oral, respiratory  aerosol, insect bite) are horizontal transmission. Transmission  can be from human to human or from animal to human.   • Most serious viral infection are systemic (i.e., the virus travels  from the portal of entry via the blood to various organs). How¬  ever, some are localized to the portal of entry, such as the com¬  mon cold, which involves only the upper respiratory tract.   • The symptoms of viral diseases are usually caused by death of  the infected cells and a consequent loss of function. For    Immunopathogenesis is the process by which the symptoms  of viral diseases are caused by the immune system rather than  by the killing of cells directly by the virus. One type of immuno¬  pathogenesis is the killing of virus-infected cell by the  attack of cytotoxic T cells that recognize viral antigens on the  cell surface. Damage to the liver caused by hepatitis viruses  occurs by this mechanism. Another is the formation of virus-  antibody complexes that are deposited in tissues. Arthritis  associated with parvovirus B19 or rubella virus infection occurs  by this mechanism.   Virulence of viruses differs markedly from one virus to another  and among different strains of the same virus. The genetic  basis for these differences is not well understood. Strains with  weakened (attenuated) virulence are often used in vaccines.  Viruses can evade host defenses by producing multiple anti¬  gens, thereby avoiding inactivation by antibodies, and by  reducing the synthesis of class I MHC proteins, thereby  decreasing the ability of a cell to present viral antigens and  blunting the ability of cytotoxic T cells to kill the virus-infected  cells. Viruses also produce receptors for immune mediators,    such as IL-1 and TNF, thereby preventing the ability of these  mediators to activate antiviral processes.   • Carrier state refers to people who produce virus for long peri¬  ods of time and can serve as a source of infection for others.  The carrier state that is frequently associated with hepatitis C  virus infection is a medically important example.   • Latent infections are those infections that are not producing  virus at the present time but can be reactivated at a subse¬  quent time. The latent infections that are frequently associated  with herpes simplex virus infection are a medically important  example.   • Slow virus infections refer to those diseases with a long incu¬  bation period, often measured in years. Some, such as progres¬  sive multifocal leukoencephalopathy, are caused by viruses,  whereas others, such as Creutzfeldt-Jakob disease, are caused  by prions. The brain is often the main site of these diseases.   1. Viruses can cause changes in individual cells that are visible in  the light microscope after suitable staining. Which one of the fol¬  lowing is most characteristic of the changes seen in rabies virus-  infected cells?   2. Many viruses use the upper respiratory tract (mouth, nasophar¬  ynx) as their important portal of entry. One feature of the portal of  entry is that it is the site where the virus first infects and replicates.  Which one of the following viruses is most likely to enter via the  upper respiratory tract?   4. Some viruses are known for their ability to cause perinatal infec¬  tions. Which one of the following viruses is most likely to cause  perinatal infections?    5. Which one of the following viruses that causes human disease has  an animal reservoir?   6. Which one of the following best describes the mechanism by  which immunopathogenesis occurs?   (B) Ability of cytotoxic T cells to block pathogenesis by viruses   (D) Ability of cytotoxic T cells to cause pathogenesis by viruses   Questions on the topics discussed in this chapter can be found  in the Basic Virology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 720. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   Host defenses against viruses fall into two major categories:  (1) nonspecific, of which the most important are interfer¬  ons and natural killer cells; and (2) specific, including both  humoral and cell-mediated immunity. Interferons are an  early, first-line defense, whereas humoral immunity and  cell-mediated immunity are effective only later because it  takes several days to induce the humoral and cell-mediated  arms of the immune response.   A description of how viruses evade our host defenses  appears in Chapter 32.   Alpha and beta interferons are a group of proteins pro¬  duced by human cells after viral infection (or after expo¬  sure to other inducers). They inhibit the growth of viruses  by blocking the synthesis of viral proteins. They do so by  two main mechanisms: One is a ribonuclease that degrades  mRNA, and the other is a protein kinase that inhibits pro¬  tein synthesis.   Interferons are divided into three types based on the cell  of origin, namely, leukocyte, fibroblast, and lymphocyte.  They are also known as alpha, beta, and gamma interfer¬  ons, respectively. Alpha and beta interferons, collectively    known as type 1 interferon are induced by viruses, whereas  gamma (T cell, immune) interferon, known as type II inter¬  feron is induced by antigens and is one of the effectors of  cell-mediated immunity (see Chapter 58). The following  discussion of alpha and beta interferons focuses on the  induction and action of their antiviral effect (Figure 33-1).   Lambda  interferon, known as type III interferon, is  active against intestinal viruses, especially rotavirus and  norovirus. It reduces the long-term persistence of virus in  intestinal mucosal cells. The role of lambda interferon in  human disease is uncertain and will not be discussed  further.   The strong inducers of these interferons are viruses and  double-stranded RNAs. Induction is not specific for a  particular virus; many DNA and RNA viruses are compe¬  tent inducers, although they differ in effectiveness. The  finding that double-stranded RNA, but not single-stranded  RNA or DNA, is a good inducer has led to the conclusion  that a double-stranded RNA is synthesized as part of the  replicative cycle of all inducing viruses. The double-  stranded RNA poly (rl-rC) is one of the strongest inducers  and was under consideration as an antiviral agent, but toxic  side effects prevented its clinical use. The weak inducers of  microbiologic interest include a variety of intracellular bac¬  teria and protozoa, as well as certain bacterial substances  such as endotoxin.   FIGURE 33-1 Induction and action of interferon. A: Virus infection induces the synthesis of interferon, which then leaves the infected  cell. B: Interferon binds to the surface receptor of an uninfected cell and induces the synthesis of three new cell-encoded enzymes (antiviral  proteins). C: A new virion enters the cell, but viral replication is inhibited by the interferon-induced antiviral proteins. One of these antiviral pro¬  teins is a ribonuclease that degrades mRNA, and another is a protein kinase that phosphorylates an initiation factor that inhibits protein synthe-  sis. (Tortora G, Microbiology: An Introduction. 1st edition, © 1982. Reprinted by permission of Pearson Education Inc, New York, New York.)    This extensive list of inducers makes it clear that  induction of these interferons is not specific. Similarly,  their inhibitory action is not specific for any particular  virus. However, they are typically specific in regard to  the host species in which they act (i.e., interferons pro¬  duced by human cells are active in human cells but are  much less effective in cells of other species). It is clear,  therefore, that other animals cannot be used as a source  of interferons for human therapy. Rather, the genes for  human interferons have been cloned, and interferon for  medical use is now produced by genetic engineering  techniques.   Interferons inhibit the intracellular replication of a wide  variety of DNA and RNA viruses but have little effect on  the metabolism of normal cells. The selectivity arises from  the presence of double-stranded RNA in virus-infected  cells, which is not present in uninfected cells.   Interferons have no effect on extracellular virus parti¬  cles. Interferons act by binding to a receptor on the cell  surface that signals the cell to synthesize three proteins,  thereby inducing the “antiviral state” (Figure 33-2). These  three proteins are inactive precursor proteins until they are  activated by double-stranded RNA synthesized during viral  replication. As a result, these proteins are active in virus-  infected cells but not in uninfected cells.   The three cellular proteins are (1) a 2,5-oligo A synthe¬  tase that synthesizes an adenine trinucleotide (2,5-oligo A),  (2) a ribonuclease that is activated by 2,5-oligo A and  degrades viral and cellular mRNAs, and (3) a protein  kinase that phosphorylates an initiation factor (eIF-2) for  protein synthesis, thereby inactivating it. The end result is  that both viral and cellular protein synthesis is inhibited  and the infected cell dies. No virus is produced by that cell,  and the spread of the virus is reduced.   Because interferons are produced within a few hours of  the initiation of viral replication, they act in the early phase  of viral diseases to limit the spread of virus. In contrast,    Uninfected ceil produces  inactive oligo A synthetase,  ribonuclease, and protein kinase    FIGURE 33-2 Interferon induces an antiviral state within an  uninfected cell. Interferon binds to the surface of the uninfected cell  and induces three proteins that remain inactive until a virus infects the  cell. These proteins are oligo A synthetase, ribonuclease, and protein  kinase. When a virus infects that cell, a double-stranded RNA (dsRNA) is  synthesized as part of the viral replicative cycle. The dsRNA activates  oligo A synthetase, which synthesizes oligo A that then activates the  ribonuclease to degrade viral (and cell) mRNA. The dsRNA also acti¬  vates the protein kinase that phosphorylates initiation factor-2 (eIF-2).  This inhibits both viral and cell protein synthesis. The cell dies without  producing progeny virus, thereby limiting the spread of infection.    antibody begins to appear in the blood several days after  infection.   Alpha interferon has been approved for use in patients  with condyloma acuminatum and chronic active hepatitis  caused by hepatitis B and C viruses. Beta interferon is used  in the treatment of multiple sclerosis. Gamma interferon  reduces recurrent infections in patients with chronic gran¬  ulomatous disease (see Chapter 68). Interferons are also  used clinically in patients with cancers such as Kaposi’s  sarcoma and hairy cell leukemia.   Natural killer (NK) cells are an important part of the innate  defenses against virus-infected cells. They are called “natu¬  ral” killer cells because they are active without the necessity  of being exposed to the virus previously and because they  are not specific for any virus. NK cells are a type of T lym¬  phocyte but do not have an antigen receptor. They recog¬  nize virus-infected cells by the absence of class I MHC  (major histocompatibility complex) proteins on the surface  of the virus-infected cell. They kill virus-infected cells by  secreting perforins and granzymes, which cause apoptosis  of the infected cells. (See page 516 for more information.)   Macrophages, particularly fixed macrophages of the reticu¬  loendothelial system and alveolar macrophages, are the  important cell types in limiting virus infection. In contrast,  polymorphonuclear leukocytes are the predominant cellu¬  lar defense in bacterial infections.   a-Defensins are a family of positively charged peptides with  antiviral activity. (They also have antibacterial activity; see  Chapter 8.) They interfere with human immunodeficiency  virus (HIV) binding to the CXCR4 receptor and block  entry of the virus into the cell. The production of a-defensins  may explain why some HIV-infected individuals are long¬  term “nonprogressors.”   APOBEC3G is an important member of the innate host  defenses against retroviral infection, especially against HIV.  APOBEC3G is an enzyme that causes hypermutation in  retroviral DNA by deaminating cytosines in both mRNA  and retroviral DNA, thereby inactivating these molecules  and reducing infectivity. HIV defends itself against this  innate host defense by producing Vif (viral infectivity pro¬  tein), which counteracts APOBEC3G, thereby preventing  hypermutation from occurring.   Elevated body temperature may play a role in host  defenses, but its importance is uncertain. Fever may act  in two ways: (1) The higher body temperature may  directly inactivate the virus particles, particularly envel¬  oped viruses, which are more heat-sensitive than nonen-  veloped viruses; and (2) Replication of some viruses is  reduced at higher temperatures; therefore, fever may  inhibit replication.   The mucociliary clearance mechanism of the respiratory  tract may protect the host. Its damage (e.g., from smoking)  results in an increased frequency of viral respiratory tract  infections, especially influenza.   There is evidence that circumcision prevents infection  by three sexually transmitted viruses: HIV, human papil¬  lomavirus (HPV), and herpes simplex virus type 2  (HSV-2).   Several factors influence host defenses in a nonspecific or  multifactorial way:   (1) Age is a significant variable in the outcome of viral  infections. In general, infections are more severe in neo¬  nates and in the elderly than in older children and young  adults. For example, influenza is typically more severe in  older people than in younger adults, and herpes simplex  virus infections are more severe in neonates than in  adults.   (2) Increased corticosteroid levels predispose to more  severe infections with some viruses, such as varicella-zoster  virus; the use of topical cortisone in herpetic keratitis can  exacerbate eye damage. It is not clear how these effects are  mediated, because corticosteroids can cause a variety of  pertinent effects, namely, lysis of lymphocytes, decreased  recruitment of monocytes, inhibition of interferon produc¬  tion, and stabilization of lysosomes.   (3) Malnutrition leads to more severe viral infections  (e.g., there is a much higher death rate from measles in  developing countries than in developed ones). Poor nutri¬  tion causes decreased immunoglobulin production and  phagocyte activity as well as reduced skin and mucous  membrane integrity.   There is evidence for natural resistance to some viruses in  certain species, which is probably based on the absence of    receptors on the cells of the resistant species. For example,  some people are resistant to HIV infection because they  lack one of the chemokine receptors that mediate entry of  the virus into the cell. However, by far the most important  type of defense is acquired immunity, either actively  acquired by exposure to the virus or passively acquired by  the transfer of immune serum. Active immunity can be  elicited by contracting the actual disease, by having an  inapparent infection, or by being vaccinated.   Active immunity, in the form of both antibodies and cyto¬  toxic T cells, is very important in the prevention of viral  diseases. The first exposure to a virus, whether it causes an  inapparent infection or symptomatic disease, stimulates the  production of antibodies and the activation of cytotoxic T  cells. The role that antibodies and cytotoxic T cells play in  the recovery from this first infection is uncertain and may  vary from virus to virus, but it is clear that they play an  essential role in protecting against disease when exposed to  the same virus at some time in the future.   The duration of protection varies; disseminated viral  infections such as measles and mumps confer lifelong  immunity against recurrences, but localized infections  such as the common cold usually impart only a brief  immunity of several months. IgA confers protection  against viruses that enter through the respiratory and gas¬  trointestinal mucosa, and IgM and IgG protect against  viruses that enter or are spread through the blood. The  lifelong protection against systemic viral infections such as  the childhood diseases measles, mumps, rubella, and  chickenpox (varicella) is a function of the anamnestic (sec¬  ondary) response of IgG. For certain respiratory viruses  such as parainfluenza and respiratory syncytial viruses, the  IgA titer in respiratory secretions correlates with protec¬  tion, whereas the IgG titer does not. Unfortunately, protec¬  tion by IgA against most respiratory tract viruses usually  lasts less than 5 years.   The role of active immunity in recovery from a viral  infection is uncertain. Because recovery usually precedes  the appearance of detectable humoral antibody, immuno¬  globulins may not be important. Also, children with agam¬  maglobulinemia recover from measles infections normally  and can be immunized against measles successfully, indi¬  cating that cell-mediated immunity plays an important  role. This is supported by the observation that children  with congenital T-cell deficiency are vulnerable to severe  infections with measles virus and herpesviruses. T cells are  important in recovery from many but not all viral  illnesses.   The protection offered by active immunity can be  affected by the phenomenon of original antigenic sin. This  term refers to the observation that when a person is  exposed to a virus that cross-reacts with another virus to    which that individual was previously exposed, more anti¬  body may be produced against the original virus than  against the current one. It appears that the immunologic  memory cells can respond to the original antigenic expo¬  sure to a greater extent than to the subsequent one. This  was observed in people with antibodies to the Aj type of  influenza virus, who, when exposed to the A 2 type, pro¬  duced large amounts of antibody to A! but very little anti¬  body to the A 2 virus. It is also the underlying cause of  severe hemorrhagic dengue fever (see Chapter 42). This  phenomenon has two practical consequences as well:   (1) attempts to vaccinate people against the different influ¬  enza virus strains may be less effective than expected; and   (2) epidemiologic studies based on measurement of anti¬  body titers may yield misleading results.   How does antibody inhibit viruses? There are two main  mechanisms. The first is neutralization of the infectivity of  the virus by antibody binding to the proteins on the outer  surface of the virus. This binding has two effects: (1) It can  prevent the interaction of the virus with cell receptors, and  (2) it can cross-link the viral proteins and stabilize the virus  so that uncoating does not occur. The virus therefore can¬  not replicate. Furthermore, antibody-coated virus is more  rapidly phagocytized than normal virus, a process similar  to the opsonizing effect of antibody on bacteria. Antibody  does not degrade the virus particle; fully infectious virus  can be recovered by dissociating the virus-antibody com¬  plex. Incomplete, also called “blocking,” antibody can inter¬  fere with neutralization and form immune complexes,  which are important in the pathogenesis of certain diseases.  Some viruses, such as herpesviruses, can spread from cell  to cell across intercellular bridges, eluding the neutralizing  effect of antibody.   Antibodies that interfere with the adherence (adsorp¬  tion and penetration) of viruses to cell surfaces are called  neutralizing antibodies. Note that neutralizing antibody is  directed against the surface proteins of the virus, typically  the proteins involved with the interaction of the virus with  receptors on the surface of the host cell. Antibodies formed  against internal components of the virus (e.g., the core  antigen of hepatitis B virus) do not neutralize the infectivity  of the virus.   The second main mechanism is the lysis of virus-  infected cells in the presence of antibody and complement.  Antibody binds to new virus-specific antigens on the cell  surface and then binds complement, which enzymatically  degrades the cell membrane. Because the cell is killed  before the full yield of virus is produced, the spread of virus  is significantly reduced.   Lysis of virus-infected cells is also caused by cytotoxic T  lymphocytes. These CD8-positive T cells recognize viral  antigen only when it is presented in association with class I  MHC proteins (see Chapter 58). They kill virus-infected  cells by three methods: (1) by releasing perforins, which  make holes in the cell membrane of the infected cells; (2) by    releasing proteolytic enzymes called granzymes into the  infected cell, which degrade the cell contents; and (3) by  activating the FAS protein, which causes programmed cell  death (apoptosis).   Not all virus infections induce antibodies. Tolerance  to viral antigens can occur when the virus infection  develops in a fetus or newborn infant. The model system  in which tolerance has been demonstrated is lympho¬  cytic choriomeningitis (LCM) infection in mice. If LCM  virus is inoculated into a newborn mouse, the virus  replicates widely, but no antibodies are formed during  the lifetime of the animal. The virus is recognized as  “self,” because it was present at the time of maturation of  the immune system. If LCM virus is given to an adult  mouse, antibodies are formed normally. There is no  example of total tolerance to a virus in humans; even in  congenital rubella syndrome, in which the virus infects  the fetus, some antibody against rubella virus is made.  However, virus production and shedding can go on for  months or years.   Suppression of the cell-mediated response can occur  during infection by certain viruses. The best-known exam¬  ple is the loss of tuberculin skin test reactivity during  measles infection. Infection by cytomegalovirus or HIV  can also cause suppression. Some viruses can “downregu-  late” (reduce) the amount of class I and class II MHC pro¬  tein made by cells, which may be a mechanism by which  these viruses suppress cell-mediated immunity.   Transfer of human serum containing the appropriate anti¬  bodies provides prompt short-term immunity for indi¬  viduals exposed to certain viruses. The term passive refers  to the administration of preformed antibodies. Two types  of immune globulin preparations are used for this pur¬  pose. One has a high titer of antibody against a specific  virus, and the other is a pooled sample from plasma  donors that contains a heterogeneous mixture of antibod¬  ies with lower titers. The immune globulins are prepared  by alcohol fractionation, which removes any viruses in the  serum. The three most frequently used high-titer prepara¬  tions are used after exposure to hepatitis B, rabies, and  varicella-zoster viruses. Low-titer immune globulin is  used mainly to prevent hepatitis A in people traveling to  areas where this infection is hyperendemic.   Two specialized examples of passive immunity include  the transfer of IgG from mother to fetus across the placenta  and the transfer of IgA from mother to newborn in  colostrum.   FIGURE 33-3 Herd immunity. Immunization of the nine  people (tan color) can protect the one unimmunized person (red  color) by interrupting transmission. Immunization levels of 90%  are generally regarded as sufficient to protect the unimmunized  individual.    virtue of the other members of the population (the herd)  being incapable of transmitting the virus to that indi¬  vidual (Figure 33-3). Herd immunity can be achieved by  immunizing a population with a vaccine that interrupts  transmission, such as the live, attenuated polio vaccine,  but not with a vaccine that does not interrupt transmis¬  sion, such as the killed polio vaccine (even though it  protects the immunized individual against disease). Note  that herd immunity occurs with the live polio vaccine  primarily because it induces secretory IgA in the gut,  which inhibits infection by virulent virus, thereby pre¬  venting its transmission to others. In addition, the live  virus in the vaccine can replicate in the immunized per¬  son and spread to other members of the population,  thereby increasing the number of people protected. How¬  ever, the important feature as far as herd immunity is  concerned is the induction of IgA, which prevents  transmission.   Herd immunity can be achieved by natural infection as  well as vaccines. For example, if a viral disease, such as  measles, occurred in approximately 90% of a group, and if  those who recovered from the disease had sufficient immu¬  nity to prevent them from becoming infected and serving  as a source of virus for others, then the remaining 10% of  the group are protected by herd immunity.   • Viruses and double-stranded RNA are the most potent  inducers of interferons. Many viruses induce interferons, and  many viruses are inhibited by interferons (i.e., neither the  induction of interferons nor its action is specific).   • Interferons act by binding to a receptor on the cell surface that  signals the cell to synthesize ribonuclease, protein kinase, and  oligo A synthetase in an inactive form. Double-stranded RNA  made by the infecting virus activates these proteins. Interferons  do not enter the cell and have no effect on extracellular viruses.   • Interferons inhibit virus replication by blocking protein synthe¬  sis, primarily by degrading mRNA and by inactivating elon¬  gation factor-2.   • Alpha and beta interferons have a stronger antiviral action  than gamma interferon.The latter acts primarily as an interleu¬  kin that activates macrophages.   • Natural killer (NK) cells are lymphocytes that destroy cells  infected by many different viruses (i.e., they are nonspe¬  cific). NK cells do not have an antigen receptor on their surface,  unlike T and B lymphocytes. Rather, NK cells recognize and  destroy cells that do not display class IMHC proteins on the  surface. They kill cells by the same mechanisms as do cytotoxic  T cells (i.e., by secreting perforins and granzymes).   • Phagocytosis by macrophages and the clearance of mucus by  the cilia of the respiratory tract are also important defenses.  Damage to these defenses predisposes to viral infection.    • Increased corticosteroid levels suppress various host  defenses and predispose to severe viral infections, especially  disseminated herpesvirus infections. Malnutrition predis¬  poses to severe measles infections in developing countries.  The very young and the very old have more severe viral  infections.   • Active immunity to viral infection is mediated by both  antibodies and cytotoxicT cells. It can be elicited either by  exposure to the virus or by immunization with a viral  vaccine.   • Passive immunity consists of antibodies preformed in  another person or animal.   • The duration of active immunity is much longer than  that of passive immunity. Active immunity is measured in  years, whereas passive immunity lasts a few weeks to a  few months.   • Passive immunity is effective immediately, whereas it takes  active immunity 7 to 10 days in the primary response (or   3-5 days in the secondary response) to stimulate detectable  amounts of antibody.   • Herd immunity is the protection of an individual that results  from immunity in many other members of the population (the  herd) that interrupts transmission of the virus to the individual.  Herd immunity can be achieved either by immunization or by  natural infection of a sufficiently high percentage of the  population.   1. Regarding the mode of action of interferon, which one of the fol¬  lowing is the most accurate?   (C) It acts by inducing a ribonuclease that degrades viral mRNA.   (D) It acts by binding to the extracellular virion, thereby preventing  entry into the cell.   (E) It acts against viruses with a DNA genome but not against  viruses that have RNA as their genome.   2. Regarding immunologic aspects of viral diseases, which one of the   (A) Antibodies protect against some viral diseases by inhibiting the  synthesis of mRNA by the RNA polymerase in the virion.   (B) IgG plays a major role in neutralizing virus infectivity during  the primary infection.   (C) IgA exerts an antiviral effect by preventing virus from  infecting the mucosal cells of the respiratory and gastroin¬  testinal tracts.    (D) IgE can prevent viral infection by activating complement,  which leads to the production of the membrane attack complex.   (E) Interleukin-2 is important in protecting uninfected cells  from viral infection by inhibiting the release of virus from  infected cells.   Questions on the topics discussed in this chapter can be found  in the Basic Virology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 720. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.    Microscopic Identification  Serologic Procedures  Detection of Viral Antigens  Detection of Viral Nucleic Acids  Pearls   There are five approaches to the diagnosis of viral dis¬  eases by the use of clinical specimens: (1) identification  of the virus in cell culture, (2) microscopic identification  directly in the specimen, (3) serologic procedures to  detect a rise in antibody titer or the presence of IgM anti¬  body, (4) detection of viral antigens in blood or body  fluids, and (5) detection of viral nucleic acids in blood or  the patient’s cells.   The growth of viruses requires cell cultures because viruses  replicate only in living cells, not on cell-free media the way  most bacteria can. Because many viruses are inactivated at  room temperature, it is important to inoculate the speci¬  men into the cell culture as soon as possible; brief transport  or storage at 4°C is acceptable.   Virus growth in cell culture frequently produces a char¬  acteristic cytopathic effect (CPE) that can provide a pre¬  sumptive identification. CPE is a change in the appearance  of the virus-infected cells. This change can be in such fea¬  tures as size, shape, and the fusion of cells to form multi-  nucleated giant cells (syncytia). CPE is usually a  manifestation of virus-infected cells that are dying or dead.  The time taken for the CPE to appear and the type of cell in  which the virus produces the CPE are important clues in  the presumptive identification.    If the virus does not produce a CPE, its presence can be  detected by several other techniques:   (1) Hemadsorption (i.e., attachment of erythrocytes to  the surface of virus-infected cells). This technique is lim¬  ited to viruses with a hemagglutinin protein on their enve¬  lope, such as mumps, parainfluenza, and influenza viruses.   (2) Interference with the formation of a CPE by a sec¬  ond virus. For example, rubella virus, which does not cause  a CPE, can be detected by interference with the formation  of a CPE by certain enteroviruses, such as echovirus or  Coxsackie virus.   (3) A decrease in acid production by infected, dying  cells. This can be detected visually by a color change in the  phenol red (a pH indicator) in the culture medium. The  indicator remains red (alkaline) in the presence of virus-  infected cells but turns yellow in the presence of metabo¬  lizing normal cells as a result of the acid produced. This  technique can be used to detect certain enteroviruses.   A definitive identification of the virus grown in cell  culture is made by using known antibody in one of several  tests. Complement fixation, hemagglutination inhibition,  and neutralization of the CPE are the most frequently used  tests. Other procedures such as fluorescent antibody, radio¬  immunoassay, enzyme-linked immunosorbent assay  (ELISA), and immunoelectron microscopy are also used in  special instances. A brief description of these tests follows.  They are described in more detail in the section on  immunology.   If the antigen (the unknown virus in the culture fluid) and  the known antibody are homologous, complement will be  fixed (bound) to the antigen-antibody complex. This  makes it unavailable to lyse the “indicator” system, which is  composed of sensitized red blood cells.   If the virus and antibody are homologous, the virus is  blocked from attaching to the erythrocytes and no hemag¬  glutination occurs. Only viruses that agglutinate red blood  cells can be identified by this method.   If the virus and antibody are homologous, the antibody  bound to the surface of the virus blocks its entry into the cell.  This neutralizes viral infectivity because it prevents viral rep¬  lication and subsequent CPE formation or animal infection.   If the virus-infected cells and the fluorescein-tagged anti¬  body are homologous, the typical apple-green color of fluo¬  rescein is seen in the cells by ultraviolet (UV) microscopy.   If the virus and the antibody are homologous, there is less  antibody remaining to bind to the known radiolabeled virus.   In the ELISA test to identify a virus, known antibody is  bound to a surface. If the virus is present in the patient’s  specimen, it will bind to the antibody. A sample of the anti¬  body linked to an enzyme is added, which will attach to the  bound virus. The substrate of the enzyme is added, and the  amount of the bound enzyme is determined.   If the antibody is homologous to the virus, aggregates of  virus-antibody complexes are seen in the electron  microscope.   Viruses can be detected and identified by direct micro¬  scopic examination of clinical specimens such as biopsy  material or skin lesions. Three different procedures can be  used. (1) Light microscopy can reveal characteristic inclu¬  sion bodies or multinucleated giant cells. The Tzanck  smear, which shows herpesvirus-induced multinucleated  giant cells in vesicular skin lesions, is a good example.  (2) UV microscopy is used for fluorescent antibody stain¬  ing of the virus in infected cells. (3) Electron microscopy    detects virus particles, which can be characterized by their  size and morphology.   A rise in the titer 1 of antibody to the virus can be used to  diagnose current infection. Seroconversion is the term used  to describe the finding of antibody to a virus (or any  microbe) in a patient’s serum when the patient previously  had no antibody. Stated another way, the patient’s serum has  converted from antibody-negative to antibody-positive.   A serum sample is obtained as soon as a viral etiology is  suspected (acute-phase), and a second sample is obtained  10 to 14 days later (convalescent-phase). If the antibody  titer in the convalescent-phase serum sample is at least  fourfold higher than the titer in the acute-phase serum  sample, the patient is considered to be infected. For exam¬  ple, if the titer in the acute-phase serum sample is 1/4 and  the titer in the convalescent-phase serum sample is 1/16 or  greater, the patient has had a significant rise in antibody  titer and has been recently infected. If, however, the titer in  the convalescent-phase serum sample is 1/8, this is not a  significant rise and should not be interpreted as a sign of  recent infection.   It is important to realize that an antibody titer on a sin¬  gle sample does not distinguish between a previous infec¬  tion and a current one. The antibody titer can be determined  by many of the immunologic tests mentioned previously.  These serologic diagnoses are usually made retrospectively  because the disease has frequently run its course by the  time the results are obtained.   In certain viral diseases, the presence of IgM antibody is  used to diagnose current infection. For example, the pres¬  ence of IgM antibody to core antigen indicates infection by  hepatitis B virus.   Other nonspecific serologic tests are available. For  example, the heterophil antibody test (Monospot) can be  used to diagnose infectious mononucleosis. In the hetero-  phile test, human serum is reacted with horse or sheep red  blood cells. If the heterophile antibody is present (i.e., if the  patient has been infected with Epstein-Barr virus), then  agglutination of the red cells occurs. (See Chapter 37 for  more information.)   Viral antigens can be detected in the patient’s blood or body  fluids by various tests, but most often by an ELISA. Tests  for the p24 antigen of human immunodeficiency virus  (HIV) and the surface antigen of hepatitis B virus are com¬  mon examples of this approach.    1 Titer is a measure of the concentration of antibodies in the patients  serum. It is defined as the highest dilution of serum that gives a positive  reaction in the test. See Chapter 64 for a discussion of titer and various  serologic tests.   Viral nucleic acids (i.e., either the viral genome or viral  mRNA) can be detected in the patients blood or tissues with  complementary DNA or RNA (cDNA or cRNA) as a probe.    If only small amounts of viral nucleic acids are present in the  patient, the polymerase chain reaction can be used to amplify  the viral nucleic acids. Assays for the RNA of HIV and hepa¬  titis C virus and the DNA of hepatitis B virus in the patient’s  blood (viral load) are commonly used to monitor the course  of the disease and to evaluate the patient’s prognosis.   • The presence of a virus in a patient's specimen can be detected  by seeing a "cytopathic effect" (CPE) in cell culture. CPE is not  specific (i.e., many viruses cause it). A specific identification of  the virus usually involves an antibody-based test such as fluo¬  rescent antibody, complement fixation, or enzyme-linked  immunosorbent assay (ELISA).   • Inclusion bodies, formed by aggregates of many virus parti¬  cles, can be seen in either the nucleus or cytoplasm of infected  cells. They are not specific. Two important examples are the  nuclear inclusions formed by certain herpesviruses and the  cytoplasmic inclusions formed by rabies virus (Negri bodies).   • Multinucleated giant cells are formed by several viruses,  notably certain herpesviruses, respiratory syncytial virus, and  measles virus.   • Fluorescent antibody staining of cells obtained from the  patient or of cells infected in culture can provide a rapid, spe¬  cific diagnosis.   • Electron microscopy is not often used in clinical diagnosis but  is useful in the diagnosis of certain viruses, such as Ebola virus   1. Regarding the diagnosis of viral infections in the clinical laboratory,   (A) Cytopathic effect produced by a virus that replicates on human  foreskin cells   (B) Cytoplasmic inclusion bodies produced by a virus that repli¬  cates in the cytoplasm   (C) Multinucleated giant cells produced by a virus that replicates in  human skin cells   (D) Neutralization of infectivity using antibody against the viral  surface protein   (E) Intranuclear inclusion bodies produced by a virus that repli¬  cates in the nucleus   2. Seeing multinucleated giant cells in a Tzanck smear can be used   to make a presumptive diagnosis of infection by which one of the    that has a characteristic appearance and is dangerous to grow  in culture.   • The presence of IgM can be used to diagnose current  infection.   • The presence of IgG cannot be used to diagnose current  infection because the antibody may be due to an infection in  the past. As a result, an acute and convalescent serum sample  should be analyzed. An antibody titer that is fourfold or greater  in the convalescent serum sample compared with the acute  sample can be used to make a diagnosis.   • The presence of viral proteins, such as p24 of FHIV and  hepatitis B surface antigen, is commonly used in diagnosis.   • The presence of viral DNA or RNA is increasingly becoming  the "gold standard" in viral diagnosis. Labeled probes are highly  specific, and results are rapidly obtained. Small amounts of viral  nucleic acids can be amplified using reverse transcriptase to  produce amounts detectable by the probes. An important  example is the "viral load" assay of HIV RNA.   Questions on the topics discussed in this chapter can be found  in the Basic Virology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 720. Also see Part XIV:  USMLE (National Board) Practice Examination starting on  page 751.   Inhibitors of Herpesviruses  Inhibitors of Retroviruses  Inhibitors of Hepatitis B Virus  Inhibitors of Hepatitis C Virus  Inhibitors of Other Viruses  Inhibition of Integrase   Compared with the number of drugs available to treat  bacterial infections, the number of antiviral drugs is very  small. The major reason for this difference is the diffi¬  culty in obtaining selective toxicity against viruses;  their replication is intimately involved with the normal  synthetic processes of the cell. Despite the difficulty, sev¬  eral virus-specific replication steps have been identified  that are the site of action of effective antiviral drugs  (Table 35-1). Table 35-2 describes the mode of action of  antiviral drugs that block early events in viral replication,  and Table 35-3 describes the mode of action of antiviral  drugs that block viral nucleic acid synthesis. Figure 35-1  shows the replication of a model virus and the site of action  of drugs used to treat various viral infections. Figure 35-2  shows the replication of human immunodeficiency virus  (HIV) and the site of action of drugs used to treat HIV  infection.   Another limitation of antiviral drugs is that they are  relatively ineffective because many cycles of viral replica¬  tion occur during the incubation period when the patient  is well. By the time the patient has a recognizable systemic    viral disease, the virus has spread throughout the body and  it is too late to interdict it. Furthermore, some viruses (e.g.,  herpesviruses) become latent within cells, and no current  antiviral drug can eradicate them.   Another limiting factor is the emergence of drug-resistant  viral mutants. For example, when drug-resistant mutants of  HIV emerge, it requires that drug regimens be changed.  Also, treatment of HIV infection uses multiple drugs, often  from different classes, so that if mutants resistant to one  drug emerge, another drug will still be effective.   Amantadine (ct-adamantanamine, Symmetrel) is a  three-ring compound (Figure 35-3) that blocks the rep¬  lication of influenza A virus. It prevents replication by  inhibiting uncoating of the virus by blocking the “ion  channel” activity of the matrix protein (M2 protein) in  the virion. Absorption and penetration occur normally,  but transcription by the virion RNA polymerase does  not because uncoating cannot occur. This drug specifi¬  cally inhibits influenza A virus; influenza B and C viruses  are not affected.   Protease inhibitors of HIV: saquinavir, indinavir, ritonavir, nelfinavir, amprenavir,  atazanavir, darunavir, lopinavir, tipranavir    Despite its efficacy in preventing influenza, it is not  widely used in the United States because the vaccine is pre¬  ferred for the high-risk population. Furthermore, most  isolates have become resistant to amantadine. The main side  effects of amantadine are central nervous system altera¬  tions such as dizziness, ataxia, and insomnia. Rimantadine  (Flumadine) is a derivative of amantadine and has the same  mode of action but fewer side effects.   Enfuvirtide (Fuzeon) is a synthetic peptide that binds  to gp41 on the surface of HIV, thereby blocking the entry of  the virus into the cell. It is the first of a new class of anti-  HIV drugs known as “fusion inhibitors” (i.e., they prevent  the fusion of the viral envelope with the cell membrane).   Maraviroc (Selzentry) blocks the binding of HIV to  CCR-5—an important coreceptor for those strains of HIV  that use CCR-5 for entry into the cell. The drug binds to  CCR-5 and blocks the interaction of gpl20, an HIV enve¬  lope protein, to CCR-5 on the cell surface.   Palivizumab (Synagis) is a monoclonal antibody  directed against the fusion protein of respiratory syncytial  virus (RSV). Palivizumab neutralizes RSV by binding to the  fusion protein on the surface of RSV, thereby preventing the    virus from binding to receptors on the surface of respiratory  tract mucosal cells. It is used to prevent bronchiolitis and  pneumonia in premature or immunocompromised infants.   These drugs are analogues of nucleosides that inhibit the  DNA polymerase of one or more members of the herpesvi¬  rus family. For example, acyclovir inhibits the DNA poly¬  merase herpes simplex virus types 1 and 2 (HSV-1 and -2)  and varicella-zoster virus but not cytomegalovirus (CMV).   1. Acyclovir —Acyclovir (acycloguanosine, Zovirax) is a  guanosine analogue that has a three-carbon fragment in  place of the normal sugar, ribose, which has five carbons  (see Figure 35-3). The term acyclo refers to the fact that  the three-carbon fragment does not have a sugar ring  structure (a = without, cyclo = ring).   Inhibits fusion by binding to gp41 of human immunodeficiency virus (HIV)   Monoclonal antibody that blocks binding of viral fusion protein to receptor  on respiratory mucosal cell   1. Nucleoside inhibitors: acyclovir, ganciclovir, valacyclovir, valganciclovir,  penciclovir, famciclovir, cidofovir, trifluridine    Acyclovir is active primarily against HSV-1 and -2  and varicella-zoster virus (VZV). It is relatively non¬  toxic, because it is activated preferentially within virus-  infected cells. This is due to the virus-encoded  thymidine kinase, which phosphorylates acyclovir  much more effectively than does the cellular thymidine  kinase. Because only HSV-1, HSV-2, and VZV encode a  kinase that efficiently phosphorylates acyclovir, the drug    is active primarily against these viruses. It has no activity  against CMV. Once the drug is phosphorylated to acy¬  clovir monophosphate by the viral thymidine kinase,  cellular kinases synthesize acyclovir triphosphate, which  inhibits viral DNA polymerase much more effectively  than it inhibits cellular DNA polymerase. Acyclovir  causes chain termination because it lacks a hydroxyl  group in the 3' position.    Interferon a and p  (Interferon a used  clinically against  HBV and HCV)  Fomivirsen (CMV)    FIGURE 35-1 Replicative cycle of a model virus showing the site of action of drugs used to treat various viral infections. CMV, cytomega¬  lovirus; HBV, hepatitis B virus; HCV, hepatitis C virus; HSV, herpes simplex virus; RSV, respiratory syncytial virus; VZV, varicella-zoster virus.    Replicative cycle of human immunodeficiency virus (HIV) showing the site of action of drugs used to treat HIV infection.   ACV-TP is incorporated into the growing herpesvirus DNA chain and, acting as a chain terminator,  inhibits viral DNA polymerase.   Because no ACV-MP is made, no ACV-DP or ACV-TP can be made either.    FIGURE 35~4 Acyclovir (ACV) is phosphorylated to ACV-MP very effectively by herpesvirus-encoded thymidine kinase but very poorly by  cell-encoded thymidine kinase. The thymidine kinases encoded by herpes simplex virus (HSV)-I, HSV-2, and varicella-zoster virus (VZV) are par¬  ticularly active on ACV; the thymidine kinases encoded by cytomegalovirus and Epstein-Barr virus are not. This accounts for the selective action  of ACV in cells infected by HSV-1, HSV-2, and VZV. The fact that ACV-TP is not made in uninfected cells explains why ACV has so few side effects  (i.e., why DNA synthesis is not inhibited in uninfected cells). ACV-MP, ACV monophosphate; ACV-DP, ACV diphosphate; ACV-TP, ACV  triphosphate.    To recap, the selective action of acyclovir is based on  two features of the drug. (1) Acyclovir is phosphorylated  to acyclovir monophosphate much more effectively by  herpesvirus-encoded thymidine kinase than by cellular  thymidine kinase. It is therefore preferentially activated in  herpesvirus-infected cells and much less so in uninfected  cells, which accounts for its relatively few side effects.  (2) Acyclovir triphosphate inhibits herpesvirus-encoded  DNA polymerase much more effectively than it does cel¬  lular DNA polymerase. It therefore inhibits viral DNA  synthesis to a much greater extent than cellular DNA  synthesis (Figure 35-4).   Topical acyclovir is effective in the treatment of primary  genital herpes and reduces the frequency of recurrences  while it is being taken. However, it has no effect on latency  or on the rate of recurrences after treatment is stopped.  Acyclovir is the treatment of choice for HSV-1 encephalitis  and is effective in preventing systemic infection by HSV-1  or VZV in immunocompromised patients.   Acyclovir-resistant mutants have been isolated from  HSV-1 and VZV-infected patients. Resistance is most often  due to mutations in the gene encoding the viral thymidine  kinase. This results in reduced activity of or the total  absence of the virus-encoded thymidine kinase.   Acyclovir is well tolerated and causes few side effects—  even in patients who have taken it orally for many years to  suppress genital herpes. Intravenous acyclovir may cause  renal or central nervous system toxicity.   Derivatives of acyclovir with various properties are now  available. Valacyclovir (Valtrex) achieves a high plasma  concentration when taken orally and is used in herpes    genitalis and in herpes zoster. Penciclovir cream (Denavir)  is used in the treatment of recurrent orolabial herpes sim¬  plex. Famciclovir (Famvir), when taken orally, is converted  to penciclovir and is used to treat herpes zoster and herpes  simplex infections.   2. Ganciclovir —Ganciclovir (dihydroxypropoxymethyl-  guanine, DHPG, Cytovene) is a nucleoside analogue of  guanosine with a four-carbon fragment in place of the  normal sugar, ribose (see Figure 35-3). It is structurally  similar to acyclovir but is more active against CMV than  is acyclovir. Ganciclovir is activated by a CMV-encoded  phosphokinase in a process similar to that by which acy¬  clovir is activated by HSV. Isolates of CMV resistant to  ganciclovir have emerged, mostly due to mutations in the  UL97 gene that encodes the phosphokinase.   Ganciclovir is effective in the treatment of retinitis  caused by CMV in patients with acquired immunodefi¬  ciency syndrome (AIDS) and is useful in other dissemi¬  nated infections, such as colitis and esophagitis, caused by  this virus. The main side effects of ganciclovir are leukope¬  nia and thrombocytopenia as a result of bone marrow sup¬  pression. Valganciclovir, which can be taken orally, is also  effective against CMV retinitis.   3. Cidofovir —Cidofovir (hydroxyphosphonylmethoxy-  propylcytosine, HPMPC, Vistide) is an analogue of cyto¬  sine that lacks a ribose ring. Cidofovir does not have to be  phosphorylated and therefore is not dependent on the  action of a virus-encoded phosphokinase. It is useful in the  treatment of retinitis caused by CMV and in severe human  papillomavirus infections. It may be useful in the treatment    of severe molluscum contagiosum in immunocompro¬  mised patients. Kidney damage is the main side effect.   4. Trifluridine —(trifluorothymidine, Viroptic) is a  nucleoside analogue in which the methyl group of thymi¬  dine contains three fluorine atoms instead of three hydro¬  gen atoms (see Figure 35-3). The drug is phosphorylated  to the triphosphate by cellular kinases and incorporated  into DNA. Because it has a high frequency of mismatched  pairing to adenine, it causes the formation of faulty prog¬  eny DNA and mRNA. However, because it is incorpo¬  rated into normal cell DNA as well as viral DNA, it is too  toxic to be used systemically. It is the drug of choice for  the topical treatment of keratoconjunctivitis caused by  herpes simplex virus.   Nonnucleoside inhibitors inhibit the DNA polymerase of  herpesviruses by mechanisms distinct from the nucleoside  analogues described previously. Foscarnet is the only  approved drug in this class at this time.   1. Foscarnet —Foscarnet (trisodium phosphonoformate,  Foscavir), unlike the previous drugs, which are nucleoside  analogues, is a pyrophosphate analogue (see Figure 35-3).  It binds to DNA polymerase at the pyrophosphate cleav¬  age site and prevents removal of the phosphates from  nucleoside triphosphates (dNTP). This inhibits the addi¬  tion of the next dNTP and, as a consequence, the exten¬  sion of the DNA strand.   Foscarnet inhibits the DNA polymerases of all herpesvi¬  ruses, especially HSV and CMV. Unlike acyclovir, it does  not require activation by thymidine kinase. It is useful in  the treatment of retinitis caused by CMV, but ganciclovir is  the drug of first choice for this disease. Foscarnet is also  used to treat patients infected with acyclovir-resistant  mutants of HSV-1 and VZV.   The selective toxicity of zidovudine, lamivudine, emtric-  itabine, didanosine, zalcitabine, stavudine, abacavir, and  tenofovir is based on their ability to inhibit DNA synthesis  by the reverse transcriptase of HIV to a much greater  extent than they inhibit DNA synthesis by the DNA poly¬  merase in human cells. These drugs are collectively called  nucleoside reverse transcriptase inhibitors (NRTIs). The  effect of these drugs on the replication of HIV is depicted  in Figure 35-2.   1. Zidovudine —Zidovudine (azidothymidine, Retrovir,  AZT) is a nucleoside analogue that causes chain termina¬  tion during DNA synthesis; it has an azido group in place  of the hydroxyl group on the ribose (see Figure 35-3). It  is particularly effective against DNA synthesis by the  reverse transcriptase of HIV and inhibits the growth of    the virus in cell culture. The main adverse effects of zid¬  ovudine are bone marrow suppression and myopathy.   2. Lamivudine —Lamivudine (dideoxythiacytidine,  Epivir, 3TC) is a nucleoside analogue that causes chain  termination during DNA synthesis by the reverse tran¬  scriptase of HIV. When used in combination with AZT, it  is very effective both in reducing the viral load and in  elevating the CD4 cell count. Lamivudine is also used in  the treatment of chronic hepatitis B because it inhibits the  reverse transcriptase of hepatitis B virus. It is one of the  best tolerated of the nucleoside inhibitors, but adverse  effects such as neutropenia, pancreatitis, and peripheral  neuropathy do occur.   3. Emtricitabine —Emtricitabine (Emtriva), a derivative  of lamivudine, is also useful and well tolerated. A combi¬  nation of emtricitabine and tenofovir (Truvada) can be  used for preexposure prophylaxis for men who have sex  with men as well as for postexposure prophylaxis.   4. Didanosine —Didanosine (dideoxyinosine, Videx,  ddl) is a nucleoside analogue that causes chain termina¬  tion during DNA synthesis; it is missing hydroxyl groups  on the ribose. The administered drug ddl is metabolized  to ddATP, which is the active compound. It is effective  against DNA synthesis by the reverse transcriptase of  HIV and is used to treat patients with AIDS who are  intolerant of or resistant to zidovudine. The main adverse  effects of didanosine are pancreatitis and peripheral  neuropathy.   5. Stavudine —Stavudine (didehydrodideoxythymidine,  d4T, Zerit) is a nucleoside analogue that causes chain  termination during DNA synthesis. It inhibits DNA syn¬  thesis by the reverse transcriptase of HIV and is used to  treat patients with advanced AIDS who are intolerant of  or resistant to other approved therapies. The main  adverse effect is peripheral neuropathy.   6. Abacavir —Abacavir (Ziagen) is a nucleoside ana¬  logue of guanosine that causes chain termination during  DNA synthesis. It is available through the “expanded  access” program to those who have failed currently avail¬  able drug regimens. Abacavir is used in combination with  either a protease inhibitor, typically darunavir and ritona¬  vir, or zidovudine plus lamivudine. The main adverse  effects are liver damage and severe hypersensitivity reac¬  tions. Patients who have an HLA-B1701 allele are more  likely to have a severe hypersensitivity reaction, such as  fever, rash, or respiratory problems, to abacavir. Patients  should be tested for this gene before being prescribed  abacavir. If patients develop hypersensitivity symptoms,  abacavir should be immediately and permanently  discontinued.   7. Tenofovir —Tenofovir (Viread) is an acyclic phospho-  nate that is an analogue of adenosine monophosphate.    It is a reverse transcriptase inhibitor that acts by chain  termination. It is approved for use in patients who have  developed resistance to other reverse transcriptase inhibi¬  tors and in those who are starting treatment for the first  time. It should be used in combination with other anti-  HIV drugs. The main adverse effects are liver damage,  lactic acidosis, and renal failure.   Unlike the drugs described earlier, the drugs in this group  are not nucleoside analogues and do not cause chain ter¬  mination. The nonnucleoside reverse transcriptase inhibi¬  tors (NNRTIs) act by binding near the active site of the  reverse transcriptase and inducing a conformational  change that inhibits the synthesis of viral DNA. NNRTIs  should not be used as monotherapy because resistant  mutants emerge rapidly. Strains of HIV resistant to one  NNRTI are usually resistant to others as well. NNRTIs are  typically used in combination with one or two nucleoside  analogues.   1. Nevirapine —Nevirapine (Viramune) is usually used  in combination with zidovudine and didanosine. There is  no cross-resistance with the nucleoside inhibitors of  reverse transcriptase described previously. The main side  effect of nevirapine is a severe skin rash (Stevens-Johnson  syndrome).   2. Delavirdine —Delavirdine (Rescriptor) is effective  in combination with either zidovudine or zidovudine  plus didanosine. The main side effect of delavirdine is a  skin rash.   3. Efavirenz —Efavirenz (Sustiva) is effective in combi¬  nation with zidovudine plus lamivudine. The most com¬  mon side effects are referable to the central nervous  system, such as dizziness, insomnia, and headaches.   4. Etravirine —Etravirine (Intelence) is a second-  generation NNRTI useful in treatment-experienced  patients who have significant viremia. It is most effective  when given in combination with two protease inhibitors,  darunavir and ritonavir. The most common adverse  effect is a rash, and Stevens-Johnson syndrome has  occurred, albeit rarely.   5. Rilpivirine —Rilpivirine (Edurant) is a second-gener¬  ation NNRTI useful in treatment-naive adult patients. It  is most effective when used in combination with either  tenofovir or emtricitabine. The most common adverse  effects are depression and insomnia.   Adefovir (Hepsera) is a nucleotide analogue of adenosine  monophosphate that inhibits the DNA polymerase (reverse    transcriptase) of hepatitis B virus (HBV). It is used for the  treatment of chronic active hepatitis caused by this virus.   Entecavir (Baraclude) is a guanosine analogue that inhibits  the DNA polymerase (reverse transcriptase) of HBV. It has  no activity against the DNA polymerase (reverse transcrip¬  tase) of HIV. It is approved for the treatment of adults with  chronic HBV infection.   Telbivudine (Tyzeka) is a thymidine analogue that inhibits  the DNA polymerase (reverse transcriptase) of HBV but  has no effect on the reverse transcriptase of HIV. It is useful  in the treatment of chronic HBV infection.   Sofosbuvir (Sovaldi) is a uridine analogue that inhibits the  RNA polymerase of HCV. It acts as a chain terminating  drug. It is useful in the treatment of chronic HCV infection  caused by genotypes 1, 2, 3, and 4.   Dasabuvir is a nonnucleoside inhibitor of the RNA poly¬  merase of HCV. Its precise mode of action is uncertain as of  this writing. It is available in combination with ombitsavir  (an NS5A inhibitor), paritaprevir (a protease inhibitor) and  ritonavir (a booster of protease inhibitor activity). This  four-drug combination is called Viekira.   Ledipasvir is an inhibitor of NS5A, an RNA-binding pro¬  tein required for the activity of the RNA polymerase of  HCV. Ledipasvir is available in combination with sofosbu¬  vir (Harvoni) and is useful in the treatment of chronic  HCV infection caused by genotype 1.   Ombitsavir is another NS5A inhibitor that is available in  combination with dasabuvir (a polymerase inhibitor), pari¬  taprevir (a protease inhibitor) and ritonavir (a booster of  protease inhibitor activity). This four-drug combination is  called Viekira.   Ribavirin (Virazole) is a nucleoside analogue in which a  triazole-carboxamide moiety is substituted in place of the  normal purine precursor aminoimidazole-carboxamide  (see Figure 35-3). The drug inhibits the synthesis of gua¬  nine nucleotides, which are essential for both DNA and  RNA viruses. It also inhibits the 5' capping of viral mRNA.  Ribavirin aerosol is used clinically to treat pneumonitis  caused by respiratory syncytial virus (RSV) in infants and  to treat severe influenza B infections.   Raltegravir (Isentress) is an integrase inhibitor (i.e., it  blocks the HIV-encoded integrase that mediates the inte¬  gration of the newly synthesized viral DNA into host cell  DNA). Two additional integrase inhibitors are available:  dolutegravir (Tivicay) and elvitegravir (Stribild).   Members of the protease inhibitor (PI) class of drugs, such  as saquinavir (Invirase, Fortovase), indinavir (Crixivan),  ritonavir (Norvir), lopinavir/ritonavir (Kaletra), atazanavir  (Reyataz), tipranavir (Aptivus), amprenavir (Agenerase)  and its prodrug fosamprenavir (Lexiva), darunavir (Prezista),  and nelfinavir (Viracept), inhibit the protease encoded by  HIV (Figure 35-5). The protease cleaves the gag and pol  precursor polypeptides to produce several nucleocapsid  proteins (e.g., p24) and enzymatic proteins (e.g., reverse  transcriptase) required for viral replication. These inhibi¬  tors contain peptide bonds that bind to the active site of the   FIGURE 35-5 Structure of the protease inhibitor saquinavir.  Note the presence of several peptide bonds, which interact with the  active site of the protease. An arrow indicates one of the peptide  bonds.    viral protease, thereby preventing the protease from cleav¬  ing the viral precursor. These drugs inhibit production of  infectious virions but do not affect the proviral DNA and  therefore do not cure the infection.   Monotherapy with Pis should not be used because resis¬  tant mutants emerge rapidly. These drugs typically are  prescribed in combination with reverse transcriptase inhib¬  itors, such as zidovudine and lamivudine. Ritonavir is typi¬  cally used in combination with another PI, as in the  commonly used combination lopinavir/ritonavir (Kaletra).  Ritonavir inhibits the enzymes that metabolize the other  PI, which effectively raise the concentration of the other  drug (e.g., lopinavir in the Kaletra combination). Ritonavir  “boosts” lopinavir is the way to remember it.   The side effects of Pis include nausea, diarrhea, and  abnormal fat accumulation in the back of the neck that can  result in a “buffalo hump” appearance. These abnormal fat  deposits can be disfiguring and cause patients to stop tak¬  ing the drug. The fat deposits are a type of lipodystrophy;  the metabolic process by which this occurs is unknown.  Indinavir can cause kidney stones; thus extra water should  be consumed to reduce the likelihood of stone formation.   Boceprevir (Victrelis), simeprevir (Olysio), telaprevir  (Incivek), and paritaprevir (component of Viekira) are Pis  that block a serine protease required for the replication of  hepatitis C virus. They are approved for the treatment of  chronic hepatitis C caused by hepatitis C virus (genotype 1)  in combination with peginterferon and ribavirin. The  most important adverse effect of these drugs is anemia.  Paritaprevir is an inhibitor of HCV protease that is avail¬  able in combination with ombitsavir (an NS5A inhibitor),  dasabuvir (a polymerase inhibitor), and ritonavir (a booster  of protease inhibitor activity). This four-drug combination  is called Viekira.   The mode of action of interferon is described in Chapter 33.  Recombinant alpha interferon is effective in the treatment  of some patients with chronic hepatitis B and chronic hepa¬  titis C infections. Note that the use of alpha interferon and  pegylated alpha inteferon (see next paragraph) for chronic  HCV infection has been significantly reduced due to the  availability of newer less toxic drug regimens.   It also causes regression of condylomata acuminata  lesions caused by human papillomavirus and the lesions of  Kaposi’s sarcoma caused by human herpesvirus-8.   Pegylated interferon (peginterferon), which is alpha  interferon conjugated to polyethylene glycol, is used for the  treatment of chronic hepatitis B and C. The advantage of   Prevention of disseminated CMV disease in immunocompromised  patients, especially retinitis in AIDS patients   Prevention of bronchiolitis and pneumonia caused by respiratory syncytial  virus in infants    AIDS = acquired immunodeficiency syndrome; CMV = cytomegalovirus; HIV = human immunodeficiency virus; HSV = herpes simplex virus; VZV = varicella-zoster virus.    pegylated interferon is that it has a longer half-life than  unconjugated alpha interferon and can be administered  once a week instead of three times a week.   Fomivirsen (Vitravene) is an antisense DNA that blocks the  replication of CMV. Antisense DNA is a single-stranded  DNA whose base sequence is the complement of the viral  mRNA. Antisense DNA binds to the mRNA within the  infected cell and prevents it from being translated into viral  protein. Fomivirsen is approved for the intraocular treat¬  ment of CMV retinitis. It is the first and, at present, the  only antisense molecule to be approved for the treatment of  human disease.   Oseltamivir (Tamiflu), zanamivir (Relenza), and peramivir  (Rapivab) inhibit the neuraminidase of influenza virus.    This enzyme is located on the surface of influenza virus  and is required for the release of the virus from infected  cells. Inhibition of release of influenza virus limits the  infection by reducing the spread of virus from one cell to  another. These drugs are effective against both influenza  A and B viruses, in contrast to amantadine, which is  effective only against influenza A virus. These drugs are  effective against strains of influenza virus resistant to  amantadine.   In most instances, the antiviral agents described in this  chapter are used to treat infectious diseases. However, there  are times when they are used to prevent diseases from  occurring—a process called chemoprophylaxis. Table 35-4  describes the drugs used for this purpose and the situations  in which they are used. For more information, see the  chapters on the individual viruses.    • Selective toxicity is the ability of a drug to inhibit viral replica¬  tion without significantly damaging the host cell. It is difficult  to achieve a high degree of selective toxicity with antiviral  drugs because the virus can only replicate within cells and uses  many cellular functions during replication.   • Amantadine inhibits the uncoating of influenza A virus by  blocking "ion channel" activity of the viral matrix protein  (M2 protein).The drug has no effect on influenza B or C viruses.    • Maraviroc inhibits the binding of the gpl 20 of HIV to the CCR-5  receptor on the cell.   • Enfuvirtide is a "fusion inhibitor." It inhibits the fusion of HIV  with the cell membrane by binding to gp41, an envelope pro¬  tein of HIV.   • Acyclovir inhibits the DNA polymerase of herpes simplex  virus (HSV) type 1, HSV-2, and varicella-zoster virus (VZV).    virus-encoded thymidine kinase that phosphorylates the  drug. Acyclovir is not phosphorylated in uninfected cells, and  cellular DNA synthesis is not inhibited. Selective toxicity is high,  and there are very few adverse effects.   • Acyclovir is a chain-terminating drug because it lacks a  hydroxyl group in the 3' position. It does not have a ribose ring  (i.e., it is acydo, meaning without a ring). The absence of this  hydroxyl group means the next nucleoside triphosphate can¬  not be added and the replicating DNA chain is terminated.   • Acyclovir inhibits viral replication but has no effect on the  latency of HSV-1, HSV-2, and VZV.   • Ganciclovir action is very similar to that of acyclovir, but it is effec¬  tive against cytomegalovirus (CMV), whereas acyclovir is not.   • Foscarnet inhibits the DNA polymerase of all herpesviruses  but is clinically useful against HSV and CMV. It is a pyrophos¬  phate analogue that inhibits the cleavage of pyrophosphate  from the nucleoside triphosphate that has been added to the  growing DNA chain.   • Zidovudine inhibits the DNA polymerase (reverse transcrip¬  tase) of HIV. It is a chain-terminating drug because it has an  azide group in place of the hydroxyl group in the 3' position.  Unlike acyclovir, it does not require a viral-encoded kinase to be  phosphorylated. Cellular kinases phosphorylate the drug, so it is  active in uninfected cells and significant adverse effects can occur.   • Other drugs with the same mode of action include lamivudine,  didanosine, emtricitabine, stavudine, abacavir, and tenofovir.   • Adefovir, entecavir, lamivudine, and telbivudine inhibit the  DNA polymerase of hepatitis B virus (HBV). These drugs are  useful in the treatment of chronic HBV infection.   • Sofosbuvir inhibits RNA polymerase and is useful in the  treatment of chronic HCV infection.   • Ledipasvir inhibits NS5A protein of HCV thereby inhibiting  synthesis of the progeny RNA genomes of HCV.   • Ribavirin is a guanosine analogue that can inhibit nucleic acid  synthesis of several viruses. It is useful in severe respiratory  syncytial virus infections.   • Raltegravir, elvitegravir, and dolutegravir inhibit the integrase  encoded by HIV, which blocks the integration of HIV DNA into  host cell DNA.   • Indinavir and other similar drugs inhibit the virus-encoded  protease of HIV. Inhibition of the protease prevents cleavage  of precursor polypeptides, which prevents formation of the  structural proteins of the virus. Synthesis of infectious virus is  inhibited, but the viral DNA integrated into the host cell DNA is  unaffected.   • Boceprevir, simeprevir, telaprevir, and paritaprevir inhibit the  protease of hepatitis C virus.   • Interferons inhibit virus replication by degrading mRNA  and blocking protein synthesis. (See Chapter 33 for more  information.) Pegylated interferon-alpha is used in the treat¬  ment of chronic hepatitis B and acute hepatitis C.   • Fomivirsen is an antisense DNA that binds to the mRNA of CMV,  which prevents the mRNA from being translated into viral  proteins.   • Zanamivir and oseltamivir inhibit the neuraminidase of  both influenza A and B viruses. This inhibits the release  of progeny virus, which reduces spread of virus to neigh¬  boring cells.   1. Regarding the mode of action of antiviral drugs, which one of the   (A) Amantadine inhibits the virus-encoded DNA polymerase that  is required to synthesize viral progeny DNA.   (B) Lamivudine inhibits the cell-encoded RNA polymerase that is  required to synthesize viral genome.   (C) Raltegravir inhibits the translation of viral mRNA into viral  proteins.    (D) Ritonavir inhibits the virus-encoded protease that is required  to cleave viral precursor polypeptides into functional proteins.   (E) Zidovudine inhibits the virus-encoded RNA polymerase that is  required to synthesize viral mRNA.   2. Which one of the following best describes the action of oseltami¬  vir (Tamiflu)?    (D) Inhibits viral protein synthesis by binding to the 60S ribosomal  subunit   (E) Inhibits the neuraminidase required for release of virus from  the infected cell   3. Which one of the following is a well-described adverse effect of the   4. Regarding acyclovir, which one of the following is the MOST   (B) It terminates the latent state of both herpes simplex virus type  1 and type 2.   (C) It inhibits the virus-encoded thymidine kinase that is required  to synthesize viral DNA.   (D) Resistance to acyclovir is primarily caused by proton pumps  that export the drug from the cell.   (E) It is a chain-terminating drug because it does not have a com¬  plete ribose ring and therefore lacks a hydroxyl group in the  correct position.   Questions on the topics discussed in this chapter can be found  in the Basic Virology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 720. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   Because few drugs are useful against viral infections, pre¬  vention of infection by the use of vaccines is very impor¬  tant. Prevention of viral diseases can be achieved by the use  of vaccines that induce active immunity or by the adminis¬  tration of preformed antibody that provides passive  immunity.   There are two types of vaccines that induce active immu¬  nity: those that contain live virus whose pathogenicity has  been attenuated and those that contain killed virus. An  attenuated virus is one that is unable to cause disease, but  retains its antigenicity and can induce protection.   Some vaccines, such as the hepatitis B vaccine, contain  purified viral proteins and are often called subunit vac¬  cines. The features of subunit vaccines resemble those of  killed vaccines because no viral replication occurs in these  vaccines. The attributes of live and killed vaccines are listed  in Table 36-1.   In general, live vaccines are preferred to vaccines con¬  taining killed virus because their protection is greater and  longer-lasting. With live vaccines, the virus multiplies in the  host, producing a prolonged antigenic stimulus, and IgA and  IgG are elicited when the vaccine is administered by the  natural route of infection (e.g., when polio vaccine is given  orally). Killed vaccines, which are usually given intramuscu¬  larly, do not stimulate a major IgA response. Killed vaccines  typically do not stimulate a cytotoxic T-cell response, because  the virus in the vaccine does not replicate. In the absence of    replication, no viral epitopes are presented in association  with class I MHC proteins, and the cytotoxic T-cell response  is not activated (see Chapter 58). Although live vaccines  stimulate a long-lasting response, booster doses are now  recommended with measles and polio vaccines.   One unique form of a live, attenuated viral vaccine is the  influenza vaccine that contains a temperature-sensitive  mutant of the virus as the immunogen. The temperature-  sensitive mutant will replicate in the cooler air passages of  the nose, where it induces IgA-based immunity, whereas it  will not replicate in the warmer lung tissue and therefore  will not cause disease.   (1) They are composed of attenuated viral mutants,  which can revert to virulence either during vaccine pro¬  duction or in the immunized person. Reversion to viru¬  lence during production can be detected by quality control  testing, but there is no test to predict whether reversion will  occur in the immunized individual. Of the commonly used  live vaccines, only polio vaccine has had problems regard¬  ing revertants; measles, mumps, rubella, and varicella vac¬  cines have not.   Even if the virus in the live vaccine does not revert, it  can still cause disease because, although attenuated (weak¬  ened), it can still be pathogenic in a host with reduced  immunity. For this reason, live viral vaccines should not be  given to immunocompromised people or to pregnant  women because the fetus may become infected.   (2) The live vaccine can be excreted by the immunized  person. This is a double-edged sword. It is advantageous if  the spread of the virus successfully immunizes others, as  occurs with the live polio vaccine. However, it could be a  problem if, for example, a virulent poliovirus revertant  spreads to a susceptible person. Rare cases of paralytic  polio occur in the United States each year by this route of  infection.   (3) A second virus could contaminate the vaccine if it  was present in the cell cultures used to prepare the vac¬  cine. This concern exists for both live and killed vaccines,  although, clearly, the live vaccine presents a greater prob¬  lem, because the process that inactivates the virus in the  killed vaccine could inactivate the contaminant as well. It  is interesting, therefore, that the most striking incidence  of contamination of a vaccine occurred with the killed  polio vaccine. In 1960, it was reported that live simian  vacuolating virus 40 (SV40 virus), an inapparent “passen¬  ger” virus in monkey kidney cells, had contaminated some  lots of polio vaccine and was resistant to the formaldehyde  used to inactivate the poliovirus. There was great concern  when it was found that SV40 virus causes sarcomas in a  variety of rodents. Fortunately, it has not caused cancer in  the individuals inoculated with the contaminated polio  vaccine.   Certain viral vaccines, namely, influenza, measles,  mumps, and yellow fever vaccines, are grown in chick  embryos. These vaccines should not be given to those who  have had an anaphylactic reaction to eggs. People with  allergies to chicken feathers can be immunized.   In addition to the disadvantages of the killed vaccines  already mentioned—namely, that they induce a shorter  duration of protection, are less protective, and induce  fewer IgA antibodies—there is the potential problem that  the inactivation process might be inadequate. Although  this is rare, it happened in the early days of the manufacture  of the killed polio vaccine. However, killed vaccines do  have two advantages: They cannot revert to virulence, and    they are more heat-stable. Therefore, they can be used  more easily in tropical climates.   Most viral vaccines are usually given before a known  exposure (i.e., they are administered preexposure). How¬  ever, there are two vaccines, the vaccines against rabies and  hepatitis B that are also effective when given postexposure  because the incubation period of these diseases is long  enough that the vaccine-induced immunity can prevent  the disease. Thus the rabies vaccine is most often used in  people after they have received a bite from a potentially  rabid animal, and the hepatitis B vaccine is used in people  who have sustained a needle-stick injury.   The prospect for the future is that some of the disadvan¬  tages of current vaccines will be bypassed by the use of  purified viral antigens produced from genes cloned in  either bacteria or yeasts by recombinant DNA techniques.  The advantages of antigens produced by the cloning  process are that they contain no viral nucleic acid and  so cannot replicate or revert to virulence, they have no  contaminating viruses from cell culture, and they can be  produced in large amounts. A disadvantage of these cloned  vaccines is that they are unlikely to stimulate a cytotoxic  T-cell response because no viral replication occurs. The  two viral subunit vaccines containing purified anti¬  gens produce by recombinant DNA techniques are the  hepatitis B virus (HBV) vaccine and the human papilloma  virus (HPV) vaccine.   Another prospect for the future is the use of “DNA vac¬  cines.” These vaccines contain purified DNA encoding the  appropriate viral proteins genetically engineered into a  viral vector or plasmid. Immunization with this composite  DNA elicits both antibody and cytotoxic T cells and pro¬  tects against disease in experimental animals.   Certain live viral vaccines, such as the vaccines contain¬  ing vaccinia virus, adenovirus, and poliovirus, are being  used experimentally to immunize against other viruses  such as HIV. This is done by splicing the HIV gene into the  live viral genome and then infecting the experimental ani¬  mal with the constructed virus. The advantage of this pro¬  cedure is that a cytotoxic T-cell response is elicited (because  the virus is replicating), whereas if the purified antigen  alone were used to immunize the animal, an antibody  response but not a cytotoxic T-cell response would be  elicited.   The viral vaccines currently in use are described in  Table 36-2. The vaccines, both viral and bacterial, recom¬  mended for children from 0 to 6 years of age are listed in  Table 36-3.   Passive immunity is provided by the administration of  preformed antibody in preparations called immune globu¬  lins. The immune globulins useful in the prevention of viral  diseases are described next. Passive-active immunity is    ’There are two vaccines that contain live varicella-zoster virus: one that prevents varicella (Varivax) and another that prevents zoster (Zostavax) (see Chapter 37).   2 Only the killed vaccine is recommended for routine immunizations in the United States.   h'he live vaccine contains a temperature-sensitive mutant of influenza virus. The killed vaccine contains two purified protein subunits (hemagglutinin and neuraminidase)  obtained after the virus is chemically inactivated.   7 Used for military personnel and certain medical personnel such as "first responders" and emergency room staff.    induced by giving both immune globulins to provide  immediate protection and a vaccine to provide long-term  protection. This approach is described in the sections on  rabies and hepatitis B. The following preparations are  available:   (1) Rabies immune globulin (RIG) is used in the pre¬  vention of rabies in people who may have been exposed to  the virus. It is administered by injecting as much RIG as  possible into the tissue at the bite site, and the remainder is  given intramuscularly. The preparation contains a high  titer of antibody made by hyperimmunizing human volun¬  teers with rabies vaccine. RIG is obtained from humans to  avoid hypersensitivity reactions. In addition to RIG, the    vaccine containing killed rabies virus made in human dip¬  loid cells should be given. RIG and the vaccine should be  given at different sites. This is an example of passive-active  immunization.   (2) Hepatitis B immune globulin (HBIG) is used in  the prevention of hepatitis B in people who may have  been exposed to the virus either by needle-stick or as a  neonate born of a mother who is a carrier of hepatitis B  virus. The preparation contains a high titer of antibody to  hepatitis B virus and is obtained from humans to avoid  hypersensitivity reactions. HBIG is often used in conjunc¬  tion with hepatitis B vaccine, an example of passive-active  immunization.   2 A complete description of the vaccine schedule is available on the Centers for Disease Control and Prevention Web site, www.cdc.gov.  3 Human papillomavirus vaccine is recommended for females aged 9-26 years.    (3) Varicella-zoster immune globulin (VZIG) is used in  the prevention of disseminated zoster in people who may  have been exposed to the virus and who are immunocom¬  promised. The preparation contains a high titer of antibody  to varicella-zoster virus and is obtained from humans to  avoid hypersensitivity reactions.   (4) Vaccinia immune globulins (VIG) can be used to treat  some of the complications of the smallpox vaccination.   (5) Immune globulins (IGs) are useful in the prevention  (or mitigation) of hepatitis A or measles in people who  may have been exposed to these viruses. IGs are commonly  used prior to traveling to areas of the world where hepatitis  A virus is endemic. IGs contain pooled serum obtained  from a large number of human volunteers who have not  been hyperimmunized. The effectiveness of IG is based on  antibody being present in many members of the pool.   Herd immunity (also known as community immunity)  occurs when a sufficiently large percentage of the popula¬  tion (the “herd”) is immunized so that an unimmunized  individual is protected (see Chapter 33). For herd immu¬  nity to occur, the vaccine must prevent transmission of the  virus as well as prevent disease. For example, the live,  attenuated polio vaccine can provide good herd immunity  because it induces intestinal IgA, which prevents poliovirus  from replicating in the gastrointestinal tract and being  transmitted to others. However, the killed polio vaccine  does not induce herd immunity because secretory IgA is  not produced, and immunized individuals (although pro¬  tected from poliomyelitis) can still serve as a source of  poliovirus for others.   • Active immunity can be elicited by vaccines containing killed  viruses, purified protein subunits, or live, attenuated (weak¬  ened) viruses.   • In general, live viral vaccines are preferable to killed vac¬  cines for three reasons: (1) they induce a higher titer of anti¬  body and hence longer-lasting protection; (2) they induce a  broader range of antibody (e.g., both IgA and IgG, not just  IgG); and (3) they activate cytotoxic T cells, which kill virus-  infected cells.   • There are some potential problems with live viral vaccines,  the most important of which is reversion to virulence.   Transmission of the vaccine virus to others who may be immu¬  nocompromised is another concern. Also there may be a sec¬  ond, unwanted virus in the vaccine that was present in the cells  used to make the vaccine virus. This second virus may cause  adverse effects.   • Live viral vaccines should not be given to immunocompro¬  mised individuals or to pregnant women.   • Vaccines grown in chick embryos, especially influenza vaccine,  should not be given to those who have had an anaphylactic  reaction to eggs.   • Passive immunity is immunity acquired by an individual by  the transfer of preformed antibodies made in either other  humans or in animals. These antibody preparations are often  called immune globulins. Passive immunity also occurs    naturally when IgG is transferred from the mother to the fetus  across the placenta and when IgA is transferred from the  mother to the newborn in colostrum.   • The main advantage of passive immunity is that it provides  immediate protection. The main disadvantage is that it does  not provide long-term protection (i.e., it is active only for a few  weeks to a few months).   • Immune globulin preparations against rabies virus, hepatitis  A virus, hepatitis B virus, and varicella-zoster virus are  effective.   • Passive-active immunity consists of administering both  immune globulins and a viral vaccine. This provides both  immediate as well as long-term protection. For example, pro¬  tection against rabies in an unimmunized person who has  been bitten by a potentially rabid animal consists of both  rabies immune globulins and the rabies vaccine.   • Herd immunity is the protection of an individual that  results from immunity in many other members of the popu¬  lation (the"herd") that interrupts transmission of the virus to  the individual. Flerd immunity can be achieved either by  active immunization or by natural infection of a sufficiently  high percentage of the population. Flerd immunity is  unlikely to be achieved by passive immunity because,  although antibodies can protect the individual against  spread of virus through the bloodstream, they are unlikely  to prevent viral replication at the portal of entry and conse¬  quent transmission to others.   (A) Killed vaccines induce a longer lasting response than do live,  attenuated vaccines.   (B) Killed vaccines are no longer used in this country because they  do not induce secretory IgA.   (C) Killed vaccines induce a broader range of immune responses  than do live, attenuated vaccines.   (D) Killed vaccines are safer to give to immunocompromised  patients than are live, attenuated vaccines.   2. Individuals who have had an anaphylactic reaction to egg proteins  should NOT receive which one of the following vaccines?   3. Induction of passive-active immunity is useful in the prevention  of which one of the following sets of two viral diseases?    4. Protection of the unimmunized individual based on immuniza¬  tion of a sufficient number of other members of the population is  a description of which one of the following?   Questions on the topics discussed in this chapter can be found  in the Basic Virology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 720. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.    Most of the clinically important viral pathogens can be catego¬  rized into groups according to their structural characteristics  (i.e., DNA enveloped viruses, DNA nonenveloped 1 viruses,  RNA enveloped viruses, and RNA nonenveloped viruses)  (see Chapters 37-40 and Table IV-1).   However, some viruses (e.g., arboviruses, tumor viruses,  and slow viruses) (see Chapters 41-45) are described best  in terms of their biologic features. Several clinically less  prominent viruses (e.g., parvoviruses and coronaviruses)  are described in Chapter 46. An overview of the viruses in  the four structural categories follows.   These viruses are noted for their ability to cause latent  infections. This family includes (1) herpes simplex virus  types 1 and 2, which cause painful vesicles on the face and  genitals, respectively; (2) varicella-zoster virus, which  causes varicella (chickenpox) typically in children and,  when it recurs, zoster (shingles); (3) cytomegalovirus, an  important cause of congenital malformations; (4) Epstein-  Barr virus, which causes infectious mononucleosis; and  (5) human herpesvirus 8, which causes Kaposi’s sarcoma.  (See Chapter 37.)   This virus is one of the important causes of viral hepatitis.  In contrast to hepatitis A virus (an RNA nucleocapsid  virus), hepatitis B virus causes a more severe form of hepa¬  titis, results more frequently in a chronic carrier state, and  is implicated in the induction of hepatocellular carcinoma,  the most common cancer worldwide. (See Chapter 41.)   Poxviruses are the largest and most complex of the viruses.  The disease smallpox has been eradicated by effective use  of the vaccine. Molluscum contagiosum virus is the only  poxvirus that causes human disease in the United States at  this time. (See Chapter 37.)   These viruses are best known for causing upper and lower  respiratory tract infections, including pharyngitis and  pneumonia. (See Chapter 38.)   These viruses cause papillomas on the skin and mucous  membranes of many areas of the body. Some types are  implicated as a cause of cancer (e.g., carcinoma of the cer¬  vix). (See Chapter 38.)   This virus causes “slapped cheeks” syndrome, hydrops feta¬  lis, and severe anemia, especially in those with hereditary  anemias such as sickle cell anemia. (See Chapter 38.)   (1) Influenza A and B viruses. Influenza A virus is the  major cause of recurrent epidemics of influenza.   (2) Parainfluenza viruses. These viruses are the leading  cause of croup in young children and an important cause  of common colds in adults.   (3) Respiratory syncytial virus. This virus is the leading  cause of bronchiolitis and pneumonia in infants. (See  Chapter 39.)   Herpesviruses (herpes simplex virus types 1 and 2, varicella-zoster virus, cytomegalovirus, Epstein-Barr virus, human  herpesvirus 8), hepatitis B virus, smallpox virus   Influenza virus, parainfluenza virus, respiratory syncytial virus, measles virus, mumps virus, rubella virus, rabies virus,  human T-cell lymphotropic virus, human immunodeficiency virus, hepatitis C virus   Enteroviruses (poliovirus, coxsackievirus, echovirus, hepatitis A virus), rhinovirus, rotavirus, noroviruses, hepatitis E  virus   These viruses cause well-known childhood diseases and are  the viral components of the MMR vaccine. Widespread use  of the vaccine has markedly reduced the incidence of these  diseases in the United States. These viruses are well known  for the complications associated with the diseases they  cause (e.g., rubella virus infection in a pregnant woman can  cause congenital malformations). (See Chapter 39.)   This virus causes almost invariably fatal encephalitis follow¬  ing the bite of a rabid animal. In the United States, wild ani¬  mals such as skunks, foxes, raccoons, and bats are the major  sources, but human infection is rare. (See Chapter 39.)   This virus causes hepatitis C, the most prevalent form of  viral hepatitis in the United States. It causes a very high rate  of chronic carriers and predisposes to chronic hepatitis and  hepatic carcinoma.   This virus causes T-cell leukemia in humans. It also causes  an autoimmune disease called tropical spastic paraparesis.  (See Chapter 43.)   Human immunodeficiency virus (HIV) causes acquired  immunodeficiency syndrome (AIDS). (See Chapter 45.)   These viruses infect the enteric tract and are transmitted by  the fecal-oral route. Poliovirus rarely causes disease in the  United States because of the vaccine but remains an impor¬  tant cause of aseptic meningitis and paralysis in developing  countries. Of more importance in the United States are  Coxsackie viruses, which cause aseptic meningitis,    myocarditis, and pleurodynia; and echoviruses, which  cause aseptic meningitis. (See Chapter 40.)   These viruses are the most common cause of the common  cold. They have a large number of antigenic types, which  may account for their ability to cause disease so frequently.  (See Chapter 40.)   These viruses possess an unusual genome composed of  double-stranded RNA in 11 segments. Rotaviruses are an  important cause of viral gastroenteritis in young children.  (See Chapter 40.)   This virus is an important cause of hepatitis. It is an entero¬  virus but is described in this book in conjunction with  hepatitis B virus. It is structurally different from hepatitis B  virus, which is a DNA enveloped virus. Furthermore, it is  epidemiologically distinct (i.e., it primarily affects children,  is transmitted by the fecal-oral route, and rarely causes a  prolonged carrier state). (See Chapter 41.)   Noroviruses are a common cause of gastroenteritis, espe¬  cially in adults. They are a well-known cause of outbreaks  of vomiting and diarrhea in hospitals, nursing homes, and  on cruise ships (see Chapter 40).   The main human pathogen in the hepevirus family is hepatitis  E virus (HEV). It causes hepatitis acquired by fecal-oral trans¬  mission similar to hepatitis A virus. HEV is a nonenveloped  virus with a positive-polarity single-stranded RNA genome.   Chapter 42 describes the large and varied group of arbovi¬  ruses, which have the common feature of being transmitted  by an arthropod. Chapter 43 covers tumor viruses, and    TABLE IV-2 The 10 Most Common Notifiable Viral  Diseases in the United States in 2012 1    Chapter 44 covers the “slow” viruses that cause degenera¬  tive central nervous system diseases primarily. Chapter 45  describes HIV, the cause of AIDS. The less common viral  pathogens are described in Chapter 46.   Table I V-2 lists the frequency of the 10 most common  notifiable viral diseases in the United States for 2012 (the  latest year for which complete data are available). Note that  the common cold, which is probably the most frequent  disease, is not listed because it is not a notifiable disease.   Molluscum Contagiosum Virus  HEPADNAVIRUSES  Hepatitis B Virus  Self-Assessment Questions  Summaries Of Organisms   The herpesvirus family contains six important human  pathogens: herpes simplex virus types 1 and 2, varicella-  zoster virus, cytomegalovirus, Epstein-Barr virus, and  human herpesvirus 8 (the cause of Kaposi’s sarcoma).   All herpesviruses are structurally similar. Each has an  icosahedral core surrounded by a lipoprotein envelope  (Figure 37-1). The genome is linear double-stranded  DNA. The virion does not contain a polymerase. They are  large (120-200 nm in diameter), second in size only to  poxviruses.   They replicate in the nucleus, form intranuclear inclu¬  sions, and are the only viruses that obtain their envelope by  budding from the nuclear membrane. The virions of her¬  pesviruses possess a tegument located between the nucleo-  capsid and the envelope. This structure contains regulatory  proteins, such as transcription and translation factors,  which play a role in viral replication.   Herpesviruses are noted for their ability to cause latent  infections. In these infections, the acute disease is followed  by an asymptomatic period during which the virus remains  in a quiescent (latent) state. When the patient is exposed to  an inciting agent or immunosuppression occurs, reactivation  of virus replication and disease can occur. 1 With some    'Note the similarity between latency with herpesviruses and lysogeny  with bacteriophage (discussed in Chapter 29).    herpesviruses (e.g., herpes simplex virus), the symptoms of  the subsequent episodes are similar to those of the initial  one; however, with others (e.g., varicella-zoster virus), they  are different (Table 37-1).   Three HSV virions are visible. Short arrow points to the envelope of an  HSV virion. Long arrow points to the nucleocapsid of the virion. The  dark area between the inner nucleocapsid and the outer envelope is  the tegument. (Source: Dr. John Hierholzer, Public Health Image Library, Centers for  Disease Control and Prevention.)   Intrauterine infection, transfusions,  sexual contact, via secretions  (e.g., saliva and urine)    CMV = cytomegalovirus; EBV = Epstein-Barr virus; HHV-8 = human herpesvirus 8; HSV = herpes simplex virus; VZV = varicella-zoster virus.  ’Primary infection is often asymptomatic.   2 ln immunocompromised patients, dissemination of virus can cause life-threatening disease.   6 A mononucleosis-like syndrome has been described. Kaposi's sarcoma itself also can result from a primary infection.    Some information is available regarding the mechanism  by which herpes simplex virus (HSV) and cytomegalovirus  (CMV) initiate and maintain the latent state. Shortly after  HSV infects neurons, a set of “latency-associated tran¬  scripts” (LATS) are synthesized. These noncoding, regula¬  tory RNAs suppress viral replication. The precise  mechanism by which they do so is unknown. The process  by which latency is terminated and reactivation of viral  replication occurs is unclear, but various triggers such as  sunlight, fever, and stress are known. CMV establishes  latency by producing microRNAs that inhibit the transla¬  tion of mRNAs required for viral replication. Also, the  CMV genome encodes a protein and an RNA that have the  ability to inhibit apoptosis in infected cells. Inhibition of  apoptosis allows the infected cell to survive.   Three of the herpesviruses, HSV types 1 and 2 and var¬  icella-zoster virus (VZV), cause a vesicular rash, both in  primary infections and in reactivations. Primary infections  are usually more severe than reactivations. The other two  herpesviruses, CMV and Epstein-Barr virus (EBV), do not  cause a vesicular rash.   Four herpesviruses, namely HSV types 1 and 2, VZV,  and CMV, induce the formation of multinucleated giant  cells, which can be seen microscopically in the lesions. The  importance of giant cells is best illustrated by the Tzanck  smear, which reveals multinucleated giant cells in a smear  taken from the painful vesicles of the genitals caused by  HSV type 2 (Figure 37-2).   The herpesvirus family can be subdivided into three  categories based on the type of cell most often infected and  the site of latency. The alpha herpesviruses, consisting of    HSV types 1 and 2 and VZV, infect epithelial cells primarily  and cause latent infection in neurons. The beta herpesvi¬  ruses, consisting of CMVs and human herpesvirus 6, infect  and become latent in a variety of tissues. The gamma her¬  pesviruses, consisting of EBV and human herpesvirus 8  (HHV-8, Kaposi’s sarcoma-associated virus), infect and  become latent primarily in lymphoid cells. Table 37-2  describes some important clinical features of the common  herpesviruses.   Certain herpesviruses are associated with or cause can¬  cer in humans (e.g., Epstein-Barr virus is associated with   giant cells in Tzanck smear. Arrow points to a multinucleated giant cell  with approximately eight nuclei. (Source: Dr. Joe Miller, Public Health Image  Library, Centers for Disease Control and Prevention.)    CMV = cytomegalovirus; EBV = Epstein-Barr virus; HHV-8 = human herpesvirus 8; HSV = herpes simplex virus; VZV = varicella-zoster virus.  ’Not used in recurrent herpes labialis.   3 Used in CMV retinitis and other severe forms of disease.    Burkitt’s lymphoma and nasopharyngeal carcinoma, and  human herpesvirus 8 causes Kaposi’s sarcoma). Several her¬  pesviruses cause cancer in animals (e.g., leukemia in mon¬  keys and lymphomatosis in chickens) (see Chapter 43).   HSV type 1 (HSV-1) and type 2 (HSV-2) are distinguished  by two main criteria: antigenicity and location of lesions.  Lesions caused by HSV-1 are, in general, above the waist,  whereas those caused by HSV-2 are below the waist.  Table 37-3 describes some important differences between  the diseases caused by HSV-1 and HSV-2.   HSV-1 causes acute gingivostomatitis, recurrent herpes  labialis (cold sores), keratoconjunctivitis (keratitis), and  encephalitis, primarily in adults. HSV-2 causes herpes  genitalis (genital herpes), neonatal encephalitis and other  forms of neonatal herpes, and aseptic meningitis. Infection  by HSV-1 or HSV-2 is a common cause of erythema  multiforme.   HSV-1 and HSV-2 are structurally and morphologically  indistinguishable. They can, however, be differentiated by  the restriction endonuclease patterns of their genome DNA  and by type-specific monoclonal antisera against glycopro¬  tein G. Humans are the natural hosts of both HSV-1 and  HSV-2.   The cycle begins when HSV-1 binds first to heparan sulfate  on the cell surface and then to a second receptor, nectin.  Following fusion of the viral envelope with the cell mem¬  brane, the nucleocapsid and the tegument proteins are  released into the cytoplasm. The viral nucleocapsid is  transported to the nucleus, where it docks to a nuclear pore  and the genome DNA enters the nucleus along with tegu¬  ment protein VP 16. The linear genome DNA now becomes  circular. VP 16 interacts with cellular transcription factors  to activate transcription of viral immediate early (IE) genes  by host cell RNA polymerase. IE mRNA is translated into  IE proteins that regulate the synthesis of early proteins such  as the DNA polymerase that replicates the genome and   ’infection acquired after birth from HSV-1-infected person,  infection acquired during passage through birth canal.    thymidine kinase. These two proteins are important  because they are involved in the action of acyclovir, which  is the most important drug effective against HSV.   Note that early protein synthesis by HSV can be subdi¬  vided into two categories: immediate early and early. Imme¬  diate early proteins are those whose mRNA synthesis is  activated by a protein brought in by the incoming parental  virion (i.e., no new viral protein synthesis is required for  the production of the five immediate early proteins). The  early proteins, on the other hand, do require the synthesis  of new viral regulatory proteins to activate the transcrip¬  tion of their mRNAs.   The viral DNA polymerase replicates the genome DNA,  at which time early protein synthesis is shut off and late  protein synthesis begins. These late, structural proteins are  transported to the nucleus, where virion assembly occurs.  The virion obtains its envelope by budding through the  nuclear membrane and exits the cell via tubules or vacuoles  that communicate with the exterior.   In latently infected cells, such as HSV-infected neurons,  circular HSV DNA resides in the nucleus and is not inte¬  grated into cellular DNA. Transcription of HSV DNA is  limited to a few latency-associated transcripts (LATS).  These noncoding, regulatory RNAs suppress viral replica¬  tion. Reactivation of viral replication can occur at a later  time when the genes encoding LATS are excised.   HSV-1 is transmitted primarily in saliva, whereas HSV-2 is  transmitted by sexual contact. As a result, HSV-1 infec¬  tions occur mainly on the face, whereas HSV-2 lesions  occur in the genital area. However, oral-genital sexual  practices can result in HSV-1 infections of the genitals and  HSV-2 lesions in the oral cavity (this occurs in about  10%-20% of cases). Although transmission occurs most  often when active lesions are present, asymptomatic shed¬  ding of both HSV-1 and HSV-2 does occur and plays an  important role in transmission.   The number of HSV-2 infections has markedly increased  in recent years, whereas that of HSV-1 infections has not.  Roughly 80% of people in the United States are infected with  HSV-1, and 40% have recurrent herpes labialis. Most pri¬  mary infections by HSV-1 occur in childhood, as evidenced  by the early appearance of antibody. In contrast, antibody to  HSV-2 does not appear until the age of sexual activity.   The virus replicates in the skin or mucous membrane at the  initial site of infection, and then migrates up the neuron by  retrograde axonal flow and becomes latent in the sensory  ganglion cells. In general, HSV-1 becomes latent in the  trigeminal ganglia, whereas HSV-2 becomes latent in the  lumbar and sacral ganglia. During latency, most—if not  all—viral DNA is located in the cytoplasm rather than inte¬  grated into nuclear DNA. The virus can be reactivated from    the latent state by a variety of inducers (e.g., sunlight, hor¬  monal changes, trauma, stress, and fever), at which time it  migrates down the neuron and replicates in the skin, caus¬  ing lesions.   The typical skin lesion is a vesicle that contains serous  fluid filled with virus particles and cell debris. When the  vesicle ruptures, virus is liberated and can be transmitted to  other individuals. Multinucleated giant cells are typically  found at the base of herpesvirus lesions.   Immunity is type-specific, but some cross-protection  exists. However, immunity is incomplete, and both reinfec¬  tion and reactivation occur in the presence of circulating  IgG. Cell-mediated immunity is important in limiting  herpesviruses, because its suppression often results in reac¬  tivation, spread, and severe disease.   (1) Gingivostomatitis occurs primarily in children and  is characterized by fever, irritability, and vesicular lesions in  the mouth. The primary disease is more severe and lasts  longer than recurrences. The lesions heal spontaneously in  2 to 3 weeks. Many children have asymptomatic primary  infections.   (2) Herpes labialis (fever blisters or cold sores) is the  milder, recurrent form and is characterized by crops of  vesicles, usually at the mucocutaneous junction of the lips  or nose (Figure 37-3). Recurrences frequently reappear at  the same site.   (3) Keratoconjunctivitis is characterized by corneal  ulcers and lesions of the conjunctival epithelium. Recur¬  rences can lead to scarring and blindness.   FIGURE 37~3 Herpes labialis—note vesicles on upper lip adja¬  cent to the Vermillion border of the lip caused by herpes simplex  virus type 1. (Used with permission from Jack Resneck, Sr., MD.)    (4) Encephalitis caused by HSV-1 is characterized by a  necrotic lesion in one temporal lobe. Fever, headache, vom¬  iting, seizures, and altered mental status are typical clinical  features. The onset may be acute or protracted over several  days. The disease occurs as a result of either a primary  infection or a recurrence. Magnetic resonance imaging  often reveals the lesion. Examination of the spinal fluid  typically shows a moderate increase of lymphocytes, a  moderate elevation in the amount of protein, and a normal  amount of glucose. HSV-1 encephalitis has a high mortality  rate and causes severe neurologic sequelae in those who  survive.   (5) Herpetic whitlow is a pustular lesion of the skin of  the finger or hand. It can occur in medical personnel as a  result of contact with patient’s lesions.   (6) Herpes gladiatorum, as the name implies, occurs in  wrestlers and others who have close body contact. It is  caused primarily by HSV-1 and is characterized by vesicu¬  lar lesions on the head, neck, and trunk.   (7) Eczema herpeticum (Kaposi’s varicelliform erup¬  tion) is an infection of the skin of a patient with atopic  dermatitis. Vesicular lesions are seen at the site of the atopic  dermatitis (eczema). Most cases occur in children.   (8) Disseminated infections, such as esophagitis and  pneumonia, occur in immunocompromised patients with  depressed T-cell function.   (1) Genital herpes is characterized by painful vesicular  lesions of the male and female genitals and anal area  (Figure 37-4). The lesions are more severe and protracted  in primary disease than in recurrences. Primary infections  are associated with fever and inguinal adenopathy.   Many infections are asymptomatic (i.e., many people  have antibody to HSV-2 but have no history of disease).  Asymptomatic infections occur in both men (in the pros¬  tate or urethra) and women (in the cervix). Shedding of  virus from those with an asymptomatic infection occurs  sporadically and can be a source of infection of other  individuals.   Approximately 80% to 90% of herpes genitalis cases are  caused by HSV-2. The remainder are caused by HSV-1 as a  result of oral-genital contact. The clinical importance of  this is that suppressive chemoprophylaxis for HSV-2 lesions  should be considered because lesions caused by HSV-2 are  more likely to recur than lesions caused by HSV-1.   (2) Neonatal herpes originates chiefly from contact  with vesicular lesions within the birth canal. In some cases,  although there are no visible lesions, HSV-2 is shed into the  birth canal (asymptomatic shedding) and can infect the  child during birth. Neonatal herpes varies from severe dis¬  ease (e.g., disseminated lesions or encephalitis) to milder  local lesions (skin, eye, mouth) to asymptomatic infection.  Neonatal disease may be prevented by performing cesarean   penis caused by herpes simplex virus type 2. (Used with permission from  Jack Resneck, Sr., MD.)   section on women with either active lesions or positive  viral cultures. Both HSV-1 and HSV-2 can cause severe  neonatal infections that are acquired after birth from car¬  riers handling the child. Despite their association with  neonatal infections, neither HSV-1 nor HSV-2 causes con¬  genital abnormalities to any significant degree.   Serious neonatal infection is more likely to occur when  the mother is experiencing a primary herpes infection than  a recurrent infection for two reasons: (1) the amount of  virus produced during a primary infection is greater than  during a secondary infection, and (2) mothers who have  been previously infected can pass IgG across the placenta,  which can protect the neonate from serious disseminated  infection.   (3) Aseptic meningitis caused by HSV-2 is usually a  mild, self-limited disease with few sequelae.   Both HSV-1 and HSV-2 infections are associated with  erythema multiforme. The rash of erythema multiforme  appears as a central red area surrounded by a ring of nor¬  mal skin outside of which is a red ring (“target” or “bull’s  eye” lesion). The lesions are typically macular or papular  and occur symmetrically on the trunk, hands, and feet. The  rash is thought to be an immune-mediated reaction to the  presence of HSV antigens. Acyclovir is useful in preventing  recurrent episodes of erythema multiforme, probably by    reducing the amount of HSV antigens. Many drugs, espe¬  cially sulfonamides among the antimicrobial drugs, com¬  monly cause erythema multiforme. Other prominent  infectious causes include Mycoplasma pneumoniae and  viruses such as hepatitis B virus and hepatitis C virus.   Erythema multiforme major, also known as Stevens-  Johnson syndrome, is characterized by fever, erosive oral  lesions, and extensive desquamating skin lesions. Mycoplasma  pneumoniae infection is the most common infectious cause  of Stevens-Johnson syndrome.   An important diagnostic procedure is isolation of the virus  from the lesion by growth in cell culture. The typical cyto-  pathic effect occurs in 1 to 3 days, after which the virus is  identified by fluorescent antibody staining of the infected  cells or by detecting virus-specific glycoproteins in enzyme-  linked immunosorbent assays (ELISAs). HSV-1 can be  distinguished from HSV-2 by using monoclonal antibody  against glycoprotein G often in an ELISA test.   A rapid presumptive diagnosis can be made from skin  lesions by using the Tzanck smear, in which cells from the  base of the vesicle are stained with Giemsa stain. The pres¬  ence of multinucleated giant cells suggests herpesvirus  infection (see Figure 37-2).   If herpes encephalitis is suspected, a rapid diagnosis can  be made by detecting E1SV DNA in the spinal fluid by using  a polymerase chain reaction (PCR) assay. The PCR assay is  more sensitive than viral culture. The diagnosis of neonatal  herpes infection typically involves the use of viral cultures  or PCR assay.   Serologic tests such as the neutralization test can be  used in the diagnosis of primary infections because a sig¬  nificant rise in antibody titer is readily observed. However,  they are of no use in the diagnosis of recurrent infections  because many adults already have circulating antibodies,  and recurrences rarely cause a rise in antibody titer.   Acyclovir (acycloguanosine, Zovirax) is the treatment of  choice for encephalitis and systemic disease caused by  HSV-1. It is also useful for the treatment of primary and  recurrent genital herpes; it shortens the duration of the  lesions and reduces the extent of shedding of the virus but  does not cure the latent state. Acyclovir is also used to treat  neonatal infections caused by HSV-2. Mutants of HSV-1  resistant to acyclovir have been isolated from patients; fos-  carnet can be used in these cases.   For HSV-1 eye infections, other nucleoside analogues  (e.g., trifluridine [Viroptic]) are used topically. Oral acy¬  clovir is also used for HSV keratitis. Penciclovir (a  derivative of acyclovir) or docosanol (a long-chain satu¬  rated alcohol) can be used to treat recurrences of orola-  bial HSV-1 infections in immunocompetent adults.    Valacyclovir (Valtrex) and famciclovir (Famvir) are used  in the treatment of genital herpes and in the suppression  of recurrences.   Note that no drug treatment of the primary infection  prevents recurrences; drugs have no effect on the latent  state, but prophylactic, long-term administration of acy¬  clovir, valacyclovir, or famciclovir can suppress clinical  recurrences.   Valacyclovir (Valtrex) and famciclovir (Famvir) are used in  the suppression of recurrent lesions, especially in those  with frequent recurrences caused by HSV-2. Suppressive  chemoprophylaxis also reduces shedding of the virus and,  as a result, transmission to others. Prevention also involves  avoiding contact with the vesicular lesion or ulcer. Cesar¬  ean section is recommended for women who are at term  and who have genital lesions or positive viral cultures.  Circumcision reduces the risk of infection by HSV-2. There  is no vaccine against HSV-1 or HSV-2.   Varicella (chickenpox) is the primary disease; zoster (shin¬  gles) is the recurrent form.   VZV is structurally and morphologically similar to other  herpesviruses but is antigenically different. It has a single  serotype. The same virus causes both varicella and zoster.  Humans are the natural hosts.   The cycle is similar to that of HSV (see page 290).   The virus is transmitted by respiratory droplets and by  direct contact with the lesions. Varicella is a highly conta¬  gious disease of childhood; more than 90% of people in the  United States have antibody by age 10 years. Varicella  occurs worldwide. Prior to 2001, there were more cases of  chickenpox than any other notifiable disease, but the wide¬  spread use of the vaccine has significantly reduced the  number of cases.   There is infectious VZV in zoster vesicles. This virus  can be transmitted, usually by direct contact, to children  and can cause varicella. The appearance of either varicella  or zoster in a hospital is a major infection control problem  because the virus can be transmitted to immunocompro¬  mised patients and cause life-threatening disseminated  infection.   VZV infects the mucosa of the upper respiratory tract, and  then spreads via the blood to the skin, where the typical  vesicular rash occurs. Multinucleated giant cells with  intranuclear inclusions are seen in the base of the lesions.  The virus infects sensory neurons and is carried by retro¬  grade axonal flow into the cells of the dorsal root ganglia,  where the virus becomes latent.   In latently infected cells, VZV DNA is located in the  nucleus and is not integrated into cellular DNA. Later in  life, frequently at times of reduced cell-mediated immunity  or local trauma, the virus is activated and causes the vesicu¬  lar skin lesions and nerve pain of zoster.   Immunity following varicella is lifelong: A person gets  varicella only once, but zoster can occur despite this immu¬  nity to varicella. Zoster usually occurs only once. The fre¬  quency of zoster increases with advancing age, perhaps as a  consequence of waning immunity.   After an incubation period of 14 to 21 days, brief prodro¬  mal symptoms of fever and malaise occur. A papulovesicu¬  lar rash then appears in crops on the trunk and spreads to  the head and extremities (Figure 37-5). The rash evolves   FIGURE 37-5 Varicella (chickenpox)—note vesicles on an  erythematous base caused by varicella-zoster virus. (Reproduced with  permission from Usatine, RP et al: The Color Atlas of Family Medicine, New York:  McGraw-Hill, 2009. Courtesy of Richard P. Usatine, MD.)   FIGURE 37-6 Zoster (shingles)—note vesicles along the  dermatome of a thoracic nerve caused by varicella-zoster virus.  (Reproduced with permission from Usatine, RP et al: The Color Atlas of Family  Medicine, New York: McGraw-Hill, 2009. Courtesy of Richard P. Usatine, MD.)    from papules to vesicles, pustules, and, finally, crusts. Itch¬  ing (pruritus) is a prominent symptom, especially when  vesicles are present. Varicella is mild in children but more  severe in adults. Varicella pneumonia and encephalitis are  the major rare complications, occurring more often in  adults. Reye’s syndrome, characterized by encephalopathy  and liver degeneration, is associated with VZV and influ¬  enza B virus infection, especially in children given aspirin.  Its pathogenesis is unknown.   The occurrence of painful vesicles along the course of a  sensory nerve of the head or trunk is the usual picture  (Figure 37-6). The pain can last for weeks, and post¬  zoster neuralgia (also known as postherpetic neuralgia)  can be debilitating. In immunocompromised patients,  life-threatening disseminated infections such as pneu¬  monia can occur.   Although most diagnoses are made clinically, laboratory  tests are available. A presumptive diagnosis can be made by  using the Tzanck smear. Multinucleated giant cells are seen  in VZV as well as HSV lesions (see Figure 37-2). The  definitive diagnosis is made by isolation of the virus in cell  culture and identification with specific antiserum. A rise in  antibody titer can be used to diagnose varicella but is less  useful in the diagnosis of zoster.   No antiviral therapy is necessary for chickenpox or zoster  in immunocompetent children. Immunocompetent adults  with either moderate or severe cases of chickenpox or    zoster often are treated with acyclovir because it can reduce  the duration and severity of symptoms. Immunocompro¬  mised children and adults with chickenpox, zoster, or dis¬  seminated disease should be treated with acyclovir. Disease  caused by acyclovir-resistant strains of VZV can be treated  with foscarnet. Two drugs similar to acyclovir, famciclovir  (Famvir) and valacyclovir (Valtrex), can be used in patients  with zoster to accelerate healing of the lesions, but none of  these drugs can cure the latent state. There is some evi¬  dence that these drugs reduce the incidence of postzoster  neuralgia.   There are two vaccines against VZV: one designed to pre¬  vent varicella, called Varivax, and the other designed to  prevent zoster, called Zostavax. Both contain live, attenu¬  ated VZV, but the zoster vaccine contains 14 times more  virus than the varicella vaccine. The zoster vaccine is effec¬  tive in preventing the symptoms of zoster, but does not  eradicate the latent state of VZV.   The varicella vaccine is recommended for children  between the ages of 1 and 12 years, whereas the zoster vac¬  cine is recommended for people older than 60 years and  who have had varicella. The varicella vaccine is given in  two doses, whereas the zoster vaccine is given in one dose.  Because these vaccines contain live virus, they should not  be given to immunocompromised people or pregnant  women.   Acyclovir is useful in preventing varicella and dissemi¬  nated zoster in immunocompromised people exposed to  the virus. Varicella-zoster immune globulin (VZIG),  which contains a high titer of antibody to the virus, is also  used for such prophylaxis.   CMV causes cytomegalic inclusion disease (especially con¬  genital abnormalities) in neonates. It is the most common  cause of congenital abnormalities in the United States.  CMV is a very important cause of pneumonia and other  diseases in immunocompromised patients such as recipi¬  ents of bone marrow (stem cell) and solid organ trans¬  plants. It also causes heterophil-negative mononucleosis in  immunocompetent individuals.   CMV is structurally and morphologically similar to other  herpesviruses but is antigenically different. It has a single  serotype. Humans are the natural hosts; animal CMV  strains do not infect humans. Giant cells are formed, hence  the name cytomegalo.   The cycle is similar to that of HSV (see page 290). One  unique feature of CMV replication is that some of its  “immediate early proteins” are translated from mRNAs  brought into the infected cell by the parental virion rather  than being translated from mRNAs synthesized in the  newly infected cell.   CMV is transmitted by a variety of modes. Early in life, it  is transmitted across the placenta, within the birth canal,  and quite commonly in breast milk. In young children, its  most common mode of transmission is via saliva. Later in  life it is transmitted sexually; it is present in both semen  and cervical secretions. It can also be transmitted during  blood transfusions and organ transplants. CMV infection  occurs worldwide, and more than 80% of adults have anti¬  body against this virus.   Infection of the fetus can cause cytomegalic inclusion  disease, characterized by multinucleated giant cells with  prominent intranuclear inclusions. Many organs are  affected, and widespread congenital abnormalities result.  Infection of the fetus occurs mainly when a primary infec¬  tion occurs in the pregnant woman (i.e., when she has no  antibodies that will neutralize the virus before it can infect  the fetus). The fetus usually will not be infected if the preg¬  nant woman has antibodies against the virus. Congenital  abnormalities are more common when a fetus is infected  during the first trimester than later in gestation, because  the first trimester is when development of organs occurs  and the death of any precursor cells can result in congeni¬  tal defects.   Infections of children and adults are usually asymp¬  tomatic, except in immunocompromised individuals.  CMV enters a latent state primarily in monocytes and can  be reactivated when cell-mediated immunity is decreased.  CMV can also persist in kidneys for years. Reactivation of  CMV from the latent state in cervical cells can result in  infection of the newborn during passage through the  birth canal.   CMV has a specific mechanism of “immune evasion”  that allows it to maintain the latent state for long periods.  In CMV-infected cells, assembly of the major histocompat¬  ibility complex (MHC) class I-viral peptide complex is  unstable, so viral antigens are not displayed on the cell  surface and killing by cytotoxic T cells does not occur. In  addition, CMV encodes several microRNAs, one of which  binds to and prevents the translation of the cell’s mRNA for  the class I MHC protein. This prevents viral proteins from  being displayed on the infected cell surface, and killing by  cytotoxic T cells does not occur.    CMV also encodes a protein that functions as a chemo-  kine receptor. When this protein is released from CMV-  infected cells, the protein binds to chemokines thereby  preventing the chemokines from serving as a signal for host  immune cells to migrate to the site of CMV infection.   CMV infection causes an immunosuppressive effect by  inhibiting T cells. Host defenses against CMV infection  include both circulating antibody and cell-mediated  immunity. Cellular immunity is more important, because  its suppression can lead to systemic disease.   Approximately 20% of infants infected with CMV during  gestation show clinically apparent manifestations of cyto¬  megalic inclusion disease such as microcephaly seizures,  deafness, jaundice, and purpura. The purpuric lesions  resemble a “blueberry muffin” and are due to thrombocy¬  topenia. Hepatosplenomegaly is very common. Cytome¬  galic inclusion disease is one of the leading causes of  mental retardation in the United States. Infected infants  can continue to excrete CMV, especially in the urine, for  several years.   In immunocompetent adults, CMV can cause hetero¬  phil-negative mononucleosis, which is characterized by  fever, lethargy, and the presence of abnormal lymphocytes  in peripheral blood smears. In immunocompromised  patients, systemic CMV infections, especially pneumoni¬  tis, esophagitis, and hepatitis, occur in a high proportion  of those individuals (e.g., those with renal and bone mar¬  row transplants). In patients with acquired immunodefi¬  ciency syndrome (AIDS), CMV commonly infects the  intestinal tract and causes intractable colitis with diarrhea.  CMV also causes retinitis in AIDS patients, which can lead  to blindness.   The preferred approach involves culturing in special tubes  called shell vials coupled with the use of immunofluores-  cent antibody, which can make a diagnosis in 72 hours. The  virus obtained in the culture can then be used to determine  the drug susceptibility to ganciclovir.   Other diagnostic methods include fluorescent anti¬  body and histologic staining of inclusion bodies in giant  cells in urine and in tissue. The inclusion bodies are intra¬  nuclear and have an oval owl’s eye shape (Figure 37-7). A  fourfold or greater rise in antibody titer is also diagnostic.  PCR-based assays for CMV DNA or RNA in tissue or  body fluids, such as spinal fluid and amniotic fluid, are  also very useful.   CMV antigenemia can be measured by detecting pp65  within blood leukocytes using an immunofluorescence  assay. pp65 is a protein located in the nucleocapsid of  CMV and can be identified within infected leukocytes  using fluorescein-labeled monoclonal antibody specific  for pp65.   Arrow points to an "owl's eye" inclusion body in the nucleus of an  infected cell. (Source: Dr. Edwin Ewing, Jr., Public Health Image Library, Centers  for Disease Control and Prevention.)   Ganciclovir (Cytovene) is moderately effective in the treat¬  ment of CMV retinitis and pneumonia in patients with  AIDS. Valganciclovir, which can be taken orally is also  effective against CMV retinitis. CMV strains resistant to  ganciclovir and valganciclovir have emerged, mostly due to  mutations in the d gene that encodes the phosphokinase.  Drug susceptibility testing can be done.   Foscarnet (Foscavir) is also effective but causes more side  effects. Unlike HSV and VZV, CMV is largely resistant to  acyclovir. Cidofovir (Vistide) is also useful in the treatment  of CMV retinitis. Fomivirsen (Vitravene) is an antisense  DNA approved for the intraocular treatment of CMV retini¬  tis. It is the first and, at present, the only antisense molecule  to be approved for the treatment of human disease.   There is no vaccine. Ganciclovir can suppress progressive  retinitis in AIDS patients. Infants with cytomegalic inclu¬  sion disease who are shedding virus in their urine should  be kept isolated from other infants. Blood for transfusion to  newborns should be CMV antibody-negative. If possible,  only organs from CMV antibody-negative donors should  be transplanted to antibody-negative recipients. A high-  titer immune globulin preparation (CytoGam) is used to  prevent disseminated CMV infections in organ transplant  patients.   EBV causes infectious mononucleosis. It is associated with  Burkitt’s lymphoma, other B-cell lymphomas, and naso¬  pharyngeal carcinoma. EBV also causes hairy leukoplakia.   EBV is structurally and morphologically similar to other  herpesviruses but is antigenically different. The most  important antigen is the viral capsid antigen (VCA),  because it is used most often in diagnostic tests. The early  antigens (EA), which are produced prior to viral DNA  synthesis, and Epstein-Barr nuclear antigen (EBNA),  which is located in the nucleus bound to chromosomes,  are sometimes diagnostically helpful as well. Two other  antigens, lymphocyte-determined membrane antigen  and viral membrane antigen, have been detected also.  Neutralizing activity is directed against the viral mem¬  brane antigen.   Humans are the natural hosts. EBV infects mainly lym¬  phoid cells, primarily B lymphocytes. EBV also infects the  epithelial cells of the pharynx, resulting in the prominent  sore throat. In latently infected cells, EBV DNA is in the  nucleus and is not integrated into cellular DNA. Some, but  not all, genes are transcribed, and only a subset of those is  translated into protein.   The cycle is similar to that of HSV (see page 290). EBV  enters B lymphocytes at the site of the receptor for the C3  component of complement.   EBV is transmitted primarily by the exchange of saliva  (e.g., during kissing). The saliva of people with a reactiva¬  tion of a latent infection as well as people with an active  infection can serve as a source of the virus. In contrast to  CMV, blood transmission of EBV is very rare.   EBV infection is one of the most common infections  worldwide; more than 90% of adults in the United States  have antibody. Infection in the first few years of life is usu¬  ally asymptomatic. Early infection tends to occur in indi¬  viduals in lower socioeconomic groups. The frequency of  clinically apparent infectious mononucleosis, however, is  highest in those who are exposed to the virus later in life  (e.g., college students).   The infection first occurs in the oropharynx and then  spreads to the blood, where it infects B lymphocytes. Cyto¬  toxic T lymphocytes react against the infected B cells. The  T cells are the “atypical lymphs” seen in the blood smear.  EBV remains latent within B lymphocytes.   The immune response to EBV infection consists first  of IgM antibody to the VCA. IgG antibody to the VCA  follows and persists for life. The IgM response is there¬  fore useful for diagnosing acute infection, whereas the  IgG response is best for revealing prior infection. Life¬  time immunity against second episodes of infectious    mononucleosis is based on antibody to the viral mem¬  brane antigen.   In addition to the EBV-specific antibodies, nonspecific  heterophil antibodies are found. The term heterophil  refers to antibodies that are detected by tests using antigens  different from the antigens that induced them. The hetero¬  phil antibodies formed in infectious mononucleosis agglu¬  tinate sheep or horse red blood cells in the laboratory.  (Cross-reacting Forssman antibodies in human serum are  removed by adsorption with guinea pig kidney extract  prior to agglutination.) Note that these antibodies do not  react with any component of EBV. It seems likely that EBV  infection modifies a cell membrane constituent such that it  becomes antigenic and induces the heterophil antibody.  Heterophil antibodies usually disappear within 6 months  after recovery. These antibodies are not specific for EBV  infection and are also seen in individuals with hepatitis B  and serum sickness.   Infectious mononucleosis is characterized primarily by  fever, sore throat, lymphadenopathy, and splenomegaly.  Anorexia and lethargy are prominent. Hepatitis is frequent;  encephalitis occurs in some patients. Spontaneous recovery  usually occurs in 2 to 3 weeks. Splenic rupture, associated  with contact sports such as football, is a feared but rare  complication of the splenomegaly.   In addition to the common form of infectious mono¬  nucleosis described in the previous paragraph, EBV causes  a severe, often fatal, progressive form of infectious mono¬  nucleosis that occurs in children with an inherited immu¬  nodeficiency called X-linked lymphoproliferative  syndrome. The mutated gene encodes a signal transduc¬  tion protein required for both T-cell and natural killer-cell  function. The mortality rate is 75% by age 10. Bone mar¬  row or cord blood transplants may cure the underlying  immunodeficiency. EBV also causes hairy leukoplakia—a  whitish, nonmalignant lesion with an irregular “hairy”  surface on the lateral side of the tongue (Figure 37-8). It  occurs in immunocompromised individuals, especially  AIDS patients.   EBV infection is associated with several cancers, namely  Burkitt’s lymphoma, some forms of Hodgkin’s lymphoma,  and nasopharyngeal carcinoma. The word associated  refers to the observation that EBV infection is the initiating  event that causes the cells to divide, but that event itself  does not cause a malignancy. It requires additional steps for  malignant transformation to occur. Reduced cell-mediated  immunity predisposes to the uncontrolled growth of the  EBV-infected cells.   Another EBV-associated disease is post-transplant  lymphoproliferative disorder (PTLD). The most common  form of PTLD is a B-cell lymphoma. PTLD occurs follow¬  ing both bone marrow transplants and solid organ trans¬  plants. The main predisposing factor to PTLD is the   eral aspect of tongue caused by Epstein-Barr virus. (Reproduced with  permission from Wolff K, Johnson R. Fitzpatrick's Color Atlas & Synopsis of Clinical Der¬  matology. 6th ed. New York: McGraw-Hill, 2009. Copyright © 2009 by The McGraw-  Hill Companies, Inc.)   FIGURE 37-9 Atypical lymphocytes in infectious mononucle¬  osis—note two atypical lymphocytes, each with an enlarged nucleus  and abundant cytoplasm on the left side. The lymphocyte on the  right side appears normal. (Reproduced with permission from Fauci AS,  Braunwald E, Kasper DL et al, eds. Harrison's Principles of Internal Medicine. 17th ed.  New York: McGraw-Hill, 2008. Copyright © 2008 by The McGraw-Hill Companies, Inc.)    immunosuppression required to prevent rejection of the  graft. The lymphoma will regress if the degree of immuno¬  suppression is reduced.   The diagnosis of infectious mononucleosis in the clinical  laboratory is based primarily on two approaches:   (1) In the hematologic approach, absolute lymphocyto¬  sis occurs, and as many as 30% abnormal lymphocytes are  seen on a smear. These atypical lymphs are enlarged, have  an expanded nucleus, and an abundant, often vacuolated  cytoplasm (Figure 37-9). They are cytotoxic T cells that are  reacting against the EBV-infected B cells.   (2) In the immunologic approach, there are two types  of serologic tests: (a) The heterophil antibody test is use¬  ful for the early diagnosis of infectious mononucleosis  because it is usually positive by week 2 of illness. However,  because the antibody titer declines after recovery, it is not  useful for detection of prior infection. The Monospot test  is often used to detect the heterophil antibody; it is more  sensitive, more specific, and less expensive than the tube  agglutination test, (b) The EBV-specific antibody tests are  used primarily in diagnostically difficult cases. The IgM  VCA antibody response can be used to detect early illness;  the IgG VCA antibody response can be used to detect prior  infection. In certain instances, antibodies to EA and EBNA  can be useful diagnostically.   Although EBV can be isolated from clinical samples  such as saliva by morphologic transformation of cord blood    lymphocytes, it is a technically difficult procedure and is  not readily available. No virus is synthesized in the cord  lymphocytes; its presence is detected by fluorescent anti¬  body staining of the nuclear antigen.   No antiviral therapy is necessary for uncomplicated infec¬  tious mononucleosis. Acyclovir has little activity against  EBV, but administration of high doses may be useful in  life-threatening EBV infections.   EBV infection is associated with cancers of lymphoid  origin: Burkitt’s lymphoma in African children, other  B-cell lymphomas, nasopharyngeal carcinoma in the  Chinese population, and thymic carcinoma in the United  States. The initial evidence of an association of EBV  infection with Burkitt’s lymphoma was the production of  EBV by the lymphoma cells in culture. In fact, this was  how EBV was discovered by Epstein and Barr in 1964.  Additional evidence includes the finding of EBV DNA  and EBNA in the cells of nasopharyngeal and thymic  carcinomas. EBV can induce malignant transformation  in B lymphocytes in vitro.   In Burkitt’s lymphoma, oncogenesis is a function of the  translocation of the c-myc oncogene to a site adjacent to    an immunoglobulin gene promoter. This enhances synthe¬  sis of the c-myc protein, a potent oncoprotein. The c-myc  protein is a transcriptional regulator that enhances the  synthesis of kinases that activate the cell cycle.   In 1994, it was reported that a new herpesvirus, now  known as human herpesvirus 8 (HHV-8), or Kaposi’s sar¬  coma-associated herpesvirus (KSHV), causes Kaposi’s  sarcoma (KS), the most common cancer in patients with  AIDS. The idea that a virus other than HIV is the cause of  KS arose from epidemiologic data showing that KS was  common in patients who acquired HIV sexually but rare  in patients who acquired HIV via blood transfusion. A  second virus transmitted sexually appeared likely to be  the cause.   The initial evidence that HHV-8 was involved was the  finding that most KS cells taken from AIDS patients con¬  tain the DNA of this virus, but tissues taken from AIDS  patients without KS had very little viral DNA. The DNA of  this virus was also found in KS cells that arose in non-HIV-  infected patients. On DNA analysis, HHV-8 resembles the  lymphotropic herpesviruses (e.g., EBV and herpesvirus  saimiri) more than it does the neurotropic herpesviruses,  such as HSV and VZV.   Additional support was provided by serologic studies  showing that most HIV-infected patients with KS had anti¬  bodies to HHV-8, whereas considerably fewer HIV-infected  patients without KS had antibodies to the virus, and very  few patients with other sexually transmitted diseases, but  who were not HIV-infected, had these antibodies. The cur¬  rent estimate of HHV-8 infection in the general population  ranges from about 3% in the United States and England to  about 50% in East Africa.   HHV-8 causes malignant transformation by a mecha¬  nism similar to that of other DNA viruses (e.g., human  papillomavirus), namely, inactivation of a tumor suppres¬  sor gene. A protein encoded by HHV-8 called latency-  associated nuclear antigen (LANA) inactivates RB and p53  tumor suppressor proteins, which causes malignant trans¬  formation of endothelial cells.   Transmission of HHV-8 occurs primarily via sex and by  saliva, but it is also transmitted in transplanted organs such  as kidneys and appears to be the cause of transplantation-  associated KS. The DNA of HHV-8 is found in the cells of  transplantation-associated KS but not in the cells of other  transplantation-associated cancers.   KS in AIDS patients is a malignancy of vascular endo¬  thelial cells that contains many spindle-shaped cells and  erythrocytes. The lesions are reddish to dark purple, flat to   FIGURE 37~10 Kaposi's sarcoma—note several dark purple  lesions on the foot caused by human herpesvirus 8 (Kaposi's  sarcoma-associated virus). (Source: Public Health Image Library, Centers for  Disease Control and Prevention.)    nodular, and often appear at multiple sites such as the skin,  oral cavity, and soles (but not the palms) (Figure 37-10).  Internally, lesions occur commonly in the gastrointestinal  tract and the lungs. The extravasated red cells give the  lesions their purplish color. HHV-8 also infects B cells,  inducing them to proliferate and produce a type of lym¬  phoma called primary effusion lymphoma.   Laboratory diagnosis of KS is often made by biopsy of  the skin lesions. HHV-8 DNA and RNA are present in most  spindle cells, but that analysis is not usually done. Virus is  not grown in culture.   The type of treatment depends on the site and number  of the lesions. Surgical excision, radiation, chemotherapy,  or immunomodulatory drugs, such as alpha interferon can  be used. In early HIV-associated KS, highly active antiret¬  roviral drugs (HAART) can be effective treatment. Note  that anti-herpesvirus drugs, such as acyclovir, foscarnet,  and cidofovir are not effective.   HAART also can prevent KS as evidenced by a decreased  incidence of KS in HIV-infected patients treated with  HAART. There is no vaccine against HHV-8.   The poxvirus family includes three viruses of medical  importance: smallpox virus, vaccinia virus, and molluscum  contagiosum virus. Poxviruses are the largest and most  complex viruses.   Smallpox virus, also called variola virus, is the agent of  smallpox, the only human disease that has been eradicated  from the face of the Earth. Eradication was achieved by the  widespread use of the smallpox vaccine. There is concern  regarding the use of smallpox virus as an agent of bioterror¬  ism. Poxviruses of animal origin, such as cowpox and mon¬  key pox, are described in Chapter 46. (Note that rinderpest,  a disease primarily of cattle, has also been eradicated by  using the vaccine against rinderpest virus (RPV). RPV is a  paramyxovirus related to measles virus.)   Poxviruses are brick-shaped particles containing linear  double-stranded DNA, a disk-shaped core within a double  membrane, and a lipoprotein envelope. The virion contains  a DNA-dependent RNA polymerase. This enzyme is  required because the virus replicates in the cytoplasm and  does not have access to the cellular RNA polymerase, which  is located in the nucleus.   Smallpox virus has a single, stable serotype, which is the  key to the success of the vaccine. If the antigenicity varied  as it does in influenza virus, eradication would not have  succeeded. Smallpox virus infects only humans; there is no  animal reservoir.   The following description of the replicative cycle is based  on studies with vaccinia virus, as it is much less likely to  cause human disease than smallpox virus. After penetra¬  tion of the cell and uncoating, the virion DNA-dependent  RNA polymerase synthesizes early mRNA, which is trans¬  lated into early, nonstructural proteins, mainly enzymes  required for subsequent steps in viral replication. The viral  DNA then is replicated, after which late, structural proteins  are synthesized that will form the progeny virions. The  virions are assembled and acquire their envelopes by bud¬  ding from the cell membrane as they are released from the  cell. Note that all steps in replication occur in the cyto¬  plasm, which is unusual for a DNA virus.   Smallpox virus is transmitted via respiratory aerosol or by  direct contact with virus either in the skin lesions or on  fomites such as bedding.    Prior to the 1960s, smallpox was widespread throughout  large areas of Africa, Asia, and South America, and millions  of people were affected. In 1967, the World Health Organi¬  zation embarked on a vaccination campaign that led to the  eradication of smallpox. The last naturally occurring case  was in Somalia in 1977.   Smallpox begins when the virus infects the upper respira¬  tory tract and local lymph nodes and then enters the blood  (primary viremia). Internal organs are infected; then the  virus reenters the blood (secondary viremia) and spreads to  the skin. These events occur during the incubation period,  when the patient is still well. The rash is the result of virus  replication in the skin, followed by damage caused by cyto¬  toxic T cells attacking virus-infected cells.   Immunity following smallpox disease is lifelong; immu¬  nity following vaccination lasts about 10 years.   After an incubation period of 7 to 14 days, there is a sud¬  den onset of prodromal symptoms such as fever and mal¬  aise. This is followed by the rash, which is worse on the  face and extremities than on the trunk (i.e., it has a cen¬  trifugal distribution). The rash evolves through stages  from macules to papules, vesicles, pustules, and, finally,  crusts in 2 to 3 weeks.   In the past when the disease occurred, the diagnosis was  made either by growing the virus in cell culture or chick  embryos or by detecting viral antigens in vesicular fluid by  immunofluorescence.   The disease was eradicated by global use of the vaccine,  which contains live, attenuated vaccinia virus. The success  of the vaccine is dependent on five critical factors: (1) small¬  pox virus has a single, stable serotype; (2) there is no animal  reservoir, and humans are the only hosts; (3) the antibody  response is prompt, and therefore exposed persons can be  protected; (4) the disease is easily recognized clinically, and  therefore exposed persons can be immunized promptly; and  (5) there is no carrier state or subclinical infection.   The vaccine is inoculated intradermally, where virus  replication occurs. The formation of a vesicle is indicative  of a “take” (success). Although the vaccine was relatively  safe, it became apparent in the 1970s that the incidence of  side effects such as encephalitis, generalized vaccinia, and  vaccinia gangrenosa exceeded the incidence of smallpox.    Routine vaccination of civilians was discontinued, and it is  no longer a prerequisite for international travel. Military  personnel are still vaccinated.   In response to the possibility of a bioterrorism attack  using smallpox virus, the U.S. federal government has insti¬  tuted a program to vaccinate “first responders” so that they  can give emergency medical care without fear of contract¬  ing the disease. To protect the unimmunized general popu¬  lation, the concept of “ring vaccination” will be used. This  is based on the knowledge that an exposed individual can  be immunized as long as 4 days after exposure and be  protected. Therefore, if an attack occurs, people known to  be exposed will be immunized as well as the direct contacts  of those people and then the contacts of the contacts, in an  expanding ring. Several military personnel and civilians  have experienced myocarditis following vaccination, and as  of this writing, caution has been urged regarding expand¬  ing this program to the general population.   Vaccinia immune globulins (VIG), containing high-titer  antibodies against vaccinia virus, can be used to treat most of  the complications of vaccination. In the past, methisazone was  used to treat the complications of vaccination and could be  useful again. Rifampin inhibits viral DNA-dependent RNA  polymerase but was not used clinically against smallpox.   Molluscum contagiosum virus (MCV) is a member of the  poxvirus family but is quite distinct from smallpox and  vaccinia viruses. The lesion of molluscum contagiosum is a  small (2-5 mm), flesh-colored papule on the skin or  mucous membrane that is painless, nonpruritic, and not  inflamed (Figure 37-11). The lesions have a characteristic  cup-shaped (umbilicated) crater with a white core. The  lesion is composed of hyperplastic epithelial cells within  which a cytoplasmic inclusion body can be seen. The inclu¬  sion body contains progeny MCV.   Note that these lesions are different from warts,  which are caused by papillomavirus, a member of the  papovavirus family.   papular lesions under the eye caused by molluscum contagiosum  virus, a member of the poxvirus family. (Reproduced with permission  from Usatine RP et al. The Color Atlas of Family Medicine. New York: McGraw-Hill,  2009. Copyright © 2009 by The McGraw-Hill Companies, Inc.)    MCV is transmitted by close personal contact, includ¬  ing sexually. The disease is quite common in children, in  whom lesions often occur around the eyes and on the  trunk. Adults often have lesions in the genital area. The  lesions can be large and numerous in patients with  reduced cellular immunity, such as AIDS patients. In  immunocompetent patients, the lesions are self-limited  but may last for months.   The diagnosis is typically made clinically; the virus is not  isolated in the clinical laboratory, and antibody titers are not  helpful. Removal of the lesions by curettage or with liquid  nitrogen is often effective. There is no established antiviral  therapy, but cidofovir may be useful in the treatment of the  extensive lesions that occur in immunocompromised  patients. In AIDS patients, antiretroviral therapy may  restore sufficient immunity to cause the lesions to resolve.  There is no vaccine.   Hepatitis B virus, a DNA enveloped virus, is described in  Chapter 41 with the other hepatitis viruses.   1. Your patient is a 30-year-old man who has frequent episodes of  herpes labialis. He asks you to tell him something about herpes  simplex virus type 1 (HSV-1). Which one of the following would  be the most accurate statement to make?    (A) Acyclovir can eradicate the latent state of HSV-1 but not  HSV-2.   (B) The main site of latency by HSV-1 is the neurons in the sen¬  sory ganglia of the face.   (C) HSV-1 is an enveloped virus that has a DNA genome and a  DNA polymerase in the virion.   (D) The lesions of primary HSV-1 infections are less extensive  and less severe than the lesions of recurrent HSV-1  infections.   (E) The laboratory diagnosis of HSV-1 infections typically  involves the detection of a greater than fourfold rise in anti¬  body titer against the virus.    2. Your patient is a woman who is due to give birth next week. She  asks you about the risk of her baby becoming infected with her¬  pes simplex virus type 2 (HSV-2). Which one of the following is  the most accurate response?   (B) The risk is higher if the mother has visible lesions than if she  does not.   (C) The risk is higher if the mother has IgG antibody to HSV-2  than if she has IgM antibody.   (D) The risk is higher if the delivery occurs by cesarean section  than if the delivery is performed vaginally.   (E) The risk is higher if the mother is having an episode of recur¬  rent disease caused by HSV-2 than if it were a primary  episode.   3. Regarding varicella-zoster virus (VZV), which one of the follow¬  ing is most accurate?   (A) High-dose acyclovir can eliminate the latent state caused by  VZV.   (B) The principal site of latency of VZV is in the nucleus of motor  neurons.   (C) Domestic animals, such as pigs and chickens, are the main  reservoir for VZV.   (D) The vaccine against varicella contains all three serotypes of  formalin-killed VZV as the immunogen.   (E) When zoster occurs in an immunocompromised patient,  acyclovir should be given to prevent disseminated infection.   4. Regarding cytomegalovirus (CMV), which one of the following  is most accurate?   (A) CMV is usually acquired by the fecal-oral route in adults.   (B) Neonates born from infected mothers should be given the  subunit vaccine.   (C) Reactivation of CMV in sensory ganglion cells leads to pain¬  ful vesicles along nerves.   (D) Lamivudine should be used to treat CMV infections in  immunocompromised patients.   (E) CMV infection of a fetus during the first trimester results in  more congenital abnormalities than infection in the third  trimester.   5. Regarding Epstein-Barr virus (EBV) and infectious mononu¬  cleosis, which one of the following is most accurate?   (A) EBV enters the latent state primarily in CD4-positive helper T  cells.   (B) Approximately 10% of people in the United States have been  exposed to EBV.   (C) People with infectious mononucleosis produce antibodies  that agglutinate sheep red cells.   (D) The atypical lymphs in the blood of people with infectious  mononucleosis are EBV-infected T helper cells.   (E) Patients with deficient cell-mediated immunity should receive  passive-active immunization against EBV.   6. Naturally occurring smallpox disease has been eradicated from  the face of the Earth. Eradication was achieved by the use of the  vaccine. Regarding this vaccine, which one of the following is the  most accurate?   (A) The vaccine should be given in conjunction with preformed  antibody to the virus.   (B) Administration of the vaccine 1 day after exposure to the  virus does not protect against disease.    (C) The vaccine contains killed smallpox virus so the virus in the  vaccine does not cause adverse effects.   (D) Smallpox virus has a single stable serotype, so new formula¬  tions of the vaccine do not have to be made each year.   (E) Because domestic animals such as cows are the main reservoir  for smallpox virus, the vaccine must interrupt transmission  from these sources.   7. Your patient is a 35-year-old man who had a grand-mal seizure  this morning. Magnetic resonance imaging revealed a lesion in  the temporal lobe. A brain biopsy showed multinucleated giant  cells with intranuclear inclusion bodies. Which one of the fol¬  lowing is the most likely cause of this disease?   8. Regarding the patient in Question 7, which one of the following  is the best choice of drug to treat his infection?   9. Your patient is a 22-year-old woman with several episodes of  bloody diarrhea. She is HIV antibody positive with a CD4 count  of 50. Stool cultures for Shigella , Salmonella , and Campylobac¬  ter were negative. An assay for Clostridium difficile toxin was  negative. Colonoscopy revealed many ulcerated lesions. Biopsy  revealed cells with “owl’s eye” inclusions in the nucleus. Which  one of the following is the most likely cause of this disease?   10. Regarding the patient in Question 9, which one of the following  is the best choice of drug to treat her infection?   Brief summaries of the organisms described in this chapter  begin on page 668. Please consult these summaries for a rapid  review of the essential material.    (National Board) Practice Questions starting on page 723. Also  see Part XIV: USMLE (National Board) Practice Examination  starting on page 751.   Questions on the topics discussed in this chapter can be  found in the Clinical Virology section of Part XIII: USMLE   Adenoviruses cause a variety of upper and lower respira¬  tory tract diseases such as pharyngitis, conjunctivitis (“pink  eye”), the common cold, and pneumonia. Keratoconjuncti¬  vitis, hemorrhagic cystitis, and gastroenteritis also occur.  Some adenoviruses cause sarcomas in rodents. Table 38-1  describes some of the important clinical features of adeno¬  viruses and compares them with features of the other two  medically important viruses in this chapter, human papil¬  lomavirus (HPV) and parvovirus B19.   Adenoviruses are nonenveloped viruses with double-  stranded linear DNA and an icosahedral nucleocapsid.  They are the only viruses with a fiber protruding from each  of the 12 vertices of the capsid. The fiber is the organ of  attachment and is a hemagglutinin. When purified free of  virions, the fiber is toxic to human cells.    There are 41 known antigenic types; the fiber protein is  the main type-specific antigen. All adenoviruses have a com¬  mon group-specific antigen located on the hexon protein.   Certain serotypes of human adenoviruses (especially 12,  18, and 31) cause sarcomas at the site of injection in labora¬  tory rodents such as newborn hamsters. There is no evi¬  dence that adenoviruses cause tumors in humans.   After attachment to the cell surface via its fiber, the virus  penetrates and uncoats, and the viral DNA moves to the  nucleus. Host cell DNA-dependent RNA polymerase tran¬  scribes the early genes, and splicing enzymes remove the  RNA representing the introns, resulting in functional  mRNA. (Note that introns and exons, which are common  in eukaryotic DNA, were first described for adenovirus  DNA.) Early mRNA is translated into nonstructural pro¬  teins in the cytoplasm. After viral DNA replication in the  nucleus, late mRNA is transcribed and then translated into  structural virion proteins. Viral assembly occurs in the    nucleus, and the virus is released by lysis of the cell, not by  budding.   Adenoviruses are transmitted by several mechanisms:  aerosol droplet, fecal-oral route, and direct inoculation  of conjunctivas by tonometers or fingers. The fecal-oral  route is the most common mode of transmission among  young children and their families. Many species of animals  are infected by strains of adenovirus, but these strains are  not pathogenic for humans.   Adenovirus infections are endemic worldwide, but out¬  breaks occur among military recruits, apparently as a  result of the close living conditions that facilitate transmis¬  sion. Certain serotypes are associated with specific syn¬  dromes (e.g., types 3,4, 7, and 21 cause respiratory disease,  especially in military recruits; types 8 and 19 cause epi¬  demic keratoconjunctivitis; types 11 and 21 cause hemor¬  rhagic cystitis; and types 40 and 41 cause infantile  gastroenteritis).   Adenoviruses infect the mucosal epithelium of several  organs (e.g., the respiratory tract [both upper and lower],  the gastrointestinal tract, and the conjunctivas). Immu¬  nity based on neutralizing antibody is type-specific and  lifelong.   In addition to acute infection leading to death of the  cells, adenoviruses cause a latent infection, particularly in  the adenoidal and tonsillar tissues of the throat. In fact,  these viruses were named for the adenoids, from which  they were first isolated in 1953.   In the upper respiratory tract, adenoviruses cause such  infections as pharyngitis, pharyngoconjunctival fever, and  acute respiratory disease, characterized by fever, sore  throat, coryza (runny nose), and conjunctivitis. In the  lower respiratory tract, they cause bronchitis and atypical  pneumonia. Hematuria and dysuria are prominent in hem¬  orrhagic cystitis. Gastroenteritis with nonbloody diarrhea  occurs mainly in children younger than 2 years of age. Most  adenovirus infections resolve spontaneously. Approxi¬  mately half of all adenovirus infections are asymptomatic.   The most frequent methods of diagnosis are isolation of the  virus in cell culture and detection of a fourfold or greater  rise in antibody titer. Complement fixation and hemagglu¬  tination inhibition are the most important serologic tests.   Three live, nonattenuated vaccines against serotypes 4, 7,  and 21 are available but are used only by the military. Each  of the three vaccines is monovalent (i.e., each contains  only one serotype). The viruses are administered sepa¬  rately because they interfere with each other when given  together. The vaccines are delivered in an enteric-coated  capsule, which protects the live virus from inactivation by  stomach acid. The virus infects the gastrointestinal tract,  where it causes an asymptomatic infection and induces  immunity to respiratory disease. This vaccine is not avail¬  able for civilian use.   Epidemic keratoconjunctivitis is an iatrogenic disease,  preventable by strict asepsis and hand washing by health  care personnel who examine eyes.   Human papillomavirus (HPV) causes papillomas, which  are benign tumors of squamous cells (e.g., warts on the  skin). Some HPV types, especially types 16 and 18, cause   Papillomaviruses are nonenveloped viruses with double-  stranded circular DNA and an icosahedral nucleocapsid.  The HPV genome has seven early genes (E1-E7) and two  late genes (LI and L2). The early genes encode proteins  involved in the synthesis of viral mRNA and in the replica¬  tion of the progeny DNA genomes, and the late genes  encode the structural proteins of the progeny virions.   Two of the early genes, E6 and E7, are implicated in  carcinogenesis. They encode proteins that inactivate pro¬  teins encoded by tumor suppressor genes in human cells  (e.g., the p53 gene and the retinoblastoma [RB] gene,  respectively). Inactivation of the p53 and RB proteins is an  important step in the process by which a normal cell  becomes a cancer cell.   There are at least 100 types of papillomaviruses, classi¬  fied primarily on the basis of DNA restriction fragment  analysis. There is a pronounced predilection of certain  types to infect certain tissues. For example, skin warts are  caused primarily by HPV-1 through HPV-4, whereas geni¬  tal warts are usually caused by HPV-6 and HPV-11.  Approximately 30 types of HPV infect the genital tract.   After attachment and uncoating, the genome DNA moves  to the nucleus. Messenger RNA is synthesized by host cell  RNA polymerase with early viral protein E2 acting as a  transcriptional activator. Early viral protein El acts as a  helicase that separates the DNA strands of the incoming    viral genome. This allows the host cell DNA polymerase to  synthesize the progeny DNA genomes. The initial progeny  genomes are maintained as episomes in the nucleus. Most  of the synthesis of progeny viral DNA occurs in conjunc¬  tion with cellular DNA synthesis during S phase.   Late mRNA’s encode both the major structural protein  (LI) and the minor structural protein (L2). LI protein com¬  prises the capsid of HPV virions. LI has the ability to self-  assemble into capsids in vitro and it is this form that is the  immunogen in the HPV vaccine. L2 protein aids in the  packaging of genome DNA into the progeny virions as well  as in uncoating the genome when they infect the next cell.   In human tissue, infectious virus particles are found in  the terminally differentiated squamous cells rather than in  the basal cells (Figure 38-1 A). Note that HPV initially  infects the cells of the basal layer in the skin, but no virus is  produced by the basal cells. Rather, infectious virions are    produced by squamous cells on the surface, which enhances  the likelihood that efficient transmission will occur.   In malignant cells, viral DNA is integrated into host cell  DNA in the vicinity of cellular proto-oncogenes, and E6  and E7 are overexpressed (Figure 38—IB). However, in  latently infected, nonmalignant cells, the viral DNA is epi-  somal, and E6 and E7 are not overexpressed. This differ¬  ence occurs because another early gene, E2, controls E6 and  E7 expression. The E2 gene is functional when the viral  DNA is episomal but is inactivated when it is integrated.   Papillomaviruses are transmitted primarily by skin-to-skin  contact, including genital contact. Micro-abrasions in the  skin allow access to the basal epithelial cells where infection  begins (see Figure 38-1A).    FIGURE 38~1 A. Replication of human papilloma virus in the skin. HPV initiates replication in the basal cells of the skin at site of abrasion  to skin surface. Small black dots in nucleus of three cells represent viral genome DNA. Early protein synthesis occurs followed by progeny  genome synthesis. Late proteins are then produced and progeny virions are released from squamous cells on the surface of the skin. Large  black dots at top of figure represent progeny virions. B. Malignant transformation by HPV in the skin. HPV initiates replication in the basal cells  of the skin. Early protein synthesis occurs. Viral DNA integrates into cell DNA and large amounts of viral E6 and E7 proteins are produced. E6 and  E7 proteins inactivate tumor suppressor proteins p53 and RB and the cell becomes malignant. No late viral proteins and no progeny virions are  produced.    Genital warts are among the most common sexually  transmitted diseases. Skin warts are more common in  children and young adults and tend to regress in older  adults. HPV transmitted from an infected mother to the  neonate during childbirth causes warts in the mouth and in  the respiratory tract, especially on the larynx, of the infant.  Many species of animals are infected with their own types  of papillomaviruses, but these viruses are not an important  source of human infection.   Papillomaviruses infect squamous epithelial cells and  induce within those cells a characteristic perinuclear cyto¬  plasmic vacuole. These vacuolated cells, called koilocytes,  are the hallmark of infection by these viruses (Figure 38-2).   Most warts are benign and do not progress to malig¬  nancy. However, HPV infection is associated with carci¬  noma of the uterine cervix and penis. The proteins encoded  by viral genes E6 and E7 interfere with the growth-inhibi¬  tory activity of the proteins encoded by the p53 and RB  tumor suppressor genes and thereby contribute to onco¬  genesis by these viruses. The E6 and E7 proteins of HPV-16  bind more strongly to p53 and RB proteins than the E6 and  E7 proteins of HPV types not implicated in carcinomas—a  finding that explains why HPV-16 causes carcinomas more  frequently than the other types of HPV.   Both cell-mediated immunity and antibody are induced  by viral infection and are involved in the spontaneous  regression of warts. Immunosuppressed patients (e.g.,  patients with acquired immunodeficiency syndrome   koilocyte, seen here in a biopsy specimen of cervical intraepithelial  neoplasia caused by human papilloma virus. Koilocytes have a  small condensed nucleus and a large perinuclear cytoplasmis vacu¬  ole. 400X magnification. (Reproduced with permission from Kemp, WL,   Burns, DK and Brown TG. Pathology: The Big picture. 1st ed. New York: McGraw-Hill,  2008. Copyright © 2008 by The McGraw-Hill Companies, Inc.)   FIGURE 38 _ 3 Papillomas (warts) on finger—note dry, raised  verrucous lesions caused by human papillomavirus. (Reproduced with  permission from Wolff K, Johnson R. Fitzpatrick's Color Atlas & Synopsis of Clinical Der¬  matology. 6th ed. New York: McGraw-Hill, 2009. Copyright © 2009 by The McGraw-  Hill Companies, Inc.)    [AIDS]) have more extensive warts, and women infected  with human immunodeficiency virus (HIV) have a very  high rate of carcinoma of the cervix.   Papillomas of various organs are the predominant finding.  These papillomas are caused by specific HPV types. For  example, skin and plantar warts (Figure 38-3) are caused  primarily by HPV-1 through HPV-4, whereas genital warts  (condylomata acuminata) (Figure 38-4) are caused pri¬  marily by HPV-6 and HPV-11. HPV-6 and HPV-11 also  cause respiratory tract papillomas, especially laryngeal  papillomas, in young children.   Carcinoma of the uterine cervix, the penis, and the anus,  as well as premalignant lesions called intraepithelial neo¬  plasia, are associated with infection by HPV-16 and HPV-  18. The premalignant lesions are named for the organ  affected (e.g., cervical intraepithelial neoplasia [CIN],  penile intraepithelial neoplasia [PIN]). Occult premalig¬  nant lesions of the cervix and penis can be revealed by   FIGURE 38~4 Papillomas (warts) on penis (condylomata acu¬  minata)—note dry, raised verrucous lesions caused by human papil¬  lomavirus. (Reproduced with permission from Wolff K, Johnson R. Fitzpatrick's  Color Atlas & Synopsis of Clinical Dermatology. 6th ed. New York: McGraw-Hill, 2009.  Copyright © 2009 by The McGraw-Hill Companies, Inc.)    applying acetic acid to the tissue. HPV-16 is also implicated  as the cause of oral cancers.   Infections are usually diagnosed clinically. The presence of  koilocytes in the lesions indicates HPV infection. A poly¬  merase chain reaction (PCR)-based test can be used to  detect the presence of the DNA of 14 high-risk genotypes,  including HPV-16 and HPV-18.   Diagnostic tests based on detection of antibodies in a  patients serum or on isolation of the virus from a patient’s  tissue are not used.   The usual treatment for genital warts is podophyllin; alpha  interferon is also effective and is better at preventing recur¬  rences than are non-antiviral treatments. Liquid nitrogen is  commonly used for skin warts. Plantar warts can be  removed surgically or treated with salicylic acid topically.  Cidofovir may be useful in the treatment of severe HPV  infections.   The HPV vaccines are very effective in preventing carci¬  noma of the cervix, anal carcinoma, and genital warts. Note  that HPV vaccines have no effect on existing papillomas.   There are three vaccines against HPV. Gardasil 9, a  recombinant vaccine against nine types of HPV, contains  the capsid proteins of types 6 and 11, which cause genital  warts, and types 16 and 18, which are the two most com¬  mon causes of cervical, penile, and anal carcinoma. It also  contains the proteins of five more types (31, 33, 45, 52, and  58) that are less common causes of these cancers. A quad¬  rivalent version of Gardasil containing the proteins of types  6, 11, 16, and 18 is also available.   The third vaccine, Cervarix, is also a recombinant vaccine  but contains the proteins only of types 16 and 18. Cervarix  contains an adjuvant called AS04 that stimulates Toll-like  receptors and thereby enhances antibody production.   The role of cesarean section in preventing transmis¬  sion of HPV from a mother with genital warts to her  newborn is uncertain. Circumcision reduces the risk of  infection by HPV.   Parvovirus B19 causes erythema infectiosum (slapped  cheek syndrome, fifth disease), aplastic anemia (especially  in patients with sickle cell anemia), and fetal infections,  including hydrops fetalis.   Parvovirus B19 is a very small (22 nm) nonenveloped virus  with a single-stranded DNA genome. The genome is    negative-strand DNA, but there is no virion polymerase. The  capsid has icosahedral symmetry. There is one serotype.   After adsorption to host cell receptors, the virion pene¬  trates and moves to the nucleus, where replication occurs.  The single-stranded genome DNA has “hairpin” loops at  both of its ends that provide double-stranded areas for the  cellular DNA polymerase to initiate the synthesis of the  progeny genomes. The viral mRNA is synthesized by cel¬  lular RNA polymerase from the double-stranded DNA  intermediate. The progeny virions are assembled in the  nucleus. B19 virus replicates only when a cell is in S phase,  which explains why the virus replicates in red cell precur¬  sors but not in mature red cells.   B19 virus is transmitted primarily by the respiratory route;  transplacental transmission also occurs. Blood donated for  transfusions also can transmit the virus. B19 virus infection  occurs worldwide, and about half the people in the United  States older than 18 years of age have antibodies to the  virus. Humans are the natural reservoir; animals are not a  source of human infection.   B19 virus infects primarily two types of cells: red blood cell  precursors (erythroblasts) in the bone marrow, which  accounts for the aplastic anemia, and endothelial cells in  the blood vessels, which accounts, in part, for the rash asso¬  ciated with erythema infectiosum. Immune complexes  composed of virus and IgM or IgG also contribute to the  pathogenesis of the rash and to the arthritis that is seen in  some adults infected with B19 virus. Infection provides  lifelong immunity against reinfection.   Hydrops fetalis manifests as massive edema of the fetus.  This is secondary to congestive heart failure precipitated  by severe anemia caused by the death of parvovirus  B19-infected erythroblasts in the fetus.   This is a mild disease, primarily of childhood, character¬  ized by a bright red rash that is most prominent on the  cheeks (Figure 38-5), accompanied by low-grade fever,  runny nose (coryza), and sore throat. A “lacy,” less intense,  erythematous rash appears on the body. The symptoms  resolve in about 1 week.   The disease in children is also called fifth disease. The  four other macular or maculopapular rash diseases of  childhood are measles, rubella, scarlet fever, and roseola.   FIGURE 38~5 Slapped cheek syndrome—note erythematous  macular rash on cheeks bilaterally caused by parvovirus B19.  (Reproduced with permission from Usatine, RP et al: The Color Atlas of Family  Medicine , New York: McGraw-Hill, 2009. Courtesy of Richard P. Usatine, MD..)   Children with chronic anemia, such as sickle cell anemia,  thalassemia, and spherocytosis, can have transient but  severe aplastic anemia (aplastic crisis) when infected with  B19 virus. People with normal red blood cells do not have  clinically apparent anemia, although their red blood cell  precursors are infected.   If a woman is infected with B19 virus during the first or  second trimester of pregnancy, the virus may cross the  placenta and infect the fetus. Infection during the first  trimester is associated with fetal death, whereas infection  during the second trimester leads to hydrops fetalis.  Third-trimester infections do not result in important clini¬  cal findings. B19 virus is not a common cause of congeni¬  tal abnormalities, probably because the fetus dies when  infected early in pregnancy.   Parvovirus B19 infection in adults, especially women, can  cause arthritis mainly involving the small joints of the  hands and feet bilaterally. It resembles rheumatoid arthritis.    Other viral infections that cause an immune complex-  related arthritis include hepatitis B and rubella.   People with immunodeficiencies, especially HIV-infected,  chemotherapy, or transplant patients, can have chronic  anemia, leukopenia, or thrombocytopenia as a result of  chronic B19 infection.   Fifth disease and aplastic anemia are usually diagnosed by  detecting IgM antibodies. B19 virus can be isolated from  throat swabs, but this is not usually done. In immunocom¬  promised patients, antibodies may not be detectable; there¬  fore, viral DNA in the blood can be assayed by PCR  methods. Fetal infection can be determined by PCR analy¬  sis of amniotic fluid.   There is no specific treatment of B19 infection. Pooled  immune globulins may have a beneficial effect on chronic  B19 infection in patients with immunodeficiencies. There  is no vaccine or chemoprophylaxis.   There are three members of the polyomavirus family that  cause disease in humans, Merkel cell polyomavirus, JC  virus, and BK virus. One member, SV40 virus, is a monkey  virus that infected humans when it contaminated the  poliovirus vaccine but has not caused human disease.   Merkel cell polyomavirus causes carcinoma of the skin  and is discussed in Chapter 43. JC virus is the cause of pro¬  gressive multifocal leukoencephalopathy and is discussed  in Chapter 44. BK virus causes nephropathy in renal trans¬  plant patients and is discussed in Chapter 46. SV40 virus  causes no detectable disease in humans but does cause  sarcomas in newborn hamsters. SV40 virus is discussed as  a tumor virus in Chapter 43 and as a contaminant of the  poliovirus vaccine in Chapter 40.   (C) They are often transmitted across the placenta and cause  hydrocephalus in the fetus.   (D) The adenovirus vaccine is recommended for all children prior  to entering first grade.   (E) Laboratory diagnosis depends on seeing multinucleated giant  cells on biopsy as the virus has not been grown in cell culture.    2. Regarding human papillomavirus (HPV), which one of the fol¬  lowing statements is most accurate?   (B) Acyclovir is effective in preventing lesions caused by HPV but  does not cure the latent state.   (C) Antigen-antibody complexes play an important role in the  pathogenesis of warts caused by HPV.   (D) The early proteins of HPV play a more important role in malig¬  nant transformation than the late proteins.   (E) The diagnosis of HPV infection is usually made by detecting  cytoplasmic inclusions within giant cells in the lesions.   3. Regarding parvovirus B19, which one of the following statements  is most accurate?   (A) A vaccine is available that contains killed virus as the  immunogen.   (B) Patients infected by parvovirus B19 can be diagnosed in the  laboratory using the cold agglutinin test.   (C) Parvovirus B19 causes a severe anemia because it preferentially  infects erythrocyte precursors such as erythroblasts.   (D) It commonly infects neutrophils, resulting in an immunodefi¬  ciency that predisposes to infections by pyogenic bacteria.   (E) Parvoviruses have a double-stranded DNA genome but require  a DNA polymerase in the virion because they replicate in the  cytoplasm.   4. A 24-year-old woman is seen by her gynecologist for a routine Pap  smear. The smear shows cervical intraepithelial neoplasia grade 3  (CIN 3). You decide to examine her long-term male sexual part¬  ner. Which one of the following is the most likely finding?   Brief summaries of the organisms described in this chapter  begin on page 670. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Clinical Virology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 723. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   Influenza viruses are important human pathogens because  they cause both outbreaks of influenza that sicken and kill  thousands of people each year as well as infrequent but  devastating worldwide epidemics (pandemics).   Influenza viruses are the only members of the ortho¬  myxovirus family. The orthomyxoviruses differ from the  paramyxoviruses primarily in that the former have a  segmented RNA genome (usually eight pieces), whereas  the RNA genome of the latter consists of a single piece. 1 * *  The term myxo refers to the observation that these  viruses interact with mucins (glycoproteins on the sur¬  face of cells).   In addition, the orthomyxoviruses are smaller (110 nm  in diameter) than the paramyxoviruses (150 nm in diame¬  ter). See Table 39-1 for additional differences.   Table 39-2 shows a comparison of influenza A virus  with several other viruses that infect the respiratory tract.  Table 39-3 describes some of the important clinical    1 The total molecular weight of influenza virus RNA is approximately   (2-4) x 10 6 , whereas the molecular weight of paramyxovirus RNA is    features of influenza virus and compares them with the  clinical features of the other medically important viruses in  this chapter.   In 1997, an outbreak of human influenza (avian influ¬  enza, bird flu) caused by an H5N1 strain of influenza A  virus began. This outbreak and subsequent outbreaks are  described on page 317. In 2009, there was an outbreak of  human influenza caused by H1N1 influenza A virus of  swine origin (swine-origin influenza virus, S-OIV). This  outbreak and the subsequent pandemic are described on  page 317. In 2013, an outbreak of influenza caused by an  H7N9 strain of influenza virus occurred.   Influenza A virus causes worldwide epidemics (pandemics)  of influenza, influenza B virus causes major outbreaks of  influenza, and influenza C virus causes mild respiratory  tract infections but does not cause outbreaks of influenza.  Pandemics occur when a variant of influenza A virus that  contains a new hemagglutinin against which people do not  have preexisting antibodies is introduced into the human  population.    TABLE 39-2 Features of Viruses That Infect the Respiratory Tract 1   'influenza virus, parainfluenza virus, respiratory syncytial virus, rubella virus, measles virus, mumps virus, and coronavirus are enveloped RNA viruses and are described in this  chapter.    The pandemics caused by influenza A virus occur infre¬  quently (the last one was in 1968), but major outbreaks  caused by this virus occur virtually every year in many  countries. Each year, influenza is the most common cause  of respiratory tract infections that result in physician visits  and hospitalizations in the United States.   In the 1918 influenza pandemic, more Americans died  than in World War I, World War II, the Korean War, and  the Vietnam War combined. Influenza B virus does not  cause pandemics, and the major outbreaks caused by this  virus do not occur as often as those caused by influenza A  virus. It is estimated that approximately 36,000 people die  of influenza each year in the United States.   Influenza virus is composed of a segmented single-  stranded RNA genome, a helical nucleocapsid, and an  outer lipoprotein envelope (Figure 39-1). The virion con-   FIGURE 39-1 Influenza virus—electron micrograph. Long  arrow points to the helical nucleocapsid of influenza virus. The  nucleocapsid contains the segmented, negative-polarity genome  RNA. Short arrow points to the spikes on the virion envelope. The  spikes are the hemagglutinin and neuraminidase proteins. (Source:  Dr. Erskine Palmer and Dr. M. Martin, Public Health Image Library, Centers for  Disease Control and Prevention.)    tains an RNA-dependent RNA polymerase, which tran¬  scribes the negative-polarity genome into mRNA.   The envelope is covered with two different types of  spikes, a hemagglutinin and a neuraminidase.” Influenza  A virus has 16 antigenically distinct types of hemagglutinin  and 9 antigenically distinct types of neuraminidase. As  discussed later, some of these types cause disease in  humans, but most of the types typically cause disease in  other animal species such as birds, horses, and pigs.   The function of the hemagglutinin is to bind to the cell  surface receptor (neuraminic acid, sialic acid) to initiate  infection of the cell. In the clinical laboratory, the hemag¬  glutinin agglutinates red blood cells, which is the basis of a  diagnostic test called the hemagglutination inhibition test.  The hemagglutinin is also the target of neutralizing anti¬  body (i.e., antibody against the hemagglutinin inhibits  infection of the cell).   The neuraminidase cleaves neuraminic acid (sialic acid)  to release progeny virus from the infected cell. The hemag¬  glutinin functions at the beginning of infection, whereas  the neuraminidase functions at the end. Neuraminidase  also degrades the protective layer of mucus in the respira¬  tory tract. This enhances the ability of the virus to gain  access to the respiratory epithelial cells.   Influenza viruses, especially influenza A virus, show  changes in the antigenicity of their hemagglutinin and  neuraminidase proteins; this property contributes to their  capacity to cause devastating worldwide epidemics (pan¬  demics). There are two types of antigenic changes: (1)  antigenic shift, which is a major change based on the reas¬  sortment of segments of the genome RNA; and (2) anti¬  genic drift, which is a minor change based on mutations in  the genome RNA. Note that in reassortment, entire seg¬  ments of RNA are exchanged, each one of which codes for  a single protein (e.g., the hemagglutinin) (Figure 39-2).   Influenza A virus has two matrix proteins: The Ml  matrix protein is located between the internal nucleopro-  tein and the envelope and provides structural integrity. The  M2 matrix protein forms an ion channel between the  interior of the virus and the external milieu. This ion  channel plays an essential role in the uncoating of the    2 Paramyxoviruses also have a hemagglutinin and a neuraminidase, but  the two proteins are located on the same spike.   FIGURE 39-2 Antigenic shift in influenza virus. A human  strain of influenza virus containing the gene encoding one antigenic  type of hemagglutinin (colored orange) infects the same lung cell as  a chicken strain of influenza virus containing the gene encoding a  different antigenic type of hemagglutinin (colored black). Reassort¬  ment of the genome RNA segments that encode the hemagglutinin  occurs, and a new strain of influenza virus is produced containing the  chicken type of hemagglutinin (colored black).   virion after it enters the cell. It transports protons into the  virion causing the disruption of the envelope, which frees  the nucleocapsid containing the genome RNA, allowing it  to migrate to the nucleus.   (1) The internal ribonucleoprotein in the nucleocapsid  is the group-specific antigen that distinguishes influenza A,  B, and C viruses.   (2) The hemagglutinin and the neuraminidase are the  type-specific antigens located on the surface. Antibody  against the hemagglutinin neutralizes the infectivity of the  virus (and prevents disease), whereas antibody against the  group-specific antigen (which is located internally) does  not. Antibody against the neuraminidase does not neutral¬  ize infectivity but does reduce disease by decreasing the  amount of virus released from the infected cell and thus  reducing spread of the virus to adjacent cells.   An important determinant of the virulence of this virus  is a nonstructural protein called NS-1 encoded by the  genome RNA of influenza virus. NS-1 has several    functions, but the one pertinent to virulence is its ability to  inhibit the production of interferon mRNA. As a result,  innate defenses are reduced and viral virulence is corre¬  spondingly enhanced.   Many species of animals (e.g., aquatic birds, chickens,  swine, and horses) have their own influenza A viruses.  These animal viruses are the source of the RNA segments  that encode the antigenic shift variants that cause epidem¬  ics among humans. For example, if an avian and a human  influenza A virus infect the same cell (e.g., in a farmer’s  respiratory tract), reassortment could occur and a new  variant of the human A virus, bearing the avian virus hem¬  agglutinin, may appear (see Figure 39-1).   There is evidence that aquatic birds (waterfowl) are a  common source of these new genes and that the reassort¬  ment event leading to new human strains occurs in pigs. In  other words, pigs may serve as the “mixing bowl” within  which the human, avian, and swine viruses reassort. There  are 16 types of hemagglutinin (HI to Hi6) and 9 types of  neuraminidase (N1 to N9) found in waterfowl. In humans,  three types of hemagglutinin (HI, H2, and H3) and two  types of neuraminidase (N1 and N2) predominate.   Because influenza B virus is only a human virus, there is  no animal source of new RNA segments. Influenza B virus  therefore does not undergo antigenic shifts. It does, how¬  ever, undergo enough antigenic drift that the current strain  must be included in the new version of the influenza vac¬  cine produced each year. Influenza B virus has no antigens  in common with influenza A virus.   A/Philippines/82 (H3N2) illustrates the nomenclature  of influenza viruses. “A” refers to the group antigen. Next  are the location and year the virus was isolated. H3N2 is the  designation of the hemagglutinin (H) and neuraminidase  (N) types. The H1N1 and H3N2 strains of influenza A  virus are the most common at this time and are the strains  included in the current vaccine. The H2N2 strain caused a  pandemic in 1957.   The virus adsorbs to the cell when the viral hemagglutinin  interacts with sialic acid receptors on the cell surface. (The  hemagglutinin on the virion surface is cleaved by extracel¬  lular proteases to generate a modified hemagglutinin that  actually mediates attachment to the cell surface.) The virus  then enters the cell in vesicles and uncoats within an endo-  some. Uncoating is facilitated by the low pH within the  endosome. Protons pass through the ion channel formed  by the M2 protein into the interior of the virion. This dis¬  rupts the virion envelope and frees the nucleocapsid to  enter the cytoplasm and then migrate to the nucleus where  the genome RNA is transcribed.   The virion RNA polymerase transcribes the eight  genome segments into eight mRNAs in the nucleus. Syn¬  thesis of the eight mRNAs occurs in the nucleus because a  methylated guanosine “cap” is required. The cap is obtained  from cellular nuclear RNAs in a process called “cap    snatching.” Most of the mRNAs move to the cytoplasm,  where they are translated into viral proteins. Some of the  viral mRNAs remain in the nucleus, where they serve as the  template for the synthesis of the negative-strand RNA  genomes for the progeny virions. Replication of the prog¬  eny genomes is performed by a different subunit of the viral  RNA polymerase (acting as a replicase) from the subunit  that functioned earlier as a transcriptase that synthesized  the mRNAs. Two newly synthesized proteins, NP protein  and matrix protein, bind to the progeny RNA genome in  the nucleus, and that complex is transported to the  cytoplasm.   The helical ribonucleoprotein assembles in the cyto¬  plasm, matrix protein mediates the interaction of the  nucleocapsid with the envelope, and the virion is released  from the cell by budding from the outer cell membrane at  the site where the hemagglutinin and neuraminidase are  located. The neuraminidase releases the virus by cleaving  neuraminic acid on the cell surface at the site of the bud¬  ding progeny virions. Influenza virus, hepatitis delta virus,  and retroviruses are the only RNA viruses that have an  important stage of their replication take place in the  nucleus.   The virus is transmitted by airborne respiratory droplets.  The ability of influenza A virus to cause epidemics is  dependent on antigenic changes in the hemagglutinin and  neuraminidase. As mentioned previously, influenza A virus  undergoes both major antigenic shifts as well as minor  antigenic drifts. Antigenic shift variants appear infre¬  quently, whereas drift variants appear virtually every year.  The last major antigenic shift that caused a pandemic in  humans was in 1968 when H3N2 emerged. Epidemics and  pandemics (worldwide epidemics) occur when the antige¬  nicity of the virus has changed sufficiently that the preexist¬  ing immunity of many people is no longer effective. The  antigenicity of influenza B virus undergoes antigenic drift  but not antigenic shift. The antigenic changes exhibited by  influenza B virus are less dramatic and less frequent than  those of influenza A virus.   Influenza occurs primarily in the winter months of  December to February in the northern hemisphere, when  influenza and bacterial pneumonia secondary to influenza  cause a significant number of deaths, especially in older  people. The morbidity of influenza in children younger  than 2 years is also very high, second only to the morbidity  in the elderly. In the southern hemisphere (e.g., in Australia  and New Zealand), influenza occurs primarily in the winter  months of June through August. In the tropics, influenza  occurs year round with little seasonal variation.   Influenza virus infection causes inflammation of the  mucosa of upper respiratory tract sites such as the nose and    pharynx, and lower respiratory tract sites such as the lar¬  ynx, trachea, and bronchi. Pneumonia, which involves the  alveoli may also occur.   After the virus has been inhaled, the neuraminidase  degrades the protective mucus layer, allowing the virus to  gain access to the cells of the upper and lower respiratory  tract. The infection is limited primarily to this area because  the proteases that cleave the hemagglutinin are located in  the respiratory tract.   Despite systemic symptoms, viremia rarely occurs. The  systemic symptoms, such as severe myalgias, are due to  cytokines circulating in the blood. There is necrosis of the  superficial layers of the respiratory epithelium. Influenza  virus pneumonia, which can complicate influenza, is inter¬  stitial in location.   Immunity depends mainly on secretory IgA in the respi¬  ratory tract. IgG is also produced but is less protective.  Cytotoxic T cells also play a protective role.   After an incubation period of 24 to 48 hours, fever, myal¬  gias, headache, sore throat, and cough develop suddenly.  Severe myalgias (muscle pains) coupled with respiratory  tract symptoms are typical of influenza. Vomiting and diar¬  rhea are rare. The symptoms usually resolve spontaneously  in 4 to 7 days, but influenzal or bacterial pneumonia may  complicate the course. One of the well-known complications  of influenza is pneumonia caused by either Staphylococcus  aureus or Streptococcus pneumoniae.   Reye’s syndrome, characterized by encephalopathy and  liver degeneration, is a rare, life-threatening complication  in children following some viral infections, particularly  influenza B and chickenpox. Aspirin given to reduce fever  in viral infections has been implicated in the pathogenesis  of Reye’s syndrome.   Although most diagnoses of influenza are made on clinical  grounds, laboratory tests are available. The test most com¬  monly used is an enzyme-linked immunosorbent assay  (ELISA) for viral antigen in respiratory secretions such as  nasal or throat washings, nasal or throat swabs, or sputum.  Several rapid ELISA tests suitable for a physician’s office  laboratory are available. Two tests (FLU OIA and QuickVue  Influenza Test) are based on detection of viral antigen using  monoclonal antibodies, and a third test (ZSTATFLU) is  based on detection of viral neuraminidase using a substrate  of the enzyme that changes color when cleaved by neur¬  aminidase. The rationale for using the rapid tests is that  treatment with the neuraminidase inhibitors should be  instituted within 48 hours of the onset of symptoms. Other  tests such as direct fluorescent antibody and polymerase  chain reaction (PCR) are also used.   Influenza can also be diagnosed by the detection of anti¬  bodies in the patient’s serum. A rise in antibody titer of at    least fourfold in paired serum samples taken early in the  illness and 10 days later is sufficient for diagnosis. Either  the hemagglutination inhibition or complement fixation  (CF) test can be used to assay the antibody titer. Because  the second sample is taken 10 days later, this approach is  used to make a retrospective diagnosis, often for epidemio¬  logic purposes.   Oseltamivir (Tamiflu) taken orally and zanamivir (Relenza)  inhaled into the nose are the two most commonly used  drugs for the treatment of influenza. A third drug, perami-  vir (Rapivab) is administered intravenously and became  available in 2015. They are members of a class of drugs  called neuraminidase inhibitors, which act by inhibiting  the release of virus from infected cells. This limits the  extent of the infection by reducing the spread of virus from  one cell to another. These drugs are effective against both  influenza A and B viruses.   Resistance to Tamiflu occurs but currently is not clini¬  cally significant. Some isolates of HINI influenza virus are  resistant to Tamiflu. However, H3N2 strains are still sus¬  ceptible to Tamiflu. Both HINI and H3N2 strains remained  susceptible to Relenza. All influenza B strains are suscepti¬  ble to Tamiflu and Relenza.   Tamiflu pills are administered orally, whereas Relenza  is delivered by inhaling the powder directly into the respi¬  ratory tract. Clinical studies showed they reduce the dura¬  tion of symptoms by 1 to 2 days. They also reduce the  amount of virus produced and therefore reduce the chance  of spread to others. These drugs are most effective when  taken within 48 hours of the onset of symptoms. In 2015,  some concern regarding the efficacy of Tamiflu and  Relenza arose. Additional studies are needed to resolve  this issue.   Amantadine (Symmetrel) is approved for both the  treatment and prevention of influenza A. However, 90% of  the H3N2 strains in the United States are resistant to  amantadine (and rimantadine, see later), and so these  drugs are no longer recommended. These drugs block the  M2 ion channel, thereby inhibiting uncoating. Resistance  is caused primarily by mutations in the gene for the M2  protein.   Note that amantadine is effective only against influenza  A, not against influenza B. Rimantadine (Flumadine), a  derivative of amantadine, can also be used for treatment  and prevention of influenza A and has fewer side effects  than amantadine. It should be emphasized that the vaccine  is preferred over these drugs in the prevention of  influenza.   The main mode of prevention is the vaccine, which con¬  tains both influenza A and B viruses. Prior to 2013, the  vaccine was trivalent and contained recent isolates of two A    strains (H1N1 and H3N2) and one B strain. In 2013, quad¬  rivalent vaccines containing two A strains and two B strains  became available. The vaccine is usually reformulated each  year to contain the current antigenic strains.   There are two main types of influenza vaccines available  in the United States, a killed vaccine and a live, attenuated  vaccine. The vaccine that has been used for many years is a  killed vaccine containing purified protein subunits of the  virus (hemagglutinin and neuraminidase). The virus is  inactivated with formaldehyde and then treated with a lipid  solvent that disaggregates the virions. Note that the hemag¬  glutinin is the most important antigen because it elicits  neutralizing antibody. This vaccine is typically adminis¬  tered intramuscularly. A high-dose killed vaccine that  contains four times as much hemagglutinin as the standard  vaccine is recommended for those over 65 years of age. In  2011, a killed influenza vaccine that can be administered  intradermally became available.   The other vaccine is a live, attenuated vaccine contain¬  ing temperature-sensitive mutants of influenza A and B  viruses. These temperature-sensitive mutants can repli¬  cate in the cooler (33°C) nasal mucosa where they induce  IgA, but not in the warmer (37°C) lower respiratory tract.  The live virus in the vaccine therefore immunizes but  does not cause disease. This vaccine is administered by  spraying into the nose (“nasal mist”). This vaccine is rec¬  ommended for children whereas the inactivated vaccine is  recommended for adults. The live vaccine should not be  given to pregnant women or to immunocompromised  individuals.   Most of the vaccines described above are made in  chicken eggs, and anyone who has a significant allergy to  egg proteins (e.g., anaphylaxis) should not receive these  vaccines. However, in 2012, the U.S. Food and Drug  Administration (FDA) approved a killed influenza vaccine  (Flucelvax) made in calf kidney cell culture. This vaccine  has two advantages: It can be given to those with egg  allergy, and it has a short turnaround time, so the latest  drift mutant can be used.   Also in 2012, the FDA approved a recombinant vaccine  (Flublok) made by inserting the gene encoding the viral  hemagglutinin into an insect virus (baculovirus) that is  propagated in insect cell culture. This vaccine contains  purified hemagglutinin as the immunogen. This vaccine  can also be given to those with egg allergy.   Note that the killed vaccine is not a good immunogen,  because little IgA is made and the titer of IgG is relatively  low. Protection lasts only 6 months. Yearly boosters are  recommended and should be given shortly before the flu  season (e.g., in October). These boosters also provide an  opportunity to immunize against the latest antigenic  changes. The vaccine should be given to all persons 6  months and older who do not have a contraindication to  receive the vaccine. It is especially important that people  with chronic diseases, particularly respiratory and cardio¬  vascular conditions, receive the vaccine. It should also be    given to health care personnel who are likely to transmit  the virus to those at high risk.   One side effect of the influenza vaccine used in the  1970s containing the swine influenza strain that caused  influenza in humans was an increased risk of Guillain-  Barre syndrome, which is characterized by an ascending  paralysis. Analysis of the side effects of the influenza vac¬  cines in use during the last 10 years has shown no increased  risk of Guillain-Barre syndrome.   In addition to the vaccine, influenza can be prevented by  using oseltamivir (Tamiflu), which is described in the treat¬  ment section earlier. Oseltamivir is particularly useful in  elderly people who have not been immunized and who may  have been exposed. Note that this drug should not be  thought of as a substitute for the vaccine. Immunization is  the most reliable mode of prevention.   In 1997, the H5N1 strain of influenza A virus that causes  avian influenza, primarily in chickens, caused an aggres¬  sive form of human influenza with high mortality in Hong  Kong. In the winter of 2003-2004, an outbreak of avian  influenza caused by H5N1 strain killed thousands of  chickens in several Asian countries. Millions of chickens  were killed in an effort to stop the spread of the disease.  Four hundred eight human cases of H5N1 influenza  occurred between 2003 and February 2009, resulting in  254 deaths (a mortality rate of 62%). Note that these 408  people were infected directly from chickens. Both the  respiratory secretions and the chicken guano contain  infectious virus.   The spread of the H5N1 strain from person to person  occurs rarely but remains a major concern because it could  increase dramatically if reassortment with the human-  adapted strains occurs. In 2005, the H5N1 virus spread  from Asia to Siberia and into eastern Europe, where it  killed thousands of birds but has not caused human dis¬  ease. As of this writing (February 2016), there have been no  cases of human influenza caused by an H5N1 virus in the  United States. However, there have been two cases of  human influenza caused by an H7N2 strain of avian influ¬  enza virus.   The ability of the H5N1 strain to infect chickens (and  other birds) more effectively than humans is due to the  presence of a certain type of viral receptor throughout the  mucosa of the chicken respiratory tract. In contrast,  humans have this type of receptor only in the alveoli, not in  the upper respiratory tract. This explains why humans are  rarely infected with the H5N1 strain. However, when the  exposure is intense, the virus is able to reach the alveoli and  causes severe pneumonia.   The virulence of the H5N1 strain is significantly greater  than the H1N1 and H3N2 strains that have been causing    disease in humans for many years. This is attributed to two  features of the H5N1 strain, namely, relative resistance to  interferon and increased induction of cytokines, especially  tumor necrosis factor. The increase in cytokines is thought  to mediate the pathogenesis of the pneumonia and acute  respiratory distress syndrome (ARDS) seen in H5N1  infection.   The H5N1 strain is sensitive to the neuraminidase  inhibitors, oseltamivir (Tamiflu) and zanamivir (Relenza),  but not to amantadine and rimantadine. Tamiflu is the  drug of choice for both treatment and prevention. There is  no human vaccine available against the H5N1 strain, but  there is one available for use in avian species. In 2008, the  FDA approved an inactivated vaccine against H5N1 influ¬  enza virus, but as of this writing, it is not available to the  public. The vaccine is being stockpiled in the National  Emergency Reserve.   In 2013, an outbreak of influenza caused by an H7N9  strain of influenza virus occurred. Prior to this time, the  H7N9 strain affected only birds, especially chickens. As  of July 2013, 133 people have been diagnosed with influ¬  enza caused by this virus, 43 of whom have died (32%  mortality rate). Cases have been limited to China and  Taiwan. There has been no sustained person-to-person  spread.   All of the genes of this virus are of avian origin. It  acquired its H7 gene from ducks and its N9 gene from wild  birds, and all the other genes are from an influenza strain  that infects bramblings, a bird common in Asia and  Europe. This H7N9 strain is susceptible to the neuramini¬  dase inhibitors, oseltamivir and zanamivir. There is no  vaccine.   In April 2009, a novel swine origin strain of influenza A  (H1N1) virus (swine-origin influenza virus, S-OIV) caused  an outbreak of human influenza, which appeared first in  Mexico, then in the United States, followed by spread to  208 countries by December 2009. The Centers for Disease  Control and Prevention (CDC) uses the name “novel influ¬  enza A (H1N1)” for this virus.   As of December 2009, millions of cases have occurred  worldwide. There have been so many cases that most coun¬  tries have stopped documenting the number of cases.  Worldwide there have been 9596 deaths, of which 1445  have occurred in the United States. On June 11, 2009, the  World Health Organization (WHO) declared a level 6 pan¬  demic (the highest level alert). By August 2010, the number  of cases had declined significantly and the pandemic warn¬  ing was rescinded. As of this writing in February 2016, the  number of cases in the United States and worldwide have  significantly declined.    The disease affected primarily young people (60% of  cases were 18 years old or younger). Symptoms were in  general mild, with the few fatalities occurring in medically  compromised patients. There was no outbreak of swine  influenza in pigs prior to this human outbreak. Eating pork  does not transmit the virus.   S-OIV is a quadruple reassortant: The hemagglutinin,  nucleoprotein, and nonstructural protein genes are of  North American swine origin, the neuraminidase and  matrix protein genes are of Eurasian swine origin, the genes  that encode two subunits of the polymerase are of North  American avian origin, and the gene that encodes the third  subunit of the polymerase is of human H3N2 origin.   A triple reassortant strain circulated in North American  swine for several years prior to 2009 but caused human  influenza only rarely. In the triple reassortant strain, all five    of the genes that are not polymerase genes are of North  American swine origin and the polymerase genes have the  same origin as the quadruple reassortant. This strain does  not have genes of Eurasian swine origin.   The key point is that most people worldwide do not  have protective antibodies against the swine hemagglutinin  of S-OIV even though they may have antibodies against the  seasonal strain of H1N1 virus acquired either by immuni¬  zation or by exposure to the virus itself. Note also that  S-OIV spreads readily from human to human in contrast to  the avian EI5N1 strain that does not.   A PCR test for the diagnosis of S-OIV infection is avail¬  able. S-OIV is sensitive to oseltamivir and zanamivir but  resistant to amantadine and rimantadine. Both an inacti¬  vated and a live, attenuated vaccine against S-OIV became  widely available in November 2009.   The paramyxovirus family contains four important human  pathogens: measles virus, mumps virus, respiratory syncy¬  tial virus, and parainfluenza viruses. They differ from  orthomyxoviruses in that their genomes are not seg¬  mented, they have a larger diameter, and their surface  spikes are different (see Table 39-1).   Paramyxoviruses are composed of one piece of single-  stranded RNA, a helical nucleocapsid, and an outer lipopro¬  tein envelope. The virion contains an RNA-dependent RNA  polymerase, which transcribes the negative-polarity  genome into mRNA. The genome is therefore not infectious.  The envelope is covered with spikes, which contain hemag¬  glutinin, neuraminidase, or a fusion protein that causes cell  fusion and, in some cases, hemolysis (Table 39-4).   This virus causes measles, a disease characterized by a  maculopapular rash. It occurs primarily in childhood. (See  “Clinical Findings” section for additional information.)   The genome RNA and nucleocapsid of measles virus are  those of a typical paramyxovirus (see earlier). The virion  has two types of envelope spikes, one with hemagglutinat-  ing activity and the other with cell-fusing and hemolytic  activities (see Table 39-4). It has a single serotype, and the  hemagglutinin is the antigen against which neutralizing  antibody is directed. Humans are the natural host.   After adsorption to the cell surface via its hemaggluti¬  nin, the virus penetrates and uncoats and the virion  RNA polymerase transcribes the negative-strand genome  into mRNA. Multiple mRNAs are synthesized, each of  which is translated into the specific viral proteins; no  polyprotein analogous to that synthesized by poliovirus  is made. The helical nucleocapsid is assembled, the  matrix protein mediates the interaction with the enve¬  lope, and the virus is released by budding from the cell  membrane.   2 ln mumps and parainfluenza viruses, the hemagglutinin and neuraminidase are on the same spike, and the fusion protein is on a different spike.   Measles virus is transmitted via respiratory droplets pro¬  duced by coughing and sneezing both during the prodro¬  mal period and for a few days after the rash appears.  Measles occurs worldwide, usually in outbreaks every 2 to  3 years, when the number of susceptible children reaches a  high level. The WHO estimates there are 30 million cases  of measles each year worldwide.   In the year 2000, CDC declared measles eliminated  from the United States. Elimination meant that sustained  transmission within the United States no longer occurred.  However, cases acquired abroad (imported cases) followed  by small outbreaks continue to occur.   The attack rate is one of the highest of viral diseases;  most children contract the clinical disease on exposure.  When this virus is introduced into a population that has  not experienced measles, such as the inhabitants of the  Hawaiian Islands in the 1800s, devastating epidemics  occur. In malnourished children, especially those in devel¬  oping countries, measles is a much more serious disease  than in well-nourished children. Vitamin A deficiency is  especially important in this regard, and supplementation of  this vitamin greatly reduces the severity of measles. Patients  with deficient cell-mediated immunity (e.g., AIDS patients)  have a severe, life-threatening disease when they contract  measles.   After infecting the cells lining the upper respiratory tract,  the virus enters the blood and infects reticuloendothelial  cells, where it replicates again. It then spreads via the blood  to the skin. The rash is caused primarily by cytotoxic T  cells attacking the measles virus-infected vascular endo¬  thelial cells in the skin. Antibody-mediated vasculitis may  also play a role. Shortly after the rash appears, the virus can  no longer be recovered and the patient can no longer  spread the virus to others. Multinucleated giant cells,  which form as a result of the fusion protein in the spikes,  are characteristic of the lesions.   Lifelong immunity occurs in individuals who have had  the disease. Although IgG antibody may play a role in neu¬  tralizing the virus during the viremic stage, cell-mediated  immunity is more important. The importance of cell-  mediated immunity is illustrated by the fact that agamma-  globulinemic children have a normal course of disease, are  subsequently immune, and are protected by immunization.  Maternal antibody passes the placenta, and infants are pro¬  tected during the first 6 months of life.   Infection with measles virus can transiently depress  cell-mediated immunity against other intracellular micro¬  organisms, such as Mycobacterium tuberculosis, leading to a  loss of purified protein derivative (PPD) skin test reactivity,  reactivation of dormant organisms, and clinical disease.  The proposed mechanism for this unusual finding is that  when measles virus binds to its receptor (called CD46) on    the surface of human macrophages, the production of  interleukin-12 (IL-12), which is necessary for cell-mediated  immunity to occur, is suppressed.   After an incubation period of 10 to 14 days, a prodromal  phase characterized by fever, conjunctivitis (causing photo¬  phobia), running nose, and coughing occurs. Koplik’s  spots are bright red lesions with a white, central dot that are  located on the buccal mucosa and are virtually diagnostic.  A few days later, a maculopapular rash appears on the face  and proceeds gradually down the body to the lower  extremities, including the palms and soles (Figure 39-3).  The rash develops a brownish hue several days later.   The complications of measles can be quite severe.  Encephalitis occurs at a rate of 1 per 1000 cases of measles.  The mortality rate of encephalitis is 10%, and there are  permanent sequelae, such as deafness and mental retarda¬  tion, in 40% of cases. In addition, both primary measles  (giant cell) pneumonia and secondary bacterial pneumonia  occur. Bacterial otitis media is quite common. Subacute  sclerosing panencephalitis (SSPE) is a rare, fatal disease of  the central nervous system that occurs several years after  measles (see Chapter 44).   papular rash. (Source: Public Health Image Library, Centers for Disease Control  and Prevention.)    Measles in a pregnant woman leads to an increased risk  of stillbirth rather than congenital abnormalities. Measles  virus infection of the fetus is more severe than rubella virus  infection, so the former typically causes fetal death, whereas  the latter causes congenital abnormalities.   Atypical measles occurs in some people who were given  the killed vaccine and were subsequently infected with  measles virus. It is characterized by an atypical rash without  Koplik’s spots. Because the killed vaccine has not been used  for many years, atypical measles occurs only in adults and  is infrequent.   Most diagnoses are made on clinical grounds, but the virus  can be isolated in cell culture. A greater than fourfold rise  in antibody titer can be used to diagnose difficult cases.  PCR assay is also used.   Prevention rests on immunization with the live, attenuated  vaccine. The vaccine is effective and causes few side effects.  It is given subcutaneously to children at 15 months of age,  usually in combination with rubella and mumps vaccines.  The vaccine should not be given to children prior to 15  months of age, because maternal antibody in the child  can neutralize the virus and reduce the immune response.  Because immunity can wane, a booster dose is recom¬  mended. The vaccine contains live virus, so it should not be  given to immunocompromised persons or pregnant  women. The vaccine has decreased the number of cases of  measles greatly in the United States; there were only 138  reported cases of measles in 1997. However, outbreaks still  occur among unimmunized individuals (e.g., children in  inner cities and in developing countries).   The killed vaccine should not be used. Immune globu¬  lin can be used to modify the disease if given to unimmu¬  nized individuals early in the incubation period. This is  especially necessary if the unimmunized individuals are  immunocompromised.   This virus causes mumps, a disease characterized by salivary  gland swelling. It occurs primarily in childhood. (See “Clini¬  cal Findings” section for a more complete description.)   The genome RNA and nucleocapsid are those of a typical  paramyxovirus. The virion has two types of envelope    spikes: one with both hemagglutinin and neuraminidase  activities and the other with cell-fusing and hemolytic  activities (see Table 39-4).   The virus has a single serotype. Neutralizing antibody is  directed against the hemagglutinin. The internal nucleo¬  capsid protein is the S (soluble) antigen detected in the CF  test used for diagnosis. Humans are the natural host.   Replication is similar to that of measles virus (see page 318).   Mumps virus is transmitted via respiratory droplets.  Mumps occurs worldwide, with a peak incidence in the  winter. About 30% of children have a subclinical (inappar-  ent) infection, which confers immunity. There were only  683 reported cases of mumps in the United States in  1997—a finding attributed to the widespread use of the  vaccine. However, in 2006, a resurgence of mumps  occurred, with 6584 cases being recorded despite a high  (87%) coverage rate for the vaccine.   The virus infects the upper respiratory tract and then  spreads through the blood to infect the salivary glands,  especially the parotid gland, testes, ovaries, pancreas, and,  in some cases, meninges. Alternatively, the virus may  ascend from the buccal mucosa up Stensens duct to the  parotid gland.   Lifelong immunity occurs in persons who have had the  disease. There is a popular misconception that unilateral  mumps can be followed by mumps on the other side.  Mumps occurs only once; subsequent cases of parotitis can  be caused by other viruses such as parainfluenza viruses, by  bacteria, and by duct stones. Maternal antibody passes the  placenta and provides protection during the first 6 months  of life.   After an incubation period of 18 to 21 days, a prodromal  stage of fever, malaise, and anorexia is followed by tender  swelling of the salivary glands, either unilateral or bilateral  (Figure 39-4). There is a characteristic increase in parotid  gland pain when drinking citrus juices. The disease is typi¬  cally benign and resolves spontaneously within 1 week.   Two complications are of significance. One is orchitis in  postpubertal males, which, if bilateral, can result in sterility.  Postpubertal males have a fibrous tunica albuginea, which  resists expansion, thereby causing pressure necrosis of the  spermatocytes. Unilateral orchitis, although quite painful,  does not lead to sterility. The other complication is menin¬  gitis, which is usually benign, self-limited, and without  sequelae. Mumps virus, Coxsackie virus, and echovirus are  the three most frequent causes of viral (aseptic) meningitis.   inflammation of salivary glands. Note also absence of a rash as  mumps is not a rash disease, unlike measles and rubella. (Source:   Dr. Patricia Smith and Dr. Barbara Rice, Public Health Image Library, Centers for  Disease Control and Prevention.)   The widespread use of the vaccine in the United States has  led to a marked decrease in the incidence of mumps  meningitis.   The diagnosis of mumps is usually made clinically, but  laboratory tests are available for confirmation. The virus  can be isolated in cell culture from saliva, spinal fluid, or  urine. PCR assay can also be used. In addition, a fourfold  rise in antibody titer in either the hemagglutination inhibi¬  tion or the CF test is diagnostic. A single CF test that assays  both the S and the V (viral) antigens can also be used.  Because antibody to S antigen appears early and is short¬  lived, it indicates current infection. If only V antibody is  found, the patient has had mumps in the past.   A mumps skin test based on delayed hypersensitivity  can be used to detect previous infection, but serologic  tests are preferred. The mumps skin test is widely used to  determine whether a patients cell-mediated immunity is  competent.   Prevention consists of immunization with the live, attenu¬  ated vaccine. The vaccine is effective and long-lasting (at  least 10 years) and causes few side effects. Two immuniza¬  tions are recommended, one at 15 months and a booster  dose at 4 to 6 years, usually in combination with measles  and rubella vaccines. Because it is a live vaccine, it should  not be given to immunocompromised persons or pregnant  women. Immune globulin is not useful for preventing or  mitigating mumps orchitis.   In the late 1980s, outbreaks of mumps occurred in  both immunized and unimmunized people. This led to  the recommendation in 1989 that a second course of the  MMR (measles, mumps, rubella) vaccine be adminis¬  tered. The incidence of mumps fell, and outbreaks did not  occur until 2006, when 6584 cases occurred, primarily in  college-age individuals who, surprisingly, had received  two doses of the vaccine. Waning immunity after the sec¬  ond dose and immunization with a different genotype  from the genotype that caused the outbreak are suggested  explanations.   In 2009 and again in 2014, outbreaks of mumps occurred  in young adults including those who had received two  doses of vaccine. In many individuals, more than 10 years  had elapsed since their last MMR immunization, indicating  that waning immunity may play a role.   Respiratory syncytial virus (RSV) is the most important  cause of pneumonia and bronchiolitis in infants. It is also  an important cause of otitis media in children and of pneu¬  monia in the elderly and in patients with chronic cardio¬  pulmonary diseases.   The genome RNA and nucleocapsid are those of a typical  paramyxovirus (see Table 39-1). Its surface spikes are  fusion proteins, not hemagglutinins or neuraminidases  (see Table 39-4). The fusion protein causes cells to fuse,  forming multinucleated giant cells (syncytia), which give  rise to the name of the virus.   Humans are the natural hosts of RSV. For many years,  RSV was thought to have one serotype; however, two sero¬  types, designated subgroup A and subgroup B, have been  detected by monoclonal antibody tests. Antibody against  the fusion protein neutralizes infectivity.   Replication is similar to that of measles virus (see page 318).   Transmission occurs via respiratory droplets and by direct  contact of contaminated hands with the nose or mouth.  RSV causes outbreaks of respiratory infections every win¬  ter, in contrast to many other “cold” viruses, which reenter  the community every few years. It occurs worldwide, and  virtually everyone has been infected by the age of 3 years.  RSV also causes outbreaks of respiratory infections in hos¬  pitalized infants; these outbreaks can be controlled by  handwashing and use of gloves, which interrupt transmis¬  sion by hospital personnel.   RSV infection in infants is more severe and more often  involves the lower respiratory tract than in older children  and adults. The infection is localized to the respiratory  tract; viremia does not occur.   The severe disease in infants may have an immuno-  pathogenic mechanism. Maternal antibody passed to the  infant may react with the virus, form immune complexes,  and damage the respiratory tract cells. Trials with a killed  vaccine resulted in more severe disease, an unexpected  finding that supports such a mechanism.   Most individuals have multiple infections caused by  RSV, indicating that immunity is incomplete. The reason  for this is unknown, but it is not due to antigenic variation  of the virus. IgA respiratory antibody reduces the fre¬  quency of RSV infection as a person ages.   In infants, RSV is an important cause of lower respiratory  tract diseases such as bronchiolitis and pneumonia. RSV is  also an important cause of otitis media in young children.  In older children and young, healthy adults, RSV causes  respiratory tract infections such as the common cold and  bronchitis. However, in the elderly (people older than 65  years of age) and in adults with chronic cardiopulmonary  diseases, RSV causes severe lower respiratory tract disease,  including pneumonia.   An enzyme immunoassay (“rapid antigen test”) that detects  the presence of RSV antigens in respiratory secretions is  commonly used. The presence of the virus can be detected  by immunofluorescence on smears of respiratory epithe¬  lium or by isolation in cell culture. The cytopathic effect in  cell culture is characterized by the formation of multinucle-  ated giant cells. A fourfold or greater rise in antibody titer  is also diagnostic. A reverse transcriptase polymerase chain  reaction (RT-PCR) test is also available.   Aerosolized ribavirin (Virazole) is recommended for  severely ill hospitalized infants, but there is uncertainty    regarding its effectiveness. A combination of ribavirin and  hyperimmune globulins against RSV may be more  effective.   There is no vaccine. Previous attempts to protect with a  killed vaccine resulted in an increase in severity of symp¬  toms. Passive immunization with a monoclonal antibody  directed against the fusion protein of RSV (palivizumab,  Synagis) can be used for prophylaxis in premature or  immunocompromised infants. Hyperimmune globulins  (RespiGam) are also available for prophylaxis in these  infants and in children with chronic lung disease. Noso¬  comial outbreaks can be limited by handwashing and use  of gloves.   These viruses cause croup (acute laryngotracheobronchitis),  laryngitis, bronchiolitis, and pneumonia in children and a  disease resembling the common cold in adults.   The genome RNA and nucleocapsid are those of a typical  paramyxovirus (see Table 39-1). The surface spikes con¬  sist of hemagglutinin (H), neuraminidase (N), and fusion  (F) proteins (see Table 39-4). The fusion protein mediates  the formation of multinucleated giant cells. The H and N  proteins are on the same spike; the F protein is on a sepa¬  rate spike. Both humans and animals are infected by para¬  influenza viruses, but the animal strains do not infect  humans. There are four types, which are distinguished by  antigenicity, cytopathic effect, and pathogenicity (see  later). Antibody to either the H or the F protein neutral¬  izes infectivity.   Replication is similar to that of measles virus (see page 318).   These viruses are transmitted via respiratory droplets.  They cause disease worldwide, primarily in the winter  months.   These viruses cause upper and lower respiratory tract  disease without viremia. A large proportion of infections  are subclinical. Parainfluenza viruses 1 and 2 are major  causes of croup. Parainfluenza virus 3 is the most com¬  mon parainfluenza virus isolated from children with  lower respiratory tract infection in the United States.    Parainfluenza virus 4 rarely causes disease, except for the  common cold.   Parainfluenza viruses are best known as the main cause of  croup in children younger than 5 years of age. Croup is  characterized by a harsh cough and hoarseness. In addition  to croup, these viruses cause a variety of respiratory dis¬  eases such as the common cold, pharyngitis, laryngitis,  otitis media, bronchitis, and pneumonia.   Most infections are diagnosed clinically. The diagnosis can  be made in the laboratory either by isolating the virus in  cell culture or by observing a fourfold or greater rise in  antibody titer. PCR assay can also be used.   Coronaviruses are an important cause of the common cold,  probably second only to rhinoviruses in frequency. In 2002,  a new disease, an atypical pneumonia called severe acute  respiratory syndrome (SARS), emerged. In 2012, another  severe pneumonia called Middle East respiratory syndrome  emerged.   Coronaviruses have a nonsegmented, single-stranded,  positive-polarity RNA genome. They are enveloped  viruses with a helical nucleocapsid. There is no virion  polymerase. In the electron microscope, prominent club-  shaped spikes in the form of a corona (halo) can be seen.  There are two serotypes called 229E and OC43. The  genome sequence of the coronavirus that caused the SARS  (CoV-SARS) outbreak is different from that of the exist¬  ing human strains. The genome sequence of different  isolates of CoV-SARS is very similar, so the antigenicity of  the virus is likely to be quite stable. The receptor for the  SARS coronavirus on the surface of cells is angiotensin¬  converting enzyme-2.   The virus adsorbs to cells via its surface spikes (hemag¬  glutinin), after which it enters the cytoplasm, where it is  uncoated. The positive-strand genome is translated into  two large polypeptides, which are self-cleaved by the  virus-encoded protease. Two of these peptides aggregate  to form the RNA polymerase that replicates the genome.  In addition, mRNAs are synthesized and then translated  into the structural proteins. The virus is assembled and  obtains its envelope from the endoplasmic reticulum,  not from the plasma membrane. Replication occurs in  the cytoplasm.   Coronaviruses are transmitted by the respiratory aero¬  sol. Infection occurs worldwide and occurs early in life,  as evidenced by finding antibody in more than half of  children. Outbreaks occur primarily in the winter on a  2- to 3-year cycle.   SARS originated in China in November 2002 and spread  rapidly to other countries. As of this writing, there have  been 8300 cases and 785 deaths—a fatality rate of approxi¬  mately 9%. Human-to-human transmission occurs, and  some patients with SARS are thought to be “super-spread¬  ers.” Early in the outbreak, many hospital personnel were  affected, but respiratory infection control procedures have  greatly reduced the spread within hospitals. There are  many animal coronaviruses, and they are suspected of  being the source of CoV-SARS. The horseshoe bat appears  to be the natural reservoir for CoV-SARS, with the civet cat  serving as an intermediate host.   In 2012-2013, a new human coronavirus caused an out¬  break of serious, often fatal pneumonia in Saudi Arabia and  other countries in the region. The disease is called Middle  East respiratory syndrome (MERS), and the virus is called  MERS coronavirus (MERS-CoV). Its closest relative is a bat  coronavirus, and bats are thought to be a reservoir. Close  contact with camels appears to be the mode of transmission  to humans. The risk of person-to-person transmission is  low but has occurred in hospitals with inadequate infection  control. Another name for the virus is human coronavirus-  EMC (HCoV-EMC).   Coronavirus infection is typically limited to the mucosal  cells of the respiratory tract. Approximately 50% of infec¬  tions are asymptomatic, and it is unclear what role they  play in the spread of infection. Immunity following infec¬  tion appears to be brief, and reinfection can occur.   Pneumonia caused by SARS coronavirus is character¬  ized by diffuse edema resulting in hypoxia. The binding of    the virus to angiotensin-converting enzyme-2 (ACE-2) on  the surface of respiratory tract epithelium may contribute  to the dysregulation of fluid balance that causes the edema  in the alveolar space. MERS-CoV binds to CD-26 on the  respiratory mucosa, not to ACE-2.   The common cold caused by coronavirus is characterized  by coryza (rhinorrhea, runny nose), scratchy sore throat,  and low-grade fever. This illness typically lasts several days  and has no long-term sequelae. Coronaviruses also cause  bronchitis.   SARS is a severe atypical pneumonia characterized by a  fever of at least 38°C, nonproductive cough, dyspnea, and  hypoxia. Chills, rigors, malaise, and headache commonly  occur, but sore throat and rhinorrhea are uncommon.    Chest X-ray reveals interstitial “ground-glass” infiltrates  that do not cavitate. Leukopenia and thrombocytopenia are  seen. The incubation period for SARS ranges from 2 to 10  days, with a mean of 5 days. The clinical findings of MERS  are similar to those of SARS.   The diagnosis of the “common cold” is primarily a clinical  one. If SARS or MERS is suspected, antibody-based and  PCR-based tests can be used.   There is no antiviral therapy or vaccine available. A combi¬  nation of ribavirin and steroids has been tried in the treat¬  ment of life-threatening cases of SARS, but their efficacy is  uncertain.   This virus causes rubella and congenital rubella syndrome.  Congenital rubella syndrome is characterized by congeni¬  tal malformations.   Rubella virus is a member of the toga virus family. It is  composed of one piece of single-stranded RNA, an icosa-  hedral nucleocapsid, and a lipoprotein envelope. How¬  ever, unlike the paramyxoviruses, such as measles and  mumps viruses, it has a positive-strand RNA and there¬  fore has no virion polymerase. Its surface spikes contain  hemagglutinin. The virus has a single antigenic type. Anti¬  body against hemagglutinin neutralizes infectivity.  Humans are the natural host.   Because knowledge of rubella virus replication is incom¬  plete, the following cycle is based on the replication of  other togaviruses. After penetration of the cell and uncoat¬  ing, the plus-strand RNA genome is translated into several  nonstructural and structural proteins. Note the difference  between togaviruses and poliovirus, which also has a plus-  strand RNA genome but translates its RNA into a single  large polyprotein, which is subsequently cleaved. One of  the nonstructural rubella proteins is an RNA-dependent  RNA polymerase, which replicates the genome first by  making a minus-strand template and then, from that, plus-  strand progeny. Both replication and assembly occur in the  cytoplasm, and the envelope is acquired from the outer  membrane as the virion exits the cell.   The virus is transmitted via respiratory droplets and from  mother to fetus transplacentally. The disease occurs  worldwide. In areas where the vaccine is not used, epidem¬  ics occur every 6 to 9 years.   In 2005, the CDC declared rubella eliminated from the  United States and in 2015 rubella was declared eliminated  from the Western hemisphere. The few cases that occur in  the United States are acquired outside and imported into  this country. Elimination was made possible by the wide¬  spread use of the vaccine. As a result, cytomegalovirus is a  much more common cause of congenital malformations in  the United States than is rubella virus.   Initial replication of the virus occurs in the nasopharynx and  local lymph nodes. From there it spreads via the blood to the  internal organs and skin. The origin of the rash is unclear; it  may be due to antigen/antibody-mediated vasculitis.   Natural infection leads to lifelong immunity. Second  cases of rubella do not occur; similar rashes are caused by  other viruses, such as Coxsackie viruses and echoviruses.  Antibody crosses the placenta and protects the newborn.   Rubella is a milder, shorter disease than measles. After an  incubation period of 14 to 21 days, a brief prodromal  period with fever and malaise is followed by a maculopapu-  lar rash, which starts on the face and progresses downward  to involve the extremities (Figure 39-5). Posterior auricular  lymphadenopathy is characteristic. The rash typically lasts    3 days. When rubella occurs in adults, especially women,  polyarthritis caused by immune complexes often occurs.   The significance of rubella virus is not as a cause of mild  childhood disease but as a teratogen. When a nonimmune  pregnant woman is infected during the first trimester,  especially the first month, significant congenital malforma¬  tions can occur as a result of maternal viremia and fetal  infection. The increased rate of abnormalities during the  early weeks of pregnancy is attributed to the very sensitive  organ development that occurs at that time. The malforma¬  tions are widespread and involve primarily the heart (e.g.,  patent ductus arteriosus), the eyes (e.g., cataracts), and the  brain (e.g., deafness and mental retardation).   In addition, some children infected in utero can con¬  tinue to excrete rubella virus for months after birth, which  is a significant public health hazard because the virus can  be transmitted to pregnant women. Some congenital shed-  ders are asymptomatic and without malformations and  hence can be diagnosed only if the virus is isolated. Con¬  genitally infected infants also have significant IgM titers  and persistent IgG titers long after maternal antibody has  disappeared.    identified by its ability to interfere with echovirus CPE. If  rubella virus is present in the patient’s specimen and has  grown in the cell culture, no CPE will appear when the  culture is superinfected with an echovirus. The diagnosis  can also be made by observing a fourfold or greater rise in  antibody titer between acute-phase and convalescent-phase  sera in the hemagglutination inhibition test or ELISA or by  observing the presence of IgM antibody in a single acute-  phase serum sample. PCR assay can also be used.   In a pregnant woman exposed to rubella virus, the pres¬  ence of IgM antibody indicates recent infection, whereas  a 1:8 or greater titer of IgG antibody indicates immunity  and consequent protection of the fetus. If recent infection  has occurred, an amniocentesis can reveal whether there is  rubella virus in the amniotic fluid, which indicates definite  fetal infection.   Prevention involves immunization with the live, attenu¬  ated vaccine. The vaccine is effective and long-lasting (at  least 10 years) and causes few side effects, except for tran¬  sient arthralgias in some women. It is given subcutaneously  to children at 15 months of age (usually in combination  with measles and mumps vaccine) and to unimmunized  young adult women if they are not pregnant and will use  contraception for the next 3 months. There is no evidence  that the vaccine virus causes malformations. Because it is a  live vaccine, it should not be given to immunocompro¬  mised patients or to pregnant women.   The vaccine has caused a significant reduction in the  incidence of both rubella and congenital rubella syndrome.  It induces some respiratory IgA, thereby interrupting the  spread of virulent virus by nasal carriage.   Immune serum globulins (IG) can be given to pregnant  women in the first trimester who have been exposed to a  known case of rubella and for whom termination of the  pregnancy is not an option. The main problems with giving  IG are that there are instances in which it fails to prevent  fetal infection and that it may confuse the interpretation of  serologic tests. If termination of the pregnancy is an option,  it is recommended to attempt to determine whether the  mother and fetus have been infected as described in the  preceding “Laboratory Diagnosis” section.   To protect pregnant women from exposure to rubella  virus, many hospitals require their personnel to demon¬  strate immunity, either by serologic testing or by proof of  immunization.   Rubella virus can be grown in cell culture, but it produces Several other medically important togaviruses are described   little cytopathic effect (CPE). It is therefore usually in the chapter on arboviruses (see Chapter 42).   Rabies virus is the only medically important member of the  rhabdovirus family. It has a single-stranded RNA enclosed  within a bullet-shaped capsid surrounded by a lipoprotein  envelope. Because the genome RNA has negative polarity,  the virion contains an RNA-dependent RNA polymerase.  Rabies virus has a single antigenic type. The antigenicity  resides in the envelope glycoprotein spikes.   Rabies virus has a broad host range: It can infect all  mammals, but only certain mammals are important sources  of infection for humans (see later).   Rabies virus attaches to the acetylcholine receptor on the  cell surface. After entry into the cell, the virion RNA poly¬  merase synthesizes five mRNAs that code for viral proteins.  After replication of the genome viral RNA by a virus-  encoded RNA polymerase, progeny RNA is assembled with  virion proteins to form the nucleocapsid, and the envelope  is acquired as the virion buds through the cell membrane.   The virus is transmitted by the bite of a rabid animal that  manifests aggressive, biting behavior induced by the viral  encephalitis. The virus is in the saliva of the rabid animal.  In the United States, transmission is usually from the bite of  wild animals such as skunks, raccoons, and bats; dogs and  cats are frequently immunized and therefore are rarely  sources of human infection. In recent years, bats have been  the source of most cases of human rabies in the United  States. Rodents and rabbits do not transmit rabies.   Human rabies has also occurred in the United States in  people who have not been bitten, so-called “nonbite” expo¬  sures. The most important example of this type of trans¬  mission is exposure to aerosols of bat secretions containing  rabies virus. Another rare example is transmission in trans¬  plants of corneas taken from patients who died of undiag¬  nosed rabies.   In the United States, fewer than 10 cases of rabies  occur each year (mostly imported), whereas in developing  countries there are hundreds of cases, mostly due to rabid  dogs. In 2007, the United States was declared “canine-  rabies free”—the result of the widespread immunization  of dogs. Worldwide, approximately 50,000 people die of  rabies each year.   The country of origin and the reservoir host of a strain of  rabies virus can often be identified by determining the base    sequence of the genome RNA. For example, a person devel¬  oped clinical rabies in the United States, but sequencing of  the genome RNA revealed that the virus was the Mexican  strain. It was later discovered that the man had been bitten  by a dog while in Mexico several months earlier.   The virus multiplies locally at the bite site, infects the sen¬  sory neurons, and moves by axonal transport to the cen¬  tral nervous system. During its transport within the nerve,  the virus is sheltered from the immune system and little, if  any, immune response occurs. The virus multiplies in the  central nervous system and then travels down the periph¬  eral nerves to the salivary glands and other organs. From  the salivary glands, it enters the saliva to be transmitted by  the bite. There is no viremic stage.   Within the central nervous system, encephalitis devel¬  ops, with the death of neurons and demyelination. Infected  neurons contain an eosinophilic cytoplasmic inclusion  called a Negri body, which is important in laboratory diag¬  nosis of rabies (Figure 39-6). Because so few individuals  have survived rabies, there is no information regarding  immunity to disease upon being bitten again.   The incubation period varies, according to the location of  the bite, from as short as 2 weeks to 16 weeks or longer. It  is shorter when bites are sustained on the head rather than  on the leg, because the virus has a shorter distance to travel  to reach the central nervous system.   Clinically, the patient exhibits a prodrome of nonspe¬  cific symptoms such as fever, anorexia, and changes in   "Negri body," an inclusion body in the cytoplasm of an infected  neuron. (Source: Public Health Image Library, Centers for Disease Control and  Prevention.)    sensation at the bite site called paresthesias. After the pro¬  drome, rabies manifests as either of two forms: “furious”  (encephalitic) or “dumb” (paralytic). The furious form  occurs in about 80% of cases. In the furious form, agitation,  delirium, seizures, and hydrophobia occur. Hydrophobia is  an aversion to swallowing water because of painful spasm  of the pharyngeal muscles. In contrast, in the dumb form,  these symptoms do not occur. Rather, the spinal cord is  primarily involved, and an ascending paralysis occurs.  Death almost invariably occurs following both forms, but  with the advent of life support systems, a few individuals  have survived.   Rapid diagnosis of rabies infection in the animal is usually  made by examination of brain tissue by using either PCR  assay, fluorescent antibody to rabies virus, or histologic  staining of Negri bodies in the cytoplasm of hippocampal  neurons (see Figure 39-6). The virus can be isolated from  the animal brain by growth in cell culture, but this takes too  long to be useful in the decision of whether to give the  vaccine.   Rabies in humans can be diagnosed by PCR assay; by  fluorescent antibody staining of a biopsy specimen, usually  taken from the skin of the neck at the hairline; by isolation  of the virus from sources such as saliva, spinal fluid, and  brain tissue; or by a rise in titer of antibody to the virus.  Negri bodies can be demonstrated in corneal scrapings and  in autopsy specimens of the brain.   There is no antiviral therapy for a patient with rabies. Only  supportive treatment is available.   In the United States, the rabies vaccine contains inactivated  virus grown in human diploid cells. (Vaccine grown in  monkey lung cells or chick embryo cells is also available.) In  other countries, the duck embryo vaccine or various nerve  tissue vaccines are available as well. Duck embryo vaccine  has low immunogenicity, and the nerve tissue vaccines can  cause an allergic encephalomyelitis as a result of a cross¬  reaction with human myelin. For these reasons, the human  diploid cell vaccine (HDCV) is preferred.    There are two approaches to prevention of rabies in  humans: preexposure and postexposure immunization.  Preexposure immunization with rabies vaccine should be  given to individuals in high-risk groups, such as veterinarians,  zookeepers, and travelers to areas of hyperendemic infection  (e.g., Peace Corps members). Preexposure immunization con¬  sists of three doses given on days 0, 7, and 21 or 28. Booster  doses are given as needed to maintain an antibody titer of 1:5.   The rabies vaccine is also used routinely postexposure  (i.e., after the person has been exposed to the virus via ani¬  mal bite). The long incubation period of the disease allows  the virus in the vaccine sufficient time to induce protective  immunity.   Postexposure immunization involves the use of both the  vaccine and human rabies immune globulin (RIG, obtained  from hyperimmunized persons) plus immediate cleaning of  the wound. This is an example of passive-active immuniza¬  tion. Tetanus immunization should also be considered.   The decision to give postexposure immunization depends  on a variety of factors, such as (1) the type of animal (all wild  animal attacks demand immunization); (2) whether an attack  by a domestic animal was provoked, whether the animal was  immunized adequately, and whether the animal is available to  be observed; and (3) whether rabies is endemic in the area.  The advice of local public health officials should be sought.  Hospital personnel exposed to a patient with rabies need not  be immunized unless a significant exposure has occurred  (e.g., a traumatic wound to the health care worker).   If the decision is to immunize, both HDCV and RIG are  recommended. Five doses of HDCV are given (on days 0,3,7,  14, and 28), but RIG is given only once with the first dose of  HDCV (at a different site). HDCV and RIG are given at differ¬  ent sites to prevent neutralization of the virus in the vaccine by  the antibody in the RIG. As much as possible of the RIG is  given into the bite site, and the remainder is given intramus¬  cularly. If the animal has been captured, it should be observed  for 10 days and euthanized if symptoms develop. The brain of  the animal should be examined by immunofluorescence.   The vaccine for immunization of dogs and cats consists  of inactivated rabies virus. The first immunization is usu¬  ally given at 3 months of age, with booster doses given  either annually or at 3-year intervals. In the United States,  an alternative vaccine used in dogs and cats contains live  canarypox virus genetically engineered to contain the gene  for the envelope protein of rabies virus.   There are two important human retroviruses: human T-cell  lymphotropic virus, which is described here, and human  immunodeficiency virus (HIV), which is described in  Chapter 45.   Human T-cell lymphotropic virus-1 (HTLV-1) causes two  distinctly different diseases: a cancer called adult T-cell  leukemia/lymphoma and a neurologic disease called  HTLV-associated myelopathy (also known as tropical spas¬  tic paraparesis or chronic progressive myelopathy). HTLV-2    also appears to cause these diseases, but the association is  less clearly documented. (All information in this section  refers to HTLV-1 unless otherwise stated.)   HTLV and HIV are the two medically important members  of the retrovirus family. Both are enveloped viruses with  reverse transcriptase in the virion and two copies of a sin¬  gle-stranded, positive-polarity RNA genome. However,  HTLV does not kill T cells, whereas HIV does. In fact,  HTLV does just the opposite; it causes malignant transfor¬  mation that “immortalizes” the infected T cells and allows  them to proliferate in an uncontrolled manner.   The genes in the HTLV genome whose functions have  been clearly identified are the three structural genes com¬  mon to all retroviruses, namely, gag, pol, and env, plus two  regulatory genes, tax and rex. In general, HTLV genes and  proteins are similar to those of HIV in size and function,  but the genes differ in base sequence, and therefore the  proteins differ in amino acid sequence (and antigenicity).  For example, p24 is the major nucleocapsid protein in both  HTLV and HIV, but they differ antigenically. The virions of  both HTLV and HIV contain a reverse transcriptase, inte-  grase, and protease. The envelope proteins of HTLV are  gp46 and gp21, whereas those of HIV are gpl20 and gp41.   The proteins encoded by the tax and rex genes play the  same functional roles as those encoded by the HIV regula¬  tory genes, tat and rev. The Tax protein is a transcriptional  activator, and the Rex protein governs the processing of  viral mRNA and its export from the nucleus to the cyto¬  plasm. Tax protein is required for malignant transforma¬  tion of T cells.   In contrast to other oncogenic retroviruses, such as  Rous sarcoma virus in chickens (see page 363), HTLV does  not possess an oncogene in its genome and does not inte¬  grate its proviral DNA at a specific site near a cellular onco¬  gene in the T-cell DNA (i.e., it does not cause insertional  mutagenesis). Rather, it is the activation of transcription of  both cellular and viral mRNA synthesis by the Tax protein  that initiates oncogenesis. The Tax protein activates the  synthesis of IL-2 (which is T-cell growth factor) and of the  IL-2 receptor. IL-2 promotes rapid T-cell growth and even¬  tually malignant transformation of the T cell.   The stability of the genes of HTLV is much greater than  that of HIV. As a consequence, HTLV does not show the  high degree of variability of the antigenicity of the envelope  proteins that occurs in HIV.   The replication of HTLV is thought to follow a typical retro¬  viral cycle, but specific information has been difficult to  obtain because the virus grows poorly in cell culture. HTLV  primarily infects CD4-positive T lymphocytes. The cellular  receptor for the virus is unknown. Within the cytoplasm,  reverse transcriptase synthesizes a DNA copy of the genome,    which migrates to the nucleus and integrates into cell DNA.  Viral mRNA is made by host cell RNA polymerase, and  transcription is upregulated by Tax protein, as mentioned  earlier. The Rex protein controls the synthesis of the gag/pol  mRNA, the env mRNA, and their subsequent transport to  the cytoplasm, where they are translated into structural viral  proteins. Full-length RNA destined to become progeny  genome RNA is also synthesized and transported to the  cytoplasm. The virion nucleocapsid is assembled in the  cytoplasm, and budding occurs at the outer cell membrane.  Cleavage of precursor polypeptides into functional struc¬  tural proteins is mediated by the virus-encoded protease.   HTLV is transmitted primarily by intravenous drug use,  sexual contact, or breast feeding. Transplacental trans¬  mission has been rarely documented. Transmission by  blood transfusion has greatly decreased in the United  States with the advent of screening donated blood for  antibodies to HTLV and discarding those that are posi¬  tive. Transmission by processed blood products, such as  immune serum globulins, has not occurred. Transmis¬  sion is thought to occur primarily by the transfer of  infected cells rather than free, extracellular virus. For  example, whole blood, but not plasma, is a major source,  and infected lymphocytes in semen are the main source  of sexually transmitted virus.   HTLV infection is endemic in certain geographic areas,  namely, the Caribbean region including southern Florida,  eastern South America, western Africa, and southern  Japan. The rate of seropositive adults is as high as 20% in  some of these areas, but infection can occur anywhere  because infected individuals migrate from these areas of  endemic infection. At least half the people in the United  States who are infected with HTLV are infected with  HTLV-2, usually acquired via intravenous drug use.   HTLV causes two distinct diseases, each with a different  type of pathogenesis. One disease is adult T-cell leuke¬  mia/lymphoma (ATL) in which HTLV infection of CD4-  positive T lymphocytes induces malignant transformation.  As described earlier, HTLV-encoded Tax protein  enhances synthesis of IL-2 (T-cell growth factor) and  IL-2 receptor, which initiates the uncontrolled growth  characteristic of a cancer cell. All the malignant T cells  contain the same integrated proviral DNA, indicating  that the malignancy is monoclonal (i.e., it arose from a  single HTLV-infected cell). HTLV remains latent within  the malignant T cells (i.e., HTLV is typically not pro¬  duced by the malignant cells).   The other disease is HTLV-associated myelopathy  (HAM), also known as tropical spastic paraparesis or  chronic progressive myelopathy. HAM is a demyelinating  disease of the brain and spinal cord, especially of the motor    neurons in the spinal cord. HAM is caused either by an  autoimmune cross-reaction in which the immune response  against HTLV damages the neurons or by cytotoxic T cells  that kill HTLV-infected neurons.   ATL is characterized by lymphadenopathy, hepatospleno-  megaly, lytic bone lesions, and skin lesions. These features  are caused by proliferating T cells infiltrating these organs.  In the blood, the malignant T cells have a distinct “flower¬  shaped” nucleus. Hypercalcemia due to increased osteoclast  activity within the bone lesions is seen. Patients with ATL  often have reduced cell-mediated immunity, and opportu¬  nistic infections with fungi and viruses are common.   The clinical features of HAM include gait disturbance,  weakness of the lower limbs, and low back pain. Loss of  bowel and bladder control may occur. Loss of motor func¬  tion is much greater than sensory loss. T cells with a  “flower-shaped” nucleus can be found in the spinal fluid.  Magnetic resonance imaging of the brain shows nonspe¬  cific findings. Progression of symptoms occurs slowly over  a period of years. HAM occurs primarily in women of  middle age. The disease resembles multiple sclerosis except  that HAM does not exhibit the remissions characteristic of  multiple sclerosis.   Both ATL and HAM are relatively rare diseases. The vast  majority of people infected with HTLV develop asymptom¬  atic infections, usually detected by the presence of anti¬  body. Only a small subset of those infected develop either  ATL or HAM.   Infection with HTLV is determined by detecting antibodies  against the virus in the patient’s serum using the ELISA test.  The Western blot assay is used to confirm a positive ELISA  result. PCR assay can detect the presence of HTLV RNA or  DNA within infected cells. The laboratory tests used to  screen donated blood contain only HTLV-1 antigens, but  because there is cross-reactivity between HTLV-1 and  HTLV-2, the presence of antibodies against both viruses is  usually detected. However, some HTLV-2 antibodies are  missed in these routine screening tests. Isolation of HTLV in  cell culture from the patient’s specimens is not done.   ATL is diagnosed by finding malignant T cells in the  lesions. The diagnosis of HAM is supported by the pres¬  ence of HTLV antibody in the spinal fluid or finding HTLV  nucleic acids in cells in the spinal fluid.   There is no specific antiviral treatment for HTLV infection,  and no antiviral drug will cure latent infections by HTLV.  ATL is treated with anticancer chemotherapy regimens.  Antiviral drugs have not been effective in the treatment of  HAM. Corticosteroids and danazol have produced  improvement in some patients.   There is no vaccine against HTLV. Preventive measures  include screening donated blood for the presence of anti¬  bodies, using condoms to prevent sexual transmission, and  encouraging women with HTLV antibodies to refrain from  breast feeding.    There are two important filoviruses that cause human  disease: Ebola virus and Marburg virus. Filoviruses are  long filamentous (filo = thread) viruses. They are the lon¬  gest viruses, often measuring thousands of nanometers  (Figure 39-7).   Ebola virus causes Ebola hemorrhagic fever (EHF). The  virus is named for the river in Zaire that was the site of  the first known outbreak of EHF in 1976. A devastating  epidemic of EHF occurred in several West African coun¬  tries, especially Liberia, Sierra Leone, and Guinea in  2014-2015.   Ebola virus has a single-stranded, nonsegmented, negative  polarity RNA genome. There is an RNA-dependent RNA   points to a typical virion of Ebola virus. Short arrow points to the  "shepherd's crook" appearance of some Ebola virions. (Source: Dr. Erskine  Palmer and Dr. Russell Regnery, Public Health Image Library, Centers for Disease  Control and Prevention.)    polymerase in the virion. The nucleocapsid has helical  symmetry. The surface proteins of Ebola virus are antigeni-  cally distinct from those of Marburg virus.   Ebola virus is one of the most virulent human viruses  and is cultured only under the highest biosafety contain¬  ment (BSL-4). It can be inactivated by lipid solvents and  bleach (hypochlorite).   There are five types: Ebola-Zaire is the most pathogenic  for humans and Ebola-Reston is pathogenic for monkeys  but not for humans. Ebola-Sudan is also highly pathogenic.  The degree of pathogenicity of Ebola-Ivory Coast (Tai For¬  est) and Ebola-Bundibugyo for humans is uncertain  because the number of cases is relatively small. The Zaire,  Sudan, Ivory Coast, and Bundibugyo types are found in  Africa whereas the Reston type originated in the  Philippines.   The general outline of its replication is similar to that of  other negative-stranded RNA enveloped viruses. After the  virion envelope glycoproteins bind to the surface of the  human cell, the nucleocapsid enters the cytoplasm where  the virion RNA polymerase transcribes the seven genes  into individual messenger RNAs. The m-RNAs are trans¬  lated into structural and nonstructural proteins. The  negative-strand progeny genome is synthesized by the  virus-encoded RNA polymerase using a plus-strand tem¬  plate. The newly synthesized nucleocapsid proteins sur¬  round both the progeny genome and the virion RNA  polymerase. The matrix protein then mediates the inter¬  action of the nucleocapsid protein with the outer cell  membrane at the site of the progeny envelope proteins.  The progeny virus then buds from the surface of the  infected cell.   The natural reservoir of Ebola virus is unknown. Fruit bats  or rodents are suspected of being the reservoir. Monkeys  can be infected but, because they become sick and die, are  unlikely to be the natural reservoir. The mode of transmis¬  sion from the reservoir host to humans is unknown.   Transmission from human to human occurs via blood  and body fluids. Elospital personnel without adequate pro¬  tection are especially at risk. Many cases arise by secondary  transmission from contact with the patients blood or secre¬  tions (e.g., in hospital staff). Reuse of needles and syringes  has been implicated in the spread in some hospitals in  resource-poor countries. There is no evidence for human  disease occurring via airborne transmission or by casual  personal contact. There is evidence of Ebola virus persist¬  ing in the semen of survivors of the disease.   Subsequent to the first recorded outbreak of EHF in  1976, there have been sporadic outbreaks in rural areas in  various sub-Saharan African countries, mostly in the 1990s    and 2000s. Then in 2014-2015, the largest epidemic  occurred in Liberia, Sierra Leone, and Guinea in which  more than 10,000 people died. The fatality rate is 60% in  this epidemic. This epidemic included cases in both rural  and highly populated areas.   The high mortality rate of Ebola virus is attributed to sev¬  eral viral virulence factors: Its glycoprotein kills endothelial  cells, resulting in hemorrhage, and two other proteins  inhibit the induction and action of interferon. Lympho¬  cytes, macrophages, and dendritic cells are killed. As a  result, the antibody response is often ineffective in prevent¬  ing disease. Hepatocytes are also killed leading to liver  failure.   The incubation period is typically 5 to 7 days but may be up  to 21 days. EHF begins with a constellation of symptoms  some of which are fever, headache, sore throat, myalgia,  arthralgia, epigastric pain, vomiting, and diarrhea. Later,  bleeding into the skin and gastrointestinal tract occurs, fol¬  lowed by shock and disseminated intravascular coagulation  leading to multiorgan failure. The hemorrhages are the  result of both severe thrombocytopenia and death of endo¬  thelial cells. Marked lymphopenia occurs. The mortality  rate associated with this virus can be up to 90%.   In some patients who recover from EHF, a post-Ebola  syndrome (PES) occurs. The findings in PES include eye  pain, blurred vision, hearing loss, headache, joint pain,  fatigue, and insomnia. In one patient with uveitis, infec¬  tious Ebola virus was recovered from fluid aspirated from  the interior of his eye several months after recovery.   Diagnosis is most often made by detecting viral antigens in  serum using an ELISA assay, by detecting viral RNA using  a PCR assay or by detecting IgM antibody in the serum.  (Extreme care must be taken when handling specimens in  the laboratory.) The virus can be cultured in monkey cells  in BSL-4 containment facility. Electron microscopy may  reveal the long rod-shape of a filovirus, implicating either  Ebola virus or Marburg virus.   No antiviral therapy is available. Supportive therapy includ¬  ing intravenous fluids and electrolytes is useful. Treatment  with immune serum globulins containing antibody to  Ebola virus has had variable results. An experimental  monoclonal antibody (ZMapp) was used in the 2014 epi¬  demic but its effectiveness is uncertain.   Prevention centers on limiting secondary spread by  proper handling of patients secretions and blood and by  the wearing of personal protective equipment (PPE).    Quarantine of individuals thought to be exposed for 21 days  is also important. There is no vaccine. Several candidate  vaccines are being tested as of this writing.   Marburg virus and Ebola virus are similar in that they both  cause hemorrhagic fever and are members of the filovirus  family; however, they are antigenically distinct. Marburg  virus was first recognized as a cause of human disease in  1967 in Marburg, Germany. The common feature of the  infected individuals was their exposure to African green  monkeys that had recently arrived from Uganda. As with  Ebola virus, the natural reservoir of Marburg virus is  unknown, although bats are suspected.   The clinical picture of this hemorrhagic fever is as  described for Ebola virus (see earlier). In 2005, an outbreak  of hemorrhagic fever caused by Marburg virus killed hun¬  dreds of people in Angola. No cases of disease caused by  Marburg virus occurred in the United States prior to 2008.  However, in that year, a U.S. traveler became ill after visit¬  ing a cave in Uganda inhabited by fruit bats. He returned to  the United States, where he was diagnosed with Marburg  hemorrhagic fever. He recovered without sequelae.   The diagnosis is made by PCR assay or detecting a rise in  IgM antibody titer. No antiviral therapy or vaccine is available.  As with Ebola virus, secondary cases among medical per¬  sonnel have occurred; therefore, stringent infection control  practices must be instituted to prevent nosocomial spread.   1. Regarding influenza virus, which one of the following statements  is most accurate?   (B) Its surface proteins, hemagglutinin and neuraminidase, have  multiple serologic types.   (C) The protein that undergoes antigenic variation most often is  the internal ribonucleoprotein.   (D) Antigenic drift involves major changes in antigenicity that  result from reassortment of the segments of its RNA genome.   (E) The neuraminidase on the virion surface mediates the inter¬  action of the virus with the receptors on the respiratory tract  epithelium.   2. Regarding influenza virus and the disease influenza, which one  of the following statements is most accurate?   (A) Both the killed and the live, attenuated vaccines induce life¬  long immunity.   (B) Influenza A virus causes more severe disease and more wide¬  spread epidemics than does influenza B virus.   (C) The genome of influenza A virus has eight segments, but the  genome of influenza B virus is in one piece.   (D) The classification of influenza viruses into A, B, and C viruses  is based on antigenic differences in their hemagglutinin.   (E) Chronic carriers (i.e., patients from whom influenza virus is  isolated at least 6 months after the acute disease) are an  important source of human infection.    3. Regarding measles virus and the disease measles, which one of  the following statements is most accurate?   (B) One of the main sequelae of measles is autoimmune glomeru¬  lonephritis and kidney failure.   (C) Measles is unlikely to be eradicated because there is a signifi¬  cant animal reservoir for this virus.   (D) Fecal-oral transmission during the diaper stage is the main  mode of acquisition of measles virus.   (E) This virus has only one antigenic type, and lifelong immunity  occurs in patients who have had measles.   4. Regarding respiratory syncytial virus (RSV), which one of the  following statements is most accurate?   (B) RSV causes tumors in newborn animals but not in  humans.   (C) The RSV vaccine is recommended for all children prior to  entering school.   (D) Amantadine should be given to elderly nursing home resi¬  dents to prevent outbreaks of disease caused by RSV.   (E) RSV forms intranuclear inclusion bodies within neutrophils  that are important in diagnosis by the clinical laboratory.   5. Regarding rubella virus, which one of the following statements is  most accurate?   (A) Systemic infection with rubella virus often causes severe liver  damage resulting in cirrhosis.   (B) If a pregnant woman is infected during the first trimester,  significant fetal abnormalities typically result.   (C) The main source of virus is adults who have recovered from  the disease but are chronic carriers of the virus.   (D) Immunization of both male and female health care workers  with the formalin-inactivated vaccine is recommended.   (E) The significant changes in the antigenicity of this virus are  attributed to reassortment of the segments of its genome.   6. Regarding rabies virus and the disease rabies, which one of the  following statements is most accurate?   (A) Finding intranuclear inclusion bodies within macrophages is  presumptive evidence of rabies virus infection.   (B) Lamivudine is used to treat rabies because it inhibits the  RNA-dependent DNA polymerase in the virion.   (C) In the United States, skunks and bats are more likely to trans¬  mit rabies virus to people than are dogs and cats.   (D) The incubation period of the disease is usually 2 to 4 days,  leading to the rapid progression of the encephalitis and  death.   (E) After the animal bite, rabies virus enters the bloodstream,  replicates in internal organs such as the liver, and then  reaches the central nervous system during the secondary  viremia.   7. A woman was hiking in an isolated area when a skunk appeared  and bit her on the leg. She now presents to your emergency room  about an hour after the bite. Which one of the following is the  most appropriate thing to do?   (B) Reassure her that rabies is not a problem because skunks do  not carry rabies.   (C) Quarantine the animal for 10 days and only treat her if signs  of rabies appear in the animal.   (D) Test the patients serum for antibodies now and in 10 days to  see if there is a rise in antibody titer before treating her.    8. Human T-cell lymphotropic virus (HTLV) causes T-cell leuke¬  mia in adults. Regarding this virus, which one of the following  statements is most accurate?   (B) Oseltamivir cures the latent state established by HTLV within  T cells.   (C) The genome of HTLV consists of double-stranded RNA;  therefore, there is no polymerase in the virion.   (D) HTLV is associated with leukemia in Japan, but the virus has  not appeared in the United States at the present time.   (E) Oncogenesis by HTLV is related to a viral transcription factor  that activates the production of interleukin-2 and its  receptor.   9. Your patient is a 75-year-old woman with fever, chills, and myal¬  gias that began yesterday. It is January and an outbreak of influ¬  enza is occurring in the retirement community in which she  lives. A rapid test for influenza antigen is positive. Which one of  the following is the best choice of drug to treat the infection?   10. Regarding Ebola virus, which one of the following is most  accurate?   (A) Skunks and raccoons are the main natural reservoirs for  Ebola virus.   (B) In endemic areas, most people are latently infected with Ebola  virus.   (C) People known to be exposed to Ebola virus should be given  ganciclovir to prevent disease.   (D) Ebola hemorrhagic fever occurs primarily in people with  deficient cell-mediated immunity.   (E) The appearance of Ebola virus in the electron microscope is  that of a long thread, which often has a curved end.   Brief summaries of the organisms described in this chapter  begin on page 671. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Clinical Virology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 723. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.    Picornaviruses are small (20-30 nm) nonenveloped viruses  composed of an icosahedral nucleocapsid and a single-  stranded RNA genome. The genome RNA has positive  polarity (i.e., on entering the cell, it functions as the viral  mRNA). There is no polymerase within the virion. Picor¬  naviruses replicate in the cytoplasm of cells. They are not  inactivated by lipid solvents, such as ether, because they do  not have an envelope.   The picornavirus family includes two groups of medical  importance: the enteroviruses and the rhinoviruses.  Among the major enteroviruses are poliovirus, Coxsackie  viruses, echoviruses, and hepatitis A virus (which is  described in Chapter 41). Enteroviruses infect primarily  the enteric tract, whereas rhinoviruses are found in the  nose and throat (rhino = nose) (Table 40-1).    Enteroviruses replicate optimally at 37°C, whereas  rhinoviruses grow better at 33°C, in accordance with the  lower temperature of the nose. Enteroviruses are stable  under acid conditions (pH 3-5), which enables them to  survive exposure to gastric acid, whereas rhinoviruses are  acid-labile. This explains why rhinovirus infections are  restricted to the nose and throat.   Important features of viruses that commonly infect the  intestinal tract are summarized in Table 40-2. These  include the picornaviruses but also rotavirus and norovirus,  which are described later in this chapter, and adenovirus,  which is described in Chapter 38.   The host range is limited to primates (i.e., humans and  nonhuman primates such as apes and monkeys). This limi¬  tation is due to the binding of the viral capsid protein to a  receptor found only on primate cell membranes. However,  note that purified viral RNA (without the capsid protein)  can enter and replicate in many nonprimate cells—the  RNA can bypass the cell membrane receptor (i.e., it is  “infectious RNA”).   There are three serologic (antigenic) types based on  different antigenic determinants on the outer capsid pro¬  teins. Because there is little cross-reaction, protection from  disease requires the presence of antibody against each of  the three types.   The virion interacts with specific cell receptors on the cell  membrane and then enters the cell. The capsid proteins  are then removed. After uncoating, the genome RNA  functions as mRNA and is translated into one very large  polypeptide called noncapsid viral protein 00. This poly¬  peptide is cleaved by a virus-encoded protease in multiple  steps to form both the capsid proteins of the progeny  virions and several noncapsid proteins, including the  RNA polymerase that synthesizes the progeny RNA  genomes. Replication of the genome occurs by synthesis  of a complementary negative strand, which then serves as  the template for the positive strands. Some of these positive  strands function as mRNA to make more viral proteins,  and the remainder become progeny virion genome RNA.    Assembly of the progeny virions occurs by coating of the  genome RNA with capsid proteins. Virions accumulate in  the cell cytoplasm and are released upon death of the cell.  They do not bud from the cell membrane.   Poliovirus is transmitted by the fecal-oral route. It repli¬  cates in the oropharynx and intestinal tract. Humans are  the only natural hosts.   As a result of the success of the vaccine, poliomyelitis  caused by naturally occurring “wild-type” virus has been  eradicated from the United States and, indeed, from the  entire Western hemisphere. The rare cases in the United  States occur mainly in (1) people exposed to virulent  revertants of the attenuated virus in the live vaccine and  (2) unimmunized people exposed to wild-type poliovirus  while traveling abroad. Before the vaccine was available,  epidemics occurred in the summer and fall.   The World Health Organization set the eradication of  paralytic polio by 2005 as a goal. Unfortunately, this goal  was not achieved. In 1988, there were 388,000 cases of para¬  lytic polio worldwide, whereas in 2005 there were fewer  than 2000. Despite this remarkable decrease, paralytic polio  continues to occur. As of 2012, there were still a total of  approximately 200 cases each year in three countries:  Afghanistan, Nigeria, and Pakistan. Thus far, smallpox is  the only infectious disease that has been eradicated, a con¬  sequence of the worldwide use of the smallpox vaccine.   After replicating in the oropharynx and small intestine,  especially in lymphoid tissue, the virus spreads through the  bloodstream to the central nervous system. It can also  spread retrograde along nerve axons.   In the central nervous system, poliovirus preferentially  replicates in the motor neurons located in the anterior  horn of the spinal cord. Death of these cells results in    paralysis of the muscles innervated by those neurons.  Paralysis is not due to virus infection of muscle cells. The  virus also affects the brainstem, leading to “bulbar” polio¬  myelitis (with respiratory paralysis), but rarely damages the  cerebral cortex.   In infected individuals, the immune response consists of  both intestinal IgA and humoral IgG to the specific sero¬  type. Infection provides lifelong type-specific immunity.   The range of responses to poliovirus infection includes  (1) inapparent, asymptomatic infection; (2) abortive polio¬  myelitis; (3) nonparalytic poliomyelitis; and (4) paralytic  poliomyelitis. Asymptomatic infection is quite common.  Roughly 1% of infections are clinically apparent. The incu¬  bation period is usually 10 to 14 days.   The most common clinical form is abortive poliomyelitis,  which is a mild, febrile illness characterized by headache,  sore throat, nausea, and vomiting. Most patients recover  spontaneously. Nonparalytic poliomyelitis manifests as asep¬  tic meningitis with fever, headache, and a stiff neck. This also  usually resolves spontaneously. In paralytic poliomyelitis,  flaccid paralysis is the predominant finding, but brainstem  involvement can lead to life-threatening respiratory paraly¬  sis. Painful muscle spasms also occur. The motor nerve dam¬  age is permanent, but some recovery of muscle function  occurs as other nerve cells take over. In paralytic polio, both  the meninges and the brain parenchyma (meningoencepha¬  litis) are often involved. If the spinal cord is also involved, the  term meningomyeloencephalitis is often used.   A postpolio syndrome that occurs many years after the  acute illness has been described. Marked deterioration of  the residual function of the affected muscles occurs many  years after the acute phase. The cause of this deterioration  is unknown.   No permanent carrier state occurs following infection  by poliovirus, but virus excretion in the feces can occur for  several months.   The diagnosis is made either by isolation of the virus or by  a rise in antibody titer. Virus can be recovered from the  throat, stool, or spinal fluid by inoculation of cell cultures.  The virus causes a cytopathic effect (CPE) and can be iden¬  tified by neutralization of the CPE with specific antisera.   There is no antiviral therapy. Treatment is limited to symp¬  tomatic relief and respiratory support, if needed. Physio¬  therapy for the affected muscles is important.   Poliomyelitis can be prevented by both the killed vaccine  (Salk vaccine, inactivated vaccine, IPV) and the live,    attenuated vaccine (Sabin vaccine, oral vaccine, OPV)  (Table 40-3). Both vaccines induce humoral antibodies,  which neutralize virus entering the blood and hence pre¬  vent central nervous system infection and disease. Both  the killed and the live vaccines contain all three serotypes.  At present, the inactivated vaccine is preferred for reasons  that are described later.   The current version of the inactivated vaccine is called  enhanced polio vaccine, or elPV. It has a higher sero¬  conversion rate and induces a higher titer of antibody  than the previous IPV. elPV also induces some mucosal  immunity IgA, making it capable of interrupting trans¬  mission, but the amount of secretory IgA induced by  elPV is significantly less than the amount induced by  OPV. OPV is therefore preferred for eradication efforts.  The only version of polio vaccine currently produced and  used in the United States is elPV. In certain countries  where polio remains endemic (e.g., India), a monovalent  oral polio vaccine is used because the rate of seroconver¬  sion is higher with the monovalent vaccine than with the  trivalent one.   In the past, the live vaccine was preferred in the United  States for two main reasons: (1) It interrupts fecal-oral  transmission by inducing secretory IgA in the gastrointes¬  tinal tract. (2) It is given orally and so is more readily  accepted than the killed vaccine, which must be injected.   The live vaccine has four disadvantages: (1) Rarely,  reversion of the attenuated virus to virulence will occur,  and disease may ensue (especially for the type 3 virus).  (2) It can cause disease in immunodeficient persons and  therefore should not be given to them. (3) Infection of the    gastrointestinal tract by other enteroviruses can limit repli¬  cation of the vaccine virus and reduce protection. (4) It  must be kept refrigerated to prevent heat inactivation of the  live virus.   Outbreaks of paralytic polio caused by vaccine-derived  poliovirus (VDPV) continue to occur, especially in areas  where there are large numbers of unimmunized people.  These VDPV strains have lost their attenuation by acquir¬  ing genes from wild-type enteroviruses by recombination.  Outbreaks of VDPV-associated paralytic polio have been  contained by campaigns to immunize people in the affected  area with the oral (Sabin) vaccine that interrupts fecal-oral  transmission.   The duration of immunity is thought to be longer with  the live than with the killed vaccine, but a booster dose is  recommended with both.   The currently approved vaccine schedule consists of  four doses of inactivated vaccine administered at 2 months,  4 months, 6 to 18 months, and upon entry to school at  4 to 6 years. One booster (lifetime) is recommended for  adults who travel to endemic areas. The use of the inacti¬  vated vaccine should prevent some of the approximately  10 cases per year of vaccine-associated paralytic polio that  arise from reversion of the attenuated virus in the  vaccine.   In 2015, WHO decided to use the bivalent oral vaccine,  containing types 1 and 3 instead of the trivalent version.  Type 2 was omitted because it has a high rate of reversion  to virulence and is the main cause of vaccine-derived para¬  lytic polio. In addition, WHO decided to use trivalent  inactivated polio vaccine to induce antibodies against all  three types that will protect against paralytic polio caused,  not only by types 1 and 3, but also by type 2 that is omitted  from the oral vaccine.   In the past, some lots of poliovirus vaccines were con¬  taminated with a papovavirus, SV40 virus, which causes  sarcomas in rodents. SV40 virus was a “passenger” virus in  the monkey kidney cells used to grow the poliovirus for the  vaccine. Fortunately, no increase in cancer occurred in  persons inoculated with the SV40 virus-containing polio  vaccine. However, there is some evidence that SV40 DNA  can be found in certain human cancers such as non-Hodg¬  kin’s lymphoma; the role of SV40 as a cause of cancer in  persons immunized with early versions of the polio vaccine  is unresolved. At present, cell cultures used for vaccine  purposes are carefully screened to exclude the presence of  adventitious viruses.   Passive immunization with immune serum globulin is  available for protection of unimmunized individuals known  to have been exposed. Passive immunization of newborns  as a result of passage of maternal IgG antibodies across the  placenta also occurs.   Quarantine of patients with disease is not effective,  because fecal excretion of the virus occurs in infected indi¬  viduals prior to the onset of symptoms and in those who  remain asymptomatic.   Coxsackie viruses are named for the town of Coxsackie,  NY, where they were first isolated.   Coxsackie viruses cause a variety of diseases. Group A  viruses cause, for example, herpangina, acute hemorrhagic  conjunctivitis, and hand-foot-and-mouth disease, whereas  group B viruses cause pleurodynia, myocarditis, and peri¬  carditis. Both types cause nonspecific upper respiratory  tract disease (common cold), febrile rashes, and aseptic  meningitis. Coxsackie viruses and echoviruses (see next  section) together cause approximately 90% of cases of viral  (aseptic) meningitis.   The size and structure of the virion and the nature of the  genome RNA are similar to those of poliovirus. The classi¬  fication of Coxsackie viruses into group A or B is based on  pathogenicity in mice. Group A viruses cause widespread  myositis and flaccid paralysis, which is rapidly fatal,  whereas group B viruses cause generalized, less severe  lesions of the heart, pancreas, and central nervous system  and focal myositis. At least 24 serotypes of Coxsackie virus  A and 6 serotypes of Coxsackie virus B are recognized.   Coxsackie viruses are transmitted primarily by the fecal-  oral route, but respiratory aerosols also play a role. They  replicate in the oropharynx and the intestinal tract. Humans  are the only natural hosts. Coxsackie virus infections occur  worldwide, primarily in the summer and fall.   Group A viruses have a predilection for skin and mucous  membranes, whereas group B viruses cause disease in vari¬  ous organs such as the heart, pleura, pancreas, and liver.  Both group A and B viruses can affect the meninges and  the motor neurons (anterior horn cells) to cause paralysis.  From their original site of replication in the oropharynx  and gastrointestinal tract, they disseminate via the  bloodstream.   Herpangina is characterized by fever, sore throat, and ten¬  der vesicles in the oropharynx. Hand-foot-and-mouth    disease is characterized by a vesicular rash on the hands  and feet and ulcerations in the mouth, mainly in children.   Pleurodynia (Bornholm disease, epidemic myalgia, “devil’s  grip”) is characterized by fever and severe pleuritic-type  chest pain. Note that pleurodynia is pain due to an infec¬  tion of the intercostal muscles (myositis), not of the pleura.   Myocarditis and pericarditis are characterized by fever,  chest pain, and signs of congestive failure. Dilated cardio¬  myopathy with global hypokinesia of the myocardium is a  feared sequela that often requires cardiac transplantation to  sustain life. Diabetes in mice can be caused by pancreatic  damage as a result of infection with Coxsackie virus B4.  This virus is suspected to have a similar role in juvenile  diabetes in humans.   Both groups of viruses can cause aseptic meningitis, mild  paresis, and acute flaccid paralysis similar to poliomyelitis.  Upper respiratory infections, pharyngitis, and minor febrile  illnesses with or without rash can occur also.   The diagnosis is made either by isolating the virus in cell  culture or suckling mice or by observing a rise in titer of  neutralizing antibodies. A rapid (2.5-hour) polymerase  chain reaction (PCR)-based test for enteroviral RNA in the  spinal fluid is useful for making a prompt diagnosis of viral  meningitis because culture techniques typically take days to  obtain a result.   There is neither antiviral drug therapy nor a vaccine avail¬  able against these viruses. No passive immunization is  recommended.   The prefix ECHO is an acronym for enteric cytopathic  human orphan. Although called “orphans” because they  were not initially associated with any disease, they are now  known to cause a variety of diseases such as aseptic menin¬  gitis, upper respiratory tract infection, febrile illness with  and without rash, infantile diarrhea, and hemorrhagic  conjunctivitis.   The structure of echoviruses is similar to that of other  enteroviruses. More than 30 serotypes have been isolated.  In contrast to Coxsackie viruses, they are not pathogenic  for mice. Unlike polioviruses, they do not cause disease in  monkeys. They are transmitted by the fecal-oral route and  occur worldwide. Pathogenesis is similar to that of the  other enteroviruses.    diagnosis is made by isolation of the virus in cell culture.  Serologic tests are of little value, because there are a large  number of serotypes and no common antigen. There is no  antiviral therapy or vaccine available.   In view of the difficulty in classifying many enteroviruses,  all new isolates have been given a simple numerical desig¬  nation since 1969.   Enterovirus 68 (EV68 and EVD68) is a common cause  of respiratory tract disease that ranges from a mild com¬  mon cold to pneumonia and respiratory failure. It is also  implicated as a cause of acute flaccid paralysis (“poliolike”)  in children. A PCR test is available. There is no antiviral  therapy and no vaccine.   Enterovirus 70 is the main cause of acute hemorrhagic  conjunctivitis, characterized by petechial hemorrhages on  the bulbar conjunctivas. Complete recovery usually occurs,  and there is no therapy.   Enterovirus 71 is one of the leading causes of viral cen¬  tral nervous system disease, including meningitis, encepha¬  litis, and paralysis. It also causes diarrhea, pulmonary  hemorrhages, hand-foot-and-mouth disease, and herpan-  gina. Enterovirus 72 is hepatitis A virus, which is described  in Chapter 41.   There are more than 100 serologic types, which explains  why the common cold is so common. As described in  Table 40-1, they replicate better at 33°C than at 37°C,  which explains why they affect primarily the nose and con¬  junctiva rather than the lower respiratory tract. Because  they are acid-labile, they are killed by gastric acid when  swallowed. This explains why they do not infect the gastro¬  intestinal tract, unlike the enteroviruses. The host range is  limited to humans and chimpanzees.   Replication is similar to that of poliovirus. The cell surface  receptor for rhinoviruses is intracellular adhesion molecule  1 (ICAM-1), an adhesion protein located on the surface of  many types of cells.   There are two modes of transmission for these viruses. In  the past, it was accepted that they were transmitted directly  from person to person via aerosols of respiratory droplets.    However, now it appears that an indirect mode, in which  respiratory droplets are deposited on the hands or on a  surface such as a table and then transported by fingers to  the nose or eyes, is also important.   The common cold is reputed to be the most common  human infection, although data are difficult to obtain  because it is not a well-defined or notifiable disease.  Millions of days of work and school are lost each year as a  result of “colds.” Rhinoviruses occur worldwide, causing  disease particularly in the fall and winter. The reason for  this seasonal variation is unclear. Low temperatures per se  do not predispose to the common cold, but the crowding  that occurs at schools, for example, may enhance transmis¬  sion during fall and winter. The frequency of colds is high  in childhood and tapers off during adulthood, presumably  because of the acquisition of immunity.   A few serotypes of rhinoviruses are prevalent during  one season, only to be replaced by other serotypes during  the following season. It appears that the population builds  up immunity to the prevalent serotypes but remains sus¬  ceptible to the others.   The portal of entry is the upper respiratory tract, and the  infection is limited to that region. Rhinoviruses rarely  cause lower respiratory tract disease, probably because they  grow poorly at 37°C.   Immunity is serotype-specific and is a function of nasal  secretory IgA rather than humoral antibody.   After an incubation period of 2 to 4 days, sneezing, nasal  discharge, sore throat, cough, and headache are common.  A chilly sensation may occur, but there are few other sys¬  temic symptoms. The illness lasts about 1 week. Note that  other viruses such as coronaviruses, adenoviruses, influ¬  enza C virus, and Coxsackie viruses also cause the common  cold syndrome.   Diagnosis can be made by isolation of the virus from nasal  secretions in cell culture, but this is rarely attempted. Sero¬  logic tests are not done.   No specific antiviral therapy is available. Vaccines appear  impractical because of the large number of serotypes. Paper  tissues impregnated with a combination of citric acid  (which inactivates rhinoviruses) and sodium lauryl sulfate  (a detergent that inactivates enveloped viruses such as  influenza virus and respiratory syncytial virus) limit trans¬  mission when used to remove viruses from fingers con¬  taminated with respiratory secretions. High doses of  vitamin C have little ability to prevent rhinovirus-induced  colds. Lozenges containing zinc gluconate are available for  the treatment of the common cold, but their efficacy  remains uncertain.   Caliciviruses are small, nonenveloped viruses with single-  stranded RNA of positive polarity. Although they share  those features with picornaviruses, caliciviruses are dis¬  tinguished from picornaviruses by having a larger genome  and having distinctive spikes on the surface. Norovirus  is the main human pathogen in the calicivirus family.  Sapporo virus, another calicivirus that causes gastroen¬  teritis, is described in the chapter on Minor Viral Patho¬  gens (Chapter 46).   Norovirus is one of the most common causes of viral gas¬  troenteritis in adults both in the United States and world¬  wide. Norovirus is also the most common cause of viral  gastroenteritis in children in the United States because the  rotavirus vaccine has lowered the incidence of disease  caused by that virus. Norwalk virus is an important norovi¬  rus and is named for an outbreak of gastroenteritis in a  school in Norwalk, OH, in 1969.   Norovirus has a nonsegmented, single-stranded, positive-  polarity RNA genome. It is a nonenveloped virus with an  icosahedral nucleocapsid. There is no polymerase within  the virion. In the electron microscope, 10 prominent spikes  and 32 cup-shaped depressions can be seen. There are  many serotypes; the exact number is uncertain. Six geno-  groups have been identified. Most human infections are  caused by members of genogroup II.   Norovirus replicates in a manner similar to that of poliovirus  (see earlier in this chapter).   Norovirus is transmitted by the fecal-oral route, often  involving the ingestion of contaminated seafood or water.  Outbreaks typically occur in group settings such as cruise  ships (especially in the Caribbean region), schools, camps,    hospitals, and nursing homes. Person-to-person transmis¬  sion also occurs, especially in group settings. There are  many animal caliciviruses, but there is no evidence that  they cause human infection.   Infection is enhanced by several features of the virus:  low infectious dose, excretion of virus in the stool both  before the onset of symptoms and for several weeks after  recovery, and resistance to inactivation by chlorination and  to drying in the environment. It is thought to remain infec¬  tious for several days in water, uncooked food, and on  environmental surfaces such as door handles.   Norovirus infection is typically limited to the mucosal cells  of the intestinal tract. Watery diarrhea without red cells or  white cells occurs. Many asymptomatic infections occur, as  determined by the detection of antibodies. Immunity fol¬  lowing infection appears to be brief, and reinfection can  occur. New strains appear every 2 to 4 years and cause  widespread infections.   Disease is characterized by sudden onset of vomiting  and diarrhea accompanied by low-grade fever and    abdominal cramping. Neither the emesis nor the stool  contains blood. The illness typically lasts 2 to 3 days, and  there are no long-term sequelae, except in some immu¬  nocompromised patients in whom chronic gastroenteri¬  tis can occur. In some outbreaks, certain patients  manifest signs of central nervous system involvement  such as headache, meningismus, photophobia, and  obtundation.   A PCR-based test on the stool is performed when a specific  diagnosis is required. However, the diagnosis is often a  clinical one.   There is no antiviral therapy or vaccine available. Dehy¬  dration and electrolyte imbalance caused by the vomit¬  ing and diarrhea may require oral rehydration or  intravenous fluids. Personal hygiene, such as handwash¬  ing, and public health measures, such as proper sewage  disposal and disinfection of contaminated surfaces, are  helpful.   REO is an acronym for respiratory enteric orphan; when  the virus was discovered, it was isolated from the respira¬  tory and enteric tracts and was not associated with any  disease. Rotaviruses are the most important human patho¬  gens in the reovirus family.   Rotavirus is a common cause of viral gastroenteritis, espe¬  cially in young children.   Rotavirus has a segmented, double-stranded RNA  genome surrounded by a double-layered icosahedral  capsid without an envelope. The rotavirus genome has  11 segments. The virion contains an RNA-dependent RNA  polymerase. A virion polymerase is required because  human cells do not have an RNA polymerase that can  synthesize mRNA from a double-stranded RNA template.   Many domestic animals are infected with their own  strains of rotaviruses, but these are not a source of human  disease. There are at least six serotypes of human rotavirus.  The outer surface protein (also known as the viral hemag¬  glutinin) is the type-specific antigen and elicits protective  antibody.   Rotavirus attaches to the cell surface at the site of the  (I-adrenergic receptor. After entry of the virion into the  cell, the RNA-dependent RNA polymerase synthesizes  mRNA from each of the 11 segments within the cyto¬  plasm. The 11 mRNAs are translated into the corre¬  sponding number of structural and nonstructural  proteins. One of these, an RNA polymerase, synthesizes  minus strands that will become part of the genome of the  progeny virus. Capsid proteins form an incomplete cap¬  sid around the minus strands, and then the plus strands  of the progeny genome segments are synthesized. The  virus is released from the cytoplasm by lysis of the cell,  not by budding.   Rotavirus is transmitted by the fecal-oral route. Infection  occurs worldwide, and by age 6 years, most children have  antibodies to at least one serotype.   Rotavirus replicates in the mucosal cells of the small intes¬  tine, resulting in the excess secretion of fluids and electro¬  lytes into the bowel lumen. The consequent loss of salt,  glucose, and water leads to diarrhea. No inflammation  occurs, and the diarrhea is nonbloody. It is thought that    this watery diarrhea is caused primarily by stimulation of  the enteric nervous system.   The virulence of certain reoviruses in mice has been  localized to the proteins encoded by several specific genome  segments. For example, one gene governs tissue tropism,  whereas another controls the inhibition of cell RNA and  protein synthesis.   Immunity to rotavirus infection is unclear. It is likely  that intestinal IgA directed against specific serotypes pro¬  tects against reinfection and that colostrum IgA protects  newborns up to the age of 6 months.   Rotavirus infection is characterized by nausea, vomiting,  and watery, nonbloody diarrhea. Gastroenteritis is most  serious in young children, in whom dehydration and elec¬  trolyte imbalance are a major concern. Adults usually have  minor symptoms.   Although the diagnosis of most cases of viral gastroenteri¬  tis does not involve the laboratory, a diagnosis can be  made by detection of rotavirus in the stool by using  radioimmunoassay or enzyme-linked immunosorbent  assay (ELISA). This approach is feasible because there are  large numbers of virus particles in the stool. The original  demonstration of rotavirus in the stool was done by  immunoelectron microscopy, in which antibody aggre¬  gated the virions, allowing them to be visualized in the  electron microscope. This technique is not feasible for  routine clinical use. In addition to antigen detection, the  diagnosis can be made by observation of a fourfold or  greater rise in antibody titer. Although the virus can be  cultured, this procedure is not routinely done.   There are two rotavirus vaccines available. Both contain  live virus and are given orally. One is a live, attenuated  vaccine (Rotarix), which contains the single most common  rotavirus serotype (Gl) causing disease in the United  States. The other is a live reassortant vaccine (Rotateq),  which contains five rotavirus strains. An increased risk of  intussusception has been reported with both vaccines.  Patients with a history of intussusception should not  receive either vaccine.   The five rotaviruses in the Rotateq vaccine are reassor-  tants in which the gene for the outer surface protein of a  human rotavirus is inserted into a bovine strain of rotavirus.  (Recall that rotavirus has a segmented genome.) The  bovine strain is nonpathogenic for humans, but the human  outer surface protein in the vaccine virus elicits protective  (IgA) immunity in the gastrointestinal tract.   A previously approved vaccine (Rotashield) was with¬  drawn when a high rate of intussusception occurred in    vaccine recipients. Hygienic measures such as proper sew¬  age disposal and handwashing are helpful. There is no   1. Regarding poliovirus and the disease poliomyelitis, which one of  the following is most accurate?   (B) New antigenic variants arise by coinfection with animal strains  of poliovirus.   (C) Paralytic poliomyelitis is the most common manifestation of  poliovirus infection.   (D) Poliovirus has single-stranded RNA as its genome and a poly¬  merase in the virion that synthesizes its mRNA.   (E) The current vaccine recommendation is to give the live, attenu¬  ated vaccine for the first three immunizations to prevent the  child from acting as a reservoir, followed by boosters using the  killed vaccine.   2. A 70-year-old retired carpenter has signed up with a volunteer  organization to build houses in a developing country where polio  is still endemic. He plans to be there about 9 months. He thinks he  has never been immunized against polio. Which one of the follow¬  ing is the most appropriate thing to do?   (C) Give the killed vaccine containing types 1, 2, and 3.   (E) Give the live vaccine containing types 1, 2, and 3.   3. Regarding rhinoviruses, which one of the following is most accurate?   (A) Rhinoviruses are an important cause of viral meningitis and  myocarditis.   (B) The rhinovirus vaccine is recommended for all children over  2 years of age.   (C) Rhinoviruses have many serologic types, so a person can have  many infections caused by these viruses.   (D) Rhinoviruses are not inactivated by stomach acid, so they  infect the upper gastrointestinal tract and are one of the causes  of viral diarrhea.   (E) An important feature of the laboratory diagnosis of rhinovi¬  ruses is finding cytopathic effect in cell culture consisting of  multinucleated giant cells.   4. Regarding norovirus, which one of the following is most accurate?   (A) The diarrhea is caused by an exotoxin that increases cyclic  adenosine monophosphate (cyclicAMP).   (B) There are no neutrophils or red cells in the stool.   (C) Ritonavir, a protease inhibitor, is the drug of choice for chronic  diarrhea caused by norovirus.   (D) Ingestion of undercooked hamburger is a common mode of  acquisition of norovirus as cattle are a major reservoir of the  virus.   (E) The diagnosis of norovirus-induced diarrhea is typically made  by the detection of a fourfold or greater rise in antibody titer to  the virus.   5. Regarding rotavirus, which one of the following is most accurate?   (A) Rotavirus is a major cause of nosocomial diarrhea in intensive  care units.   (B) The vaccine against rotavirus contains live, attenuated virus as  the immunogen.    (C) Rotavirus has a nonsegmented, single-stranded RNA genome,  and there is no polymerase in the virion.   (D) The diagnosis of rotavirus diarrhea is typically made by the  detection of a fourfold or greater rise in antibody titer to the  virus.   (E) Diarrhea caused by rotavirus is due to a viral protein that  increases the release of IgA from many submucosal B  lymphocytes.   Brief summaries of the organisms described in this chapter  begin on page 674. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Clinical Virology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 723. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   Many viruses cause hepatitis. Of these, five medically  important viruses are commonly described as “hepatitis  viruses” because their main site of infection is the liver.  These five are hepatitis A virus (HAV), hepatitis B virus  (HBV), hepatitis C virus (HCV), hepatitis D virus (HDV,  delta virus), and hepatitis E virus (HEV) (Tables 41-1 and  41-2). Other viruses, such as Epstein-Barr virus (the cause  of infectious mononucleosis), cytomegalovirus, and yellow    fever virus, infect the liver but also infect other sites in the  body and therefore are not exclusively hepatitis viruses.  They are discussed elsewhere.   IgM HAV Ab IgM antibody to HAV; best test to detect acute hepatitis A   HBV Hepatitis B virus, a hepadnavirus (enveloped, partially double-stranded DNA virus); also known as Dane particle   HBsAg Antigen found on surface of HBV, also found on noninfectious particles in patient's blood; positive during acute disease;   HBcAb Antibody to HBcAg; positive during window phase; IgM HBcAb is an indicator of recent disease   HBeAg A second, different antigenic determinant in the HBV core; important indicator of transmissibility   HCV Hepatitis C virus, a flavivirus (enveloped RNA virus); one of the non-A, non-B viruses   HDV Hepatitis D virus, small RNA virus with HBsAg envelope; defective virus that replicates only in HBV-infected cells   HEV Hepatitis E virus, a hepevirus (nonenveloped RNA virus) one of the non-A, non-B viruses   HAV is a typical enterovirus classified in the picornavirus  family. It has a single-stranded RNA genome and a nonen-  veloped icosahedral nucleocapsid and replicates in the  cytoplasm of the cell. It is also known as enterovirus 72. It  has one serotype, and there is no antigenic relationship to  HBV or other hepatitis viruses.   HAV has a replicative cycle similar to that of other entero¬  viruses (the replicative cycle of poliovirus is discussed in  Chapter 40).   HAV is transmitted by the fecal-oral route. Humans are the  reservoir for HAV. Virus appears in the feces roughly 2 weeks  before the appearance of symptoms, so quarantine of patients  is ineffective. Children are the most frequently infected  group, and outbreaks occur in special living situations such  as summer camps and boarding schools. Common-source  outbreaks arise from fecally contaminated water or food  such as oysters grown in polluted water and eaten raw.  Unlike HBV, HAV is rarely transmitted via the blood,  because the level of viremia is low and chronic infection does  not occur. About 50% to 75% of adults in the United States  have been infected, as evidenced IgG antibody.   The pathogenesis of HAV infection is not completely  understood. The virus probably replicates in the gastroin¬  testinal tract and spreads to the liver via the blood. Hepato-  cytes are infected, but the mechanism by which cell damage  occurs is unclear. HAV infection of cultured cells produces  no cytopathic effect. It is likely that attack by cytotoxic  T cells causes the damage to the hepatocytes. The infection  is cleared, the damage is repaired, and no chronic infection  ensues. Hepatitis caused by the different viruses cannot be  distinguished pathologically.   The immune response consists initially of IgM antibody,  which is detectable at the time jaundice appears. It is there¬  fore important in the laboratory diagnosis of hepatitis A.  The appearance of IgM is followed 1 to 3 weeks later by the  production of IgG antibody, which provides lifelong  protection.   The clinical manifestations of hepatitis are virtually the  same, regardless of which hepatitis virus is the cause  (Table 41-3). Fever, anorexia, nausea, vomiting, and jaun¬  dice are typical. Dark urine, pale feces, and elevated trans¬  aminase levels are seen. Most cases resolve spontaneously  in 2 to 4 weeks. Hepatitis A has a short incubation period  (3-4 weeks) in contrast to that of hepatitis B, which is 10 to    12 weeks. Most HAV infections are asymptomatic and are  detected solely by the presence of IgG antibody. No chronic  hepatitis or chronic carrier state occurs, and there is no  predisposition to hepatocellular carcinoma.   The detection of IgM antibody is the most important test.  A fourfold rise in IgG antibody titer can also be used. Isola¬  tion of the virus in cell culture is possible but not available  in the clinical laboratory.   No antiviral therapy is available. Active immunization  with a vaccine containing inactivated HAV is available. The  virus is grown in human cell culture and inactivated with  formalin. Two doses, an initial dose followed by a booster  6 to 12 months later, should be given. No subsequent  booster dose is recommended. The vaccine is recom¬  mended for travelers to developing countries, for children  ages 2 to 18 years, and for men who have sex with men.  If an unimmunized person must travel to an endemic  area within 4 weeks, then passive immunization (see later)  should be given to provide immediate protection and the  vaccine given to provide long-term protection. This is an  example of passive-active immunization.   Because many adults have antibodies to HAV, it may be  cost-effective to determine whether antibodies are present  before giving the vaccine. The vaccine is also effective in  postexposure prophylaxis if given within 2 weeks of expo¬  sure. A combination vaccine that immunizes against both  HAV and HBV called Twinrix is available. Twinrix contains  the same immunogens as the individual HAV and HBV  vaccines.   Passive immunization with immune serum globulin  prior to infection or within 14 days after exposure can pre¬  vent or mitigate the disease. Observation of proper hygiene  (e.g., sewage disposal and handwashing after bowel move¬  ments) is of prime importance.   HBV is a member of the hepadnavirus family. It is a 42-nm  enveloped virion, 1 with an icosahedral nucleocapsid core  containing a partially double-stranded circular DNA  genome (Figure 41-1 and Table 41-2).    FIGURE 41 -1 Hepatitis B virus (HBV). Left: Cross-section of  the HBV virion. Right: The 22-nm spheres and filaments composed  only of hepatitis B surface antigen. Because there is no viral DNA in  the spheres and filaments, they are not infectious. (Reproduced with  permission from Ryan K et al. Sherris Medical Microbiology. 3rd ed. Originally  published by Appleton & Lange. Copyright 1994 McGraw-Hill.)    The envelope contains a protein called the surface anti¬  gen (HBsAg), which is important for laboratory diagnosis  and immunization. 2   Within the core is a DNA polymerase. The genome  contains four genes (four open reading frames) that encode  five proteins; namely, the S gene encodes the surface anti¬  gen, the C gene encodes the core antigen and the e antigen,  the P gene encodes the polymerase, and the X gene encodes  the X protein (HBx). HBx is an activator of viral RNA tran¬  scription and may be involved in oncogenesis because it  can inactivate the p53 tumor suppressor protein (see  Chapter 43). The DNA polymerase has both RNA-depen-  dent (reverse transcriptase) and DNA-dependent activity.   Electron microscopy of a patient’s serum reveals three  different types of particles: a few 42-nm virions and many  22-nm spheres and long filaments 22 nm wide, which are  composed of surface antigen (Figure 41-2). HBV is the  only human virus that produces these spheres and fila¬  ments in such large numbers in the patient’s blood. The  ratio of filaments and small spheres to virions is 1000:1.   In addition to HBsAg, there are two other important  antigens both located in the core of the virus: the core anti¬  gen (HBcAg) and the e antigen (HBeAg). The core anti¬  gen, as the name implies, is located on the nucleocapsid  protein that forms the core of the virion, whereas the  e antigen is soluble and is released from infected cells into  the blood. The e antigen is an important indicator of  transmissibility.   For vaccine purposes, HBV has one serotype based on  HBsAg. However, for epidemiologic purposes, there are    ^lso known as a Dane particle (named for the scientist who first pub¬  lished electron micrographs of the virion).    2 HBsAg was known as Australia antigen because it was first found in the  serum of an Australian aborigine.   arrow points to a typical virion of hepatitis B virus. Short arrow points  to a small sphere (just left of arrowhead) and a long rod (just right of  arrowhead), both composed only of hepatitis B surface antigen.  (Source: Public Health Image Library, Centers for Disease Control and Prevention.)    four serologic subtypes of HBsAg based on a group-specific  antigen, “a,” and two sets of mutually exclusive epitopes, d  or y and w or r. This leads to four serotypes—adw, adr, ayw,  and ayr—which are useful in epidemiologic studies because  they are concentrated in certain geographic areas.   The specificity of HBV for liver cells is based on two  properties: virus-specific receptors located on the hepato-  cyte cell membrane (facilitate entry) and transcription  factors found only in the hepatocyte that enhance viral  mRNA synthesis (act post-entry).    Humans are the only natural hosts of HBV. There is no  animal reservoir.   The replicative cycle of HBV is depicted in Figure 41-3.  After entry of the virion into the cell and its uncoating, the  nucleocapsid moves to the nucleus. In the nucleus, the  virion DNA polymerase synthesizes the missing portion of  DNA, and a double-stranded circular DNA is formed. This  DNA serves as a template for mRNA synthesis by cellular  RNA polymerase. After the individual mRNAs are made, a  full-length positive-strand RNA is made, which is the tem¬  plate for the minus strand of the progeny DNA. The minus  strand then serves as the template for the plus strand of the  genome DNA. This RNA-dependent DNA synthesis cata¬  lyzed by reverse transcriptase encoded by HBV takes place  within the newly assembled virion nucleocapsid core in the  cytoplasm. The RNA-dependent DNA synthesis that pro¬  duces the genome and the DNA-dependent DNA synthesis  that fills in the missing portion of DNA soon after infection  of the next cell are carried out by the same enzyme (i.e., the  HBV genome encodes only one polymerase). Progeny HBV  with its HBsAg-containing envelope are released from the  cell by budding through the cell membrane.   Hepadnaviruses are the only viruses that produce  genome DNA by reverse transcription with viral RNA as  the template. (Note that this type of RNA-dependent DNA  synthesis is similar to but different from the process in  retroviruses, in which the genome RNA is transcribed into  a DNA intermediate.)   FIGURE 41 ~3 Replication cycle of hepatitis B virus. Note virus-encoded reverse transcriptase synthesizes the progeny DNA genomes using  viral RNA as the template. CCC DNA is covalently closed circular DNA in the nucleus (blue circle).    In chronic HBV infection, a carrier state occurs and  progeny HBV continues to be made. In this carrier state,  most of the circular HBV DNA is found free in the nucleus  as an episome. A small amount of HBV DNA is integrated  into host cell DNA. How episomal HBV DNA is main¬  tained in the carrier state for many years is unclear.   The three main modes of transmission are via blood, dur¬  ing sexual intercourse, and perinatally from mother to  newborn. The observation that needle-stick injuries can  transmit the virus indicates that only very small amounts of  blood are necessary. HBV infection is especially prevalent  in addicts who use intravenous drugs. Screening of blood  for the presence of HBsAg has greatly decreased the num¬  ber of transfusion-associated cases of hepatitis B. 3   However, because blood transfusion is a modern proce¬  dure, there must be another, natural route of transmission.  HBV is found in semen and vaginal fluids, so it is likely that  sexual transmission is important. Transmission from  mother to child during birth is another important natural  route. Transplacental transmission, if it occurs, is rare.  There is no evidence that transmission of HBV occurs dur¬  ing breast feeding.   Note that enveloped viruses, such as HBV, are more  sensitive to the environment than nonenveloped viruses  and hence are more efficiently transmitted by intimate con¬  tact (e.g., sexual contact). Nonenveloped viruses, such as  HAV, are quite stable and are transmitted well via the envi¬  ronment (e.g., fecal-oral transmission).   Hepatitis B is found worldwide but is particularly preva¬  lent in Asia. Globally, more than 300 million people are  chronically infected with HBV, and about 75% of them are  Asian. There is a high incidence of hepatocellular carci¬  noma (hepatoma) in many Asian countries—a finding that  indicates that HBV is a human tumor virus (see Chapter  43). Immunization against HBV has significantly reduced  the incidence of hepatoma in children. It appears that the  HBV vaccine is the first vaccine to prevent a human  cancer.   After entering the blood, the virus infects hepatocytes, and  viral antigens are displayed on the surface of the cells. Cyto¬  toxic T cells mediate an immune attack against the viral  antigens, and inflammation and necrosis occur. Immune  attack against viral antigens on infected hepatocytes is  mediated by cytotoxic T cells. The pathogenesis of hepatitis B  is probably the result of this cell-mediated immune injury,    3 In the United States, donated blood is screened for HBsAg and antibod¬  ies to HBcAg, HCV, HIV-1, HIV-2, and HTLV-1. Two other tests are also  performed: a VDRL test for syphilis and a transaminase assay, which, if  elevated, indicates liver damage and is a surrogate marker of viral  infection.    because HBV itself does not cause a cytopathic effect. Anti-  gen-antibody complexes cause some of the early symptoms  (e.g., arthralgias, arthritis, and urticaria) and some of the  complications in chronic hepatitis (e.g., glomerulonephri¬  tis, cryoglobulinemia, and vasculitis).   About 5% of adult patients with HBV infection become  chronic carriers. In contrast, 90% of infected newborns  become chronic carriers (see later). A chronic carrier is  someone who has HBsAg persisting in their blood for  6 months or longer. The chronic carrier state is attributed  to a persistent infection of the hepatocytes, which results  in the prolonged presence of HBV and HBsAg in the  blood. The main determinant of whether a person clears  the infection or becomes a chronic carrier is the adequacy  of the cytotoxic T-cell response. HBV DNA exists primar¬  ily as an episome in the nucleus of persistently infected  cells; a small number of copies of HBV DNA are integrated  into cell DNA.   A high rate of hepatocellular carcinoma (HCC) occurs  in chronic carriers. The HBx gene may be an oncogene  because the HBx protein inactivates the p53 tumor sup¬  pressor protein (see Chapter 43). In addition, HCC may be  the result of persistent cellular regeneration that attempts to  replace the dead hepatocytes. Alternatively, malignant  transformation could be the result of insertional mutagen¬  esis, which could occur when the HBV DNA integrates into  the hepatocyte DNA. Integration of the HBV DNA could  activate a cellular oncogene, leading to a loss of growth  control. Almost all HCC cells have HBV DNA integrated  into the cell DNA.   Chronic carriage is more likely to occur when infection  occurs in a newborn than in an adult, probably because a  newborn’s immune system is less competent than that of an  adult’s. Approximately 90% of infected neonates become  chronic carriers. Chronic carriage resulting from neonatal  infection is associated with a high risk of HCC.   Some chronic carriers make e antigen (they are said to  be e antigen positive) and therefore have a high probability  of making infectious virions and being able to transmit the  disease. The e antigen is the indicator of transmissibility  because it is encoded by the same gene that encodes the  core protein indicating that the HBV DNA genome is pres¬  ent. Some chronic carriers do not make e antigen (they are  said to be e antigen negative) and therefore have a low  probability of making infectious virions and are less likely  to transmit the disease.   Lifelong immunity occurs after the natural infection  and is mediated by humoral antibody against HBsAg. Anti¬  body against HBsAg (HBsAb) is protective because it binds  to surface antigen on the virion and prevents it from inter¬  acting with receptors on the hepatocyte. Another way of  saying this is that HBsAb neutralizes the infectivity of HBV.  Note that antibody against the core antigen (HBcAb) is not  protective because the core antigen is inside the virion and  the antibody cannot interact with it.   Many HBV infections are asymptomatic and are detected  only by the presence of antibody to HBsAg. The mean  incubation period for hepatitis B is 10 to 12 weeks, which  is much longer than that of hepatitis A (3-4 weeks). The  clinical appearance of acute hepatitis B is similar to that  of hepatitis A. However, with hepatitis B, symptoms tend  to be more severe, and life-threatening hepatitis can  occur. Most chronic carriers are asymptomatic, but some  have chronic active hepatitis, which can lead to cirrhosis  and death.   Patients coinfected with both HBV and human immu¬  nodeficiency virus (HIV) may have increased hepatic dam¬  age if HIV is treated prior to treating HBV. This occurs  because the “immune reconstitution” that results when  HIV is treated successfully leads to increased damage to the  hepatocytes by the restored, competent cytotoxic T cells.  For this reason, it is suggested that HBV be treated prior to  treating HIV.   The two most important serologic tests for the diagnosis of  early hepatitis B are the tests for HBsAg and for IgM anti¬  body to the core antigen. Both appear in the serum early  in the disease. HBsAg appears during the incubation  period and is detectable in most patients during the pro¬  drome and acute disease (Figure 41-4). It falls to undetect¬  able levels during convalescence in most cases; its  prolonged presence (at least 6 months) indicates the car¬  rier state and the risk of chronic hepatitis and hepatic car¬  cinoma. As described in Table 41 -4, HBsAb is not detectable  in the chronic carrier state. Note that HBsAb is, in fact,  being made but is not detectable in the laboratory tests  because it is bound to the large amount of HBsAg present  in the blood. HBsAb is also being made during the acute  disease but is similarly undetectable because it is bound in  antigen-antibody complexes.   Note that there is a period of several weeks when  HBsAg has disappeared but HBsAb is not yet detectable.  This is the window phase. At this time, the HBcAb is  always positive and can be used to make the diagnosis.    HBcAb is present in those with acute infection and  chronic infection, as well as in those who have recovered  from acute infection. Therefore, it cannot be used to dis¬  tinguish between acute and chronic infection. The IgM  form of HBcAb is present during acute infection and  disappears approximately 6 months after infection. The  test for HBcAg is not readily available. Table 41-4  describes the serologic test results that characterize the  four important stages of HBV infection.   HBeAg arises during the incubation period and is pres¬  ent during the prodrome and early acute disease and in  certain chronic carriers. Its presence in chronic carriers  indicates a high likelihood of transmissibility, and, con¬  versely, the absence of HBeAg indicates a low likelihood of  transmission. In addition, the finding of HBeAb indicates a  lower likelihood, but transmission can still occur. DNA  polymerase activity is detectable during the incubation  period and early in the disease, but the assay is not available  in most clinical laboratories.   The detection of viral DNA (viral load) in the serum is  strong evidence that infectious virions are present. Reduc¬  tion of the viral load in patients with chronic hepatitis B is  used to monitor the success of drug therapy.   No antiviral therapy is typically used in acute hepatitis B.  For chronic hepatitis B, entecavir (Baraclude) or tenofovir  (Viread) are the drugs of choice. They are nucleoside ana¬  logues that inhibit the reverse transcriptase of HBV. Inter¬  feron in the form of peginterferon alfa-2a (Pegasys) is also  used. Other nucleoside analogues such as lamivudine  (Epivir-HBV), adefovir (Hepsera), and telbivudine (Tyzeka)  are used less frequently. A combination of tenofovir and  emtricitabine (Emtriva) is also used.   These drugs reduce hepatic inflammation and lower the  viral load of HBV in patients with chronic active hepatitis.  Neither interferon nor the nucleoside analogues cure the  HBV infection. In most patients when the drug is stopped,  HBV replication resumes.   Patients coinfected with HBV and HIV should be pre¬  scribed highly active antiretroviral therapy (HAART) with    ’Chronic carriers who are HBeAg-positive are highly likely to transmit HBV whereas those who are HBeAb-positive are less likely to transmit HBV.   2 Chronic carriers have negative antibody tests, but HBsAb is being made by these individuals. It is undetected in the tests because it is bound to the large amount of HBsAg  present in the plasma. They are not tolerant to HbsAg.   3 lgM is found in the acute stage; IgG is found in subsequent stages.   Note: People immunized with HBV vaccine have HBsAb but not HBcAb because the immunogen in the vaccine is purified HBsAg.   FIGURE 41-4 A: Important diagnostic tests during various  stages of hepatitis B. B: Serologic findings in a patient with acute  hepatitis B. C: Duration of increased liver enzyme activity and of  symptoms in a patient with acute hepatitis B. D: Serologic findings in  a patient with chronic hepatitis B. anti-HBc, hepatitis B core antibody;  anti-HBe, hepatitis B e antibody; anti-HBs, hepatitis B surface anti¬  body; HBeAg, hepatitis B e antigen; HBsAg, hepatitis B surface anti¬  gen; HBV, hepatitis B virus. (Adapted from Hollinger FB, Dienstag JL. Hepatitis  viruses. In: Lennette EH, et at, eds. Manual of Clinical Microbiology. 4th ed. Washington,  DC: ASM Press; 1985.)   caution because recovery of cell-mediated immunity can  result in an exacerbation of hepatitis (immune reconstitu¬  tion syndrome, IRIS). Consideration should be given to  treat the HBV infection prior to starting HAART.   Prevention involves the use of either the vaccine or hyper¬  immune globulin or both.   (1) The vaccine (e.g., Recombivax) contains HBsAg  produced in yeasts by recombinant DNA techniques. The  vaccine is highly effective in preventing hepatitis B and has  few side effects. The seroconversion rate is approximately  95% in healthy adults. It is indicated for people who are  frequently exposed to blood or blood products, such as  certain health care personnel (e.g., medical students, sur¬  geons, and dentists), patients receiving multiple transfu¬  sions or dialysis, patients with frequent sexually transmitted  disease, and abusers of illicit intravenous drugs. Travelers  who plan a long stay in areas of endemic infection, such as  many countries in Asia and Africa, should receive the vac¬  cine. The U.S. Public Health Service recommends that all  newborns and adolescents receive the vaccine.   At present, booster doses after the initial three-dose regi¬  men are not recommended. However, if antibody titers have  declined in immunized patients who are at high risk, such as  dialysis patients, then a booster dose should be considered.   Widespread immunization with the HBV vaccine has  significantly reduced the incidence of hepatocellular carci¬  noma in children. A vaccine called Twinrix that contains  both HBsAg and inactivated HAV provides protection  against both hepatitis B and hepatitis A.   (2) Hepatitis B immune globulin (HBIG) contains a high  titer of HBsAb. It is used to provide immediate, passive pro¬  tection to individuals known to be exposed to HBsAg-  positive blood (e.g., after an accidental needle-stick injury).   Precise recommendations for use of the vaccine and  HBIG are beyond the scope of this book. However, the  recommendation regarding one common concern of medi¬  cal students, the needle-stick injury from a patient with  HBsAg-positive blood, is that both the vaccine and HBIG  be given (at separate sites). This is true even if the patients  blood is HBeAb positive.   Both the vaccine and HBIG should also be given to a  newborn whose mother is HBsAg-positive. This regimen is  very effective in reducing the infection rate of newborns  whose mothers are chronic carriers. The regimen of vac¬  cine plus HBIG in those with needle-stick injuries and in  neonates is a good example of passive-active immuniza¬  tion, in which both immediate protection and long-term  protection are provided.   The effectiveness of Cesarean section to reduce HBV  infection of neonates is uncertain. It is currently not recom¬  mended. Breast feeding of immunized neonates by mothers  who are chronic carriers entails little risk of infection of the  neonate.   All blood for transfusion should be screened for HBsAg.  No one with a history of hepatitis (of any type) should  donate blood, because non-A, non-B viruses may be pres¬  ent. Screening of high-risk populations to detect chronic  carriers using serologic testing should be done because   The term “non-A, non-B hepatitis” was coined to describe  the cases of hepatitis for which existing serologic tests had  ruled out all known viral causes. The term is not often used  because the main cause of non-A, non-B hepatitis, namely,  HCV, has been identified. In addition, HDV and HEV have  been described. Cross-protection experiments indicate  additional hepatitis viruses exist.   HCV is a member of the flavivirus family. It is an enveloped  virion containing a genome of single-stranded, positive-  polarity RNA. It has no virion polymerase.   HCV has at least six genotypes and multiple subgeno¬  types based on differences in the genes that encode one of  its two envelope glycoproteins. This genetic variation  results in a “hypervariable” region in the envelope glyco¬  protein. The genetic variability is due to the high mutation  rate in the envelope gene coupled with the absence of a  proofreading function in the virion-encoded RNA poly¬  merase. As a result, multiple subspecies (quasispecies)  often occur in the blood of an infected individual at the  same time. Genotype 1 causes approximately 75% of infec¬  tions in the United States.   The replication of HCV is uncertain because it has not been  grown in cell culture. Other flaviviruses replicate in the  cytoplasm and translate their genome RNA into large poly¬  proteins, from which functional viral proteins are cleaved by  a virion-encoded protease. This protease is the target of  potent anti-HCV therapy (see treatment section). In addi¬  tion, HCV genome RNA encodes a protein called NS5A that  cooperates with the RNA polymerase of the virus to synthe¬  size progeny genome RNA’s. The NS5A protein is also the  target of potent anti-HCV therapy (see treatment section).   The replication of HCV in the liver is enhanced by a  liver-specific micro-RNA called miR-122. This micro-RNA  acts by increasing the synthesis of HCV mRNA. (Micro-  RNAs are known to enhance cellular mRNA synthesis in  many tissues.) In 2013, a clinical trial of an antisense  nucleotide that bound to and blocked the activity of  miR-122 showed prolonged reduction in HCV RNA levels  in infected patients.   Humans are the reservoir for HCV. It is transmitted pri¬  marily via blood. At present, injection drug use accounts  for almost all new HCV infections. Transmission from  mother to child during birth is another very common  mode of transmission. Transmission via blood transfu¬  sion rarely occurs because donated blood containing  antibody to HCV is discarded. Transmission via needle-  stick injury occurs, but the risk is lower than for HBV.  Sexual transmission is uncommon, and there is no evi¬  dence for transmission across the placenta or during  breast feeding.   the United States. (In the nationally reported incidence  data, HCV ranks below HIV and HBV as a blood-borne  pathogen, but it is estimated that HCV is more prevalent.)  Approximately 4 million people in the United States (1%-  2% of the population) are chronically infected with HCV.  Unlike yellow fever virus, another flavivirus that infects the  liver and is transmitted by mosquitoes, there is no evidence  for an insect vector for HCV. Worldwide, it is estimated  that 180 million people are infected with HCV.   Many infections are asymptomatic, so screening of  high-risk individuals for HCV antibody should be done. In  addition, screening of those who were born between 1945  and 1965 should be done because they have a high rate of  infection.   In the United States, about 1% of blood donors have  antibody to HCV. People who share needles when taking  intravenous drugs are very commonly infected. Commer¬  cially prepared immune globulin preparations are generally  very safe, but several instances of the transmission of HCV  have occurred. This is the only example of an infectious  disease transmitted by commercial preparations of immune  globulins.   HCV infects hepatocytes primarily, but there is no evidence  for a virus-induced cytopathic effect on the liver cells.  Rather, death of the hepatocytes is probably caused by  immune attack by cytotoxic T cells. HCV infection  strongly predisposes to hepatocellular carcinoma, but  there is no evidence for an oncogene in the viral genome or  for insertion of a copy of the viral genome into the DNA of  the cancer cells.   Alcoholism greatly enhances the rate of hepatocellular  carcinoma in HCV-infected individuals. This supports the  idea that the cancer is caused by prolonged liver damage  and the consequent rapid growth rate of hepatocytes as the  cells attempt to regenerate rather than by a direct onco¬  genic effect of HCV. Added support for this idea is the  observation that patients with cirrhosis of any origin, not  just alcoholic cirrhosis, have an increased risk of hepatocel¬  lular carcinoma.    Antibodies against HCV are made, but approximately  75% of patients are chronically infected and continue to  produce virus for at least 1 year. (Note that the rate of  chronic carriage of HCV is much higher than the rate of  chronic carriage of HBV.) Chronic active hepatitis and  cirrhosis occur in approximately 10% of these patients. For  patients who clear the infection, it is not known whether  reinfection can occur or whether there is lifelong  immunity.   The acute infection is often asymptomatic. If symptoms,  such as malaise, nausea, and right upper quadrant pain do  occur, they are milder than occur with infection by the  other hepatitis viruses. Fever, anorexia, nausea, vomiting,  and jaundice are common. Dark urine, pale feces, and ele¬  vated transaminase levels are seen.   Hepatitis C resembles hepatitis B as far as the ensuing  chronic liver disease, cirrhosis, and the predisposition to  hepatocellular carcinoma are concerned. Note that a  chronic carrier state occurs more often with HCV infection  than with HBV. Liver biopsy is often done in patients with  chronic infection to evaluate the extent of liver damage and  to guide treatment decisions. Many infections with HCV,  including both acute and chronic infections, are asymp¬  tomatic and are detected only by the presence of antibody.  The mean incubation period is 8 weeks. Cirrhosis resulting  from chronic HCV infection is the most common indica¬  tion for liver transplantation.   HCV infection also leads to significant autoimmune  reactions, including vasculitis, arthralgias, purpura, and  membranoproliferative glomerulonephritis. HCV is the  main cause of essential mixed cryoglobulinemia. Cryo¬  globulins are defined by their ability to precipitate at  cold temperature (cryo = cold). The cryoprecipitates are  immune complexes composed of HCV antigens and  antibodies.   HCV infection is diagnosed by detecting antibodies to  HCV in an enzyme-linked immunosorbent assay (ELISA)  (see Table 41-5). The antigen in the assay is a recombinant    protein formed from three immunologically stable  HCV proteins and does not include the highly variable enve¬  lope proteins. The test does not distinguish between IgM  and IgG and does not distinguish between an acute,  chronic, or resolved infection.   If the result of ELISA antibody test is positive, a poly¬  merase chain reaction-based test that detects the presence  of viral RNA (viral load) in the serum should be performed  to determine whether active disease exists. Reduction of the  viral load in patients with hepatitis C is used to monitor the  success of drug therapy. Isolation of the virus from patient  specimens is not done. A chronic infection is characterized  by elevated transaminase levels, a positive ELISA antibody  test, and detectable viral RNA for at least 6 months.   Treatment of acute hepatitis C with peginterferon alfa sig¬  nificantly decreases the number of patients who become  chronic carriers.   The treatment of choice for chronic hepatitis C is a com¬  bination of drugs from three classes: RNA polymerase inhibi¬  tors, NS5 A inhibitors, and protease inhibitors (see Table 41 -6).  These drugs are administered orally which is an improve¬  ment over the drugs in previous regimens that often included  pegylated interferon-alpha which is administered parenter-  ally and has significant adverse effects. One currently avail¬  able combination is Harvoni which is a combination of  sofosbuvir (RNA polymerase inhibitor) and ledipasvir (NS5A  inhibitor). Another is Viekera which is a four-drug combina¬  tion: dasabuvir (RNA polymerase inhibitor), ombitsavir  (NS5A inhibitor), paritaprevir (protease inhibitor) and rito¬  navir (a booster of protease activity; see Chapter 35, page 276).  These drug combinations are very effective against genotype  1, the most common type in the United States. Daklinza, a  combination of sofosbuvir (RNA polymerase inhibitor) and  daclatasvir (NS5A inhibitor) is effective against genotype 3  HCV. These various drug combinations offer the prospect of  a “cure” for chronic hepatitis C.   There is no vaccine and hyperimmune globulins are not  available. Pooled immune serum globulins are not useful    for postexposure prophylaxis. There is no effective regimen  for prophylaxis following needle-stick injury; only moni¬  toring is recommended.   Blood found to contain antibody is discarded—a  procedure that has prevented virtually all cases of transfu¬  sion-acquired HCV infection since 1994, when screening  began. Screening of individuals born in the United States  between 1945 and 1965 for HCV antibody is recommended  because they have a high rate of infection. Treatment of those  who are antibody-positive should reduce transmission.   Patients with chronic HCV infection should be advised  to reduce or eliminate their consumption of alcoholic bev¬  erages to reduce the risk of hepatocellular carcinoma and  cirrhosis. Patients with chronic HCV infection and cirrho¬  sis should be monitored with alpha-fetoprotein tests and  liver sonograms to detect carcinoma at an early stage.  Patients with liver failure due to HCV infection can receive  a liver transplant, but infection of the graft with HCV typi¬  cally occurs.   Patients coinfected with HCV and HIV should be  prescribed highly active antiretroviral therapy (HAART)  with caution because recovery of cell-mediated immu¬  nity can result in an exacerbation of hepatitis (immune  reconstitution syndrome, IRIS). Consideration should  be given to treat the HCV infection prior to starting  HAART.    FIGURE 41 ~5 Hepatitis D virus. Note that hepatitis B surface  antigen (HBsAg) forms the outer envelope and the genome  consists of circular RNA. (Reproduced with permission from Ryan KJ,   Ray C. Hepatitis Viruses. In: Ryan KJ, Ray C. eds. Sherris Medical Microbiology, 6e.  New York, NY: McGraw-Hill; 2014.)   HDV is unusual in that it is a defective virus (i.e., it cannot  replicate by itself because it does not have the genes for its  envelope protein). HDV can replicate only in cells also  infected with HBV because HDV uses the surface antigen  of HBV (HBsAg) as its envelope protein. HBV is therefore  the helper virus for HDV (Figure 41-5).   HDV is an enveloped virus with an RNA genome that  is a single-stranded, negative-polarity, covalently closed  circle. The RNA genome of HDV is very small and encodes  only one protein, the internal core protein called delta  antigen. HDV genome RNA has no sequence homology to  HBV genome DNA. HDV has no virion polymerase; the  genome RNA is replicated and transcribed by the host cell  RNA polymerase. HDV genome RNA is a “ribozyme” (i.e.,  it has the ability to self-cleave and self-ligate—properties  that are employed during replication of the genome). HDV  replicates in the nucleus, but the specifics of the replicative  cycle are complex and beyond the scope of this book.   HDV has one serotype because HBsAg has only one  serotype. There is no evidence for the existence of an ani¬  mal reservoir for HDV.   HDV is transmitted by the same means as is HBV (i.e.,  sexually, by blood, and perinatally). In the United States,  most HDV infections occur in intravenous drug users who  share needles. HDV infections occur worldwide, with a  similar distribution to that of HBV infections.   It seems likely that the pathogenesis of hepatitis caused by  HDV and HBV is the same (i.e., the virus-infected hepa-  tocytes are damaged by cytotoxic T cells). There is some  evidence that delta antigen is cytopathic for hepatocytes.   IgG antibody against delta antigen is not detected for  long periods after infection; it is therefore uncertain  whether long-term immunity to HDV exists.   Because HDV can replicate only in cells also infected with  HBV, hepatitis delta can occur only in a person infected  with HBV. A person can either be infected with both HDV  and HBV at the same time (i.e., be “coinfected”) or be previ¬  ously infected with HBV and then “superinfected” with HDV  Hepatitis in patients coinfected with HDV and HBV is  more severe than in those infected with HBV alone, but the  incidence of chronic hepatitis is about the same in patients  infected with HBV alone. However, hepatitis in chronic  carriers of HBV who become superinfected with HDV is  much more severe, and the incidence of fulminant, life-  threatening hepatitis, chronic hepatitis, and liver failure is  significantly higher.   The diagnosis of HDV infection in the laboratory is made  by detecting either delta antigen or IgM antibody to delta  antigen in the patient’s serum.   Peginterferon alfa can mitigate some of the effects of the  chronic hepatitis caused by HDV but does not eradicate the  chronic carrier state. There is no specific antiviral therapy  against HDV. There is no vaccine against HDV, but a per¬  son immunized against HBV will not be infected by HDV  because HDV cannot replicate unless HBV infection also  occurs.   HEV is a major cause of hepatitis transmitted by the fecal-  oral route. It is thought to be more common than HAV in  many developing countries. It is a common cause of water¬  borne epidemics of hepatitis in Asia, Africa, India, and  Mexico but is uncommon in the United States. HEV is a  nonenveloped, single-stranded RNA virus classified as a  member of the hepevirus family.   Clinically the disease resembles hepatitis A, with the  exception of a high mortality rate in pregnant women.  Chronic infection resulting in chronic hepatitis and cirrho¬  sis but not hepatocellular carcinoma, occurs in immuno¬  compromised individuals such as HIV-infected patients,  those who are receiving cancer chemotherapy, and patients  who are receiving immunosuppressive drugs to prevent  rejection of solid-organ transplants.   The diagnosis is typically made by detecting IgM anti¬  body to HEV. A PCR assay that detects HEV RNA in  patient specimens is available. There is no antiviral drug  available for acute infection in immunocompetent patients.  In immunocompromised patients, ribavirin cleared HEV  viremia in solid organ transplant recipients. There is no  vaccine.   In 1996, hepatitis G virus (HGV) was isolated from patients  with posttransfusion hepatitis. HGV is a member of the  flavivirus family, as is HCV. However, unlike HCV, which is  clearly the cause of both acute hepatitis and chronic active  hepatitis and predisposes to hepatocellular carcinoma,  HGV has not been documented to cause any of these clini¬  cal findings. The role of HGV in the causation of liver dis¬  ease has yet to be established, but it can cause a chronic  infection lasting for decades. Approximately 60% to 70% of  those infected clear the virus and develop antibodies.   HGV is transmitted via sexual intercourse and blood. It  is carried in the blood of millions of people worldwide. In  the United States, it is found in the blood of approximately  2% of random blood donors, 15% of those infected with  HCV, and 35% of those infected with HIV. Patients coin¬  fected with HIV and HGV have a lower mortality rate and  have less HIV in their blood than those infected with HIV  alone. It is hypothesized that HGV may interfere with the  replication of HIV. (HGV is also known as GB virus C.)   1. An outbreak of jaundice occurs in several young children who  attend the same day care center. If the outbreak was caused by a  virus, which one of the following is the most likely cause?   2. Regarding hepatitis A virus (HAV), which one of the following  statements is most accurate?   (A) The HAV vaccine contains live, attenuated virus as the  immunogen.   (B) The screening of blood for transfusion has greatly reduced the  spread of this virus.   (C) The diagnosis is typically made by serologic tests rather than by  culturing the virus.   (D) Multiple episodes of hepatitis A are common because it has  three serotypes.   (E) It has a segmented, negative-polarity, single-stranded RNA  genome and an RNA polymerase in the virion.   3. A woman who is hepatitis B surface antigen (HBsAg) positive and  hepatitis B surface antibody (HBsAb) negative has just given birth.  Which one of the following is the most appropriate thing to do for  the newborn?   4. Regarding hepatitis B virus (HBV) and the disease hepatitis B,  which one of the following is most accurate?   (A) The most reliable indicator that a person can transmit HBV is  the presence of HBsAg in the blood.   (B) HBV has a circular, partially double-stranded DNA as its  genome and has a DNA polymerase in the virion.    (C) Health care personnel who sustain a needle-stick injury while  taking blood from a person with hepatitis B should receive  acyclovir.   (D) HBV infection induces antibody to HBcAg (core antigen),  which protects the person from a second attack by the same  strain of HBV.   (E) A person in the “window period” can be diagnosed as having  been infected by HBV if antibody to the surface antigen  (HBsAg) is present.   5. Regarding hepatitis C virus (HCV), which one of the following is  most accurate?   (B) HCV is a defective virus that requires concurrent hepatitis B  virus (HBV) infection in order to replicate.   (C) Chronic infection with HCV occurs less frequently than  chronic infection with HBV.   (D) The killed vaccine against HCV is poorly immunogenic, so  booster doses must be given at least every 5 years.   6. Regarding hepatitis D virus (HDV), which one of the following is  most accurate?   (A) Alpha interferon can eradicate the latent state established by HDV   (B) Immunization against hepatitis B virus (HBV) will reduce the  incidence of hepatitis caused by HDV.   (C) HDV has DNA as its genome and an RNA-dependent DNA  polymerase in the virion.   (D) The laboratory diagnosis of HDV infection is made by growing  HDV in cells coinfected with HBV.   (E) Many HDV infections occur in young children in the diaper  stage in day care centers because the virus is transmitted pri¬  marily by the fecal-oral route.   7. Your patient is a 35-year-old man who complains that the whites  of his eyes have turned yellow. After taking a history and doing a  physical, you order serologic tests to determine whether he has  viral hepatitis. On the basis of the results, you tell him that he has  a mild form of hepatitis that does not cause long-term damage to  the liver. Your conclusion is based on a positive result on which  one of the following tests?   8. Your patient is a 20-year-old woman with chronic hepatitis B that  was diagnosed by detecting hepatitis B antigen in her blood more    than 6 months after her acute infection. Which one of the follow¬  ing is the best choice of drug to treat her chronic hepatitis B?   9. Your patient is a 27-year-old man with a history of intravenous  drug use who now is diagnosed with chronic hepatitis C. He is  HIV antibody negative. Which one of the following is the best  choice of drugs to treat his chronic hepatitis C?   Brief summaries of the organisms described in this chapter  begin on page 675. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Clinical Virology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 723. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   Eastern Equine Encephalitis Virus  Western Equine Encephalitis Virus  St. Louis Encephalitis Virus  California Encephalitis Virus   Arbovirus is an acronym for arthropod-borne virus and  highlights the fact that these viruses are transmitted by  arthropods, primarily mosquitoes and ticks. It is a collective  name for a large group of diverse viruses, more than 600 at  last count. In general, they are named either for the diseases  they cause (e.g., yellow fever virus) or for the place where  they were first isolated (e.g., St. Louis encephalitis virus).   A new group of viruses called roboviruses has recently  emerged. The term robo refers to the fact that these viruses  are rodent-borne (i.e., they are transmitted directly from  rodents to humans without an arthropod vector). Trans¬  mission occurs when dried rodent excrement is inhaled  into the human lung, as when sweeping the floor of a cabin.  Two roboviruses cause a respiratory distress syndrome that  is often fatal: Sin Nombre virus (a hantavirus) and White-  water Arroyo virus (an arenavirus). These viruses are  described in Chapter 46.   Most arboviruses are classified in three families, 1 namely,  togaviruses, flaviviruses, and bunyaviruses (Table 42-1).    *A few arboviruses belong to two other families. For example, Colorado  tick virus is a reovirus; Kern Canyon virus and vesicular stomatitis virus  are rhabdoviruses.    (1) Togaviruses 2 are characterized by an icosahedral  nucleocapsid surrounded by an envelope and a single-  stranded, positive-polarity RNA genome. They are 70 nm  in diameter, in contrast to the flaviviruses, which are 40 to  50 nm in diameter (see later). Togaviruses are divided into  two families, alphaviruses and rubiviruses. Only alphavi-  ruses are considered here. The only rubivirus is rubella  virus, which is discussed in Chapter 39.   (2) Flaviviruses 3 are similar to togaviruses in that they  also have an icosahedral nucleocapsid surrounded by an  envelope and a single-stranded, positive-polarity RNA  genome, but the flaviviruses are only 40 to 50 nm in diam¬  eter, whereas the togaviruses have a diameter of 70 nm.   (3) Bunyaviruses 4 have a helical nucleocapsid sur¬  rounded by an envelope and a genome consisting of three  segments of negative-polarity RNA that are hydrogen-  bonded together.   The life cycle of the arboviruses is based on the ability of  these viruses to multiply in both the vertebrate host and    ’Alphaviruses of other regions include Chikungunya, Mayaro, O'Nyong-Nyong, Ross  River, and Semliki Forest viruses.   2 Flaviviruses of other regions include Japanese encephalitis, Kyasanur Forest, Murray  Valley encephalitis, Omsk hemorrhagic fever, Powassan encephalitis viruses, and  West Nile viruses.   3 Bunyaviruses of other regions include the Bunyamwera complex of viruses and  Oropouche virus.   the bloodsucking vector (Figure 42-1). For effective  transmission to occur, the virus must be present in the  bloodstream of the vertebrate host (viremia) in suffi¬  ciently high titer to be taken up in the small volume of    blood ingested during an insect bite. After ingestion, the  virus replicates in the gut of the arthropod and then  spreads to other organs, including the salivary glands.  Only the female of the species serves as the vector of the  virus, because only she requires a blood meal in order for  progeny to be produced. An obligatory length of time,  called the extrinsic incubation period, 3 must pass before  the virus has replicated sufficiently for the saliva of the  vector to contain enough virus to transmit an infectious  dose. For most viruses, the extrinsic incubation period  ranges from 7 to 14 days.   In addition to transmission through vertebrates, some  arboviruses are transmitted by vertical “transovarian” pas¬  sage from the mother tick to her offspring. Vertical trans¬  mission has important survival value for the virus if a  vertebrate host is unavailable.   Humans are involved in the transmission cycle of arbo¬  viruses in two different ways. Usually, humans are dead¬  end hosts, because the concentration of virus in human  blood is too low and the duration of viremia is too brief for  the next bite to transmit the virus. However, in some dis¬  eases (e.g., yellow fever and dengue), humans have a high-  level viremia and act as reservoirs of the virus.    FIGURE 42-1 Arbovirus transmission cycle. Arboviruses typically cycle between the vertebrate reservoir host, often a bird, and the  vector, often a mosquito. The infected vector can also bite other hosts, such as humans and horses, which are "dead-end" hosts because their  viremia is too low to provide the vector with an infectious dose. (Source: Centers for Disease Control and Prevention.)    The intrinsic incubation period is the interval between the time of the bite and the appearance of symptoms in the human host.    Infection by arboviruses usually does not result in dis¬  ease either in the arthropod vector or in the vertebrate  animal that serves as the natural host. Disease occurs pri¬  marily when the virus infects dead-end hosts. For example,  yellow fever virus cycles harmlessly among the jungle mon¬  keys in South America, but when the virus infects a human,  yellow fever can occur.   Most human arboviral infections are asymptomatic. Of  those infections that are symptomatic, most are acute  febrile illnesses. A minority of infections cause neuroinva-  sive disease, such as encephalitis and meningitis. The dis¬  eases caused by arboviruses range in severity from mild to  rapidly fatal.   The clinical picture usually fits one of three categories:  (1) encephalitis; (2) hemorrhagic fever; or (3) fever with  myalgias, arthralgias, and nonhemorrhagic rash. The  pathogenesis of these diseases involves not only the cytoci-  dal effect of the virus, but also, in some, a prominent  immunopathologic component. After recovery from the  disease, immunity is usually lifelong.   The arboviral diseases occur primarily in the tropics but  are also found in temperate zones such as the United States  and as far north as Alaska and Siberia. They have a tendency  to cause sudden outbreaks of disease, generally at the inter¬  face between human communities and jungle or forest areas.   Of the four encephalitis viruses listed in Table 42-2, east¬  ern equine encephalitis (EEE) virus causes the most    severe disease and is associated with the highest fatality  rate (approximately 50%). In its natural habitat, the virus  is transmitted primarily by the swamp mosquito, Culi-  seta, among the small wild birds of the Atlantic and Gulf  Coast states. Species of Aedes mosquitoes are suspected of  carrying the virus from its wild bird reservoir to the  principal dead-end hosts, horses, and humans. The  number of cases of human encephalitis caused by EEE  virus in the United States usually ranges from zero to four  per year, but outbreaks involving hundreds of cases also  occur. Subclinical infections greatly exceed the number of  overt cases.   The encephalitis is characterized by the sudden onset of  severe headache, nausea, vomiting, and fever. Changes in  mental status, such as confusion and stupor, ensue. A rap¬  idly progressive downhill course with nuchal rigidity, sei¬  zures, and coma occurs. If the patient survives, the central  nervous system sequelae are usually severe. Immunity fol¬  lowing the infection is lifelong.   The diagnosis is made by either isolating the virus or  demonstrating a rise in antibody titer. Clinicians should  have a high index of suspicion in the summer months in  the appropriate geographic areas. The disease does not  occur in the winter because mosquitoes are not active. It is  not known how the virus survives the winter—in birds,  mosquitoes, or perhaps some other animal.   No antiviral therapy is available. A killed vaccine is  available to protect horses but not humans. The disease is  too rare for production of a human vaccine to be economi¬  cally feasible.   Western equine encephalitis (WEE) virus causes disease  more frequently than does EEE virus, but the illness is less  severe. Inapparent infections outnumber the apparent by at  least 100:1. The number of cases in the United States    TABLE 42-2 Epidemiology of Important Arbovirus Diseases in the United States    CE = California encephalitis; CTF = Colorado tick fever; EEE = eastern equine encephalitis virus; SLE = St. Louis encephalitis; WEE = western equine encephalitis virus.  Venezuelan equine encephalitis virus causes disease in the United States too rarely to be included.    usually ranges between 5 and 20 per year, and the fatality  rate is roughly 2%.   The virus is transmitted primarily by Culex mosquitoes  among the wild bird population of the western states, espe¬  cially in areas with irrigated farmland.   The clinical picture of WEE virus infection is similar but  less severe than that caused by EEE virus. Sequelae are less  common. The diagnosis is made by isolating the virus or  observing a rise in antibody titer. There is no antiviral ther¬  apy. There is a killed vaccine for horses but not for humans.   St. Louis encephalitis (SLE) virus causes disease over a wider  geographic area than do EEE and WEE viruses. It is found in  the southern, central, and western states and causes 10 to  30 cases of encephalitis per year in the United States.   The virus is transmitted by several species of Culex  mosquitoes that vary depending on location. Again, small  wild birds, especially English sparrows, are the reservoir,  and humans are dead-end hosts. Although EEE and WEE  viruses are predominantly rural, SLE virus occurs in urban  areas because these mosquitoes prefer to breed in stagnant  wastewater.   SLE virus causes moderately severe encephalitis with a  fatality rate that approaches 10%. Most infections are inap-  parent. Sequelae are uncommon.   The diagnosis is usually made serologically, because the  virus is difficult to isolate. No antiviral therapy or vaccine  is available.   California encephalitis (CE) virus was first isolated from  mosquitoes in California in 1952, but its name is something  of a misnomer because most human disease occurs in the  north-central states. The strain of CE virus that causes  encephalitis most frequently is called La Crosse for the city  in Wisconsin where it was isolated. CE virus is the only one  of the four major encephalitis viruses in the United States  that is a member of the bunyavirus family.   La Crosse virus is transmitted by the mosquito Aedes  triseriatus among forest rodents. The virus is passed trans-  ovarially in mosquitoes and thus survives the winter when  mosquitoes are not active. The clinical picture can be mild,  resembling enteroviral meningitis, or severe, resembling  herpes encephalitis. Death rarely occurs. Diagnosis is usu¬  ally made serologically rather than by isolation of the virus.  No antiviral therapy or vaccine is available.   Of the five diseases described in Table 42-2, Colorado tick  fever (CTF) is the most easily distinguished from the oth¬  ers, both biologically and clinically. CTF virus is a reovirus  transmitted by the wood tick Dermacentor andersoni  among the small rodents (e.g., chipmunks and squirrels) of    the Rocky Mountains. There are approximately 100 to 300  cases per year in the United States.   The disease occurs primarily in people hiking or camp¬  ing in the Rocky Mountains and is characterized by fever,  headache, retro-orbital pain, and severe myalgia. The diag¬  nosis is made either by isolating the virus from the blood or  by detecting a rise in antibody titer. No antiviral therapy or  vaccine is available. Prevention involves wearing protective  clothing and inspecting the skin for ticks.   West Nile virus (WNV) is the most common cause of  neuroinvasive (encephalitis, meningitis) arboviral dis¬  ease in the United States. WNV caused an outbreak of  encephalitis in New York City and environs in July, August,  and September 1999. This is the first time WNV caused  disease in the United States. In this outbreak, there were 27  confirmed cases and 23 probable cases, including 5 deaths.  Many birds, especially crows, died as well. No human cases  occurred after area-wide spraying of mosquito-control  compounds and the onset of cooler weather.   In the summer of the year 2000, there were 18 cases and  1 death, and by July 2001, the virus had spread to many  states along the East Coast (from New Hampshire to  Florida) and as far west as Louisiana. In 2002, there was a  marked increase in the number of cases. There were more  than 4000 cases, 274 people died, and the virus had spread  as far west as Colorado. In 2003, there were 7700 cases, of  whom 166 died, and the virus had spread to California. In  2012, there were 3142 reported cases and 134 deaths. Each  year, WNV causes the highest number of deaths due to a  mosquito-borne encephalitis in the United States. It is not  known how WNV entered the United States, but either an  infected traveler or an infected mosquito brought by an  airplane seems likely to be involved.   WNV is a flavivirus that is classified in the same anti¬  genic group as SLE virus. It is endemic in Africa but has  caused encephalitis in areas of Europe and Asia as well.  Wild birds are the main reservoir of this virus, which is  transmitted by mosquitoes, especially Culex species.  Humans are dead-end hosts. Transmission of the virus via  solid organ transplants has also occurred.   The most important clinical picture is encephalitis with  or without signs of meningitis, typically in a person over  60 years of age. Encephalitis occurs in about 1% of infections,  fever and headache without encephalitis occur in about 20%,  and roughly 80% of infections are asymptomatic.   The laboratory diagnosis can be made by either isola¬  tion of the virus from brain tissue, blood, or spinal fluid or  by detection of antibodies in spinal fluid or blood. Poly¬  merase chain reaction (PCR)-based assays are also avail¬  able. No antiviral therapy or vaccine is available. In an  attempt to prevent blood-borne transmission, blood banks  screen donated blood for the presence of WNV using  nucleic acid probes specific for the virus.   Although yellow fever and dengue are not endemic in the  United States, extensive travel by Americans to tropical  areas means that imported cases occur. It is reasonable,  therefore, that physicians in the United States be  acquainted with these two diseases. Both yellow fever  virus and dengue virus are classified as flaviviruses.  Table 42-3 describes the epidemiology of the important  arboviral diseases that occur primarily outside the United  States. Japanese encephalitis virus, also a flavivirus and an  important cause of epidemic encephalitis in Asia, is  described in Chapter 46.   As the name implies, yellow fever is characterized by jaun¬  dice and fever. It is a severe, life-threatening disease that  begins with the sudden onset of fever, headache, myalgias,  and photophobia. After this prodrome, the symptoms  progress to involve the liver, kidneys, and heart. Prostration  and shock occur, accompanied by upper gastrointestinal  tract hemorrhage with hematemesis (“black vomit”).   Yellow fever occurs primarily in the tropical areas of  Africa and South America. In the epidemiology of yellow  fever, two distinct cycles exist in nature, with different  reservoirs and vectors.   (1) Jungle yellow fever is a disease of monkeys in tropi¬  cal Africa and South America; it is transmitted primarily by  the treetop mosquitoes of the Haemagogus species. Mon¬  keys are the permanent reservoir, whereas humans are  accidental hosts. Humans (e.g., tree cutters) are infected  when they enter the jungle occupationally.   (2) In contrast, urban yellow fever is a disease of  humans that is transmitted by the mosquito Aedes aegypti,  which breeds in stagnant water. In the urban form of the  disease, humans are the reservoir. For effective transmis¬  sion to occur, the virus must replicate in the mosquito dur¬  ing the 12- to 14-day extrinsic incubation period. After the    infected mosquito bites the person, the intrinsic incuba¬  tion period is 3 to 6 days.   Diagnosis in the laboratory can be made either by isolat¬  ing the virus or by detecting a rise in antibody titer. No  antiviral therapy is available, and the mortality rate is high.  If the patient recovers, no chronic infection ensues and  lifelong immunity is conferred.   Prevention of yellow fever involves mosquito control  and immunization with the vaccine containing live, attenu¬  ated yellow fever virus. Travelers to and residents of  endemic areas should be immunized. Protection lasts up to  10 years, and boosters are required every 10 years for trav¬  elers entering certain countries. Epidemics still occur in  parts of tropical Africa and South America. Because it is a  live vaccine, it should not be given to immunocompro¬  mised people or to pregnant women.   Although dengue fever is not endemic in the United States,  some tourists to the Caribbean and other tropical areas  return with this disease. In recent years, there were 100 to  200 cases per year in the United States, mostly in the south¬  ern and eastern states. No indigenous transmission  occurred within the United States. It is estimated that about  20 million people are infected with dengue virus each year  worldwide. Dengue is the most common insect-borne viral  disease in the world.   Classic dengue fever (breakbone fever) begins sud¬  denly with an influenzalike syndrome consisting of fever,  malaise, retro-orbital pain, and headache. Severe pains in  muscles (myalgia) and joints (arthralgia, breakbone) occur.  Enlarged lymph nodes, facial flushing, a maculopapular  rash, and leukopenia are common. After a week or so, the  symptoms regress but weakness may persist. Although  unpleasant, this typical form of dengue is rarely fatal and  has few sequelae.   In contrast, dengue hemorrhagic fever is a much more  severe disease, with a fatality rate that approaches 10%. The  initial picture is the same as classic dengue, but then shock  and hemorrhage, especially into the gastrointestinal tract    TABLE 42-3 Epidemiology of Important Arboviral Diseases Outside the United States    and skin, develop. Dengue hemorrhagic fever occurs par¬  ticularly in southern Asia, whereas the classic form is found  in tropical areas worldwide.   Hemorrhagic shock syndrome is due to the produc¬  tion of large amounts of cross-reacting antibody at the  time of a second dengue infection. The pathogenesis is as  follows: The patient recovers from classic dengue caused  by one of the four serotypes, and antibody against that  serotype is produced. When the patient is infected with  another serotype of dengue virus, an anamnestic, hetero¬  typic response occurs, and large amounts of cross-reacting  antibody to the first serotype are produced. There are two  hypotheses about what happens next. One is that immune  complexes composed of virus and antibody are formed  that activate complement, causing increased vascular  permeability and thrombocytopenia. The other is that  the antibodies increase the entry of virus into monocytes  and macrophages, with the consequent liberation of a  large amount of cytokines. In either scenario, shock and  hemorrhage result.   Dengue virus is transmitted by the Aedes aegypti mos¬  quito, which is also the vector of yellow fever virus.  Humans are the reservoir for dengue virus, but a jungle  cycle involving monkeys as the reservoir and other Aedes  species as vectors is suspected.   The diagnosis can be made in the laboratory either by  isolation of the virus in cell culture or by serologic tests that  demonstrate the presence IgM antibody or a fourfold or  greater rise in antibody titer in acute and convalescent sera.  A PCR assay that detects virus in the blood is also available.   No antiviral therapy or vaccine for dengue is available.  Outbreaks are controlled by using insecticides and draining  stagnant water that serves as the breeding place for the  mosquitoes. Personal protection includes using mosquito  repellent and wearing clothing that covers the entire body  (Note that a dengue vaccine composed of live attenuated  yellow fever vaccine virus genetically engineered to pro¬  duce proteins from dengue virus that serve as the immuno¬  gen was approved by Mexico in 2015 but is not available in  the United States.)   This virus causes chikungunya fever characterized by the  sudden onset of high fever and joint pains, especially of the  wrists and ankles. Joint involvement is bilateral and sym¬  metric. A macular or maculopapular rash over much of the  body is common. Encephalitis may occur. Outbreaks  involving millions of people in India, Africa, and the  islands in the Indian Ocean have occurred in the years  from 2004 to 2006. In 2013 to 2014, this virus moved to the  western hemisphere causing outbreaks involving thou¬  sands of people on many Caribbean islands and in the state  of Florida.   Chikungunya virus is an RNA enveloped virus and is  a member of the Togavirus family. It has a    single-stranded, positive-polarity RNA genome. It is  transmitted by species of Aedes mosquitoes, both A.  aegypti and Aedes albopictus. The latter mosquito is  found in the United States, so the potential for outbreaks  exists. Humans are the most important reservoir but  infection of nonhuman primates is thought to sustain the  virus in nonpopulated areas.   Individuals returning to the United States from areas  where outbreaks have occurred have been diagnosed with  chikungunya fever. Laboratory diagnosis involves detecting  the virus in blood either by PCR assay for viral RNA or by  enzyme-linked immunosorbent assay (ELISA) for IgM anti¬  body. There is no antiviral therapy and no vaccine is available.   1. An outbreak of dengue hemorrhagic fever (DHF) recently  occurred in two Central American countries. Regarding dengue  and DHF, which one of the following is the most accurate?   (B) DHF occurs primarily in individuals who are deficient in the  late-acting complement components.   (C) Dengue virus is transmitted by Aedes mosquitoes, and mon¬  keys are an important natural reservoir.   (D) The vaccine containing live, attenuated dengue virus is recom¬  mended for those living or traveling in endemic areas.   (E) DHF occurs more often in people infected for the first time  than when they are reinfected because antibody protects  against reinfection.   2. Yellow fever still exists in many tropical areas of the globe. Which  one of the following is the best reason yellow fever still exists?   (B) Both humans and monkeys are reservoirs for yellow fever virus.   (C) The virus has mutated, so the existing vaccine is no longer  effective.   (D) The vaccine has been withdrawn because it was found to have  unacceptable side effects.   (E) The people in developing countries cannot afford to take  amantadine when they enter endemic areas.   3. Regarding West Nile virus (WNV), which one of the following is  the most accurate?   (D) Most infections are asymptomatic, but the elderly are at risk for  encephalitis.   (E) The live, attenuated vaccine should be administered to elderly  adults in endemic areas.   4. An outbreak of febrile disease involving severe joint pain and an  erythematous macular rash has occurred on several Caribbean  islands. Infection with chikungunya virus is suspected. Which one  of the following is correct regarding this virus?   (D) Laboratory diagnosis involves electron microscopy to observe  the very long filamentous shape of the virus.   (E) The killed vaccine should be administered to travelers to  endemic regions.   Brief summaries of the organisms described in this chapter  begin on page 676. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Clinical Virology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 723. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   Role of Tumor Viruses In Malignant Transformation  Proviruses & Oncogenes  Outcome of Tumor Virus Infection  Transmission of Tumor Viruses  Human Tumor Viruses   Viruses can cause benign or malignant tumors in many  species of animals (e.g., frogs, fishes, birds, and mammals).  Despite the common occurrence of tumor viruses in ani¬  mals, only a few viruses are associated with human tumors,  and evidence that they are truly the causative agents exists  for very few.   Tumor viruses have no characteristic size, shape, or  chemical composition. Some are large, and some are small;  some are enveloped, and others are naked (i.e., nonenvel-  oped); some have DNA as their genetic material, and others  have RNA. The factor that unites all of them is their com¬  mon ability to cause tumors.   Tumor viruses are at the forefront of cancer research for  two main reasons:   (1) They are more rapid, reliable, and efficient tumor  producers than either chemicals or radiation. For example,  many of these viruses can cause tumors in all susceptible  animals in 1 or 2 weeks and can produce malignant trans¬  formation in cultured cells in just a few days.   (2) They have a small number of genes compared with a  human cell (only three, four, or five for many retroviruses),  and hence their role in the production of cancer can be    Epstein-Barr Virus  Human Herpesvirus 8  Hepatitis B Virus  Merkel Cell Polyomavirus   Do Animal Tumor Viruses Cause Cancer In Humans?  Animal Tumor Viruses    readily analyzed and understood. To date, the genomes of  many tumor viruses have been sequenced and the number  of genes and their functions have been determined; all of  this has provided important information.   The term malignant transformation refers to changes in the  growth properties, shape, and other features of the tumor  cell (Table 43-1). Malignant transformation can be induced  by tumor viruses not only in animals but also in cultured  cells. In culture, the following changes occur when cells  become malignantly transformed.   Malignant cells lose their characteristic differentiated shape  and appear rounded and more refractile when seen in a  microscope. The rounding is due to the disaggregation of  actin filaments, and the reduced adherence of the cell to the  surface of the culture dish is the result of changes in the  surface charge of the cell.   Rounded as a result of disaggregation of actin  filaments and decreased adhesion to surface   Reduced requirement for serum growth factors  Increased ability to be cloned from a single cell  Increased ability to grow in suspension   (1) Malignant cells grow in a disorganized, piled-up pat¬  tern in contrast to normal cells, which have an organized,  flat appearance. The term applied to this change in growth  pattern in malignant cells is loss of contact inhibition.  Contact inhibition is a property of normal cells that refers  to their ability to stop their growth and movement upon  contact with another cell. Malignant cells have lost this  ability and consequently move on top of one another, con¬  tinue to grow to large numbers, and form a random array  of cells.   (2) Malignant cells are able to grow in vitro at a much  lower concentration of serum than are normal cells.   (3) Malignant cells grow well in suspension, whereas  normal cells grow well only when they are attached to a  surface (e.g., a culture dish).   (4) Malignant cells are easily cloned (i.e., they can grow  into a colony of cells starting with a single cell), whereas  normal cells cannot do this effectively.   (5) Infection of a cell by a tumor virus “immortalizes”  that cell by enabling it to continue growing long past the  time when its normal counterpart would have died. Nor¬  mal cells in culture have a lifetime of about 50 generations,  but malignantly transformed cells grow indefinitely.   (1) DNA synthesis is induced. If cells resting in the Gj  phase are infected with a tumor virus, they will promptly  enter the S phase (i.e., synthesize DNA and go on to divide).   (2) The karyotype becomes altered (i.e., there are  changes in the number and shape of the chromosomes as a  result of deletions, duplications, and translocations).    (3) Antigens different from those in normal cells appear.  These new antigens can be either virus-encoded proteins,  preexisting cellular proteins that have been modified, or  previously repressed cellular proteins that are now being  synthesized. Some new antigens are on the cell surface and  elicit either circulating antibodies or a cell-mediated  response that can kill the tumor cell. These new antigens  are the recognition sites for immune surveillance against  tumor cells.   (4) Agglutination by lectins is enhanced. Lectins are  plant glycoproteins that bind specifically to certain sugars  on the cell membrane surface (e.g., wheat germ agglutinin).  The increased agglutination of malignant cells may be due  to the clustering of existing receptor sites rather than to the  synthesis of new ones.   (1) Levels of cyclic adenosine monophosphate (AMP)  are reduced in malignant cells. Addition of cyclic AMP will  cause malignant cells to revert to the appearance and  growth properties of normal cells.   (2) Malignant cells secrete more plasminogen activator  than do normal cells. This activator is a protease that con¬  verts plasminogen to plasmin, the enzyme that dissolves  the fibrin clot.   (3) Increased anaerobic glycolysis leads to increased  lactic acid production (Warburg effect). The mechanism  for this change is unknown.   (4) There is a loss of high-molecular-weight glycopro¬  tein called fibronectin. The effect of this loss is unknown.   (5) There are changes in the sugar components of gly¬  coproteins and glycolipids in the membranes of malig¬  nant cells.   Malignant transformation is a permanent change in the  behavior of the cell. Must the viral genetic material be pres¬  ent and functioning at all times, or can it alter some cell  component and not be required subsequently? The answer  to this question was obtained by using a temperature-sen¬  sitive mutant of Rous sarcoma virus. This mutant has an  altered transforming gene that is functional at the low,  permissive temperature (35°C) but not at the high, restric¬  tive temperature (39°C). When chicken cells were infected  at 35°C they transformed as expected, but when incubated  at 39°C, they regained their normal morphology and  behavior within a few hours. Days or weeks later, when  these cells were returned to 35°C, they recovered their  transformed phenotype. Thus continued production of  some functional virus-encoded protein is required for the  maintenance of the transformed state.   Although malignant transformation is a permanent  change, revertants to normality do appear, albeit rarely.    In the revertants studied, the viral genetic material remains  integrated in cellular DNA, but changes in the quality and  quantity of the virus-specific RNA occur.   The two major concepts of the way viral tumorigenesis  occurs are expressed in the terms provirus and oncogene.  These contrasting ideas address the fundamental question  of the source of the genes for malignancy.   (1) In the provirus model, the genes enter the cell at the  time of infection carried by the tumor virus.   (2) In the oncogene model, the genes for malignancy are  already present in all cells of the body by virtue of being  present in the initial sperm and egg. These oncogenes  encode proteins that encourage cell growth (e.g., fibroblast  growth factor). In the oncogene model, carcinogens such as  chemicals, radiation, and tumor viruses activate cellular  oncogenes to overproduce these growth factors. This initi¬  ates inappropriate cell growth and malignant transforma¬  tion (Figure 43-1).   Both proviruses and oncogenes may play a role in  malignant transformation. Evidence for the provirus mode  consists of finding copies of viral DNA integrated into cell  DNA only in cells that have been infected with the tumor  virus. The corresponding uninfected cells have no copies of  the viral DNA.   The first direct evidence that oncogenes exist in normal  cells was based on results of experiments in which a DNA  copy of the one gene of the chicken retrovirus Rous sar¬  coma virus was used as a probe. DNA in normal chicken  embryonic cells hybridized to the probe, indicating that the   FIGURE 43-1 Effect of cellular oncogenes and tumor suppres¬  sor genes on the cell cycle. The oncoproteins encoded by cellular  oncogenes activate the cell cycle by allowing passage from the G,  phase into the S phase. The proteins encoded by tumor suppressor  genes, notably p53 and RB, inhibit the cell cycle in the G, phase. Inac¬  tivation of these proteins activates the cell cycle by allowing passage  from the G, phase into the S phase. G,, gap 1; G 2 , gap 2; M, mitosis; S,  synthesis of DNA. (Used with permission from Murray RK et al. Harp¬  er's Illustrated Biochemistry. 29th ed. New York: McGraw-Hill, 2012.)   FIGURE 43-2 Functions of cellular oncoproteins. Cellular  oncogenes encode proteins with a variety of functions that are  shown in the figure. These oncoproteins activate the cell cycle and  cause cell to grow in an unregulated manner. (Used with permission  from Murray RK et al. Harper's Illustrated Biochemistry. 29th ed. New  York: McGraw-Hill, 2012.)   cells contain a gene homologous to the viral gene. It is  hypothesized that the cellular oncogenes may be the pre¬  cursors of viral oncogenes. Proto-oncogenes are the nor¬  mal precursors of the cellular oncogenes. Proto-oncogenes  encode normal cellular proteins and are under regulatory  control. Cellular oncogenes have acquired mutations that  cause them to escape regulatory control and overproduce  altered proteins. Figure 43-2 shows the functions of impor¬  tant oncoproteins encoded by the cellular oncogenes.   Although cellular oncogenes and viral oncogenes are  similar, they are not identical. They differ in base sequence  at various points; and cellular oncogenes have exons and  introns, whereas viral oncogenes do not. It seems likely that  viral oncogenes were acquired by incorporation of cellular  oncogenes into retroviruses lacking these genes. Retrovi¬  ruses can be thought of as transducing agents, carrying  oncogenes from one cell to another. (See Chapter 4 for a  discussion of transduction.)   Since this initial observation, more than 20 cellular  oncogenes have been identified by using either the Rous  sarcoma virus DNA probe or probes made from other viral  oncogenes. Table 43-2 describes the function of several  important cellular oncogenes and their relationship to vari¬  ous human cancers. Many cells contain several different  cellular oncogenes. In addition, the same cellular onco¬  genes have been found in species as diverse as fruit flies,  rodents, and humans. Such conservation through evolution  suggests a normal physiologic function for these genes.  Some are known to be expressed during normal embryonic  development.   A marked diversity of viral oncogene function has been  found. Some, such as the sre gene, encode a protein kinase  that specifically phosphorylates the amino acid tyrosine, in  contrast to the commonly found protein kinase of cells,    which preferentially phosphorylates serine. There is evi¬  dence that the kinase phosphorylates signal transduction  factors that activate synthesis of cyclins. This drives the cell  into S phase and subsequent mitosis.   Other oncogenes have a base sequence almost identical  to that of the gene for certain cellular growth factors (e.g.,  epidermal growth factor). Several proteins encoded by  oncogenes have their effect at the cell membrane (e.g., the  ras oncogene encodes a G protein), whereas some act in the  nucleus by binding to DNA (e.g., the myc oncogene  encodes a transcription factor). These observations suggest  that growth control is a multistep process and that carcino¬  genesis can be induced by affecting one or more of several  steps.   On the basis of the known categories of oncogenes, the  following model of growth control can be constructed.  After a growth factor binds to its receptor on the cell  membrane, membrane-associated G proteins and tyrosine  kinases are activated. These, in turn, interact with cyto¬  plasmic proteins or produce second messengers, which  are transported to the nucleus and interact with nuclear  factors. DNA synthesis is activated, and cell division  occurs. Overproduction or inappropriate expression of any  of the preceding factors in boldface type can result in  malignant transformation.   Note that not all tumor viruses of the retrovirus family  contain one genes. How do these viruses cause malignant  transformation? It appears that the DNA copy of the viral  RNA integrates near a cellular oncogene, causing a marked  increase in its expression. This process is called insertional  mutagenesis. Overexpression of the cellular oncogene  may play a key role in malignant transformation by these  viruses.   Although it has been demonstrated that viral oncogenes  can cause malignant transformation, it has not been   DNA isolated from tumor cells can transform  normal cells. This DNA has a c-onc gene with  a mutation consisting of a single base  change.   Movement of c -one gene to a new site on a  different chromosome results in malignancy  accompanied by increased expression of the  gene.   The number of copies of c -one gene is  increased, resulting in enhanced expression  of their mRNA and proteins.   Proviral DNA inserts near c -one gene, which  alters its expression and causes tumors.   Addition of an active promoter site enhances  expression of the c -one gene, and malignant  transformation occurs.    directly shown that cellular oncogenes can do so. However,  as described in Table 43-3, the following evidence suggests  that they do:   (1) DNA-containing cellular oncogenes isolated from  certain tumor cells can transform normal cells in culture.  When the base sequence of these “transforming” cellular  oncogenes was analyzed, it was found to have a single base  change from the normal cellular oncogene (i.e., it had  mutated). In several tumor cell isolates, the altered sites in  the gene are the same.   (2) In certain tumors, characteristic translocations of  chromosomal segments can be seen. In Burkitt’s lym¬  phoma cells, a translocation occurs that moves a cellular  oncogene ( c-myc ) from its normal site on chromosome 8 to  a new site adjacent to an immunoglobulin heavy chain  gene on chromosome 14. This shift enhances expression of  the c -myc gene.   In chronic myelogenous leukemia (CML) cells, a trun¬  cated chromosome called a “Philadelphia” chromosome is  seen. This chromosome has a characteristic translocation  that results in the overexpression of the bcr-abl oncogene  that encodes a tyrosine kinase. Increased kinase activity  increases the rate of cell division and inhibits DNA repair  resulting in leukemia. Drugs that inhibit this kinase, such  as imatinib (Gleevec), induce a prolonged remission and  are well tolerated.   (3) Some tumors have multiple copies of the cellular  oncogenes, either on the same chromosome or on multiple  tiny chromosomes. The amplification of these genes  results in overexpression of their mRNA and proteins.   (4) Insertion of the DNA copy of the retroviral RNA  (proviral DNA) near a cellular oncogene stimulates expres¬  sion of the c-onc gene.    (5) Certain cellular oncogenes isolated from normal  cells can cause malignant transformation if they have been  modified to be overexpressed within the recipient cell.   In summary, two different mechanisms— mutation and  increased expression —appear to be able to activate the  quiescent “proto-oncogene” into a functioning oncogene  capable of transforming a cell. Cellular oncogenes provide  a rationale for carcinogenesis by chemicals and radiation  (e.g., a chemical carcinogen might act by enhancing the  expression of a cellular oncogene). Furthermore, DNA iso¬  lated from cells treated with a chemical carcinogen can  malignantly transform other normal cells. The resulting  tumor cells contain cellular oncogenes from the chemically  treated cells, and these genes are expressed with high  efficiency.   There is another mechanism of carcinogenesis involving  cellular genes, namely, mutation of a tumor suppressor  gene (Figure 43-1). A well-documented example is the reti¬  noblastoma susceptibility gene, which normally acts as a  suppressor of retinoblastoma formation. When both alleles  of this anti oncogene are mutated (made nonfunctional),  retinoblastoma occurs. Human papillomavirus and SV40  virus produce a protein that binds to and inactivates the  protein encoded by the retinoblastoma gene. Human papil¬  lomavirus also produces a protein that inactivates the pro¬  tein encoded by the p53 gene, another tumor suppressor  gene in human cells. The p53 gene encodes a transcription  factor that activates the synthesis of a second protein,  which blocks the cyclin-dependent kinases required for cell  division to occur. The p53 protein also promotes apoptosis  of cells that have sustained DNA damage or contain acti¬  vated cellular oncogenes. Apoptosis-induced death of these  cells has a “tumor-suppressive” effect by killing those cells  destined to become cancerous.   Inactivation of tumor suppressor genes appears likely to  be an important general mechanism of viral oncogenesis.  Tumor suppressor genes are involved in the formation of  other cancers as well (e.g., breast and colon carcinomas  and various sarcomas). For example, in many colon carci¬  nomas, two genes are inactivated, the p53 gene and the  DCC (deleted in colon carcinoma) gene. Table 43-4 lists  several important tumor suppressor genes and their rela¬  tionship to various human cancers. More than half of  human cancers have a mutated p53 gene in the DNA of  malignant cells.   Micro-RNA genes do not encode proteins but rather exert  their regulatory effect by being transcribed into micro-  RNA that can bind to sequences in mRNA and prevent that    TABLE 43-4 Examples of Tumor Suppressor Genes  Involved in Human Cancer    mRNA from being translated into proteins. For example,  there are micro-RNAs that bind to (“silence”) mRNA tran¬  scribed from a tumor suppressor gene. As a result, the  tumor suppressor protein is not synthesized, which  enhances the likelihood of tumorigenesis.   The outcome of tumor virus infection is dependent on the  virus and the type of cell. Some tumor viruses go through  their entire replicative cycle with the production of progeny  virus, whereas others undergo an interrupted cycle, analo¬  gous to lysogeny, in which the proviral DNA is integrated  into cellular DNA and limited expression of proviral genes  occurs. Therefore, malignant transformation does not  require that progeny virus be produced. Rather, all that is  required is the expression of one or, at most, a few viral  genes. Note, however, that some tumor viruses transform  by inserting their proviral DNA in a manner that activates  a cellular oncogene.   In most cases, the DNA tumor viruses such as the papo-  vaviruses transform only cells in which they do not repli¬  cate. These cells are called “nonpermissive” because they do  not permit viral replication. Cells of the species from which  the DNA tumor virus was initially isolated are “permissive”  (i.e., the virus replicates and usually kills the cells, and no  tumors are formed). For example, SV40 virus replicates in  the cells of the African green monkey (its species of origin)  and causes a cytopathic effect but no tumors. However, in  rodent cells, the virus does not replicate, expresses only its  early genes, and causes malignant transformation. In the  “nonproductive” transformed cell, the viral DNA is inte¬  grated into the host chromosome and remains there  through subsequent cell divisions. The underlying concept  applicable to both DNA and RNA tumor viruses is that  only viral gene expression, not replication of the viral  genome or production of progeny virus, is required for  transformation.   The essential step required for a DNA tumor virus (e.g.,  SV40 virus) to cause malignant transformation is expres¬  sion of the “early” genes of the virus (Table 43-5). (The  early genes are those expressed prior to the replication of    ’Some retroviruses have one genes that code for other proteins such as platelet-  derived growth factor and epidermal growth factor.   the viral genetic material.) These required early genes pro¬  duce a set of early proteins called T antigens . 1   The large T antigen, which is both necessary and suffi¬  cient to induce transformation, binds to SV40 virus DNA at  the site of initiation of viral DNA synthesis. This is compat¬  ible with the finding that the large T antigen is required for  the initiation of cellular DNA synthesis in the virus-  infected cell. Biochemically, large T antigen has protein  kinase and adenosine triphosphate (ATPase) activity.  Almost all of the large T antigen is located in the cell  nucleus, but some of it is in the outer cell membrane. In  that location, it can be detected as a transplantation antigen  called tumor-specific transplantation antigen (TSTA).  TSTA is the antigen that induces the immune response  against the transplantation of virally transformed cells.  Relatively little is known about the SV40 virus small T anti¬  gen, except that if it is not synthesized, the efficiency of  transformation decreases. In polyomavirus-infected cells,  the middle T antigen plays the same role as the SV40 virus  large T antigen.   In RNA tumor virus-infected cells, this required gene  has one of several different functions, depending on the  retrovirus. The oncogene of Rous sarcoma virus and sev¬  eral other viruses codes for a protein kinase that phos-  phorylates tyrosine. Some viruses have a gene for a factor  that regulates cell growth (e.g., epidermal growth factor or  platelet-derived growth factor), and still others have a gene    1 In SV40 virus-infected cells, two T antigens, large (molecular weight  [MW] 100,000) and small (MW 17,000), are produced, whereas in poly¬  omavirus-infected cells, three T antigens, large (MW 90,000), middle  (MW 60,000), and small (MW 22,000), are made. Other tumor viruses  such as adenoviruses also induce T antigens, which are immunologically  distinct from those of the two papovaviruses.    that codes for a protein that binds to DNA. The conclusion  is that normal growth control is a multistep process that  can be affected at any one of several levels. The addition of  a viral oncogene perturbs the growth control process, and  a tumor cell results.   The viral genetic material remains stably integrated in  host cell DNA by a process similar to lysogeny. In the lyso¬  genic cycle, bacteriophage DNA becomes stably integrated  into the bacterial genome. The linear DNA genome of the  temperate phage, lambda, forms a double-stranded circle  within the infected cell and then covalently integrates into  bacterial DNA (Table 43-6). A repressor is synthesized that  prevents transcription of most of the other lambda genes.  Similarly, the double-stranded circular DNA of the DNA  tumor virus covalently integrates into eukaryotic-cell DNA,  and only early genes are transcribed. Thus far, no repressor  has been identified in any DNA tumor virus-infected cell.  With RNA tumor viruses (retroviruses), the single-stranded  linear RNA genome is transcribed into a double-stranded  linear DNA that integrates into cellular DNA. In summary,  despite the differences in their genomes and in the nature  of the host cells, these viruses go through the common  pathway of a double-stranded DNA intermediate followed  by covalent integration into cellular DNA and subsequent  expression of certain genes.   Just as a lysogenic bacteriophage can be induced to  enter the replicative cycle by ultraviolet radiation and cer¬  tain chemicals, tumor viruses can be induced by several  mechanisms. Induction is one of the approaches used to  determine whether tumor viruses are present in human  cancer cells (e.g., human T-cell lymphotropic virus was  discovered by inducing the virus from leukemic cells with  iododeoxyuridine).   Three techniques have been used to induce tumor  viruses to replicate in the transformed cells:   (1) The most frequently used method is the addition of  nucleoside analogues (e.g., iododeoxyuridine). The mecha¬  nism of induction by these analogues is uncertain.   (2) The second method involves fusion with “helper”  cells (i.e., the transformed, nonpermissive cell is fused with  a permissive cell) in which the virus undergoes a normal  replicative cycle. Within the heterokaryon (a cell with two  or more nuclei that is formed by the fusion of two different  cell types), the tumor virus is induced and infectious virus  is produced. The mechanism of induction is unknown.   (3) In the third method, helper viruses provide a miss¬  ing function to complement the integrated tumor virus.  Infection with the helper virus results in the production of  both the integrated tumor virus and the helper virus.   The process of rescuing tumor viruses from cells  revealed the existence of endogenous viruses. Treatment of  normal, uninfected embryonic cells with nucleoside ana¬  logues resulted in the production of retroviruses. Retroviral  DNA is integrated within the chromosomal DNA of all  cells and serves as the template for viral replication. This    TABLE 43-6 Lysogeny as a Model for the Integration of Tumor Viruses   1 Limited transcription in some cells or under certain conditions but full transcription with viral replication in others.    proviral DNA probably arose by retrovirus infection of the  germ cells of some prehistoric ancestor.   Endogenous retroviruses, which have been rescued from  the cells of many species (including humans), differ depend¬  ing on the species of origin. Endogenous viruses are xeno-  tropic (xeno means foreign; tropism means to be attracted  to; i.e., they infect cells of other species more efficiently than  they infect the cells of the species of origin). Entry of the  endogenous virus into the cell of origin is limited as a result  of defective viral envelope-cell receptor interaction.  Although they are retroviruses, most endogenous viruses  are not tumor viruses (i.e., only a few cause leukemia).   Tumor virus transmission in experimental animals can  occur by two processes, vertical and horizontal. Vertical  transmission indicates movement of the virus from mother  to newborn offspring, whereas horizontal transmission  describes the passage of virus between animals that do not  have a mother-offspring relationship. Vertical transmis¬  sion occurs by three methods: (1) the viral genetic material  is in the sperm or the egg; (2) the virus is passed across the  placenta; and (3) the virus is transmitted in the breast milk.    When vertical transmission occurs, exposure to the  virus early in life can result in tolerance to viral antigens  and, as a consequence, the immune system will not elimi¬  nate the virus. Large amounts of virus are produced, and a  high frequency of cancer occurs. In contrast, when hori¬  zontal transmission occurs, the immunocompetent animal  produces antibody against the virus, and the frequency of  cancer is low. If an immunocompetent animal is experi¬  mentally made immunodeficient, the frequency of cancer  increases greatly.   Llorizontal transmission probably does not occur in  humans; those in close contact with cancer patients (e.g.,  family members and medical personnel) do not have an  increased frequency of cancer. There have been “outbreaks”  of leukemia in several children at the same school, but  these have been interpreted statistically to be random, rare  events that happen to coincide.   There are seven known human tumor viruses (Table 43-7).  Two are RNA viruses, namely human T-cell lymphotropic  virus and hepatitis C virus. The other five are DNA viruses,  namely human papillomavirus, Epstein-Barr virus, human  herpesvirus 8 (Kaposi’s sarcoma virus), hepatitis B virus,  and Merkel cell polyomavirus.   Sarcoma, leukemia, and carcinoma viruses in many avian and  mammalian species   Herpesvirus saimiri causes lymphomas in monkeys; Marek's  disease virus of chickens   Human adenovirus serotypes 12, 18, and 31 cause sarcomas  in rodents   There are two human T-cell lymphotropic virus (HTLV)  isolates so far, HTLV-1 and HTLV-2, both of which are  associated with leukemias and lymphomas. HTLV-1 was  isolated in 1980 from the cells of a patient with a cutaneous  T-cell lymphoma. Its RNA and proteins are different from  those of all other retroviruses. In addition to cancer, HTLV  is the cause of tropical spastic paraparesis, an autoimmune  disease in which progressive weakness of the legs occurs.  (Additional information regarding HTLV can be found in  Chapter 39.)   HTLV-1 may cause cancer by a mechanism different  from that of other retroviruses. It has no viral oncogene.  Rather, it has two special genes (in addition to the standard  retroviral genes gag, pol, and env) called tax and rex that  play a role in oncogenesis by regulating mRNA transcrip¬  tion and translation. The Tax protein has two activities: (1)  it acts on the viral long terminal repeat (LTR) sequences to  stimulate viral mRNA synthesis; and (2) it induces nuclear  factor-KB (NF-kB), which stimulates the production of  interleukin-2 (IL-2) and the IL-2 receptor. The increase in  levels of IL-2 and its receptor stimulates the T cells to con¬  tinue growing, thus increasing the likelihood that the cells  will become malignant. The Rex protein determines which  viral mRNAs can exit the nucleus and enter the cytoplasm  to be translated.   HTLV-1 is not an endogenous virus (i.e., proviral DNA  corresponding to its RNA genome is not found in normal  human cell DNA). It is an exogenously acquired virus  because its proviral DNA is found only in the DNA of the  malignant lymphoma cells. It infects CD4-positive T cells  preferentially and will induce malignant transformation in  these cells in vitro. Some (but not all) patients with T-cell  lymphomas have antibodies against the virus, indicating  that it may not be the cause of all T-cell lymphomas. Anti¬  bodies against the virus are not found in the general popu¬  lation, indicating that infection is not widespread.   Transmission occurs primarily by breast feeding, by  sexual contact, and by exchange of contaminated blood  (e.g., in transfusions and intravenous drug users). In the  United States, blood for transfusions is screened for anti¬  bodies to HTLV-1 and HTLV-2 and discarded if positive. In  recent years, HTLV-1 and HTLV-2 were found in equal  frequency in donated blood. Serologic tests for HTLV do  not cross-react with human immunodeficiency virus (HIV).   At about the same time that HTLV-1 was found, a simi¬  lar virus was isolated from malignant T cells in Japan. In  that country, a clustering of cases in the rural areas of the  west coast of Kyushu was found. Antibodies in the sera of  leukemic individuals and in the sera of 25% of the normal  population of Kyushu react with the Japanese isolate and  with HTLV-1. (Only a small fraction of infected individu¬  als contract leukemia, indicating that HTLV infection    alone is insufficient to cause cancer.) In addition, HTLV-1  is endemic in some areas of Africa and on several Carib¬  bean islands, as shown by the high frequency of antibodies.  The number of people with positive antibody titers in the  United States is quite small, except in certain parts of the  southeastern states.   HTLV-2 has 60% genetic homology with HTLV-1. Like  HTLV-1, it is transmitted primarily by blood and semen  and infects CD4-positive cells. Routine serologic tests do  not distinguish between HTLV-1 and HTLV-2; therefore,  other techniques (e.g., polymerase chain reaction) are  required.   Chronic infection with hepatitis C virus (HCV), like hepa¬  titis B virus (HBV), also predisposes to hepatocellular car¬  cinoma. HCV is an RNA virus that has no oncogene and  forms no DNA intermediate during replication. It does  cause chronic hepatitis, which seems likely to be the main  predisposing event. (Additional information regarding  HCV can be found in Chapter 41.)   Human papillomavirus (HPV) is one of the two viruses  definitely known to cause tumors in humans. Papillomas  (warts) are benign but can progress to form carcinomas,  especially in an immunocompromised person. HPV pri¬  marily infects keratinizing or mucosal squamous epithe¬  lium. (Additional information regarding HPV can be  found in Chapter 38.)   Papillomaviruses are DNA nucleocapsid viruses with  double-stranded, circular, supercoiled DNA and an icosa-  hedral nucleocapsid. Carcinogenesis by HPV involves two  proteins encoded by HPV genes E6 and E7 that interfere  with the activity of the proteins encoded by two tumor sup¬  pressor genes, p53 and Rb (retinoblastoma), found in nor¬  mal cells. In cancer cells, the viral DNA is integrated into  the cellular DNA, and the E6 and E7 proteins are  produced.   There are at least 100 different types of HPV, many of  which cause distinct clinical entities. For example, HPV-1  through HPV-4 cause plantar warts on the soles of the feet,  whereas HPV-6 and HPV-11 cause anogenital warts (con-  dylomata acuminata) and laryngeal papillomas. Certain  types of HPV, especially types 16 and 18, are implicated as  the cause of carcinoma of the cervix, penis, and anus.   Epstein-Barr virus (EBV) is a herpesvirus that was isolated  from the cells of an East African individual with Burkitt’s  lymphoma. EBV, the cause of infectious mononucleosis,  transforms B lymphocytes in culture and causes    lymphomas in marmoset monkeys. It is also associated  with nasopharyngeal carcinoma, a tumor that occurs pri¬  marily in China, and with thymic carcinoma and B-cell  lymphoma in the United States. However, cells from  Burkitfs lymphoma patients in the United States show no  evidence of EBV infection. (Additional information regard¬  ing EBV can be found in Chapter 37.)   Cells isolated from East African individuals with  Burkitfs lymphoma contain EBV DNA and EBV nuclear  antigen. Only a small fraction of the many copies of EBV  DNA is integrated; most viral DNA is in the form of closed  circles in the cytoplasm.   The difficulty in proving that EBV is a human tumor  virus is that infection by the virus is widespread but the  tumor is rare. The current hypothesis is that EBV infection  induces B cells to proliferate, thus increasing the likelihood  that a second event (e.g., activation of a cellular oncogene)  will occur. In Burkitfs lymphoma cells, a cellular oncogene,  c-myc, which is normally located on chromosome 8, is  translocated to chromosome 14 at the site of immuno¬  globulin heavy chain genes. This translocation brings the  c-myc gene in juxtaposition to an active promoter, and  large amounts of c-myc RNA are synthesized. It is known  that the c-myc oncogene encodes a transcription factor, but  the role of this factor in oncogenesis is uncertain.   Human herpesvirus 8 (HHV-8), also known as Kaposi’s  sarcoma-associated herpesvirus (KSHV), causes Kaposi’s  sarcoma (KS). KS is a malignancy of vascular endothelial  cells that contains many spindle-shaped cells and erythro¬  cytes. It is the most common cancer in patients with  acquired immunodeficiency syndrome (AIDS). KSHV is  transmitted both sexually and by saliva. A protein encoded  by KSHV called latency-associated nuclear antigen (LANA)  inactivates RB and p53 tumor suppressor proteins, which  causes malignant transformation of the endothelial cells.  (Additional information regarding HHV-8 can be found in  Chapter 37.)   HBV infection is significantly more common in patients  with primary hepatocellular carcinoma (hepatoma) than  in control subjects. This relationship is striking in areas of  Africa and Asia, where the incidence of both HBV infec¬  tion and hepatoma is high. Chronic HBV infection com¬  monly causes cirrhosis of the liver; these two events are the  main predisposing factors to hepatoma. Part of the HBV  genome is integrated into cellular DNA in malignant cells.  However, no HBV gene has been definitely implicated in  oncogenesis. The integration of HBV DNA may cause  insertional mutagenesis, resulting in the activation of a cel¬  lular oncogene. In addition, the HBx protein may play a  role because it inhibits the p53 tumor suppressor protein.   Merkel cell polyomavirus (MCPV) causes a carcinoma of  Merkel cells in the skin. (Merkel cells are neuroreceptors  for pressure and touch.) The carcinoma occurs most often  on skin exposed to the sun such as the face and neck.  Immunosuppressed individuals and the elderly are predis¬  posed to this cancer.   Members of the polyomavirus family are small, nonen-  veloped, double-stranded DNA viruses known to cause  cancer in animals (see later section on animal tumor  viruses). Infection with MCPV is common as indicated by  the presence of antibody to the virus in many healthy blood  donors. The mode of transmission is uncertain.   In carcinoma cells, the DNA of MCPV is integrated into  cell DNA. The gene for the large T antigen is mutated so the  virus cannot replicate but the T antigen continues to be  synthesized. The T antigen causes the cell to become malig¬  nant by inhibiting tumor suppressor proteins such as p53  and RB. Because MCPV does not replicate in the carci¬  noma cells, patients are not infectious to others.   Diagnosis is made by pathologic analysis of surgical  specimens. There is no virus-based laboratory test clini¬  cally available. There is no antiviral drug or vaccine avail¬  able. Prevention involves reducing sun exposure, use of  sunscreen, and frequent skin examinations to detect the  cancer before it metastasizes.   There are two vaccines designed to prevent human cancer:  the HBV vaccine and the HPV vaccine. The widespread use  of the HBV vaccine in Asia has significantly reduced the  incidence of hepatocellular carcinoma. The vaccine against  HPV, the cause of carcinoma of the cervix, was approved  for use in the United States in 2006.   There is no evidence that animal tumor viruses cause  tumors in humans. In fact, the only available information  suggests that they do not, because (1) people who were  inoculated with poliovirus vaccine contaminated with  SV40 virus have no greater incidence of cancers than do  uninoculated controls, (2) soldiers inoculated with yellow  fever vaccine contaminated with avian leukemia virus do  not have a high incidence of tumors, and (3) members of  families whose cats have died of leukemia caused by feline  leukemia virus show no increase in the occurrence of leu¬  kemia over control families. Note, however, that some  human tumor cells, namely, non-Hodgkin’s lymphoma,    contain SV40 DNA, but the relationship of that DNA to  malignant transformation is uncertain.   RNA tumor viruses have been isolated from a large number  of species, namely, snakes, birds, and mammals, including  nonhuman primates. The important RNA tumor viruses are  listed in Table 43-7. They are important because of their  ubiquity their ability to cause tumors in the host of origin,  their small number of genes, and the relationship of their  genes to cellular oncogenes (see page 363).   These viruses belo ng to the retrovirus family (the pre¬  fix retro means reverse), so named because a reverse tran¬  scriptase is located in the virion. This enzyme transcribes  the genome RNA into double-stranded proviral DNA and  is essential to their replication. The viral genome consists  of two identical molecules of positive-strand RNA. Each  molecule has a molecular weight of approximately 2 x 10 6  (these are the only viruses that are diploid [i.e., have two  copies of their genome in the virion]). The two molecules  are hydrogen-bonded together by complementary bases  located near the 5' end of both RNA molecules. Also  bound near the 5' end of each RNA is a transfer RNA  (tRNA) that serves as the primer 2 for the transcription of  the RNA into DNA.   The icosahedral capsid is surrounded by an envelope  with glycoprotein spikes. Some internal capsid proteins are  group-specific antigens, which are common to retroviruses  within a species. There are three important morphologic  types of retroviruses, labeled B, C, and D, depending pri¬  marily on the location of the capsid or core. Most of the  retroviruses are C-type particles, but mouse mammary  tumor virus is a B-type particle, and HIV, the cause of  AIDS, is a D-type particle.   The gene sequence of the RNA of a typical avian sar¬  coma virus is gag, pol, env, and src. The nontransforming  retroviruses have three genes; they are missing src. The gag  region codes for the group-specific antigens, the pol gene  codes for the reverse transcriptase, the env gene codes for  the two envelope spike proteins, and the src gene codes for  the protein kinase. There is evidence that the kinase phos-  phorylates signal tranduction factors that activate synthesis  of cyclins that drive the cell into S phase and subsequent  mitosis.   The sequences at the 5' and 3' ends function in the inte¬  gration of the proviral DNA and in the transcription of  mRNA from the integrated proviral DNA by host cell RNA    2 The purpose of the primer tRNA is to act as the point of attachment for  the first deoxynucleotide at the start of DNA synthesis. The primers are  normal-cell tRNAs that are characteristic for each retrovirus.    polymerase II. At each end is a sequence 3 called an LTR  that is composed of several regions, one of which, near the  5' end, is the binding site for the primer tRNA.   After infection of the cell by a retrovirus, the following  events occur: Using the genome RNA as the template, the  reverse transcriptase (RNA-dependent DNA polymerase)  synthesizes double-stranded proviral DNA. The DNA then  integrates into cellular DNA. Integration of the proviral  DNA is an obligatory step, but there is no specific site of  integration. Insertion of the viral LTR can enhance the  transcription of adjacent host cell genes. If this host gene is  a cellular oncogene, malignant transformation may result.  This explains how retroviruses without viral oncogenes can  cause transformation.   The two best-characterized oncogenic papovaviruses are  polyomavirus and SV40 virus. Polyomavirus (poly means  many; oma means tumor) causes a wide variety of histo¬  logically different tumors when inoculated into newborn  rodents. Its natural host is the mouse. SV40 virus, which  was isolated from normal rhesus monkey kidney cells,  causes sarcomas in newborn hamsters.   Polyomavirus and SV40 virus share many chemical and  biologic features (e.g., double-stranded, circular, super-  coiled DNA of molecular weight 3 x 10 6 and a 45-nm ico¬  sahedral nucleocapsid). However, the sequence of their  DNA and the antigenicity of their proteins are quite dis¬  tinct. Both undergo a lytic (permissive) cycle in the cells of  their natural hosts, with the production of progeny virus.  However, when they infect the cells of a heterologous spe¬  cies, the nonpermissive cycle ensues, no virus is produced,  and the cell is malignantly transformed.   In the transformed cell, the viral DNA integrates into  the cell DNA, and only early proteins are synthesized. Some  of these proteins (e.g., the T antigens described on page 353)  are required for induction and maintenance of the trans¬  formed state.   JC virus, a human papovavirus, is the cause of progres¬  sive multifocal leukoencephalopathy (see Chapter 44). It  also causes brain tumors in monkeys and hamsters. There  is no evidence that it causes human cancer.   Some human adenoviruses, especially serotypes 12,18, and 31,  induce sarcomas in newborn hamsters and transform  rodent cells in culture. There is no evidence that these    3 The length of the sequence varies from 250 to 1200 bases, depending on  the virus.    viruses cause tumors in humans, and no adenoviral DNA  has been detected in the DNA of any human tumor cells.   Adenoviruses undergo both a permissive cycle in some  cells and a nonpermissive, transforming cycle in others.  The linear genome DNA circularizes within the infected  cell, but—in contrast to the papovaviruses, whose entire  genome integrates—only a small region (10%) of the ade¬  novirus genome does so; yet transformation still occurs.  This region codes for several proteins, one of which is the  T (tumor) antigen. Adenovirus T antigen is required for  transformation and is antigenically distinct from the poly-  omavirus and SV40 virus T antigens.   Several animal herpesviruses are known to cause tumors.  Four species of herpesviruses cause lymphomas in nonhu¬  man primates. Herpesviruses saimiri and ateles induce  T-cell lymphomas in New World monkeys, and herpesvi¬  ruses pan and papio transform B lymphocytes in chimpan¬  zees and baboons, respectively.   A herpesvirus of chickens causes Marek’s disease, a con¬  tagious, rapidly fatal neurolymphomatosis. Immunization  of chickens with a live, attenuated vaccine has resulted in a  considerable decrease in the number of cases. A herpesvi¬  rus is implicated as the cause of kidney carcinomas in frogs.   Two poxviruses cause tumors in animals; these are the  fibroma-myxoma virus, which causes fibromas or myxo¬  mas in rabbits and other animals, and Yaba monkey tumor  virus, which causes benign histiocytomas in animals and  human volunteers. Little is known about either of these  viruses.   1. Regarding viruses that play a role in human carcinogenesis, which  one of the following statements is the most accurate?   (A) Epstein-Barr virus is implicated as the cause of nasopharyngeal  carcinoma primarily in Asia, where it is transmitted by mos¬  quitoes in rural areas.   (B) Evidence for hepatitis C virus (HCV) as a cause of hepatocel¬  lular carcinoma includes finding a DNA copy of the HCV  genome integrated into the DNA of hepatocytes.    (C) Hepatitis B virus is implicated as the cause of hepatocellular  carcinoma because countries with a high incidence of chronic  hepatitis B also have a high incidence of hepatocellular  carcinoma.   (D) Human T-cell leukemia virus is a retrovirus that was found  associated with leukemia in Japan but is not found in the  United States.   2. Regarding the oncogenes of DNA tumor viruses, which one of the   (C) They encode cellular growth factors that activate S-phase DNA  synthesis.   (D) They encode proteins that bind to the proteins encoded by  tumor suppressor genes.   (B) They carry the genes for proteins that act as cellular growth  factors.   (C) They synthesize a protein that inhibits the action of the cellular  p53 protein.   (D) They encode a recombinase that causes translocation of certain  chromosomes.   (E) They encode a DNA polymerase that increases the rate of cel¬  lular DNA synthesis.   Brief summaries of the organisms described in this chapter  begin on page 677. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Clinical Virology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 723. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   “Slow” infectious diseases are caused by a heterogeneous  group of agents containing both conventional viruses and  unconventional agents that are not viruses (e.g., prions).  Prions are protein-containing particles with no detect¬  able nucleic acid that are highly resistant to inactivation by  heat, formaldehyde, and ultraviolet light at doses that will  inactivate viruses. Note that prions are resistant to the tem¬  peratures usually employed in cooking, a fact that may be  important in their suspected ability to be transmitted by  food (see variant Creutzfeldt-Jakob disease [CJD] later).  Prions are, however, inactivated by protein- and lipid-dis¬  rupting agents such as phenol, ether, NaOH, and hypochlo¬  rite (see Chapter 28).   The prion protein is encoded by a normal cellular gene  and is thought to function in a signal transduction pathway  in neurons. The normal prion protein (known as PrP c , or  prion protein cellular) has a significant amount of alpha-  helical conformation. When the alpha-helical conforma¬  tion changes to a beta-pleated sheet (known as PrP sc , or  prion protein scrapie), these abnormal forms aggregate into  filaments, which disrupt neuron function and cause cell  death. Prions, therefore, “reproduce” by the abnormal beta-  pleated sheet form recruiting normal alpha-helical forms to  change their conformation. Note that the normal alpha-  helical form and the abnormal beta-pleated sheet form  have the same amino acid sequence. It is only their confor¬  mation that differs. A specific cellular RNA enhances this    conformational change. Prions are described in more detail  in Chapter 28.   Pathogenic prion proteins can be thought of conceptu¬  ally as misfolded proteins. These misfolded proteins not  only cause CJD in humans and “mad cow” disease in cattle  but are suspected of being involved in the pathogenesis of  other important diseases of the central nervous system,  such as Alzheimer’s disease and Parkinson’s disease.   In humans, the “slow” agents cause central nervous  system diseases characterized by a long incubation period,  a gradual onset, and a progressive, invariably fatal course.  There is no antimicrobial therapy for these diseases. Note  that the term slow refers to the disease, not to the rate of  replication of those viruses that cause these slow diseases.  The replication rate of these viruses is similar to that of  most other viruses.   The human prion-mediated diseases (e.g., kuru and  CJD) are called transmissible spongiform encephalopa¬  thies (TSE). The term spongiform refers to the spongy, Swiss  cheese-like holes seen in the brain parenchyma that are  caused by the death of the neurons (Figure 44-1). No virus  particles are seen in the brain of people with these diseases.   The term encephalopathy refers to a pathologic process  in the brain without signs of inflammation. In contrast,  encephalitis refers to an inflammatory brain process in  which either neutrophils or lymphocytes are present. In  TSEs, there are no inflammatory changes in the brain.   The transmissibility of the agent of kuru and CJD  (“prions”) was initially established by inoculation of   FIGURE 44-1 Prion-mediated spongiform encephalopathy  (mad cow disease)—two arrows point to the spongiform appearance  (Swiss cheese-like holes) in the brain of a cow with mad cow disease.  The brain of a patient with Creutzfeldt-Jakob disease has a similar  appearance. (Source: Dr. Al Jenny, Public Health Image Library, Centers for Dis¬  ease Control and Prevention.)   material from the brains of infected patients into the  brains of primates followed by serial transfer to the brains  of other primates.   Note, however, that both kuru and variant CJD (and  bovine spongiform encephalopathy [BSE]—“mad cow”  disease) are acquired by ingestion. In this route, the prion  protein must survive digestion in the intestinal tract and  then penetrate the gut mucosa. The prion protein is then  amplified within follicle dendritic cells in lymphatic tissue,  such as Peyer’s patches. Prions then spread to the spleen,  carried by migrating dendritic cells. From the spleen, pri¬  ons spread to the central nervous system probably via the  sympathetic nerves.   It is also possible that prions reach the brain within lym¬  phocytes, as there is a documented case of CJD that was  acquired by transfused blood. In addition, CJD has been  transmitted iatrogenically (i.e., in a medical context, via  corneal transplants, dura mater grafts, implanted brain  electrodes, and growth hormone extracts made from  human pituitary glands).    There is evidence that quinacrine and other acridine  analogues inhibit the formation of the pathologic PrP sc  form in cell culture. These drugs are currently being tested  in animal models for their ability to treat or prevent prion  diseases.   Prion-caused diseases can be classified into three cate¬  gories: some are clearly transmissible (infectious), such as  kuru; some are clearly hereditary (genetic), such as fatal  familial insomnia; and others are sporadic (neither infec¬  tious nor hereditary), such as most cases of CJD. The spo¬  radic cases seem likely to be due to spontaneous somatic  mutations in the affected individual.   Progressive multifocal leukoencephalopathy (PML) is a  fatal demyelinating disease of the white matter (i.e., leuko¬  encephalopathy) and involves multiple areas of the brain  (i.e., multifocal). Note that it is not an encephalitis because  there is no inflammation in the brain.   The clinical picture includes visual field defects, mental  status changes, and weakness. The disease rapidly pro¬  gresses to blindness, dementia, and coma, and most patients  die within 6 months. It occurs primarily in individuals with  compromised cell-mediated immunity, especially patients  with acquired immunodeficiency syndrome (AIDS) and  those who are receiving cancer chemotherapy and immu¬  nosuppressive drugs following organ transplantation. Some  patients undergoing treatment for multiple sclerosis with  the monoclonal antibody natalizumab develop PML, and  others receiving mycophenolate to prevent transplant rejec¬  tion have also developed PML. Table 44-1 describes some  important features of slow viral diseases in humans caused  by conventional viruses.   PML is caused by JC virus, a member of the polyomavi-  rus family. Polyomaviruses are nonenveloped viruses with  a circular, double-stranded DNA genome. JC virus infects  and kills oligodendroglia, causing demyelination. Neurons  are unaffected. Antibodies to JC virus are found in approxi¬  mately 75% of normal human sera, indicating that    infection is widespread. Disease occurs when latent JC  virus is activated in an immunocompromised patient. The  virus persists in the kidney and is excreted in the urine. The  diagnosis is typically made by polymerase chain reaction  assay of a brain biopsy specimen or spinal fluid. There is no  effective antiviral treatment, but cidofovir may be  beneficial.   Subacute sclerosing panencephalitis (SSPE) is a slowly pro¬  gressive disease characterized by inflammatory lesions in  many areas of the brain. It is a rare disease of children who  were infected by measles virus several years earlier. Unlike  PML, immunosuppression is not a predisposing factor.  SSPE begins with mild changes in personality and ends  with dementia and death.   SSPE is a persistent infection by a variant of measles  virus that cannot complete its replication. The evidence for  this is as follows:   (1) Inclusion bodies containing helical nucleocapsids,  which react with antibody to measles virus, are seen in the  affected neurons.   (2) A virus very similar to measles virus can be induced  from these cells by cocultivation with permissive cells in  culture. The induced virus has a different matrix protein;  this protein is important in viral assembly.   (3) Patients have high titers of measles antibody in the  blood and spinal fluid.   (4) SSPE has virtually disappeared in the United States  since the onset of widespread immunization with measles  vaccine.   A progressive panencephalitis can also occur in patients  with congenital rubella.   AIDS is caused by human immunodeficiency virus (HIV),  a member of the lentivirus group of retroviruses. AIDS is a  disease with a long latent period and a progressive course    and can involve the central nervous system. See Chapter 45  for more information.   There are five human TSEs caused by prions: kuru, CJD,  variant CJD, Gerstmann-Straussler-Scheinker (GSS) syn¬  drome, and fatal familial insomnia. Table 44-2 describes  some important features of slow viral diseases in humans  caused by prions.   This fatal disease is characterized by progressive tremors  and ataxia but not dementia. It occurs only among the Fore  tribes in New Guinea. It was transmitted during a ritual in  which the skulls of the dead were opened and the brains  eaten. There are two ways the disease could have been  acquired: either by eating the brains or via cuts in the skin  that occurred during the preparation of the brains at which  time brain tissue was introduced into the body. Since the  practice was stopped, kuru has almost disappeared. The  agents of kuru and CJD (see next) have been transmitted  serially in primates.   Pathologic examination of the brains of patients with CJD  and kuru reveals a spongiform (sponge or Swiss cheese)  appearance similar to that associated with scrapie in sheep  (see later). The spongiform changes are the result of neuro¬  nal vacuolation and neuronal loss rather than demyelin-  ation. No inflammatory cells are seen in the brains. Prions  cause scrapie and have been found in the brains of CJD  patients.   In contrast to kuru, CJD is found sporadically world¬  wide and affects both sexes. The incidence of CJD is  approximately 1 case per 1 million population, and there is  no increased risk associated with dietary habits, occupation,   Caused by ingesting or handling brain tissue; occurred in New  Guinea tribes people   No relationship to any known cause; possible new mutation in  somatic cells; most common form   Probably acquired by eating meat or nervous tissue from animals  with mad cow disease    or animal exposure. Vegetarians and meat eaters have the  same rate. The rate of CJD is the same in countries whose  animals have scrapie and those whose animals do not. There  is no evidence for person-to-person or transplacental  transmission.   There is no increased risk for medical caregivers; there¬  fore, gowns and masks are unnecessary. The standard pre¬  cautions for obtaining infectious specimens should be  observed. It has been transmitted iatrogenically (e.g., in a  corneal transplant, via intracerebral electrodes, in hor¬  mones extracted from human pituitaries, and in grafts of  cadaveric dura mater). There is only one confirmed case of  CJD being transmitted by blood transfusion, and intrave¬  nous drug use does not increase the risk. Proper steriliza¬  tion of CJD agent-contaminated material consists of either  autoclaving or treating with sodium hypochlorite.   The main clinical findings of CJD are dementia (includ¬  ing behavioral changes, memory loss, and confusion) and  myoclonic jerking. Additional findings include ataxia,  aphasia, visual loss, and hemiparesis. The symptoms typi¬  cally appear gradually and progress inexorably. In the ter¬  minal stage, the patient becomes mute and akinetic, then  comatose. About 80% of those affected die within 1 year.  Most cases occur in people who are 50 to 70 years of age.   A presumptive diagnosis of CJD can be made pathologi¬  cally by detecting spongiform changes in a brain biopsy  specimen. Neuronal loss and gliosis are seen. Amyloid  plaques are also seen in some cases of CJD. In variant CJD,  “florid” plaques composed of flowerlike amyloid plaques  surrounded by a halo of vacuoles are seen. Brain imaging  and the electroencephalogram may show characteristic  changes. There is no evidence of inflammation (i.e., no  neutrophils or lymphocytes are seen). The blood count and  routine spinal fluid test results are normal. The finding of a  normal brain protein called 14-3-3 in the spinal fluid sup¬  ports the diagnosis.   The specific diagnosis of CJD is typically made by  immunohistochemistry in which labeled antiprion anti¬  bodies are used to stain the patient’s brain specimen.  Because we do not make antibodies to prion proteins, there  are no serologic diagnostic tests. No antibodies are made in  humans because humans are tolerant to our prion proteins.  (The antibodies used in the immunohistochemical lab tests  are made in other animals in which the human prions are  immunogenic.) Unlike viruses, prions cannot be grown in  culture, so there are no culture-based diagnostic tests.   Tonsillar tissue obtained from patients with variant CJD  was positive for prion protein using monoclonal antibody-  based assays. The use of tonsillar or other similar lymphoid  tissue may obviate the need for a brain biopsy. Pathologic  prion proteins have also been detected in the olfactory epi¬  thelium of patients with CJD.   There is no treatment for CJD, and there is no drug or  vaccine available for prevention.   Although most cases of CJD are sporadic, about 10%  are hereditary. The hereditary (familial) form is inherited    as an autosomal dominant trait. In these patients, 12 dif¬  ferent point mutations and several insertion mutations in  the prion protein gene have been found. One of these, a  point mutation in codon 102, is the same mutation found  in patients with GSS syndrome —another slow central  nervous system disease of humans. The main clinical fea¬  tures of GSS syndrome are cerebellar ataxia and spastic  paraparesis. The hereditary forms of these diseases may be  prevented by the detection of carriers and genetic  counseling.   The origin of these spongiform encephalopathies is  threefold: infectious, hereditary, and sporadic. The infec¬  tious forms are kuru and probably variant CJD (see next  section). Transmission of the infectious agent was docu¬  mented by serial passage of brain material from a person  with CJD to chimpanzees. The hereditary form is best  illustrated by GSS syndrome (see preceding paragraph) and  by a disease called fatal familial insomnia. The term spo¬  radic refers to the appearance of the disease in the absence  of either an infectious or a hereditary cause.   Fatal familial insomnia is a very rare disease character¬  ized by progressive insomnia, dysautonomia (dysfunction  of the autonomic nervous system) resulting in various  symptoms, dementia, and death. A specific mutation in the  prion protein is found in patients with this disease.   In 1996, several cases of CJD occurred in Great Britain due  to ingestion of beef. These cases are a new variant of CJD  (vCJD, also called nvCJD) because they occurred in much  younger people than usual and had certain clinical and  pathologic findings different from those found in the typi¬  cal form of the disease. None of those affected had con¬  sumed cattle or sheep brains, but brain material may have  been admixed into processed meats such as sausages.   Only people whose native prion protein is homozygous  for methionine at amino acid 129 contract vCJD. People  whose native prion protein is homozygous for valine at  amino acid 129 or who are heterozygotic do not contract  vCJD. These findings indicate that prion proteins with  methionine are more easily folded into the pathologic beta-  pleated sheet form.   The prions isolated from the “variant CJD” cases in  humans chemically resemble the prions isolated from mad  cow disease more than they resemble other prions, which is  evidence to support the hypothesis that variant CJD origi¬  nated by eating beef. There is no evidence that eating lamb  is associated with variant CJD. As of February 2009, vCJD  has been diagnosed in 209 people, 165 of whom have lived  in the United Kingdom. Three cases of vCJD have occurred  in the United States; two of them are thought to have  acquired it in the United Kingdom. All cases of vCJD have  occurred in individuals who lived or traveled in a country  where BSE has been detected.    It is unknown how many people harbor the pathogenic  prion in a latent (asymptomatic) form. The possibility that  there may be people who are asymptomatic carriers of the  vCJD prion and who could be a source for infection of oth¬  ers (e.g., via blood transfusions) has led blood banks in the  United States to eliminate from the donor pool people who  have lived in Great Britain for more than 6 months.   The slow transmissible diseases of animals are important  models for human diseases. Scrapie and visna are diseases  of sheep, and BSE (mad cow disease) is a disease of cattle  that appears to have arisen from the ingestion of sheep tis¬  sue by the cattle. Chronic wasting disease occurs in deer  and elk. Visna is caused by a virus, whereas the other three  are prion-mediated diseases.   Scrapie is a disease of sheep, characterized by tremors,  ataxia, and itching, in which the sheep scrape off their  wool against fence posts. It has an incubation period of  many months. Spongiform degeneration without inflam¬  mation is seen in the brain tissue of affected animals. It has  been transmitted to mice and other animals via a brain  extract that contained no recognizable virus particles.  Studies of mice revealed that the infectivity is associated  with a 27,000-molecular-weight protein known as a prion  (see page 229).   Visna is a disease of sheep that is characterized by pneumo¬  nia and demyelinating lesions in the brain. It is caused by  visna virus, a member of the lentivirus subgroup of retrovi¬  ruses. As such, it has a single-stranded, diploid RNA  genome and an RNA-dependent DNA polymerase in the  virion. It is thought that integration of the DNA provirus  into the host cell DNA may be important in the persistence  of the virus within the host and, consequently, in its long  incubation period and prolonged, progressive course.   BSE is also known as mad cow disease. The cattle become  aggressive, ataxic, and eventually die. Cattle acquire BSE by  eating feed supplemented with organs (e.g., brains)  obtained from sheep infected with scrapie prions. (It is also  possible that BSE arose in cattle by a mutation in the gene  encoding the prion protein.)   BSE is endemic in Great Britain. Supplementation of  feed with sheep organs was banned in Great Britain in 1988  and thousands of cattle were destroyed, two measures that  have led to a marked decline in the number of new cases of  BSE. BSE has been found in cattle in other European coun¬  tries such as France, Germany, Italy, and Spain, and there is    significant concern in those countries that variant CJD may  emerge in humans. Two cases of BSE in cattle in the United  States have been reported.   Chronic wasting disease (CWD) of deer and elk is a  prion-mediated disease that exists in the United States.  Because vCJD is strongly suspected to be transmitted by  ingesting meat, there is concern regarding the conse¬  quences of eating deer and elk meat (venison). In 2002, it  was reported that neurodegenerative diseases occurred in  three men who ate venison in the 1990s. One of these  diseases was confirmed as CJD. Whether there is a causal  relationship is unclear, and surveillance continues. This  concern was heightened in 2006 when prions were  detected in the muscle of deer with CWD but not in the  muscle of normal deer.   (A) The viruses that cause slow diseases, such as progressive multi¬  focal leukoencephalopathy (PML), have a slow rate of replica¬  tion that accounts for the long latent period and slow  progression of the disease.   (B) PML is caused by a virus that causes widespread inapparent  infections early in life but causes the disease PML primarily in  people with reduced cell-mediated immunity.   (C) Creutzfeldt-Jakob disease (CJD) is caused by CJ virus, a retro¬  virus that integrates a DNA copy of its genome into the DNA  of brain neurons.   (D) CJD occurs primarily in immunocompromised people, but  infection with the virus that causes CJD is common, as evi¬  denced by the presence of antibodies.   2. Regarding prions, which one of the following is the most accurate?   (A) The genome of prions consists of a negative-polarity RNA that  has a defective polymerase gene.   (B) Prion proteins are characterized by having changes in confor¬  mation from the alpha-helical form to the beta-pleated sheet  form.   (C) Prions are very sensitive to ultraviolet (UV) light, which is why  UV light is used in hospital operating rooms to prevent their  transmission.   (D) The main host defense against prions consists of an inflam¬  matory response composed primarily of macrophages and  CD4-positive T cells.   (A) It is caused by a defective mutant of measles virus.   (C) Lesions occur in several areas of the brain, resulting in diverse  symptoms.   (D) Acyclovir is the drug of choice for patients in the early stages of  PML.   (E) It is characterized by an inflammatory reaction in the brain  containing many neutrophils.   (A) Prion-mediated diseases are characterized by vacuoles in the  brain called “spongiform changes.”   (B) Variant Creutzfeldt-Jakob disease is a disease of cattle caused  by the ingestion of sheep brain mixed into cattle feed.   (C) Kuru is a prion-mediated disease for which the diagnosis can  be confirmed in the laboratory by a fourfold or greater rise in  antibody titer.   (D) In Creutzfeldt-Jakob disease, only neurons latently infected by  JC virus produce the prion filaments that disrupt neuronal  function.   (E) Creutzfeldt-Jakob disease occurs primarily in children under  the age of 2 years because they cannot mount an adequate  immune response to the prion protein.   Brief summaries of the organisms described in this chapter  begin on page 677. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Clinical Virology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 723. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   Human immunodeficiency virus (HIV) is the cause of  acquired immunodeficiency syndrome (AIDS).   Both HIV-1 and HIV-2 cause AIDS, but HIV-1 is found  worldwide, whereas HIV-2 is found primarily in West  Africa. This chapter refers to HIV-1 unless otherwise noted.   HIV is one of the two important human T-cell lympho-  tropic retroviruses (human T-cell leukemia virus is the  other). HIV preferentially infects and kills helper (CD4)  T lymphocytes, resulting in the loss of cell-mediated  immunity and a high probability that the host will develop  opportunistic infections. Other cells (e.g., macrophages  and monocytes) that have CD4 proteins on their surfaces  can be infected also.   HIV belongs to the lentivirus subgroup of retroviruses,  which cause “slow” infections with long incubation periods  (see Chapter 44). HIV has a cylinder-shaped (type D) core  surrounded by an envelope containing virus-specific glyco¬  proteins (gpl20 and gp41) (Figures 45-1 and 45-2). The  genome of HIV consists of two identical molecules of  single-stranded, positive-polarity RNA and is said to be  diploid. The HIV genome is the most complex of the known  retroviruses (Figure 45-3). In addition to the three typical  retroviral genes gag pol, and env, which encode the struc¬  tural proteins, the genome RNA has six regulatory genes  (Table 45-1). Two of these regulatory genes, tat and rev, are    required for replication, and the other four, nef, vif, vpr, and  vpu, are not required for replication and are termed “acces¬  sory” genes.   The gag gene encodes the internal “core” proteins, the  most important of which is the p24 protein. It is important   FIGURE 45-1 Human immunodeficiency virus (HIV)—electron  micrograph. Large arrow points to a mature virion of HIV that has just  been released from the infected lymphocyte at the bottom of the  figure. Small arrow (in bottom left of image) points to several nascent  virions in the cytoplasm just prior to budding from the cell membrane.  (Source: Dr. A. Harrison, Dr. P. Feirino, and Dr. E. Palmer, Public Health Image Library,  Centers for Disease Control and Prevention.)    FIGURE 45-2 Cross-section of human immunodeficiency virus  (HIV). In the interior, two molecules of viral RNA are shown associ¬  ated with reverse transcriptase. Surrounding those structures is a  rectangular nucleocapsid composed of p24 proteins. Note that the  viral protease and integrase are also located within the nucleocapsid  (in addition to the reverse transcriptase), but, for lack of space, are  not shown in the figure. On the exterior are the two envelope pro¬  teins, gp120 and gp41, which are embedded in the lipid bilayer  derived from the cell membrane. (Reproduced with permission from Green  WC. Mechanisms of disease: the molecular biology of human immunodeficiency  virus type I infection. NEJM. 1991 ;324[5]:309.)    medically as it is the antigen in the initial serological test  that determines whether the patient has antibody to HIV  (i.e., has been infected with HIV). (See “Laboratory Diag¬  nosis” section in this chapter.)   The pol gene encodes several proteins, including the  virion “reverse transcriptase,” which synthesizes DNA by  using the genome RNA as a template, an integrase that  integrates the viral DNA into the cellular DNA, and a pro¬  tease that cleaves the various viral precursor proteins. The  env gene encodes gpl60, a precursor glycoprotein that is    FIGURE 45-3 The genome of human immunodeficiency virus  (HIV). Above the line are the three genes for the main structural pro¬  teins: (1) gag encodes the internal group-specific antigens (e.g., p24);  (2) pol encodes proteins that have four enzymatic activities: protease  (PROT), polymerase that functions as a reverse transcriptase (POL),  RNase H (H), and integrase (INT); (3) env encodes the two envelope  glycoproteins, gp120 and gp41. Below the line are five regulatory  proteins: viral infectivity factor (VIF), transactivating protein (TAT),  viral protein U (VPU), regulator of expression of virion protein (REV),  and negative regulatory factor (NEF). At both ends are long terminal  repeats (LTR), which are transcription initiation sites. Within the 5' LTR  is the binding site for the TAT protein, called the transactivation  response element (TAR). TAT enhances the initiation and elongation  of viral mRNA transcription. (*p24 and other smaller proteins such as  pi 7 and p7 are encoded by the gag gene.)    cleaved to form the two envelope (surface) glycoproteins,  gpl20 and gp41.   Differences in the base sequence of the gpl20 gene are  used to subdivide HIV into subtypes called clades. Differ¬  ent clades are found in different areas of the world. For  example, the B clade is the most common subtype in North  America. Subtype B preferentially infects mononuclear  cells and appears to be passed readily during anal sex,  whereas subtype E preferentially infects female genital tract  cells and appears to be passed readily during vaginal sex.   Three enzymes are located within the nucleocapsid of  the virion: reverse transcriptase, integrase, and protease  (see Figure 45-2).   Reverse transcriptase is the RNA-dependent DNA poly¬  merase that is the source of the family name retroviruses.  This enzyme transcribes the RNA genome into the proviral  DNA. Reverse transcriptase is a bifunctional enzyme; it  also has ribonuclease H activity. Ribonuclease H degrades  RNA when it is in the form of an RNA-DNA hybrid mol¬  ecule. The degradation of the viral RNA genome is an  essential step in the synthesis of the double-stranded provi¬  ral DNA. Integrase, another important enzyme within the  virion, mediates the integration of the proviral DNA into  the host cell DNA. The viral protease cleaves the precursor  polyproteins into functional viral polypeptides.   One essential regulatory gene is the tat (transactivation  of transcription) 1 gene, which encodes a protein that  enhances viral (and perhaps cellular) gene transcription.   The Tat protein and another HIV-encoded regulatory  protein called Nef repress the synthesis of class I major his¬  tocompatibility complex (MHC) proteins, thereby reducing  the ability of cytotoxic T cells to kill HIV-infected cells. The  other essential regulatory gene, rev, controls the passage of  late mRNA from the nucleus into the cytoplasm. The func¬  tion of the four accessory genes is described in Table 45-1.   The accessory protein Vif (viral m/ectivity) enhances  HIV infectivity by inhibiting the action of APOBEC3G, an  enzyme that causes hypermutation in retroviral DNA.  APOBEC3G is “apolipoprotein B RNA-editing enzyme”  that deaminates cytosines in both mRNA and retroviral  DNA, thereby inactivating these molecules and reducing  infectivity. APOBEC3G is considered to be an important  member of the innate host defenses against retroviral infec¬  tion. HIV defends itself against this innate host defense by  producing Vif, which counteracts APOBEC3G, thereby  preventing hypermutation from occurring.   (1) gpl20 and gp41 are the type-specific envelope gly¬  coproteins. gpl20 protrudes from the surface and interacts  with the CD4 receptor (and a second protein, a chemokine    1 Transactivation refers to activation of transcription of genes distant from  the gene (i.e., other genes on the same proviral DNA or on cellular  DNA). One site of action of the Tat protein is the long terminal repeat at  the 5' end of the viral genome.   II. Regulatory genes found in human immunodeficiency virus that are required for replication   III. Regulatory genes found in human immunodeficiency virus that are not required for replication (accessory genes)   Decreases CD4 proteins and class 1 MHC proteins on surface of infected cells; induces death of  uninfected cytotoxic T cells; important for pathogenesis by SIV 2   Enhances infectivity by inhibiting the action of APOBEC3G (an enzyme that causes hypermutation  in retroviral DNA)   1 Reverse transcriptase also contains ribonuclease H activity, which degrades the genome RNA to allow the second strand of DNA to be made.  2 Mutants of the nef gene of simian immunodeficiency virus (SIV) do not cause acquired immunodeficiency syndrome in monkeys.    receptor) on the cell surface. gp41 is embedded in the enve¬  lope and mediates the fusion of the viral envelope with the  cell membrane at the time of infection. The gene that  encodes gpl20 mutates rapidly, resulting in many anti¬  genic variants. The most immunogenic region of gpl20 is  called the V3 loop; it is one of the sites that varies antigeni-  cally to a significant degree. Antibody against gpl20 neu¬  tralizes the infectivity of HIV, but the rapid appearance of  gpl20 variants has made production of an effective vaccine  difficult. The high mutation rate may be due to lack of an  editing function in the reverse transcriptase.   (2) The group-specific antigen, p24, is located in the  nucleocapsid core and is not known to vary. Antibodies  against p24 do not neutralize HIV infectivity but serve as  important serologic markers of infection.   The natural host range of HIV is limited to humans,  although certain primates can be infected in the laboratory  HIV is not an endogenous virus of humans (i.e., no HIV  sequences are found in normal human cell DNA). The ori¬  gin of HIV and how it entered the human population  remains uncertain. There is evidence that chimpanzees  living in West Africa were the source of HIV-1. If chimpan¬  zees are the source of HIV in humans, it would be a good  example of a virus “jumping the species barrier.”   In addition to HIV-1, two other similar retroviruses are  worthy of comment:    (1) Human immunodeficiency virus type 2 (HIV-2) was  isolated from AIDS patients in West Africa in 1986. The  proteins of HIV-2 are only about 40% identical to those of  the original HIV isolates. HIV-2 remains localized primar¬  ily to West Africa and is much less transmissible than  HIV-1.   (2) Simian immunodeficiency virus (SIV) was isolated  from monkeys with an AIDS-like illness. Antibodies in  some African women cross-react with SIV. The proteins of  SIV resemble those of HIV-2 more closely than they resem¬  ble those of the original HIV isolates.   In general, the replication of HIV follows the typical retro¬  viral cycle (Figure 45-4). The initial step in the entry of  HIV into the cell is the binding of the virion gpl20 enve¬  lope protein to the CD4 protein on the cell surface. The  virion gpl20 protein then interacts with a second protein  on the cell surface, one of the chemokine receptors. Next,  the virion gp41 protein mediates fusion of the viral enve¬  lope with the cell membrane, and the virion core contain¬  ing the nucleocapsid, RNA genome, and reverse  transcriptase enters the cytoplasm.   Chemokine receptors, such as CXCR4 and CCR5 pro¬  teins, are required for the entry of HIV into CD4-positive  cells. The T cell-tropic strains of HIV bind to CXCR4,   FIGURE 45-4 Replicative cycle of human immunodeficiency virus (HIV). Showing the sites of action of the important drugs  used to treat HIV infection. The mode of action of the reverse transcriptase inhibitors, the entry inhibitors, the integrase inhibitor, and  the protease inhibitors is described in Chapter 35. On the right side of the figure, "cleavage by protease" describes the process by  which the virus-encoded protease cleaves the Gag-Pol polyprotein into functional viral proteins as the virion buds from the cell mem¬  brane. These newly formed functional proteins are transported by the mature virion to the next cell and function within that newly  infected cell. The viral reverse transcriptase and integrase are two such proteins. (Reproduced with permission from Ryan K et al.  Sherris Medical Microbiology. 3rd ed. Originally published by Appleton & Lange. Copyright 1994 McGraw-Hill.)    whereas the macrophage-tropic strains bind to CCR5.  Mutations in the gene encoding CCR5 endow the individ¬  ual with protection from infection with HIV. People who  are homozygotes are completely resistant to infection, and  heterozygotes progress to disease more slowly. Approxi¬  mately 1% of people of Western European ancestry have  homozygous mutations in this gene, and about 10% to 15%  are heterozygotes. One of the best-characterized mutations  is the delta-32 mutation, in which 32 base pairs are deleted  from the CCR5 gene.   In the cytoplasm, reverse transcriptase transcribes the  genome RNA into double-stranded DNA, which migrates  to the nucleus, where it integrates into the host cell DNA.  The viral DNA can integrate at different sites in the host  cell DNA, and multiple copies of viral DNA can integrate.  Integration is mediated by a virus-encoded endonuclease  (integrase). Viral mRNA is transcribed from the integrated  (proviral) DNA by host cell RNA polymerase (augmented  by virus-encoded Tat protein) and translated into several  large polyproteins. The Gag and Pol polyproteins are  cleaved by the viral protease, whereas the Env polyprotein  is cleaved by a cellular protease.   The Gag polyprotein is cleaved to form the main core  protein (p24), the matrix protein (pl7), and several smaller    proteins. The Pol polyprotein is cleaved to form the reverse  transcriptase, integrase, and protease. The immature virion  containing the precursor polyproteins forms in the cyto¬  plasm, and cleavage by the viral protease occurs as the  immature virion buds from the cell membrane. It is this  cleavage process that results in the mature, infectious  virion.   Note that HIV replication is dependent on cell proteins  as well as viral proteins. First there are the cell proteins  required during the early events, namely CD4, and the che-  mokine receptors, CCR5 and CXCR4. Cell proteins, such as  actin and tubulin, are involved with the movement of viral  DNA into the nucleus. The cell protein cyclin T1 and the  viral protein Tat are part of the complex that transcribes  viral mRNA. Cell proteins are also involved in the budding  process by which the virus exits the cell.   Transmission of HIV occurs primarily by sexual contact  and by transfer of infected blood. Perinatal transmission  from infected mother to neonate also occurs, either across  the placenta, at birth, or via breast milk. It is estimated that  more than 50% of neonatal infections occur at the time of    delivery and that the remainder is split roughly equally  between transplacental transmission and transmission via  breast feeding. There is no evidence for airborne, water¬  borne, or insect transmission of HIV.   Infection occurs by the transfer of either HIV-infected  cells or free HIV (i.e., HIV that is not cell-associated).  Although small amounts of virus have been found in other  fluids (e.g., saliva and tears), there is no evidence that they  play a role in infection. In general, transmission of HIV  follows the pattern of hepatitis B virus (HBV), except that  HIV infection is much less efficiently transferred (i.e., the  dose of HIV required to cause infection is much higher  than that of HBV). People with sexually transmitted dis¬  eases, especially those with ulcerative lesions such as syphi¬  lis, chancroid, and herpes genitalis, have a significantly  higher risk of acquiring HIV. Uncircumcised males have a  higher risk of acquiring HIV than do circumcised males.   Transmission of HIV via blood transfusion has been  greatly reduced by screening donated blood for the pres¬  ence of antibody to HIV. However, there is a “window”  period early in infection when the blood of an infected  person can contain HIV but antibodies are not detectable.  Blood banks now test for the presence of p24 antigen in an  effort to detect blood that contains HIV.   The Centers for Disease Control and Prevention (CDC)  estimates that at the end of 2011, there were approximately  1.1 million people infected with HIV living in the United  States. The transmission rate has declined markedly, pri¬  marily due to increased prevention efforts and improved  treatments for HIV; the latter reduces the number of people  with high titers of HIV. CDC estimates that approximately  50,000 people new infections occur each year. CDC also  estimates that 15% of those who are infected with HIV do  not know it because they have not been tested.   Approximately 630,000 people have died of AIDS in the  United States since 1981, when AIDS was first recognized.   As of 2011, it is estimated that approximately 34 million  people worldwide are infected, two-thirds of whom live in  sub-Saharan Africa. Three regions, Africa, Asia, and Latin  America, have the highest rates of new infections. AIDS is  the fourth leading cause of death worldwide. (Ischemic  heart disease, cerebrovascular disease, and acute lower  respiratory disease are ranked first, second, and third,  respectively.)   In the United States and Europe during the 1980s, HIV  infection and AIDS occurred primarily in men who have  sex with men (especially those with multiple partners),  intravenous drug users, and hemophiliacs. Heterosexual  transmission was rare in these regions in the 1980s but is  now rising significantly. Heterosexual transmission is the  predominant mode of infection in African countries.   Very few health care personnel have been infected  despite continuing exposure and needle-stick injuries, sup¬  porting the view that the infectious dose of HIV is high.  The risk of being infected after percutaneous exposure to    HIV-infected blood is estimated to be about 0.3%. The  transmission of HIV from health care personnel to patients  is exceedingly rare.   HIV infects helper T cells (CD4-positive cells) and kills  them, resulting in suppression of cell-mediated immu¬  nity. This predisposes the host to various opportunistic  infections and certain cancers such as Kaposi’s sarcoma and  lymphoma. HIV does not directly cause these tumors  because HIV genes are not found in these cancer cells. The  initial infection of the genital tract occurs in dendritic cells  that line the mucosa (Langerhans cells), after which the  local CD4-positive helper T cells become infected. HIV is  first found in the blood 4 to 11 days after infection.   HIV infection also targets a subset of CD4-positive cells  called Thl7 cells. These cells are an important mediator of  mucosal immunity, especially in the gastrointestinal tract.  Many mucosal Thl7 cells are killed early in HIV infection.  Thl7 cells produce interleukin-17 (IL-17), which attracts  neutrophils to the site of bacterial infection. The loss of  Thl7 cells predisposes HIV-infected individuals to blood¬  stream infections by bacteria in the normal flora of the  colon, such as Escherichia coli.   HIV also infects brain monocytes and macrophages,  producing multinucleated giant cells and significant central  nervous system symptoms. The fusion of HIV-infected  cells in the brain and elsewhere mediated by gp41 is one of  the main pathologic findings. The cells recruited into the  syncytia ultimately die. The death of HIV-infected cells is  also the result of immunologic attack by cytotoxic CD8  lymphocytes. Effectiveness of the cytotoxic T cells may be  limited by the ability of the viral Tat and Nef proteins to  reduce class I MHC protein synthesis (see later).   Another mechanism hypothesized to explain the death  of helper T cells is that HIV acts as a “superantigen,” which  indiscriminately activates many helper T cells and leads to  their demise. The finding that one member of the retrovi¬  rus family, mouse mammary tumor virus, can act as a  superantigen lends support to this theory. Superantigens  are described in Chapter 58.   Persistent noncytopathic infection of T lymphocytes  also occurs. Persistently infected cells continue to produce  HIV, which may help sustain the infection in vivo. Lym¬  phoid tissue (e.g., lymph nodes) is the main site of ongoing  HIV infection.   In addition, a true latent infection can occur in which  no HIV is produced. This occurs in resting CD4-positive  memory T cells within which an integrated HIV genome is  found. The latent period can last for months to years but if  the resting cell is activated, HIV can be produced. HIV  replication depends on host cell transcription factors made  in activated, but not resting, CD4-positive cells.   A person infected with HIV is considered to be  infected for life. This seems likely to be the result of    integration of viral DNA into the DNA of infected cells.  Although the use of powerful antiviral drugs (see “Treat¬  ment” section later) can significantly reduce the amount  of HIV being produced, the silent, latent infection in  CD4-positive memory T cells can be activated and serve  as a continuing source of virus.   Elite controllers are a rare group of HIV-infected peo¬  ple (less than 1% of those infected) who have no detectable  HIV in their blood. Their CD4 counts are normal without  using antiretroviral drugs. The ability to be an elite control¬  ler does not depend on gender, race, or mode of acquisition  of the virus. Although the mechanism is unclear, there is  evidence that certain HLA alleles are protective and that an  inhibitor of the cyclin-dependent kinase known as p21  plays an important role.   In addition, there is a group of HIV-infected individuals  who have lived for many years without opportunistic infec¬  tions and without a reduction in the number of their helper  T (CD4) cells. The strain of HIV isolated from these indi¬  viduals has mutations in the nef gene, indicating the impor¬  tance of this gene in pathogenesis. The Nef protein  decreases class I major histocompatibility complex (MHC)  protein synthesis, and the inability of the mutant virus to  produce functional Nef protein allows the cytotoxic T cells  to retain their activity.   Another explanation why some HIV-infected individu¬  als are long-term “nonprogressors” may lie in their ability  to produce large amounts of a-defensins. a-Defensins are a  family of positively charged peptides with antibacterial  activity that also have antiviral activity. They interfere with  HIV binding to the CXCR4 receptor and block entry of the  virus into the cell.   In addition to the detrimental effects on T cells, abnor¬  malities of B cells occur. Polyclonal activation of B cells is  seen, with resultant high immunoglobulin levels. Autoim¬  mune diseases, such as thrombocytopenia, occur.   The main immune response to HIV infection consists  of cytotoxic CD8-positive lymphocytes. These cells  respond to the initial infection and control it for many  years. Mutants of HIV, especially in the env gene encoding  gpl20, arise, but new clones of cytotoxic T cells proliferate  and control the mutant strain. It is the ultimate failure of  these cytotoxic T cells that results in the clinical picture of  AIDS. Cytotoxic T cells lose their effectiveness because so  many CD4 helper T cells have died; thus the supply of lym-  phokines, such as interleukin-2 (IL-2), required to activate  the cytotoxic T cells is no longer sufficient.   There is evidence that “escape” mutants of HIV are able  to proliferate unchecked because the patient has no clone of  cytotoxic T cells capable of responding to the mutant  strain. Furthermore, mutations in any of the genes encod¬  ing class I MHC proteins result in a more rapid progression  to clinical AIDS. The mutant class I MHC proteins cannot  present HIV epitopes, which results in cytotoxic T cells  being incapable of recognizing and destroying HIV-  infected cells.    Antibodies against various HIV proteins, such as p24,  gpl20, and gp41, are produced, but they neutralize the  virus poorly in vivo and appear to have little effect on the  course of the disease.   HIV has three main mechanisms by which it evades the  immune system: (1) integration of viral DNA into host cell  DNA, resulting in a persistent infection; (2) a high rate of  mutation of the env gene; and (3) the production of the Tat  and Nef proteins that downregulate class I MHC proteins  required for cytotoxic T cells to recognize and kill HIV-  infected cells. The ability of HIV to infect and kill CD4-  positive helper T cells further enhances its capacity to avoid  destruction by the immune system.   The clinical picture of HIV infection can be divided into  three stages: an early, acute stage; a middle, latent stage; and  a late, immunodeficiency stage (Figure 45-5). In the acute  stage, which usually begins 2 to 4 weeks after infection, a  mononucleosis-like picture of fever, lethargy, sore throat,  and generalized lymphadenopathy occurs. A maculopapu-  lar rash on the trunk, arms, and legs (but sparing the palms  and soles) is also seen. Leukopenia occurs, but the number  of CD4 cells is usually normal. A high-level viremia typi¬  cally occurs, and the infection is readily transmissible dur¬  ing this acute stage. This acute stage typically resolves  spontaneously in about 2 weeks. Resolution of the acute    FIGURE 45-5 Time course of human immunodeficiency  virus (HIV) infection. The three main stages of HIV infection—  acute, latent, and immunodeficiency—are shown in conjunction  with several important laboratory findings. Note that the levels of  virus and viral RNA (viral load) are high early in the infection,  become low for several years, and then rise during the immuno¬  deficiency stage. The level of CD4 lymphocytes remains more or  less normal for many years but then falls. This results in the immu¬  nodeficiency stage, which is characterized by opportunistic infec¬  tions and malignancies. Not shown in the figure is the marked  loss of theTh-17 subset of CD4-positive T cells early in the infec¬  tion. (Adapted from Weiss RA. How does HIV cause AIDS? Science.  1993:260:1273.)    stage is usually accompanied by a lower level of viremia and  a rise in the number of CD8-positive (cytotoxic) T cells  directed against HIV.   Antibodies to HIV typically appear 10 to 14 days after  infection, and most patients will have seroconverted by 3 to  4 weeks after infection. Note that the inability to detect  antibodies prior to that time can result in “false-negative”  serologic tests (i.e., the person is infected, but antibodies  are not detectable at the time of the test). This has impor¬  tant implications because HIV can be transmitted to others  during this period. If the antibody test is negative but HIV  infection is still suspected, then a polymerase chain reac¬  tion (PCR)-based assay for viral RNA in the plasma should  be done.   Of those who become seropositive during the acute  infection, approximately 87% are symptomatic (i.e., about  13% experience an asymptomatic initial infection).   After the initial viremia, a viral set point occurs, which  can differ from one person to another. The set point repre¬  sents the amount of virus produced (i.e., the viral load) and  tends to remain “set,” or constant, for years. The higher the  set point at the end of the initial infection, the more likely  the individual is to progress to symptomatic AIDS. It is  estimated that an infected person can produce up to 10 bil¬  lion new virions each day. This viral load can be estimated  by using an assay for viral RNA in the patient’s plasma.  (The assay detects the RNA in free virions in the plasma,  not cell-associated virions.)   The amount of viral RNA serves to guide treatment  decisions and the prognosis. For example, if a drug regimen  fails to reduce the viral load, the drugs should be changed.  As far as the prognosis is concerned, a patient with more  than 10,000 copies of viral RNA/mL of plasma is signifi¬  cantly more likely to progress to AIDS than a patient with  fewer than 10,000 copies.   The number of CD4-positive T cells is another impor¬  tant measure that guides the management of infected  patients. It is used to determine whether a patient needs  chemoprophylaxis against opportunistic organisms, to  determine whether a patient needs anti-HIV therapy, and  to determine the response to this therapy. The lower limit  of CD4 count considered as normal is 500 cells/ pL. People  with this level or higher are usually asymptomatic. The  frequency and severity of opportunistic infections signifi¬  cantly increase when the CD4 counts fall below 200/pL. A  CD4 count of 200/pL or below is an AIDS-defining  condition.   In the middle stage of HIV infection, a long latent  period, measured in years, usually ensues. In untreated  patients, the latent period typically lasts for 7 to 11 years.  The patient is asymptomatic during this period.  Although the patient is asymptomatic and viremia is low  or absent, a large amount of HIV is being produced by  lymph node cells but remains sequestered within the  lymph nodes. This indicates that during this period of    clinical latency, the virus itself does not enter a latent  state.   A syndrome called AIDS-related complex (ARC) can  occur during the latent period. The most frequent manifes¬  tations are persistent fevers, fatigue, weight loss, and  lymphadenopathy. ARC often progresses to AIDS.   The late stage of HIV infection is AIDS, manifested by a  decline in the number of CD4 cells to below 200/pL and an  increase in the frequency and severity of opportunistic  infections. Table 45-2 describes some of the common  opportunistic infections and their causative organisms seen  in HIV-infected patients during the late, immunocompro¬  mised stage of the infection.   The two most characteristic manifestations of AIDS are  Pneumocystis pneumonia and Kaposi’s sarcoma. However,  many other opportunistic infections occur with some fre¬  quency. These include viral infections such as dissemi¬  nated herpes simplex, herpes zoster, and cytomegalovirus  infections and progressive multifocal leukoencephalopa¬  thy; fungal infections such as thrush (caused by Candida  albicans), cryptococcal meningitis, and disseminated his¬  toplasmosis; protozoal infections such as toxoplasmosis  and cryptosporidiosis; and disseminated bacterial infec¬  tions such as those caused by Mycobacterium avium-  intracellulare and Mycobacterium tuberculosis. Many AIDS  patients have severe neurologic problems (e.g., dementia  and neuropathy), which can be caused by either HIV  infection of the brain or by many of these opportunistic  organisms.   The presumptive diagnosis of HIV infection is often made  by the detection of antibodies in the patient’s serum to the  p24 protein of HIV using the enzyme-linked immunosor¬  bent assay (ELISA) test. Because there are some false¬  positive results with this test, the definitive diagnosis is  made by Western blot (also known as Immunoblot)  analysis, in which the viral proteins are displayed by acryl¬  amide gel electrophoresis, transferred to nitrocellulose  paper (the blot), and reacted with the patient’s serum. If  antibodies are present in the patient’s serum, they will bind  to the viral proteins (predominantly to the gp41 or p24  protein). Enzymatically labeled antibody to human IgG is  then added. A color reaction reveals the presence of the  HIV antibody in the infected patient’s serum. Figure 64-9  depicts a Western blot (Immunoblot) test used to diagnose  HIV infection.   OraQuick is a rapid, screening immunoassay for HIV  antibody that uses an oral swab sample in an ELISA-type  test that can be done at home. Results are available in  20 minutes. Positive results for HIV antibody require con¬  firmation by a Western blot test.   The PCR test is a very sensitive and specific technique  that can be used to detect HIV DNA within infected cells.    Some individuals who do not have detectable antibodies  have been shown by this test to be infected. As already  mentioned, the amount of viral RNA in the plasma (i.e.,  the viral load) can also be determined using PCR-based  assays.   During the first month after infection, antibody tests  may be negative. These false-negative tests are due to insuf¬  ficient antibody being made early in infection to be detected  in the ELISA test. The average time for seroconversion is  10 to 14 days, and most of those infected, but not all, will  have seroconverted by 4 weeks.   In view of this, the diagnosis of acute HIV infection may  not be able to be made using only tests for antibody in the  serum. The presence of HIV can be detected during acute  infection by the plasma HIV RNA assay (viral load), as  viremia is typically high at this early stage. Also useful for  the diagnosis of early infections is the HIV antigen/  antibody “Combo” test that detects the presence of p24  antigen as well as antibodies to both HIV-1 and HIV-2.  This combination test is useful for the diagnosis of early  infections because p24 antigen is typically detectable earlier  in infection than antibody.   Other laboratory tests that are important in the manage¬  ment of an HIV-infected person include CD4 cell counts,  viral load assays, and tests for drug resistance of the strain  of HIV infecting the patient. Drug resistance tests are  described at the end of the “Treatment” section in this  chapter. HIV can be grown in culture from clinical speci¬  mens, but this procedure is available only at a few medical  centers.   The treatment of HIV infection has resulted in a remark¬  able reduction in mortality and improvement in the quality  of life of infected individuals. The two specific goals of  treatment are (1) to restore immunologic function by  increasing the CD4 count, which reduces opportunistic  infections and certain malignancies, and (2) to reduce viral  load, which reduces the chance of transmission to others.  There is evidence that starting drug therapy as soon as pos¬  sible after making the diagnosis of HIV infection is the best  way to achieve these goals.   Unfortunately, no drug regimen results in a “cure” (i.e.,  eradicates the virus from the body), but long-term suppres¬  sion can be achieved. However, if drugs are stopped, the  virus resumes active replication, and large amounts of  infectious virus reappear.   Treatment of HIV infection typically involves multiple  antiretroviral drugs. The use of a single drug (monother¬  apy) for treatment is not done because of the high rate of  mutation to drug resistance.   The choice of drugs is complex and depends on several  factors (e.g., whether it is an initial infection or an estab¬  lished infection, the number of CD4 cells, the viral load, the  resistance pattern of the virus, and whether the patient is  pregnant or is coinfected with HBV or hepatitis C virus  [HCV]). Table 45-3 describes the mechanism of action of  the drugs and their main adverse effects. The number of  drugs and the various determining factors mentioned pre¬  viously make describing all the treatments beyond the  scope of this book. The reader is advised to consult the   A prodrug of amprenavir; metabolized by phosphatases in gut  epithelium to amprenavir. AE: rash   Ritonavir inhibits CYP3A metabolism of lopinavir, thereby increasing  the effective concentration of lopinavir   May contain levels of ethyl methanesulfonate (ethyl mesylate) that  may be carcinogenic, mutagenic, or teratogenic. AE; diarrhea. No  longer recommended   Invirase must be taken with ritonavir; Fortovase can be taken without  ritonavir. Infrequently used   AE: if taken with ritonavir (Norvir), severe liver disease may occur   Binds to viral gp41 and blocks fusion of virus with cell membrane   Blocks binding of viral gp120 to CCR5 coreceptor on cell membrane;  effective against CCR5-tropic viruses but not against CXCR4-tropic  viruses   'All protease inhibitors cause lipodystrophy (“buffalo hump”) and central obesity as an adverse effect. Nausea and diarrhea are also  quite common. Hepatotoxicity occurs, especially in those infected with hepatitis B or C virus.    Department of Health and Human Services Antiretroviral  Therapy Guidelines or other reliable sources, such as the  Medical Letter.   As of 2015, the preferred approach to initial antiretrovi¬  ral therapy consists of one of four regimens, all of which  consist of three or four drugs. Each regimen includes  emtricitabine and tenofovir (“the backbone”), to which  either efavirenz, raltegravir, rilpivirine, or a combination of    two protease inhibitors (either ritonavir plus atazanavir or  ritonavir plus darunavir) or a combination of elvitegravir  plus the enhancer cobicistat is added. A single pill given in  once-a-day dosing improves compliance so a variety of  these three or four drug combinations are now available in  a single pill formulation.   These combinations are known as highly active antiret¬  roviral therapy (HAART). HAART is very effective in    prolonging life, improving quality of life, and reducing viral  load but does not cure the chronic HIV infection (i.e., rep¬  lication of HIV within CD4-positive cells continues indefi¬  nitely). Discontinuation of HA ART almost always results in  viremia (a return of the viral load to its pretreatment set  point) and a fall in the CD4 count.   Table 45-3 describes six nucleoside reverse transcriptase  inhibitors (abacavir, didanosine, emtricitabine, lamivudine,  stavudine, and zidovudine) and a single nucleotide reverse  transcriptase inhibitor (tenofovir). These drugs are charac¬  terized by not having a 3' hydroxyl group on the ribose ring  and therefore are chain-terminating drugs. They inhibit  HIV replication by interfering with proviral DNA synthesis  by reverse transcriptase. They cannot cure an infected cell  of an already integrated copy of proviral DNA. Additional  information on these “nucleoside analogue” drugs and the  other antiretroviral drugs can be found in Chapter 35. Note  that zalcitabine (Hivid), an NRTI analogue of cytosine, is  no longer available.   Two main problems limit the use of NTRIs: the emer¬  gence of resistance and adverse effects. The main adverse  effects are described in Table 45-3. For example, the long¬  term use of zidovudine (ZDV) is limited by suppression of  the bone marrow leading to anemia and neutropenia. This  hematotoxicity is due to the inhibition of the mitochondrial  DNA polymerase. Nevertheless, ZDV is used in postexpo¬  sure prophylaxis and to prevent vertical transmission from  mother to fetus. Lamivudine and its analogue emtricitabine  have the same mechanism of action as ZDV but are better  tolerated, and one or another is a common component of  HAART. Abacavir is also commonly used. Patients who  have an HLA-B1701 allele are more likely to have a severe  hypersensitivity reaction to abacavir. Patients should be  tested for this gene before being prescribed abacavir.   Table 45-3 describes five nonnucleoside reverse transcrip¬  tase inhibitors (delavirdine, efavirenz, etravirine, nevirap¬  ine, and rilpivirine) that are effective against HIV. Unlike  the NRTIs, these drugs are not base analogues. Efavirenz  (Sustiva) and nevirapine (Viramune) are the most com¬  monly used drugs in this class. Efavirenz is a common  component of HAART regimens, especially a single pill  containing efavirenz, tenofovir, and emtricitabine (Atri-  pla). Nevirapine is often used to prevent vertical transmis¬  sion of HIV from mother to fetus. Both nevirapine and  efavirenz can cause skin rashes and Stevens-Johnson  syndrome.   Table 45-3 describes the currently available protease inhib¬  itors (amprenavir atazanavir, darunavir, fosamprenavir,    indinavir, nelfinavir, ritonavir, saquinavir, tipranavir, and a  combination of lopinavir and ritonavir). Protease inhibitors  when combined with nucleoside analogues are very effec¬  tive in inhibiting viral replication and increasing CD4 cell  counts and are commonly used in HAART regimens. Lopi¬  navir and ritonavir are given in combination because rito¬  navir inhibits the degradation of lopinavir thereby  increasing the concentration of lopinavir. A briefer way of  saying that is ritonavir “boosts” lopinavir.   Mutants of HIV resistant to protease inhibitors can be a  significant clinical problem. Resistance to one protease  inhibitor often conveys resistance to all; however, the com¬  bination of two protease inhibitors, namely, ritonavir and  lopinavir (Kaletra), is effective against both mutant and  nonmutant strains of HIV. Also, darunavir is effective  against many strains of HIV that are resistant to other pro¬  tease inhibitors. Mutants of HIV resistant to both protease  inhibitors and reverse transcriptase inhibitors have been  recovered from patients.   A major side effect of protease inhibitors is abnormal  fat deposition in specific areas of the body, such as the  back of the neck (Figure 45-6). The fat deposits in the  back of the neck are said to give the person a “buffalo  hump” appearance. These abnormal fat deposits are a  type of lipodystrophy; the metabolic process by which  this occurs is unknown. Saquinavir and indinavir are  infrequently used because of toxicity and nelfinavir is no  longer recommended.   Treatment for acute HIV infection with two reverse  transcriptase inhibitors and a protease inhibitor is often  used. With this regimen, the viral load drops below the  level of detection, CD4 cell counts rise, and CD8 activity   FIGURE 45~6 Lipodystrophy —note enlarged fat pad on back  of neck. This is known as a "buffalo hump" and is an adverse effect of  the protease inhibitor class of antiretroviral drugs. (Reproduced with  permission from Wolff K, Johnson R. Fitzpatrick's Color Atlas & Synopsis  of Clinical Dermatology. 6th ed. New York: McGraw-Hill, 2009. Copy¬  right © 2009 by The McGraw-Hill Companies, Inc.)    increases. The long-term effect of this approach on rate of  progression to AIDS has yet to be determined.   Pregnant women infected with HIV should be treated  with two nucleosides and a protease inhibitor. A typical  regimen would include lamivudine, ZDV, and lopinavir/  ritonavir. In addition, ZDV should be given to the neonate.  These drugs appear not to damage the fetus, although rare  instances of mitochondrial dysfunction and death attrib¬  uted to ZDV have been reported. The reader is urged to  consult the current information regarding the use of these  drugs in pregnancy. A full discussion is beyond the scope  of this book.   Table 45-3 describes two entry inhibitors, enfuvirtide and  maraviroc. Enfuvirtide (Fuzeon) is the first of a new class  of anti-HIV drugs known as fusion inhibitors (i.e., they  prevent the fusion of the viral envelope with the cell mem¬  brane). Enfuvirtide is a synthetic peptide that binds to gp41  on the viral envelope, thereby blocking the entry of HIV  into the cell. It must be administered by injection and is  quite expensive.   Maraviroc (Selzentry) also prevents the entry of HIV  into cells. It blocks the binding of the gpl20 envelope  protein of HIV to CCR-5, which is an important corecep¬  tor on the cell surface. Before prescribing maraviroc, a  laboratory test (Trofile assay) should be performed to  ensure that the tropism of the patient’s strain of HIV is  CCR5. Maraviroc should be used in combination with  other antiretroviral drugs in patients infected with CCR5-  tropic strains of HIV and in treatment-experienced adults  infected with an HIV strain that is resistant to other anti¬  retroviral drugs.   Raltegravir (Isentress) is the first drug to inhibit the  HIV-encoded integrase (see Table 45-3). It is recom¬  mended for use in patients who have been treated with  other antiretroviral drugs but continue to produce sig¬  nificant levels of HIV. Two additional integrase inhibi¬  tors are available: dolutegravir (Tivicay) and elvitegravir  (Stribild).   Drug-resistant mutants of HIV have emerged that signifi¬  cantly affect the ability of both reverse transcriptase inhibi¬  tors and protease inhibitors to sustain their clinical efficacy.  Approximately 10% of newly infected patients are infected  with a strain of HIV resistant to at least one antiretroviral  drug. Laboratory tests to detect mutant strains include both  genotypic and phenotypic analysis. Genotyping reveals the  presence of specific mutations in either the reverse tran¬  scriptase (RT) or protease {PR) genes. Phenotyping deter¬  mines the ability of the virus to grow in cell culture in the  presence of the drug. One method of phenotyping recovers    the RT and PR genes from the patient’s virus and splices  them into a test strain of HIV, which is then used to infect  cells in culture. Another laboratory test can determine the  tropism of the patient’s isolate (i.e., whether it uses CCR5 as  its coreceptor). If so, then maraviroc can be used for  treatment.   may occur in HIV-infected patients who are treated with  a HAART regimen and who are coinfected with other  microbes such as HBV, HCV, M. tuberculosis, M. avium  complex, Cryptococcus neoformans, and Toxoplasma  gondii. In this syndrome, an exacerbation of clinical  symptoms occurs because the antiretroviral drugs  enhance the ability to mount an inflammatory response.  HIV-infected patients with a low CD4 count have a  reduced capacity to produce inflammation, but HAART  restores the inflammatory response, and as a result,  symptoms become more pronounced. To avoid IRIS, the  coinfection should be treated prior to instituting HAART  whenever possible.   No vaccine is available. Multiple trials of a variety of  experimental vaccines have failed to induce protective  antibodies, protective cytotoxic T cells, or mucosal immu¬  nity. Prevention consists of taking measures to avoid  exposure to the virus (e.g., using condoms, not sharing  needles, and discarding donated blood that is contami¬  nated with HIV).   Postexposure prophylaxis (PEP), such as that given  after a needle-stick injury or a high-risk nonoccupational  exposure, employs three drugs: the preferred regimen con¬  sists of the combination of tenofovir and emtricitabine  (given as Truvada) plus raltegravir. Three alternative drug  regimens are available. PEP should be given as soon as pos¬  sible after exposure and continued for 28 days. Truvada can  also be used for preexposure prophylaxis (PrEP) in indi¬  viduals at high risk of infection, such as men who have sex  with men.   Two steps can be taken to reduce the number of cases of  HIV infection in children: antiretroviral therapy should be  given to HIV-infected mothers and neonates, and HIV-  infected mothers should not breast feed. The choice of  antiretroviral drugs is dependent on several factors, so cur¬  rent guidelines should be consulted. In addition, the risk of  neonatal HIV infection is lower if delivery is accomplished  by cesarean section rather than by vaginal delivery. Cir¬  cumcision reduces HIV infection.   Several drugs are commonly taken by patients in the  advanced stages of AIDS to prevent certain opportunistic  infections (Table 45-4). Some examples are trimethoprim-  sulfamethoxazole to prevent Pneumocystis pneumonia, flu¬  conazole to prevent recurrences of cryptococcal meningitis,    TABLE 45-4 Drugs Used for the Prevention of  Opportunistic Infections in AIDS Patients    ganciclovir to prevent recurrences of retinitis caused by  cytomegalovirus, and oral preparations of antifungal  drugs, such as clotrimazole, to prevent thrush caused by  C. albicans.   1. Regarding the structure and replication of human immuno¬  deficiency virus (HIV), which one of the following is most  accurate?   (A) Viral mRNA is the template for the synthesis of the genome  RNA.   (B) During entry of HIV into the cell, the viral p24 protein inter¬  acts with the CD4 protein on the cell surface.   (C) HIV contains an integrase within the virion that integrates cop¬  ies of the viral genome into the progeny virions.   (D) HIV has an enzyme in the virion that synthesizes double-  stranded DNA using the single-stranded genome RNA as the  template.   (E) The HIV genome encodes a protease that cleaves cellular ribo-  somal proteins, resulting in the inhibition of cell-specific pro¬  tein synthesis.   2. Regarding clinical aspects of human immunodeficiency virus  (HIV), which one of the following is most accurate?   (A) During the primary infection with HIV, Pneumocystis pneu¬  monia commonly occurs.   (B) During the long asymptomatic period that can last for years, no  HIV is synthesized.   (C) During the period when many opportunistic infections occur,  HIV usually cannot be detected in the blood.   (D) The antibody response to a primary HIV infection usually is  detected within 7 to 10 days after infection.   (E) People with a high level of viral RNA in their plasma are more  likely to have symptomatic AIDS (i.e., opportunistic infections)  than those with low levels.   3. Regarding the laboratory diagnosis of human immunodeficiency  virus (HIV), which one of the following is most accurate?   (A) The initial screening of blood for antibodies to HIV is done by  the complement fixation test.   (B) Viral load is the term used to describe the amount of infectious  virus produced by the patients CD4-positive T lymphocytes in  cell culture.    (C) After infection with HIV, antibodies to the virus can be  detected before the polymerase chain reaction (PCR) test can  detect nucleic acids specific to HIV.   (D) Because false-positive results occur in the screening test for  HIV, a confirmatory test called the Western blot assay should  be performed for those with a positive result on the screen¬  ing test.   4. Regarding the mode of action of drugs used in the treatment of  human immunodeficiency virus (HIV) infection, which one of  the following is most accurate?   (A) Maraviroc acts by inhibiting the reverse transcriptase in the  virion.   (B) Raltegravir inhibits the integration of HIV DNA into host cell  DNA.   (C) Zidovudine is a nucleoside analog that inhibits messenger RNA  (mRNA) synthesis of HIV.   (D) Ritonavir acts by binding to the Tat protein, which prevents  budding and release of the HIV virion.   (E) Lamivudine is a “chain-terminating” drug because it inhibits  the growing polypeptide chain by causing misreading of the  viral mRNA.   5. Regarding the adverse effects of drugs used in the treatment of  human immunodeficiency virus (HIV) infection, which one of  the following is most likely to cause bone marrow suppression?   6. Regarding the adverse effects of drugs used in the treatment of  human immunodeficiency virus (HIV) infection, which one of  the following is most likely to cause lipodystrophy (i.e., abnormal  fat deposits)?   7. Regarding the adverse effects of drugs used in the treatment  of human immunodeficiency virus (HIV) infection, which  one of the following is most likely to cause Stevens-Johnson  syndrome?   8. Which of the following modes of transmission of human immu¬  nodeficiency virus (HIV) occurs significantly MORE often than  the others?   9. Your patient is a 25-year-old man who was just found to be  infected with HIV based on a positive enzyme-linked immuno¬  sorbent assay (ELISA) and a positive Western blot test. His CD4  count is 125, and his viral load is 7000. He has not received any  antiretroviral medications. Which one of the following is the best  regimen to treat his infection?    (A) Acyclovir, foscarnet, and ribavirin SU A/1 A/1 ARIES OF ORG ANIS A/IS   (C) Lamivudine, ribavirin, and ritonavir/lopinavir Brief summaries of the organisms described in this chapter   (D) Zidovudine, lamivudine, and efavirenz begin on page 678. Please consult these summaries for a rapid   Questions on the topics discussed in this chapter can be found  in the Clinical Virology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 723. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   These viruses are presented in alphabetical order. They are  listed in Table 46-1 in terms of their nucleic acid and pres¬  ence of an envelope.   especially in children. Most adults have antibodies against  astroviruses, suggesting that infection occurs commonly.   Astroviruses are nonenveloped RNA viruses similar in size  to polioviruses. They have a characteristic five- or six-  pointed morphology. These viruses cause watery diarrhea,   BK virus is a member of the polyomavirus family. Poly-  omaviruses are nonenveloped viruses with a circular,  double-stranded DNA genome. BK virus and JC virus   Herpes B virus, human herpesvirus 6, poxviruses of animal origin (cowpox virus, monkeypox virus)   Borna virus, Cache Valley virus, hantaviruses, Heartland virus, Hendra virus, human metapneumovirus, Jamestown  Canyon virus, Japanese encephalitis virus, Lassa fever virus, Lujo virus, lymphocytic choriomeningitis virus, Nipah  virus, Powassan virus, spumaviruses, Tacaribe complex of viruses (e.g., Junin and Machupo viruses), Whitewater  Arroyo virus, Zika virus   BK virus infection is widespread as determined by the  presence of antibody and is typically acquired in child¬  hood, and infection is not associated with any disease at  that time. It does, however, cause nephropathy and graft  loss in immunosuppressed renal transplant patients.  Asymptomatic shedding of BK virus in the urine of immu¬  nocompromised patients and pregnant women in the third  trimester occurs. There is no antiviral therapy effective  against BK virus.   Borna virus is an enveloped virus with a nonsegmented,  single-strand, negative-polarity RNA genome. It has the  smallest genome of any virus with this type of RNA and is  the only virus of this type to replicate in the nucleus of the  infected cell. DNA sequences homologous to the Borna  virus genome are integrated into human cellular DNA. It is  a neurotropic virus known to infect regions of the brain  such as the hippocampus.   Borna is the name of a town in Germany where the virus  caused a disease in horses in 1885. It is primarily a zoonotic  virus causing disease in domestic animals, such as cattle,  sheep, dogs, and cats. In humans, Borna virus causes an  often fatal encephalitis. In addition, there is evidence that it  is associated with human psychiatric diseases characterized  by abnormal behavior, such as bipolar disorder.   This virus was first isolated in Utah in 1956 but is found  throughout the western hemisphere. It is a bunyavirus  transmitted by Aedes, Anopheles, or Culiseta mosquitoes  from domestic livestock to people. It is a rare cause of  encephalitis in humans. There is no treatment or vaccine  for Cache Valley virus infections.   Hantaviruses are members of the bunyavirus family. The  prototype virus is Hantaan virus, the cause of Korean hem¬  orrhagic fever (KHF). KHF is characterized by headache,  petechial hemorrhages, shock, and renal failure. It occurs  in Asia and Europe but not North America and has a mor¬  tality rate of about 10%. Hantaviruses are part of a hetero¬  geneous group of viruses called roboviruses, which stands  for “rodent-borne” viruses. Roboviruses are transmitted  from rodents directly (without an arthropod vector),  whereas arboviruses are “arthropod-borne.”   In 1993, an outbreak of a new disease, characterized by  influenzalike symptoms followed rapidly by acute respira¬  tory failure, occurred in the western United States, centered  in New Mexico and Arizona. This disease, now called    hantavirus pulmonary syndrome, is caused by a hantavirus  (Sin Nombre virus) endemic in deer mice ( Peromyscus ) and  is acquired by inhalation of aerosols of the rodents urine  and feces. It is not transmitted from person to person. Very  few people have antibody to the virus, indicating that  asymptomatic infections are not common.   The diagnosis is made by detecting viral RNA in lung  tissue with the polymerase chain reaction (PCR) assay, by  performing immunohistochemistry on lung tissue, or by  detecting IgM antibody in serum. The mortality rate of  hantavirus pulmonary syndrome is very high, approxi¬  mately 35%. Between 1993 and December 2009, a total of  534 cases of hantavirus pulmonary syndrome have been  reported in the United States. Most cases occurred in the  states west of the Mississippi, particularly in New Mexico,  Arizona, California, and Colorado, in that order.   There is no effective drug; ribavirin has been used but  appears to be ineffective. There is no vaccine for any  hantavirus.   This virus was first recognized as a human pathogen in  2012, when it caused fever, thrombocytopenia, and leuko¬  penia in two men in the state of Missouri. It is a member of  the bunyavirus family. It is transmitted by the bite of the  Lone Star tick, Amblyomma. There is no antiviral treatment  or vaccine for this virus.   This virus was first recognized as a human pathogen in  1994, when it caused severe respiratory disease in Hendra,  Australia. It is a paramyxovirus resembling measles virus  and was previously called equine morbillivirus. The human  infections were acquired by contact with infected horses,  but fruit bats appear to be the natural reservoir. There is no  treatment or vaccine for Hendra virus infections.   This virus (monkey B virus or herpesvirus simiae) causes a  rare, often fatal encephalitis in persons in close contact  with monkeys or their tissues (e.g., zookeepers or cell cul¬  ture technicians). The virus causes a latent infection in  monkeys that is similar to herpes simplex virus (HSV)-l  infection in humans.   Herpes B virus and HSV-1 cross-react antigenically, but  antibody to HSV-1 does not protect from herpes B enceph¬  alitis. The presence of HSV-1 antibody can, however, con¬  fuse serologic diagnosis by making the interpretation of a  rise in antibody titer difficult. The diagnosis can therefore  be made only by recovering the virus. Acyclovir may be  beneficial. Prevention consists of using protective clothing  and masks to prevent exposure to the virus. Immune    globulin containing antibody to herpes B virus should be  given after a monkey bite.   Human bocavirus (HBoV) is a parvovirus isolated from  young children with respiratory tract infections. Antibody  to HBoV is found in most adults worldwide. A description  of this virus was first reported in 2005, and its precise role  in respiratory tract disease has yet to be defined.   This herpesvirus is the cause of exanthem subitum (rose¬  ola infantum), a common disease in infants that is charac¬  terized by a high fever and a transient macular or  maculopapular rash. The virus is found worldwide, and up  to 80% of people are seropositive. The virus is lympho-  tropic and infects both T and B cells. It remains latent  within these cells but can be reactivated in immunocom¬  promised patients and cause pneumonia. Many virologic  and clinical features of human herpesvirus 6 are similar to  those of cytomegalovirus, another member of the herpes¬  virus family.   This paramyxovirus was first reported in 2001 as a cause  of severe bronchiolitis and pneumonia in young children  in the Netherlands. It is similar to respiratory syncytial  virus (also a paramyxovirus) in the range of respiratory  tract disease it causes. Serologic studies showed that most  children have been infected by 5 years of age and that this  virus has been present in the human population for at least  50 years.   Jamestown Canyon virus (JCV) is a member of the bunya-  virus family that causes encephalitis. It is transmitted by  mosquito bite, most commonly by Aedes species. JCV cir¬  culates widely among deer in North America but human  disease is rare. In the United States, cases are primarily in  the northeastern and midwestern states. There is no antivi¬  ral treatment or vaccine for JCV infections.   This virus is the most common cause of epidemic enceph¬  alitis in rural areas of Asia. The disease is characterized by  fever, headache, nuchal rigidity, altered states of conscious¬  ness, tremors, incoordination, and convulsions. The mor¬  tality rate is high, and neurologic sequelae are severe and  can be detected in most survivors. The disease occurs    throughout Asia but is most prevalent in Southeast Asia.  The rare cases seen in the United States have occurred in  travelers returning from that continent. American military  personnel in Asia have been affected.   Japanese encephalitis virus is a member of the flavivirus  family. It is transmitted to humans by certain species of  Culex mosquitoes endemic to Asian rice fields. There are  two main reservoir hosts—birds and pigs. The diagnosis  can be made by isolating the virus, by detecting IgM anti¬  body in serum or spinal fluid, or by staining brain tissue  with fluorescent antibody. There is no antiviral therapy.  Prevention consists of an inactivated vaccine and pesticides  to control the mosquito vector. Immunization is recom¬  mended for individuals living in areas of endemic infection  for several months or longer.   Lassa fever virus was first seen in 1969 in the Nigerian town  of that name. It causes a severe, hemorrhagic fever charac¬  terized by multiorgan involvement. The disease begins  slowly with fever, headache, vomiting, and diarrhea and  progresses to involve the lungs, heart, kidneys, and brain. A  petechial rash and gastrointestinal tract hemorrhage ensue,  followed by death from vascular collapse. The fatality rate  is approximately 20%.   Lassa fever virus is a member of the arenavirus family,  which includes other infrequent human pathogens such as  lymphocytic choriomeningitis virus and certain members  of the Tacaribe group. Arenaviruses (“arena” means sand)  are united by their unusual appearance in the electron  microscope. Their most striking feature is the “sandlike”  particles on their surface, which are ribosomes. The func¬  tion, if any, of these ribosomes is unknown. Arenaviruses  are enveloped viruses with surface spikes, a helical nucleo-  capsid, and single-stranded RNA with negative polarity.   The natural host for Lassa fever virus is the small rodent  Mastomys, which undergoes a chronic, lifelong infection.  The virus is transmitted to humans by contamination of  food or water with animal urine. Secondary transmission  among hospital personnel occurs also. Asymptomatic  infection is widespread in areas of endemic infection.   The diagnosis is made either by isolating the virus or by  detecting a rise in antibody titer. Ribavirin reduces the  mortality rate if given early, and hyperimmune serum,  obtained from persons who have recovered from the dis¬  ease, has been beneficial in some cases. No vaccine is avail¬  able, and prevention centers on proper infection control  practices and rodent control.   Lujo virus is an arenavirus that causes a hemorrhagic fever  similar to Lassa fever. This virus emerged in Zambia in  2008 and caused an outbreak in which four of the five    infected patients died. The one survivor was treated with  ribavirin. The identification of this virus was made by  sequencing the viral RNA from the liver and serum of  patients. The animal reservoir and mode of transmission  are unknown, but other arenaviruses are transmitted by  rodent excreta.   Lymphocytic choriomeningitis virus is a member of the  arenavirus family. It is a rare cause of aseptic meningitis  and cannot be distinguished clinically from the more fre¬  quent viral causes (e.g., echovirus, Coxsackie virus, or  mumps virus). The usual picture consists of fever, head¬  ache, vomiting, stiff neck, and changes in mental status.  Spinal fluid shows an increased number of cells, mostly  lymphocytes, with an elevated protein level and a normal or  low sugar level.   The virus is endemic in the mouse population, in  which chronic infection occurs. Animals infected trans-  placentally become healthy lifelong carriers. The virus is  transmitted to humans via food or water contaminated by  mouse urine or feces. There is no human-to-human  spread (i.e., humans are accidental dead-end hosts),  although transmission of the virus via solid organ trans¬  plants has occurred. In 2005, seven of eight transplant  recipients who became infected died. Diagnosis is made  by isolating the virus from the spinal fluid or by detecting  an increase in antibody titer. No antiviral therapy or vac¬  cine is available.   This disease is the prototype used to illustrate immuno-  pathogenesis. If immunocompetent adult mice are inocu¬  lated, meningitis and death ensue. If, however, newborn  mice or X-irradiated immunodeficient adults are inocu¬  lated, no meningitis occurs despite extensive viral replica¬  tion. If sensitized T cells are transplanted to the  immunodeficient adults, meningitis and death occur. The  immunodeficient adult mice, who are apparently well,  slowly develop glomerulonephritis. It appears that the mice  are partially tolerant to the virus in that their cell-mediated  immunity is inactive, but sufficient antibody is produced to  cause immune complex disease.   Nipah virus is a paramyxovirus that causes encephalitis,  primarily in the South Asian countries of Bangladesh,  Malaysia, and Singapore. The natural reservoir appears to  be fruit bats. People who have contact with pigs are particu¬  larly at risk for encephalitis, and some human-to-human  transmission occurs. In general, paramyxoviruses are  transmitted by saliva or sputum and that is the likely mode    of transmission. There is no treatment, or vaccine for  Nipah virus infections.   Powassan virus is a flavivirus that causes severe encephali¬  tis with significant sequelae. It is transmitted by Ixodes  ticks, and rodents are the reservoir. It is the only flavivirus  transmitted by ticks.   It is named for the town of Powassan, Ontario, Canada,  where one of the first cases occurred. Most cases in the  United States occur in Minnesota and Wisconsin. There are  typically 0 to 10 cases in the United States each year. The  diagnosis can be made by PCR or serologic tests. There is  no antiviral drug or vaccine.   Four poxviruses cause disease in animals and also cause  poxlike lesions in humans on rare occasions. They are  transmitted by contact with the infected animals, usually in  an occupational setting.   Cowpox virus causes vesicular lesions on the udders of  cows and can cause similar lesions on the skin of persons  who milk cows. Pseudocowpox virus causes a similar pic¬  ture but is antigenically distinct. Orf virus is the cause of  contagious pustular dermatitis in sheep and of vesicular  lesions on the hands of sheepshearers.   Monkeypox virus is different from the other three; it  causes a human disease that resembles smallpox. It occurs  almost exclusively in Central Africa. In 2003, an outbreak  of monkeypox occurred in Wisconsin, Illinois, and Indi¬  ana. In this outbreak, the source of the virus was animals  imported from Africa. It appears that the virus from the  imported animals infected local prairie dogs, which then  were the source of the human infection. None of those  affected died. In Africa, monkeypox has a death rate of  between 1% and 10%, in contrast to 50% for smallpox.  There is no effective antiviral treatment. The vaccine  against smallpox appears to have some protective effect  against monkeypox.   Any new case of smallpox-like disease must be pre¬  cisely diagnosed to ensure that it is not due to smallpox  virus. There has not been a case of smallpox in the world  since 1977, 1 and smallpox immunization has been allowed  to lapse.   For these reasons, it is important to ensure that new  cases of smallpox-like disease are due to monkeypox virus.  Monkeypox virus can be distinguished from smallpox virus  in the laboratory both antigenically and by the distinctive  lesions it causes on the chorioallantoic membrane of  chicken eggs.   Sapporo virus is a calicivirus that causes acute gastroenteri¬  tis, primarily in adults. The main symptoms are vomiting  and diarrhea. It is transmitted by ingestion of food or water  contaminated with human feces. There is no antiviral drug  or vaccine. It is named after Sapporo, Japan, the city where  it was first isolated.   Spumaviruses are a subfamily of retroviruses that cause a  foamy appearance in cultured cells. They can present a  problem in the production of viral vaccines if they con¬  taminate the cell cultures used to make the vaccine. There  are no known human pathogens.   The Tacaribe complex contains several human pathogens,  all of which cause hemorrhagic fever.   The best known are Sabia virus in Brazil, Junin virus in  Argentina, and Machupo virus in Bolivia. Hemorrhagic fevers,  as the name implies, are characterized by fever and bleeding  into the gastrointestinal tract, skin, and other organs. The  bleeding is due to thrombocytopenia. Death occurs in up to  20% of cases, and outbreaks can involve thousands of people.  Agricultural workers are particularly at risk.   Similar to other arenaviruses such as Lassa fever virus  and lymphocytic choriomeningitis virus, these viruses are  endemic in the rodent population and are transmitted to  humans by accidental contamination of food and water by  rodent excreta. The diagnosis can be made either by isolat¬  ing the virus or by detecting a rise in antibody titer. In a  laboratory-acquired Sabia virus infection, ribavirin was an  effective treatment. No vaccine is available.   This virus is the cause of a hemorrhagic fever/acute  respiratory distress syndrome in the western part of the  United States. It is a member of the arenavirus family, as is  Lassa fever virus, a cause of hemorrhagic fever in Africa  (see page 393). Wood rats are the reservoir of this virus,  and it is transmitted by inhalation of dried rat excrement.  This mode of transmission is the same as that of the hanta¬  virus, Sin Nombre virus (see page 392). There is no estab¬  lished antiviral therapy, and there is no vaccine.   Zika virus (ZIKV) is a flavivirus that causes an illness  similar to dengue characterized by fever, arthralgia,  maculopapular rash, and non-purulent conjunctivitis.    Infection in pregnant women can cause serious fetal  abnormalities, including microcephaly. Infection also  predisposes to Guillain-Barre syndrome. Approximately  80% of infections are asymptomatic. The typical illness  lasts a few days to a week. Most symptomatic adults  recover without sequelae.   The vector is the Aedes mosquito and the vertebrate  hosts are non-human primates and humans. Almost all  infections are transmitted by mosquitoes but semen can  contain the virus and sexual transmission occurs.   There is no confirmed report of transmission by blood  transfusion or breast milk.   Disease caused by ZIKV was very rare until 2007  when an outbreak occurred on Yap, an island in  Micronesia. In 2015, ZIKV caused outbreaks in the  Caribbean and South America. By March 2016, ZIKV  had spread to 33 countries in the Americas, causing  disease in thousands of people.   The diagnosis is established either by finding viral RNA  in the serum using a PCR assay or by finding IgM antibody  in the serum using an ELISA test. Interpretation of the  antibody test is complicated by cross-reacting antibody in  people infected with other flaviviruses, such as dengue  virus, or who have received the yellow fever vaccine. Treat¬  ment is symptomatic. There is no effective antiviral drug  and no vaccine against ZIKV.   The main goal of prevention is to eliminate infection  in pregnant women. Prevention involves protective  clothing, mosquito repellent, bed nets, and screened  windows. Pregnant women should not travel to areas  where outbreaks are occurring. Men who may have  become infected should use condoms or refrain from  sexual intercourse.   1. Regarding Sin Nombre virus (a hantavirus), which one of the fol¬  lowing is most accurate?   (C) Infection is acquired by inhalation of dried mouse feces and  urine.   (D) Oseltamivir is an effective prophylactic drug if given within  48 hours of exposure.   (E) Immunization of children at the age of 15 months with the  killed vaccine has greatly reduced the incidence of disease.   2. Regarding Japanese encephalitis virus (JEV), which one of the fol¬  lowing is most accurate?   (B) It is transmitted by the bite of the dog tick, Dermacentor.   (C) Acyclovir is the drug of choice for encephalitis caused by  JEV.   (D) The killed vaccine should be given to those living in an  endemic area.   (E) JEV is a nonenveloped virus with a circular double-stranded  RNA genome.   Brief summaries of the organisms described in this chapter  begin on page 678. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Clinical Virology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 723. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   Because fungi (yeasts and molds) are eukaryotic organ¬  isms, whereas bacteria are prokaryotic, they differ in sev¬  eral fundamental respects (Table 47-1). Two fungal cell  structures are important medically:   (1) The fungal cell wall consists primarily of chitin (not  peptidoglycan as in bacteria); thus fungi are insensitive to  certain antibiotics, such as penicillins and cephalosporins,  that inhibit peptidoglycan synthesis.   (2) Chitin is a polysaccharide composed of long chains  of N-acetylglucosamine. The fungal cell wall contains other  polysaccharides as well, the most important of which is  (3-glucan, a long polymer of D-glucose. The medical impor¬  tance of (3-glucan is that it is the site of action of the anti¬  fungal drug caspofungin.   (3) The fungal cell membrane contains ergosterol, in  contrast to the human cell membrane, which contains cho¬  lesterol. The selective action of amphotericin B and azole  drugs, such as fluconazole and ketoconazole, on fungi is  based on this difference in membrane sterols.   There are two types of fungi: yeasts and molds. Yeasts  grow as single cells that reproduce by asexual budding.    Molds grow as long filaments (hyphae) and form a mat  (mycelium). Some hyphae form transverse walls (septate  hyphae), whereas others do not (nonseptate hyphae).  Nonseptate hyphae are multinucleated (coenocytic). The  growth of hyphae occurs by extension of the tip of the  hypha, not by cell division all along the filament.   Several medically important fungi are thermally dimor¬  phic (i.e., they form different structures at different tem¬  peratures). They exist as molds in the environment at  ambient temperature and as yeasts (or other structures) in  human tissues at body temperature.   Most fungi are obligate aerobes; some are facultative  anaerobes; but none are obligate anaerobes. All fungi  require a preformed organic source of carbon—hence their  frequent association with decaying matter. The natural  habitat of most fungi is, therefore, the environment. An  important exception is Candida albicans, which is part of  the normal human flora.   Some fungi reproduce sexually by mating and forming  sexual spores (e.g., zygospores, ascospores, and basidio-  spores). Zygospores are single large spores with thick walls;  ascospores are formed in a sac called ascus; and basidio-  spores are formed externally on the tip of a pedestal called  a basidium. The classification of these fungi is based on  their sexual spores. Fungi that do not form sexual spores  are termed “imperfect” and are classified as fungi  imperfecti.   Most fungi of medical interest propagate asexually by  forming conidia (asexual spores) from the sides or ends of  specialized structures (Figure 47-1). The shape, color, and  arrangement of conidia aid in the identification of fungi.   FIGURE 47-1 Asexual spores. A: Blastoconidia and pseudohy-  phae (Candida). B: Chlamydospores (Candida). C: Arthrospores  (Coccidioides). D: Sporangia and sporangiospores (Mucor). E: Microco-  nidia (Aspergillus). F: Microconidia and macroconidia (Microsporum).  (Reproduced with permission from Conant NF et al. Manual of Clinical Mycology.    Some important conidia are (1) arthrospores, 1 which arise  by fragmentation of the ends of hyphae and are the mode  of transmission of Coccidioides immitis; (2) chlamydo¬  spores, which are rounded, thick-walled, and quite resis¬  tant (the terminal chlamydospores of C. albicans aid in its  identification); (3) blastospores, which are formed by the  budding process by which yeasts reproduce asexually  (some yeasts, e.g., C. albicans, can form multiple buds that  do not detach, thus producing sausagelike chains called  pseudohyphae, which can be used for identification); and  (4) sporangiospores, which are formed within a sac (spo¬  rangium) on a stalk by molds such as Rhizopus and Mucor.   Although this book focuses on the fungi that are human  pathogens, it should be remembered that fungi are used in  the production of important foods (e.g., bread, cheese,  wine, and beer). Fungi are also responsible for the spoilage  of certain foods. Because molds can grow in a drier, more  acidic, and higher osmotic pressure environment than bac¬  teria, they tend to be involved in the spoilage of fruits,  grains, vegetables, and jams.   The response to infection with many fungi is the formation of  granulomas. Granulomas are produced in the major systemic  fungal diseases (e.g., coccidioidomycosis, histoplasmosis, and  blastomycosis, as well as several others). The cell-mediated  immune response is involved in granuloma formation. Acute  suppuration, characterized by the presence of neutrophils in the  exudate, also occurs in certain fungal diseases such as aspergil¬  losis and sporotrichosis. Fungi do not have endotoxin in their  cell walls and do not produce bacterial-type exotoxins.   Activation of the cell-mediated immune system results  in a delayed hypersensitivity skin test response to certain  fungal antigens injected intradermally. A positive skin test  indicates exposure to the fungal antigen. It does not imply  current infection, because the exposure may have occurred  in the past. A negative skin test makes the diagnosis  unlikely unless the patient is immunocompromised.  Because most people carry Candida as part of the normal  flora, skin testing with Candida antigens can be used to  determine whether cell-mediated immunity is normal.   The transmission and geographic locations of some  important fungi are described in Table 47-2.   Intact skin is an effective host defense against certain  fungi (e.g., Candida, dermatophytes), but if the skin is dam¬  aged, organisms can become established. Fatty acids in the  skin inhibit dermatophyte growth, and hormone-associ¬  ated skin changes at puberty limit ringworm of the scalp  caused by Trichophyton. The normal flora of the skin and  mucous membranes suppress fungi. When the normal flora  is inhibited (e.g., by antibiotics), overgrowth of fungi such  as C. albicans can occur.    1 The term spores can be replaced with conidia (e.g., arthroconidia).   Mississippi and Ohio River valleys in  United States; many other  countries    In the respiratory tract, the important host defenses are the  mucous membranes of the nasopharynx, which trap inhaled  fungal spores, and alveolar macrophages. Circulating IgG and  IgM are produced in response to fungal infection, but their  role in protection from disease is uncertain. The cell-mediated  immune response is protective; its suppression can lead to  reactivation and dissemination of asymptomatic fungal infec¬  tions and to disease caused by opportunistic fungi.   In addition to mycotic infections, there are two other kinds  of fungal disease: (1) mycotoxicoses, caused by ingested  toxins, and (2) allergies to fungal spores. The best-known  mycotoxicosis occurs after eating Amanita mushrooms.  These fungi produce five toxins, two of which—amanitin  and phalloidin—are among the most potent hepatotoxins.  The toxicity of amanitin is based on its ability to inhibit cel¬  lular RNA polymerase, which prevents mRNA synthesis.  Another mycotoxicosis, ergotism, is caused by the mold  Claviceps purpurea, which infects grains and produces alka¬  loids (e.g., ergotamine and lysergic acid diethylamide [LSD])  that cause pronounced vascular and neurologic effects.   Other ingested toxins, aflatoxins, are coumarin derivatives  produced by Aspergillus flavus that cause liver damage and  tumors in animals and are suspected of causing hepatic carci¬  noma in humans. Aflatoxins are ingested with spoiled grains  and peanuts and are metabolized by the liver to the epoxide, a  potent carcinogen. Aflatoxin B1 induces a mutation in the p53  tumor suppressor gene, leading to a loss of p53 protein and a  consequent loss of growth control in the hepatocyte.   Allergies to fungal spores, particularly those of Aspergil¬  lus, are manifested primarily by an asthmatic reaction  (rapid bronchoconstriction mediated by IgE), eosinophilia,  and a “wheal and flare” skin test reaction. These clinical  findings are caused by an immediate hypersensitivity  response to the fungal spores.   There are four approaches to the laboratory diagnosis of  fungal diseases: (1) direct microscopic examination, (2) cul¬  ture of the organism, (3) DNA probe tests, and (4) serologic  tests. Direct microscopic examination of clinical specimens  such as sputum, lung biopsy material, and skin scrapings  depends on finding characteristic asexual spores, hyphae, or  yeasts in the light microscope. The specimen is either treated  with 10% potassium hydroxide (KOH) to dissolve tissue  material, leaving the alkali-resistant fungi intact, or stained  with special fungal stains. Some examples of diagnostically  important findings made by direct examination are (1) the  spherules of C. immitis and (2) the wide capsule of Crypto¬  coccus neoformans seen in India ink preparations of spinal  fluid. Calcofluor white is a fluorescent dye that binds to  fungal cell walls and is useful in the identification of fungi in  tissue specimens. Methenamine silver stain is also useful in  the microscopic diagnosis of fungi in tissue.   Fungi are frequently cultured on Sabouraud’s agar,  which facilitates the appearance of the slow-growing fungi  by inhibiting the growth of bacteria in the specimen. Inhi¬  bition of bacterial growth is due to the low pH of the  medium and to the penicillin, streptomycin, and cyclohexi-  mide that are frequently added. The appearance of the  mycelium and the nature of the asexual spores are fre¬  quently sufficient to identify the organism.   Tests involving DNA probes can identify colonies grow¬  ing in culture at an earlier stage of growth than can tests  based on visual detection of the colonies. As a result, the  diagnosis can be made more rapidly. At present, DNA  probe tests are available for Coccidioides, Histoplasma, Blas¬  tomyces, and Cryptococcus.   Tests for the presence of antibodies in the patient’s  serum or spinal fluid are useful in diagnosing systemic  mycoses but less so in diagnosing other fungal infections.  As is the case for bacterial and viral serologic testing,    TABLE 47-3 Mechanism of Action and Adverse Effects of Antifungal Drugs   Renal toxicity, fever, and chills; moni¬  tor kidney function; use test dose;  liposomal preparation reduces  toxicity   Ketoconazole inhibits human cyto¬  chrome P450; this decreases synthe¬  sis of gonadal steroids resulting in  gynecomastia    a significant rise in the antibody titer must be observed to  confirm a diagnosis. The complement fixation test is most  frequently used in suspected cases of coccidioidomycosis,  histoplasmosis, and blastomycosis. In cryptococcal menin¬  gitis, the presence of the polysaccharide capsular antigens  of C. neoformans in the spinal fluid can be detected by the  latex agglutination test.   The drugs used to treat bacterial diseases have no effect on  fungal diseases. For example, penicillins and aminoglyco¬  sides inhibit the growth of many bacteria but do not affect  the growth of fungi. This difference is explained by the  presence of certain structures in bacteria (e.g., peptidogly-  can and 70S ribosomes) that are absent in fungi.    The most effective antifungal drugs, amphotericin B  and the various azoles, exploit the presence of ergosterol  in fungal cell membranes that is not found in bacterial or  human cell membranes. Amphotericin B (Fungizone)  disrupts fungal cell membranes at the site of ergosterol  and azole drugs inhibit the synthesis of ergosterol, which  is an essential component of fungal membranes. Another  antifungal drug, caspofungin (Cancidas), inhibits the  synthesis of (3-glucan, which is found in fungal cell walls  but not in bacterial cell walls. Human cells do not have a  cell wall.   The mode of action of these drugs is described in  Chapter 10. Table 47-3 summarizes the mode of action and  the important adverse effects of the major antifungal drugs.  Clinically significant resistance to antifungal drugs is uncom¬  mon. Resistance to azole drugs is rare but is increasing.   • Fungi are eukaryotic organisms that exist in two basic forms:  yeasts and molds. Yeasts are single cells, whereas molds con¬  sist of long filaments of cells called hyphae. Yeasts reproduce  by budding, a process in which the daughter cells are unequal  in size, whereas molds reproduce by cell division (daughter  cells are equal in size).    Some fungi are dimorphic (i.e., they can exist either as yeasts  or molds, depending on the temperature). At room tempera¬  ture (e.g., 25°C), dimorphic fungi are molds, whereas at body  temperature they are yeasts (or some other form such as a  spherule).   The fungal cell wall is made of chitin; the bacterial cell wall is  made of peptidoglycan. Therefore, antibiotics that inhibit    peptidoglycan synthesis such as penicillins, cephalosporins,  and vancomycin are not effective against fungi.   • The fungal cell membrane contains ergosterol, whereas the  bacterial cell membrane does not contain ergosterol. There¬  fore, antibiotics that inhibit ergosterol synthesis (e.g., the azole  drugs) are not effective against bacteria. Similarly, amphoteri¬  cin B that binds to fungal cell membranes at the site of ergos¬  terol is not effective against bacteria.   • Infection with certain systemic fungi, such as Histoplasma and  Coccidioides, elicits a granulomatous host defense response   (composed of macrophages and helper T cells). Infection with  other fungi, notably Aspergillus, Mucor, and Sporothrix, elicits a  pyogenic response (composed of neutrophils).   • Infection with the systemic fungi, such as Histoplasma and Coc¬  cidioides, can be detected by using skin tests. An antigen  extracted from the organism injected intradermally elicits a  delayed hypersensitivity reaction, manifested as induration  (thickening) of the skin. Note that a positive skin test only indi¬  cates that infection has occurred, but it is not known whether  that infection occurred in the past or at the present time.  Therefore, a positive skin test does not indicate that the disease  the patient has now is caused by that organism. Note also that  a false-negative skin test can occur in patients with reduced  cell-mediated immunity, such as those with a low CD4 count.  To determine whether the patient can mount a delayed hyper¬  sensitivity response, a control skin test with a common antigen,  such as C. albicans, can be used.   • Reduced cell-mediated immunity predisposes to disseminated  disease caused by the systemic fungi, such as Histoplasma and  Coccidioides, whereas a reduced number of neutrophils predis¬  poses to disseminated disease caused by fungi such as Asper¬  gillus and Mucor.   • Ingestion of Amanita mushrooms causes liver necrosis due to  the presence of two fungal toxins, amanitin and phalloidin.   • Ingestion of peanuts and grains contaminated with A. flavus  causes liver cancer due to the presence of aflatoxin. Aflatoxin  epoxide induces a mutation in the p53 gene that results in a  loss of the p53 tumor suppressor protein.   • Inhalation of the spores of Aspergillus fumigatus can cause  allergic bronchopulmonary aspergillosis. This is an IgE-  mediated immediate hypersensitivity response.   • Microscopic examination of a KOH preparation can reveal the  presence of fungal structures. The purpose of the KOH is to dis¬  solve the human cells, allowing visualization of the fungi.   • Sabouraud's agar is often used to grow fungi because its low  pH inhibits the growth of bacteria, allowing the slower-grow¬  ing fungi to emerge.   • DNA probes can be used to identify fungi growing in culture at  a much earlier stage (i.e., when the colony size is much smaller).   • Tests for the presence of fungal antigens and for the presence  of antibodies to fungal antigens are often used. Two commonly  used tests are those for cryptococcal antigen in spinal fluid and  for Coccidioides antibodies in the patient's serum.   • The selective toxicity of amphotericin B and the azole group of  drugs is based on the presence of ergosterol in fungal cell mem¬  branes, in contrast to the cholesterol found in human cell mem¬  branes and the absence of sterols in bacterial cell membranes.   • Amphotericin B binds to fungal cell membranes at the site of  ergosterol and disrupts the integrity of the membranes.   • Azole drugs, such as itraconazole, fluconazole, and ketocon-  azole, inhibit the synthesis of ergosterol.   • The selective toxicity of echinocandins, such as caspofungin, is  based on the presence of a cell wall in fungi, whereas human  cells do not have a cell wall. Echinocandins inhibit the synthesis  of D-glucan, which is a component of the fungal cell wall.   1. Regarding the structure and reproduction of fungi, which one of   (A) Peptidoglycan is an important component of the cell wall of  fungi.   (B) Molds are fungi that grow as single cells and reproduce by  budding.   (C) Some fungi are dimorphic (i.e., they are yeasts at room tem¬  perature and molds at body temperature).   (D) The fungal cell membrane contains ergosterol, whereas the  human cell membrane contains cholesterol.   (E) As most fungi are anaerobic, they should be cultured under  anaerobic conditions in the clinical laboratory.    2. Regarding fungal pathogenesis, which one of the following is most   (B) The host response to infection by the systemic fungi, such as  Histoplasma and Coccidioides , consists of granulomas  formation.   (C) The fever seen in systemic fungal infections is caused by  endotoxin-induced release of interlukin-1.   (D) Ingestion of aflatoxin produced by Aspergillus flavus can cause  adenocarcinoma of the colon.   (E) A positive result in the skin test to fungal antigens, such as coc-  cidioidin, is caused by an immediate hypersensitivity reaction.    3. Regarding the mode of action of antifungal drugs, which one of   (A) Azole drugs, such as fluconazole, act by inhibiting ergosterol  synthesis.   (B) Amphotericin B acts by inhibiting fungal protein syntheses at  the 40S ribosomal subunit.   (C) Terbinafine acts by inhibiting fungal DNA synthesis but has no  effect on DNA synthesis in human cells.   (D) Echinocandins, such as caspofungin, act by inhibiting messen¬  ger RNA synthesis in yeasts but not in molds.   Questions on the topics discussed in this chapter can be found  in the Mycology section of Part XIII: USMLE (National Board)  Practice Questions starting on page 728. Also see Part XIV:  USMLE (National Board) Practice Examination starting on  page 751.   Medical mycoses can be divided into four categories: (1)  cutaneous, (2) subcutaneous, (3) systemic, and (4)  opportunistic. Some features of the important fungal  diseases are described in Table 48-1. Cutaneous and    subcutaneous mycoses are discussed in this chapter, and  important features of the causative organisms are  described in Table 48-2. The systemic and opportunistic  mycoses are discussed in Chapters 49 and 50,  respectively.    1 1 + = not serious, treatment may or may not be given; 2+ = moderately serious, treatment often given; 4+ = serious, treatment given especially in disseminated disease.   Tinea capitis, tinea pedis, etc.,  "ringworm" Ring of inflammatory,  pruritic vesicles with a healing  center   Potassium hydroxide (KOH)  prep shows septate  hyphae culture on Sab-  ouraud's agar   Tinea capitis, tinea pedis, etc.,  "ringworm” Ring of inflammatory,  pruritic vesicles with a healing center   KOH prep shows cigar¬  shaped yeasts culture at  20°C shows hyphae with  daisy-like conidia   Dermatophytoses are caused by fungi (dermatophytes)  that infect only superficial keratinized structures (skin,  hair, and nails), not deeper tissues. The most important  dermatophytes are classified in three genera: Trichophyton,  Epidermophyton, and Microsporum. They are spread from  infected persons by direct contact. Microsporum is also  spread from animals such as dogs and cats. This indicates  that to prevent reinfection, the animal must be treated also.   Dermatophytoses (tinea, ringworm) are chronic infec¬  tions often located in the warm, humid areas of the body  (e.g., athlete’s foot and jock itch). 1 Typical ringworm lesions  have an inflamed circular border containing papules and  vesicles surrounding a clear area of relatively normal skin.  The lesions are typically pruritic. Broken hairs and dam¬  aged nails are often seen. The disease is typically named for  the affected body part (i.e., tinea capitis [head], tinea cor¬  poris [body], tinea cruris [groin], and tinea pedis [foot])  (Figure 48-1). Tinea unguium, also called onychomycosis,  is a disease of the nails, especially toe nails. The nails  become thickened, broken, and discolored.   Trichophyton tonsurans is the most common cause of out¬  breaks of tinea capitis in children and is the main cause of  endothrix (inside the hair) infections. Trichophyton rubrum is  also a very common cause of tinea capitis. Trichophyton  schoenleinii is the cause of favus, a form of tinea capitis in  which crusts are seen on the scalp. Trichophyton species also  cause an inflammatory pustular lesion on the scalp called a  kerion. The marked inflammation is caused by an intense  T-cell-mediated reaction to the presence of the fungus.   In some infected persons, hypersensitivity causes dermato-  phytid (“id”) reactions (e.g., vesicles on the fingers). Id lesions  are a response to circulating fungal antigens; the lesions do not    1 These infections are also known as tinea pedis and tinea cruris,  respectively.    contain hyphae. Patients with tinea infections show positive  skin tests with fungal extracts (e.g., trichophytin).   Scrapings of skin or nail placed in 10% potassium  hydroxide (KOH) on a glass slide show septate hyphae  under microscopy. Cultures on Sabouraud’s agar at room  temperature develop typical hyphae and conidia. Tinea  capitis lesions caused by Microsporum species can be  detected by seeing fluorescence when the lesions are  exposed to ultraviolet light from a Wood’s lamp.   Treatment involves local antifungal creams, such as (ter-  binafine (Lamisil), undecylenic acid (Desenex), micon¬  azole (Micatin), or tolnaftate (Tinactin). Oral griseofulvin  (Fulvicin) or oral itraconazole (Sporanox) can also be used.  Tinea unguium can be treated with efinaconazole solution  applied topically to the nails. Prevention centers on keeping  skin dry and cool.   FIGURE 48-1 Tinea corporis (ringworm). Note oval, ring-shaped  inflamed lesion with central clearing. Caused by dermatophytes such as  Epidermophyton, Trichophyton, and Microsporum. (Reproduced with permis¬  sion from Fauci AS, Braunwald E, Kasper DL et al, eds. Harrison's Principles of Internal Med¬  icine. 17th ed. New York: McGraw-Hill, 2008. Copyright © 2008 by The McGraw-Hill  Companies, Inc.)   Tinea versicolor (pityriasis versicolor), a superficial skin  infection of cosmetic importance only, is caused by Malas-  sezia species. The lesions are usually noticed as hypopig-  mented areas, especially on tanned skin in the summer.  There may be slight scaling or itching, but usually the infec¬  tion is asymptomatic. It occurs more frequently in hot,  humid weather. The lesions contain both budding yeast  cells and hyphae. Diagnosis is usually made by observing  this mixture in KOH preparations of skin scrapings. Cul¬  ture is not usually done. The treatment of choice is topical  miconazole, but the lesions have a tendency to recur. Oral  antifungal drugs, such as fluconazole or itraconazole, can  be used to treat recurrences.   Tinea nigra is an infection of the keratinized layers of the  skin. It appears as a brownish spot caused by the melanin¬  like pigment in the hyphae. The causative organism, Clado-  sporium werneckii, is found in the soil and transmitted  during injury. In the United States, the disease is seen in the  southern states. Diagnosis is made by microscopic exami¬  nation and culture of skin scrapings. The infection is  treated with a topical keratolytic agent (e.g., salicylic acid).    In human immunodeficiency virus (HlV)-infected  patients with low CD4 counts, disseminated sporotrichosis  can occur. Sporotrichosis occurs most often in gardeners,  especially those who prune roses, because they may be  stuck by a rose thorn.   In the clinical laboratory, round or cigar-shaped bud¬  ding yeasts are seen in tissue specimens. In culture at room  temperature, hyphae occur bearing oval conidia in clusters  at the tip of slender conidiophores (resembling a daisy).  The drug of choice for skin lesions is itraconazole (Spo-  ranox). It can be prevented by protecting skin when touch¬  ing plants, moss, and wood.   This is a slowly progressive granulomatous infection that is  caused by several soil fungi ( Fonsecaea, Phialophora, Clado-  sporium, etc.) when introduced into the skin through  trauma. These fungi are collectively called dematiaceous  fungi, so named because their conidia or hyphae are dark-  colored, either gray or black. Wartlike lesions with crusting  abscesses extend along the lymphatics. The disease occurs  mainly in the tropics and is found on bare feet and legs. In  the clinical laboratory, dark brown, round fungal cells are  seen in leukocytes or giant cells. The disease is treated with  oral flucytosine or thiabendazole, plus local surgery.   These are caused by fungi that grow in soil and on vegeta¬  tion and are introduced into subcutaneous tissue through   Sporothrix schenckii is a dimorphic fungus. The mold form  lives on plants, and the yeast form occurs in human tissue.  When spores of the mold are introduced into the skin, typi¬  cally by a thorn, it causes a local pustule or ulcer with nod¬  ules along the draining lymphatics (Figure 48-2). The  lesions are typically painless, and there is little systemic  illness. Untreated lesions may wax and wane for years.   FIGURE 48~2 Sporotrichosis. Note papular lesions on left  hand and forearm. Caused by Sporothrix schenckii. (Reproduced with per-  mission from Wolff K, Johnson R. Fitzpatrick's Color Atlas & Synopsis of Clinical Derma¬  tology. 6th ed. New York: McGraw-Hill, 2009. Copyright © 2009 by The McGraw-Hill  Companies, Inc.)   Soil fungi ( Petriellidium, Madurella) enter through wounds  on the feet, hands, or back and cause abscesses, with pus  discharged through sinuses. The pus contains compact  colored granules. Actinomycetes such as Nocardia can  cause similar lesions (actinomycotic mycetoma). Sulfon¬  amides may help the actinomycotic form. There is no effec¬  tive drug against the fungal form; surgical excision is  recommended.   1. Regarding ringworm and the dermatophytes, which one of the fol¬  lowing is most accurate?   (B) The drug of choice for the treatment of ringworm lesions is  amphotericin B.   (C) The purpose of the KOH prep is to observe fungal antigens  within infected cells.   (D) The dermatophytid reaction refers to the necrotic area typically  seen in the center of ringworm lesions.   (E) The principal reservoir of dermatophytes in the genus Tricho¬  phyton is domestic animals such as dogs and cats.   (B) Laboratory diagnosis involves seeing a nonseptate mold in an  aspirate of the lesion.    (C) Sporothrix is often acquired by penetrating wounds sustained  while gardening.   (D) The treatment of choice for sporotrichosis is surgical removal  of the lesion because there is no effective drug.   (E) Disease occurs primarily in patients who are deficient in the  late-acting complement components.   3. Your patient is a 65-year-old woman with a 2-cm ulcerated lesion  on the palm of her hand that has been gradually getting bigger  during the past month. The lesion is only slightly tender and is not  red, hot, or painful. A careful history reveals that she was mak¬  ing holly wreaths for use at Christmas. (Holly leaves have sharp  points.) She is afebrile and otherwise well. An aspirate of the lesion  was obtained. Which one of the following would best support a  diagnosis of sporotrichosis?   (A) A culture on blood agar at 25°C revealed white, beta-hemolytic  colonies.   (B) A methenamine silver stain examined in the light microscope  revealed budding yeasts.   (C) A KOH preparation examined in the light microscope revealed  septate hyphae.   (D) A culture on Sabouraud’s agar at 37°C revealed a brownish  mycelium with green spores.   (E) An unstained sample examined in the dark field microscope  revealed non-septate hyphae.   4. Your patient is a 10-year-old boy with tinea pedis (athletes feet).  Which one of the following is the best choice of drug to treat his  infection?   Brief summaries of the organisms described in this chapter  begin on page 679. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Mycology section of Part XIII: USMLE (National Board)  Practice Questions starting on page 728. Also see Part XIV:  USMLE (National Board) Practice Examination starting on  page 751.   These infections result from inhalation of the spores of  dimorphic fungi that have their mold forms in the soil.  Within the lungs, the spores differentiate into yeasts or  other specialized forms, such as spherules.   Most lung infections are asymptomatic and self-limited.  However, in some persons, disseminated disease develops  in which the organisms grow in other organs, cause  destructive lesions, and may result in death. Infected per¬  sons do not communicate these diseases to others.   Important features of the systemic fungal diseases are  described in Table 49-1. Systemic fungi are also called   Coccodioides immitis is a dimorphic fungus that exists as a  mold in soil and as a spherule in tissue (Figure 49-1).   Valley fever in immunocompetent;  dissemination to bone and menin¬  ges in immunocompromised,  pregnant women, African Ameri¬  cans, and Filipinos   Culture at 20°C grows mold with  arthrospores; serologic test for  IgM and IgG   Ohio and Mississippi River  valleys; worldwide; asso¬  ciated with bird and bat  guano   Cavitary lung lesions; granulomas in  liver and spleen; pancytopenia  and tongue ulcer in  immunocompromised   Culture at 20°C grows mold with  tuberculate macroconidia;  serologic test for IgM and IgG;  urinary antigen   Culture at 20°C grows mold;  serologic test for IgM and IgG   FIGURE 49~1 Stages of Coccidioides immitis. A: Arthrospores form at the ends of hyphae in the soil. They germinate in the soil to  form new hyphae. If inhaled, the arthrospores differentiate into spherules. B: Endospores form within spherules in tissue. When spherules  rupture, endospores disseminate and form new spherules. (Reproduced with permission from Brooks GF et al. Medical Microbiology. 20th ed. Originally pub-  lished by Appleton & Lange. Copyright 1995 McGraw-Hill.)   The fungus is endemic in arid regions of the southwestern  United States and Latin America. People who live in Cen¬  tral and Southern California, Arizona, New Mexico, Western  Texas, and Northern Mexico, a geographic region called the  Lower Sonoran Life Zone, are often infected. In soil, it forms  hyphae with alternating arthrospores and empty cells  (Figure 49-2). Arthrospores are very light and are carried by  the wind. They can be inhaled and infect the lungs.   In the lungs, arthrospores form spherules that are large  (30 mm in diameter), have a thick, doubly refractive wall,  and are filled with endospores (Figure 49-3). Upon rupture   shaped, rectangular arthrospores appear blue with lactophenol  -cotton blue stain. Arthrospores are also called arthroconidia.  (Source: Dr. Hardin, Public Health Image Library, Centers for Disease Control and  Prevention.)    of the wall, endospores are released and differentiate to  form new spherules. The organism can spread within a  person by direct extension or via the bloodstream. Granu¬  lomatous lesions can occur in virtually any organ but are  found primarily in bones and the central nervous system  (meningitis).   Dissemination from the lungs to other organs occurs in  people who have a defect in cell-mediated immunity. Most  people who are infected by C. immitis develop a cell-  mediated (delayed hypersensitivity) immune response that  restricts the growth of the organism. One way to determine  whether a person has produced adequate cell-mediated  immunity to the organism is to do a skin test (see later). In  general, a person who has a positive skin test reaction has  developed sufficient immunity to prevent disseminated   points to a spherule in lung tissue. Spherules are large thick-walled  structures containing many endospores. Short arrow points to an  endospore. (Source: Dr. L. Georg, Public Health Image Library, Centers for Disease  Control and Prevention.)    disease from occurring. If, at a later time, a person’s cellular  immunity is suppressed by drugs or disease, disseminated  disease can occur.   Infection of the lungs is often asymptomatic and is evident  only by a positive skin test and the presence of antibodies.  Some infected persons have an influenza-like illness with  fever and cough. About 50% have changes in the lungs  (infiltrates, adenopathy, or effusions) as seen on chest  X-ray, and 10% develop erythema nodosum (see later) or  arthralgias. This syndrome is called “valley fever” (in the  San Joaquin Valley of California) or “desert rheumatism”  (in Arizona); it tends to subside spontaneously.   Disseminated disease can occur in almost any organ; the  meninges (meningitis), bone (osteomyelitis), and skin  (nodules) are important sites. The overall incidence of dis¬  semination in persons infected with C. immitis is 1%,  although the incidence in Filipinos and African Americans  is 10 times higher. Women in the third trimester of preg¬  nancy also have a markedly increased incidence of dissemi¬  nation. Erythema nodosum (EN) manifests as red, tender  nodules (“desert bumps”) on extensor surfaces such as the  skin over the tibia and ulna. It is a delayed (cell-mediated)  hypersensitivity response to fungal antigens and thus is an  indicator of a good prognosis. There are no organisms in  these lesions; they are not a sign of disseminated disease.  EN is not specific for coccidioidomycosis; it occurs in other  granulomatous diseases (e.g., histoplasmosis, tuberculosis,  and leprosy).   In infected persons, skin tests with fungal extracts (coc-  cidioidin or spherulin) cause at least a 5-mm induration  48 hours after injection (delayed hypersensitivity reaction).  Skin tests become positive within 2 to 4 weeks of infection  and remain so for years but are often negative (anergy) in  patients with disseminated disease.   In tissue specimens, spherules are seen microscopically.  Cultures on Sabouraud’s agar incubated at 25°C show sep¬  tate hyphae with arthrospores (see Figure 49-2). ( Caution:  Cultures are highly infectious; precautions against inhaling  arthrospores must be taken.) In serologic tests, IgM and  IgG precipitins appear within 2 to 4 weeks of infection and  then decline in subsequent months. Complement-fixing  antibodies occur at low titer initially, but the titer rises  greatly if dissemination occurs. A PCR assay that detects  nucleic acids of Coccidioides is available.   No treatment is needed in asymptomatic or mild primary  infection. Amphotericin B (Fungizone) or itraconazole is  used for persisting lung lesions or disseminated disease.    Fluconazole is also effective in lung disease. If meningitis  occurs, fluconazole is the drug of choice. Intrathecal  amphotericin B may be required and may induce remis¬  sion, but long-term results are often poor. There are no  means of prevention except avoiding travel to endemic  areas. Patients who have recovered from coccidioidal men¬  ingitis should receive long-term suppressive therapy with  fluconazole to prevent a recurrence.   Histoplasma capsulatum is a dimorphic fungus that exists  as a mold in soil and as a yeast in tissue. It forms two types  of asexual spores (Figure 49-4): (1) tuberculate macroco-  nidia, with typical thick walls and fingerlike projections  that are important in laboratory identification; and (2)  microconidia, which are smaller, thin, smooth-walled  spores that, if inhaled, transmit the infection.   This fungus occurs in many parts of the world. In the  United States, it is endemic in central and eastern states,  especially in the Ohio and Mississippi River valleys. It  grows in soil, particularly if the soil is heavily contaminated  with bird droppings, especially from starlings. Although  the birds are not infected, bats can be infected and can  excrete the organism in their guano. In areas of endemic  infection, excavation of the soil during construction or  exploration of bat-infested caves has resulted in a signifi¬  cant number of infected individuals.   A: Tuberculate macroconidia. B: Microconidia. (Reproduced with permis-  sion from Brooks GF et al. Medical Microbiology. 19th ed. Originally published by  Appleton & Lange. Copyright 1991 McGraw-Hill.)   FIGURE 49-5 Histoplasma capsulatum. Yeasts are located  within the macrophage. (Reproduced with permission from Brooks GF et al.  Medical Microbiology. 19th ed. Originally published by Appleton & Lange. Copyright  1991 McGraw-Hill.)   In several tropical African countries, histoplasmosis  is caused by Histoplasma duboisii. The clinical picture is  different from that caused by H. capsulatum. A descrip¬  tion of the differences between African histoplasmosis  and that seen in the United States is beyond the scope of  this book.   Inhaled spores are engulfed by macrophages and develop  into yeast forms. In tissues, H. capsulatum occurs as an oval  budding yeast inside macrophages (Figures 49-5 and  49-6). The yeasts survive within the phagolysosome of the  macrophage by producing alkaline substances, such as  bicarbonate and ammonia, which raise the pH and thereby  inactivate the degradative enzymes of the phagolysosome.   The organisms spread widely throughout the body,  especially to the liver and spleen, but most infections  remain asymptomatic, and the small granulomatous foci  heal by calcification. With intense exposure (e.g., in a  chicken house or bat-infested cave), pneumonia and cavi¬  tary lung lesions may become clinically manifest. Severe   FIGURE 49~6 Histoplasma capsulatum —yeasts within mac¬  rophages. Arrow points to a macrophage containing several  purple-stained yeasts in the cytoplasm. Yeasts within macro¬  phages can be seen in many macrophages in this specimen of  spleen. (Source: Dr. M. Hicklin, Public Health Image Library, Centers for Disease  Control and Prevention.)    disseminated histoplasmosis develops in a small minority  of infected persons, especially infants and individuals with  reduced cell-mediated immunity, such as patients with  acquired immunodeficiency syndrome (AIDS). In AIDS  patients, pancytopenia and ulcerated lesions on the tongue  are typical of disseminated histoplasmosis. In immuno¬  competent people, EN can occur (see description of EN in  earlier section on Coccidioides). EN is a sign that cell-  mediated immunity is active and the organism will proba¬  bly be contained.   A skin test using histoplasmin (a mycelial extract)  becomes positive (i.e., shows at least 5 mm of induration)  within 2 to 3 weeks after infection and remains positive for  many years. However, because there are many false-positive  reactions (due to cross-reactivity) and many false-negative  reactions (in disseminated disease), the skin test is not use¬  ful for diagnosis. Furthermore, the skin test can stimulate  an antibody response and confuse the serologic tests. The  skin test is useful for epidemiologic studies, and up to 90%  of individuals have positive results in areas of endemic  infection.   In tissue biopsy specimens or bone marrow aspirates, oval  yeast cells within macrophages are seen microscopically  (see Figure 49-6). Cultures on Sabouraud’s agar show  hyphae with tuberculate macroconidia when grown at low  temperature (e.g., 25°C) and yeasts when grown at 37°C.  Tests that detect a Histoplasma polysaccharide antigen by  enzyme-linked immunosorbent assay (ELISA) and Histo¬  plasma RNA with DNA probes are also useful. In immuno¬  compromised patients with disseminated disease, tests for  Histoplasma antigen in the urine are especially useful  because antibody tests may be negative.   Two serologic tests are useful for diagnosis: comple¬  ment fixation (CF) and immunodiffusion (ID). An anti¬  body titer of 1:32 in the CF test with yeast phase antigens  is considered to be diagnostic. However, cross-reactions  with other fungi, especially Blastomyces, occur. CF titers  fall when the disease becomes inactive and rise in dis¬  seminated disease. The ID test detects precipitating anti¬  bodies (precipitins) by forming two bands, M and H, in an  agar-gel diffusion assay. The ID test is more specific but  less sensitive than the CF test.   No therapy is needed in asymptomatic or mild primary  infections. With progressive lung lesions, oral itraconazole  is effective. In disseminated disease, parenteral itraconazole  (or amphotericin B) is the treatment of choice. Liposomal  amphotericin B should be used in patients with preexisting  kidney damage. In meningitis, fluconazole is often used  because it penetrates the spinal fluid well. Oral itraconazole  is used for chronic suppression in patients with AIDS.    There are no means of prevention except avoiding exposure  in areas of endemic infection.   Blastomyces dermatitidis is a dimorphic fungus that exists  as a mold in soil and as a yeast in tissue. The yeast is round  with a doubly refractive wall and a single broad-based bud  (Figures 49-7 and 49-8). Note that this organism forms a  broad-based bud, whereas Cryptococcus neoformans is a  yeast that forms a narrow-based bud.   This fungus is endemic primarily in eastern North America,  especially in the region bordering the Ohio, Mississippi,  and St. Lawrence rivers, and the Great Lakes region. Less  commonly, blastomycosis has also occurred in Central and  South America, Africa, and the Middle East. It grows in  moist soil rich in organic material, forming hyphae with  small pear-shaped conidia. Inhalation of the conidia causes  human infection.   Infection occurs mainly via the respiratory tract. Asymp¬  tomatic or mild cases are rarely recognized. Dissemina¬  tion may result in ulcerated granulomas of skin, bone, or  other sites.   FIGURE 49~8 Blastomyces dermatitidis —broad-based bud¬  ding yeast. Arrow points to the broad base of the budding yeast.  (Source: Dr. L. Ajello, Public Health Image Library, Centers for Disease Control and  Prevention.)   In tissue biopsy specimens, thick-walled yeast cells with  single broad-based buds are seen microscopically (see  Figure 49-8). Hyphae with small pear-shaped conidia are  visible on culture. The skin test lacks specificity and has  little value. Serologic tests have little value. A PCR assay  that detects nucleic acids of Blastomyces is available.   Itraconazole is the drug of choice for most patients, but  amphotericin B should be used to treat severe disease. Sur¬  gical excision may be helpful. There are no means of  prevention.   FIGURE 49~7 Blastomyces dermatitidis. A: Yeast with a broad-based bud at 37°C. B: Mold with microconidia at 20°C. (Reproduced   with permission from Brooks GF et al. Medical Microbiology. 19th ed. Originally published by Appleton & Lange. Copyright 1991 McGraw-Hill.)   FIGURE 49-9 Paracoccidioides brasiliensis. Note the multiple  buds of the yeast form of Paracoccidioides, in contrast to the single  bud of Blastomyces.    Skin tests are rarely helpful. Serologic testing shows that  when significant antibody titers (by ID or CF) are found,  active disease is present.   The drug of choice is itraconazole taken orally for several  months. There are no means of prevention.   Paracoccidioides brasiliensis causes paracoccidioidomyco¬  sis, also known as South American blastomycosis.   Paracoccidioides brasiliensis is a dimorphic fungus that  exists as a mold in soil and as a yeast in tissue. The yeast is  thick-walled with multiple buds, in contrast to B. derma-  titidis, which has a single bud (Figures 49-9 and 49-10).   This fungus grows in the soil and is endemic in rural Latin  America. Disease occurs only in that region.   The spores are inhaled, and early lesions occur in the  lungs. Asymptomatic infection is common. Alternatively,  oral mucous membrane lesions, lymph node enlargement,  and sometimes dissemination to many organs develop.   resembling a "ship captain's wheel." Methenamine silver stain. (Source:  Dr. Lucille Georg, Public Health Image Library, Centers for Disease Control and  Prevention.)   1. Regarding coccidioidomycosis and C. immitis , which one of the  following is most accurate?   (A) C. immitis is a mold in the soil and a yeast in the body.   (B) The diagnosis of acute coccidioidomycosis can be made by  detecting IgM antibodies in the patient’s serum.   (C) Travelers to the Philippines are at high risk of acquiring the  disease.   (D) The nodules of erythema nodosum are a typical finding in dis¬  seminated coccidioidomycosis.   (E) Infection typically occurs when arthrospores enter the skin  (e.g., through a wound caused by a rose thorn).   2. Regarding histoplasmosis and H. capsulatum , which one of the  following is most accurate?   (A) In tissue biopsies, H. capsulatum is found as a yeast within  macrophages.   (B) The laboratory diagnosis is made by seeing germ tubes when  incubated at 37°C.   (C) Histoplasmosis occurs primarily in the tropical areas of Central  and South America.   (D) To prevent disease, people who live in endemic areas should  receive the vaccine containing histoplasmin.   (E) Most infections are acquired by ingesting food accidentally  contaminated with fungal spores from the soil.   3. Regarding B. dermatitidis , which one of the following is most  accurate?   (A) It forms a mycelium in culture at 37°C in the clinical lab.   (B) Humoral immunity is the main host defense against this  organism.   (C) It causes a dermatophytid (“id”) reaction when it disseminates  to the skin.   (D) The most important virulence factor of this organism is endo¬  toxin in its cell wall.   (E) It is a dimorphic fungus that exists as a mold in the soil and a  yeast in the body.   4. Your patient is a 30-year-old woman who is in her third trimester  of pregnancy, is of Filipino origin, and lives in the Central Valley  of California. She complains of severe low back pain of several  weeks in duration. An X-ray reveals a lesion in the fourth lumbar  vertebra. Material from a needle biopsy of the lesion is examined  by a pathologist who calls to tell you the patient has coccidioido¬  mycosis. Of the following, which one did the pathologist see in the  biopsy?    5. Your patient is a 30-year-old man who is human immunodeficiency  virus (HIV) antibody positive with a CD4 count of 100. He has an  ulcerated lesion on his tongue, and biopsy of the lesion reveals yeasts  within macrophages. A diagnosis of disseminated histoplasmosis is  made. Which one of the following is the best choice of drug to treat  his disseminated histoplasmosis?   Brief summaries of the organisms described in this chapter  begin on page 679. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Mycology section of Part XIII: USMLE (National Board)  Practice Questions starting on page 728. Also see Part XIV:  USMLE (National Board) Practice Examination starting on  page 751.    Penicillium marneffei  Pseudallescheria boydii  Fusarium solani  Self-Assessment Questions  Summaries of Organisms   Opportunistic fungi fail to induce disease in most immu¬  nocompetent persons but can do so in those with impaired  host defenses. There are five genera of medically important  fungi: Candida, Cryptococcus, Aspergillus, Mucor, and Rhi¬  zopus. Important features of the opportunistic fungal dis¬  eases are described in Table 50-1.   Candida albicans, the most important species of Candida,  causes thrush, vaginitis, esophagitis, diaper rash, and  chronic mucocutaneous candidiasis. It also causes    disseminated infections such as right-sided endocarditis  (especially in intravenous drug users), bloodstream infec¬  tions (candidemia), and endophthalmitis. Infections related  to indwelling intravenous and urinary catheters are also  important. Candida glabrata is the second most common  cause of disseminated candidal infections and is more drug  resistant than C. albicans.   Candida albicans is an oval yeast with a single bud  (Figures 50-1 and 50-2). It is part of the normal flora of  mucous membranes of the upper respiratory, gastrointesti¬  nal, and female genital tracts. In tissues it appears most  often as yeasts or as pseudohyphae (Figures 50-1 and 50-3).  Pseudohyphae are elongated yeasts that visually resemble   Fungus ball in lung; wound and burn infec¬  tions; indwelling catheter infections; sinusitis   Necrotic lesion formed when mold invades  blood vessels; predisposing factors are dia¬  betic ketoacidosis, renal acidosis, and cancer   Culture grows mold with black  spores; conidia enclosed in a sac  called a sporangium    FIGURE 50-1 Candida albicans. A: Budding yeasts and pseudohyphae in tissues or exudate. B: Pseudohyphae and chlamydospores in  culture at 20°C. C: Germ tubes at 37°C. (Reproduced with permission from Brooks GF et al. Medical Microbiology. 20th ed. Originally published by  Appleton & Lange. Copyright 1995 McGraw-Hill.)    hyphae but are not true hyphae. True hyphae are also  formed when C. albicans invades tissues.   Carbohydrate fermentation reactions can be used to dif¬  ferentiate it from other species (e.g., Candida tropicalis,  Candida parapsilosis, Candida krusei, and C. glabrata ) that  cause human infections.   Candida dubliniensis is closely related to C. albicans. It  also causes opportunistic infections in immunocompro¬  mised patients, especially AIDS patients. Both species form  chlamydospores but C. albicans grows at 42°C whereas  C. dubliniensis does not.   As a member of the normal flora, C. albicans is already pres¬  ent on the skin and mucous membranes. In addition to the   budding yeast. Short arrow points to the outer membrane of a vagi¬  nal epithelial cell. In this Gram-stained specimen, various bacteria  that are part of the normal flora of the vagina can be seen. (Source:  Dr. S. Brown, Public Health Image Library, Centers for Disease Control  and Prevention.)    skin, C. albicans is found throughout the GI tract (especially  the mouth and esophagus) and in the vagina. Thrush in the  newborn is the result of passage through a birth canal heavily  colonized by the organism. The presence of C. albicans on  the skin predisposes to infections involving instruments that  penetrate the skin, such as needles (intravenous drug use)  and indwelling catheters. It is often found in the urine of  patients with indwelling urinary (Foley) catheters.   The first line of defense against Candida infections is intact  skin and mucous membranes. The second line is cell-  mediated immunity, especially Th-1 cells producing  gamma-interferon that activates efficient killing by macro¬  phages. Neutrophils are also important as evidenced by the   FIGURE 50-3 Candida albicans —pseudohyphae. Two arrows  point to pseudohyphae of Candida albicans. (Source: Dr. S. Brown,  Public Health Image Library, Centers for Disease Control and  Prevention.)   ish plaques on tongue. (Reproduced with permission from Usatine,  RP et al: The Color Atlas of Family Medicine, New York: McGraw-Hill,  2009. Courtesy of Richard P. Usatine, MD.)   When local or systemic host defenses are impaired, dis¬  ease may result. Overgrowth of C. albicans in the mouth  produces white patches called thrush (Figure 50-4). (Note  that thrush is a pseudomembrane, a term that is defined in  Chapter 7 on page 39.) Vaginitis with itching and discharge  is favored by high pH, diabetes, or use of antibiotics. Anti¬  biotics suppress the normal flora Lactobacillus, which keep  the pH low. As a result, the pH rises, which favors the  growth of Candida.   Skin invasion occurs in warm, moist areas, which  become red and weeping. Fingers and nails become  involved when repeatedly immersed in water; persons  employed as dishwashers in restaurants are commonly  affected. Thickening or loss of the nail can occur. Diaper  rash in infants occurs when wet diapers are not changed  promptly (Figure 50-5).   In immunosuppressed individuals, Candida may dis¬  seminate to many organs or cause chronic mucocutaneous  candidiasis (CMC). CMC is a prolonged infection of the  skin, oral and genital mucosa, and nails that occurs in indi¬  viduals deficient in T-cell immunity. Patients with muta¬  tions in the gene encoding interleukin-17 (IL-17) and the  receptor for IL-17 are predisposed to CMC. After organ  transplantation, patients receiving immunosuppressive  drugs to prevent rejection are predisposed to invasive Can¬  dida infections.   Intravenous drug abuse, indwelling intravenous cathe¬  ters, and hyperalimentation also predispose to dissemi¬  nated candidiasis, especially right-sided endocarditis and  endophthalmitis (infection within the eye). Candida  esophagitis, often accompanied by involvement of the  stomach and small intestine, is seen in patients with   area of inflammation in perineal region. (Reproduced with permis¬  sion from Wolff K, Johnson R. Fitzpatrick's Color Atlas & Synopsis of  Clinical Dermatology. 6th ed. New York: McGraw-Hill, 2009. Copyright  © 2009 by The McGraw-Hill Companies, Inc.)    leukemia and lymphoma. Subcutaneous nodules are often  seen in neutropenic patients with disseminated disease.  Candida albicans is the most common species to cause dis¬  seminated disease in these patients, but C. tropicalis and C.  parapsilosis are important pathogens also.   In exudates or tissues, budding yeasts and pseudohyphae  appear gram-positive and can be visualized by using  calcofluor-white staining. In culture, typical yeast colonies  are formed that resemble large staphylococcal colonies.  Candida albicans forms germ tubes in serum at 37°C,  whereas most other species of pathogenic Candida species  do not (see Figure 50-1). Chlamydospores are typically  formed by C. albicans but not by most other species of Can¬  dida. Note that C. dubliniensis also forms chlamydospores  but will not grow at 42°C whereas C. albicans will. Serologic  testing is rarely helpful.   A laboratory test that can identify C. albicans and four  other Candida species in blood cultures in 3 to 5 hours  instead of the usual several days was approved in 2014. The  test uses magnetic resonance technology to detect the pres¬  ence of yeast DNA and then to identify the species.   Skin tests with Candida antigens are uniformly positive  in immunocompetent adults and are used as an indicator  that the person can mount a cellular immune response. A  person who does not respond to Candida antigens in the  skin test is presumed to have deficient cell-mediated immu¬  nity. Such a person is anergic, and other skin tests cannot be  interpreted. Thus if a person has a negative Candida skin    test, a negative purified protein derivative (PPD) skin test for  tuberculosis could be a false-negative result.   The drug of choice for most candidal infections is flucon¬  azole, including oropharyngeal or esophageal thrush. Itra¬  conazole and voriconazole are also effective. An  echinocandin, such as caspofungin or micafungin can also  be used for esophageal candidiasis. Treatment of skin infec¬  tions consists of topical antifungal drugs (e.g., clotrimazole  or nystatin). Candida vaginitis is treated either with topical  (intravaginal) azole drugs, such as clotrimazole or micon¬  azole, or with oral fluconazole. Chronic mucocutaneous  candidiasis can be controlled by fluconazole or itraconazole.  Treatment of disseminated candidiasis consists of either flu¬  conazole or an echinocandin such as caspofungin.   Treatment of candidal infections with antifungal drugs  should be supplemented by reduction of predisposing fac¬  tors. Strains of C. albicans resistant to azole drugs have  emerged in patients with acquired immunodeficiency syn¬  drome (AIDS) receiving long-term prophylaxis with fluco¬  nazole. Most isolates of C. glabrata are resistant to  fluconazole and voriconaziole. An echinocandin such as  caspofungin or amphoptericin B should be used.   Certain candidal infections (e.g., thrush) can be pre¬  vented by oral clotrimazole troches, buccal miconazole  tablets, or nystatin “swish and swallow.” Fluconazole is use¬  ful in preventing candidal infections in high-risk patients,  such as those undergoing bone marrow transplantation  and premature infants. Micafungin can also be used. There  is no vaccine.   Cryptococcus neoformans causes cryptococcosis, especially  cryptococcal meningitis. Cryptococcosis is the most com¬  mon, life-threatening invasive fungal disease worldwide. It  is especially important in AIDS patients. Another species,  Cryptococcus gattii, causes human disease less frequently  than C. neoformans.   Cryptococcus neoformans is an oval, budding yeast sur¬  rounded by a wide polysaccharide capsule (Figures 50-6  and 50-7). It is not dimorphic. Note that this organism  forms a narrow-based bud, whereas the yeast form of  Blastomyces dermatitidis forms a broad-based bud.   Cryptococcus neoformans occurs widely in nature and  grows abundantly in soil containing bird (especially  pigeon) droppings. The birds are not infected. Human   FIGURE 50~6 Cryptococcus neoformans. India ink preparation  shows budding yeasts with a wide capsule. India ink forms a dark  background; it does not stain the yeast itself. (Reproduced with  permission from Brooks GF et al. Medical Microbiology. 20th ed. Origi¬  nally published by Appleton & Lange. Copyright 1995 McGraw-Hill.)   infection results from inhalation of the organism. There is  no human-to-human transmission. Cryptococcus gattii is  associated with eucalyptus trees, most often in the north¬  western states of the United States. It is also found in sub¬  tropical and tropical areas of many countries.   Lung infection is often asymptomatic or may produce  pneumonia. Disease caused by C. neoformans occurs  mainly in patients with reduced cell-mediated immunity,  especially AIDS patients, in whom the organism dissemi¬  nates to the central nervous system (meningitis) and other  organs. Subcutaneous nodules are often seen in dissemi¬  nated disease. Note, however, that roughly half the patients  with cryptococcal meningitis fail to show evidence of  immunosuppression.   In some patients with AIDS who are infected with Cryp¬  tococcus, treating the patient with highly active antiretrovi¬  ral therapy (HAART) causes an exacerbation of symptoms.   tion. Arrow points to a budding yeast of Cryptococcus neoformans.  Note the thick, translucent polysaccharide capsule outlined by the  dark India ink particles. (Source: Dr. L. Haley, Public Health Image  Library, Centers for Disease Control and Prevention.)    This phenomenon is called immune reconstitution inflam¬  matory syndrome (IRIS). The explanation of the exacerba¬  tion of symptoms is that HAART increases the number of  CD4 cells, which increases the inflammatory response.  Some patients have died as a result of cryptococcal IRIS. To  prevent IRIS, patients should be treated for the underlying  infection before starting HAART.   Cryptococcus gattii causes human disease less frequently  but is more capable of causing disease in an immunocom¬  petent person than C. neoformans. Cryptococcus gattii is  more likely to cause cryptococcomas (granulomas), espe¬  cially in the brain, than C. neoformans.   In spinal fluid mixed with India ink, the yeast cell is seen  microscopically surrounded by a wide, unstained capsule.  Appearance of the organism in Gram stain is unreliable,  but stains such as periodic acid-Schiff (PAS stain), methe-  namine silver, and mucicarmine will allow the organism  to be visualized (Figure 50-8). The organism can be cul¬  tured from spinal fluid and other specimens. The colonies  are highly mucoid—a reflection of the large amount of  capsular polysaccharide produced by the organism.   Serologic tests can be done for both antibody and anti¬  gen. In infected spinal fluid, capsular antigen occurs in  high titer and can be detected by the latex particle aggluti¬  nation test. This test is called the cryptococcal antigen test,  often abbreviated as “crag.”   Distinguishing between C. neoformans and C. gattii in the  laboratory requires specialized media not generally available,  so many C. gattii infections may go undiagnosed.   Combined treatment with amphotericin B and flucytosine  is used in meningitis and other disseminated disease.   FIGURE 50~8 Cryptococcus neoformans —Mucicarmine stain.  Note many red, oval yeasts of C. neoformans in lung tissue of patient  with AIDS. (Source: Dr. Edwin P. Ewing, Jr, Public Health Image Library,  Centers for Disease Control and Prevention.)    Liposomal amphotericin B should be used in patients with  preexisting kidney damage. There are no specific means of  prevention. Fluconazole is used in AIDS patients for long¬  term suppression of cryptococcal meningitis. Cryptococcus  gattii is less responsive to antifungal drugs than is C.  neoformans.   Aspergillus species, especially Aspergillus fumigatus, cause  infections of the skin, eyes, ears, and other organs; “fungus  ball” in the lungs; and allergic bronchopulmonary  aspergillosis.   Aspergillus species exist only as molds; they are not dimor¬  phic. They have septate hyphae that form V-shaped  (dichotomous) branches (Figures 50-9 and 50-10). The  walls are more or less parallel, in contrast to Mucor and  Rhizopus walls, which are irregular (Figures 50-9 and  50-11). The conidia of Aspergillus form radiating chains, in  contrast to those of Mucor and Rhizopus, which are  enclosed within a sporangium (Figure 50-12).   These molds are widely distributed in nature. They grow on  decaying vegetation, producing chains of conidia. Trans¬  mission is by airborne conidia.   Aspergillus fumigatus can colonize and later invade abraded  skin, wounds, burns, the cornea, the external ear, or para¬  nasal sinuses. It is the most common cause of fungal sinus¬  itis. In immunocompromised persons, especially those  with neutropenia, it can invade the lungs producing  hemoptysis and the brain causing an abscess. Neutropenic  patients are also predisposed to intravenous catheter infec¬  tions caused by this organism. In 2012, an outbreak of A.  fumigatus infections, especially meningitis, occurred  caused by injectable corticosteroid solutions that were con¬  taminated with the fungus.   Aspergilli are well-known for their ability to grow in  cavities within the lungs, especially cavities caused by   FIGURE 50~9 Aspergillus and Mucor in tissue. A: Aspergillus  has septate hyphae with V-shaped branching. B: Mucor has nonsep-  tate hyphae with right-angle branching.   Long arrow points to the septate hyphae of Aspergillus. Note the  straight parallel cell walls of this mold. Short arrow points to the  typical low-angle, Y-shaped branching. (Used with permission of  Prof. Henry Sanchez, University of California, San Francisco School  of Medicine.)    tuberculosis. Within the cavities, they produce an aspergil-  loma (fungus ball), which can be seen on chest X-ray as a  radiopaque structure that changes its position when the  patient is moved from an erect to a supine position.   Allergic bronchopulmonary aspergillosis (ABPA) is a  hypersensitivity reaction to the presence of Aspergillus in  the bronchi. Patients with ABPA have asthmatic symptoms  and a high IgE titer against Aspergillus antigens, and they  expectorate brownish bronchial plugs containing hyphae.  Asthma caused by the inhalation of airborne conidia, espe¬  cially in certain occupational settings, also occurs.  Aspergillus flavus growing on cereals or nuts produces  aflatoxins that may be carcinogenic or acutely toxic.   FIGURE 50-11 Mucor species—nonseptate hyphae. Arrow  points to irregular-shaped, nonseptate hyphae of Mucor. (Source:   Dr. L. Ajello, Public Health Image Library, Centers for Disease Control  and Prevention.)    FIGURE 50~12 Aspergillus and Mucor in culture. A: Aspergillus  spores form in radiating columns. B: Mucor spores are contained  within a sporangium.   Biopsy specimens show septate, branching hyphae invad¬  ing tissue (Figure 50-10). Cultures show colonies with  characteristic radiating chains of conidia (Figure 50-12).  However, positive cultures do not prove disease because  colonization is common. In persons with invasive aspergil¬  losis, there may be high titers of galactomannan antigen in  serum. Patients with ABPA have high levels of IgE specific  for Aspergillus antigens and prominent eosinophilia. IgG  precipitins are also present.   Voriconazole is the drug of choice for invasive aspergillosis.  Liposomal amphotericin B, posaconazole, and caspofungin  are alternative drugs. A fungus ball growing in a sinus or in  a pulmonary cavity can be surgically removed. Patients  with ABPA can be treated with corticosteroids and antifun¬  gal agents, such as itraconazole. There are no specific  means of prevention.   Mucormycosis (zygomycosis, phycomycosis) is a disease  caused by saprophytic molds (e.g., Mucor, Rhizopus, and  Absidia) found widely in the environment. They are not  dimorphic. These organisms are transmitted by airborne  asexual spores and invade tissues of patients with reduced  host defenses. They proliferate in the walls of blood vessels,  particularly of the paranasal sinuses, lungs, or gut, and  cause infarction and necrosis of tissue distal to the blocked  vessel (Figure 50-13).   Patients with diabetic ketoacidosis, burns, bone mar¬  row transplants, or leukemia are particularly susceptible.  Diabetic patients are particularly susceptible to rhinocere-  bral mucormycosis, in which mold spores in the sinuses  germinate to form hyphae that invade blood vessels that  supply the brain. One species, Rhizopus oryzae, causes  about 60% of cases of mucormycosis.   In biopsy specimens, organisms are seen microscopi¬  cally as nonseptate hyphae with broad, irregular walls and  branches that form more or less at right angles (Figures 50-9  and 50-11). Cultures show colonies with spores contained  within a sporangium (Figure 50-12). These organisms are   area involving the nose and face. (Reproduced with permission from  Lichtman MA et al, eds. Lichtman's Atlas of Hematology. New York:  McGraw-Hill, 2007. Copyright © 2007 by The McGraw-Hill  Companies, Inc.)    difficult to culture because they are a single, very long cell,  and damage to any part of the cell can limit its ability to  grow.   If diagnosis is made early, treatment of the underlying  disorder, plus administration of amphotericin B and surgi¬  cal removal of necrotic infected tissue, has resulted in some  remissions and cures. Liposomal amphotericin B should be  used in patients with preexisting kidney damage. Posacon-  azole can also be used.   Pneumocystis jiroveci is classified as a yeast on the basis of  molecular analysis, but it has many characteristics of a pro¬  tozoan. Some regard it as an “unclassified” organism. A  summary of the important clinical information is presented  here and a more detailed description is presented in  Chapter 52 with the blood and tissue protozoa. In 2002,  taxonomists renamed the human species of Pneumocystis  as P. jiroveci and recommended that P. carinii be used only  to describe the rat species of Pneumocystis.   Pneumocystis is acquired by inhalation of airborne  organisms into the lungs. An inflammatory exudate com¬  posed primarily of plasma cells occurs, oxygen exchange is  reduced, and dyspnea occurs. A reduced number of CD  4-postive T lymphocytes, such as occurs in AIDS, predis¬  poses to pneumonia. Most immunocompetent people have  asymptomatic infections.   The clinical findings of Pneumocystis pneumonia  include fever, nonproductive cough, and dyspnea. Rales are  heard bilaterally and the chest X-ray shows a “ground-  glass” pattern. The mortality rate of untreated Pneumocystis  pneumonia is approximately 100%.   The diagnosis is typically made by finding the cysts of  Pneumocystis in bronchial lavage specimens. Fluorescent  antibody stains or tissue stains, such as methenamine silver  or Giemsa, are used to identify the organism. PCR-based  tests are also used. Serological tests are not useful.   The drug of choice for Pneumocystis pneumonia is trim¬  ethoprim - sulfamethoxazole. Trimethoprim- sulfamethoxa¬  zole or aerosolized pentamidine can be used for prophylaxis  in patients with CD4 counts below 200.   Penicillium marneffei is a dimorphic fungus that causes  tuberculosis-like disease in AIDS patients, particularly in  Southeast Asian countries such as Thailand. It grows as a mold  that produces a rose-colored pigment at 25°C but at 37°C  grows as a small yeast that resembles Histoplasma capsulatum.  Bamboo rats are the only other known hosts. The diagnosis is  made either by growing the organism in culture or by using  fluorescent antibody staining of affected tissue. The treatment  of choice consists of amphotericin B for 2 weeks followed by  oral itraconazole for 10 weeks. Relapses can be prevented with  prolonged administration of oral itraconazole.   Pseudallescheria boydii is a mold that causes disease primar¬  ily in immunocompromised patients. The clinical findings  and the microscopic appearance of the septate hyphae in    tissue closely resemble those of Aspergillus. In culture, the  appearance of the conidia (pear-shaped) and the color of the  mycelium (brownish-gray) of P. boydii are different from  those of Aspergillus. The drug of choice is either ketocon-  azole or itraconazole because the response to amphotericin B  is poor. Debridement of necrotic tissue is important as well.   Fusarium solani is a mold that causes disease primarily in  neutropenic patients. Fever and skin lesions are the most com¬  mon clinical features. The organism is similar to Aspergillus in  that it is a mold with septate hyphae that tends to invade blood  vessels. Blood cultures are often positive in disseminated dis¬  ease. In culture, banana-shaped conidia are seen. Liposomal  amphotericin B is the drug of choice. Indwelling catheters  should be removed or replaced. In 2006, an outbreak of  Fusarium keratitis (infection of the cornea) occurred in people  who used a certain contact lens solution.   1. Regarding C. albicans , which one of the following is most accurate?   (A) The diagnosis of disseminated candidiasis is typically made by  detecting IgM antibodies.   (B) It exists as a yeast on mucosal surfaces but forms pseudohyphae  when it invades tissue.   (C) Antibody-mediated immunity is a more important host defense  than cell-mediated immunity.   (D) A positive skin test can be used to confirm the diagnosis of skin  infection caused by C. albicans.   (E) In the clinical laboratory, it is diagnosed by isolating a mold  with nonseptate hyphae when cultures are grown at room  -temperature.   2. Regarding Cryptococcus neoformans , which one of the following is  most accurate?   (A) It is a dimorphic fungus, growing as a mold in the soil and a  yeast in the body.   (B) It is acquired primarily by ingestion of food contaminated with  pigeon guano.   (C) Dark field microscopy is typically used to visualize the organ¬  ism in spinal fluid.   (D) Pathogenesis involves an exotoxin that acts as a superantigen  recruiting lymphocytes into the spinal fluid.   (E) Laboratory diagnosis of cryptococcal meningitis can be  achieved by detecting the capsular polysaccharide of the organ¬  ism in the spinal fluid.   3. Regarding Aspergillus fumigatus and aspergillosis, which one of  the following is most accurate?   (A) The natural habitat of A. fumigatus is the hair follicles of the  human skin.   (B) In the clinical laboratory, cultures of A. fumigatus incubated at  37°C form yeast colonies.   (C) The India ink stain is typically used to visualize A. fumigatus in  the clinical laboratory.   (E) The main predisposing factor to allergic bronchopulmonary  aspergillosis is neutropenia.   4. Regarding Mucor species, which one of the following is most  accurate?   (A) Infection is acquired by the ingestion of food contaminated by  spores of the organism.   (B) Diabetic ketoacidosis is a major predisposing factor for inva¬  sive mucormycosis.   (C) Mucor species have septate hyphae in contrast to Aspergillus  species, which have nonseptate hyphae.   (D) In biopsy specimens obtained from patients with invasive dis¬  ease, Mucor species appear as pseudohyphae.   (E) Skin tests using mucoroidin as the immunogen are used to deter¬  mine whether the patient has been infected with Mucor species.   5. Your patient is a 20-year-old woman who is human immunode¬  ficiency virus (HIV) antibody positive with a CD4 count of 50.  She has recovered from cryptococcal meningitis. Which one of    the following is the best choice of drug to use as long-term pro¬  phylaxis to prevent another episode of cryptococcal meningitis?   6. Your patient is a 1-month-old infant with whitish lesions in the  mouth that are diagnosed as oropharyngeal candidiasis (thrush).  Which one of the following is the best choice of drug to treat this  infection?   7. Your patient is a 50-year-old woman with leukemia who is neutro¬  penic from her cancer chemotherapy. She now has disseminated  aspergillosis that does not respond to amphotericin B. Which one  of the following is the best choice of drug to treat this infection?   Brief summaries of the organisms described in this chapter  begin on page 680. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Mycology section of Part XIII: USMLE (National Board)  Practice Questions starting on page 728. Also see Part XIV:  USMLE (National Board) Practice Examination starting on  page 751.    Parasites occur in two distinct forms: single-celled proto¬  zoa and multicellular metazoa called helminths or worms.  For medical purposes, protozoa can be subdivided into  four groups: Sarcodina (amebas), Sporozoa (sporozoans),  Mastigophora (flagellates), and Ciliata (ciliates). Metazoa  are subdivided into two phyla: the Platyhelminthes (flat-  worms) and the Nemathelminthes (roundworms, nema¬  todes). The phylum Platyhelminthes contains two medically  important classes: Cestoda (tapeworms) and Trematoda  (flukes). This classification is shown in Figure VI-1.   Understanding the life cycle and pathogenesis of proto¬  zoa and helminths requires an explanation of certain terms.  Many protozoa have a life cycle consisting of a trophozoite,  which is the motile, feeding, reproducing form surrounded  by a flexible cell membrane, and a cyst, which is the non-  motile, nonmetabolizing, nonreproducing form sur¬  rounded by a thick wall. The cyst form survives well in the  environment and so is often involved in transmission.  Certain protozoa, such as Leishmania and Trypanosoma,  have flagellated forms called promastigotes or trypomas-  tigotes and nonflagellated forms called amastigotes.    Many helminths have a life cycle that progresses from  egg to larva to adult. The egg contains an embryo that,  upon hatching, differentiates into a larval form, which then  matures into the adult form that produces the eggs.   There are special terms applied to the host of certain  parasites as they proceed through their life cycle. A defini¬  tive host is one in which the sexual cycle occurs or the  adult is present, and the intermediate host is one in which  the asexual cycle occurs or the larva is present.   Regarding the laboratory diagnosis of these infections,  examination of the stool for ova and parasites (O&P) is  often done. Ova refers to the eggs, and parasites refers to  the larval or adult forms.   Eosinophilia is associated with several helminth infections,  especially when roundworm larvae migrate through tissue.  High eosinophil counts are seen in infections caused by  the following roundworms: Ascaris, Strongyloides, Trichinella,  Toxocara, and the hookworms, Necator and Ancylostoma.  Infections with the flatworm (fluke) Schistosoma (a trema-  tode) also elicit eosinophilia. Eosinophils are an important  component of the host defense against these parasites.   In this book, the major protozoan pathogens are grouped  according to the location in the body where they most fre¬  quently cause disease. The intestinal and urogenital proto¬  zoa are described in this chapter, and the blood and tissue  protozoa are described in Chapter 52.   (1) Within the intestinal tract, three organisms—the ameba  Entamoeba histolytica, the flagellate Giardia lamblia, and the  sporozoan Cryptosporidium hominis —are the most important.   (2) In the urogenital tract, the flagellate Trichomonas  vaginalis is the important pathogen.   (3) The blood and tissue protozoa are a varied group  consisting of the flagellates Trypanosoma and Leishmania    and the sporozoans Plasmodium and Toxoplasma. The  important opportunistic lung pathogen Pneumocystis will  be discussed in this group, although there is molecular evi¬  dence that it should be classified as a fungus.   The major and minor pathogenic protozoa are listed in  Table 51-1.   Although immigrants and Americans returning from  abroad can present to physicians in the United States with  any parasitic disease, certain parasites are much more likely  to occur outside the United States. The features of the  medically important protozoa, including their occurrence  in the United States, are described in Table 51-2.   The medically important stages in the life cycle of the  intestinal protozoa are described in Table 51-3.   The life cycle of E. histolytica is shown in Figure 51-1. The  life cycle has two stages: the motile ameba (trophozoite) and  the nonmotile cyst (Figures 51-2A and B, 51-3, and 51-4).  The trophozoite is found within the intestinal and extraint-  estinal lesions and in diarrheal stools. The cyst predominates    in nondiarrheal stools. These cysts are not highly resistant  and are readily killed by boiling but not by chlorination of  water supplies. They are removed by filtration of water.   The cyst has four nuclei, an important diagnostic crite¬  rion. Upon excystation in the intestinal tract, an ameba  with four nuclei emerges and then divides to form eight  trophozoites. The mature trophozoite has a single nucleus  with an even lining of peripheral chromatin and a promi¬  nent central nucleolus (karyosome).   Antibodies are formed against trophozoite antigens in  invasive amebiasis, but they are not protective; previous  infection does not prevent reinfection. The antibodies are  useful, however, for serologic diagnosis.    ’Also known as T. brucei gambiense and T. brucei rhodesiense, respectively.   2 L tropica and L. mexicana cause Old World and New World cutaneous leishmaniasis,  respectively.   The organism is acquired by ingestion of cysts that are  transmitted primarily by the fecal-oral route in contami¬  nated food and water. Anal-oral transmission (e.g.,  among male homosexuals) also occurs. There is no ani¬  mal reservoir. The ingested cysts differentiate into tro¬  phozoites in the ileum but tend to colonize the cecum  and colon.   The trophozoites invade the colonic epithelium and  secrete enzymes that cause localized necrosis. Little inflam¬  mation occurs at the site. As the lesion reaches the muscu-  laris layer, a typical “flask-shaped” ulcer forms that can  undermine and destroy large areas of the intestinal epithe¬  lium (Figure 51-5). Progression into the submucosa leads  to invasion of the portal circulation by the trophozoites. By  far the most frequent site of systemic disease is the liver,  where abscesses containing trophozoites form.    Infection by E. histolytica is found worldwide but occurs  most frequently in tropical countries, especially in areas  with poor sanitation. About 1% to 2% of people in the  United States are affected. Infection is common in men  who have sex with men.   Acute intestinal amebiasis presents as dysentery (i.e.,  bloody, mucus-containing diarrhea) accompanied by lower  abdominal discomfort, flatulence, and tenesmus. Chronic  amebiasis with low-grade symptoms such as occasional  diarrhea, weight loss, and fatigue also occurs. Roughly 90%  of those infected have asymptomatic infections, but they  may be carriers, whose feces contain cysts that can be  transmitted to others. In some patients, a granulomatous  lesion called an ameboma may form in the cecal or recto¬  sigmoid areas of the colon. These lesions can resemble an  adenocarcinoma of the colon and must be distinguished  from them.   Amebic abscess of the liver is characterized by right-  upper-quadrant pain, weight loss, fever, and a tender,  enlarged liver. Right-lobe abscesses can penetrate the  diaphragm and cause lung disease. Most cases of amebic  liver abscess occur in patients who have not had overt  intestinal amebiasis. Aspiration of the liver abscess yields  brownish-yellow pus with the appearance and consistency  of anchovy paste.   Diagnosis of intestinal amebiasis rests on finding either tro¬  phozoites in diarrheal stools or cysts in formed stools (See  Figures 51-3 and 51-4). Diarrheal stools should be exam¬  ined within 1 hour of collection to see the ameboid motility  of the trophozoite. Trophozoites characteristically contain  ingested red blood cells. The most common error is to mis¬  take fecal leukocytes for trophozoites. Because cysts are  passed intermittently, at least three specimens should be  examined. The O&P test is insensitive and false negatives  commonly occur. Also, about half of the patients with  extraintestinal amebiasis have negative stool examinations.   Entamoeba histolytica can be distinguished from other  amebas by two major criteria: (1) The first is the nature of  the nucleus of the trophozoite. The E. histolytica nucleus  has a small central nucleolus and fine chromatin granules  along the border of the nuclear membrane. The nuclei of  other amebas are quite different. (2) The second is cyst size  and number of its nuclei. Mature cysts of E. histolytica are  smaller than those of Entamoeba coli and contain four  nuclei, whereas E. coli cysts have eight nuclei.   The trophozoites of Entamoeba dispar, a nonpathogenic  species of Entamoeba, are morphologically indistinguish¬  able from those of E. histolytica; therefore, a person who  has trophozoites in the stool is only treated if symptoms  warrant it. Two tests are highly specific for E. histolytica in   Chloroquine if sensitive; mefloquine  or quinine plus doxycycline or  Malarone or artemisinins if resistant    the stool: one detects E. histolytica antigen, and the other  detects nucleic acids of the organism in a polymerase chain  reaction (PCR)-based assay.   A complete examination for cysts includes a wet mount  in saline, an iodine-stained wet mount, and a fixed,  trichrome-stained preparation, each of which brings out  different aspects of cyst morphology. These preparations  are also helpful in distinguishing amebic from bacillary    dysentery. In the latter, many inflammatory cells such as  polymorphonuclear leukocytes are seen, whereas in amebic  dysentery, they are not.   Serologic testing is useful for the diagnosis of invasive  amebiasis. The indirect hemagglutination test is usually  positive in patients with invasive disease but is frequently  negative in asymptomatic individuals who are passing  cysts.    TABLE 51 -3 Medically Important Stages in Life Cycle of Intestinal Protozoa    FIGURE 51-1 Entamoeba histolytica. Life cycle. Top blue arrow shows cysts being ingested. Within the intestine, the cyst produces tro¬  phozoites that cause amebic dysentery in the colon and can spread to the liver (most often), lung, and brain (Boxes A and B). Bottom blue arrow  shows cysts and trophozoites being passed in the stool and entering the environment. Red arrow indicates survival of cysts in the environment.  (Source: Centers for Disease Control and Prevention.)   FIGURE 51-2 A: Entamoeba histolytica trophozoite with one  ingested red blood cell and one nucleus (circle with inner dotted line  represents a red blood cell). B: f. histolytica cyst with four nuclei.   C: Giardia lamblia trophozoite. D: G. lamblia cyst. E: Trichomonas  vaginalis trophozoite (1200x).   points to trophozoite of f. histolytica. Short arrow points to the  nucleus of the trophozoite. Arrowhead points to one of the six  ingested red blood cells. (Source: Centers for Disease Control and Prevention.)   FIGURE 51-4 Entamoeba histolytica —cyst. Arrow points to a  cyst of £ histolytica. Two of the four nuclei are visible just to the left of  the head of the arrow. (Source: Centers for Disease Control and Prevention.)   The treatment of choice for symptomatic intestinal amebia¬  sis or hepatic abscesses is metronidazole (Flagyl) or tinida-  zole. Hepatic abscesses need not be drained. Asymptomatic  cyst carriers should be treated with iodoquinol or  paromomycin.   Prevention involves avoiding fecal contamination of food  and water and observing good personal hygiene such as  handwashing. Purification of municipal water supplies is  usually effective, but outbreaks of amebiasis in city dwellers  still occur when contamination is heavy. The use of “night  soil” (human feces) for fertilization of crops should be pro¬  hibited. In areas of endemic infection, vegetables should be  cooked.   in colonic mucosa resulting in bloody diarrhea. (Source: Dr. Mae Melvin,  Centers for Disease Control and Prevention.)   The life cycle of G. lamblia is shown in Figure 51-6. The life  cycle consists of two stages: the trophozoite and the cyst  (Figures 51-2C and D, and 51-7). The trophozoite is pear-  shaped with two nuclei, four pairs of flagella, and a suction  disk with which it attaches to the intestinal wall. The oval  cyst is thick-walled with four nuclei and several internal  fibers. Each cyst gives rise to two trophozoites during  excystation in the intestinal tract.   Transmission occurs by ingestion of cysts in fecally con¬  taminated food and water. Excystation takes place in the  duodenum, where the trophozoite attaches to the gut wall  but does not invade the mucosa and does not enter the  bloodstream. The trophozoite causes inflammation of the  duodenal mucosa, leading to malabsorption of protein  and fat.   The organism is found worldwide; about 5% of stool  specimens in the United States contain Giardia cysts.  Approximately half of those infected are asymptomatic car¬  riers who continue to excrete the cysts for years. IgA defi¬  ciency greatly predisposes to symptomatic infection.   In addition to being endemic, giardiasis occurs in out¬  breaks related to contaminated water supplies. Chlorina¬  tion does not kill the cysts, but filtration removes them.  Hikers who drink untreated stream water are frequently  infected. Many species of mammals as well as humans act  as the reservoirs. They pass cysts in the stool, which then  contaminates water sources. Giardiasis is common in male  homosexuals as a result of oral-anal contact. The incidence  is high among children in day care centers and among  patients in mental hospitals.   Watery (nonbloody), foul-smelling diarrhea is accompa¬  nied by nausea, anorexia, flatulence, and abdominal cramps  persisting for weeks or months. There is no fever.   Diagnosis is made by finding trophozoites or cysts or both  in diarrheal stools (See Figure 51-7). In formed stools (e.g.,  in asymptomatic carriers), only cysts are seen. An enzyme-  linked immunosorbent assay (ELISA) test that detects  Giardia antigen in the stool is also very useful. Tests for  antibody in the serum are not routinely available.   If those tests are negative and symptoms persist, the  string test, which consists of swallowing a weighted piece    Trophozoites are also  passed in stool but  they do not survive in  the environment.    FIGURE 51-6 Giardia lamblia. Life cycle. Top blue arrow shows cysts being ingested. Within the intestine, the cyst produces trophozoites  that cause diarrhea. Bottom blue arrow shows cysts and trophozoites being passed in the stool and entering the environment. Red arrow indi¬  cates survival of cysts in the environment. (Source: Dr. Alexander J. da Silva and Melanie Moser, Centers for Disease Control and Prevention.)   FIGURE 51-7 Giardia lamblia —trophozoite. Arrow points to a  pear-shaped trophozoite of G. lamblia. (Source: Dr. M. Mosher, Centers for  Disease Control and Prevention.)    of string until it reaches the duodenum, may be useful. The  trophozoites adhere to the string and can be visualized after  withdrawal of the string.   The treatment of choice is either tinidazole (Tinidamax) or  metronidazole (Flagyl). Tinidazole is better tolerated.   Prevention involves drinking boiled, filtered, or iodine-  treated water in endemic areas and while hiking. No pro¬  phylactic drug or vaccine is available.   Cryptosporidium hominis causes cryptosporidiosis, the  main symptom of which is diarrhea. The diarrhea is most  severe in immunocompromised patients (e.g., those    with acquired immunodeficiency syndrome [AIDS]).  Cryptosporidium parvum is the former name that is no  longer used.   The life cycle of C. hominis is shown in Figure 51-8. Some  aspects of the life cycle remain uncertain, but the following  stages have been identified. Oocysts release sporozoites,  which form trophozoites. Several stages ensue, involving  the formation of schizonts and merozoites. Eventually  microgametes and macrogametes form; these unite to pro¬  duce a zygote, which differentiates into an oocyst. This  cycle has several features in common with other sporozoa  (e.g., Isospora ). Taxonomically, Cryptosporidium is in the  subclass Coccidia.   The organism is acquired by fecal-oral transmission of  oocysts from either human sources (primarily) or from    animal sources, for example, cattle (occasionally). The  oocysts excyst in the small intestine, where the trophozo¬  ites (and other forms) attach to the gut wall. Invasion does  not occur. The jejunum is the site most heavily infested.  The pathogenesis of the diarrhea is uncertain; no toxin has  been identified.   Cryptosporidia cause diarrhea worldwide. Large out¬  breaks of diarrhea caused by Cryptosporidia in several cities  in the United States are attributed to inadequate purifica¬  tion of drinking water. Other outbreaks are related to  swimming in fecally contaminated pools and lakes. The  cysts are highly resistant to chlorination but are killed by  pasteurization and can be removed by filtration.   The disease in immunocompromised patients presents  primarily as a watery, nonbloody diarrhea causing large  fluid loss. Symptoms persist for long periods in immuno¬  compromised patients, whereas they are self-limited in    FIGURE 51 ~8 Cryptosporidium hominis. Life cycle.Top blue arrow shows cysts being ingested. Within the intestine, the oocyst produces  trophozoites that cause diarrhea. Bottom blue arrow shows cysts being passed in the stool and entering the environment. Red arrow indicates  survival of cysts in the environment. (Source: Dr. Alexander J. da Silva and Melanie Moser, Centers for Disease Control and Prevention.)    immunocompetent patients. Although immunocompro¬  mised patients usually do not die of cryptosporidiosis, the  fluid loss and malnutrition are severely debilitating.   Diagnosis is made by finding oocysts in fecal smears when  using a modified Kinyoun acid-fast stain (Figure 51-9). A  test for Cryptosporidium antigen in the stool is also useful.   Nitazoxanide is the drug of choice for patients not infected  with human immunodeficiency virus (HIV). There is no  effective drug therapy for severely immunocompromised  patients, but paromomycin may be useful in reducing diar¬  rhea. There is no vaccine or other specific means of preven¬  tion. Purification of the water supply, including filtration to  remove the cysts, which are resistant to the chlorine used  for disinfection, can prevent cryptosporidiosis.   FIGURE 51 ~9 Cryptosporidium hominis —cysts. Acid-fast stain  of cysts in stool. Cysts appear red on a blue background. (Source: Centers  for Disease Control and Prevention; J Infect Dis. 1983;147(5):824-8.)   Trichomonas vaginalis is a pear-shaped organism with a  central nucleus and four anterior flagella (Figures 51-2E  and 51-10). It has an undulating membrane that extends  about two-thirds of its length. It exists only as a trophozo¬  ite; there is no cyst form.   FIGURE 51-10 Trichomonas vaginalis —trophozoite. Arrows  point to two trophozoites. (Source: Centers for Disease Control and Prevention.)   The organism is transmitted by sexual contact, and hence  there is no need for a durable cyst form. The primary loca¬  tions of the organism are the vagina and the prostate. It is  found only in humans; there is no animal reservoir.   Trichomoniasis is one of the most common infections  worldwide. Roughly 25% to 50% of women in the United  States harbor the organism. The frequency of symptomatic  disease is highest among sexually active women in their  thirties and lowest in postmenopausal women. Asymptom¬  atic infections are common in both men and women.   In women, a watery, foul-smelling, greenish vaginal dis¬  charge accompanied by itching and burning occurs. Infec¬  tion in men is usually asymptomatic, but about 10% of  infected men have urethritis.   In a wet mount of vaginal discharge, the pear-shaped tro¬  phozoites have a typical jerky motion (See Figure 51-10).  Neutrophils are often seen in the fluid. There is no sero¬  logic test.   The treatment of choice is either tinidazole (Tinidamax) or  metronidazole (Flagyl) for both partners to prevent rein¬  fection. Tinidazole is better tolerated. Maintenance of the  low pH of the vagina is helpful. Condoms limit transmis¬  sion. No prophylactic drug or vaccine is available.   1. Regarding Entamoeba histolytica , which one of the following is  most accurate?   (B) Domestic animals such as dogs and cats are the main reservoir  of E. histolytica.   (C) In the microscope, E. histolytica is recognized by having two  sets of paired flagella.   (D) Entamoeba histolytica infections are limited to the intestinal  mucosa and do not spread to other organs.   (E) The infection is typically acquired by the ingestion of the tro¬  phozoite in contaminated food and water.   2. Regarding Giardia lamblia , which one of the following is most  accurate?   (B) In giardiasis, ova and parasite (O&P) analysis of the stool  reveals sporozoites in the feces.   (C) Giardia lamblia produces an enterotoxin that increases cyclic  AMP within the enterocyte, resulting in diarrhea.   (D) Giardia lamblia infection is acquired by ingestion of food or  water contaminated with human feces only (i.e., there is no  animal reservoir for this organism).   (E) Infection by G. lamblia occurs principally in the small intestine,  frequently resulting in the malabsorption of fat and foul¬  smelling, frothy, fat-containing stools.   3. Regarding Cryptosporidium hominis, which one of the following is  most accurate?   (B) Microscopic examination of the diarrheal stool reveals both  red cells and white cells.   (C) Laboratory diagnosis involves seeing cysts of the organism in  an acid-fast stain of the stool.   (D) Cryptosporidium hominis is typically acquired by the ingestion  of trophozoites in contaminated food or water.   (E) In immunocompromised patients, such as AIDS patients with  a very low CD4 count, disseminated disease occurs that typi¬  cally involves the brain and meninges.   4. Regarding Trichomonas vaginalis , which one of the following is  most accurate?   (B) Domestic animals, such as dogs and cats, are the principal res¬  ervoir of the organism.   (C) Trichomonas vaginalis is typically acquired by contact with the  cysts of the organism during sexual intercourse.   (D) Laboratory diagnosis typically involves the detection of a  greater than fourfold rise in the titer of IgA antibody.   (E) The asymptomatic male sex partner of a woman with T. vaginalis  infection should not be treated because asymptomatic men are  rarely the source of the organism.   5. Your patient is a 30-year-old woman who returned from traveling  in Eastern Europe 1 week ago. While on the trip, she experienced  anorexia, nausea but no vomiting, and abdominal bloating. For  the last 2 days, she has had explosive watery diarrhea. An exami¬  nation of her stool revealed pear-shaped, flagellated, motile organ¬  isms. Of the following, which one is the most likely cause of this  infection?   6. Regarding the patient in Question 5, which one of the following is  the best antibiotic to treat the infection?   7. Your patient is a 30-year-old Peace Corps volunteer who has  recently returned from Central America. She now has fever and  right-upper-quadrant pain. She reports that she had bloody diar¬  rhea 2 months ago. A CT scan reveals a radiolucent area in the liver  that is interpreted to be an abscess. Aspiration of material from the  abscess was performed. Microscopic examination revealed motile,  nonflagellated trophozoites with ameboid movement. Of the fol¬  lowing, which one is the most likely cause of this infection?   8. Your patient is a 30-year-old man with persistent watery diar¬  rhea for 2 weeks. He is HIV antibody positive with a CD4 count  of 10. Routine stool culture revealed no bacterial pathogen. Ova  and parasite analysis revealed cysts that stained red in an acid-fast  stain. Of the following, which one is the most likely cause of this  infection?   Brief summaries of the organisms described in this chapter  begin on page 682. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Parasitology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 731. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   The medically important organisms in this category of pro¬  tozoa consist of the sporozoans Plasmodium and Toxoplasma  and the flagellates Trypanosoma and Leishmania. Pneumocystis  is discussed in this book as a protozoan because it is con¬  sidered as such from a medical point of view. However,  molecular data indicate that it is related to yeasts such as  Saccharomyces cerevisiae. Table 51-2 summarizes several  important features of these blood and tissue protozoa.   The medically important stages in the life cycle of the  blood and tissue protozoa are described in Table 52-1.   Malaria is caused primarily by four plasmodia: Plasmodium  vivax, Plasmodium ovale, Plasmodium malariae, and  Plasmodium falciparum. Plasmodium vivax and P. falciparum  are more common causes of malaria than are P. ovale and  P. malariae. Plasmodium vivax is most widely distributed  and P. falciparum causes the most serious disease. A fifth  species, Plasmodium knowlesi, is found in Southeast Asia.   Worldwide, malaria is one of the most common infec¬  tious diseases and one of the leading causes of death.   The life cycle of Plasmodium species is shown in Figure 52-1.  The vector and definitive host for plasmodia is the female  Anopheles mosquito (only the female takes a blood meal).    There are two phases in the life cycle: the sexual cycle, which  occurs primarily in mosquitoes, and the asexual cycle,  which occurs in humans, the intermediate hosts. 1   The sexual cycle is called sporogony because sporo¬  zoites are produced (sporogonic cycle is labeled C in  Figure 52-1), and the asexual cycle is called schizogony  because schizonts are made.   The life cycle in humans begins with the introduction of  sporozoites into the blood from the saliva of the biting  mosquito. The sporozoites are taken up by hepatocytes  within 30 minutes. This “exoerythrocytic” phase (labeled A  in Figure 52-1) consists of cell multiplication and differen¬  tiation into merozoites. Plasmodium vivax and P. ovale  produce a latent form (hypnozoite) in the liver; this form  is the cause of relapses seen with vivax and ovale malaria.   Merozoites are released from the liver cells and infect  red blood cells. During the erythrocytic phase (labeled B in  Figure 52-1), the organism differentiates into a ring-  shaped trophozoite (Figures 52-2A and B and 52-3). The  ring form grows into an ameboid form and then differenti¬  ates into a schizont filled with merozoites (Figure 52-2C).  After release, the merozoites infect other erythrocytes (step  6 in Figure 52-1). This cycle in the red blood cell repeats at  regular intervals typical for each species. The periodic  release of merozoites causes the typical recurrent symp¬  toms of chills, fever, and sweats seen in malaria patients.    lr The sexual cycle is initiated in humans with the formation of gameto-  cytes within red blood cells (gametogony) and completed in mosquitoes  with the fusion of the male and female gametes, oocyst formation, and  production of many sporozoites (sporogony).    TABLE 52-1 Medically Important Stages in Life Cycle of Blood and Tissue Protozoa   Tissue cyst (pseudocysts)  in undercooked meat or  oocyst in cat feces   Rapidly multiplying  trophozoites (tachyzoites)  within various cell types;  tachyzoites can pass placenta  and infect fetus; slowly  multiplying trophozoites  (bradyzoites) in tissue cysts    The sexual cycle begins in the human red blood cells  when some merozoites develop into male and others into  female gametocytes (Figures 52-2D to F and 52-4, and  step 7 in Figure 52-1). The gametocyte-containing red blood  cells are ingested by the female Anopheles mosquito and,  within her gut, produce a female macrogamete and eight  spermlike male microgametes. After fertilization, the dip¬  loid zygote differentiates into a motile ookinete that bur¬  rows into the gut wall, where it grows into an oocyst within  which many haploid sporozoites are produced. The sporo¬  zoites are released and migrate to the salivary glands, ready  to complete the cycle when the mosquito takes her next  blood meal.   A very important feature of P. falciparum is chloroquine  resistance. Chloroquine-resistant strains now predominate  in most areas of the world where malaria is endemic. Chlo¬  roquine resistance is mediated by a mutation in the gene  encoding the chloroquine transporter in the cell membrane  of the organism.   Most of the pathologic findings of malaria result from the  destruction of red blood cells. Red cells are destroyed  both by the release of the merozoites and by the action of  the spleen to first sequester the infected red cells and then    to lyse them. The enlarged spleen characteristic of malaria  is due to congestion of sinusoids with erythrocytes, coupled  with hyperplasia of lymphocytes and macrophages.   Malaria caused by P. falciparum is more severe than that  caused by other plasmodia. It is characterized by infection  of far more red cells than the other malarial species and by  occlusion of the capillaries with aggregates of parasitized  red cells. This leads to life-threatening hemorrhage and  necrosis, particularly in the brain (cerebral malaria). Fur¬  thermore, extensive hemolysis and kidney damage occur,  with resulting hemoglobinuria. The dark color of the  patient’s urine has given rise to the term “blackwater fever”.  The hemoglobinuria can lead to acute renal failure.   The timing of the fever cycle is 72 hours for P. malariae  and 48 hours for the other plasmodia. Disease caused by  P. malariae is called quartan malaria because it recurs every  fourth day, whereas malaria caused by the other plasmodia  is called tertian malaria because it recurs every third day.  Tertian malaria is subdivided into malignant malaria,  caused by P. falciparum, and benign malaria, caused by  P. vivax and P. ovale.   Plasmodium falciparum causes a high level of parasitemia  because it can infect red cells of all ages. In contrast, P. vivax  infects only reticulocytes and P. malariae infects only mature  red cells; therefore, they produce much lower levels of para¬  sites in the blood. Individuals with sickle cell trait    FIGURE 52-1 Plasmodium species. Life cycle. Right side of figure describes the stages within the human (blue arrows). Cycle A (top right) is  the exoerythrocyte stage that occurs in the liver. Note that only P. vivax and P. ovale produce hypnozoites (a latent form) in liver cells (not shown).  Cycle B (bottom right) is the erythrocyte stage that occurs in the red blood cell. Note that at step 6 in the cycle, merozoites released from the ruptured  schizonts then infect other red blood cells. The synchronized release of merozoites causes the periodic fever and chills characteristic of malaria. Left side  of figure describes the stages within the mosquito (red arrows). Humans are infected at step 1 when mosquito injects sporozoites. Mosquito is infected  at Step 8 when mosquito ingests gametocytes in human blood. (Source: Dr. Alexander J. da Silva and Melanie Moser, Centers for Disease Control and Prevention.)    (heterozygotes) are protected against malaria because their  red cells have too little ATPase activity and cannot produce  sufficient energy to support the growth of the parasite. Peo¬  ple with homozygous sickle cell anemia are also protected  but rarely live long enough to obtain much benefit.   The receptor for P. vivax is the Duffy blood group anti¬  gen. People who are homozygous recessive for the genes  that encode this protein are resistant to infection by P. vivax.  More than 90% of black West Africans and many of their  American descendants do not produce the Duffy antigen  and are thereby resistant to vivax malaria.   People with glucose-6-phosphate dehydrogenase (G6PD)  deficiency are also protected against the severe effects of  falciparum malaria. G6PD deficiency is an X-linked    hemoglobinopathy found in high frequency in tropical  areas where malaria is endemic. Both male and female  carriers of the mutated gene are protected against  malaria.   Malaria is transmitted primarily by mosquito bites, but  transmission across the placenta, in blood transfusions,  and by intravenous drug use also occurs.   Partial immunity based on humoral antibodies that  block merozoites from invading the red cells occurs in  infected individuals. A low level of parasitemia and low-  grade symptoms result; this condition is known as premu¬  nition. In contrast, a nonimmune person, such as a  first-time traveler to an area where falciparum malaria is  endemic, is at risk of severe, life-threatening disease.   FIGURE 52-2 A: Plasmodium vivax signet-ring trophozoite within a red blood cell. B: Plasmodium vivax ameboid trophozoite  within a red blood cell, showing Schuffner's dots. C: Plasmodium vivax mature schizont with merozoites inside. D: Plasmodium vivax  microgametocyte. E: Plasmodium vivax macrogametocyte. F: Plasmodium falciparum "banana-shaped" gametocyte with attached red  cell ghost. G: Toxoplasma gondii trophozoites within macrophage. H: Pneumocystis jiroveci cysts. (A-G, 1200x; H, 800x.)    More than 200 million people worldwide have malaria,  and more than 1 million die of it each year, making it the  most common lethal infectious disease. It occurs primarily  in tropical and subtropical areas, especially in Asia, Africa,  and Central and South America. Malaria in the United   ite. Long arrow points to a red blood cell containing a ring-shaped  trophozoite. Arrowhead points to a red blood cell containing four  ring-shaped trophozoites. Note the very high percentage of red cells  containing ring forms. This high-level parasitemia is more often seen  in Plasmodium falciparum infection than in infection by the other  plasmodia. (Source: Dr. S. Glenn, Public Health Image Library, Centers for Disease  Control and Prevention.)    States is seen in Americans who travel to areas of endemic  infection without adequate chemoprophylaxis and in  immigrants from areas of endemic infection. It is not  endemic in the United States. Certain regions in Southeast  Asia, South America, and east Africa are particularly  affected by chloroquine-resistant strains of P. falciparum.  People who have lived or traveled in areas where malaria  occurs should seek medical attention for febrile illnesses up  to 3 years after leaving the malarious area.   FIGURE 52-4 Plasmodium falciparum —gametocyte. Arrow  points to a "banana-shaped" gametocyte of Plasmodium falciparum.  (Source: Dr. S. Glenn, Public Health Image Library, Centers for Disease Control and  Prevention.)   Malaria presents with abrupt onset of fever and chills,  accompanied by headache, myalgias, and arthralgias, about  2 weeks after the mosquito bite. Fever may be continuous  early in the disease; the typical periodic cycle does not  develop for several days after onset. The fever spike, which  can reach 41°C, is frequently accompanied by shaking  chills, nausea, vomiting, and abdominal pain. The fever is  followed by drenching sweats. Patients usually feel well  between the febrile episodes. Splenomegaly is seen in most  patients, and hepatomegaly occurs in roughly one-third.  Anemia is prominent.   Untreated malaria caused by P. falciparum is potentially  life-threatening as a result of extensive brain (cerebral  malaria) and kidney (blackwater fever) damage. Malaria  caused by the other three plasmodia is usually self-limited,  with a low mortality rate. However, relapses of P. vivax and  P. ovale malaria can occur up to several years after the ini¬  tial illness as a result of hypnozoites latent in the liver.   Diagnosis rests on microscopic examination of blood,  using both thick and thin Giemsa-stained smears. The  thick smear is used to screen for the presence of organisms,  and the thin smear is used for species identification. It is  important to identify the species because the treatment of  different species can differ. Ring-shaped trophozoites can  be seen within infected red blood cells (see Figure 52-3).  The gametocytes of P. falciparum are crescent-shaped  (“banana-shaped”), whereas those of the other plasmodia  are spherical (Figure 52-2F). If more than 5% of red blood  cells are parasitized, the diagnosis is usually P. falciparum  malaria.   Plasmodium species typically produce hemozoin pig¬  ment in infected red blood cells whereas Babesia species  (see Chapter 53) do not. Plasmodia metabolize heme in the  red cells to produce hemozoin. Also found within P. vivax  and P. ovfl/e-infected red cells are Schiiffner’s dots. These  are intracytoplasmic granules that stain red using the  Romanovsky stain.    If blood smears do not reveal the diagnosis, then a poly¬  merase chain reaction (PCR)-based test for Plasmodium  nucleic acids or an enzyme-linked immunosorbent assay  (ELISA) test for a protein specific for P. falciparum can be  useful.   The treatment of malaria is complicated, and the details are  beyond the scope of this book. Table 52-2 presents the  drugs commonly used in the United States. The main crite¬  ria used for choosing specific drugs are the severity of the  disease and whether the organism is resistant to chloro-  quine. Chloroquine resistance is determined by the geo¬  graphical location where the infection was acquired rather  than by laboratory testing.   Chloroquine is the drug of choice for treatment of  uncomplicated malaria caused by non-falciparum species  in areas without chloroquine resistance. Chloroquine kills  the merozoites, thereby reducing the parasitemia, but does  not affect the hypnozoites of P. vivax and P. ovale in the  liver. These are killed by primaquine, which must be used  to prevent relapses. Primaquine may induce severe hemo¬  lysis in those with G6PD deficiency, so testing for this  enzyme should be done before the drug is given. Prima¬  quine should not be given if the patient is severely G6PD  deficient. If primaquine is not given, one approach is to  wait to see whether symptoms recur and then treat with  chloroquine.   Uncomplicated, chloroquine-resistant P. falciparum  infection is treated with either Coartem (artemether plus  lumefantrine) or Malarone (atovaquone and proguanil). In  severe complicated cases of chloroquine-resistant falci¬  parum malaria, intravenous administration of either arte-  sunate or quinidine is used.   Outside the United States, the artemisinins, such as arte-  sunate or artemether, are widely used in combination with  other antimalarial drugs. The artemisinins are inexpensive  and have few side effects. However, P. falciparum has devel¬  oped resistance to artemisinins in mainland Southeast Asia  (e.g., Vietnam, Cambodia, Myanmar, and Thailand).    TABLE 52-2 Drugs Commonly Used for the Treatment of Malaria in the United States   ’Available in the United States through the Centers for Disease Control and Prevention.  2 lf intravenous quinidine is used, cardiac monitoring should be in place.   Chemoprophylaxis of malaria for travelers to areas where  chloroquine-resistant P. falciparum is endemic consists of  mefloquine or doxycycline. A combination of atovaquone  and proguanil (Malarone), in a fixed dose, can also be used.  Chloroquine should be used in areas where P. falciparum is  sensitive to that drug. Travelers to areas where the other  three plasmodia are found should take chloroquine starting  2 weeks before arrival in the endemic area and continuing  for 4 weeks after leaving the endemic area. This should be  followed by a 2-week course of primaquine if exposure was  high. Primaquine will kill the hypnozoites of P. vivax and  P. ovale.   Other preventive measures include the use of mosquito  netting, window screens, protective clothing, and insect  repellents. The mosquitoes feed from dusk to dawn, so  protection is particularly important during the night.  Communal preventive measures are directed against  reducing the mosquito population. Many insecticide  sprays, such as DDT, are no longer effective because the  mosquitoes have developed resistance. Drainage of stag¬  nant water in swamps and ditches reduces the breeding  areas. There is no vaccine.   The life cycle of T. gondii is shown in Figure 52-5. The  definitive host is the domestic cat and other felines;  humans and other mammals are intermediate hosts. Infec¬  tion of humans begins with the ingestion of cysts in under¬  cooked meat or from accidental contact with cysts in cat  feces. In the small intestine, the cysts rupture and release  forms that invade the gut wall, where they are ingested by  macrophages and differentiate into rapidly multiplying  trophozoites (tachyzoites), which kill the cells and infect  other cells (Figures 52-2G and 52-6). Cell-mediated  immunity usually limits the spread of tachyzoites, and the  parasites enter host cells in the brain, muscle, and other  tissues, where they develop into cysts in which the parasites  multiply slowly. These forms are called bradyzoites. These  tissue cysts are both an important diagnostic feature and a  source of organisms when the tissue cyst breaks in an  immunocompromised patient.   The cycle within the cat begins with the ingestion of  cysts in raw meat (e.g., mice). Bradyzoites are released from  the cysts in the small intestine, infect the mucosal cells, and  differentiate into male and female gametocytes, whose  gametes fuse to form oocysts that are excreted in cat feces.  The cycle is completed when soil contaminated with cat    feces is accidentally ingested. Human infection usually  occurs from eating undercooked meat (e.g., lamb and pork)  from animals that grazed in soil contaminated with infected  cat feces.   Toxoplasma gondii is usually acquired by ingestion of cysts  in uncooked meat or in food accidentally contaminated by  cat feces.   Transplacental transmission from an infected mother  to the fetus occurs also. Human-to-human transmission,  other than transplacental transmission, does not occur.  After infection of the intestinal epithelium, the organisms  spread to other organs, especially the brain, lungs, liver, and  eyes. Progression of the infection is usually limited by a  competent immune system. Cell-mediated immunity  plays the major role, but circulating antibody enhances kill¬  ing of the organism. Most initial infections are asymptom¬  atic. When contained, the organisms persist as cysts within  tissues. There is no inflammation, and the individual  remains well unless immunosuppression allows activation  of organisms in the cysts.   Congenital infection of the fetus occurs only when  the mother is infected during pregnancy. If she is infected  before the pregnancy, the organism will be in the cyst  form and there will be no trophozoites to pass through  the placenta. The mother who is reinfected during preg¬  nancy but who has immunity from a previous infection  will not transmit the organism to her child. Roughly one-  third of mothers infected during pregnancy give birth to  infected infants, but only 10% of these infants are  symptomatic.   Infection by T. gondii occurs worldwide. Serologic sur¬  veys reveal that in the United States antibodies are found in  5% to 50% of people in various regions. Infection is usually  sporadic, but outbreaks associated with ingestion of raw  meat or contaminated water occur. Approximately 1% of  domestic cats in the United States shed Toxoplasma cysts.   Most primary infections in immunocompetent adults are  asymptomatic, but some resemble infectious mononucleo¬  sis, except that the heterophil antibody test is negative.  Congenital infection can result in abortion, stillbirth, or  neonatal disease with encephalitis, chorioretinitis, and  hepatosplenomegaly. Fever, jaundice, and intracranial cal¬  cifications are also seen. Most infected newborns are  asymptomatic, but chorioretinitis or mental retardation  will develop in some children months or years later. Con¬  genital infection with Toxoplasma is one of the leading  causes of blindness in children. In patients with reduced  cell-mediated immunity (e.g., patients with acquired  immunodeficiency syndrome [AIDS]), life-threatening  disseminated disease, primarily encephalitis, occurs.    Both oocysts and tissue cysts transform  into tachyzoites shortly after ingestion.   Tachyzoites localize in neural and  muscle tissue and develop into tissue  cyst bradyzoites. If a pregnant woman  becomes infected, tachyzoites can infect  the fetus via the bloodstream.   2. Direct identification of the parasite from peripheral  blood, amniotic fluid, or in tissue sections    FIGURE 52~5 Toxoplasma gondii. Life cycle. Top red arrows show the natural life cycle as T. gondii circulates between cats (#1), which  excrete oocysts in the feces that are eaten by mice, but also by domestic animals such as pigs and sheep. Cysts form in tissue such as muscle  and brain. The natural cycle is completed when cats eat mice. Humans are accidental hosts. They can be infected by the ingestion of under¬  cooked pork and lamb (blue arrow #2) containing tissue cysts in muscle or by ingestion of food contaminated with cat feces containing oocysts  (blue arrow #3). (Source: Dr. Alexander J. da Silva and Melanie Moser, Centers for Disease Control and Prevention.)   For the diagnosis of acute and congenital infections, an  immunofluorescence assay for IgM antibody is used. IgM  is used to diagnose congenital infection, because IgG can    be maternal in origin. Tests of IgG antibody can be used to  diagnose acute infections if a significant rise in antibody  titer in paired sera is observed.   Microscopic examination of Giemsa-stained prepara¬  tions shows crescent-shaped trophozoites during acute   a tachyzoite of T. gondii in cardiac muscle. (Source: Dr. E. Ewing, Jr, Public  Health Image Library, Centers for Disease Control and Prevention.)   infections. Cysts may be seen in the tissue. The organism  can be grown in cell culture. Inoculation into mice can  confirm the diagnosis.   Congenital toxoplasmosis, whether symptomatic or asymp¬  tomatic, should be treated with a combination of sulfadia¬  zine and pyrimethamine. These drugs also constitute the  treatment of choice for disseminated disease in immuno¬  compromised patients. Acute toxoplasmosis in an immu¬  nocompetent individual is usually self-limited, but any  patient with chorioretinitis should be treated.   The most effective means of preventing toxoplasmosis is to  cook meat thoroughly to kill the cysts. Pregnant women  should be especially careful to avoid undercooked meat and  contact with cat feces. They should refrain from emptying  cat litter boxes. Cats should not be fed raw meat. Trime¬  thoprim-sulfamethoxazole is used to prevent Toxoplasma  encephalitis in patients infected with human immunodefi¬  ciency virus (HIV).   Pneumocystis jiroveci is an important cause of pneumonia  in immunocompromised individuals. In 2002, taxonomists  renamed the human species of Pneumocystis as P. jiroveci  and recommended that Pneumocystis carinii be used only  to describe the rat species of Pneumocystis.   The classification and life cycle of Pneumocystis are unclear.  Many aspects of its biochemistry indicate that it is a yeast,   P. jiroveci in lung tissue. (Source: Dr. E. Ewing, Jr., Public Health Image Library,  Centers for Disease Control and Prevention.)   but it also has several attributes of a protozoan. An analysis  of rRNA sequences published in 1988 indicates that  Pneumocystis should be classified as a fungus related to  yeasts such as Saccharomyces cerevisiae. Subsequent analysis  of mitochondrial DNA and of various enzymes supports  the idea that it is a fungus. However, it does not have ergos-  terol in its membranes as do the fungi. It has cholesterol.   Medically, it is still thought of as a protozoan. In tissue,  it appears as a cyst that resembles the cysts of protozoa  (Figures 52-2H and 52-7). The findings that it does not  grow on fungal media and that antifungal drugs are ineffec¬  tive have delayed acceptance of its classification as a  fungus.   Pneumocystis species are found in domestic animals  such as horses and sheep and in a variety of rodents, but it  is thought that these animals are not a reservoir for human  infection. Each mammalian species is thought to have its  own species of Pneumocystis.   Pneumocystis species have a major surface glycoprotein  that exhibits significant antigenic variation in a manner  similar to that of Trypanosoma brucei. Pneumocystis species  have multiple genes encoding these surface proteins, but  only one is expressed at a time. This process of pro¬  grammed rearrangements was first observed in T. brucei.   Transmission occurs by inhalation, and infection is pre¬  dominantly in the lungs. The presence of cysts in the alveoli  induces an inflammatory response consisting primarily of  plasma cells, resulting in a frothy exudate that blocks oxy¬  gen exchange. (The presence of plasma cells has led to the  name “plasma cell pneumonia”.) The organism does not  invade the lung tissue.   Pneumonia occurs when host defenses (e.g., the number  of CD4-positive [helper] T cells) are reduced. This accounts  for the prominence of Pneumocystis pneumonia in patients    with AIDS and in premature or debilitated infants. Hospital  outbreaks do not occur, and patients with Pneumocystis  pneumonia are not isolated.   Pneumocystis jiroveci is distributed worldwide. It is  estimated that 70% of people have been infected. Most  5-year-old children in the United States have antibodies  to this organism. Asymptomatic infection is therefore  quite common. Prior to the advent of immunosuppres¬  sive therapy, Pneumocystis pneumonia was rarely seen in  the United States. Its incidence has paralleled the increase  in immunosuppression and the rise in the number of  AIDS cases.   Most Pneumocystis infections in AIDS patients are new  rather than a reactivation of a prior latent infection. This  conclusion is based on the finding that Pneumocystis recov¬  ered from AIDS patients shows resistance to drugs that the  patients have not taken.   The sudden onset of fever, nonproductive cough, dyspnea,  and tachypnea is typical of Pneumocystis pneumonia.  Bilateral rales and rhonchi are heard, and the chest X-ray  shows a diffuse interstitial pneumonia with “ground glass”  infiltrates bilaterally. In infants, the disease usually has a  more gradual onset. Extrapulmonary Pneumocystis infec¬  tions occur in the late stages of AIDS and affect primarily  the liver, spleen, lymph nodes, and bone marrow. The  mortality rate of untreated Pneumocystis pneumonia  approaches 100%.   Diagnosis is made by finding the typical cysts by micro¬  scopic examination of lung tissue or fluids obtained by  bronchoscopy, bronchial lavage, or open lung biopsy  (see Figure 52-7). Sputum is usually less suitable. The  cysts can be visualized with methenamine silver, Giemsa,  or other tissue stains. Fluorescent-antibody staining is  also commonly used for diagnosis. PCR-based tests  using respiratory tract specimens are also useful. The  organism stains poorly with Gram stain. There is no  serologic test, and the organism has not been grown in  culture.   The treatment of choice is a combination of trimethoprim  and sulfamethoxazole (Bactrim, Septra). Pentamidine and  atovaquone are alternative drugs.   Trimethoprim-sulfamethoxazole or aerosolized pentami¬  dine should be used as chemoprophylaxis in patients whose  CD4 counts are below 200.   The genus Trypanosoma includes three major pathogens:  Trypanosoma cruzi, Trypanosoma gambiense, and Trypano¬  soma rhodesiense . 2   The life cycle of T. cruzi is shown in Figure 52-8. The life  cycle involves the reduviid bug ( Triatoma , cone-nose or  kissing bug) as the vector, and both humans and animals as  reservoir hosts. The animal reservoirs include domestic  cats and dogs and wild species such as the armadillo, rac¬  coon, and rat. The cycle in the reduviid bug begins with  ingestion of trypomastigotes in the blood of the reservoir  host. In the insect gut, they multiply and differentiate first  into epimastigotes and then into trypomastigotes. When  the bug bites again, the site is contaminated with feces con¬  taining trypomastigotes, which enter the blood of the per¬  son (or other reservoir) and form nonflagellated amastigotes  within host cells. Many cells can be affected, but myocar¬  dial, glial, and reticuloendothelial cells are the most fre¬  quent sites. To complete the cycle, amastigotes differentiate  into trypomastigotes, which enter the blood and are taken  up by the reduviid bug (Figures 52-9A to C and 52-10).   Chagas’ disease occurs primarily in rural Central and  South America. Acute Chagas’ disease occurs rarely in the  United States, but the chronic form causing myocarditis  and congestive heart failure is seen with increasing fre¬  quency in immigrants from Latin America. The disease is  seen primarily in rural areas because the reduviid bug  lives in the walls of rural huts and feeds at night. It bites  preferentially around the mouth or eyes, hence the name  “kissing bug”.   The amastigotes can kill cells and cause inflammation,  consisting mainly of mononuclear cells. Cardiac muscle is  the most frequently and severely affected tissue. In addi¬  tion, neuronal damage leads to cardiac arrhythmias and  loss of tone in the colon (megacolon) and esophagus  (megaesophagus). During the acute phase, there are both  trypomastigotes in the blood and amastigotes intracellu-  larly in the tissues. In the chronic phase, the organism  persists in the amastigote form.    2 Taxonomically, the last two organisms are morphologically identical  species called T. brucei gambiense and T. brucei rhodesiense, but the  shortened names are used here.   Intracellular amastigotes  transform into trypomastigotes,  then burst out of the cell  and enter the bloodstream    Trypomastigotes  can infect other cells  and transform into  I intracellular amastigotes  in new infection sites.    Triatomine bug takes a blood meal  . (passes metacyclic trypomastigotes in feces,  r trypomastigotes enter bite wound or  mucosal membrane, such as the conjunctiva)    Metacyclic trypomastigotes  penetrate various cells at bite  wound site. Inside cells they  transform into amastigotes    FIGURE 52-8 Trypanosoma cruzi. Life cycle. Right side of figure describes the stages within the human (blue arrows). Humans are  infected at step 1 when triatomine (reduviid) bug bites human and defecates at bite site. Trypomastigotes in feces enter bite wound.  Amastigotes form within cells, especially heart muscle and neural tissue. Reduviid bug is infected at step 5 when it ingests trypomastigotes  in human blood. Left side of figure describes the stages within the reduviid bug (red arrows). (Source: Dr. Alexander J. da Silva and Melanie Moser,  Centers for Disease Control and Prevention.)    Chagas’ disease has occurred in the United States in  recipients of either blood transfusions or organ transplants  from infected donors. The organism can also be transmit¬  ted congenitally from an infected mother to the fetus across  the placenta.   The acute phase of Chagas’ disease consists of facial edema  and a nodule (chagoma) near the bite, coupled with fever,  lymphadenopathy, and hepatosplenomegaly. A bite around  the eye can result in unilateral palpebral swelling called  Romana’s sign. The acute phase resolves in about 2 months.    Most individuals then remain asymptomatic, but some  progress to the chronic form with myocarditis and megaco¬  lon. Death from chronic Chagas’ disease is usually due to  cardiac arrhythmias or congestive heart failure.   Acute disease is diagnosed by demonstrating the presence  of trypomastigotes in thick or thin films of the patient’s  blood. Both stained and wet preparations should be exam¬  ined, the latter for motile organisms. Because the trypomas¬  tigotes are not numerous in the blood, other diagnostic  methods maybe required, namely (1) a stained preparation   FIGURE 52~9 A: Trypanosoma cruzi trypomastigote found in human blood (1200x). B: T. cruzi amastigotes found in cardiac muscle  (850x). C: T. cruzi epimastigote found in reduviid bug (1200x). D: Trypanosoma brucei gambiense or rhodesiense trypomastigote found in human  blood (1200x). E: Leishmania donovani amastigotes within splenic macrophages (lOOOx). (Circle with inner dotted line represents a red blood cell.)    of a bone marrow aspirate or muscle biopsy specimen  (which may reveal amastigotes); (2) culture of the organism  on special medium; and (3) xenodiagnosis, which consists  of allowing an uninfected, laboratory-raised reduviid bug to  feed on the patient and, after several weeks, examining the  intestinal contents of the bug for the organism.   Serologic tests can be helpful also. The indirect fluores¬  cent antibody test is the earliest to become positive. Indi¬  rect hemagglutination and complement fixation tests are  also available. Diagnosis of chronic disease is difficult  because there are few trypomastigotes in the blood. Xeno¬  diagnosis and serologic tests are used.   The drug of choice for the acute phase is nifurtimox, which  kills trypomastigotes in the blood but is much less effective   points to an amastigote (nonflagellated form) in cytoplasm.  (Source: Dr. A. J. Sulzer, Public Health Image Library, Centers for Disease Control  and Prevention.)    against amastigotes in tissue. Benznidazole is an alternative  drug. There is no effective drug against the chronic form.   Prevention involves protection from the reduviid bite,  improved housing, and insect control. No prophylactic  drug or vaccine is available. Blood for transfusion is tested  for the presence of antibodies to T. cruzi. Blood containing  antibodies should not be used.   These organisms cause sleeping sickness (African trypano¬  somiasis). They are also known as Trypanosoma brucei  gambiense and Trypanosoma brucei rhodesiense.   The life cycle of Trypanosoma brucei is shown in  Figure 52-11. The morphology and life cycle of the two  species are similar. The vector for both is the tsetse fly,  Glossina, but different species of fly are involved for each.  Humans are the reservoir for T. gambiense, whereas  T. rhodesiense has reservoirs in both domestic animals  (especially cattle) and wild animals (e.g., antelopes).   The 3-week life cycle in the tsetse fly begins with inges¬  tion of trypomastigotes in a blood meal from the reservoir  host. They multiply in the insect gut and then migrate to  the salivary glands, where they transform into epimasti-  gotes, multiply further, and then form metacyclic trypo¬  mastigotes, which are transmitted by the tsetse fly bite. The  organisms in the saliva are injected into the skin, where  they enter the bloodstream, differentiate into blood-form  trypomastigotes, and multiply, thereby completing the  cycle (Figures 52-9D and 52-12). Note that these species    Bloodstream trypomastigotes  transform into procyclic  trypomastigotes in tsetse  fly’s midgut. Procyclic  trypomastigotes  multiply by binary fission    Trypomastigotes multiply  by binary fission in various  body fluids, e.g., blood,  lymph, and spinal fluid    FIGURE 52~11 Trypanosoma brucei. Life cycle. Right side of figure describes the stages within the human (blue arrows). Humans are  infected at step 1 when the tsetse fly bites human and injects trypomastigotes into bloodstream. Tsetse fly is infected at step 5 when it ingests  trypomastigotes in human blood. Left side of figure describes the stages within the tsetse fly (red arrows). (Source: Dr. Alexander J. da Silva and Melanie  Moser, Centers for Disease Control and Prevention.)    are rarely found as amastigotes in tissue, in contrast to  T. cruzi and Leishmania species, in which amastigotes are  commonly found.   These trypanosomes exhibit remarkable antigenic vari¬  ation of their surface glycoproteins, with hundreds of anti¬  genic types found. One antigenic type will coat the surface  of the parasites for approximately 10 days, followed by  other types in sequence in the new progeny. This variation  is due to sequential movement of the glycoprotein genes to  a preferential location on the chromosome, where only that  specific gene is transcribed into mRNA. These antigenic  variations allow the organism to continually evade the host  immune response.   The trypomastigotes spread from the skin through the  blood to the lymph nodes and the brain. The typical som¬  nolence (sleeping sickness) progresses to coma as a result  of a demyelinating encephalitis.   In the acute form, a cyclical fever spike (approximately  every 2 weeks) occurs that is related to antigenic variation.  As antibody-mediated agglutination and lysis of the trypo¬  mastigotes occur, the fever subsides. However, a few anti¬  genic variants survive, multiply, and cause a new fever  spike. This cycle repeats itself over a long period. The lytic  antibody is directed against the surface glycoprotein.   points to a trypomastigote (the flagellated form) in the blood. (Source:  Dr. M. Schultz, Public Health Image Library, Centers for Disease Control and  Prevention.)   The disease is endemic in sub-Saharan Africa, the natu¬  ral habitat of the tsetse fly. Both sexes of fly take blood  meals and can transmit the disease. The fly is infectious  throughout its 2- to 3-month lifetime. Trypanosoma gambiense  is the species that causes the disease along water courses in  west Africa, whereas T. rhodesiense is found in the arid  regions of east Africa. Both species are found in central  Africa.   Although both species cause sleeping sickness, the progress  of the disease differs. Trypanosoma gambiense-induced  disease runs a low-grade chronic course over a few years,  whereas T. rhodesiense causes a more acute, rapidly pro¬  gressive disease that, if untreated, is usually fatal within  several months.   The initial lesion is an indurated skin ulcer (“trypano-  somal chancre”) at the site of the fly bite. After the organ¬  isms enter the blood, intermittent weekly fever and  lymphadenopathy develop. Enlargement of the posterior  cervical lymph nodes (Winterbottom’s sign) is commonly  seen. The encephalitis is characterized initially by head¬  ache, insomnia, and mood changes, followed by muscle  tremors, slurred speech, and apathy that progress to som¬  nolence and coma. Untreated disease is usually fatal as a  result of pneumonia.   During the early stages, microscopic examination of the  blood (either wet films or thick or thin smears) reveals  trypomastigotes (see Figure 52-12). An aspirate of the  chancre or enlarged lymph node can also demonstrate the  parasites. The presence of trypanosomes in the spinal fluid,  coupled with an elevated protein level and pleocytosis,    indicates that the patient has entered the late, encephalitic  stage. Serologic tests, especially the ELISA for IgM anti¬  body, can be helpful.   Treatment must be initiated before the development of  encephalitis, because suramin, the most effective drug,  does not pass the blood-brain barrier well. Suramin will  effect a cure if given early. Pentamidine is an alternative  drug. If central nervous system symptoms are present,  suramin (to clear the parasitemia) followed by melarsoprol  should be given.   The most important preventive measure is protection  against the fly bite, using netting and protective clothing.  Clearing the forest around villages and using insecticides  are helpful measures. No vaccine is available.   The genus Leishmania includes four major pathogens:  Leishmania donovani, Leishmania tropica, Leishmania  mexicana, and Leishmania braziliensis.   The life cycle of L. donovani is shown in Figure 52-13. The  life cycle involves the sandfly 3 as the vector and a variety of  mammals such as dogs, foxes, and rodents as reservoirs.   Only female flies are vectors because only they take blood  meals (a requirement for egg maturation). When the sandfly  sucks blood from an infected host, it ingests macrophages-  containing amastigotes (Figures 52-9E and 52-14). 4   After dissolution of the macrophages, the freed amasti¬  gotes differentiate into promastigotes in the gut. They mul¬  tiply and then migrate to the pharynx and proboscis, where  they can be transmitted during the next bite. The cycle in  the sandfly takes approximately 10 days.   Shortly after an infected sandfly bites a human, the pro¬  mastigotes are engulfed by macrophages, where they trans¬  form into amastigotes (Figure 52-9E). Amastigotes can  remain in the cytoplasm of macrophages because they can  prevent fusion of the vacuole with lysosomes.    3 Phlebotomus species in the Old World; Lutzomyia species in South  America.   4 Amastigotes are nonflagellated, in contrast to promastigotes, which have  a flagellum with a characteristic anterior kinetoplast.    FIGURE 52-13 Leishmania donovani. Life cycle. Right side of figure describes the stages within the human (blue arrows). Humans are  infected at step 1 when the sandfly bites human and injects promastigotes. Sandfly is infected at step 5 when it ingests macrophages contain¬  ing amastigotes in human blood. Left side of figure describes the stages within the sandfly (red arrows). (Source: Dr. Alexander J. da Silva and Melanie  Moser, Centers for Disease Control and Prevention.)   FIGURE 52-14 Leishmania donovani —amastigotes. Arrow  points to an amastigote (nonflagellated form) in cytoplasm of bone  marrow cell. (Source: Dr. Francis Chandler, Public Health Image Library, Centers  for Disease Control and Prevention.)    The infected cells die and release progeny amastigotes  that infect other macrophages and reticuloendothelial cells.  The cycle is completed when the fly ingests macrophages  containing the amastigotes.   In visceral leishmaniasis, the organs of the reticuloendo¬  thelial system (liver, spleen, and bone marrow) are the  most severely affected. Reduced bone marrow activity,  coupled with cellular destruction in the spleen, results in  anemia, leukopenia, and thrombocytopenia. This leads  to secondary infections and a tendency to bleed. The  striking enlargement of the spleen is due to a combina¬  tion of proliferating macrophages and sequestered blood  cells. The marked increase in IgG is neither specific nor  protective.   Kala-azar occurs in three distinct epidemiologic pat¬  terns. In one area, which includes the Mediterranean basin,    the Middle East, southern Russia, and parts of China, the  reservoir hosts are primarily dogs and foxes. In sub-  Saharan Africa, rats and small carnivores (e.g., civets) are  the main reservoirs. A third pattern is seen in India and  neighboring countries (and Kenya), in which humans  appear to be the only reservoir.   Symptoms begin with intermittent fever, weakness, and  weight loss. Massive enlargement of the spleen is character¬  istic. Hyperpigmentation of the skin is seen in light¬  skinned patients (kala-azar means black sickness). The  course of the disease runs for months to years. Initially,  patients feel reasonably well despite persistent fever. As  anemia, leukopenia, and thrombocytopenia become more  profound, weakness, infection, and gastrointestinal bleed¬  ing occur. Untreated severe disease is nearly always fatal as  a result of secondary infection.   Diagnosis is usually made by detecting amastigotes in a  bone marrow, spleen, or lymph node biopsy or “touch”  preparation (see Figure 52-14). The organisms can also be  cultured. Serologic (indirect immunofluorescence) tests are  positive in most patients. Although not diagnostic, a very  high concentration of IgG is indicative of infection. A skin  test using a crude homogenate of promastigotes (leishma-  nin) as the antigen is available. The skin test is negative  during active disease but positive in patients who have  recovered.   The drug of choice is either liposomal amphotericin B or  sodium stibogluconate. With proper therapy, the mortality  rate is reduced to almost 5%. Recovery results in perma¬  nent immunity.   Prevention involves protection from sandfly bites (use of  netting, protective clothing, and insect repellents) and  insecticide spraying.   Leishmania tropica and L. mexicana both cause cutaneous  leishmaniasis; the former organism is found in the Old  World, whereas the latter is found only in the Americas.  Leishmania braziliensis causes mucocutaneous leishmania¬  sis, which occurs only in Central and South America.   Sandflies are the vectors for these three organisms, as they  are for L. donovani, and forest rodents are their main reser¬  voirs. The life cycle of these parasites is essentially the same  as that of L. donovani.   The lesions are confined to the skin in cutaneous leish¬  maniasis and to the mucous membranes, cartilage, and  skin in mucocutaneous leishmaniasis. A granulomatous  response occurs, and a necrotic ulcer forms at the bite site.  The lesions tend to become superinfected with bacteria.   Old World cutaneous leishmaniasis (Oriental sore,  Delhi boil), caused by L. tropica, is endemic in the Middle  East, Africa, and India. New World cutaneous leishmania¬  sis (chicle ulcer, bay sore), caused by L. mexicana, is found  in Central and South America. Mucocutaneous leishmani¬  asis (espundia), caused by L. braziliensis, occurs mostly in  Brazil and Central America, primarily in forestry and con¬  struction workers.   The initial lesion of cutaneous leishmaniasis is a red papule  at the bite site, usually on an exposed extremity. This  enlarges slowly to form multiple satellite nodules that  coalesce and ulcerate. There is usually a single lesion that  heals spontaneously in patients with a competent immune  system. However, in certain individuals, if cell-mediated  immunity does not develop, the lesions can spread to  involve large areas of skin and contain enormous numbers  of organisms.   Mucocutaneous leishmaniasis begins with a papule at  the bite site, but then metastatic lesions form, usually at the  mucocutaneous junction of the nose and mouth. Disfigur¬  ing granulomatous, ulcerating lesions destroy nasal carti¬  lage but not adjacent bone. These lesions heal slowly, if at  all. Death can occur from secondary infection.   Diagnosis is usually made microscopically by demonstrat¬  ing the presence of amastigotes in a smear taken from the  skin lesion. The leishmanin skin test becomes positive  when the skin ulcer appears and can be used to diagnose  cases outside the area of endemic infection.   The drug of choice is sodium stibogluconate, but the results  are frequently unsatisfactory.   Prevention involves protection from sandfly bites by using  netting, window screens, protective clothing, and insect  repellents.   (A) These organisms are transmitted by the bite of female  Anopheles mosquitoes.   (B) The bite of the vector injects merozoites into the bloodstream  that then infect red blood cells.   (C) Both male and female gametocytes are formed in the vector  and are injected into the person at the time of the bite.   (D) Hypnozoites are produced by P. falciparum and can cause  relapses of malaria after the acute phase is over.   (E) Malaria caused by P. vivax is characterized by a cerebral  malaria and blackwater fever more often than malaria caused  by the other three species.   2. Regarding drugs used to treat or prevent malaria, which one of   (A) The combination of atovaquone and proguanil is useful for  the treatment of acute malaria but not for prevention.   (B) Chloroquine is the drug of choice in malaria caused by  P. falciparum because resistance to the drug is rare.   (C) Mefloquine is useful for the prevention of chloroquine-  sensitive P. falciparum but not for chloroquine-resistant strains.   (D) Artemisinin derivatives, such as artesunate and artemether,  are effective in the treatment of multiple-drug resistant  P. falciparum.   (E) Primaquine is useful in the treatment of infections caused by  P. falciparum because it kills the hypnozoites residing in the  liver.   3. Regarding T. gondii, which one of the following is most accurate?   (A) One way to prevent this infection is to advise pregnant  women not to drink unpasteurized milk.   (B) The form of Toxoplasma found in the tissue cysts in humans  is the rapidly dividing tachyzoite.   (C) The most important definitive host (the host in which the  sexual cycle occurs) for Toxoplasma is the domestic cat.   (D) Infection in people with reduced cell-mediated immunity,  such as AIDS patients, is characterized by persistent watery  (nonbloody) diarrhea.   (E) If your patient is a pregnant woman who has IgM antibody to  Toxoplasma in her blood, then you can tell her that it is  unlikely that her fetus is at risk for infection.   4. Regarding P. jiroveci , which one of the following is most accurate?   (A) The treatment of choice is a combination of penicillin G and  an aminoglycoside.   (B) Finding oval cysts in bronchial lavage fluid supports a diagno¬  sis of Pneumocystis pneumonia.   (C) Large domestic animals such as cows and sheep are an impor¬  tant reservoir of human infection with this organism.   (D) Patients with a CD4 count below 200 should receive the vac¬  cine containing the surface glycoprotein as the immunogen.   (E) Transmission occurs by the ingestion of food contaminated  with the organism, after which it enters the bloodstream and  is transported to the lung.   (B) The drug of choice for the acute phase of Chagas’ disease is  chloroquine.   (C) The vector for T. cruzi, the cause of Chagas’ disease, is the  reduviid (cone-nosed) bug.    (D) Seeing trypomastigotes in a muscle biopsy supports the diag¬  nosis of Chagas’ disease.   (E) The main site of disease caused by T. cruzi is skeletal muscle,  resulting in severe muscle pain.   6. Regarding leishmaniasis, which one of the following is most  accurate?   (A) Mefloquine is effective in preventing disease caused by  L. donovani.   (B) Large domestic animals such as cattle are the principal reser¬  voir of L. donovani.   (C) Both visceral leishmaniasis and cutaneous leishmaniasis are  transmitted by the bite of sandflies.   (D) Marked enlargement of the heart on chest X-ray is a typical  finding of visceral leishmaniasis.   (E) Pathologists examining a specimen for the presence of  L. donovani should look primarily at eosinophils in the  peripheral blood.   7. Your patient is a 20-year-old man who, while playing soccer,  experienced palpitations and dizziness and then fainted. An  electrocardiogram showed right bundle branch block. Holter  monitoring showed multiple runs of ventricular tachycardia.  A ventricular myocardial biopsy was performed. Microscopic  examination revealed a lymphocytic inflammatory process sur¬  rounding areas containing amastigotes. The patient was born and  raised in rural El Salvador and came to this country 2 years ago.  Of the following, which one is the most likely cause?   8. Your patient is a 25-year-old man with fever and weight loss for  the past 3 weeks. He is a soldier in the U.S. Army who recently  returned from a tour of duty in the Middle East. Physical exam  was noncontributory. Laboratory tests revealed anemia and leu¬  kopenia. Multiple blood cultures for bacteria and fungi were  negative, as was a test for the p24 antigen of HIV. CT scan of  the abdomen revealed splenomegaly. A bone marrow biopsy was  performed, and a stained sample revealed amastigotes within  mononuclear cells. Of the following, which one is the most likely  cause?   9. Your patient is a 55-year-old man with fever and increasing  fatigue during the past week. Today, he was so weak he “could  barely stand up”. He had been working in Cameroon and Chad  for 2 months and returned 2 weeks ago. On examination, he was  febrile to 40°C, hypotensive, and tachycardic. Pertinent lab work  revealed anemia and thrombocytopenia. Blood smear revealed  ring-shaped trophozoites within red blood cells. Of the follow¬  ing, which one is the most likely cause?   10. Your patient is a 35-year-old woman who has just had a seizure.  A CT scan shows a ring-enhancing lesion in her brain. History    reveals that she is an intravenous drug user and is HIV anti¬  body positive with a CD4 count of 30. Serologic tests confirm  that the patient is infected with Toxoplasma gondii. Which one  of the following is the best choice of drug to treat her cerebral  toxoplasmosis?   11. Regarding the patient in Question 10, she was treated and recov¬  ered without sequelae. Antiretroviral therapy was instituted. As  long as her CD4 count remains below 100, she should receive  chemoprophylaxis to prevent recurrent disease caused by T. gondii.  Which one of the following is the best chemoprophylactic drug?   Brief summaries of the organisms described in this chapter  begin on page 682. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Parasitology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 731. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.    The medically important stages in the life cycle of certain  minor protozoa are described in Table 53-1.   Acanthamoeba castellanii and Naegleria fowleri are free-  living amebas that cause meningoencephalitis. The organ¬  isms are found in warm freshwater lakes and in soil. Their  life cycle involves trophozoite and cyst stages. Cysts are  quite resistant and are not killed by chlorination.   Naegleria trophozoites usually enter the body through  mucous membranes while an individual is swimming.  They can penetrate the nasal mucosa and cribriform plate  to produce a purulent meningitis and encephalitis that are  usually rapidly fatal (Figure 53-1). Acanthamoeba is car¬  ried into the skin or eyes during trauma. Acanthamoeba  infections occur primarily in immunocompromised indi¬  viduals, whereas Naegleria infections occur in otherwise  healthy persons, usually children. In the United States,  these rare infections occur mainly in the southern states  and California.   Diagnosis is made by finding amebas in the spinal fluid.  The prognosis is poor even in treated cases. Amphotericin  B may be effective in Naegleria infections. Pentamidine,   two ameba-shaped trophozoites in brain tissue. (Source: Public Health  Image Library, Centers for Disease Control and Prevention.)   Acanthamoeba also causes keratitis—an inflammation  of the cornea that occurs primarily in those who    TABLE 53-1 Medically Important Stages in Life Cycle of Certain Minor Protozoa    wear contact lenses. With increasing use of contact lenses,  keratitis has become the most common disease associated  with Acanthamoeba infection. The amebas have been  recovered from contact lenses, lens cases, and lens disinfec¬  tant solutions. Tap water contaminated with amebas is the  source of infection for lens users.   Babesia microti causes babesiosis—a zoonosis acquired  chiefly in the coastal areas and islands off the northeastern  coast of the United States (e.g., Nantucket Island). The  sporozoan organism is endemic in rodents and is trans¬  mitted by the bite of the tick Ixodes dammini (renamed  I. scapularis), the same species of tick that transmits Borrelia  burgdorferi, the agent of Lyme disease. Babesia infects red  blood cells, causing them to lyse, but unlike plasmodia, it  has no exoerythrocytic phase. Asplenic patients are affected  more severely.   The influenzalike symptoms begin gradually and may  last for several weeks. Hepatosplenomegaly and anemia  occur. Diagnosis is made by seeing intraerythrocytic ring-  shaped parasites on Giemsa-stained blood smears. The  intraerythrocytic ring-shaped trophozoites are often in  tetrads in the form of a Maltese cross (Figure 53-2). Unlike  the case with plasmodia, there is no pigment in the eryth¬  rocytes. The treatment of choice for mild to moderate dis¬  ease is a combination of atovaquone and azithromycin.  Patients with severe disease should receive a combination  of quinidine and clindamycin. Exchange transfusion should  also be considered in patients with severe disease. Preven¬  tion involves protection from tick bites and, if a person is  bitten, prompt removal of the tick.   points to a red blood cell containing four trophozoites in a tetrad  resembling a "Maltese cross". (Source: Dr. S. Glenn, Public Health Image  Library, Centers for Disease Control and Prevention.)   Balantidium coli is the only ciliated protozoan that  causes human disease (i.e., diarrhea). It is found world¬  wide but only infrequently in the United States. Domestic  animals, especially pigs, are the main reservoir for the  organism, and humans are infected after ingesting the  cysts in food or water contaminated with animal or  human feces. The trophozoites excyst in the small intes¬  tine, travel to the colon, and, by burrowing into the wall,  cause an ulcer similar to that of Entamoeba histolytica.  However, unlike the case with E. histolytica, extraintesti-  nal lesions do not occur.   Most infected individuals are asymptomatic; diarrhea  rarely occurs. Diagnosis is made by finding large ciliated  trophozoites or large cysts with a characteristic V-shaped  nucleus in the stool. There are no serologic tests. The treat¬  ment of choice is tetracycline. Prevention consists of  avoiding contamination of food and water by domestic  animal feces.   Cyclospora cayetanensis is an intestinal protozoan that  causes watery diarrhea in both immunocompetent and  immunocompromised individuals. It is classified as a  member of the Coccidia. 1   The organism is acquired by fecal-oral transmission,  especially via contaminated water supplies. One outbreak  in the United States was attributed to the ingestion of con¬  taminated raspberries. There is no evidence for an animal  reservoir.   The diarrhea can be prolonged and relapsing, especially  in immunocompromised patients. Infection occurs world¬  wide. The diagnosis is made microscopically by observing  the spherical oocysts in a modified acid-fast stain of a stool  sample. There are no serologic tests. The treatment of  choice is trimethoprim-sulfamethoxazole.   Isospora belli is an intestinal protozoan that causes diar¬  rhea, especially in immunocompromised patients (e.g.,  those with acquired immunodeficiency syndrome [AIDS]).  Its life cycle parallels that of other members of the Coccidia.  The organism is acquired by fecal-oral transmission of  oocysts from either human or animal sources. The oocysts  excyst in the upper small intestine and invade the mucosa,  causing destruction of the brush border.   The disease in immunocompromised patients presents  as a chronic, profuse, watery diarrhea. The pathogenesis  of the diarrhea is unknown. Diagnosis is made by finding    the typical oocysts in fecal specimens. Serologic tests are 3.  not available. The treatment of choice is trimethoprim-  sulfamethoxazole.   Microsporidia are a group of protozoa characterized by  obligate intracellular replication and spore formation. As  the name implies, the spores are quite small, approximately  1 to 3 mm, about the size of Escherichia coli. One unique  feature of these spores is a “polar tube,” which is coiled  within the spore and extrudes to attach to the human cells  upon infection. The protoplasm of the spore then enters the  human cell via the polar tube.   Enterocytozoon bieneusi and Encephalitozoon intestinalis  are two important microsporidial species that cause severe,  persistent, watery diarrhea in AIDS patients. The organ¬  isms are transmitted from human to human by the fecal-  oral route. Microsporidia are also implicated in infections  of the central nervous system, the genitourinary tract, and  the eye. It is uncertain whether an animal reservoir exists.  Diagnosis is made by visualization of spores in stool sam¬  ples or intestinal biopsy samples. The treatment of choice is  albendazole.   (A) They are free-living amebas that live in warm fresh water.   (B) Naegleria is a well-recognized cause of otitis media, primarily  in children.   (C) The drug of choice for infections caused by these organisms is  chloroquine.   (D) Their main clinical presentation is pneumonia acquired when  water is aspirated into the lung.   2. Regarding B. microti, which one of the following is most accurate?   (D) Seeing sporozoites within red cells supports the diagnosis of  babesiosis.   (E) Babesia microti causes disease primarily in the northeastern  region of the United States.    Your patient is a 10-year-old girl with a fever and a severe head¬  ache for the past 2 days. Pertinent history includes swimming in  a pond near their home in rural California in August. On exam,  nuchal rigidity was observed and a lumbar puncture was per¬  formed. The spinal fluid white blood cell count was 12,200 with  80% neutrophils. Microscopic examination of a wet mount of spi¬  nal fluid revealed motile trophozoites. Of the following, which one  is the most likely cause?   4. Your patient is a 50-year-old man with a fever and shaking chills  who had been vacationing 2 weeks ago on one of the islands off the  coast of Massachusetts. Microscopic examination of a blood smear  reveals ring-shaped trophozoites in tetrads within red blood cells.  Of the following, which one is the most likely cause?   Brief summaries of the organisms described in this chapter  begin on page 684. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Parasitology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 731. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   Platyhelminthes ( platy means flat; helminth means worm)  are divided into two classes: Cestoda (tapeworms) and  Trematoda (flukes). The trematodes are described in  Chapter 55.   Tapeworms consist of two main parts: a rounded head  called a scolex and a flat body consisting of multiple seg¬  ments. Each segment is called a proglottid. The scolex has  specialized means of attaching to the intestinal wall,  namely, suckers, hooks, or sucking grooves. The worm  grows by adding new proglottids from its germinal center  next to the scolex. The oldest proglottids at the distal end  are gravid and produce many eggs, which are excreted in  the feces and transmitted to various intermediate hosts  such as cattle, pigs, and fish.   Humans usually acquire the infection when under¬  cooked meat or fish containing the larvae is ingested.    However, in two important human diseases, cysticercosis  and hydatid disease, it is the eggs that are ingested and the  resulting larvae cause the disease.   There are four medically important cestodes: Taenia  solium, Taenia saginata, Diphyllobothrium latum, and  Echinococcus granulosus. Their features are summarized in  Table 54-1, and the medically important stages in the life  cycle of these organisms are described in Table 54-2. Three  cestodes of lesser importance, Echinococcus multilocularis,  Hymenolepis nana, and Dipylidium caninum, are described  at the end of this chapter.   There are two important human pathogens in the genus  Taenia: T. solium (the pork tapeworm) and T. saginata  (the beef tapeworm).   (B) Ingest eggs in food or  water contaminated with  human feces    TABLE 54-2 Medically Important Stages in Life Cycle of Cestodes (Tapeworms)   The adult form of T. solium causes taeniasis. Taenia solium  larvae cause cysticercosis.   The life cycle of T. solium is shown in Figure 54-1. Taenia  solium can be identified by its scolex, which has four suck¬  ers and circle of hooks, and by its gravid proglottids,  which have 5 to 10 primary uterine branches (Figures  54-2A, B and 54-3). The eggs appear the same micro¬  scopically as those of T. saginata and Echinococcus species  (Figure 54-4A).   In taeniasis, the adult tapeworm is located in the human  intestine (see Figure 54-1). This occurs when humans are  infected by eating raw or undercooked pork containing the  larvae, called cysticerci. (A cysticercus consists of a pea¬  sized fluid-filled bladder with an invaginated scolex.) In the  small intestine, the larvae attach to the gut wall and take  about 3 months to grow into adult worms measuring up to  5 m. The gravid terminal proglottids containing many eggs  detach daily, are passed in the feces, and are accidentally  eaten by pigs. Note that pigs are infected by the worm eggs;  therefore, it is the larvae (cysticerci) that are found in the  pig. A six-hooked embryo (oncosphere) emerges from each  egg in the pig’s intestine. The embryos burrow into a blood  vessel and are carried to skeletal muscle. They develop into  cysticerci in the muscle, where they remain until eaten by a  human. Humans are the definitive hosts, and pigs are the  intermediate hosts.   In cysticercosis, a more dangerous sequence occurs  when a person ingests the worm eggs in food or water that  has been contaminated with human feces (Figure 54-5).  Note that in cysticercosis, humans are infected by eggs  excreted in human feces, not by ingesting undercooked  pork. Also, pigs do not have the adult worm in their intes¬  tine, so they are not the source of the eggs that cause  human cysticercosis. The eggs hatch in the small intestine,  and the oncospheres burrow through the wall into a blood    vessel. They can disseminate to many organs, especially  the eyes and brain, where they encyst to form cysticerci  (Figure 54-6). Each cysticercus contains a larva.   The adult tapeworm attached to the intestinal wall causes  little damage. The cysticerci, on the other hand, can  become very large, especially in the brain, where they  manifest as a space-occupying lesion (see Figure 54-6).  Living cysticerci do not cause inflammation, but when they  die, they can release substances that provoke an inflamma¬  tory response. Eventually, the cysticerci calcify.   The epidemiology of taeniasis and cysticercosis is related  to the access of pigs to human feces and to consumption of  raw or undercooked pork. The disease occurs worldwide  but is endemic in areas of Asia, South America, and Eastern  Europe. Most cases in the United States are imported.   Most patients with adult tapeworms are asymptomatic, but  anorexia and diarrhea can occur. Some may notice proglot¬  tids in the stools. Cysticercosis in the brain causes head¬  ache, vomiting, and seizures. Cysticercosis in the eyes can  appear as uveitis or retinitis, or the larvae can be visualized  floating in the vitreous. Subcutaneous nodules containing  cysticerci commonly occur. Cysts also are commonly found  in skeletal muscle.   Identification of T. solium consists of finding gravid pro¬  glottids with 5 to 10 primary uterine branches in the  stools. In contrast, T. saginata proglottids have 15 to 20  primary uterine branches. Eggs are found in the stools less  often than are proglottids. Diagnosis of cysticercosis  depends on demonstrating the presence of the cyst in tis¬  sue, usually by surgical removal or computed tomography  (CT) scan. Serologic tests (e.g., enzyme-linked immuno¬  sorbent assay [ELISA]) that detect antibodies to T. solium  antigens are available, but they may be negative in  neurocysticercosis.    Cattle (7. saginata) and pigs (7. solium)  become infected by ingesting vegetation  contaminated by eggs or gravid proglottids    FIGURE 54-1 Taenia solium and Taenia saginata. Life cycle. Right side of figure describes the stages within the human (blue arrows).  Humans are infected at step 1 when they ingest undercooked pork (T. solium ) or beef (I saginata) containing cysticerci (larval stage). Adult  tapeworms form in intestine and lay eggs. Pigs and cattle are infected when they ingest either the eggs or proglottids in human stool. Left  side of figure describes the stages within the pigs and cattle (red arrows). (Source: Public Health Image Library, Centers for Disease Control and  Prevention.)   The treatment of choice for the intestinal worms is praziqu¬  antel. The treatment for cysticercosis is either praziquantel  or albendazole, but surgical excision may be necessary.   Prevention of taeniasis involves cooking pork adequately  and disposing waste properly so that pigs cannot ingest  human feces. Prevention of cysticercosis consists of treat¬  ment of patients to prevent autoinfection plus observation  of proper hygiene, including handwashing, to prevent con¬  tamination of food with the eggs.   Taenia saginata causes taeniasis. Taenia saginata larvae do  not cause cysticercosis.   Taenia saginata has a scolex with four suckers but, in contrast  to T. solium, no hooklets. Its gravid proglottids have 15 to 25  primary uterine branches, in contrast to T. solium proglottids,  which have 5 to 10 (see Figure 54-2C and D). The eggs are  morphologically indistinguishable from those of T. solium.   FIGURE 54-2 A: Taenia solium scolex with suckers and hooks (1 Ox). B: Taenia solium gravid proglottid. This has fewer uterine branches  than does the proglottid of Taenia saginata (see panel D) (2x). C: T. saginata scolex with suckers (1 Ox). D: T. saginata gravid proglottid (2x).  E: Diphyllobothrium latum scolex with sucking grooves (7x). F: Entire adult worm of Echinococcus granulosus (7x). G: £ granulosus adult scolex (70x).   Long arrow points to one of the four suckers on the scolex of  T. solium. Short arrow points to the circle of hooklets. Proglottids can  be seen extending from the scolex toward the left side of the image.  (Source: Dr. M. Melvin, Public Health Image Library, Centers for Disease Control and  Prevention.)   FIGURE 54-4 A: Taenia solium egg containing oncosphere  embryo. Four hooklets are visible. Taenia saginata and Echinococcus  granulosus eggs are very similar to the T. solium egg but do not have  hooklets. B: Diphyllobothrium latum egg with an operculum on the  top (300x).    I Oncospheres develop  into cysticerci in muscles  of pigs or humans    Cysticercosis  Humans acquire the infection  by ingesting eggs in human feces.  Cysticerci may develop  in any organ, but are more  common in the brain,  eyes and skin    Humans acquire the  infection by ingesting  undercooked pork from  infected animal host    Embryonated eggs or gravid proglottids  i ingested by pigs (red arrow)  Embryonated eggs ingested by humans  ^ ^ (blue arrow)    FIGURE 54-5 Taenia solium. Life cycle including cysticercosis stage. Center and left side of figure describes the cycle of T. solium within  the human and the pig similar to Figure 54-1. Note, however, that there are now blue arrows between the egg at the bottom that go up the left  side of the figure to the person at the top right. In cysticercosis, humans are infected when they ingest the eggs of T. solium in food contami¬  nated with human feces. The eggs differentiate into cysticerci primarily in brain, eyes, and skin. (Source: Dr. Alexander J. da Silva and Melanie Moser,  Centers for Disease Control and Prevention.)    The life cycle of T. saginata is shown in Figure 54-1.  Humans are infected by eating raw or undercooked beef  containing larvae (cysticerci). In the small intestine, the  larvae attach to the gut wall and take about 3 months to  grow into adult worms measuring up to 10 m (Figure 54-7).  The gravid proglottids detach, are passed in the feces, and  are eaten by cattle. The embryos (oncospheres) emerge  from the eggs in the cows intestine and burrow into a  blood vessel, where they are carried to skeletal muscle. In  the muscle, they develop into cysticerci. The cycle is    completed when the cysticerci are ingested. Humans are  the definitive hosts and cattle the intermediate hosts.  Unlike T. solium, T. saginata does not cause cysticercosis  in humans.   Little damage results from the presence of the adult worm  in the small intestine. The epidemiology of taeniasis caused  by T. saginata is related to the access of cattle to human  feces and to the consumption of raw or undercooked beef.   arrow points to a larva of T. solium. Short arrow points to the wall of  the cysticercus (sac) that surrounds the larva. (Used with permission from  Rhodes B. Holliman, PhD, Professor Emeritus, Virgina Tech.)   The disease occurs worldwide but is endemic in Africa,  South America, and Eastern Europe. In the United States,  most cases are imported.   Most patients with adult tapeworms are asymptomatic, but  malaise and mild cramps can occur. In some, proglottids  appear in the stools and may even protrude from the anus.  The proglottids are motile and may cause pruritus ani as  they move on the skin adjacent to the anus.   Identification of T. saginata consists of finding gravid pro¬  glottids with 15 to 20 uterine branches in the stools. Eggs  are found in the stools less often than are the proglottids.   scolex on the right side of the image and the gravid proglottids on  the left side of the image. White arrow points to the scolex. Ruler is  1 2 inches long. (Source: Public Health Image Library, Centers for  Disease Control and Prevention.)   Prevention involves cooking beef adequately and disposing  waste properly so that cattle cannot consume human feces.   In contrast to the other cestodes, which have suckers,  the scolex of D. latum has two elongated sucking  grooves by which the worm attaches to the intestinal wall  (see Figure 54-2E). The scolex has no hooks, unlike  T. solium and Echinococcus. The proglottids are wider than  they are long, and the gravid uterus is in the form of a  rosette. Unlike other tapeworm eggs, which are round,  D. latum eggs are oval and have a lidlike opening (operculum)  at one end (see Figure 54-4B). Diphyllobothrium latum is  the longest of the tapeworms, measuring up to 13 m.   Humans are infected by ingesting raw or undercooked  fish containing larvae (called plerocercoid or sparganum  larvae). In the small intestine, the larvae attach to the gut  wall and develop into adult worms. Gravid proglottids  release fertilized eggs through a genital pore, and the eggs  are then passed in the stools. The immature eggs must be  deposited in fresh water for the life cycle to continue. The  embryos emerge from the eggs and are eaten by tiny cope-  pod Crustacea (first intermediate hosts). There, the embryos  differentiate and form procercoid larvae in the body cavity.  When the copepod is eaten by freshwater fish (e.g., pike,  trout, and perch), the larvae differentiate into plerocercoids  in the muscle of the fish (second intermediate host). The  cycle is completed when raw or undercooked fish is eaten  by humans (definitive hosts).   Infection by D. latum causes little damage in the small  intestine. In some individuals, megaloblastic anemia occurs  as a result of vitamin B 12 deficiency caused by preferential  uptake of the vitamin by the worm.   The epidemiology of D. latum infection is related to the  ingestion of raw or inadequately cooked fish and to con¬  tamination of bodies of fresh water with human feces. The  disease is found worldwide but is endemic in areas where  eating raw fish is the custom, such as Scandinavia, northern  Russia, Japan, Canada, and certain north-central states of  the United States.   Most patients are asymptomatic, but abdominal discomfort  and diarrhea can occur.   Diagnosis depends on finding the typical eggs (i.e., oval,  yellow-brown eggs with an operculum at one end, in the  stools). There is no serologic test.   Prevention involves adequate cooking of fish and proper  disposal of human feces.   Echinococcus granulosus (dog tapeworm) causes echino¬  coccosis. The larva of E. granulosus causes unilocular  hydatid cyst disease. Multilocular hydatid disease is caused  by E. multilocularis, which is a minor pathogen and is  discussed later.   Echinococcus granulosus is composed of a scolex and only  three proglottids, making it one of the smallest tapeworms  (see Figures 54-2F and G). The scolex has a circle of hooks  and four suckers similar to T. solium. Dogs are the most  important definitive hosts. The intermediate hosts are usu¬  ally sheep. Humans are almost always dead-end intermedi¬  ate hosts.   The life cycle of E. granulosus is shown in Figure 54-8.  In the typical life cycle, worms in the dog’s intestine liberate  thousands of eggs, which are ingested by sheep (or humans)  (see Figure 54-4). The oncosphere embryos emerge in the  small intestine and migrate primarily to the liver but also to  the lungs, bones, and brain. The embryos develop into large  fluid-filled hydatid cysts, the inner germinal layer of which  generates many protoscoleces within “brood capsules.” The  life cycle is completed when the entrails (e.g., liver contain¬  ing hydatid cysts) of slaughtered sheep are eaten by dogs.   Echinococcus granulosus usually forms one large fluid-filled  cyst (unilocular) that contains thousands of individual  scoleces as well as many daughter cysts within the large  cyst. Individual scoleces lying at the bottom of the large  cyst are called “hydatid sand”. The cyst acts as a space-  occupying lesion, putting pressure on adjacent tissue. The  outer layer of the cyst is thick, fibrous tissue produced by    the host. The cyst fluid contains parasite antigens, which  can sensitize the host. Later, if the cyst ruptures spontane¬  ously or during trauma or surgical removal, life-threatening  anaphylactic shock can occur. Rupture of a cyst can also  spread protoscoleces widely.   The disease is found primarily in shepherds living in the  Mediterranean region, the Middle East, and Australia. In  the United States, the western states report the largest num¬  ber of cases.   Many individuals with hydatid cysts are asymptomatic, but  liver cysts may cause hepatic dysfunction. Cysts in the lungs  can erode into a bronchus, causing bloody sputum, and  cerebral cysts can cause headache and focal neurologic  signs. Rupture of the cyst can cause fatal anaphylactic shock.   Diagnosis is based either on microscopic examination  demonstrating the presence of brood capsules containing  multiple protoscoleces or on serologic tests (e.g., the indi¬  rect hemagglutination test).   Treatment involves albendazole with or without surgical  removal of the cyst. Extreme care must be exercised to pre¬  vent release of the protoscoleces during surgery. A proto-  scolicidal agent (e.g., hypertonic saline) should be injected  into the cyst to kill the organisms and prevent accidental  dissemination.   Many of the features of this organism are the same as those  of E. granulosus, but the definitive hosts are mainly foxes  and the intermediate hosts are various rodents. Humans are  infected by accidental ingestion of food contaminated with  fox feces. The disease occurs primarily in hunters and trap¬  pers and is endemic in northern Europe, Siberia, and the  western provinces of Canada. In the United States, it occurs  in North and South Dakota, Minnesota, and Alaska.   Within the human liver, the larvae form multiloculated  cysts with few protoscoleces. No outer fibrous capsule  forms, so the cysts continue to proliferate, producing a  honey-comb effect of hundreds of small vesicles. The clini¬  cal picture usually involves jaundice and weight loss. The  prognosis is poor. Albendazole treatment may be successful  in some cases. Surgical removal may be feasible.    FIGURE 54~8 Echinococcus granulosus. Life cycle. Center and left side of figure describes the natural cycle off. granulosus within dogs  (top half) and sheep (bottom half). Dogs are the definitive hosts and contain the adult tapeworm in the intestines. Sheep are an important  intermediate host and ingest the eggs in dog feces. Hydatid cysts containing larvae form in the sheep. Humans are accidental intermediate  hosts when they ingest food contaminated with dog feces containing the eggs (#2 at blue arrow at right). Eggs hatch oncosphere embryos in  human intestine (#3 in human figure). Hydatid cysts form primarily in the liver, lung, brain, and bone (#4 in human figure). (Source: Dr. Alexander  J. da Silva and Melanie Moser, Centers for Disease Control and Prevention.)   Hymenolepis nana (dwarf tapeworm) is the most fre¬  quently found tapeworm in the United States. It is only 3 to  5 cm long and is different from other tapeworms because  its eggs are directly infectious for humans (i.e., ingested  eggs can develop into adult worms without an intermediate  host). Within the duodenum, the eggs hatch and differenti¬  ate into cysticercoid larvae and then into adult worms.  Gravid proglottids detach, disintegrate, and release fertil¬  ized eggs. The eggs either pass in the stool or can reinfect  the small intestine (autoinfection). In contrast to infection    by other tapeworms, where only one adult worm is present,  many H. nana worms (sometimes hundreds) are found.   Infection causes little damage, and most patients are  asymptomatic. The organism is found worldwide, com¬  monly in the tropics. In the United States, it is most preva¬  lent in the southeastern states, usually in children. Diagnosis  is based on finding eggs in stools. The characteristic feature  of H. nana eggs is the 8 to 10 polar filaments lying between  the membrane of the six-hooked larva and the outer shell.  The treatment is praziquantel. Prevention consists of good  personal hygiene and avoidance of fecal contamination of  food and water.   Dipylidium caninum is the most common tapeworm of  dogs and cats. It occasionally infects humans, usually  young children, while playing with their pets. Human  infection occurs when dog or cat fleas carrying cysticerci  are ingested. The cysticerci develop into adult tapeworms  in the small intestine. Most human infections are asymp¬  tomatic, but diarrhea and pruritus ani can occur. The diag¬  nosis in animals and humans is made by observing the  typical “barrel-shaped” proglottids in the stool or diapers.  Niclosamide is the drug of choice.   1. Regarding Taenia solium, which one of the following is most   (A) The scolex of T. solium has four suckers and a circle of  hooklets.   (B) The drug of choice for the adult worm in humans is  metronidazole.   (C) The cysticercus of T. solium contains the mature eggs of the  organism.   (D) In the laboratory, identification of adult worms is based on  finding the typical scolex in the stool.   (E) Ingestion of the terminal proglottids of T. solium by pigs results  in mature tapeworms in the pig’s intestine.   3. Regarding D. latum, which one of the following is most accurate?   (B) Megaloblastic anemia may occur as a result of vitamin B 12  deficiency.   (C) The laboratory diagnosis depends on finding a scolex with  hooklets in the stool.   (D) Infection is acquired by the ingestion of eggs in food or water  contaminated with human feces.   (E) Larvae migrate from the gastrointestinal tract via the portal  circulation to the liver, where abscesses can occur.   4. Regarding E. granulosus, which one of the following is most   (A) The drug of choice for E. granulosus infection is  metronidazole.   (B) Dogs are a required part of the life cycle of the causative  organism.   (C) Echinococcus granulosus is one of the longest tapeworms, some¬  times measuring 10 ft in length.   (D) Echinococcus granulosus larvae typically migrate to skeletal  muscle, where they cause an abscess.   (E) The main mode of transmission to humans is ingestion of eggs  in food or water contaminated with human feces.    Your patient is a 15-year-old girl with a 2-week history of head¬  ache and vomiting and a 3-day history of confusion and incoher¬  ent speech. She was born in Ecuador but moved to this country  5 years ago. MRI of the brain reveals multiple lesions bilaterally.  The following day, she has a seizure and dies. On autopsy, the  brain lesions consist of a cyst-like sac containing a larva. Of the  following, which one is the most likely cause?   Your patient is a 40-year-old man with occasional mild right upper  abdominal discomfort but is otherwise well. On examination, his  liver is enlarged. An MRI reveals a cystic mass in the liver. On  questioning, he says that he was born and raised in rural Argen¬  tina on a sheep ranch and came to this country 10 years ago. Of the  following, which one is the most likely cause?   7. Your patient is a 20-year-old woman who is a recent immigrant  from Central America. On routine exam, a stool ova and parasite  test reveal eggs resembling those of T. solium. Which one of the  following is the best choice of drug to treat this patient?   Brief summaries of the organisms described in this chapter  begin on page 685. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Parasitology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 731. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   Trematoda (flukes) and Cestoda (tapeworms) are the two  large classes of parasites in the phylum Platyhelminthes.  The most important trematodes are Schistosoma species  (blood flukes), Clonorchis sinensis (liver fluke), and  Paragonimus westermani (lung fluke). Schistosomes have  by far the greatest impact in terms of the number of people  infected, morbidity, and mortality. Features of the medi¬  cally important trematodes are summarized in Table 55-1,  and the medically important stages in the life cycle of these  organisms are described in Table 55-2. Three trematodes  of lesser importance, Fasciola hepatica, Fasciolopsis buski,  and Heterophyes heterophyes, are described at the end of  this chapter.   The life cycle of the medically important trematodes  involves a sexual cycle in humans (definitive host) and  asexual reproduction in freshwater snails (intermediate  hosts) (Figure 55-1). Transmission to humans takes place  either via penetration of the skin by the free-swimming  cercariae of the schistosomes (Figures 55-2D and 55-3) or  via ingestion of cysts in undercooked (raw) fish or crabs in  Clonorchis and Paragonimus infection, respectively.   Trematodes that cause human disease are not endemic  in the United States. However, immigrants from tropical  areas, especially Southeast Asia, are frequently infected.   Schistosoma causes schistosomiasis. Schistosoma mansoni  and Schistosoma japonicum affect the gastrointestinal tract, 1  whereas Schistosoma haematobium affects the urinary tract.   The life cycle of Schistosoma species is shown in Figure  55-1. In contrast to the other trematodes, which are her¬  maphrodites, adult schistosomes exist as separate sexes but  live attached to each other. The female resides in a groove  in the male, the gynecophoric canal (“schist”), where he  continuously fertilizes her eggs (see Figure 55-2A). The  three species can be distinguished by the appearance of  their eggs in the microscope: S. mansoni eggs have a prom¬  inent lateral spine, whereas S. japonicum eggs have a very  small lateral spine and S. haematobium eggs have a terminal  spine (Figures 55-4A and B, 55-5, and 55-6). Schistosoma  mansoni and S. japonicum adults live in the mesenteric  veins, whereas S. haematobium lives in the veins draining  the urinary bladder. Schistosomes are therefore known as  blood flukes.    Humans are infected when the free-swimming, fork¬  tailed cercariae penetrate the skin (see Figures 55-2D and  55-3). They differentiate to larvae (schistosomula), enter  the blood, and are carried via the veins into the arterial  circulation. Those that enter the superior mesenteric artery  pass into the portal circulation and reach the liver, where  they mature into adult flukes. Schistosoma mansoni and  S. japonicum adults migrate against the portal flow to reside  in the mesenteric venules. Schistosoma haematobium adults  reach the bladder veins through the venous plexus between  the rectum and the bladder.   In their definitive venous site, the female lays fertilized  eggs, which penetrate the vascular endothelium and enter  the gut or bladder lumen, respectively. The eggs are excreted  in the stools or urine and must enter fresh water where they  release ciliated, swimming larvae called miracidia. The  miracidia then penetrate snails and undergo further devel¬  opment and multiplication to produce many cercariae. (The  three schistosomes use different species of snails as interme¬  diate hosts.) Cercariae leave the snails, enter fresh water, and  complete the cycle by penetrating human skin.   Most of the pathologic findings are caused by the presence  of eggs in the liver, spleen, or wall of the gut or bladder.  Eggs in the liver induce granulomas, which lead to fibrosis,  hepatomegaly, and portal hypertension. The granulomas    are formed in response to antigens secreted by the eggs.  Hepatocytes are usually undamaged, and liver function  tests remain normal. Portal hypertension leads to   Schistosoma mansoni eggs damage the wall of the distal  colon (inferior mesenteric venules), whereas S. japonicum  eggs damage the walls of both the small and large intestines  (superior and inferior mesenteric venules). The damage is  due both to digestion of tissue by proteolytic enzymes pro¬  duced by the egg and to the host inflammatory response that  forms granulomas in the venules. The eggs of S. haematobium  in the wall of the bladder induce granulomas and fibrosis,  which can lead to carcinoma of the bladder.   Schistosomes have evolved a remarkable process for  evading the host defenses. There is evidence that their  surface becomes coated with host antigens, thereby limit¬  ing the ability of the immune system to recognize them as  foreign.   The epidemiology of schistosomiasis depends on the  presence of the specific freshwater snails that serve as  intermediate hosts. Schistosoma mansoni is found in Africa  and Latin America (including Puerto Rico), whereas  S. haematobium is found in Africa and the Middle East.  Schistosoma japonicum is found only in Asia and is the  only one for which domestic animals (e.g., water buffalo  and pigs) act as important reservoirs. More than 150 million  people in the tropical areas of Africa, Asia, and Latin  America are affected.    TABLE 55-2 Medically Important Stages in Life Cycle of Trematodes (Flukes)   Adult flukes living in mesenteric  or bladder veins lay eggs that  cause granulomas   (laying eggs that circulate to  the liver and shed in stools)    FIGURE 55-1 Schistosoma species. Life cycle. Right side of figure describes the stages within the human (blue arrows). Humans are  infected at step 2 when free-swimming cercariae penetrate human skin. Cercariae differentiate into adult worms (two sexes) that migrate to the  mesenteric veins ( Schistosoma mansoni and Schistosoma japonicum) or the venous plexus of the urinary bladder ( Schistosoma haematobium).  The adult worms lay eggs, which appear in the stool (S. mansoni and S. japonicum) or the urine (S. haematobium). The eggs pass into fresh water,  where the miracidia stage infects snails, which produce cercariae. Left side of figure describes the stages in fresh water and in the snail (red  arrows). (Source: Dr. Alexander J. da Silva and Melanie Moser, Centers for Disease Control and Prevention.)   Most patients are asymptomatic, but chronic infections  may become symptomatic. The acute stage, which begins  shortly after cercarial penetration, consists of itching and  dermatitis followed 2 to 3 weeks later by fever, chills,    diarrhea, lymphadenopathy, and hepatosplenomegaly.  Eosinophilia is seen in response to the migrating larvae.  This stage usually resolves spontaneously.   The chronic stage can cause significant morbidity and  mortality. In patients with S. mansoni or S. japonicum  infection, gastrointestinal hemorrhage, hepatomegaly, and   FIGURE 55-2 A: Male and female Schistosoma mansoni adults. The female lives in the male's schist (shown as a ventral opening) (6x).  B: Clonorchis sinensis adult (6x). C: Paragonimus westermani adult (0.6x). D: 5. mansoni cercaria (300x).   caria of Schistosoma. Note the typical forked tail on the left side of  the image. (Source: the Minnesota Department of Health, R.N. Barr Library;  Librarians M. Rethlefson and M. Jones; Prof. W.Wiley, Public Health Image Library,  Centers for Disease Control and Prevention.)    FIGURE 55-4 A: Schistosoma mansoni egg with lateral spine.  B: Schistosoma haematobium egg with terminal spine. C: Clonorchis  sinensis egg with operculum. D: Paragonimus westermani egg with  operculum (300x). (Circles represent red blood cells.)   FIGURE 55-5 Schistosoma mansoni —egg. Long arrow  points to an egg of S. mansoni. Short arrow points to its large  lateral spine. (Source: Public Health Image Library, Centers for Disease Control  and Prevention.)   FIGURE 55-6 Schistosoma haematobium —egg. Long arrow  points to an egg of S. haematobium. Short arrow points to its termi¬  nal spine. (Source: Public Health Image Library, Centers for Disease Control and  Prevention.)    massive splenomegaly can develop. The most common  cause of death is exsanguination from ruptured esophageal  varices. Patients infected with S. haematobium have hema¬  turia as their chief early complaint. Superimposed bacterial  urinary tract infections occur frequently.   “Swimmer’s itch,” which consists of pruritic papules, is a  frequent problem in many lakes in the United States. The  papules are an immunologic reaction to the presence in the  skin of the cercariae of nonhuman schistosomes. The pru¬  ritic papules appear within minutes to hours after expo¬  sure, indicating that this is an immediate (immunoglobulin  [Ig] E-mediated) hypersensitivity. These nonhuman schis¬  tosomes are incapable of replicating in humans and do not  cause disseminated disease.   Diagnosis depends on finding the characteristic ova in the  feces or urine. The large lateral spine of S. mansoni and  the rudimentary spine of S. japonicum are typical, as is the  large terminal spine of S. haematobium (see Figures 55-4A  and B, 55-5, and 55-6). Serologic tests are not useful.  Moderate eosinophilia occurs.   Prevention involves proper disposal of human waste and  eradication of the snail host when possible. Swimming in  areas of endemic infection should be avoided.   Humans are infected by eating raw or undercooked fish  containing the encysted larvae (metacercariae). After  excystation in the duodenum, immature flukes enter the  biliary ducts and differentiate into adults (see Figure 55-2B).  The hermaphroditic adults produce eggs, which are excreted  in the feces (see Figure 55-4C). Upon reaching fresh  water, the eggs are ingested by snails, which are the first  intermediate hosts. The eggs hatch within the gut and dif¬  ferentiate first into larvae (rediae) and then into many free-  swimming cercariae. Cercariae encyst under the scales of  certain freshwater fish (second intermediate hosts), which  are then eaten by humans.   In some infections, the inflammatory response can cause  hyperplasia and fibrosis of the biliary tract, but often there  are no lesions. Clonorchiasis is endemic in China, Japan,  Korea, and Indochina, where it affects about 20 million  people. It is seen in the United States among immigrants  from these areas.   Most infections are asymptomatic. In patients with a heavy  worm burden, upper abdominal pain, anorexia, hepato¬  megaly, and eosinophilia can occur.   Diagnosis is made by finding the typical small, brownish,  operculated eggs in the stool (see Figure 55-4C). Serologic  tests are not useful.   Prevention centers on adequate cooking of fish and proper  disposal of human waste.   Humans are infected by eating raw or undercooked crab  meat (or crayfish) containing the encysted larvae (metacer¬  cariae). After excystation in the small intestine, immature  flukes penetrate the intestinal wall and migrate through the  diaphragm into the lung parenchyma. They differentiate  into hermaphroditic adults (see Figure 55-2C) and produce  eggs that enter the bronchioles and are coughed up or swal¬  lowed (see Figure 55-4D). Eggs in either sputum or feces  that reach fresh water hatch into miracidia, which enter  snails (first intermediate hosts). There, they differentiate  first into larvae (rediae) and then into many free-swimming  cercariae. The cercariae infect and encyst in freshwater  crabs (second intermediate hosts). The cycle is completed  when undercooked infected crabs are eaten by humans.   Within the lung, the worms exist in a fibrous capsule that  communicates with a bronchiole. Secondary bacterial  infection frequently occurs, resulting in bloody sputum.    Paragonimiasis is endemic in Asia and India. In the United  States, it occurs in immigrants from these areas.   The main symptom is a chronic cough with bloody sputum.  Dyspnea, pleuritic chest pain, and recurrent attacks of bacte¬  rial pneumonia occur. The disease can resemble tuberculosis.   Diagnosis is made by finding the typical operculated eggs  in sputum or feces (see Figure 55-4D). Serologic tests are  not useful.   Fasciola hepatica, the sheep liver fluke, causes disease pri¬  marily in sheep and other domestic animals in Latin  America, Africa, Europe, and China. Humans are infected  by eating watercress (or other aquatic plants) contami¬  nated by larvae (metacercariae) that excyst in the duode¬  num, penetrate the gut wall, and reach the liver, where they  mature into adults. Hermaphroditic adults in the bile ducts  produce eggs, which are excreted in the feces. The eggs  hatch in fresh water, and miracidia enter the snails. Mira-  cidia develop into cercariae, which then encyst on aquatic  vegetation. Sheep and humans eat the plants, thus complet¬  ing the life cycle.   Symptoms are due primarily to the presence of the adult  worm in the biliary tract. In early infection, right-upper-  quadrant pain, fever, and hepatomegaly can occur, but  most infections are asymptomatic. Months or years later,  obstructive jaundice can occur. Halzoun is a painful phar¬  yngitis caused by the presence of adult flukes on the poste¬  rior pharyngeal wall. The adult flukes are acquired by  eating raw sheep liver.   Diagnosis is made by identification of eggs in the feces.  There is no serologic test. The drug of choice is triclaben-  dazole. Adult flukes in the pharynx and larynx can be  removed surgically. Prevention involves not eating wild  aquatic vegetables or raw sheep liver.   Fasciolopsis buski is an intestinal parasite of humans and  pigs that is endemic to Asia and India. Humans are infected    by eating aquatic vegetation that carries the cysts. After  excysting in the small intestine, the parasites attach to the  mucosa and differentiate into adults. Eggs are passed in the  feces; on reaching fresh water, they differentiate into mira¬  cidia. The ciliated miracidia penetrate snails and, after  several stages, develop into cercariae that encyst on aquatic  vegetation. The cycle is completed when plants carrying the  cysts are eaten.   Pathologic findings are due to damage of the intestinal  mucosa by the adult fluke. Most infections are asymptom¬  atic, but ulceration, abscess formation, and hemorrhage  can occur. Diagnosis is based on finding typical eggs in the  feces. Praziquantel is the treatment of choice. Prevention  consists of proper disposal of human sewage.   Heterophyes heterophyes is an intestinal parasite of people  living in Africa, the Middle East, and Asia who are infected  by eating raw fish containing cysts. Larvae excyst in the  small intestine, attach to the mucosa, and develop into  adults. Eggs are passed in the feces and, on reaching brack¬  ish water, are ingested by snails. After several developmen¬  tal stages, cercariae are produced that encyst under the  scales of certain fish. The cycle is completed when fish  carrying the infectious cysts are eaten.   Pathologic findings are due to inflammation of the  intestinal epithelium as a result of the presence of the adult  flukes. Most infections are asymptomatic, but abdominal  pain and nonbloody diarrhea can occur. Diagnosis is based  on finding the typical eggs in the feces. Praziquantel is the  treatment of choice. Prevention consists of proper disposal  of human sewage.   (A) The visual appearance of male and female schistosomes is the  same.   (B) Humans are infected by schistosomes when cercariae penetrate  the skin.   (C) Infection of freshwater fish is a required part of the life cycle of  schistosomes.   (D) The pathology of schistosomiasis is principally caused by the  cercariae entering hepatocytes and killing them.   (E) Infection by nonhuman schistosomes can cause meningitis in  people who swim in certain lakes in the United States.   2. Regarding Schistosoma mansoni, which one of the following state¬  ments is the most accurate?   (A) The main site of S. mansoni in the human body is the mesen¬  teric venules.   (B) Schistosomiasis caused by S. mansoni has been eradicated from  the Western hemisphere.   (C) The laboratory diagnosis of S. mansoni depends on seeing eggs  with a terminal spine in the stool.    (D) Adult schistosomes are passed in the stool, and it is obligatory  that they be ingested by freshwater snails to continue the life  cycle.   (E) Swimmers itch occurs when S. mansoni eggs spread from the  liver to the skin, where they induce a histamine-mediated  immediate (type 1) hypersensitivity reaction.   3. Which one of the following is the drug of choice for infections  with S. mansoni and S. haematobium ?   4. Your patient is a 30-year-old man with low-grade perineal pain  for several weeks who had an episode of painful ejaculation and  postcoital hematuria yesterday. He is in a long-standing monoga¬  mous relationship. He has traveled extensively throughout the  world during the past 10 years. Urinalysis and urine culture were  negative. Cytologic examination of cells in the urine revealed no  tumor cells. Cystoscopy revealed several polypoid lesions, and a  biopsy of a lesion was taken. The tissue was examined in the light  microscope, and eggs with a terminal spine were seen. Of the fol¬  lowing, which one is the MOST likely cause?   Brief summaries of the organisms described in this chapter  begin on page 686. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Parasitology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 731. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   Nematodes (also known as Nemathelminthes) are round-  worms with a cylindrical body and a complete digestive  tract, including a mouth and an anus. The body is covered  with a noncellular, highly resistant coating called a cuticle.  Nematodes have separate sexes; the female is usually larger  than the male. The male typically has a coiled tail.   The medically important nematodes can be divided into  two categories according to their primary location in the  body, namely, intestinal and tissue nematodes.   (1) The intestinal nematodes include Enterobius (pin-  worm), Trichuris (whipworm), Ascaris (giant roundworm),  Necator and Ancylostoma (the two hookworms), Strongyloides  (small roundworm), and Trichinella. Enterobius, Trichuris,  and Ascaris are transmitted by ingestion of eggs; the others  are transmitted as larvae. There are two larval forms: the  first- and second-stage (rhabditiform) larvae are noninfec-  tious, feeding forms; the third-stage (filariform) larvae are  the infectious, nonfeeding forms. As adults, these nematodes  live within the human body, except for Strongyloides, which  can also exist in the soil.   (2) The important tissue nematodes Wuchereria,  Onchocerca, and Loa are called the “filarial worms,” because  they produce motile embryos called microfilariae in blood    and tissue fluids. These organisms are transmitted from per¬  son to person by bloodsucking mosquitoes or flies. A fourth  species is the guinea worm, Dracunculus, whose larvae  inhabit tiny crustaceans (copepods) and are ingested in  drinking water.   The nematodes described above cause disease as a result  of the presence of adult worms within the body. In addi¬  tion, several species cannot mature to adults in human tis¬  sue, but their larvae can cause disease. The most serious of  these diseases is visceral larva migrans, caused primarily by  the larvae of the dog ascarid, Toxocara canis. Cutaneous  larva migrans, caused mainly by the larvae of the dog and  cat hookworm, Ancylostoma caninum, is less serious. A  third disease, anisakiasis, is caused by the ingestion of  Anisakis larvae in raw seafood.   In infections caused by certain nematodes that migrate  through tissue (e.g., Strongyloides, Trichinella, Ascaris,  and the two hookworms Ancylostoma and Necator), a  striking increase in the number of eosinophils (eosino-  philia) occurs. Eosinophils do not ingest the organisms;  rather, they attach to the surface of the parasite via IgE  and secrete cytotoxic enzymes contained within their  eosinophilic granules. Host defenses against helminths  are stimulated by interleukins synthesized by the Th-2  subset of helper T cells (e.g., the production of IgE is    increased by interleukin-4, and the number of eosinophils  is increased by interleukin-5 [IL-5]) (see Chapter 58).  Cysteine proteases produced by the worms to facilitate  their migration through tissue are the stimuli for IL-5  production.    Features of the medically important nematodes are  summarized in Table 56-1. The medically important stages  in the life cycle of the intestinal nematodes are described in  Table 56-2, and those of the tissue nematodes are described  in Table 56-3.   The life cycle of E. vermicularis is shown in Figure 56-1.  Infection occurs only in humans; there is no animal    reservoir or vector. The infection is acquired by ingest¬  ing the worm eggs. The eggs hatch in the small intes¬  tine, where the larvae differentiate into adults and  migrate to the colon. The adult male and female worms  live in the colon, where mating occurs (Figure 56-2A).  At night, the female migrates from the anus and releases  thousands of fertilized eggs on the perianal skin and  into the environment. Within 6 hours, the eggs develop  into embryonated eggs (Figures 56-3A and 56-4) and  become infectious. Reinfection can occur if they are    TABLE 56-2 Medically Important Stages in Life Cycle of Intestinal Nematodes (Roundworms)   Female worm migrates out anus and lays  eggs on perianal skin, causing itching    carried to the mouth by fingers after scratching the itch¬  ing skin.   Perianal pruritus is the most prominent symptom. Pruri¬  tus is thought to be an allergic reaction to the proteins of  either the adult female or the eggs. Scratching predisposes  to secondary bacterial infection.   Enterobius is found worldwide and is the most common  helminth in the United States. Children younger than 12  years of age are the most commonly affected group.   The eggs are recovered from perianal skin by using the  Scotch tape technique and can be observed microscopi¬  cally (see Figure 56-4).    Unlike those of other intestinal nematodes, these  eggs are not found in the stools. The small, whitish  adult worms can be found in the stools or near the  anus of diapered children. No serologic tests are  available.   The drug of choice is either albedazole, mebendazole or  pyrantel pamoate. These drugs kill the adult worms in the  colon but not the eggs, so retreatment in 2 weeks is sug¬  gested. Reinfection is very common. Household members  should also be treated.   There are no specific means of prevention but  washing hands when preparing food and washing bed  sheets, towels, diapers, and clothing to remove eggs is  helpful.    TABLE 56-3 Medically Important Stages in Life Cycle of Tissue Nematodes (Roundworms)   FIGURE 56-1 Enterobius vermicularis. Life cycle. Top: Blue arrow at top left shows eggs being ingested. Adult pinworms form in colon.  Female migrates out anus and lays eggs on perianal skin. Bottom: Red arrow indicates survival of eggs in the environment. (Source: Public Health  Image Library, Centers for Disease Control and Prevention.)   Humans are infected by ingesting worm eggs in food or  water contaminated with human feces (see Figures 56-3B  and 56-5). The eggs hatch in the small intestine, where  the larvae differentiate into immature adults. These  immature adults migrate to the colon, where they mature,    mate, and produce thousands of fertilized eggs daily,  which are passed in the feces. Eggs deposited in warm,  moist soil form embryos. When the embryonated eggs  are ingested, the cycle is completed. Figure 56-2B illus¬  trates the characteristic “whiplike” appearance of the  adult worm.   Although adult Trichuris worms burrow their hairlike ante¬  rior ends into the intestinal mucosa, they do not cause sig¬  nificant anemia, unlike the hookworms. Trichuris may  cause diarrhea, but most infections are asymptomatic.    FIGURE 56-2 A: Enterobius vermicularis female adult (6x). B: Trichuris trichiura female adult. Note the thin anterior (whiplike) end (6x). C: Ascaris  lumbricoides female adult (0.6x). D: Ancylostoma duodenale female adult (6x). E: Ancylostoma duodenale filariform larva (60x). F: Ancylostoma  duodenale head with teeth (25x). G: Necatoramericanus head with cutting plates (25x). H: Strongyloides stercoralis female adult (60x). I: Strongyloides  stercoralis filariform larva (60x). J: Strongyloides stercoralis rhabditiform larva (60x). K: Trichinella spiralis cyst containing two larvae in muscle (60x).    Trichuris may also cause rectal prolapse in children  with heavy infection. Prolapse results from increased peri¬  stalsis that occurs in an effort to expel the worms. The  whitish worms may be seen on the prolapsed mucosa.   Whipworm infection occurs worldwide, especially in the  tropics; more than 500 million people are affected. In the  United States, it occurs mainly in the southern states.   Diagnosis is based on finding the typical eggs (i.e., barrel¬  shaped [lemon-shaped] with a plug at each end) in the  stool (see Figures 56-3B and 56-5).   The life cycle of A. lumbricoides is shown in Figure 56-6.  Flumans are infected by ingesting worm eggs in food or   FIGURE 56-3 A: Enterobius vermicularis egg. B: Trichuris trichiura egg. C: Ascaris lumbricoides egg. D: Ancylostoma duodenale or Necator  americanus egg (300x). (Circles represent red blood cells.)   points to an egg of the pinworm, E. vermicularis recovered on  "Scotch tape". Short arrow points to the embryo inside the egg.  (Source: Public Health Image Library, Centers for Disease Control and Prevention.)   The major damage occurs during larval migration rather  than from the presence of the adult worm in the intestine.  The principal sites of tissue reaction are the lungs, where  inflammation with an eosinophilic exudate occurs in  response to larval antigens. Because the adults derive their  nourishment from ingested food, a heavy worm burden  may contribute to malnutrition, especially in children in  developing countries.   Most infections are asymptomatic. Ascaris pneumonia  with fever, cough, and eosinophilia can occur with a heavy  larval burden. Abdominal pain and even obstruction can  result from the presence of adult worms in the intestine.   Ascaris infection is very common, especially in the tropics;  hundreds of millions of people are infected. In the United  States, most cases occur in the southern states.    water contaminated with human feces (Figures 56-3C and  56-7). The eggs hatch in the small intestine, and the larvae  migrate through the gut wall into the bloodstream and then  to the lungs. They enter the alveoli, pass up the bronchi and  trachea, and are swallowed. Within the small intestine, they  become adults (Figures 56-2C and 56-8). They live in the  lumen, do not attach to the wall, and derive their suste¬  nance from ingested food. The adults are the largest intes¬  tinal nematodes, often growing to 25 cm or more. Ascaris  lumbricoides is known as the “giant roundworm.” Thou¬  sands of eggs are laid daily, are passed in the feces, and  differentiate into embryonated eggs in warm, moist soil  (see Figure 56-3C). Ingestion of the embryonated eggs  completes the cycle.   egg of T. trichiura. Short arrow points to one of the two "plugs" on  each end of the egg. (Source: Public Dr. M. Melvin, Health Image Library,  Centers for Disease Control and Prevention.)   Diagnosis is usually made microscopically by detecting  eggs in the stools. The egg is oval with an irregular surface  (see Figures 56-3C and 56-7). Occasionally, the patient  sees adult worms in the stools.   Ancylostoma duodenale (Old World hookworm) and Necator  americanus (New World hookworm) cause hookworm  infection.   The life cycle of the hookworms is shown in Figure 56-9.  Humans are infected when filariform larvae in moist soil  penetrate the skin, usually of the feet or legs (Figures  56-2E and 56-10). They are carried by the blood to the  lungs, migrate into the alveoli and up the bronchi and tra¬  chea, and then are swallowed. They develop into adults in  the small intestine, attaching to the wall with either cutting  plates ( Necator ) or teeth ( Ancylostoma ) (Figures 56-2D, F,  and G, and 56-11). They feed on blood from the capillaries  of the intestinal villi. Thousands of eggs per day are passed  in the feces (Figures 56-3D and 56-12). Eggs develop first  into noninfectious, feeding (rhabditiform) larvae and then   FIGURE 56-6 Ascaris lumbricoides. Life cycle. Top: Blue arrow at top left shows eggs being ingested. Larvae emerge in the intestinal tract,  enter the bloodstream, and migrate to the lungs. They then enter the alveoli, ascend into the bronchi and trachea, migrate to the pharynx, and  are swallowed. Adult Ascaris worms form in small intestine. Eggs pass in human feces. Bottom: Red arrow indicates maturation of eggs in the  soil. (Figure courtesy of Public Health Image Library, Centers for Disease Control and Prevention.)    into third-stage, infectious, nonfeeding (filariform) larvae  (see Figure 56-2E), which penetrate the skin to complete  the cycle.   The major damage is due to the loss of blood at the site of  attachment in the small intestine. Up to 0.1 to 0.3 mL per  worm can be lost per day. Blood is consumed by the worm  and oozes from the site in response to an anticoagulant  made by the worm. Weakness and pallor accompany the  microcytic anemia caused by blood loss. These symptoms  occur in patients whose nutrition cannot compensate for    the blood loss. “Ground itch,” a pruritic papule or vesicle,  can occur at the site of entry of the larvae into the skin. The  human hookworms also cause cutaneous larva migrans.  Pneumonia with eosinophilia can be seen during larval  migration through the lungs.   Hookworm is found worldwide, especially in tropical areas.  In the United States, Necator is endemic in the rural south¬  ern states. Walking barefooted on soil predisposes to infec¬  tion. An important public health measure was requiring  children to wear shoes to school.   Diagnosis is made microscopically by observing the eggs in  the stools (see Figures 56-3D and 56-12). Occult blood in  the stools is frequent. Eosinophilia is typical.   The drug of choice is either albendazole, mebendazole, or  pyrantel pamoate.    FIGURE 56-7 Ascaris lumbricoides —egg. Arrow points to an  egg of Ascaris. Note the typical "scalloped" edge of the Ascaris egg.  (Source: Public Health Image Library, Centers for Disease Control and Prevention.)   Disposing of sewage properly and wearing shoes are effec¬  tive means of prevention.   FIGURE 56-8 Ascaris lumbricoides — adult worms. (Source: Dr. Henry  Bishop, Public Health Image Library, Centers for Disease Control and Prevention.)   The life cycle of S. stercoralis is shown in Figure 56-13.  S. stercoralis has two distinct life cycles, one within the  human body and the other free-living in the soil. The life  cycle in the human body begins with the penetration of  the skin, usually of the feet, by infectious (filariform) lar¬  vae (see Figures 56-21 and 56-10) and their migration to  the lungs. They enter the alveoli, pass up the bronchi and  trachea, and then are swallowed. In the small intestine, the  larvae molt into adults (see Figure 56-2H) that enter the  mucosa and produce eggs.   The eggs usually hatch within the mucosa, forming  rhabditiform larvae (see Figure 56-2J) that are passed in  the feces. Some larvae molt to form filariform larvae, which  penetrate the intestinal wall directly without leaving the  host and migrate to the lungs (autoinfection). Filariform  larvae can also exit the anus and reinfect through the peri¬  anal skin. In immunocompetent patients, this is an infre¬  quent, clinically unimportant event.   However, in immunocompromised patients (e.g., those  who have acquired immunodeficiency syndrome [AIDS]  or are taking high-dose corticosteroids or TNF inhibitors)  or patients who are severely malnourished, autoinfection  can lead to massive reinfection (hyperinfection), with  larvae passing to many organs and with severe, sometimes  fatal consequences. Reinfection can also occur in those  infected with human T-cell lymphotropic virus (HTLV)  because their ability to mount a protective T-cell response  is diminished.   If larvae are passed in the feces and enter warm, moist  soil, they molt through successive stages to form adult male  and female worms. After mating, the entire life cycle of egg,  larva, and adult can occur in the soil. After several free-  living cycles, filariform larvae are formed. When they   migrate through lung and may cause pneumonia. Adult hookworms attach to intestinal mucosa and cause bleeding and anemia. Eggs pass in  human feces. Bottom: Red arrow indicates maturation of eggs in the soil to form rhabditiform larvae and then infective filariform larvae. (Source:  Public Health Image Library, Centers for Disease Control and Prevention.)   Most patients are asymptomatic, especially those with a low  worm burden. Adult female worms in the wall of the small  intestine can cause inflammation, resulting in watery diar¬  rhea. Larvae in the lungs can produce a pneumonitis simi¬  lar to that caused by Ascaris. Pruritus (ground itch) can    occur at the site of larval penetration of the skin, as with  hookworm. Strongyloides stercoralis also causes cutaneous  larva migrans.   Autoinfection can result in chronic strongyloidiasis  characterized by intermittent abdominal pain, fluctuating  rashes, and intermittent eosinophilia. In hyperinfection,  the penetrating larvae may cause sufficient damage to the  intestinal mucosa that sepsis caused by enteric bacteria,  such as Escherichia coli and Bacteroides fragilis, can occur.   FIGURE 56-10 Necator and Strongyloides —filariform larvae. Filariform larva of Necator on the left and Strongyloides on the right. Filari¬  form larva is the infective form that penetrates the skin. (Source: Public Health Image Library, Centers for Disease Control and Prevention.)   Strongyloidiasis occurs primarily in the tropics, especially  in Southeast Asia. Its geographic pattern is similar to that  of hookworm because the same type of soil is required. In  the United States, Strongyloides is endemic in the southeast¬  ern states.   Diagnosis depends on finding larvae, rather than eggs, in  the stool (see Figure 56-10). As with many nematode infec¬  tions in which larvae migrate through tissue, eosinophilia  can be striking. Serologic tests are useful when the larvae  are not visualized. An enzyme immunoassay that detects   FIGURE 56~11 Ancylostoma duodenate —head of the adult  hookworm. Arrows point to the four cutting teeth in the mouth of  Ancylostoma. (Source: Dr. M. Melvin, Public Health Image Library, Centers for  Disease Control and Prevention.)    antibody to larval antigens is available through the Centers  for Disease Control and Prevention (CDC) in Atlanta.   Ivermectin is the drug of choice. Albendazole is an alterna¬  tive drug.   Prevention involves disposing of sewage properly and  wearing shoes. To prevent Strongyloides hyperinfection in  patients scheduled to received immunosuppressive drugs  (e.g., corticosteroids, TNF inhibitors) and who have lived  in an area of Strongyloides endemicity. serological tests to   FIGURE 56-12 Necator and Ancylostoma (hookworms)—egg.  Arrow points to an egg of a hookworm. The eggs of Necator and  Ancylostoma are indistinguishable. Note the embryo coiled up  inside. (Source: Public Health Image Library, Centers for Disease Control and  Prevention.)    Infective filariform larvae  in the soil penetrate  the intact skin initiating  the infection ■■    The filariform larvae enter the  circulatory system, are  transported to the lungs, and  penetrate the alveolar spaces.  They are carried to the trachea  and pharynx, swallowed, and  reach the small intestine  where they become adults    FIGURE 56 - 13 Strongyloides stercoralis. Life cycle. Center and right side of figure describe the stages within the human (blue arrows).   Filariform larvae penetrate the skin (step 1). Larvae migrate through lung and may cause pneumonia. Adult Strongyloides worms form in small  intestine. Eggs hatch in intestinal mucosa, and rhabditiform larvae are excreted in human feces, not worm eggs. Curved blue arrow ascending  from step 5 describes the autoinfection cycle in which filariform larvae form in the gastrointestinal tract and infect by penetrating the gut  mucosa or perianal skin. Left side of figure describes the maturation in the soil (red arrows). Note that steps 6, 7, and 8 constitute the free-living  life cycle in the soil. (Source: Dr. Alexander J. da Silva and Melanie Moser, Centers for Disease Control and Prevention.)    determine whether antibodies to Strongyloides are present  should be performed. If antibodies are found, the patient  should be treated with ivermectin before immunosuppres¬  sion is undertaken, if possible.   Trichinella spiralis causes trichinosis. Trichinella spiralis is  also called the trichina worm.   The life cycle of T. spiralis is shown in Figure 56-14. Any  mammal can be infected, but pigs are the most important  reservoirs of human disease in the United States (except in  Alaska, where bears constitute the main reservoir). Humans  are infected by eating raw or undercooked meat contain¬  ing larvae encysted in the muscle (see Figure 56-2K). The  larvae excyst and mature into adults within the mucosa of  the small intestine. Eggs hatch within the adult females,  and larvae are released and distributed via the bloodstream   FIGURE 56~14 Trichinella spiralis. Life cycle. Top: Blue arrow on left shows ingestion of meat (muscle) containing encysted Trichinella  larva. Adult worms form in intestine and produce larvae that enter bloodstream and encyst in human muscle. Bottom: Red circular arrow  describes the natural cycle in which Trichinella circulates between pigs and various carnivores such as bears. (Source: Public Health Image Library, Cen-  ters for Disease Control and Prevention.)    to many organs; however, they develop only in striated  muscle cells. Within these “nurse cells,” they encyst within  a fibrous capsule and can remain viable for several years but  eventually calcify (Figure 56-15).    The parasite is maintained in nature by cycles within  reservoir hosts, primarily swine and rats. Humans are end-  stage hosts, because the infected flesh is not consumed by  other animals.   FIGURE 56~15 Trichinella spiralis —larvae in skeletal muscle.  Three arrows point to Trichinella larvae within "nurse cells" in skeletal  muscle. (Source: Public Health Image Library, Centers for Disease Control and  Prevention.)   A few days after eating undercooked meat, usually pork,  the patient experiences diarrhea followed 1 to 2 weeks later  by fever, muscle pain, periorbital edema, and eosino-  philia. Subconjunctival hemorrhages are an important  diagnostic criterion. Signs of cardiac and central nervous  system disease are frequent, because the larvae migrate to    these tissues as well. Death, which is rare, is usually due to  congestive heart failure or respiratory paralysis.   Trichinosis occurs worldwide, especially in Eastern Europe  and west Africa. In the United States, it is related to eating  home-prepared sausage, usually on farms where the pigs  are fed uncooked garbage. Bear and seal meat also are  sources. In many countries, the disease occurs primarily in  hunters who eat undercooked wild game.   Muscle biopsy reveals larvae within striated muscle (see  Figures 56-2K and 56-15). Serologic tests, especially the  bentonite flocculation test, become positive 3 weeks after  infection.   There is no effective treatment for trichinosis when the  larvae have infected the muscle, but for patients with severe  symptoms, steroids plus albendazole can be useful. Meben¬  dazole is effective against the adult intestinal worms early  in infection.   The disease can be prevented by properly cooking pork and  by feeding pigs only cooked garbage.   Wuchereria bancrofti causes filariasis. 1 Elephantiasis is a  striking feature of this disease. Tropical pulmonary eosino-  philia is an immediate hypersensitivity reaction to W. ban¬  crofti in the lung.   The life cycle of W. bancrofti is shown in Figure 56-16.  Humans are infected when the female mosquito (espe¬  cially Anopheles and Culex species) deposits infective larvae  on the skin while biting. The larvae penetrate the skin,  enter a lymph node, and, after 1 year, mature to adults that  produce microfilariae (Figure 56-17A and 56-18). These  circulate in the blood, chiefly at night, and are ingested by  biting mosquitoes. Within the mosquito, the microfilariae  produce infective larvae that are transferred with the next  bite. Humans are the only definitive hosts.   Adult worms in the lymph nodes cause inflammation  that eventually obstructs the lymphatic vessels, causing  edema. Massive edema of the legs is called elephantiasis  (Figure 56-19). Note that microfilariae do not cause  symptoms.   Early infections are asymptomatic. Later, fever, lym¬  phangitis, and cellulitis develop. Gradually, the obstruction  leads to edema and fibrosis of the legs and genitalia, espe¬  cially the scrotum. Elephantiasis occurs mainly in patients  who have been repeatedly infected over a long period.  Tourists, who typically are infected only once, do not get  elephantiasis.   Wolbachia species are Rickettsia-like bacteria found  intracellularly within filarial nematodes such as  Wuchereria and Onchocerca. Wolbachia release endo-  toxin-like molecules that are thought to play a role in the  pathogenesis of Wuchereria and Onchocerca infections.  Evidence for this includes the use of doxycycline, which  kills the Wolbachia, resulting in a reduction in the number  of microfilaria and in the inflammatory response to the  nematode infection.    0 Adult worms produce sheathed  microfilariae that migrate  into lymph and blood channels    Microfilariae shed sheaths,  penetrate mosquito’s midgut,  and migrate to thoracic muscles    FIGURE 56-16 Wuchereria bancrofti. Life cycle. Right side of figure describes the stages within the human (blue arrows). Humans are  infected at step 1 when mosquito bites human and larvae enter blood stream. Adult Wuchereria worms are formed in lymphatics. Mosquito is  infected at step 4 when it ingests microfilariae in human blood. Left side of figure describes the stages within the mosquito (red arrows).  (Source: Dr. Alexander J. da Silva and Melanie Moser, Centers for Disease Control and Prevention.)    Tropical pulmonary eosinophilia is characterized by  coughing and wheezing, especially at night. These symp¬  toms are caused by microfilariae in the lung that elicit an  immediate hypersensitivity reaction characterized by a  high IgE concentration and eosinophilia.   This disease occurs in the tropical areas of Africa, Asia, and  Latin America. The species of mosquito that acts as the  vector varies from area to area. Altogether, 200 to 300 mil¬  lion people are infected.   Thick blood smears taken from the patient at night reveal  the microfilariae (see Figure 56-18). Serologic tests are not  useful.   Diethylcarbamazine is effective only against microfilariae;  no drug therapy for adult worms is available. Treatment of  patients with Wuchereria (and Onchocerca ) infections with  doxycycline to kill Wolbachia results in a significant  decrease in the number of microfilariae in the patient.   Prevention involves mosquito control with insecticides and  the use of protective clothing, mosquito netting, and  repellents.   FIGURE 56-17 A: Wuchereria bancrofti microfilaria in blood. Note that the pointed tail is free of nuclei (225-300 x 8-10 pm).   B: Onchocerca volvulus microfilaria in skin (rare) in blood) (300-350 x 5-9 pm). C: Loa loa microfilaria in blood. Note that the pointed tail  contains nuclei (250-300 x 6-9 pm). (Circles represent red blood cells.)   Humans are infected when the female blackfly, Simulium,  deposits infective larvae while biting. The larvae enter  the wound and migrate into the subcutaneous tissue,  where they differentiate into adults, usually within der¬  mal nodules. The female produces microfilariae (see  Figure 56-17B) that are ingested when another blackfly  bites. The microfilariae develop into infective larvae in  the fly to complete the cycle. Humans are the only  definitive hosts.   FIGURE 56-18 Wuchereria bancrofti —filarial worm in blood.  Arrow points to filarial worm in blood smear. (Source: Dr. M. Melvin, Public  Health Image Library, Centers for Disease Control and Prevention.)   Inflammation occurs in subcutaneous tissue, and pruritic  papules and nodules form in response to the adult worm  proteins. Microfilariae migrate through subcutaneous    tissue, ultimately concentrating in the eyes. There they  cause lesions that can lead to blindness. Loss of subcutane¬  ous elastic fibers leads to wrinkled skin, which is called  “hanging groin” when it occurs in the inguinal region.  Thickening, scaling, and dryness of the skin accompanied  by severe itching are the manifestations of a dermatitis  often called “lizard skin.”   The role of Wolbachia in the pathogenesis of onchocer¬  ciasis has been discussed earlier in “Wuchereria.”   Millions of people are affected in Africa and Central America.  The disease is a major cause of blindness. It is called river  blindness, because the blackflies develop in rivers and peo¬  ple who live along those rivers are affected. Infection rates  are often greater than 80% in areas of endemic infection.   Biopsy of the affected skin reveals microfilariae (see  Figure 56-17B). Examination of the blood for microfilariae  is not useful because they do not circulate in the blood.  Eosinophilia is common. Serologic tests are not helpful.   Ivermectin is effective against microfilariae but not adults.  Suramin kills adult worms but is quite toxic and is used  particularly in those with eye disease. Skin nodules can be  removed surgically, but new nodules can develop; therefore,  a surgical cure is unlikely in areas of endemic infection.   Humans are infected by the bite of the deer fly (mango fly),  Chrysops, which deposits infective larvae on the skin. The  larvae enter the bite wound, wander in the body, and  develop into adults. The females release microfilariae  (see Figure 56-17C) that enter the blood, particularly during  the day. The microfilariae are taken up by the fly during a  blood meal and differentiate into infective larvae, which  continue the cycle when the fly bites the next person.   There is no inflammatory response to the microfilariae or  adults, but a hypersensitivity reaction causes transient,    localized, nonerythematous, subcutaneous edema (Calabar  swellings). The most dramatic finding is an adult worm  crawling across the conjunctiva of the eye, a harmless but  disconcerting event.   The disease is found only in tropical central and west  Africa, the habitat of the vector Chrysops.   Diagnosis is made by visualization of the microfilariae in a  blood smear (see Figure 56-17C). There are no useful sero¬  logic tests.   Diethylcarbamazine eliminates the microfilariae and may  kill the adults. Worms in the eyes may require surgical  excision.   Humans are infected when tiny crustaceans (copepods)  containing infective larvae are swallowed in drinking  water. The larvae are released in the small intestine and  migrate into the body, where they develop into adults.  Meter-long adult females cause the skin to ulcerate and  then release motile larvae into fresh water. Copepods eat  the larvae, which molt to form infective larvae. The cycle is  completed when these are ingested in the water.   The adult female produces a substance that causes inflam¬  mation, blistering, and ulceration of the skin, usually of  the lower extremities. The inflamed papule burns and  itches, and the ulcer can become secondarily infected.  Diagnosis is usually made clinically by finding the worm  in the skin ulcer.   The global eradication campaign sponsored by the World  Health Organization (WHO) has greatly reduced the num¬  ber of cases. During the year 2015, only 22 new cases were  detected worldwide. The cases occurred in four African    countries (Chad, Ethiopia, Mali, and South Sudan). Prior to  the campaign, the disease occurred over large areas of  tropical Africa, the Middle East, and India, where tens of  millions of people were infected.   The time-honored treatment consists of gradually extract¬  ing the worm by winding it up on a stick over a period of  days.   Toxocara canis is the major cause of visceral larva migrans.  Toxocara cati and several other related nematodes also  cause this disease.   The definitive host for T. canis is the dog. The adult T. canis  female in the dog intestine produces eggs that are passed in  the feces into the soil. Humans ingest soil containing the  eggs, which hatch into larvae in the small intestine. The  larvae migrate to many organs, especially the liver, brain,  and eyes. The larvae eventually are encapsulated and die.  The life cycle is not completed in humans; humans are  therefore accidental, dead-end hosts.   Pathology is related to the granulomas that form around  the dead larvae as a result of a delayed hypersensitivity  response to larval proteins. The most serious clinical find¬  ing is blindness associated with retinal involvement. Fever,  hepatomegaly, and eosinophilia are common.   Young children are primarily affected, because they are  likely to ingest soil containing the eggs. Toxocara canis is a  common parasite of dogs in the United States.   Serologic tests are commonly used, but the definitive diag¬  nosis depends on visualizing the larvae in tissue. The pres¬  ence of hypergammaglobulinemia and eosinophilia  supports the diagnosis.   The treatment of choice is either albendazole or mebenda¬  zole, but there is no proven effective treatment. Many  patients recover without treatment.   Dogs should be dewormed, and children should be pre¬  vented from eating soil.   Cutaneous larva migrans is caused by the filariform larvae of  A. caninum (dog hookworm) and Ancylostoma braziliense  (cat hookworm), as well as other nematodes. The organism  cannot complete its life cycle in humans. The larvae pene¬  trate the skin and migrate through subcutaneous tissue,  causing an inflammatory response. The lesions (“creeping  eruption”) are extremely pruritic (Figure 56-20). The disease  occurs primarily in the southern United States, in children  and construction workers who are exposed to infected soil.  The diagnosis is made clinically; the laboratory is of little  value. Albendazole or ivermectin is usually effective.   The larvae of the rat lung nematode Angiostrongylus canto-  nensis cause eosinophilic meningitis (i.e., a meningitis   FIGURE 56~20 Ancylostoma caninum —cutaneous larva  migrans. Note serpiginous rash on foot. (Reproduced with permission from  Dr. Richard P. Usatine. From Usatine RP: A rash on the feet and buttocks. West J Med.  170:334, 1999.)    characterized by many eosinophils in the spinal fluid and in  the blood). Usually at least 10% of the white cells are  eosinophils. The larvae are typically ingested in under¬  cooked seafood, such as crabs, prawns, and snails. Infection  by this organism most often occurs in Asian countries. The  diagnosis is made primarily on clinical grounds, but occa¬  sionally, the laboratory will find a larva in the spinal fluid.  There is no treatment. Most patients recover spontaneously  without major sequelae.   Eosinophilic meningitis is also caused by the larvae of  two additional nematodes. Gnathostoma spinigerum, an  intestinal nematode of cats and dogs, is acquired by eating  undercooked fish, and Baylisascaris procyonis, a raccoon  roundworm, is acquired by accidentally ingesting raccoon  feces. These organisms cause more severe disease than  Angiostrongylus, and fatalities occur. Albendazole may be  effective against Gnathostoma, but there is no treatment for  Baylisascaris.   Anisakiasis is caused by the larvae of the nematode,  Anisakis simplex. The larvae are ingested in raw seafood  and can penetrate the submucosa of the stomach or intes¬  tine. The adult worms live in the intestines of marine mam¬  mals such as whales, dolphins, and seals. The eggs produced  by the adults are eaten by crustaceans, which are then eaten  by marine fish such as salmon, mackerel, and herring. Gas¬  troenteritis, abdominal pain, eosinophilia, and occult blood  in the stool typically occur. Acute infection can resemble  appendicitis, and chronic infection can resemble gastroin¬  testinal cancer.   Most cases in the United States have been traced to eat¬  ing sushi and sashimi (especially salmon and red snapper)  in Japanese restaurants. The diagnosis is typically made  endoscopically or on laparotomy. Microbiologic and sero¬  logic tests are not helpful in the diagnosis. There are no  effective drugs. Surgical removal may be necessary. Preven¬  tion consists of cooking seafood adequately or freezing it  for 24 hours before eating.   Another member of the Anisakid family of nematodes is  Pseudoterranova decepiens, whose larvae cause a noninva-  sive form of anisakiasis. The larvae are acquired by eating  undercooked fish and cause vomiting and abdominal pain.  The diagnosis is made by finding the larvae in the intestinal  tract or in the vomitus. There is no drug treatment. The  larvae can be removed during endoscopy.   1. You are a volunteer with Doctors Without Borders in sub-  Saharan Africa. In certain villages, you detect anemia in a signifi¬  cant number of children. This is most likely due to infection with  which one of the following?   2. In the same villages as described in Question 1, you observe that  some people are eating unwashed raw vegetables. Which one of  the following organisms is most likely to cause infection in these  people?   3. Which one of the following nematodes is transmitted by a filari¬  form larva penetrating the skin?   4. One of the most important public health measures in the United  States in the twentieth century was recommending that chil¬  dren in rural areas wear shoes. This effort was designed to pre¬  vent infection through the feet with which one of the following  organisms?   5. The larvae of certain nematodes migrate through the lung and  cause pneumonitis characterized by cough or wheezing. Infec¬  tion by which one of the following nematodes is most likely to  cause this clinical picture?   7. Your patient is a 60-year-old man with abdominal pain, vomit¬  ing, and weight loss for the past 2 months. He has a history of  asthma that requires 20 mg of prednisone daily to control. He  lived most of his life in Cuba, moved to Spain 10 years ago, and  has lived in this country for 1 year. Abdominal exam is normal,  and radiographic studies are unrevealing. His white blood cell  count is 10,900 with 16% eosinophils. Examination of the stool  reveals rhabditiform larvae. Of the following, which organism is  the MOST likely cause?    8. Regarding the patient in Question 7, which one of the following  is the best drug to treat the infection?   9. Your patient is a 40-year-old man with fever, myalgia, and facial  swelling. White blood cell count was 14,400 with 24% eosino¬  phils. Additional history reveals that he shot a bear in Canada  and ate some of it about 6 weeks ago. He emphasized that he likes  his meat rare. A muscle biopsy was performed, and a H&E stain  of the tissue showed coiled larvae within skeletal muscle. Of the  following, which one is the most likely cause?   10. Your patient is a 35-year-old woman with severe upper abdomi¬  nal pain for the past hour. There is no nausea, vomiting, or diar¬  rhea. You suspect she may have cholecystitis, pancreatitis, or a  perforated viscus but first ask her if she has ingested raw fish  recently. She says yes, and tells you that she had sashimi the night  before last. Endoscopy reveals a larva in the gastric mucosa. Of  the following, which one is the most likely cause?   11. Your patient is a 5-year-old boy who complains of perianal itch¬  ing, especially at night. A “Scotch tape” preparation reveals the  eggs of Enterobius in the microscope. Which one of the following  is the best drug to treat his pinworm infection?   Brief summaries of the organisms described in this chapter  begin on page 687. Please consult these summaries for a rapid  review of the essential material.   Questions on the topics discussed in this chapter can be found  in the Parasitology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 731. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.    Function of the Immune System  Specificity of the Immune Response  7. Cell-Mediated Immunity  2. Antibody-Mediated Immunity  Innate & Adaptive Immunity  7. Innate Immunity   The main function of the immune system is to prevent or  limit infections, fungi, and parasites, such as protozoa and  worms. The first line of defense against microorganisms is  the intact skin and mucous membranes. If microorgan¬  isms breach this line and enter the body, then the innate  arm of the immune system (second line of defense) is avail¬  able to destroy the invaders. Because the components of the  innate arm (Table 57-1) are preformed and fully active,  they can function immediately upon entry of the    microorganisms. The ability of the innate arm to kill  microorganisms is not specific. For example, a neutrophil  can ingest and destroy many different kinds of bacteria.   Highly specific protection is provided by the adaptive  (acquired) arm of the immune system (third line of  defense), but it takes several days for this arm to become  fully functional. The two components of the adaptive arm  are cell-mediated immunity and antibody-mediated  (humoral) immunity. An overview of the functions and  interactions between many of the important members of  the innate and adaptive arms of the immune response is  provided in Figure 57-1. (The features of the innate and the    TABLE 57-1 Main Components of Innate and Adaptive Immunity That Contribute to Humoral  (Antibody-Mediated) Immunity and Cell-Mediated Immunity   FIGURE 57-1 Introduction to the interactions and functions of the major components of the immune system. Left: Antibody-mediated  (humoral) immunity. This is our main defense against extracellular, encapsulated, pyogenic bacteria such as staphylococci and streptococci.  Antibodies also neutralize toxins, such as tetanus toxin, as well as viruses, such as hepatitis B virus. Right: Cell-mediated immunity. There are  two distinct components. (1) Helper T cells and macrophages are our main defense against intracellular bacteria, such as M. tuberculosis, and  fungi, such as Histoplasma capsulatum. (2) Cytotoxic T cells are an important defense against viruses and act by destroying virus-infected cells.  (IL-4 and IL-5 are interleukin-4 and interleukin-5, respectively.)    adaptive arms of the immune system are contrasted in  Table 57-2.)   The cell-mediated arm consists primarily of T lympho¬  cytes (e.g., helper T cells and cytotoxic T cells), whereas the  antibody-mediated arm consists of antibodies (immuno¬  globulins) and B lymphocytes that can differentiate into  plasma cells. Some of the major functions of T cells and B  cells are shown in Table 57-3.   The main functions of antibodies are (1) to neutralize  toxins and viruses and (2) to opsonize bacteria, making  them easier to phagocytize. Opsonization is the process by  which immunoglobulin G (IgG) antibody and the C3b com¬  ponent of complement enhance phagocytosis. The process of  phagocytosis and the killing of the ingested microbe within  the phagocyte is described in Chapter 8, especially Figure 8-3.   The main functions of cell-mediated immunity (CMI)  are to kill virus-infected cells and to inhibit organisms such    as fungi, parasites, and certain intracellular bacteria such as  Mycobacterium tuberculosis. CMI can also kill cancer cells  that often form new antigens on their surface which are  recognized as foreign.   Both the cell-mediated and antibody-mediated responses  are characterized by three important features: (1) they exhibit  remarkable diversity (i.e., they can respond to millions of  different antigens); (2) they have a long memory (i.e., they  can respond many years after the initial exposure because  memory T cells and memory B cells are produced); and (3)  they exhibit exquisite specificity (i.e., their actions are spe¬  cifically directed against the antigen that initiated the  response).   The combined effects of certain cells (e.g., T cells, B  cells, macrophages, and neutrophils) and certain proteins  (e.g., interleukins, antibodies, and complement) produce  an inflammatory response, one of the body’s main defense    mechanisms. The process by which these components  interact to cause inflammation is described in Chapter 8.   Macrophages and certain other phagocytic cells such as  dendritic cells participate in both the innate and adaptive  arms of the immune response. They are, in effect, a bridge  between the two arms. As part of the innate arm, they  ingest and kill various microbes. They also present antigen  to helper T cells, which is the essential first step in the acti¬  vation of the adaptive arm (see later). It is interesting to  note that neutrophils, which are also phagocytes and have  excellent microbicidal abilities, do not present antigen to  helper T cells and therefore function in innate but not  acquired immunity.   Cell-mediated immunity and antibody are both highly spe¬  cific for the invading organism. How do these specific  protective mechanisms originate? The process by which  these host defenses originate can be summarized by three  actions: (1) the recognition of the foreign organism by  specific immune cells, (2) the activation of these immune  cells to produce a specific response (e.g., antibodies), and  (3) the response that specifically targets the organism for  destruction. The following examples briefly describe how  specific immunity to microorganisms occurs. An overview  of these processes with a viral infection as the model is  shown in Figure 57-2. A detailed description is presented  in Chapter 58.   In the following example, a bacterium (e.g., M. tuberculosis)  enters the body and is ingested by a macrophage. The bac¬  terium is broken down, and fragments of it called antigens  or epitopes appear on the surface of the macrophage in  association with class II major histocompatibility com¬  plex (MHC) proteins. The antigen-class II MHC protein  complex interacts with an antigen-specific receptor on the  surface of a helper T lymphocyte. Activation and clonal  proliferation of this antigen-specific helper T cell occur as  a result of the production of interleukins, the most impor¬  tant of which are interleukin-2 (T cell growth factor) and  gamma interferon (activates macrophages). These acti¬  vated helper T cells, aided by activated macrophages,    mediate one important component of cellular immunity  (i.e., a delayed hypersensitivity reaction specifically  against M. tuberculosis).   Cytotoxic (cytolytic) T lymphocytes are also specific  effectors of the cellular immune response, particularly  against virus-infected cells. In this example, a virus (e.g.,  influenza virus) is inhaled and infects a cell of the respira¬  tory tract. Viral envelope glycoproteins appear on the sur¬  face of the infected cell in association with class I MHC  proteins. A cytotoxic T cell binds via its antigen-specific  receptor to the viral antigen-class I MHC protein complex  and is stimulated to grow into a clone of cells by interleu¬  kin-2 produced by helper T cells. These cytotoxic T cells  specifically kill influenza virus-infected cells (and not cells  infected by other viruses) by recognizing viral antigen-class   I MHC protein complexes on the cell surface and releasing  perforins that destroy the membrane of the infected cell.   Antibody synthesis typically involves the cooperation of  three cells: antigen-presenting cells (e.g., dendritic cells  and macrophages), helper T cells, and B cells. After pro¬  cessing by an antigen-presenting cell, fragments of antigen  appear on the surface of that cell in association with class   II MHC proteins. The antigen-class II MHC protein com¬  plex binds to receptors on the surface of a helper T cell  specific for that antigen. This activates the helper T cells to  produce interleukins such as interleukin-2 (IL-2), IL-4, and  IL-5. These interleukins activate the B cell to produce anti¬  bodies specific for that antigen. (Note that the interleukins  are nonspecific; the specificity lies in the T cells and B cells  and is mediated by the antigen receptors on the surface of  these cells.) The activated B cell proliferates and differenti¬  ates to form many plasma cells that secrete large amounts  of immunoglobulins (antibodies).   Although antibody formation usually involves helper T  cells, certain antigens (e.g., bacterial polysaccharides) can  activate B cells directly, without the help of T cells, and are  called T-cell-independent antigens. In this T-cell-  independent response, only IgM is produced by B cells  because it requires IL-4 and IL-5 made by the helper T cell  for the B cell to “class switch” to produce IgG, IgA, and IgE.  See Chapter 59 for a discussion of “class switching,” the  process by which the B cell switches the antibody it pro¬  duces from IgM to one of the other classes.   FIGURE 57-2 Overview of the process by which cell-mediated immunity and antibody-mediated immunity are induced by exposure to a  virus. Note that the figure shows a virus as the immunogen in the top left corner, but the same processes occur for other microbes, such as bac¬  teria or fungi. IL, interleukin; MHC, major histocompatibility complex. (Reproduced with permission from Stites D, Terr A, ParslowT, eds. Basic &  Clinical Immunology. 9th ed. Originally published by Appleton & Lange. Copyright 1997 McGraw-Hill.)    Figure 57-3 summarizes the human host defenses against  virus-infected cells and illustrates the close interaction of  various cells in mounting a coordinated attack against the  pathogen. The specificity of the response is provided by the  antigen receptor (T-cell receptor [TCR]) on the surface of    both the CD4-positive T cell and the CD8-positive T cell and  by the antigen receptor (IgM) on the surface of the B cell.  The interleukins, on the other hand, are not specific.   As depicted in Figure 57-3, B cells can perform two  important functions during the induction process: (1) they   FIGURE 57-3 Induction of cell-mediated immunity and antibody against a viral infection. Right: Virus released by an infected cell is  ingested and processed by an antigen-presenting cell (APC) (e.g., a macrophage). The viral epitope is presented in association with a class II  major histocompatibility complex (MHC) protein to the virus-specific T-cell receptor (TCR) on the CD4 cell. The macrophage makes interleukin  (IL)-1, which helps activate the CD4 cell. The activated CD4 cell makes interleukins (e.g., IL-2, which activates the CD8 cell to attack the virus-  infected cell, and IL-4 and IL-5, which activate the B cell to produce antibody). The specificity of the cytotoxic response mounted by the CD8 cell  is provided by its TCR, which recognizes the viral epitope presented by the virus-infected cell in association with a class I MHC protein. Left:  Virus released by an infected cell interacts with the antigen receptor (IgM monomer) specific for that virus located on the surface of a B cell. The  virus is internalized, and the viral proteins are broken down into small peptides. B cells (as well as macrophages) can present viral epitopes in  association with class II MHC proteins and activate CD4 cells. The CD4-positive helper cell produces IL-4 and IL-5, which induce the B cell to  differentiate into a plasma cell that produces antibody specifically against this virus.    recognize antigens with their surface IgM that acts as an  antigen receptor, and (2) they present epitopes to helper T  cells in association with class II MHC proteins. Note that  the IgM antigen receptor on the B cell can recognize not  only foreign proteins but also carbohydrates, lipids, DNA,  RNA, and other types of molecules. The class II MHC pro¬  teins of the B cell, however, can only present peptide frag¬  ments to the helper T cells. This distinction will become  important when haptens are discussed later in this chapter.  It is this remarkable ability of the IgM antigen receptor on  the B cell to bind to an incredibly broad range of molecules  that enables B cells to produce antibodies against virtually  every molecule known. How the B cell generates such a  diverse array of antibodies is described on page 528.   Our immune host defenses can be divided into two major  categories: innate (natural) and adaptive (acquired). The  features of these two important components of our host  defenses are compared in Table 57-2.   Innate immunity is resistance that exists prior to exposure  to the microbe (antigen). It is nonspecific and includes  host defenses such as barriers to infectious agents (e.g., skin    and mucous membranes), certain cells (e.g., natural killer  cells), and certain proteins (e.g., the complement cascade  and interferons) and involves processes such as phagocyto¬  sis and inflammation (Table 57-4). Innate immunity does  not improve after exposure to the organism, in contrast to  acquired immunity, which does. In addition, innate  immune processes have no memory, whereas acquired  immunity is characterized by long-term memory.   Note that the innate arm of our host defenses performs  two major functions: killing invading microbes and activat¬  ing adaptive immune processes. Some components of the  innate arm, such as neutrophils, only kill microbes, whereas  others, such as macrophages and dendritic cells, perform  both functions (i.e., they kill microbes and present antigen to  helper T cells, which activates adaptive immune processes).   Although innate immunity is often successful in elimi¬  nating microbes and preventing infectious diseases, it is not  sufficient for human survival. This conclusion is based on  the observation that children with severe combined immu¬  nodeficiency disease (SCID), who have intact innate  immunity but no adaptive immunity, suffer from repeated,  life-threatening infections.   Several components of the innate arm recognize what is  foreign by detecting certain carbohydrates or lipids on the   Ingest and destroy microbes and present antigen to helper T  cells   C3b is an opsonin; membrane attack complex creates holes in  bacterial membranes    surface of microorganisms that are different from those on  human cells. Components of the innate arm have receptors  called pattern-recognition receptors that recognize a  molecular pattern called a pathogen-associated molecular  pattern (PAMP) that is present on the surface of many  microbes but—very importantly—is not present on human  cells. By using this strategy, these components of the innate  arm do not have to have a highly specific receptor for every  different microbe but can still distinguish between what is  foreign and what is self.   There are two classes of receptors on the surface of cells  (Toll-like receptors and mannan-binding lectin receptors)  that recognize microbes outside of cells and two classes of  receptors in the cytoplasm of cells (NOD receptors and  RIG-I helicase receptors) that recognize microbes within  cells. Mutations in the genes encoding these pattern recep¬  tors result in a failure to recognize the pathogen and predis¬  pose to severe bacterial, viral, and fungal infections.   The most important of these pattern-recognition recep¬  tors are the Toll-like receptors (TLR). This is a family of 10  receptors found mainly on the surface of three types of  cells: macrophages, dendritic cells, and mast cells. TLRs  recognize various microbial components and then activate  transcription factors that enhance the synthesis of several  proinflammatory cytokines. This initiates an immune  response appropriate to defend against that type of microbe.   Note that the type of host defense mounted by the body  differs depending on the type of organism. For example, a    humoral (antibody-mediated) response is produced against  one type of bacteria, but a cell-mediated response occurs in  response to a different type of bacteria. The process that  determines the type of response depends on the cytokines  produced by the macrophages, and this in turn depends on  which pattern-recognition receptor is activated by the  organism, as described in the next paragraph.   (1) Endotoxin is a lipopolysaccharide (LPS) found on  the surface of most gram-negative bacteria (but not on  human cells). The lipid A portion of LPS is the most impor¬  tant cause of septic shock and death in hospitalized  patients. When released from the bacterial surface, LPS  combines with LPS-binding protein, a normal component  of plasma. This binding protein transfers LPS to a receptor  on the surface of macrophages called CD 14. LPS stimulates  a pattern-recognition receptor called Toll-like receptor 4  (TLR4), which transmits a signal, via several intermediates,  to the nucleus of the cell. This induces the production of  cytokines, such as IL-1, IL-6, IL-8, and tumor necrosis fac¬  tor (TNL), and induces the costimulator protein, B7, which  is required to activate helper T cells and to produce anti¬  bodies. Note that a different Toll-like receptor, TLR2, sig¬  nals the presence of gram-positive bacteria and yeasts  because they have a different molecular pattern on their  surface. Drugs that modify the action of these Toll-like    receptors may become important in preventing endotoxin-  mediated septic shock, a leading cause of death in hospital¬  ized patients.   (2) Many bacteria and yeasts have a polysaccharide  called mannan on their surface that is not present on  human cells. (Mannan is a polymer of the sugar, mannose.)  A pattern-recognition receptor called mannan-binding  lectin (MBL) (also known as mannose-binding protein) on  the surface of dendritic cells and macrophages binds to the  mannan on the surface of the microbes, which then acti¬  vates complement (see Chapter 63), resulting in death of  the microbe. MBL also enhances phagocytosis (acts as an  opsonin) via receptors to which it binds on the surface of  phagocytes, such as macrophages. MBL is a normal serum  protein whose concentration in the plasma is greatly  increased during the acute-phase response (see later).   MBL is a member of a group of pattern receptors called  C-type lectin receptors. A different C-type lectin receptor  recognizes beta-glucan in the cell wall of fungi such as  Candida albicans.   (3) Part of the peptidoglycan (cell wall) of bacteria is  recognized by NOD receptors. These receptors are located  within the cytoplasm of human cells (e.g., macrophages,  dendritic cells, and epithelial cells); hence they are impor¬  tant in the innate response to intracellular bacteria such as  Listeria.   (4) RIG-I helicase receptors recognize the nucleic acids  of viruses in the cytoplasm of infected cells. For example,  members of the orthomyxovirus, paramyxovirus, and rhab-  dovirus families synthesize double-stranded RNA during  replication that are recognized by RIG-I helicase receptors.  Activation of these receptors results in the synthesis of alpha  and beta interferons that induce the antiviral state (see later).   The important cellular effectors of innate immunity namely,  macrophages, dendritic cells, and natural killer cells are dis¬  cussed in some detail in Chapter 58 starting on page 514. For  this discussion, it will suffice to say that macrophages and  dendritic cells ingest and kill various microbes whereas natu¬  ral killer cells primarily kill virus-infected cells.   Innate immunity has noncellular effectors as well.  Complement, a cascade of approximately 20 proteins, is  described in Chapter 63. Other protein effectors are dis¬  cussed in the following paragraphs.   The acute-phase response, which consists of an increase  in the levels of various plasma proteins (e.g., C-reactive  protein and mannose-binding protein), is an important  part of innate immunity. These proteins are synthesized by  the liver and are nonspecific responses to microorganisms  and other forms of tissue injury. The liver synthesizes these  proteins in response to certain cytokines, namely, IL-1,  IL-6, and TNF, produced by the macrophage after exposure  to microorganisms. These cytokines, IL-1, IL-6, and TNF,  are often called the proinflammatory cytokines, meaning    that they enhance the inflammatory response. The inflam-  masome is a multi-protein complex with protease activity  that enhances inflammation by producing IL-1 from its  precursor protein. Anti-inflammatory cytokines, such as  IL-10 and transforming growth factor-(3 (TGF-(3), restore  homeostasis after the inflammatory response is no longer  needed.   Some acute-phase proteins bind to the surface of bacte¬  ria and activate complement, which can kill the bacteria.  For example, C-reactive protein binds to a carbohydrate in  the cell wall of Streptococcus pneumoniae and, as men¬  tioned earlier, MBL binds to mannan (mannose) on the  surface of many bacteria.   Defensins are another important component of innate  immunity. Defensins are highly positively charged (i.e.,  cationic) peptides that create pores in the membranes of  bacteria and thereby kill them. How they distinguish  between microbes and our cells is unknown. Defensins are  located primarily in the gastrointestinal and lower respira¬  tory tracts. Neutrophils and Paneth cells in the intestinal  crypts contain one type of defensin (a-defensins), whereas  the respiratory tract produces different defensins called  -defensins.   a-Defensins also have antiviral activity. They interfere  with human immunodeficiency virus (HIV) binding to the  CXCR4 receptor and block entry of the virus into the cell.  The production of a-defensins may explain why some  HIV-infected individuals are long-term “nonprogressors.”   APOBEC3G (apolipoprotein B RNA-editing enzyme) is  an important member of the innate host defenses against  retroviral infection, especially against HIV. APOBEC3G is  an enzyme that causes hypermutation in retroviral DNA by  deaminating cytosines in both mRNA and retroviral DNA,  thereby inactivating these molecules and reducing infectiv-  ity. HIV defends itself against this innate host defense by  producing Vif (viral infectivity protein), which counteracts  APOBEC3G, thereby preventing hypermutation from  occurring.   Alpha and beta interferons are important antiviral pro¬  teins. They are synthesized early in infection within virus-  infected cells. They exit that cell, bind to the surface of an  adjacent cell, and induce an antiviral state in that adjacent  cell. The antiviral state is mediated by a ribonuclease and a  protein kinase that, acting together, inhibit viral protein  synthesis. See Chapter 33 for more information on the  action of these interferons. Gamma interferon is an impor¬  tant mediator of inflammation but has only modest antiviral  activity. It acts primarily to enhance killing by macrophages  and other phagocytes, and to increase the synthesis of class  1 and class 2 MHC proteins. See pages 509 and 520 for more  information on the action of gamma interferon.   FIGURE 57~4 Macrophages and other antigen-presenting  cells, such as dendritic cells, participate in both the innate arm and  the adaptive arm of the immune system. These cells are considered  part of the innate arm because they phagocytose and kill many types  of microbes and also produce cytokines that cause inflammation.  They are also part of the adaptive arm because they present antigen  in association with class II major histocompatibility complex (MHC)  proteins to CD4-positive helperT cells. (In common with all other  nucleated cells, they also can present antigen in association with  class I MHC proteins to CD8-positive cytotoxic T cells.)    mediated by antibody produced by B lymphocytes and by  two types of T lymphocytes, namely, helper T cells and  cytotoxic T cells. The cells responsible for adaptive immu¬  nity have long-term memory for a specific antigen. Adap¬  tive immunity can be active or passive. Chapter 58 describes  how the specificity and memory of acquired immunity is  produced.   Macrophages and other antigen-presenting cells such as  dendritic cells play an important role in both the innate and  the adaptive arms of the immune system (Figure 57-4).  When they phagocytose and kill microbes, they function as  part of the innate arm, but when they present antigen to a  helper T lymphocyte, they activate the adaptive arm that  leads to the production of antibody and of cells such as  cytotoxic T lymphocytes. Note that the adaptive arm can be  activated only after the innate arm has interacted with the  microbe.   Active immunity is resistance induced after contact with  foreign antigens (e.g., microorganisms). This contact may  consist of clinical or subclinical infection, immunization  with live or killed infectious agents or their antigens, or    exposure to microbial products (e.g., toxins and toxoids).  In all these instances, the host actively produces an immune  response consisting of antibodies and activated helper and  cytotoxic T lymphocytes.   The main advantage of active immunity is that resistance  is long-term (Table 57-5). Its major disadvantage is its slow  onset, especially the primary response (see Chapter 60).   Passive immunity is resistance based on antibodies pre¬  formed in another host. Administration of antibody against  diphtheria, tetanus, botulism, etc., makes large amounts of  antitoxin immediately available to neutralize the toxins.  Likewise, preformed antibodies to certain viruses (e.g.,  rabies and hepatitis A and B viruses) can be injected during  the incubation period to limit viral multiplication. Other  forms of passive immunity are IgG passed from mother to  fetus during pregnancy and IgA passed from mother to  newborn during breast feeding.   The main advantage of passive immunization is the  prompt availability of large amounts of antibody; disad¬  vantages are the short life span of these antibodies and  possible hypersensitivity reactions if globulins from another  species are used. (See serum sickness in Chapter 65.)   Passive-active immunity involves giving both pre¬  formed antibodies (immune globulins) to provide immedi¬  ate protection and a vaccine to provide long-term  protection. These preparations should be given at different  sites in the body to prevent the antibodies from neutraliz¬  ing the immunogens in the vaccine. This approach is used  in the prevention of tetanus (see Chapters 12 and 17),  rabies (see Chapters 36 and 39), and hepatitis B (see Chap¬  ters 36 and 41).   Antigens are molecules that react with antibodies, whereas  immunogens are molecules that induce an immune  response. In most cases, antigens are immunogens, and the  terms are used interchangeably. However, there are certain  important exceptions (e.g., haptens). A hapten is a mole¬  cule that is not immunogenic by itself but can react with  specific antibody. Haptens are usually small molecules, but  some high-molecular-weight nucleic acids are haptens as  well. Many drugs (e.g., penicillins) are haptens, and the  catechol in the plant oil that causes poison oak and poison  ivy is a hapten.   Haptens are not immunogenic because they cannot acti¬  vate helper T cells. The failure of haptens to activate is due  to their inability to bind to MHC proteins; they cannot    bind because they are not polypeptides and only polypep¬  tides can be presented by MHC proteins. Furthermore,  haptens are univalent and therefore cannot activate B cells  by themselves. (Compare with the T-independent response  of multivalent antigens discussed earlier in this chapter and  in Chapter 58.)   Although haptens cannot stimulate a primary or sec¬  ondary response by themselves, they can do so when cova¬  lently bound to a “carrier” protein (Figure 57-5). In this  process, the hapten interacts with an IgM receptor on the B  cell and the hapten-carrier protein complex is internalized.  A peptide of the carrier protein is presented in association  with class II MHC protein to the helper T cells. The acti¬  vated helper T cell then produces interleukins, which  stimulate the B cells to produce antibody to the hapten (see  Chapter 58, page 512, for additional information).   Two additional ideas are needed to understand how  haptens interact with our immune system. The first is that  many haptens, such as drugs (e.g., penicillin) and poison  oak oil, bind to our normal proteins, to which we are tol¬  erant. The hapten-protein combination now becomes  immunogenic (i.e., the hapten modifies the protein suffi¬  ciently such that when the hapten-peptide combination is  presented by the MHC protein, it is recognized as  foreign).   The second idea is that although most haptens are uni¬  valent, type I hypersensitivity reactions such as anaphylaxis  (see Chapter 65) require cross-linking of adjacent IgEs to  trigger the release of the mediators. By itself, a univalent  hapten cannot cross-link, but when many hapten molecules  are bound to the carrier protein, they are arranged in such  a way that cross-linking can occur. This is how a univalent  hapten, such as penicillin, causes anaphylaxis. Sufficient    penicillin binds to one of our proteins to cross-link IgE. An  excellent example of this is penicilloyl polylysine, which is  used in skin tests to determine whether a patient is allergic  to penicillin. Each lysine in the polylysine has a penicillin  molecule attached to it. These univalent penicillin mole¬  cules form a “multivalent” array and can cross-link adja¬  cent IgEs on the surface of mast cells. The consequent  release of mediators causes a “wheal and flare” reaction in  the skin of the penicillin-allergic patient.   Another medically important concept that is related to  the hapten-carrier protein model is that of conjugate vac¬  cines such as the pneumococcal and meningococcal vac¬  cines and the vaccine against Haemophilus influenzae. In  these conjugate vaccines, the capsular polysaccharide is  conjugated to a carrier protein. The capsular polysaccha¬  ride is not a hapten because it can induce IgM via the  T-independent response. However, adding a carrier protein  causes helper T cells to be involved, and large amounts of  IgG are produced via the T-dependent response.   The interaction of antigen and antibody is highly spe¬  cific, and this characteristic is frequently used in the diag¬  nostic laboratory to identify microorganisms. Antigen and  antibody bind by weak forces such as hydrogen bonds and  van der Waals forces rather than by covalent bonds. The  strength of the binding (the affinity) is proportionate to the  fit of the antigen with its antibody-combining site (i.e., its  ability to form more of these bonds). The affinity of anti¬  bodies increases with successive exposures to the specific  antigen (see Chapter 60). Another term, avidity, is also used  to express certain aspects of binding. It need not concern us  here.   The features of molecules that determine immunoge-  nicity are as follows.   Hapten part of  hapten-carrier  conjugate interacts  with hapten-specific  IgM on the surface  of a B cell    B cell presents  epitope of carrier  protein to helper  T cell that carries  aTCR specific  for that epitope    Activated helper T cell produces  interleukins that stimulate the  B cell to become a plasma cell  that produces antibodies against  the hapten    FIGURE 57~5 Hapten -carrier conjugate induces antibody against the hapten. A hapten covalently bound to a carrier protein can induce   antibody to a hapten by the mechanism depicted in the figure. A hapten alone cannot induce antibody, because the helperT cells are not  activated by the hapten. Although the hapten alone (without the carrier protein) can bind to the IgM receptor on the B-cell surface, the  interleukins essential for the B cell to become a plasma cell are not made.TCR,T-cell receptor.   In general, molecules recognized as “self” are not immuno¬  genic (i.e., we are tolerant to those self-molecules) (see  Chapter 66). To be immunogenic, molecules must be rec¬  ognized as “nonself” (i.e., foreign).   The most potent immunogens are proteins with high molec¬  ular weights (i.e., above 100,000). Generally, molecules with  molecular weight below 10,000 are weakly immunogenic,  and very small ones (e.g., an amino acid) are nonimmuno-  genic. Certain small molecules (e.g., haptens) become  immunogenic only when linked to a carrier protein.   A certain amount of chemical complexity is required (e.g.,  amino acid homopolymers are less immunogenic than het¬  eropolymers containing two or three different amino acids).   Epitopes are small chemical groups on the antigen mole¬  cule that can elicit and react with antibody. An antigen can  have one or more determinants (epitopes). Most antigens  have many determinants (i.e., they are multivalent). In gen¬  eral, a determinant is roughly five amino acids or sugars in  size. The overall three-dimensional structure is the main  criterion of antigenic specificity.   These factors also affect immunogenicity. In addition, the  genetic constitution of the host (HLA genes) determines  whether a molecule is immunogenic. Different strains of  the same species of animal may respond differently to the  same antigen.   Adjuvants enhance the immune response to an immuno¬  gen. They are chemically unrelated to the immunogen and  differ from a carrier protein because the adjuvant is not  covalently bound to the immunogen, whereas the carrier  protein is. Adjuvants can act in a variety of ways; they can  cause slow release of immunogen, thereby prolonging the  stimulus; enhance uptake of immunogen by antigen-pre¬  senting cells; and induce costimulatory molecules (“second  signals”). (See Chapter 58 regarding costimulators.)  Another important mechanism of action of some adjuvants  is to stimulate Toll-like receptors (see pages 494 and 505)  on the surface of macrophages, which results in cytokine  production that enhances the response of T cells and B cells  to the immunogen (antigen). Some human vaccines con¬  tain adjuvants such as aluminum hydroxide or lipids.   Immunity is less than optimal at both ends of life (i.e., in  the newborn and the elderly). The reason for the relatively  poor immune response in newborns is unclear, but new¬  borns appear to have less effective T-cell function than do  adults. In newborns, antibodies are provided primarily by  the transfer of maternal IgG across the placenta. Because  maternal antibody decays over time (little remains by 3-6  months of age), the risk of infection in the child is high.  Colostrum also contains antibodies, especially secretory  IgA, which can protect the newborn against various respi¬  ratory and intestinal infections.   The fetus can mount an IgM response to certain anti¬  gens (e.g., to Treponema pallidum, the cause of syphilis,  which can be acquired congenitally). IgG and IgA begin to  be made shortly after birth. The response to protein anti¬  gens is usually good; hence hepatitis B vaccine can be given  at birth and poliovirus immunization can begin at 2  months of age. However, neonates and children under the  age of 2 years respond poorly to polysaccharide antigens  unless they are conjugated to a carrier protein. For exam¬  ple, the pneumococcal vaccine containing the unconju¬  gated polysaccharides does not induce protective immunity  when given prior to 18 months of age, but the pneumococ¬  cal vaccine containing the polysaccharides conjugated to a  carrier protein is effective when given as early as 2 months  of age. This indicates the children under the age of two  years do not mount a protective T-independent response  (see Chapter 58, page 512).   In the elderly, immunity generally declines. There is a  reduced IgG response to certain antigens, fewer T cells, and  a reduced delayed hypersensitivity response. As in the very  young, the frequency and severity of infections are high.   The frequency of autoimmune diseases is also high in the  elderly, possibly because of a decline in the number of regu¬  latory T cells, which allows autoreactive T cells to prolifer¬  ate and cause disease.   1. Which one of the following is an attribute of the innate, rather   (D) Is part of our host defense against bacteria but not against fungi   (E) Is as effective the first time it is exposed to bacteria as it is  subsequent times    3. Which one of the following is most likely to induce an IgM anti¬  body response without the participation of helper T cells?   4. Regarding haptens, which one of the following is the most  accurate?   (A) A hapten is the antigen-binding site in the hypervariable region  of IgG.   (B) A hapten cannot induce antibody by itself but can do so when  covalently bound to a carrier protein.   (C) A hapten can bind to the antigen receptor of CD4-positive  T cells without being processed by macrophages.   (D) A hapten is defined by its ability to bind to the smaller of the  two polypeptides that comprise the class I MHC proteins.   5. Certain components of our immune system are characterized by  two attributes: being able (1) to respond specifically to microbes  and (2) to exhibit memory of having responded to a particular  microbe previously. Which one of the following has BOTH speci¬  ficity and memory?   6. Your patient says that she must travel on business 3 days from now  to a country where hepatitis A is endemic. She just read in the  newspaper that there are two types of protection against this dis¬  ease: one is a vaccine that contains killed hepatitis A virus, and the  other is serum globulin preparation that contains antibodies to the  virus. She asks which you would recommend and for what reason?    (A) The vaccine containing killed hepatitis A virus is best because  it induces the most antibody.   (B) The vaccine containing killed hepatitis A virus is best because  it provides the most long-lived immunity.   (C) The serum globulin preparation containing antibodies against  the virus is best because it provides immunity in the shortest  time.   (D) The serum globulin preparation containing antibodies against  the virus is best because it provides the most long-lived  immunity.   Questions on the topics discussed in this chapter can be found  in the Immunology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 733. Also see Part  XIV: USMLE (National Board) Practice Examinations starting  on page 751.   Costimulation Is Required to ActivateT Cells  T Cells Recognize Only Peptides  Memory T Cells  T-Cell Receptor   Effect of Superantigens onT Cells  Features ofT Cells  Effector Functions ofT Cells  Regulatory Functions ofT Cells   Summary of the Interaction Of Antigen-Presenting  Cells, T Cells, &B Cells  Follicular Dendritic Cells  Natural Killer Cells  Neutrophils  Eosinophils  Basophils & Mast Cells  Important Cytokines   Cytokines Affecting Lymphocytes  Cytokines Affecting Macrophages & Monocytes  Cytokines Affecting Polymorphonuclear Leukocytes  Cytokines Affecting Stem Cells  Cytokines Produced by Macrophages That Affect  Other Cells   The capability of responding to immunologic stimuli rests  mainly with lymphoid cells. During embryonic develop¬  ment, blood cell precursors originate mainly in the fetal  liver and yolk sac; in postnatal life, the stem cells reside in  the bone marrow. Stem cells differentiate into cells of the  erythroid, myeloid, or lymphoid series. The latter evolve  into two main lymphocyte populations: T cells and B cells  (Figure 58-1 and Table 58-1). The formation ofT cells and  B cells from stem cells is enhanced by interleukin-7 (IL-7)  produced by the stromal cells of the thymus and bone mar¬  row, respectively.   The ratio ofT cells to B cells is approximately 3:1.  Figure 58-1 describes the origin of B cells and the two types  ofT cells: helper T cells and cytotoxic T cells. Table 58-1    compares various important features of B cells and T cells.  These features will be described in detail later in the  chapter.   T-cell precursors differentiate into immunocompetent T  cells within the thymus. Prior to entering the thymus, stem  cells lack antigen receptors and lack CD3, CD4, and CD8  proteins on their surface. During passage through the thy¬  mus, they differentiate into T cells that can express both  antigen receptors and the various CD proteins. The stem  cells, which initially express neither CD4 nor CD8 (double¬  negatives), first differentiate to express both CD4 and CD8  (double-positives) and then proceed to express either CD4  or CD8. A double-positive cell will differentiate into a   FIGURE 58-1 Origi n of T and B cells. Stem cells in the bone marrow (or fetal liver) are the precursors of both T and B lymphocytes. Stem  cells differentiate intoT cells in the thymus, whereas they differentiate into B cells in the bone marrow. Within the thymus, T cells become either  CD4-positive (helper) cells or CD8-positive (cytotoxic) cells. B cells can differentiate into plasma cells that produce large amounts of antibodies  (immunoglobulins). Dotted lines indicate interactions mediated by interleukins. (Reproduced with permission from Brooks GF et al. Medical Microbiology.  20th ed. Originally published by Appleton & Lange. Copyright 1995 McGraw-Hill.)   Antigen receptor recognizes only processed peptides in association with MHC protein   Antigen receptor recognizes whole, unprocessed proteins and has no requirement for  presentation by MHC protein    IgM = immunoglobulin M; IL = interleukin; MHC = major histocompatibility complex.    CD4-positive cell if it contacts a cell bearing class II major  histocompatibility complex (MHC) proteins but will dif¬  ferentiate into a CD8-positive cell if it contacts a cell bear¬  ing class I MHC proteins. (Mutant mice that do not make  class II MHC proteins will not make CD4-positive cells,  indicating that this interaction is required for differentia¬  tion into single-positive cells to occur.) The double-nega¬  tive cells and the double-positive cells are located in the  cortex of the thymus, whereas the single-positive cells are  located in the medulla, from which they migrate out of the  thymus into the blood and extrathymic tissue.   Within the thymus, two very important processes called  thymic education occur:   (1) CD4-positive, CD8-positive cells bearing antigen  receptors for “self” proteins are killed (clonal deletion)  by a process of programmed cell death called apoptosis  (Figure 58-2). The removal of these self-reactive cells, a  process called negative selection, results in tolerance to  our own proteins (i.e., self-tolerance) and prevents autoim¬  mune reactions (see Chapter 66).   For negative selection to be efficient, the thymic epithe¬  lial cells must display a vast repertoire of self proteins. A  transcriptional regulator called the autoimmune regulator  (AIRE) enhances the synthesis of this array of self proteins.  Mutations in the gene encoding the AIRE protein results in  the development of an autoimmune disease called autoim¬  mune polyendocrinopathy.   (2) CD4-positive, CD8-positive cells bearing antigen  receptors that do not react with self MHC proteins  (See Figure 58-2) are also killed. This results in a positive  selection for T cells that react well with self MHC proteins.   These two processes produce T cells that are selected for  their ability to react both with foreign antigens via their  antigen receptors and with self MHC proteins. Both of  these features are required for an effective immune response  by T cells.   Note that MHC proteins perform two essential func¬  tions in the immune response: one is the positive selection  of T cells in the thymus, as just mentioned, and the other,  which is described later, is the presentation of antigens to  T cells, the initial step required to activate those cells. MHC  proteins are also the most important antigens recognized in  the graft rejection process (see Chapter 62).   During their passage through the thymus, each double¬  positive T cell synthesizes a different, highly specific anti¬  gen receptor called the T-cell receptor (TCR). The  rearrangement of the variable, diversity, and joining genes  (see Chapter 59) that encode the receptor occurs early in  T-cell differentiation and accounts for the remarkable abil¬  ity of T cells to recognize millions of different antigens.   Some T lymphocytes, perhaps as much as 40% of the  total, do not develop in the thymus but rather in the gut-  associated lymphoid tissue (GALT). These intraepithelial  lymphocytes (IELs) are thought to provide protection    against intestinal pathogens. Their antigen receptors and  surface proteins are different from those of thymus-derived  lymphocytes. IELs cannot substitute for thymus-derived  lymphocytes because patients with DiGeorge’s syndrome  who lack a thymus (see Chapter 68) are profoundly immu-  nodeficient and have multiple infections.   The thymus involutes in adults, yet T cells continue to  be made. Two explanations have been offered for this  apparent paradox. One is that a remnant of the thymus  remains functional throughout life and the other is that an  extrathymic site takes over for the involuted thymus. Indi¬  viduals who have had their thymus removed still make T  cells, which supports the latter explanation.   B-cell precursors differentiate into immunocompetent B  cells in the bone marrow; they do not pass through the  thymus. Analogous to T cells, B cells also undergo clonal  deletion (apoptosis) of those cells bearing antigen receptors  for self proteins, a process that induces tolerance and  reduces the occurrence of autoimmune diseases (see Chap¬  ter 66). Note that B cells bearing an antigen receptor for a  self protein can escape clonal deletion by a process called  receptor editing. In this process, a new, different light  chain is produced that changes the specificity of the recep¬  tor so that it no longer recognizes a self protein. It is esti¬  mated that as many as 50% of self-reactive B cells undergo  receptor editing. T cells do not undergo receptor editing.   Natural killer (NK) cells are large granular lymphocytes  that do not pass through the thymus, do not have an anti¬  gen receptor, and do not bear CD4 or CD8 proteins. They  recognize and kill target cells, such as virus-infected cells  and tumor cells, without the requirement that the antigens  be presented in association with class I or class II MHC  proteins. Rather, NK cells target those cells to be killed by  detecting that they do not display class I MHC proteins on  the cell surface. This detection process is effective because  many cells lose their ability to synthesize class I MHC pro¬  teins after they have been infected by a virus (see page 516).   In contrast to T cells, B cells, and NK cells, which differen¬  tiate from lymphoid stem cells, macrophages arise from  myeloid precursors. Macrophages have two important  functions, namely, phagocytosis and antigen presentation.  They do not pass through the thymus and do not have an  antigen receptor. On their surface, they display class II  MHC proteins, which play an essential role in antigen pre¬  sentation to helper T cells. Macrophages also display class I  MHC proteins, as do all nucleated cells. The cell surface  proteins that play an important role in the immune  response are listed in Table 58-2.   FIGURE 58~2 Development of T cells. Note the positive and negative selection that occurs in the thymus. MHC, major histocompatibility   T cells perform several important functions, which can be  divided into two main categories, namely, regulatory and  effector. The regulatory functions are mediated primarily    by helper (CD4-positive) T cells, which produce interleukins  (Table 58-3). For example, helper T cells make (1) interleu¬  kin (IL)-2, which activates CD4 and CD8 cells; (2) IL-4,  which help B cells make antibodies, especially IgE; and  (3) gamma interferon, which enhances killing by    TABLE 58-2 Cell Surface Proteins That Play an  Important Role in the Immune Response 1    IgM = immunoglobulin M; MHC = major histocompatibility complex; TCR = T-cell  antigen receptor.   ’There are many other cell surface proteins that play a role in the immune response,  but the proteins listed in this table are the most important for understanding the  fundamental aspects of this response.   3 Mature red blood cells do not synthesize class I MHC proteins because they do not  have a functioning nucleus.   macrophages. The effector functions are carried out pri¬  marily by cytotoxic (CD8-positive) T cells, which kill virus-  infected cells, tumor cells, and allografts.   Within the thymus, perhaps within the outer cortical epithe¬  lial cells (nurse cells), T-cell progenitors differentiate under  the influence of thymic hormones (thymosins and thymo-  poietins) into T-cell subpopulations. These cells are charac¬  terized by certain surface glycoproteins (e.g., CD3, CD4, and  CD8). All T cells have CD3 proteins on their surface in  association with antigen receptors (TCR [see later]). The  CD3 complex of five transmembrane proteins is involved  with transmitting, from the outside of the cell to the inside,  the information that the antigen receptor is occupied. One  of the CD3 transmembrane proteins, the zeta chain, is linked  to a tyrosine kinase called fyn, which is involved with signal  transduction. The signal is transmitted via several second  messengers, which are described in the section on activation  (see later). CD4 is a single transmembrane polypeptide,  whereas CD8 consists of two transmembrane polypeptides.  They may signal via tyrosine kinase (the lek kinase) also.   T cells are subdivided into two major categories on the  basis of whether they have CD4 or CD8 proteins on their   Activates the antigen-specific helperT cell  to produce a clone of these cells    surface. Mature T cells have either CD4 or CD8 proteins  but not both.   CD4 lymphocytes perform the following helper func¬  tions: (1) they help B cells develop into antibody-producing  plasma cells; (2) they help CD8 T cells to become activated  cytotoxic T cells; and (3) they help macrophages kill  ingested microbes. These functions are performed by three  distinct subpopulations of CD4 cells:   (1) The Th-1 cell subpopulation produces gamma  interferon that activates macrophages to become more  effective killers of intracellular microbes such as Mycobac¬  terium tuberculosis (Figure 58-3). The activated macro¬  phages have more lysosomal proteases, more hypochlorite,  and more reactive oxygen species such as superoxides, all of  which collaborate in the killing of the intracellular microbes.   Also, Th-1 cells are an important source of IL-2 which is  T-cell growth factor, that is, it stimulates the growth of both  CD4 and CD8 T cells. Note also that the cytokines produced by  Th-1 cells (e.g., gamma interferon) help B cells to class switch  (see page 512) to produce two subclasses of IgG (namely IgG 1  and IgG 3) that are very effective opsonizers of bacteria.   (2) The Th-2 cell subpopulation performs the B-cell  helper function primarily by producing IL-4 and IL-5 (see  Figure 58-3). IL-4 and IL-5 enhance the growth and dif¬  ferentiation of B cells into plasma cells. Also, IL-4 induces  class switching to IgE thereby enhancing the host defense  against worms. IL-5 increases the number and activity of  eosinophils which further enhances the host defense  against worms. IL-5 also induces class switching to IgA  thereby enhancing gut immunity.   (3) The Th-17 cell subpopulation produces IL-17 that  recruits neutrophils to a site of inflammation. Th-17 cells  enhance mucosal immunity, especially in the gastrointesti¬  nal tract ( not shown in Figure 58-3).   One important regulator of the balance between Th-1  cells and Th-2 cells is IL-12, which is produced by macro¬  phages. IL-12 increases the number of Th-1 cells, thereby  enhancing host defenses against organisms that are con¬  trolled by a delayed hypersensitivity response (Table 58-4).  Another important regulator is gamma interferon, which  inhibits the production of Th-2 cells. CD4 cells make up  about 65% of peripheral T cells and predominate in the  thymic medulla, tonsils, and blood.   To mount a protective immune response against a spe¬  cific microbe requires that the appropriate subpopulation  (i.e., either Th-1 or Th-2 cells) play a dominant role in the  response. For example, if an individual is infected with M.  tuberculosis and Th-2 cells are the major responders, then  humoral immunity will be stimulated rather than cell-  mediated immunity. Humoral immunity is not protective  against M. tuberculosis, and the patient will suffer severe  tuberculosis. Similarly, if an individual is infected with  Streptococcus pneumoniae and Th-1 cells are the major  responders, then humoral immunity will not be stimulated  and the patient will have severe pneumococcal disease.    FIGURE 58-3 The origin of Th-1 and Th-2 cells. On the left side, the origin of Th-1 cells is depicted. Microorganisms are ingested by mac¬  rophages, and interleukin (IL)-12 is produced. IL-12 induces naive Th-0 cells to become Th-1 cells that produce gamma interferon and IL-2. These  interleukins activate macrophages and cytotoxic T cells, respectively, and cell-mediated immunity occurs. On the right side, the origin of Th-2  cells is depicted. Microorganisms are ingested by an unknown type of cell (see footnote below), and IL-4 is produced. IL-4 induces naive Th-0  cells to become Th-2 cells that produce IL-4 and IL-5. These interleukins activate B cells to become plasma cells, and antibodies are produced.  Not shown is the differentiation of Th-17 cells from naive helperT cells (Th-0) under the direction of IL-6 andTGF-(3 produced by dendritic cells.  Also not shown in the figure is an important regulatory step, namely, that IL-10 produced by Th-2 cells inhibits IL-12 production by macro¬  phages and drives the system toward an antibody response and away from a cell-mediated response. *The human cell that produces the IL-4,  which induces naive helperT cells to become Th-2 cells, has not been identified.    Precisely what component of a microbe activates either  Th-1 or Th-2 cells is unknown.   How the appropriate response is stimulated is known for  one medically important organism, namely, M. tuberculosis.  A lipoprotein of that bacterium interacts with a specific  Toll-like receptor on the surface of the macrophage, which  induces the production of IL-12 by the macrophage. IL-12  drives the differentiation of naive helper T cells to form the  Th-1 type of helper T cells that are required to mount a  cell-mediated (delayed hypersensitivity) response against  the organism.   A subset of CD4 cells called Th-17 cells play an impor¬  tant role in mucosal immunity, especially in the mucosa of  the gastrointestinal (GI) tract. These cells are characterized    by producing IL-17 rather than the typical cytokines pro¬  duced by Th-1 cells, namely gamma interferon, and Th-2  cells, namely IL-4. IL-17 acts to recruit neutrophils to the  site of bacterial infections. Th-17 cells develop from naive  Th-0 cells in the presence of IL-6 and TGF-f) produced by  dendritic cells.   One clinical finding related to Th-17 cells is that they  are selectively killed by human immunodeficiency virus  (HIV). The loss of Th-17 cells results in a high rate of  bloodstream infections caused by colonic bacteria, such as  Escherichia coli and Klebsiella. IL-17 also contributes to our  host defenses against certain fungal infections, such as  chronic mucocutaneous candidiasis. The signature cyto¬  kines produced by the subsets of CD4-positive helper T  cells are described in Table 58-5.   CD8 lymphocytes perform cytotoxic functions (i.e.,  they kill virus-infected tumor and allograft cells). They kill  by either of two mechanisms, namely, the release of perfo¬  rins, which destroy cell membranes, or the induction of  programmed cell death (apoptosis). CD8 cells predominate  in human bone marrow and gut lymphoid tissue and con¬  stitute about 35% of peripheral T cells.   The activation of helper T cells requires that their TCR  recognize a complex on the surface of antigen-presenting  cells (APCs) (e.g., dendritic cells) consisting of both the  antigen and a class II MHC protein. The activation of  cytotoxic T cells requires that their TCR recognize a com¬  plex on the surface of APCs consisting of both the antigen  and class I MHC protein. Note that this can occur because  APCs have both class I and class II proteins on their sur¬  face. Dendritic cells, macrophages, and B cells are the  most important antigen-presenting cells (i.e., have class  II proteins on their surface).   The activation of helper T cells begins with the ingestion  of the foreign protein (or microbe) into the APC. Within  the cytoplasm of the APC, the foreign protein is cleaved  into small peptides that associate with the class II MHC  proteins. The complex is transported to the surface of the    TABLE 58-5 Signature Cytokine Produced by  Subsets of CD4-Positive Helper T Cells    APC, where the antigen, in association with a class II MHC  protein, is presented to the receptor on the CD4-positive  helper cell. This plus the action of costimulators (see later)  activates the helper T cell.   Note that APCs (e.g., dendritic cells) are typically under  an epithelial surface, whereas T cells are primarily in lymph  nodes. How do the two cells get together? After the APC  ingests the microbe, it produces a chemokine receptor  called CCR7. T cells in the lymph node continuously pro¬  duce the chemokine CCL21, and the dendritic cell migrates  from the epithelium to the lymph node via the lymphatics  by ascending the gradient of CCL21.   The activation of cytotoxic T cells can occur when the  APC itself is infected with a virus and viral proteins are  synthesized and then presented on the surface in associa¬  tion with class I MHC proteins. Activation of cytotoxic T  cells can also occur when the APC ingests pieces of a dying  virus-infected cell. Viral antigens from the infected cell are  then presented in association with class I MHC proteins, a  process called cross-presentation.   Similarly, within a virus-infected cell that is not an APC,  the newly synthesized viral peptide associates with class I  MHC protein and the complex is transported to the sur¬  face, where the viral antigen is presented to the receptor on  a CD8-positive cytotoxic cell. Remember the rule of eight:  CD4 cells interact with class II (4 x 2 = 8), and CD8 cells  interact with class I (8 x 1 = 8).   There are many different alleles within the class I and  class II MHC genes; hence, there are many different MHC  proteins. These various MHC proteins bind to different  peptide fragments. The polymorphism of the MHC genes  and the proteins they encode are a means of presenting  many different antigens to the TCR. Note that class I and  class II MHC proteins can only present peptides; other  types of molecules do not bind and therefore cannot be  presented. Note also that MHC proteins present peptides  derived from self proteins as well as from foreign proteins;  therefore, whether an immune response occurs is deter¬  mined by whether a T cell bearing a receptor specific for  that peptide has survived the positive and negative selec¬  tion processes in the thymus.   Two signals are required to activate both helper and  cytotoxic T cells. The first signal in the activation pro¬  cess is the interaction of the antigen and the MHC pro¬  tein with the TCR specific for that antigen (Figure 58-4  left). Note that when the TCR interacts with the antigen-  MHC protein complex, the CD4 protein on the surface  of the helper T cell also interacts with the class II MHC  protein. In addition to the binding of the CD4 protein  with the MHC class II protein, other proteins interact to  help stabilize the contact between the T cell and the APC  (e.g., lymphocyte function-associated antigen 1 [LFA-1]   FIGURE 58-4 Activation ofT cells. Left: An antigen-presenting cell (APC) presents processed antigen in association with a class II major  histocompatibility complex (MHC) protein. The antigen is recognized by theT-cell receptor (TCR) specific for that antigen, and the helper T cell  is activated to produce interleukin-2 (IL-2). IL-2 binds to its receptor on the helperT cell and further activates it. Note that CD4 protein on the  helper T cell binds to the MHC class II protein on the APC, which stabilizes the interaction between the two cells, and that B7 on the APC must  interact with CD28 on the helperT cell for full activation of helperT cells to occur. Note, for simplicity, the activation of cytotoxic T cells is not  shown but that process requires costimulation as well. To activate cytotoxic T cells, B7 and CD28 must interact when the APC presents viral anti¬  gen in association with class I MHC protein. Right: A virus-infected cell presents viral antigen in association with class I MHC protein. The viral  antigen is recognized by the TCR specific for that antigen, and in conjunction with IL-2 produced by the helperT cell, the cytotoxic T cell is acti¬  vated to kill the virus-infected cell. The CD8 protein on the cytotoxic T cell binds to the class I protein on the virus-infected cell, which stabilizes  the interaction between the two cells. Note that for this effector function of cytotoxic T cells, no B7:CD28 costimulation is required. Note also  that the class II MHC protein consists of two polypeptides, both of which are encoded by genes in the human leukocyte antigen (HLA) locus.  The class I protein, in contrast, consists of one polypeptide encoded by the HLA locus and p 2 -microglobulin (fi 2 MG), which is encoded  elsewhere.    protein 1 on T cells [both CD4-positive and CD8-positive]  binds to intracellular adhesion molecule 1 [ICAM-1]  protein on APCs).   A second costimulatory signal is also required (i.e., B7  protein on the APC must interact with CD28 protein on the  helper or cytotoxic T cell) (See Figure 58-4). If the costim¬  ulatory signal occurs, IL-2 is made by the helper T cell, and  it is this step that is crucial to producing a helper and  cytotoxic T cell capable of performing its regulatory, effec¬  tor, and memory functions. Note that in Figure 58-4 left,  only the B7:CD28 interaction for the helper T cell is shown  but the same costimulation process is required to activate  cytotoxic T cells as well.   If, on the other hand, the TCR interacts with its antigen  (epitope) and the costimulatory signal does not occur, a  state of unresponsiveness called anergy ensues (see Chap¬  ter 66). The anergic state is specific for that epitope. Other  T cells specific for other epitopes are not affected. Produc¬  tion of the costimulatory protein depends on activation of  the Toll-like receptor on the APC surface. Foreign antigens,  such as bacterial proteins, induce B7 protein, whereas self  antigens do not.    1 Lymphocyte function-associated antigen proteins belong to a family of  cell surface proteins called integrins, which mediate adhesion to other  cells. Integrin proteins are embedded in the surface membrane and have  both extracellular and intracellular domains. Hence, they interact with  other cells externally and with the cytoplasm internally.    After the T cells have served their function and the  infection is resolved, a different protein called cytotoxic T  lymphocyte antigen-4 (CTLA-4) appears on the T-cell sur¬  face and binds to B7 by displacing CD28. The interaction  of B7 with CTLA-4 inhibits T-cell activation by blocking  IL-2 synthesis (Figure 58-5). This restores the activated T  cell to a quiescent state and thereby plays an important role  in T-cell homeostasis. Mutant T cells that lack CTLA-4 and  therefore cannot be deactivated cause autoimmune reac¬  tions. Furthermore, administration of CTLA-4 reduced the  rejection of organ transplants in experimental animals.   The clinical importance of CTLA-4 is dramatically illus¬  trated by the effectiveness of abatacept (Orencia) in rheu¬  matoid arthritis. Abatacept is CTLA-4-IG, a fusion protein  composed of CTLA-4 and a fragment of the Fc domain of  human IgG. The Fc fragment provides resistance against  degradation, resulting in increased plasma levels of CTLA-4  for a longer duration than CTLA-4 alone. The mechanism  of action of abatacept is the binding of CTLA-4 to B7,  thereby displacing CD28 from its binding to B7. This  results in a reduction of the helper T-cell activity and a  reduction in the inflammatory response.   Administration of antibody against CTLA-4 can enhance  the immune response against some human cancer cells and  cause the cancer to regress. Note that in this instance, the  antibody is an inhibitor of an inhibitory molecule (CTLA-  4), resulting in an enhancement of the immune response  against the cancer cells. The activity of both helper and    FIGURE 58-5 Inhibition of activated helper T cells. When the activated helper T cells are no longer needed, a return to a quiescent state  occurs when an inhibitory protein called CTLA-4 is displayed on the surface of the helper T cell. CTLA-4 binds more strongly to B7 than does  CD28 and so displaces CD28 from its interaction with B7.This inhibits the synthesis of interleukin-2 (IL-2), and theT cell enters a resting state.  Left: Activation of the helper T cells occurs because B7 protein is displayed on the surface of the antigen-presenting cell and interacts with  CD28 on the helperT cell. (This is the same process as that depicted on the left side of Figure 58-4.) Right: CTLA-4 protein is displayed on the  surface of the helperT cell and interacts with B7 on the antigen-presenting cell. As a result, IL-2 is no longer synthesized. MHC, major histocom¬  patibility complex; TCR, T-cell receptor.    cytotoxic T cells is enhanced with cytotoxic T cells killing  the cancer cells displaying new, nonself antigens.   In addition to CTLA-4, there is another inhibitory pro¬  tein on the surface of T cells called PD-1 (programmed cell  death-1). When PD-1 interacts with its ligand (PDL-1) on  the surface of APCs, such as dendritic cells and macro¬  phages, the immune response is inhibited. Monoclonal  antibodies against PD-1 that enhance the immune response  are effective as anticancer drugs in clinical trials.   T cells recognize only polypeptide antigens. Furthermore,  they recognize those polypeptides only when they are pre¬  sented in association with MHC proteins. Helper T cells  recognize antigen in association with class II MHC pro¬  teins, whereas cytotoxic T cells recognize antigen in asso¬  ciation with class I MHC proteins. This is called MHC  restriction (i.e., the two types of T cells [CD4 helper and  CD8 cytotoxic] are “restricted” because they are able to  recognize antigen only when the antigen is presented with  the proper class of MHC protein). This restriction is medi¬  ated by specific binding sites primarily on the TCR, but also  on the CD4 and CD8 proteins that bind to specific regions  on the class II and class I MHC proteins, respectively.   Generally speaking, class I MHC proteins present  endogenously synthesized antigens (e.g., viral proteins),  whereas class II MHC proteins present the antigens of  extracellular microorganisms that have been phagocytized  (e.g., bacterial proteins). One important consequence of  these observations is that killed viral vaccines do not acti¬  vate the cytotoxic (CD8-positive) T cells, because the virus  does not replicate within cells and therefore viral epitopes  are not presented in association with class I MHC proteins.    Class I and class II proteins are described in more detail in  Chapter 62.   This distinction between endogenously synthesized and  extracellularly acquired proteins is achieved by processing  the proteins in different compartments within the cyto¬  plasm. The endogenously synthesized proteins (e.g., viral  proteins) are cleaved by a proteasome, and the peptide frag¬  ments associate with a “TAP transporter” that transports  the fragment into the rough endoplasmic reticulum, where  it associates with the class I MHC protein. The complex of  peptide fragment and class I MHC protein then migrates  via the Golgi apparatus to the cell surface. In contrast, the  extracellularly acquired proteins are cleaved to peptide  fragments within an endosome, where the fragment associ¬  ates with class II MHC proteins. This complex then  migrates to the cell surface.   An additional protection that prevents endogenously  synthesized proteins from associating with class II MHC  proteins is the presence of an “invariant chain” that is  attached to the class II MHC proteins when these proteins  are outside of the endosome. The invariant chain is  degraded by proteases within the endosome, allowing the  peptide fragment to attach to the class II MHC proteins  only within that compartment.   B cells, on the other hand, can interact directly with  antigens via their surface immunoglobulins (IgM and IgD).  Antigens do not have to be presented to B cells in associa¬  tion with class II MHC proteins, unlike T cells. Note that B  cells can then present the antigen, after internalization and  processing, to helper T cells in association with class II  MHC proteins located on the surface of the B cells (see the  section on B cells, later). Unlike the antigen receptor on T  cells, which recognizes only peptides, the antigen receptors  on B cells (IgM and IgD) recognize many different types of    molecules, such as peptides, polysaccharides, nucleic acids,  and small molecules (e.g., drugs such as penicillin).   These differences between T cells and B cells explain the  hapten-carrier relationship described in Chapter 57. To  stimulate hapten-specific antibody, the hapten must be  covalently bound to the carrier protein. The hapten binds  to the IgM receptor on the B-cell surface. That IgM is spe¬  cific for the hapten, not the carrier protein. The hapten-  carrier conjugate is internalized and the carrier protein  processed into small peptides that are presented in associa¬  tion with class II MHC proteins to a helper T cell bearing a  receptor for that peptide. The helper T cell then secretes  lymphokines that activate the B cell to produce antibodies  to the hapten.   When the antigen-MHC protein complex on the APC  interacts with the TCR, a signal is transmitted by the CD3  protein complex through several pathways that eventually  lead to a large influx of calcium into the cell. (The details of  the signal transduction pathway are beyond the scope of  this book, but it is known that stimulation of the TCR acti¬  vates a series of phosphokinases, which then activate phos¬  pholipase C, which cleaves phosphoinositide to produce  inositol triphosphate, which opens the calcium channels.)  Calcium activates calcineurin, a serine phosphatase. Calci-  neurin moves to the nucleus and is involved in the activa¬  tion of the genes for IL-2 and the IL-2 receptor. (Calcineurin  function is blocked by cyclosporine, one of the most effec¬  tive drugs used to prevent rejection of organ transplants  [see Chapter 62].)   The end result of this series of events is the activation of  the helper T cell to produce various lymphokines (e.g., IL-2),  as well as the IL-2 receptor. IL-2, also known as T-cell growth  factor, stimulates the helper T cell to multiply into a clone of  antigen-specific helper T cells. Most cells of this clone per¬  form effector and regulatory functions, but some become  memory cells (see later), which are capable of being rapidly  activated upon exposure to antigen at a later time. (Cytotoxic  T cells and B cells also form memory cells.) Note that IL-2  stimulates CD8 cytotoxic T cells as well as CD4 helper T cells.  Activated CD4-positive T cells also produce another lympho-  kine called gamma interferon, which increases the expres¬  sion of class II MHC proteins on APCs. This enhances the  ability of APCs to present antigen to T cells and upregulates  the immune response. (Gamma interferon also enhances the  microbicidal activity of macrophages.)   The process of activating T cells does not function as a  simple “on-off” switch. The binding of an epitope to the  TCR can result in either full activation, partial activation in  which only certain lymphokines are made, or no activation,  depending on which of the signal transduction pathways is  stimulated by that particular epitope. This important  observation may have profound implications for our under¬  standing of how helper T cells shape our response to infec¬  tious agents.   There are three genes at the class I locus (A, B, and C)  and three genes at the class II locus (DP, DQ, and DR).    We inherit one set of class I and one set of class II genes  from each parent. Therefore, our cells can express as many  as six different class I and six different class II proteins (see  Chapter 62). Furthermore, there are multiple alleles at each  gene locus. Each of these MHC proteins can present pep¬  tides with a different amino acid sequence. This explains, in  part, our ability to respond to many different antigens.   Memory T (and B) cells, as the name implies, endow our  host defenses with the ability to respond rapidly and vigor¬  ously for many years after the initial exposure to a microbe  or other foreign material. This memory response to a spe¬  cific antigen is due to several features: (1) many memory  cells are produced, so that the secondary response is greater  than the primary response, in which very few cells respond;  (2) memory cells live for many years or have the capacity to  reproduce themselves; (3) memory cells are activated by  smaller amounts of antigen and require less costimulation  than do naive, unactivated T cells; and (4) activated mem¬  ory cells produce greater amounts of interleukins than do  naive T cells when they are first activated.   The TCR for antigen consists of two polypeptides, alpha  and beta, 2 which are associated with CD3 proteins.   TCR polypeptides are similar to immunoglobulin  heavy chains in that (1) the genes that code for them are  formed by rearrangement of multiple regions of DNA (see  Chapter 59); (2) there are V (variable), D (diversity), J (join¬  ing), and C (constant) segments that rearrange to provide  diversity, giving rise to an estimated number of more than  100 million different receptor proteins; (3) the variable  regions have hypervariable domains; and (4) the two genes  (RAG-1 and RAG-2 ) that encode the recombinase enzymes  that catalyze these gene rearrangements are similar in T cells  and B cells.   Note that each T cell has a unique TCR on its surface,  which means that hundreds of millions of different T cells  exist in each person. Activated T cells, like activated B cells,  clonally expand to yield large numbers of cells specific for  that antigen.   Although TCRs and immunoglobulins (antibodies) are  analogous in that they both interact with antigen in a  highly specific manner, the TCR is different in two impor¬  tant ways: (1) it has two chains rather than four, and (2) it  recognizes antigen only in conjunction with MHC pro¬  teins, whereas immunoglobulins recognize free antigen.    2 Some TCRs have a different set of polypeptides called gamma and delta.  These TCRs are unusual because they do not require that antigen be pre¬  sented in association with MHC proteins. Gamma/delta T cells constitute  approximately 10% of all T cells. Some of the T cells bearing these TCRs  are involved in cell-mediated immunity against M. tuberculosis.    Note that the receptor on the surface of B cells (either IgM  or IgD) recognizes antigen directly without the need for  presentation by MHC proteins. Also TCR proteins are  always anchored into the outer membrane of T cells. There  is no circulating form as there is with certain antibodies  (e.g., monomeric IgM is in the B-cell membrane, but pen-  tameric IgM circulates in the plasma).   Certain proteins, particularly staphylococcal enterotoxins  and toxic shock syndrome toxin, act as “superantigens”  (Figure 58-6). In contrast to the typical (nonsuper) anti¬  gen, which activates one (or a few) helper T cell, superanti¬  gens are “super” because they activate a large number of  helper T cells. For example, toxic shock syndrome toxin  binds directly to class II MFIC proteins without internal  processing of the toxin. This complex interacts with the  variable portion of the beta chain (V(3) of the TCR of many  T cells. 3   This activates the T cells, causing the release of IL-2  from the T cells and IL-1 and tumor necrosis factor (TNF)  from macrophages. These interleukins account for many of  the findings seen in toxin-mediated staphylococcal dis¬  eases. Certain viral proteins (e.g., those of mouse mam¬  mary tumor virus [a retrovirus]) also possess superantigen  activity.   T cells constitute 65% to 80% of the recirculating pool of  small lymphocytes. Within lymph nodes, they are located  in the inner, subcortical region, not in the germinal centers.  (B cells make up most of the remainder of the pool of small  lymphocytes and are found primarily in the germinal cen¬  ters of lymph nodes.) The life span of T cells is long:  months or years. They can be stimulated to divide when  exposed to certain mitogens (e.g., phytohemagglutinin or  concanavalin A [endotoxin, a lipopolysaccharide found on  the surface of gram-negative bacteria, is a mitogen for B  cells but not T cells]). Most human T cells have receptors  for sheep erythrocytes on their surface and can form  “rosettes” with them; this finding serves as a means of iden¬  tifying T cells in a mixed population of cells.   The four types of T cells (Th-1, Th-2, and Th-17 types of  CD4 cells, and CD8 cells) mediate different aspects of our  host defenses. Th-1 cells mediate delayed hypersensitivity  reactions against intracellular organisms. Th-2 cells medi¬  ate protection against helminths (worms). Th-17 cells pro¬  tect against the spread of bacterial infections by recruiting    3 Each superantigen (e.g., the different staphylococcal enterotoxins)  interacts with different V(5 chains. This explains why many, but not all,  helper T cells are activated by the various superantigens.   FIGURE 58-6 Activation of helper T cells by superantigen.  Top: The helper T cell is activated by the presentation of processed  antigen in association with class II major histocompatibility complex  (MHC) protein to the antigen-specific portion of the T-cell receptor.  Note that superantigen is not involved and that only one or a small  number of helper T cells specific for the antigen are activated.  Bottom: The helper T cell is activated by the binding of superantigen  to the V[3 portion of the T-cell receptor outside of its antigen-specific  site without being processed by the antigen-presenting cell. Because  it bypasses the antigen-specific site, superantigen can activate many  helper T cells. (Used with permission from Pantaleo G et al. Mechanisms of dis-  ease: The immunopathogenesis of human immunodeficiency virus infection. N Engl  J Med. 1993;328:327.)    neutrophils to the site of infection. CD8 cells protect  against viral infection by killing virus-infected cells.   Th-1 cells and macrophages are the main effectors of  delayed hypersensitivity reactions that protect against  intracellular microorganisms including certain fungi  (e.g., Histoplasma and Coccidioid.es) and certain intracellu¬  lar bacteria (e.g., M. tuberculosis). The most important  interleukin for these reactions is gamma interferon, but  others such as macrophage activation factor and macro¬  phage migration inhibition factor (MIF) also play a role.  Th-1 cells produce the interleukins that activate the macro¬  phages, and macrophages are the ultimate effectors that    kill the organisms. A reduced ability to mount this response  manifests itself as a marked susceptibility to disease caused  by such microorganisms.   In the case of M. tuberculosis, a lipoprotein of the bacte¬  rium stimulates a specific Toll-like receptor on the macro¬  phage, which signals the cell to synthesize IL-12. IL-12 then  induces naive helper T cells to differentiate into the Th-1  type of helper T cells that participates in the delayed hyper¬  sensitivity response.   Th-1 cells produced gamma interferon, which activates  macrophages, thereby enhancing their ability to kill M.  tuberculosis. This IL-12-gamma interferon axis is very  important in the ability of our host defenses to control  infections by intracellular pathogens, such as M. tuberculo¬  sis and Listeria monocytogenes.   Th-2 cells and eosinophils are the main effectors of reactions  that protect against helminths (worms) such as Schistosoma  and Strongyloides. The most important interleukins for these  reactions are IL-4, which increases the production of IgE, and  IL-5, which activates eosinophils. IgE binds to the surface of  the worm. Eosinophils then bind to the heavy chain of IgE and  secrete enzymes that destroy the worm.   Th-17 cells protect against the spread of bacterial infec¬  tions at mucosal surfaces by producing IL-17. IL-17 attracts  neutrophils to the site of infection whereupon the bacteria  are ingested and destroyed. Patients with a defective Th-17  response are predisposed to bloodstream infections caused  by bacteria in the colon such as Escherichia and Klebsiella.  A defective Th-17 response also predisposes to chronic  mucocutaneous candidiasis caused by C. albicans.   CD8 cells mediate the cytotoxic response that is con¬  cerned primarily with destroying virus-infected cells and  tumor cells but also play an important role in graft rejec¬  tion. In response to virus-infected cells, the CD8 lympho¬  cytes must recognize both viral antigens and class I  molecules on the surface of infected cells. To kill the virus-  infected cell, the cytotoxic T cell must be activated by IL-2  produced by a helper (CD4-positive) T cell. To become  activated to produce IL-2, helper T cells recognize viral  antigens bound to class II molecules on an APC (e.g., a  dendritic cell or macrophage). The activated helper T cells  secrete cytokines such as IL-2, which stimulates the virus-  specific cytotoxic T cell to form a clone of activated cyto¬  toxic T cells.   Activated cytotoxic T cells kill virus-infected cells pri¬  marily by inserting perforins and degradative enzymes  called granzymes into the infected cell. Perforins form a  channel through the membrane, the cell contents are lost,  and the cell dies. Granzymes are proteases that degrade    proteins in the cell membrane, which also leads to the loss  of cell contents. Granzymes also activate caspases (a type of  protease) that initiate apoptosis, resulting in cell death.  After killing the virus-infected cell, the cytotoxic T cell  itself is not damaged and can continue to kill other cells  infected with the same virus. Cytotoxic T cells have no  effect on free virus, only on virus-infected cells.   Note that the effector functions of cytotoxic T cells do  not require costimulation. No B7-CD28 interaction is  required. This allows cytotoxic T cells to kill any virus-  infected cell. The activation of the cytotoxic T cells by the  antigen-presenting cell (e.g., dendritic cell) does require  costimulation but the effector function does not.   Another mechanism by which cytotoxic T cells kill tar¬  get cells is the Fas-Fas ligand (FasL) interaction. Fas is  a protein displayed on the surface of many cells. When  a cytotoxic TCR recognizes an epitope on the surface of a  target cell, FasL is induced in the cytotoxic T cell. When Fas  and FasL interact, apoptosis (death) of the target cell  occurs. NK cells can also kill target cells by Fas-FasL-  induced apoptosis.   In addition to direct killing by cytotoxic T cells, virus-  infected cells can be destroyed by a combination of IgG and  phagocytic cells. In this process, called antibody-dependent  cellular cytotoxicity (ADCC), antibody bound to the sur¬  face of the infected cell is recognized by IgG receptors on  the surface of phagocytic cells (e.g., macrophages or NK  cells), and the infected cell is killed. The ADCC process can  also kill helminths (worms). In this case, IgE is the antibody  involved, and eosinophils are the effector cells. IgE binds to  surface proteins on the worm, and the surface of eosino¬  phils displays receptors for the epsilon heavy chain. The  major basic protein located in the granules of the eosino¬  phils is released and damages the surface of the worm.   Many tumor cells develop new antigens on their surface.  These antigens bound to class I proteins are recognized by  cytotoxic T cells, which are stimulated to proliferate by  IL-2. The resultant clone of cytotoxic T cells can kill the  tumor cells, a phenomenon called immune surveillance.   In response to allografts, cytotoxic (CD8) cells recog¬  nize the class I MHC molecules on the surface of the for¬  eign cells. Helper (CD4) cells recognize the foreign class II  molecules on certain cells in the graft (e.g., macrophages  and lymphocytes). The activated helper cells secrete IL-2,  which stimulates the cytotoxic cell to form a clone of cells.  These cytotoxic cells kill the cells in the allograft.   T cells play a central role in regulating both the humoral  (antibody) and cell-mediated arms of the immune system.   Antibody production by B cells usually requires the partici¬  pation of helper T cells (T-cell-dependent response), but  antibodies to some antigens (e.g., polymerized [multivalent]   FIGURE 58-7 A: B-cell activation by helper T cells. B° is a resting B cell to which a multivalent antigen is attaching to monomer IgM  receptors (Y). The antigen is internalized, and a fragment (▲) is returned to the surface in conjunction with a class II molecule (■). A receptor  on an activated T cell recognizes the complex on the B-cell surface, and the T cell produces interleukins that induce the B 1 cell to form B 2 and  B 3 cells, which then differentiate into antibody-producing (e.g., pentamer IgM) plasma cells (PC). Memory B cells are also produced.   B: Inducible protein B7 (w) on the B cell must interact with CD28 protein on the helper T cell in order for the helper T cell to be fully  activated, and CD40L (CD40 ligand) on the helper T cell must interact with CD40 on the B cell for the B cell to be activated and synthesize  the full range of antibodies. (Reproduced with permission from Stites DP, Terr A, eds. Basic & Clinical Immunology. 7th ed. Originally published by Appleton & Lange.  Copyright 1991 McGraw-Hill.)    macromolecules such as bacterial capsular polysaccharide)  are T-cell-independent. These polysaccharides are long  chains consisting of repeated subunits of several sugars. The  repeated subunits act as a multivalent antigen that cross¬  links the IgM antigen receptors on the B cell and activates it  in the absence of help from CD4 cells. Other macromole¬  cules, such as DNA, RNA, and many lipids, also elicit a  T-cell-independent response.   In the following example illustrating the T-cell-  dependent response, B cells are used as the APC. This  process begins when antigen binds to IgM or IgD on the  surface of the B cell, is internalized within the B cell, and is  fragmented. Some of the fragments return to the surface in  association with class II MHC molecules (Figure 58-7A). 4  These interact with the receptor on the helper T cell, and, if  the costimulatory signal is given by the B7 protein on the B  cell interacting with CD28 protein on the helper T cell, the  helper T cell is then stimulated to produce interleukins  (e.g., IL-2, IL4, and IL-5). IL-4 and IL-5 induce “class  switching” from IgM, which is the first class of immuno¬  globulins produced, to other classes, namely, IgG, IgA, and  IgE (see the end of Chapter 59). These interleukins stimu¬  late the B cell to divide and differentiate into many anti-  body-producing plasma cells.   Note that interleukins alone are not sufficient to activate  B cells. A membrane protein on activated helper T cells,  called CD40 ligand (CD40L), must interact with a protein  called CD40 on the surface of the resting B cells to stimu¬  late the differentiation of B cells into antibody-producing  plasma cells (Figure 58-7B). Furthermore, other proteins    4 Note that one important difference between B cells and T cells is that B  cells recognize antigen itself, whereas T cells recognize antigen only in  association with MHC proteins.    on the surface of these cells serve to strengthen the interac¬  tion between the helper T cell and the antigen-presenting B  cell (e.g., CD28 on the T cell interacts with B7 on the B cell,  and LFA-1 on the T cell interacts with ICAM-1 on the B  cell). (There are also ICAM proteins on the T cell that inter¬  act with LFA proteins on the B cell.)   In the T-cell-dependent response, all classes of antibody  are made (IgG, IgM, IgA, etc.), whereas in the T-cell-inde-  pendent response, primarily IgM is made. This indicates  that lymphokines produced by the helper T cell are needed  for class switching. The T-cell-dependent response gener¬  ates memory B cells, whereas the T-cell-independent  response does not; therefore, a secondary antibody response  (see Chapter 60) does not occur in the latter. The T-cell-  independent response is the main response to bacterial  capsular polysaccharides, because these molecules are not  processed and presented by APCs and hence do not acti¬  vate helper T cells. The reason for this is that polysaccha¬  rides do not bind to class II MHC proteins, whereas peptide  antigens do.   In the cell-mediated response, the initial events are similar  to those described previously for antibody production. The  antigen is processed by macrophages, is fragmented, and is  presented in conjunction with class II MHC molecules on  the surface. These interact with the receptor on the helper  T cell, which is then stimulated to produce lymphokines  such as IL-2 (T-cell growth factor), which stimulates the  specific helper and cytotoxic T cells to grow.   A subset of T cells called regulatory T cells (TReg) can sup¬  press (inhibit) the effector functions of CD4 (helper) and    CD8 (cytotoxic) T cells. (These cells are also called suppres¬  sor T cells.) TReg cells are 5% to 10% of the CD4-positive  cells and are characterized by possessing the CD25 marker.   TReg cells produce FoxP3, a regulator of transcription  of various genes. A hallmark of TReg cells that are  expressing FoxP3 is the synthesis of the inhibitory sur¬  face protein, CTLA-4. (Recall that CTLA-4 inhibits  costimulation by the B7:CD28 complex.) Individuals whose  TReg cells lack the ability to make FoxP3 are predisposed  to autoimmune diseases such as systemic lupus erythema¬  tosus and a rare X-linked disease characterized by polyen-  docrinopathy and enteropathy (IPEX).   When there is an imbalance in numbers or activity  between CD4 and CD8 cells, cellular immune mechanisms  are greatly impaired. For example, in lepromatous leprosy  there is unrestrained multiplication of Mycobacterium  leprae, a lack of delayed hypersensitivity to M. leprae anti¬  gens, a lack of cellular immunity to that organism, and an  excess of CD8 cells in lesions. Removal of some CD8 cells  can restore cellular immunity in such patients and limit  M. leprae multiplication. In acquired immunodeficiency  syndrome (AIDS), the normal ratio of CD4: CD8 cells  (>1.5) is greatly reduced. Many CD4 cells are destroyed by  HIV, and the number of CD8 cells increases. This imbal¬  ance (i.e., a loss of helper activity and an increase in sup¬  pressor activity) results in a susceptibility to opportunistic  infections and certain tumors.   One important part of the host response to infection is  the increased expression of class I and class II MHC pro¬  teins induced by various cytokines, especially interferons  such as gamma interferon. The increased amount of MHC  proteins leads to increased antigen presentation and a more  vigorous immune response. However, certain viruses can  suppress the increase in MHC protein expression, thereby  enhancing their survival. For example, hepatitis B virus,  adenovirus, and cytomegalovirus can prevent an increase  in class I MHC protein expression, thereby reducing the  cytotoxic T-cell response against cells infected by these  viruses.   B cells perform two important functions: (1) they differen¬  tiate into plasma cells and produce antibodies, and (2) they  can present antigen to helper T cells.   During embryogenesis, B-cell precursors are recognized first  in the fetal liver. From there they migrate to the bone mar¬  row, which is their main location during adult life. Unlike T  cells, they do not require the thymus for maturation. Pre-B  cells lack surface immunoglobulins and light chains but do  have p heavy chains in the cytoplasm. The maturation of B  cells has two phases: the antigen-independent phase consists  of stem cells, pre-B cells, and B cells, whereas the antigen-  dependent phase consists of the cells that arise subsequent to  the interaction of antigen with the B cells (e.g., activated B  cells and plasma cells) (Figure 58-8).   For pre-B cells to differentiate into B cells, a signal  transduction protein called Bruton’s tyrosine kinase is  required. A mutation in the gene encoding this protein  causes X-linked agammaglobulinemia in which immuno¬  globulins (e.g., IgM, IgG) are not made and B cells are  absent. Severe infections caused by pyogenic bacteria occur  in these patients.   B cells display surface IgM, which serves as a receptor  for antigens. This surface IgM is a monomer, in contrast to  circulating IgM, which is a pentamer. The monomeric IgM  on the surface has an extra transmembrane domain that  anchors the protein in the cell membrane that is not present  in the circulating pentameric form of IgM. Surface IgD on  some B cells may also be an antigen receptor. Pre-B cells  are found in the bone marrow, whereas B cells circulate in  the bloodstream.   B cells constitute about 30% of the recirculating pool of  small lymphocytes, and their life span is short (i.e., days or  weeks). Approximately 10 9 B cells are produced each day.  Within lymph nodes, they are located in germinal centers;  within the spleen, they are found in the white pulp. They    FIGURE 58-8 Maturation of B cells. B cells arise from stem cells and differentiate into pre-B cells expressing p heavy chains in the cyto¬  plasm and then into B cells expressing monomer IgM on the surface. This occurs independent of antigen. Activation of B cells and differentia¬  tion into plasma cells is dependent on antigen. Cells to the left of the vertical dotted line do not have IgM on their surface, whereas B cells, to  the right of the vertical line, do have IgM. p, mu heavy chains in cytoplasm; Y, IgM. (Reproduced with permission from Stites DP, Terr A, eds. Basic & Clinical  Immunology. 7th ed. Originally published by Appleton & Lange. Copyright 1991 McGraw-Hill.)    are also found in the gut-associated lymphoid tissue  (GALT) such as Peyer’s patches.   How do antibodies arise? Does the antigen “instruct” the B  cell to make an antibody, or does the antigen “select” a B  cell endowed with the preexisting capacity to make the  antibody?   It appears that the latter alternative (i.e., clonal selec¬  tion) accounts for antibody formation. Each individual has  a large pool of B lymphocytes (about 10 7 ). Each immuno-  logically responsive B cell bears a surface receptor (either  IgM or IgD) that can react with one antigen (or closely  related group of antigens). It is estimated that there are at  least 10 million different specificities. An antigen interacts  with the B lymphocyte that shows the best “fit” with its  immunoglobulin surface receptor. After the antigen binds,  the B cell is stimulated to proliferate and form a clone of  cells. These selected B cells soon become plasma cells and  secrete antibody specific for the antigen. Plasma cells syn¬  thesize the immunoglobulins with the same antigenic  specificity (i.e., they have the same heavy chain and the  same light chain) as those carried by the selected B cell.  Antigenic specificity does not change when heavy chain  class switching occurs (see Chapter 59).   Note that clonal selection also occurs with T cells. The  antigen interacts with a specific receptor located on the  surface of either a CD4-positive or a CD8-positive T cell.  This “selects” this cell and activates it to expand into a clone  of cells with the same specificity.   In the following example, the B cell is the APC. Multivalent  antigen binds to surface IgM (or IgD) and cross-links adja¬  cent immunoglobulin molecules. The immunoglobulins  aggregate to form “patches” and eventually migrate to one  pole of the cell to form a cap. Endocytosis of the capped  material follows, the antigen is processed, and epitopes  appear on the surface in conjunction with class II MHC  proteins. This complex is recognized by a helper T cell with  a receptor for the antigen on its surface. 5 The T cell now  produces various interleukins (IL-2, IL-4, and IL-5) that  stimulate the growth and differentiation of the B cell.   The activation of B cells to produce the full range of anti¬  bodies requires two other interactions in addition to recog¬  nition of the epitope by the T-cell antigen receptor and the  production of IL-4 and IL-5 by the helper T cell. These    5 Macrophages bearing antigen bound to class II MHC proteins can also  present antigen to the T cell, resulting in antibody formation. In general,  B cells are poor activators of “virgin” T cells in the primary response  because B cells do not make IL-1. B cells are, however, very good activa¬  tors of memory T cells because little, if any, IL-1 is needed.    costimulatory interactions, which occur between surface  proteins on the T and B cells, are as follows: (1) CD28 on the  T cell must interact with B7 on the B cell, and (2) CD40L on  the T cell must interact with CD40 on the B cell. The CD28-  B7 interaction is required for activation of the T cell to pro¬  duce interleukins, and the CD40L-CD40 interaction is  required for class switching from IgM to other immuno¬  globulin classes, such as IgG and IgA, to occur.   Hyper-IgM syndrome is caused by a mutation in the  gene encoding CD40L. Patients have very high IgM levels  and very little IgG, IgA, and IgE because they cannot “class-  switch.” This syndrome is characterized by severe pyogenic  infections (see Chapter 68).   The end result of the activation process is the production of  many plasma cells that produce large amounts of immuno¬  globulins specific for the epitope. Plasma cells secrete thou¬  sands of antibody molecules per second for a few days and  then die. Some activated B cells form memory cells, which  can remain quiescent for long periods but are capable of  being activated rapidly upon reexposure to antigen. Most  memory B cells have surface IgG that serves as the antigen  receptor, but some have IgM. Memory T cells secrete  interleukins that enhance antibody production by the  memory B cells. The presence of these cells explains the  rapid appearance of antibody in the secondary response  (see Chapter 60).   Macrophages have three main functions: phagocytosis, anti¬  gen presentation, and cytokine production (Table 58-6).   (1) Phagocytosis. Macrophages ingest bacteria, viruses,  and other foreign particles. They have surface Fc receptors  that interact with the Fc portion of IgG, thereby enhancing   Ingestion and killing of microbes in phagolyso¬  somes. Killing caused by reactive oxygen inter¬  mediates such as superoxides, reactive nitrogen  intermediates such as nitric oxide, and lyso¬  somal enzymes such as proteases, nucleases,  and lysozyme.   Presentation of antigen in association with class II  MHC proteins to CD4-positive helper T cells.   Synthesis and release of cytokines, such as IL-1  and TNF, and chemokines such as IL-8.    the uptake of opsonized organisms. Macrophages also have  receptors for C3b, another important opsonin. After inges¬  tion, the phagosome containing the microbe fuses with a  lysosome. The microbe is killed within this phagolysosome  by reactive oxygen and reactive nitrogen compounds and  by lysosomal enzymes.   (2) Antigen presentation. Foreign material is ingested  and degraded, and fragments of antigen are presented on  the macrophage cell surface (in conjunction with class II  MHC proteins) for interaction with the TCR of CD4-  positive helper T cells. Degradation of the foreign protein  stops when the fragment associates with the class II MHC  protein in the cytoplasm. The complex is then transported  to the cell surface by specialized “transporter” proteins.   (3) Cytokine production. Macrophages produce sev¬  eral cytokines, the most important of which are IL-1 and  TNF. Both IL-1 (endogenous pyrogen) and TNF are  important mediators of inflammation. In addition, macro¬  phages produce IL-8, an important chemokine that attracts  neutrophils and T cells to the site of infection.   These three functions are greatly enhanced when a pro¬  cess called macrophage activation occurs. Macrophages  are activated initially by substances such as bacterial lipo-  polysaccharide (LPS, endotoxin), by bacterial peptidogly-  can, and by bacterial DNA. (Human DNA is methylated,  whereas bacterial DNA is unmethylated and therefore is  perceived as foreign.) These substances interact with Toll¬  like receptors on the macrophage surface and signal the cell  to produce certain cytokines. Macrophages are also acti¬  vated by gamma interferon produced by helper T cells.  Gamma interferon increases the synthesis of class II MHC  proteins, which enhances antigen presentation and  increases the microbicidal activity of macrophages by  inducing the synthesis of NADPH oxidase which produces  reactive oxygen species that are highly microbicidal.   Macrophages are derived from bone marrow histiocytes  and exist both free (e.g., monocytes) and fixed in tissues  (e.g., Kupffer cells of the liver). Macrophages migrate to the  site of inflammation, attracted by certain mediators, espe¬  cially C5a, a chemokine released in the complement cascade.   Dendritic cells are a third type of cell that function as “pro¬  fessional” APCs (macrophages and B cells are the other  two) (i.e., they express class II MHC proteins and present  antigen to CD4-positive T cells). They are particularly  important because they are the main inducers of the pri¬  mary antibody response. The name dendritic describes  their many long, narrow processes (that resemble neuronal  dendrites), which make them very efficient at making con¬  tact with foreign material.   Dendritic cells are primarily located under the skin and  the mucosa (e.g., Langerhans’ cells in the skin). Dendritic  cells migrate from their peripheral location under the skin  and mucosa to local lymph nodes, where they present    antigen to helper T cells. Migration of dendritic cells to the  lymph nodes is a response to the chemokine, CCR7, pro¬  duced by T cells in the lymph nodes.   B cells also present antigen to helper T cells. IgM on the  surface of B cells serves as the antigen receptor. The cap¬  tured antigens are typically proteins rather than bacteria  that usually enter the APC by phagocytosis. The proteins  are degraded within the B cells into smaller peptides (epit¬  opes) that are presented in the context of Class II MHC  proteins to helper T cells. The epitope is recognized by the  TCR of the helper T cells. The helper T cells are then acti¬  vated to produce IL-2, IL-4, and IL-5 that promote the  growth and differentiation of the B cells into antibody-  producing plasma cells (see activation of B cells earlier).   The interactive process is initiated by the ingestion of a  microbe by an APC, for example, the ingestion of a bacte¬  rium by a dendritic cell in the skin. The dendritic cell  migrates to the lymph node via lymph vessels, attracted  there by chemokines. In the lymph node, the dendritic cell  presents antigen to the T cell bearing a receptor specific for  that antigen. While this process is occurring, fragments of  the microbe circulate to the lymph node and bind directly  to the B-cell antigen receptor (membrane IgM). The anti¬  gen is internalized, processed, and presented to helper T  cells with the correct receptor. Various chemokines and  chemokine receptors (e.g., CCR7) facilitate the migration  of these cells to a junctional area in the lymph node where  they have a high probability of interacting with each other.  The proximity of the B cell to the helper T cell allows inter¬  leukins produced by the helper T cell to efficiently activate  antibody synthesis by the B cell.   These cells have a similar appearance to the dendritic cells  mentioned earlier but are quite different from them in their  location and function. Follicular dendritic cells (FDCs) are  located in the B-cell-containing germinal centers of the  follicles in the spleen and lymph nodes. They do not pres¬  ent antigen to helper T cells because they do not produce  class II MHC proteins. Rather, they capture antigen-  antibody complexes via Fc receptors located on their sur¬  face. The antigen-antibody complexes are then detected by  activated B cells. The antibody produced by these B cells  undergoes affinity maturation. (Affinity maturation is the  improvement in the affinity of an antibody for the antigen  that occurs upon repeated exposure to the antigen.)    Affinity maturation is described in Chapter 60. In addition,  FDCs produce chemokines that attract B cells to the folli¬  cles in the spleen and lymph nodes.   NK cells play two important roles in our innate host  defenses: (1) they kill virus-infected cells, and (2) they pro¬  duce gamma interferon that activates macrophages to kill  bacteria ingested by the macrophage (Table 58-7).   NK cells specialize in killing virus-infected cells and  tumor cells by secreting cytotoxins (perforins and gran-  zymes) similar to those of cytotoxic T lymphocytes and by  participating in Fas-Fas ligand-mediated apoptosis. They  are called “natural” killer cells because they are active with¬  out prior exposure to the virus, are not enhanced by expo¬  sure, and are not specific for any virus. They can kill  without antibody, but antibody (IgG) enhances their effec¬  tiveness, a process called antibody-dependent cellular cyto¬  toxicity (ADCC) (see the section on effector functions of T  cells [earlier]). IL-12 produced by macrophages and inter¬  ferons alpha and beta produced by virus-infected cells are  potent activators of NK cells. Approximately 5% to 10% of  peripheral lymphocytes are NK cells.   NK cells are lymphocytes with some T-cell markers, but  they do not have to pass through the thymus in order to  mature. They have no immunologic memory and, unlike  cytotoxic T cells, have no TCR; also, killing does not  require recognition of MHC proteins. In fact, NK cells have  receptors that detect the presence of class I MHC proteins  on the cell surface. If a cell displays sufficient class I MHC  proteins, that cell is not killed by the NK cell. Many virus-  infected cells and tumor cells display a significantly reduced  amount of class I MHC proteins, and it is those cells that  are recognized and killed by the NK cells. Humans who  lack NK cells are predisposed to life-threatening infections  with varicella-zoster virus and cytomegalovirus.   • Lack T-cell receptor, CD3 proteins, and surface IgM and IgD   • Killing is nonspecific and is not dependent on foreign antigen  presentation by class I or II MHC proteins   • Produce gamma interferon that activates macrophages to kill  ingested bacteria    NK cells detect the presence of cancer cells by recogniz¬  ing a protein called MICA that is found on the surface of  many cancer cells but not normal cells. Interaction of  MICA with a receptor on NK cells triggers the production  of cytotoxins by the NK cell and death of the tumor cell.   Neutrophils are a very important component of our innate  host defenses, and severe bacterial infections occur if they  are too few in number (neutropenia) or are deficient in  function, as in chronic granulomatous disease. They have  cytoplasmic granules that stain a pale pink (neutral) color  with blood stains such as Wright stain, in contrast to  eosinophils and basophils, whose granules stain red and  blue, respectively. These granules are lysosomes, which  contain a variety of degradative enzymes that are important  in the bactericidal action of these cells. The process of  phagocytosis and the bactericidal action of neutrophils are  described in detail in Chapter 8.   Neutrophils have receptors for IgG on their surface so  IgG is the only immunoglobulin that opsonizes (i.e., makes  bacteria more easily phagocytosed). Note that neutrophils  do not display class II MHC proteins on their surface and  therefore do not present antigen to helper T cells. This is in  contrast to macrophages that are also phagocytes but do  present antigen to helper T cells.   Neutrophils can be thought of as a “two-edged” sword.  The positive edge of the sword is their powerful microbici¬  dal activity, but the negative edge is the tissue damage  caused by the release of degradative enzymes. An excellent  example of the latter is the damage to the glomeruli in acute  poststreptococcal glomerulonephritis. The damage is  caused by enzymes released by neutrophils attracted to the  glomeruli by C5a activated by the antigen-antibody com¬  plexes deposited on the glomerular membrane.   Eosinophils are white blood cells with cytoplasmic granules  that appear red when stained with Wright stain. The red  color is caused by the negatively charged eosin dye binding  to the positively charged major basic protein in the gran¬  ules. The eosinophil count is elevated in two medically  important types of diseases: parasitic diseases, especially  those caused by nematodes (see Chapter 56), and hyper¬  sensitivity diseases, such as asthma and serum sickness  (see Chapter 65). Diseases caused by protozoa are typically  not characterized by eosinophilia.   The function of eosinophils has not been clearly estab¬  lished. It seems likely that their main function is to defend  against the migratory larvae of nematodes, such as Strongy-  loides and Trichinella. They attach to the surface of the lar¬  vae and discharge the contents of their granules, which in  turn damages the cuticle of the larvae. Attachment to the    larvae is mediated by receptors on the eosinophil surface  for the Fc portion of the heavy chain of IgG and IgE.   Another function of eosinophils may be to mitigate the  effects of immediate hypersensitivity reactions because the  granules of eosinophils contain histaminase, an enzyme  that degrades histamine, which is an important mediator of  immediate reactions. However, the granules of the eosino¬  phils also contain leukotrienes and peroxidases, which can  damage tissue and cause inflammation. The granules also  contain major basic protein that damages respiratory epi¬  thelium and contributes to the pathogenesis of asthma.   Eosinophils can phagocytose bacteria but they do so  weakly and are not sufficient to protect against pyogenic bac¬  terial infections in neutropenic patients. Although they can  phagocytose, they do not present antigen to helper T cells.  The growth and differentiation of eosinophils are stimulated  by IL-5. Eotaxin is a chemokine that attracts eosinophils to  the site of nematode larvae and allergic reactions.   Basophils are white blood cells with cytoplasmic granules  that appear blue when stained with Wright stain. The blue  color is caused by the positively charged methylene blue  dye binding to several negatively charged molecules in the  granules. Basophils circulate in the bloodstream, whereas  mast cells, which are similar to basophils in many ways, are  fixed in tissue, especially under the skin and in the mucosa  of the respiratory and GI tracts.   Basophils and mast cells have receptors on the cell sur¬  face for the Fc portion of the heavy chain of IgE. When    adjacent IgE molecules are cross-linked by antigen, immu-  nologically active mediators, such as histamine, and  enzymes, such as peroxidases and hydrolases, are released.  These cause inflammation and, when produced in large  amounts, cause severe immediate hypersensitivity reac¬  tions such as systemic anaphylaxis.   Mast cells also play an important role in the innate  response to bacteria and viruses. The surface of mast cells  contains Toll-like receptors that recognize bacteria and  viruses. The mast cells respond by releasing cytokines and  enzymes from their granules that mediate inflammation  and attract neutrophils and dendritic cells to the site of  infection. Dendritic cells are important APCs that initiate  the adaptive response. The role of mast cells in inflamma¬  tion has been demonstrated in rheumatoid arthritis. These  cells produce both inflammatory cytokines and the  enzymes that degrade the cartilage in the joints.   The important functions of the main cytokines are  described in Table 58-8. Note that the three important  proinflammatory cytokines are IL-1, IL-6, and TNF. The  term proinflammatory means “to stimulate or enhance  inflammation.” The main anti-inflammatory cytokines are  IL-10 and transforming growth factor (3.   (1) IL-l is produced mainly by macrophages. It is a pro-  inflammatory cytokine (i.e., plays an important role, along  with IL-6 and TNF, in inducing inflammation). In addition,   Proinflammatory cytokine. Low concentration: activates neutrophils and increases their  adhesion to endothelial cells. High concentration: mediates septic shock, acts as cachectin,  causes necrosis of tumors.   T-cell growth factor. Stimulates growth of both helper (CD4) and cytotoxic (CD8) T cells.   Stimulates phagocytosis and killing by macrophages. Increases class 1 and II MHC protein  expression. Inhibits growth ofTh-2 cells.   Recruits neutrophils to site of infection. Important in gut mucosal immunity.   Anti-inflammatory cytokine. Inhibits activation ofT cells. Increases isotype switching to IgA.    IL-1 is endogenous pyrogen, which acts on the hypothala¬  mus to cause the fever associated with infections and other  inflammatory reactions. (Exogenous pyrogen is endotoxin,  a lipopolysaccharide found in the cell wall of gram-negative  bacteria [see Chapter 7].)   (2) IL-2 is produced mainly by helper T cells. It stimulates  both helper and cytotoxic T cells to grow. IL-2 is T-cell  growth factor. Resting T cells are stimulated by antigen (or  other stimulators) both to produce IL-2 and to display IL-2  receptors on their surface, thereby acquiring the capacity to  respond to IL-2. Interaction of IL-2 with its receptor stimu¬  lates DNA synthesis, allowing cell division to occur.   (3) IL-4 is produced by the Th-2 class of helper T cells.  IL-4 stimulates the development of Th-2 cells from T cells  that have been activated by exposure to antigen. It also  induces class switching to IgE. IL-4 is the “signature” (most  characteristic) cytokine produced by Th-2 cells (Figure 58-3  and Table 58-5).   (4) IL-5 is produced by the Th-2 class of helper T cells.  It induces class switching to IgA, thereby increasing muco¬  sal immunity. It also increases the number and activity of  eosinophils. Eosinophils are an important host defense  against many helminths (worms), (e.g., Strongyloides) (see  Chapter 56) and are increased in immediate hypersensitiv¬  ity (allergic) reactions (see Chapter 65).   (5) IL-6 is produced mainly by macrophages. It is a pro-  inflammatory cytokine that induces fever by affecting the  hypothalamus and induces the production of acute-phase  proteins by the liver. Acute-phase proteins are described on  page 481.   (6) IL-7 is produced by stromal cells in the thymus and  bone marrow. It is required for stem cells to differentiate into  T cells and B cells. A mutation in the gene for the y chain of  the IL-7 receptor results in severe combined immunodefi¬  ciency because neither T cells nor B cells are formed.   (7) IL-10 and IL-12 regulate the production of Th-1  cells, the cells that mediate delayed hypersensitivity (See  Figure 58-3). IL-12 is produced by macrophages and pro¬  motes the development of Th-1 cells, whereas IL-10 is  produced by Th-2 cells and inhibits the development of  Th-1 cells. The relative amounts of IL-4, IL-10, and IL-12  drive the differentiation of Th-1 and Th-2 cells and there¬  fore enhance either cell-mediated or humoral immunity,  respectively. This is likely to have important medical conse¬  quences because the main host defense against certain  infections is either cell-mediated or humoral immunity. For  example, Leishmania infections in mice are lethal if a  humoral response predominates but are controlled if a vig¬  orous cell-mediated response occurs.   The IL-12-gamma interferon axis is very important in  the ability of our host defenses to control infections by  intracellular pathogens, such as M. tuberculosis and  L. monocytogenes. IL-12 increases the number of Th-1 cells,  and Th-1 cells produce the gamma interferon that activates  the macrophages that phagocytose and kill the intracellular  bacterial pathogens mentioned earlier.    (8) IL-13 is implicated as the mediator of allergic airway  disease (asthma). IL-13 is made by Th-2 cells and binds to  a receptor that shares a chain with the IL-4 receptor. In  animals, IL-13 was shown to be necessary and sufficient to  cause asthma. IL-13 is involved in producing the airway  hyperresponsiveness seen in asthma but not in increasing  the amount of IgE.   (9) The main function of transforming growth factor-13  (TGF-(3) is to inhibit the growth and activation of T cells.  It is an “anti-inflammatory” cytokine. Although it is a  “negative regulator” of the immune response, it stimulates  wound healing by enhancing the synthesis of collagen. It is  produced by many types of cells, including T cells, B cells,  and macrophages. In summary, the role of TGF-f) is to  dampen or suppress the immune response when it is no  longer needed after an infection and to promote the healing  process.   Chemokines are a group of cytokines that can attract either  macrophages or neutrophils to the site of infection. The  term chemokine is a contraction of chemotactic and cyto¬  kine. Chemokines are produced by various cells in the  infected area, such as endothelial cells and resident macro¬  phages. The circulating neutrophils and macrophages  (monocytes) are attracted to the site by an increasing gradi¬  ent of chemokines and then bind to selectins on the endo¬  thelial cell surface. Chemokines also activate integrins on  the surface of the neutrophils and macrophages that bind to  ICAM proteins on the endothelial cell surface. The interac¬  tion between integrin and ICAM facilitates the movement  of the white cells into the tissue to reach the infected area.   Approximately 50 chemokines have been identified;  they are small polypeptides ranging in size from 68 to 120  amino acids. The alpha-chemokines have two adjacent  cysteines separated by another amino acid (Cys-X-Cys),  whereas the beta-chemokines have two adjacent cysteines  (Cys-Cys) (Table 58-9). The alpha-chemokines attract neu¬  trophils and are produced by activated mononuclear cells.  IL-8 is a very important member of this group. The beta-  chemokines attract macrophages and monocytes and are  produced by activated T cells. RANTES and MCAF are  important beta-chemokines.   There are specific receptors for chemokines on the sur¬  face of cells, such as neutrophils and monocytes. Interac¬  tion of the chemokine with its receptor results in changes in  cell surface proteins that allow the cell to adhere to and  migrate through the endothelium to the site of infection.   (1) TNF activates the phagocytic and killing activities of  neutrophils and increases the synthesis of adhesion    ’RANTES is an abbreviation for regulated upon activation, normal T expressed and secreted.  2 MCAF is an abbreviation for macrophage chemoattractant and activating /actor.    molecules by endothelial cells. The adhesion molecules  mediate the attachment of neutrophils at the site of  infection.   (2) Chemotactic factors for neutrophils, basophils, and  eosinophils selectively attract each cell type. Interleukin-8  and complement component C5a are important attractants  for neutrophils. (See the discussion of chemokines on this  page and Table 58-9.)   (3) Leukocyte-inhibitory factor inhibits migration of  neutrophils, analogous to migration-inhibitory factor (see  later discussion). Its function is to retain the cells at the site  of infection.   (4) IL-17 produced by Th-17 T cells recruits neutrophils  to the site of infection. IL-17 plays an important role in  mucosal immunity, especially in the GI tract. Reduced  numbers of Th-17 T cells, as occurs in HIV-infected  patients, predispose to sepsis caused by E. coli and Klebsiella.  Mutations in the genes encoding IL-17 and the receptor for  IL-17 predispose to chronic mucocandidiasis caused by  Candida albicans.   IL-3 is made by activated helper T cells and supports the  growth and differentiation of bone marrow stem cells.  Granulocyte-macrophage colony-stimulating factor (GM-  CSF; sargramostim) is made by T lymphocytes and macro¬  phages. It stimulates the growth of granulocytes and  macrophages and enhances the antimicrobial activity of  macrophages. It is used clinically to improve regeneration  of these cells after bone marrow transplantation. Granulo¬  cyte colony-stimulating factor (G-CSF; filgrastim) is made  by various cells (e.g., macrophages, fibroblasts, and endo¬  thelial cells). It enhances the development of neutrophils  from stem cells and is used clinically to prevent infections  in patients who have received cancer chemotherapy. The  stimulation of neutrophil production by G-CSF and GM-  CSF results in the increased number of these cells in the  peripheral blood after infection.   (1) TNF-a is a proinflammatory cytokine produced pri¬  marily by macrophages. It has many important effects that  differ depending on the concentration. At low concentrations,    it increases the synthesis of adhesion molecules by endothelial  cells, which allows neutrophils to adhere to blood vessel walls  at the site of infection. It also activates the respiratory burst  within neutrophils, thereby enhancing the killing power of  these phagocytes. It also causes fever.   At high concentrations, it is an important mediator  of endotoxin-induced septic shock; antibody to TNF-a  prevents the action of endotoxin. (The action of endo¬  toxin is described in Chapter 7.) TNF mediates septic  shock by inducing fever and causing hypotension  through vasodilation and an increase in capillary  permeability.   TNF-a is also known as cachectin because it inhibits  lipoprotein lipase in adipose tissue, thereby reducing the  utilization of fatty acids. This results in cachexia. TNF-a, as  its name implies, causes the death and necrosis of certain  tumors in experimental animals. It may do this by promot¬  ing intravascular coagulation that causes infarction of the  tumor tissue. Note the similarity of this intravascular  coagulation with the disseminated intravascular coagula¬  tion (DIC) of septic shock, both of which are caused by  TNF-a.   (2) Nitric oxide (NO) is an important mediator made  by macrophages in response to the presence of endotoxin,  a lipopolysaccharide found in the cell wall of gram-negative  bacteria. NO causes vasodilation, which contributes to the  hypotension seen in septic shock. Inhibitors of NO syn¬  thase, the enzyme that catalyzes the synthesis of NO from  arginine, can prevent the hypotension associated with sep¬  tic shock.   (3) Macrophage migration inhibitory factor (MIF) is  another important mediator made by macrophages in  response to endotoxin. The function of MIF is to retain the  macrophages at the site of infection. Recent studies have  shown that MIF plays a major role in the induction of sep¬  tic shock. Antibody against MIF can prevent septic shock  in animals genetically incapable of producing TNF. The  mechanism of action of MIF in septic shock is unclear at  this time.   (1) Interferons are glycoproteins that block virus  replication and exert many immunomodulating func¬  tions. Alpha interferon (from leukocytes) and beta  interferon (from fibroblasts) are induced by viruses (or    double-stranded RNA). These interferons exert a pow¬  erful antiviral activity by inducing the synthesis of a  ribonuclease that degrades viral mRNA, thereby inhib¬  iting viral replication. They also activate NK cells,  causing those cells to kill virus-infected cells more  effectively. (See Chapter 33.)   Gamma interferon is a lymphokine produced primarily  by the Th-1 subset of helper T cells and is the “signature”  cytokine involved in the inflammation mediated by those  cells (Table 58-5). It is one of the most potent activators of  the phagocytic activity of macrophages, NK cells, and neu¬  trophils, thereby enhancing their ability to kill microorgan¬  isms and tumor cells. It does so by inducing the synthesis  of NADPH oxidase which produces reactive oxygen species  that are highly microbicidal. For example, it greatly  increases the killing of intracellular bacteria, such as M.  tuberculosis, by macrophages. It also increases the synthesis  of class I and IIMHC proteins in a variety of cell types. This  enhances antigen presentation by these cells.   (2) Lymphotoxin (also known as TNF-(5) is made by  activated T lymphocytes and causes effects similar to those  of TNF-a. It binds to the same receptor as TNF-a and  hence has the same effects as TNF-a.   1. One of the cells involved in certain autoimmune diseases is described  as a CD3-positive, CD4-positive cell. Regarding this cell, which one  of the following is the most accurate regarding its function?   2. Which one of the following sets of cells can present antigen to  helper T cells?   3. The activation of a CD8-positive T lymphocyte requires presenta¬  tion of antigen in association with which one of the following?   (A) Class I MHC protein and synthesis of interleukin-2 by CD4 T  lymphocytes   (B) Class I MHC protein and synthesis of gamma-interferon by  macrophages   (C) Class II MHC protein and synthesis of interleukin-1 by  macrophages   (D) Class II MHC protein and synthesis of interleukin-4 by CD4 T  lymphocytes   4. Regarding Th-1, Th-2, and Th-17 cells, which one of the following  is the most accurate?   (A) Th-17 cells produce interleukin-17, which stimulates the pro¬  duction of Th-2 cells.   (B) The production of Th-1 cells is enhanced by interleukin-4,  whereas the production of Th-2 cells is enhanced by  interleukin-2.    (C) Th-2 cells synthesize gamma interferon, which is important in  controlling infections caused by Staphylococcus aureus and  other pyogenic bacteria.   (D) Th-1 cells are involved with delayed hypersensitivity reactions,  such as those that control infections caused by M. tuberculosis.   5. Regarding events that occur in the thymus during the maturation  of T cells, which one of the following is the most accurate?   (A) T cells bearing antigen receptors that recognize self antigens  are deleted, a process known as negative selection.   (B) Superantigens are “super” because they play a selective role in  both the positive and the negative selection that occurs in the  thymus.   (C) T cells bearing antigen receptors that recognize antigen in asso¬  ciation with foreign MHC proteins survive, a process known as  positive selection.   (D) Most mature T cells have both CD4 and CD8 proteins in their  surface that ensures their ability to react with antigen presented  by either MHC class I or MHC class II proteins.   6. Regarding interleukins, which one of the following is the most  accurate?   (A) IL-2 is made by B cells and increases class switching from IgM  to IgG.   (B) IL-4 is made by cytotoxic T cells and mediates the killing of  virus-infected cells.   (C) IL-12 is made by eosinophils and enhances the production of  cells that mediate immediate hypersensitivity.   (D) Gamma interferon is made by Th-1 cells and activates macro¬  phages to phagocytose more effectively.   7. Regarding chemokines, which one of the following is the most  accurate?   (A) Chemokines penetrate the membranes of target cells during  attack by cytotoxic T cells.   (B) Chemokines bind to the T-cell receptor outside of the antigen¬  binding site and activate many T cells.   (C) Chemokines attract neutrophils to the site of bacterial infec¬  tion, thereby playing a role in the inflammatory response.   (D) Chemokines induce gene switching in B cells, which increases  the amount of IgE synthesized, thereby predisposing to  allergies.   8. Your patient is a 20-year-old woman who experienced the sud¬  den onset of fever, vomiting, myalgias, and diarrhea. This was fol¬  lowed by hypotension and a sunburn-like rash over most of her  body. You make a presumptive diagnosis of toxic shock syndrome.  Which one of the following is the most accurate description of the  pathogenesis of this disease?   (A) It is caused by the release of large amounts of histamine from  basophils.   (B) It is caused by an insufficient amount of inhibitor of the Cl  component of complement.   (C) It is caused by a superantigen that induces an overproduction  of cytokines from helper T cells.   (D) It is caused by a delayed hypersensitivity response to procain¬  amide, which she was taking for her atrial fibrillation.   (E) It is caused by a mutation in the gene for ZAP-70, one of the  signal transduction proteins in T lymphocytes.   Questions on the topics discussed in this chapter can be found  in the Immunology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 733. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   Antibodies are globulin proteins (immunoglobulins [Ig])  that react specifically with the antigen that stimulated their  production. They make up about 20% of the protein in  blood plasma. Blood contains three types of globulins,  alpha, beta, and gamma, based on their electrophoretic  migration rate. Antibodies are gamma globulins. There are  five classes of antibodies: IgG, IgM, IgA, IgD, and IgE. Anti¬  bodies are subdivided into these five classes based on dif¬  ferences in their heavy chains.   The most important functions of antibodies are to neu¬  tralize toxins and viruses, to opsonize microbes so they  are more easily phagocytosed, to activate complement,  and to prevent the attachment of microbes to mucosal  surfaces. The specific antibody classes that mediate these  functions are described in Table 59-1. In addition to these  functions, antibodies have a catalytic (enzymatic) capa¬  bility that is described in a separate section at the end of  this chapter.   Antibodies that arise in an animal in response to typical  antigens are heterogeneous, because they are formed by  several different clones of plasma cells (i.e., they are poly¬  clonal). Antibodies that arise from a single clone of cells   Immunoglobulin Class Switching (Isotype Switching)  Allelic Exclusion  Catalytic Antibody  Self-Assessment Questions    (e.g., in a plasma cell tumor [myeloma]) 1 are homogeneous  (i.e., they are monoclonal).   Monoclonal antibodies also can be made in the labora¬  tory by fusing a myeloma cell with an antibody-producing  cell (Figure 59-1; also see box “Hybridomas & Monoclonal  Antibodies”). Such hybridomas produce virtually unlim¬  ited quantities of monoclonal antibodies that are useful in  diagnostic tests and in research (see box “Hybridomas &  Monoclonal Antibodies”).   Immunoglobulins are glycoproteins made up of light (L)  and heavy (H) polypeptide chains. The terms light and  heavy refer to molecular weight; light chains have a molec¬  ular weight of about 25,000, whereas heavy chains have a  molecular weight of 50,000 to 70,000. The simplest anti¬  body molecule has a Y shape (Figure 59-2) and consists of  four polypeptide chains: two H chains and two L chains.    1 Multiple myeloma is a malignant disease characterized by an over¬  production of plasma cells (B cells). All the myeloma cells in a patient  produce the same type of immunoglobulin molecule, which indicates  that all the cells arose from a single progenitor. Excess k or X L chains  are synthesized and appear as dimers in the urine. These are known as  Bence Jones proteins and have the unusual attribute of precipitating at  50°C to 60°C but dissolving when the temperature is raised to the  boiling point.    ’The 11S form is found in secretions (e.g., saliva, milk, and tears) and fluids of the respiratory, intestinal, and genital tracts.  2 lgM opsonizes indirectly by activating complement. This produces C3b, which is an opsonin.    The four chains are linked by disulfide bonds. An individ¬  ual antibody molecule always consists of identical H chains  and identical L chains. This is primarily the result of two  phenomena: allelic exclusion (see page 530) and regulation    within the B cell, which ensure the synthesis of either kappa  (k) or lambda  L chains, but not both.   L and H chains are subdivided into variable and constant  regions. The regions are composed of three-dimensionally    Hybridoma cells have the remarkable ability to produce  large quantities of a single molecular species of immuno¬  globulin. These immunoglobulins, which are known as  monoclonal antibodies, are called “monoclonal” because  they are made by a clone of cells that arose from a single cell.  Note, however, that this single cell is, in fact, formed by the  fusion of two different cells (i.e., it is a hybrid), hence the  term hybridoma. Hybridoma cells are made in the following  manner: (1) An animal (e.g., a mouse) is immunized with  the antigen of interest. (2) Spleen cells from this animal are  grown in a culture dish in the presence of mouse myeloma  cells. The myeloma cells have two important attributes:  They grow indefinitely in culture, and they do not produce  immunoglobulins. (3) Fusion of the cells is encouraged by  adding certain chemicals (e.g., polyethylene glycol). (4) The  cells are grown in a special culture medium (HAT medium)  that supports the growth of the fused, hybrid cells but not of  the “parental” cells. (5) The resulting clones of cells are  screened for the production of antibody to the antigen of  interest.    Chimeric monoclonal antibodies consisting of mouse  variable regions and human constant regions are being made  for use in treating human diseases such as leukemia. The  advantages of the human constant chain are that human  complement is activated (whereas it is not if the constant  region is mouse-derived) and that antibodies against the  monoclonal antibody are not formed (whereas antibodies  are formed if the constant region is mouse-derived). The  advantage of the mouse variable region is that it is much  easier to obtain monoclonal antibodies against, for example,  a human tumor antigen by inoculating a mouse with the  tumor cells. Chimeric antibodies can kill tumor cells either  by complement-mediated cytotoxicity or by delivering toxins  (e.g., diphtheria toxin) specifically to the tumor cell.   Monoclonal antibodies are now used in a variety of clini¬  cal situations, such as immunosuppression related to organ  transplants, treatment of autoimmune disease, treatment of  cancer, and the prevention of infectious disease. Table 62-2  describes these monoclonal antibodies, their cellular targets,  and their clinical use.    Fused cells (hybridoma cells)  are selected in special medium  and unfused cells die    Single hybridoma cell is placed  in a well and monitored for production  of monoclonal antibody against antigen  of interest    folded, repeating segments called domains. An L chain con¬  sists of one variable (V L ) and one constant (C L ) domain.  Most H chains consist of one variable (V H ) and three con¬  stant (C H ) domains. (IgG and IgA have three C H domains,  whereas IgM and IgE have four.) Each domain is  approximately 110 amino acids long. The variable regions of    both the light and heavy chain are responsible for antigen¬  binding, whereas the constant region of the heavy chain is  responsible for various biologic functions (e.g., comple¬  ment activation and binding to cell surface receptors). The  complement binding site is in the C H 2 domain. The constant  region of the light chain has no known biologic function.   FIGURE 59-2 Structure of immunoglobulin G (IgG). The Y-shaped IgG molecule consists of two light chains and two heavy chains. Each light  chain consists of a variable region and a constant region. Each heavy chain consists of a variable region and a constant region that is divided into  three domains: C H 1, C H 2, and C H 3.The C H 2 domain contains the complement-binding site, and the C H 3 domain is the site of attachment of IgG to  receptors on neutrophils and macrophages.The antigen-binding site is formed by the variable regions of both the light and heavy chains. The speci¬  ficity of the antigen-binding site is a function of the amino acid sequence of the hypervariable regions (see Figure 59-3). (Reproduced with permission  from Brooks GF et al. Medical Microbiology. 20th ed. Originally published by Appleton & Lange. Copyright 1995 McGraw-Hill.)    FIGURE 59-3 The antigen-binding site is formed by the  hypervariable regions. A: Hypervariable regions on immunoglobulin  G (IgG). B: Magnified view of antigen-binding site. (Reproduced with per-  mission from Stites DP, Terr A, ParslowT, eds. Basic & Clinical Immunology. 8th ed.  Originally published by Appleton & Lange. Copyright 1994 McGraw-Hill.)    The variable regions of both L and H chains have three  extremely variable (hypervariable) amino acid sequences  at the amino-terminal end that form the antigen-binding  site (Figure 59-3). Only 5 to 10 amino acids in each hyper¬  variable region form the antigen-binding site. Antigen-  antibody binding involves electrostatic and van der Waals’  forces and hydrogen and hydrophobic bonds rather than  covalent bonds. The remarkable specificity of antibodies is  due to these hypervariable regions (see the discussion of  idiotypes on page 527).   L chains belong to one of two types, k (kappa) or \  (lambda), on the basis of amino acid differences in their  constant regions. Both types occur in all classes of immu¬  noglobulins (IgG, IgM, etc.), but any one immunoglobulin  molecule contains only one type of L chain. 2    2 In humans, the ratio of immunoglobulins containing k chains to those  containing A chains is approximately 2:1.   Main antibody in the secondary response.  Opsonizes bacteria, making them easier to  phagocytize. Fixes complement, which  enhances bacterial killing. Neutralizes  bacterial toxins and viruses. Crosses the  placenta.   Secretory IgA prevents attachment of bacteria  and viruses to mucous membranes. Does not  fix complement.   Produced in the primary response to an  antigen. Fixes complement. Does not cross  the placenta. Antigen receptor on the surface  of B cells.   Uncertain. Found on the surface of many B cells  as well as in serum.   Mediates immediate hypersensitivity by  causing release of mediators from mast cells  and basophils upon exposure to antigen  (allergen). Defends against worm infections by  causing release of enzymes from eosinophils.  Does not fix complement. Main host defense  against helminth infections.    The amino-terminal portion of each L chain partici¬  pates in the antigen-binding site. H chains are distinct for  each of the five immunoglobulin classes and are desig¬  nated y, a, p, e, and 6 (Table 59-2). The amino-terminal  portion of each H chain participates in the antigen¬  binding site; the carboxy terminal forms the Fc fragment,  which has the biologic activities described earlier and in  Table 59-2.   If an antibody molecule is treated with a proteolytic  enzyme such as papain, peptide bonds in the “hinge” region  are broken, producing two identical Fab fragments, which  carry the antigen-binding sites, and one Fc fragment,  which is involved in placental transfer, complement fixa¬  tion, attachment site for various cells, and other biologic  activities (see Figure 59-2).   Each IgG molecule consists of two L chains and two FI  chains linked by disulfide bonds (molecular formula  FI2L2). Because it has two identical antigen-binding sites, it  is said to be divalent. There are four subclasses, IgGl-  IgG4, based on antigenic differences in the FI chains and on  the number and location of disulfide bonds. IgGl makes up  most (65%) of the total IgG. IgG2 antibody is directed  against polysaccharide antigens and is an important host  defense against encapsulated bacteria.    IgG is the predominant antibody in the secondary  response and constitutes an important defense against bac¬  teria and viruses (See Table 59-1). IgG is the only antibody  to cross the placenta; only its Fc portion binds to receptors  on the surface of placental cells. This receptor, called FcRn,  transports maternal IgG across the placenta into the fetal  blood. IgG is therefore the most abundant immunoglobu¬  lin in newborns. This is an example of passive immunity as  the IgG is made by the mother, not by the fetus (see Chap¬  ter 57). Another important attribute of IgG is that it is one  of the two immunoglobulins that can activate complement;  IgM is the other (see Chapter 63).   IgG is the immunoglobulin that opsonizes. It can opso¬  nize (i.e., enhance phagocytosis) because there are recep¬  tors for the yH chain on the surface of phagocytes. IgM  does not opsonize directly, because there are no receptors  on the phagocyte surface for the pH chain. However, IgM  activates complement, and the resulting C3b can opsonize  because there are receptors for C3b on the surface of  phagocytes. Note that there are four subclasses of IgG. Sub¬  classes IgGl and IgG3 are more effective opsonizers than  are IgG2 and IgG4.   IgG has various sugars attached to the heavy chains,  especially in the CH2 domain. The medical importance  of these sugars is that they determine whether IgG will  have a proinflammatory or anti-inflammatory effect. For  example, if the IgG molecule has a terminal N-acetyl  glucosamine, it is proinflammatory because it will bind  to mannose-binding ligand and activate complement  (see Chapter 63 and Figure 63-1). In contrast, if the IgG  has a sialic acid side chain, then it will not bind and  becomes anti-inflammatory. Thus IgG proteins specific for  a single antigen that are made by a single plasma cell can,  at various times, possess different properties depending on  these sugar modifications.   IgA is the main immunoglobulin in secretions such as  colostrum, saliva, tears, and respiratory, intestinal, and  genital tract secretions. It prevents attachment of microor¬  ganisms (e.g., bacteria and viruses) to mucous membranes.  Each secretory IgA molecule consists of two H2L2 units  plus one molecule each of J (joining) chain 3 and secretory  component (Figure 59-4). The two heavy chains in IgA are  a heavy chains.   The secretory component is a polypeptide synthesized  by epithelial cells that provides for IgA passage to the  mucosal surface. It also protects IgA from being degraded  in the intestinal tract. In serum, some IgA exists as mono¬  meric H2L2.    3 Only IgA and IgM have J chains. Only these immunoglobulins exist as  multimers (dimers and pentamers, respectively). The J chain initiates the  polymerization process, and the multimers are held together by disulfide  bonds between their Fc regions.   IgM is the main immunoglobulin produced early in the  primary response. It is present as a monomer on the sur¬  face of virtually all B cells, where it functions as an anti¬  gen-binding receptor. 4 In serum, it is a pentamer  composed of five H2L2 units plus one molecule of J (join¬  ing) chain (see Figure 59-4). IgM has a p heavy chain.  Because the pentamer has 10 antigen-binding sites, it is  the most efficient immunoglobulin in agglutination,  complement fixation (activation), and other antibody  reactions and is important in defense against bacteria and  viruses. It can be produced by the fetus in certain infec¬  tions. It has the highest avidity of the immunoglobulins;  its interaction with antigen can involve all 10 of its bind¬  ing sites.   This immunoglobulin has no known antibody function but  may function as an antigen receptor; it is present on the  surface of many B lymphocytes. It is present in small  amounts in serum.   IgE is medically important for two reasons: (1) it mediates  immediate (anaphylactic) hypersensitivity (see Chapter 65),  and (2) it participates in host defenses against certain  parasites (e.g., helminths [worms]) (see Chapter 56). The  Fc region of IgE binds to the surface of mast cells and  basophils. Bound IgE serves as a receptor for antigen  (allergen). When the antigen-binding sites of adjacent  IgEs are cross-linked by allergens, several mediators are  released by the cells, and immediate (anaphylactic) hyper¬  sensitivity reactions occur (see Figure 65-1). Although  IgE is present in trace amounts in normal serum (approx¬  imately 0.004%), persons with allergic reactivity have  greatly increased amounts, and IgE may appear in exter¬  nal secretions. IgE does not fix complement and does not  cross the placenta.   IgE is the main host defense against certain important  helminth (worm) infections, such as Strongyloides,  Trichinella, Ascaris, and the hookworms Necator and  Ancylostoma. The serum IgE level is usually increased in  these infections. Because worms are too large to be  ingested by phagocytes, they are killed by eosinophils  that release worm-destroying enzymes. IgE specific for  worm proteins binds to receptors on eosinophils, trigger¬  ing the antibody-dependent cellular cytotoxicity (ADCC)  response.    4 The surface monomer IgM and the serum IgM both have p heavy  chains, but the heavy chain of the surface IgM has a hydrophobic  sequence that mediates binding within the cell membrane, whereas the  serum IgM does not.   FIGURE 59-4 Structure of serum immunoglobulin (Ig) A (A), secretory IgA (B), and IgM (C). Note that both IgA and IgM have a J chain  but that only secretory IgA has a secretory component. (Reproduced with permission from Stites D,Terr A, ParslowT, eds. Basic & Clinical Immunology. 8th ed.  Originally published by Appleton & Lange. Copyright 1994 McGraw-Hill.)   Because immunoglobulins are proteins, they are antigenic,  and that property allows them to be subdivided into iso-  types, allotypes, and idiotypes.   (1) Isotypes are defined by antigenic (amino acid) dif¬  ferences in their constant regions. Although different anti-  genically, all isotypes are found in all normal humans. For  example, IgG and IgM are different isotypes; the constant  region of their H chains (y and p) is different antigenically  (the five immunoglobulin classes—IgG, IgM, IgA, IgD, and  IgE—are different isotypes; their H chains are antigenically  different). The IgG isotype is subdivided into four sub-  types, IgGl, IgG2, IgG3, and IgG4, based on antigenic dif¬  ferences of their heavy chains. Similarly, IgAl and IgA2 are    different isotypes (the antigenicity of the constant region of  their H chains is different), and k and A chains are different  iso types (their constant regions also differ antigenically).   (2) Allotypes, on the other hand, are additional anti¬  genic features of immunoglobulins that vary among  individuals. They vary because the genes that code for  the L and H chains are polymorphic, and individuals can  have different alleles. For example, the AH chain con¬  tains an allotype called Gm, which is due to a one- or  two-amino acid difference that provides a different anti¬  genicity to the molecule. Each individual inherits differ¬  ent allelic genes that code for one or another amino acid  at the Gm site. 5    5 Allotypes related to \H chains are called Gm (an abbreviation of  gamma); allotypes related to kL chains are called Inv (an abbreviation of  a patients name).   V H 1 V h 2 V H n a b c x 1 2 3 4 5 6 ° Cg   FIGURE 59-5 Gene rearrangement to produce a pH chain. The antigen-binding site is formed by randomly choosing one of the V H  genes, one of the D H genes, and one of the J H genes. After transcription and RNA splicing, the mRNA is translated to produce an immuno¬  globulin M (IgM) heavy chain. V, variable regions; D, diversity segments; J, joining segments; C, constant region; IVS, intervening sequence.  (Reproduced with permission from Stites DP, Terr A, ParslowT, eds. Basic & Clinical Immunology. 8th ed. Originally published by Appleton & Lange. Copyright 1994  McGraw-Hill.)    (3) Idiotypes are the antigenic determinants formed by  the specific amino acids in the hypervariable region. 6 Each  idiotype is unique for the immunoglobulin produced by a  specific clone of antibody-producing cells. Anti-idiotype  antibody reacts only with the hypervariable region of the  specific immunoglobulin molecule that induced it.   To produce the very large number of different immuno¬  globulin molecules (estimated to be as many as 100 mil¬  lion) without requiring excessive numbers of genes, special  genetic mechanisms (e.g., DNA rearrangement and RNA  splicing) are used. The DNA rearrangements are per¬  formed by recombinases. Two important genes that encode  recombinases are RAG-1 and RAG-2 (recombination-acti¬  vating genes). Mutations in these genes arrest the develop¬  ment of lymphocytes and result in severe combined  immunodeficiency (see page 580).   Each of the four immunoglobulin chains consists of two  distinct regions: a variable (V) and a constant (C) region.  For each type of immunoglobulin chain (i.e., kappa light    6 Any one of these antigen determinants is called an idiotope.    chain [kL], lambda light chain [XL], and the five heavy  chains [\H, aH, pH, eH, and SH]), there is a separate pool  of gene segments located on different chromosomes. 7 Each  pool contains a set of different V gene segments widely  separated from the D (diversity, seen only in H chains),  J (joining), and C gene segments (Figure 59-5). In the syn¬  thesis of an H chain, for example, a particular V region is  translocated to lie close to a D segment, several J segments,  and a C region. These genes are transcribed into mRNA,  and all but one of the J segments are removed by splicing  the RNA. During B-cell differentiation, the first transloca¬  tion brings a V H gene near a Cp gene, leading to the forma¬  tion of IgM as the first antibody produced in a primary  response. Note that the J (joining) gene does not encode the  J chain found in IgM and IgA. Note also that the DNA of  the unused V, D, and J genes is discarded so a particular B  cell is committed to making antibody with only one  specificity.   The V region of each L chain is encoded by two gene  segments (V + J). The V region of each H chain is encoded  by three gene segments (V + D + J). These various seg¬  ments are united into one functional V gene by DNA    7 The genes for kL, XL, and the five heavy chains are on chromosomes 2,  22, and 14, respectively.    rearrangement. Each of these assembled V genes is then  transcribed with the appropriate C genes and spliced to  produce an mRNA that codes for the complete peptide  chain. L and H chains are synthesized separately on poly¬  somes and then assembled in the cytoplasm by means of  disulfide bonds to form H2L2 units. Finally, an oligosac¬  charide is added to the constant region of the heavy chain,  and the immunoglobulin molecule is released from the cell.   The gene organization mechanism outlined above per¬  mits the assembly of a very large number of different mol¬  ecules. Antibody diversity depends on (1) multiple gene  segments, (2) their rearrangement into different sequences,  (3) the combining of different L and H chains in the assem¬  bly of immunoglobulin molecules, and (4) mutations. A  fifth mechanism called junctional diversity applies primar¬  ily to the antibody heavy chain. Junctional diversity occurs  by the addition of new nucleotides at the splice junctions  between the V-D and D-J gene segments.   The diversity of the T-cell antigen receptor is also  dependent on the joining of V, D, and J gene segments and  the combining of different alpha and beta polypeptide  chains. However, unlike antibodies, mutations do not play  a significant role in the diversity of the T-cell receptor.   Several lymphoid cancers manifest chromosomal trans¬  locations involving the VDJ region and a cellular oncogene.  For example, in Burkitfs lymphoma, the c-myc oncogene  on chromosome 8 is translocated to a position adjacent to  the VDJ region of a heavy-chain gene. The active promoter    of the heavy-chain gene increases transcription of the  c-myc oncogene, which predisposes to malignancy.   Initially, all B cells carry IgM specific for an antigen and  produce IgM antibody in response to exposure to that anti¬  gen. Later, gene rearrangement permits the elaboration of  antibodies of the same antigenic specificity but of different  immunoglobulin classes (Figure 59-6). Note that the anti¬  genic specificity remains the same for the lifetime of the B  cell and plasma cell because the specificity is determined by  the variable region genes (V, D, and J genes on the heavy  chain and V and J genes on the light chain) no matter  which heavy-chain constant region is being utilized.   In class switching, the same assembled V H gene can  sequentially associate with different C H genes so that the  immunoglobulins produced later (IgG, IgA, or IgE) are  specific for the same antigen as the original IgM but have  different biologic characteristics. This is illustrated in the  “class switch” section of Figure 59-6. A different molecular  mechanism is involved in the switching from IgM to IgD.  In this case, a single mRNA consisting of VDJ CpC6 is  initially transcribed and is then spliced into separate VDJ  Cp and VDJ C6 mRNAs. Mature B cells can, in this man¬  ner, express both IgM and IgD (see Figure 59-6, alternative    FIGURE 59-6 Gene rearrangement to produce different immunoglobulin (Ig) classes. IgM is formed first because the p constant region is  closest to the VDJ DNA. Later the p constant region can be switched with a y, e, or a constant region to form the heavy chain of IgG, IgE, or IgA,  respectively. Note that the antigenic specificity of the B cell remains the same because the VDJ DNA remains the same. V, variable regions; D,  diversity segments; J, joining segments; C, constant regions; S, switch sites. (Reproduced with permission from Stites DP, Terr A, ParslowT, eds. Basic & Clinical  Immunology. 8th ed. Originally published by Appleton & Lange. Copyright 1994 McGraw-Hill.)    RNA splicing). Note that once a B cell has “class” switched  past a certain H chain gene, it can no longer make that class  of H chain because the intervening DNA is excised and  discarded. Class switching occurs only with heavy chains;  light chains do not undergo class switching. “Switch recom-  binase” is the enzyme that catalyzes the rearrangement of  the VDJ genes during class switching.   The control of class switching is dependent on at least  two factors. One is the concentration of various interleu¬  kins. For example, interleukin-4 (IL-4) enhances the pro¬  duction of IgE, whereas IL-5 increases IgA (see Table 58-7).  The other is the interaction of the CD40 protein on the B  cell with CD40 ligand protein on the helper T cell. In  hyper-IgM syndrome, the failure to interact properly  results in an inability of the B cell to switch to the produc¬  tion of IgG, IgA, or IgE. Therefore, only IgM is made (see  Chapter 68).   A single B cell expresses only one L chain gene (either k or \)  and one H chain gene. In theory, a B cell could express two  sets of immunoglobulin genes, a maternal set and a pater¬  nal set. But this is not what happens. Only one set of genes  is expressed, either maternal or paternal, and the other set  is silent (i.e., it is excluded). This is called allelic exclusion.  Each individual contains a mixture of B cells, some express¬  ing the paternal genes and others the maternal ones. The  mechanism of this exclusion is unknown.   Antibody can act as an enzyme to catalyze the synthesis of  ozone (0 3 ) that has microbicidal activity. Antibody can  take the singlet oxygen produced by neutrophils and react  it with water to produce hydrogen peroxide and 0 3 . The 0 3  generated can kill Escherichia coli. The catalytic function of  antibodies is independent of their antigen specificity and  of the requirement to bind to any antigen. The importance  of these observations to our host defenses remains to be  determined.   1. It’s time to play “Who am I?” I am the first class of antibody to  appear, so my presence indicates an active infection rather than an  infection that occurred in the past. I can fix complement, which  is an important defense against many bacterial infections. I am  found in plasma as a pentamer.    2. Regarding IgG, which one of the following is the most accurate?   (B) It is the most important antigen receptor on the surface of  neutrophils.   (C) During the primary response, it is made in larger amounts than  is IgM.   (D) The ability of IgG to fix complement resides on the constant  region of the light chain.   (E) It is the only one of the five immunoglobulins that is trans¬  ferred from mother to fetus in utero.   3. If a person had a mutation in the gene encoding J (joining)  chains, which of the following classes of antibodies could NOT be  produced?   4. Regarding the function of the different classes of antibodies, which  one of the following statements is the most accurate?   (A) IgE blocks the binding of viruses to the gut mucosa.   (B) IgA acts as an antigen receptor on the surface of B cells.   (C) IgD is our most important defense against worm parasites,  such as hookworms.   (D) IgG can activate the alternative pathway of complement, result¬  ing in the production of C3a that degrades the bacterial cell  wall.   (E) There are receptors for the heavy chain of IgG on the surface of  neutrophils that mediate a host defense process called  opsonization.   (A) Hypervariable regions are encoded by the genes of both the  light and heavy chains.   (B) The genes for the light and heavy chains are linked on the same  chromosome adjacent to the HLA locus.   (C) During the production of IgG, the light and the heavy chains  acquire the same antigen binding sites by translocation of the  same variable genes.   (D) The gene for the constant region of the gamma heavy chain is  first in the sequence of heavy chain genes, and that is why IgG  is made in greatest amounts.   Questions on the topics discussed in this chapter can be found  in the Immunology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 733. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.    Introduction  The Primary Response  The Secondary Response  Response to Multiple Antigens Administered  Simultaneously   Humoral (antibody-mediated) immunity is directed  primarily against (1) exotoxin-mediated diseases such as  tetanus and diphtheria, (2) infections in which virulence is  related to polysaccharide capsules (e.g., pneumococci,  meningococci, Haemophilus influenzae), and (3) certain  viral infections. In this chapter, the kinetics of antibody  synthesis (i.e., the primary and secondary responses) are  described. The functions of the various immunoglobulins  are summarized in this chapter and are described in detail  in Chapter 59.   The primary response occurs the first time that antigen is  encountered. In the primary response, antibodies are  detectable in the serum after a longer lag period than  occurs in the secondary response (see Table 60-1). This  concept is medically important because the protection    afforded by a vaccine the first time it is given is delayed  compared with the protection afforded by a booster shot in  which a faster secondary response occurs.   The lag period of the primary response is typically 7 to  10 days but can be longer depending on the nature and  dose of the antigen and the route of administration (e.g.,  parenteral or oral). A small clone of B cells and plasma cells  specific for the antigen is formed. The serum antibody  concentration continues to rise for several weeks, then  declines and may drop to very low levels (Figure 60-1). The  first antibodies to appear in the primary response are IgM,  followed by IgG or IgA. IgM levels decline earlier than IgG  levels.   When there is a second encounter with the same antigen or  a closely related (or cross-reacting) one, months or years  after the primary response, there is a rapid antibody  response (the lag period is typically only 3-5 days) to    TABLE 60-1 Important Features of the Primary and Secondary Antibody Response   FIGURE 60~1 Antibody synthesis in the primary and secondary responses. In the primary response, immunoglobulin (Ig) M is the first  type of antibody to appear. In the secondary response, IgG appears earlier and shows a more rapid rise and a higher final concentration than  the primary response. If at the time of the second exposure to the antigen (Ag 1), a second, non-cross-reacting antigen (Ag2) was injected, a  primary response to Ag2 would occur while a secondary response to Ag 1 was occurring.    higher levels than the primary response. This is attributed  to the persistence of antigen-specific “memory cells” after  the first contact. These memory cells proliferate to form a  large clone of specific B cells and plasma cells, which medi¬  ate the secondary antibody response.   During the secondary response, the amount of IgM  produced is similar to that after the first contact with  antigen. However, a much larger amount of IgG antibody  is produced, and the levels tend to persist much longer  than in the primary response.   With each succeeding exposure to the antigen, the anti¬  bodies tend to bind antigen more firmly. Antibody binding  improves because mutations occur in the DNA that encodes  the antigen-binding site, a process called somatic hyper¬  mutation. Some mutations result in the insertion of differ¬  ent amino acids in the hypervariable region that result in a  better fit and cause the antigen to be bound more strongly.  The subset of plasma cells with these improved hypervari¬  able regions are more strongly (and more frequently)  selected by antigen and therefore constitute an increasingly  larger part of the population of antibody-producing cells.  This process is called affinity maturation. One important  effect of booster doses of vaccines is to improve antibody  binding by enhancing the affinity maturation process.   Affinity maturation occurs in the germinal centers of  the follicles in the spleen and lymph nodes. Follicle den¬  dritic cells capture antigen-antibody complexes on their  surface via Fc receptors. The complexes interact with an  activated B cell bearing the immunoglobulin that best fits  the antigen, and it is that B cell that is stimulated to form a  clone of many B cells capable of synthesizing the improved  antibody.   When two or more antigens are administered at the same  time, the host reacts by producing antibodies to all of them.    Competition of antigens for antibody-producing mecha¬  nisms occurs experimentally but appears to be of little sig¬  nificance in medicine. Combined immunization is widely  used (e.g., the diphtheria, tetanus, and pertussis [DTP]  vaccine or the measles, mumps, rubella [MMR] vaccine).   The primary function of antibodies is to protect against  infectious agents or their products (see Table 59-2). Anti¬  bodies provide protection because they can (1) neutralize  toxins and viruses and (2) opsonize microorganisms.  Opsonization is the process by which antibodies make  microorganisms more easily ingested by phagocytic cells.  This occurs by either of two reactions: (1) the Fc portion of  IgG interacts with its receptors on the phagocyte surface to  facilitate ingestion or (2) IgG or IgM activates complement  to yield C3b, which interacts with its receptors on the sur¬  face of the phagocyte.   Antibodies can be induced actively in the host or  acquired passively and are thus immediately available for  defense. In medicine, passive immunity is used in the neu¬  tralization of the toxins of diphtheria, tetanus, and botu¬  lism by antitoxins and in the inhibition of such viruses as  rabies and hepatitis A and B viruses early in the incubation  period.   Antibodies in the fetus are primarily IgG acquired by trans¬  fer of maternal IgG across the placenta. Some antibodies  can be made by the fetus if infection occurs, such as in  congenital syphilis. Newborn infants can make IgG (and  other isotypes, such as IgM and IgA) to certain protein  antigens. For example, the vaccine against hepatitis B that  contains hepatitis B surface antigen is effective when given  to newborns. After birth, maternal IgG declines and pro¬  tection from maternal IgG is lost by 3 to 6 months.   Evaluation of humoral immunity consists primarily of  measuring the amount of each of the three important  immunoglobulins (i.e., IgG, IgM, and IgA) in the patient’s  serum. This is usually done by nephelometry. Immunoelec¬  trophoresis can also provide valuable information. These  techniques are described in Chapter 64.   (A) The IgM made in the primary response is made primarily by  memory B cells.   (B) The lag phase is shorter in the primary response than in the  secondary response.   (C) In the primary response, memory B cells are produced, but  memory T cells are not.   (D) The amount of IgG made in the secondary response is greater  than the amount made in the primary response.    2. Which one of the following is a function of humoral (antibody-  mediated) immunity?   (D) Inhibit the growth of virus-infected cells by enhancing the  production of perforins   Questions on the topics discussed in this chapter can be found  in the Immunology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 733. Also see Part XIV:  USMLE (National Board) Practice Examination starting on  page 751.   In Vivo Tests for Lymphoid Cell Competence (Skin Tests)  In Vitro Tests for Lymphoid Cell Competence   Although humoral (antibody-mediated) immunity is an  important host defense against many bacterial and viral dis¬  eases, in many other bacterial infections (especially intracel¬  lular infections such as tuberculosis) and viral infections, it is  primarily the cell-mediated arm that imparts resistance and  aids in recovery. Furthermore, cell-mediated immunity is  important in defense against fungi, parasites, and cancers. It  is also the main mechanism involved in the rejection of  organ transplants. The strongest evidence for the importance  of cell-mediated immunity comes from clinical situations in  which its suppression (by immunosuppressive drugs or dis¬  ease, e.g., acquired immunodeficiency syndrome [AIDS])  results in overwhelming infections or tumors.   The constituents of the cell-mediated immune system  include several cell types: (1) macrophages, which present  the antigen to T cells; (2) helper T cells, which participate  in antigen recognition and in regulation (helper and sup¬  pressor) functions (see Chapter 58); (3) natural killer (NK)  cells, which can inactivate pathogens; and (4) cytotoxic  T cells, which can kill virus-infected cells with or without  antibody. Macrophages and helper T cells produce cyto¬  kines that activate helper and cytotoxic T cells, leading to  the killing of the pathogen or tumor cell.   An excellent example of cell-mediated immunity is the  delayed hypersensitivity response to the tuberculin skin  test in people who have been infected with Mycobacterium  tuberculosis. An area of skin induration occurs at the site of  the PPD injection subcutaneously. The induration is caused  by the presence of macrophages and helper T cell that  respond to the purified protein derivative (PPD) extracted  from M. tuberculosis.   Infection with some viruses, namely, measles virus and  cytomegalovirus, can suppress cell-mediated immunity    Role of Adjuvants & Lipids in Establishing  Cell-Mediated Reactivity  Self-Assessment Questions    against other microorganisms. In particular, measles virus  infection in people infected with M. tuberculosis can result in  a loss of purified protein derivative (PPD) skin test reactiv¬  ity, reactivation of dormant organisms, and clinical disease.  A proposed explanation for these findings is that when  measles virus binds to its receptor on the surface of human  macrophages, the production of interleukin-12 (IL-12) by  the macrophages, which is necessary for cell-mediated  immunity to occur, is suppressed.   The terms primary and secondary response are associated  primarily with antibody formation as described in Chapter  60, but the timing of the T-cell response also follows the same  pattern. After the initial exposure to the antigen, the specific  T cell proliferates to form a small clone of cells (i.e., a primary  response occurs). Then, on subsequent exposure to the anti¬  gen, the small clone expands, and many more specific T cells  are formed. These cells constitute the secondary response.   Although the interactions between various cells and  various cytokines are complex, the result is relatively sim¬  ple: In the person with competent cellular immunity,  opportunistic pathogens rarely or never cause disease, and  the spread of other agents—for example, certain viruses  (e.g., herpesviruses) or tumors (e.g., Kaposi’s sarcoma)—is  limited. The assessment of the competence of cell-mediated  immunity is therefore important.   Evaluation of the immunocompetence of persons depends  either on the demonstration of delayed-type hypersensitiv¬  ity to commonly present antigens (equating the ability to  respond with the competence of cell-mediated immunity)  or on laboratory assessments of T cells.   Most normal persons respond with delayed-type reactions  to skin test antigens of Candida and other antigens because  of past exposure to these antigens. Absence of reactions to  several of these skin tests suggests impairment of cell-  mediated immunity.   Most normal persons readily develop reactivity to simple  chemicals (e.g., dinitrochlorobenzene [DNCB]) applied to  their skin in lipid solvents. When the same chemical is  applied to the same area 7 to 14 days later, they respond  with a delayed-type skin reaction. Immunocompromised  persons with incompetent cell-mediated immunity fail to  develop such delayed-type hypersensitivity.   When sensitized T lymphocytes are exposed to the specific  antigen, they transform into large blast cells with greatly  increased DNA synthesis, as measured by incorporation of  tritiated thymidine. This specific effect involves relatively few  cells. A larger number of T cells undergo nonspecific blast  transformation when exposed to certain mitogens. The  mitogens phytohemagglutinin and concanavalin A are plant  extracts that stimulate T cells specifically. (Bacterial endo¬  toxin, a lipopolysaccharide, stimulates B cells specifically.)   Macrophage migration inhibitory factor is elaborated by  cultured T cells when exposed to the antigen to which they  are sensitized. Its effect can be measured by observing the  reduced migration of macrophages in the presence of the  factor compared with the level in controls.   The number of each type of cell can be counted by use of a  machine called a fluorescence-activated cell sorter (FACS)  (see Chapter 64). In this approach, cells are labeled with  monoclonal antibody tagged with a fluorescent dye, such as  fluorescein or rhodamine. The various monoclonal anti¬  bodies are specific for proteins found on the surface of  different types of cells. Single cells are passed through a  laser light beam, and the number of cells that fluoresce a  particular color is registered.   B cells (and plasma cells) making different classes of  antibodies can be detected by using monoclonal antibodies  against the various heavy chains. The total number of    B cells can be counted by using fluorescein-labeled anti¬  body against all immunoglobulin classes. Specific mono¬  clonal antibodies directed against T-cell markers permit the  enumeration of T-cells, CD4 helper cells, CD8 suppressor  cells, and others. The normal ratio of CD4 to CD8 cells is  1.5 or greater, whereas in some immunodeficiencies (e.g.,  AIDS), it is less than 1.   Weak antigens or simple chemicals tend not to elicit cell-  mediated hypersensitivity when administered alone, but  they do so when given as a mixture with an adjuvant. The  roles of the adjuvant are to enhance the uptake of the anti¬  gen by antigen-presenting cells (e.g., macrophages), to  stimulate the expression of costimulators, such as B7, and  to enhance the production of cytokines, such as IL-12, that  promote the development of Th-1 cells. A common experi¬  mental adjuvant is a mixture of mineral oil, lanolin, and  killed mycobacteria (Freund’s adjuvant), which stimulates  the formation of local granulomas. It is prohibited for  human use.   1. Cell-mediated immunity is the main host defense against which  one of the following organisms?   2. Infection by which one of the following sets of viruses causes a  reduction in cell-mediated immunity?   Questions on the topics discussed in this chapter can be found  in the Immunology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 733. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.    HLA Typing in the Laboratory  The Fetus Is an Allograft That Is Not Rejected  Results of Organ Transplants  Graft-Versus-Host Reaction   The success of tissue and organ transplants depends on  the donor’s and recipient’s human leukocyte antigens  (HLA) encoded by the HLA genes. These proteins are  alloantigens (i.e., they differ among members of the same  species). If the HLA proteins on the donor’s cells differ  from those on the recipient’s cells, then an immune  response occurs in the recipient. The genes for the HLA  proteins are clustered in the major histocompatibility  complex (MHC), located on the short arm of chromosome 6.  Three of these genes (HLA-A, HLA-B, and HLA-C) code  for the class I MHC proteins. Several HLA-D loci deter¬  mine the class II MHC proteins (i.e., DP, DQ, and DR)  (Figure 62-1). The features of class I and class II MHC  proteins are compared in Table 62-1.    Each person has two haplotypes (i.e., two sets of these  genes: one on the paternal and the other on the maternal  chromosome 6). These genes are very diverse (polymor¬  phic) (i.e., there are many alleles of the class I and class II  genes). For example, there are at least 47 HLA-A genes, 88  HLA-B genes, 29 HLA-C genes, and more than 300 HLA-D  genes, but any individual inherits only a single allele at each  locus from each parent and thus can make no more than  two class I and II proteins at each gene locus. Expression of  these genes is codominant (i.e., the proteins encoded by  both the paternal and maternal genes are produced). Each  person can make as many as 12 different HLA proteins: 3  at class I loci and 3 at class II loci, from both chromosomes.  A person can make fewer than 12 different HLA proteins if  the person is homozygous at any of the 6 loci (i.e., if both  parents have the same HLA allele).    FIGURE 62-1 The human leukocyte antigen (HLA)-gene complex. A, B, and C are class I loci. DP, DQ, and DR are class II loci. C2 and C4  are complement loci. LT, lymphotoxin; TNF, tumor necrosis factor. PGM 3 , GLO, and Pg5 are adjacent, unrelated genes. (Reproduced with permission  from Stites DP, Terr A, ParslowT, eds. Basic & Clinical Immunology. 9th ed. Originally published by Appleton & Lange. Copyright 1997 McGraw-Hill.)    TABLE 62-1 Comparison of Class I and Class II MHC Proteins   Found on surface of "professional" antigen-presenting cells, such as dendritic  cells, macrophages, and B cells   Composed of one peptide encoded in the FILA locus and a P 2 -microglobulin    1 Note that class I MHC proteins are found on the surface of all nucleated cells, including those that have class II MHC proteins on their surface. Mature red blood cells are  nonnucleated; therefore, they do not synthesize class I MHC-proteins.    In addition to the major antigens encoded by the HLA  genes, there is an unknown number of minor antigens  encoded by genes at sites other than the HLA locus. These  minor antigens can induce a weak immune response that can  result in slow rejection of a graft. The cumulative effect of  several minor antigens can lead to a more rapid rejection  response. These minor antigens are various normal body  proteins that have one or more amino acid differences from  one person to another (i.e., they are “allelic variants”).  Because these proteins have an amino acid difference, they  are immunogenic when introduced as part of the donor graft  tissue. There are no laboratory tests for minor antigens.   Between the class I and class II gene loci is a third locus  (See Figure 62-1), sometimes called class III. This locus  contains several immunologically important genes, encod¬  ing two cytokines (tumor necrosis factor and lymphotoxin)  and two complement components (C2 and C4), but it does  not have any genes that encode histocompatibility  antigens.   These are glycoproteins found on the surface of virtually  all nucleated cells. There are approximately 20 different  proteins encoded by the allelic genes at the A locus, 40 at  the B locus, and 8 at the C locus. The complete class I pro¬  tein is composed of a 45,000-molecular-weight heavy chain  noncovalently bound to a p 2 -microglobulin. The heavy  chain is highly polymorphic and is similar to an immuno¬  globulin molecule; it has hypervariable regions in its  N-terminal region. The polymorphism of these molecules  is important in the recognition of self and nonself. Stated  another way, if these molecules were more similar, our abil¬  ity to accept foreign grafts would be correspondingly  improved. The heavy chain also has a constant region  where the CD8 protein of the cytotoxic T cell binds.   These are glycoproteins found on the surface of antigen-  presenting cells, such as macrophages, B cells, dendritic  cells of the spleen, and Langerhans’ cells of the skin. They  are highly polymorphic glycoproteins composed of two  polypeptides that are noncovalently bound. Like class I  proteins, they have hypervariable regions that provide  much of the polymorphism. Unlike class I proteins,  which have only one chain encoded by the MHC locus  ((3 2 -microglobulin is encoded on chromosome 15), both  chains of the class II proteins are encoded by the MHC  locus. The two peptides also have a constant region where  the CD4 proteins of the helper T cells bind.   The ability of T cells to recognize antigen is dependent on  association of the antigen with either class I or class II pro¬  teins. For example, cytotoxic T cells respond to antigen in  association with class I MHC proteins. Thus a cytotoxic T  cell that kills a virus-infected cell will not kill a cell infected  with the same virus if the cell does not also express the  appropriate class I proteins. This finding was determined by  mixing cytotoxic T cells bearing certain class I MHC pro¬  teins with virus-infected cells bearing different class I MHC  proteins and observing that no killing of the virus-infected  cells occurred. Helper T cells recognize class II proteins.  Helper cell activity depends in general on both the recogni¬  tion of the antigen on antigen-presenting cells and the pres¬  ence on these cells of “self” class II MHC proteins. This  requirement to recognize antigen in association with a “self”  MHC protein is called MHC restriction. Note that T cells  recognize antigens only when the antigens are presented on  the surface of cells (in association with either class I or II  MHC proteins), whereas B cells do not have that require¬  ment and can recognize soluble antigens in plasma with their  surface monomer IgM acting as the antigen receptor.    MHC genes and proteins are also important in two  other medical contexts. One is that many autoimmune  diseases occur in people who carry certain MHC genes (see  Chapter 66), and the other is that the success of organ  transplants is, in large part, determined by the compatibil¬  ity of the MHC genes of the donor and recipient (see later).   An autograft (transfer of an individual’s own tissue to  another site in the body) is always permanently accepted  (i.e., it always “takes”). A syngeneic graft 1 is a transfer of  tissue between genetically identical individuals (i.e., identi¬  cal twins) and almost always “takes.” A xenograft, 1 a trans¬  fer of tissue between different species, is always rejected by  an immunocompetent recipient.   An allograft 1 is a graft between genetically different  members of the same species (e.g., from one human to  another). Allografts are usually rejected unless the recipient  is given immunosuppressive drugs. The severity and rapid¬  ity of the rejection will vary depending on the degree of the  differences between the donor and the recipient at the  MHC loci.   Unless immunosuppressive measures are taken, allografts  are rejected by a process called the allograft reaction. In an  acute allograft reaction, vascularization of the graft is nor¬  mal initially, but in 11 to 14 days, marked reduction in  circulation and mononuclear cell infiltration occurs, with  eventual necrosis. This is called a primary (first-set) reac¬  tion. A T-cell-mediated reaction is the main cause of  rejection of many types of grafts (e.g., skin), but antibodies  contribute to the rejection of certain transplants, especially  bone marrow. In experimental animals, rejection of most  types of grafts can be transferred by cells, not serum. Also,  T-cell-deficient animals do not reject grafts, but B-cell-  deficient animals do. The role of cytotoxic T cells in  allograft rejection is described on page 511.   If a second allograft from the same donor is applied to a  sensitized recipient, it is rejected in 5 to 6 days. This accel¬  erated (second-set) reaction is caused primarily by presen¬  sitized cytotoxic T cells.   The acceptance or rejection of a transplant is deter¬  mined, in large part, by the class I and class II MHC pro¬  teins on the donor cells, with class II playing the major  role. The proteins encoded by the DR locus of class II are  especially important. These donor alloantigens activate  recipient T cells, both helper and cytotoxic, which bear  T-cell receptors specific for the alloantigens. The activated    1 Previously used synonyms for these terms include isograft (syngeneic  graft), heterograft (xenograft), and homograft (allograft).    T cells proliferate and then react against the alloantigens on  the donor cells. CD8-positive cytotoxic T cells do most of  the killing of the allograft cells.   Foreign MHC proteins typically activate many more  T cells (i.e., they elicit a much stronger reaction) than do  foreign proteins that are not MHC proteins. The strength of  the response to foreign MHC proteins can be explained by  the observation that there are two processes by which the  recipient’s immune response is stimulated.   (1) The direct process in which the donor’s antigen-  presenting cells (e.g., macrophages and dendritic cells)  (APC) from the graft can present either the donor’s or the  recipient’s proteins in association with their class I and class  II MHC proteins and activate the recipient’s immune  response in the draining lymph node. This process results  in activation of the recipient’s cytotoxic T cells that kill  the cells of the graft. This is the main mechanism of the  acute graft rejection process. Note that the recipient’s pro¬  teins can be recognized as foreign by the recipient’s immune  cells (to which they would ordinarily be tolerant) when  presented by a foreign MHC protein.   (2) The indirect process in which the recipient’s anti¬  gen-presenting cells present the donor’s proteins. The  donor’s self proteins and class I and class II MHC proteins  can be shed and subsequently processed by the recipient’s  APC that enter the graft. The APC then migrates to the  draining lymph node where it activates the recipient’s  helper T cells and B cells. This process typically results in  production of antibodies against the graft and is thought  to be important in the chronic rejection process.   A graft that survives an acute allograft reaction can nev¬  ertheless become nonfunctional as a result of chronic rejec¬  tion. This can occur months to years after engraftment. The  main pathologic finding in grafts undergoing chronic rejec¬  tion is atherosclerosis of the vascular endothelium. The  immunologic stimulus that causes chronic rejection is likely  to be incompatibility of minor histocompatibility antigens  which are allelic variants of normal proteins that differ in  the donor and recipient. The adverse effects of long-term  use of immunosuppressive drugs are likely to play a role in  chronic rejection as well.   In addition to acute and chronic rejection, a third type  called hyperacute rejection can occur. Hyperacute rejec¬  tion typically occurs within minutes of engraftment and is  due to the reaction of preformed anti-ABO antibodies in  the recipient with ABO antigens on the surface of the endo¬  thelium of the graft. Hyperacute rejection is often called the  “white graft” reaction, because the graft turns white as a  result of the loss of blood supply caused by spasm and  occlusion of the vessels serving the graft. In view of this  severe rejection reaction, the ABO blood group of donors  and recipients must be matched and a cross-matching test  (see later) must be done. The laboratory tests used to deter¬  mine ABO blood groups are described in Chapter 64.   Prior to transplantation surgery, laboratory tests, com¬  monly called HLA typing or tissue typing, are performed  to determine the closest MHC match between the donor  and the recipient.   There are two methods commonly used in the labora¬  tory to determine the haplotype (i.e., the class I and class II  alleles on both chromosomes) of both the potential donors  and the recipient. One method is DNA sequencing using  polymerase chain reaction (PCR) amplification and spe¬  cific probes to detect the different alleles. This method is  highly specific and sensitive and is the method of choice  when available. The other method is serologic assays, in  which cells from the donor and recipient are reacted with a  battery of antibodies, each one of which is specific for a  different class I and class II protein. Complement is then  added, and any cell bearing an MHC protein homologous  to the known antibody will lyse. This method is satisfactory  in most instances but has failed to identify certain alleles  that have been detected by DNA sequencing.   If sufficient data cannot be obtained by DNA sequenc¬  ing or serologic assays, then additional information regard¬  ing the compatibility of the class II MHC proteins can be  determined by using the mixed lymphocyte culture  (MLC) technique. This test is also known as the mixed  lymphocyte reaction (MLR). In this test, “stimulator” lym¬  phocytes from a potential donor are first killed by irradia¬  tion and then mixed with live “responder” lymphocytes  from the recipient; the mixture is incubated in cell culture  to permit DNA synthesis, which is measured by incorpora¬  tion of tritiated thymidine. The greater the amount of DNA  synthesis in the responder cells, the more foreign are the  class II MHC proteins of the donor cells. A large amount of  DNA synthesis indicates an unsatisfactory “match” (i.e.,  donor and recipient class II [HLA-D] MHC proteins are  not similar and the graft is likely to be rejected). Therefore,  the best donor is the person whose cells stimulated the  incorporation of the least amount of tritiated thymidine in  the recipient cells.   In addition to the tests used for matching, preformed  cytotoxic antibodies in the recipient’s serum reactive  against the graft are detected by observing the lysis of  donor lymphocytes by the recipient’s serum plus comple¬  ment. This is called cross-matching and is done to prevent  hyperacute rejections from occurring. The donor and  recipient are also matched for the compatibility of their  ABO blood groups. The laboratory tests used to determine  ABO blood groups are described in Chapter 64.   Among siblings in a single family, there is a 25% chance  for both haplotypes to be shared, a 50% chance for one  haplotype to be shared, and a 25% chance for no haplotypes  to be shared. For example, if the father is haplotype AB, the  mother is CD, and the recipient child is AC, there is a 25%  chance for a sibling to be AC (i.e., a two-haplotype  match), a 50% chance for a sibling to be either BC or AD    (i.e., a one-haplotype match), and a 25% chance for a sib¬  ling to be BD (i.e., a zero-haplotype match).   A fetus has MHC genes inherited from the father that are  foreign to the mother, yet allograft rejection of the fetus  does not occur. This is true despite many pregnancies from  the same mother-father combination that produce off¬  spring with the same MHC haplotypes. The reason that the  mother fails to reject the fetus is unclear. The mother forms  antibodies against the foreign paternal MHC proteins;  therefore, the reason is not that the mother is not exposed  to fetal antigens. One possible explanation is that the  trophoblast layer of the placenta does not allow maternal  T cells to enter the fetus.   If the donor and recipient are well-matched by mixed-  lymphocyte culture and histocompatibility antigen typing,  the long-term survival of a transplanted organ or tissue is  enhanced. Also, long-term survival is better if the donor is  living rather than deceased. In 2010, the 5-year survival  rate of kidney transplants from living donors, who are often  family members and share at least one haplotype with the  recipient, is approximately 90%, whereas the 5-year sur¬  vival rate of kidney transplants from deceased donors (i.e.,  a zero-haplotype match) is approximately 80%. The 5-year  survival rate of heart transplants that are from deceased  donors (i.e., a zero-haplotype match) is approximately 75%.  Corneal transplants have a very high rate of success for a  different reason, namely, because corneas are avascular and  the lymphatic supply of the eye prevents many antigens  from triggering an immune response. Because corneal  transplants elicit a weak rejection response, immunosup¬  pression is usually minimal. In contrast, most other trans¬  plants require long-term immunosuppression, although  the dose of immunosuppressive drugs typically decreases  with time, and in some recipients, a state of tolerance  ensues and the drugs can be stopped.   Well-matched transplants of bone marrow may establish  themselves initially in 85% of recipients, but subsequently a  graft-versus-host (GVH) reaction develops in about two-  thirds of the recipients. 2    2 GVH reactions can also occur in immunodeficient patients given a  blood transfusion because there are immunocompetent T cells in the  donors blood that react against the recipient’s cells.    resulting in severe organ dysfunction. The donor’s cytotoxic  T cells play a major role in destroying the recipient’s cells.  Among the main symptoms are maculopapular rash, jaun¬  dice, hepatosplenomegaly, and diarrhea. Many GVH reac¬  tions end in overwhelming infections and death.   There are three requirements for a GVH reaction to  occur: (1) the graft must contain immunocompetent  T cells, (2) the host must be immunocompromised, and  (3) the recipient must express antigens (e.g., MHC pro¬  teins) foreign to the donor (i.e., the donor T cells recognize  the recipient cells as foreign). Note that even when donor  and recipient have identical class I and class II MHC pro¬  teins (i.e., identical haplotypes), a GVH reaction can occur  because it can be elicited by differences in minor antigens.  The GVH reaction can be reduced by treating the donor  tissue with antithymocyte globulin or monoclonal antibod¬  ies before grafting; this eliminates mature T cells from the  graft. Cyclosporine (see later) is also used to reduce the  GVH reaction.   To reduce the rejection of transplanted tissue, immunosup¬  pressive measures (e.g., cyclosporine, tacrolimus [FK506,  Prograf], sirolimus [rapamycin, Rapamune], corticoste¬  roids, azathioprine, monoclonal antibodies, belatacept, and  radiation) are used.   Cyclosporine prevents the activation of T lymphocytes  by inhibiting the synthesis of interleukin (IL)-2 and IL-2  receptor. It does so by inhibiting calcineurin—a protein  (a serine phosphatase) involved in the activation of tran¬  scription of the genes for IL-2 and the IL-2 receptor. Cyclo¬  sporine is well-tolerated and is remarkably successful in    preventing the rejection of transplants. Cyclosporine and  tacrolimus have the same mode of action; tacrolimus is  more immunosuppressive but causes more side effects.  Rapamycin also inhibits signal transduction but at a site  different from that of cyclosporine and tacrolimus.   Corticosteroids act primarily by inhibiting cytokine  (e.g., IL-1 and tumor necrosis factor) production by macro¬  phages and by lysing certain types of T cells. Corticoste¬  roids inhibit cytokine production by blocking transcription  factors, such as NFkB and AP-1, which prevents the mRNA  for these cytokines from being synthesized. Azathioprine is  an inhibitor of DNA synthesis and blocks the growth of  T cells. Mycophenolate mofetil also inhibits DNA synthesis  and has fewer side effects than azathioprine. However,  some recipients of mycophenolate have contracted progres¬  sive multifocal leukoencephalopathy—an often fatal dis¬  ease (see Chapter 44).   Monoclonal antibodies are used in immunosuppressive  regimens, both to prevent rejection and to treat rejection  episodes. Muromonab (OKT3) is a monoclonal antibody  against CD3, and basiliximab (Simulect) is a monoclonal  antibody against the IL-2 receptor. Table 62-2 describes  these monoclonal antibodies as well as others in clinical use.   Antithymocyte globulin (Thymoglobulin), which is a  polyclonal antibody against human thymocytes, is also used  in immunosuppressive regimens. Thymoglobulin contains  antibodies against many lymphocyte antigens (e.g., CD3,  CD4, CD8, CD25, and others). As a consequence, it has a  broader immunosuppressive effect than do the monoclonal  antibodies described in the previous paragraph.   Belatacept (Nulojix) is a fusion protein consisting of  cytotoxic T lymphocyte antigen-4 (CTLA-4) fused to the Fc  fragment of human IgG. CTLA-4 inhibits costimulation of  helper T cells, thereby reducing graft rejection. It is similar  in structure and mode of action to abatacept (Orencia),    ’Note that most of the names end in mab, which is an abbreviation for monoclonal antibodies.    which is used to reduce the inflammatory response in rheu¬  matoid arthritis (see Chapter 66).   Unfortunately, immunosuppression greatly enhances  the recipient’s susceptibility to opportunistic infections and  neoplasms. For example, some patients undergoing treat¬  ment for multiple sclerosis with the monoclonal antibody  natalizumab developed progressive multifocal leukoen-  cephalopathy (see Chapter 44 for a description of this viral  disease). The incidence of cancer is increased as much as  100-fold in transplant recipients who have been immuno-  suppressed for a long time. Common cancers in these  patients include squamous cell carcinoma of the skin, ade¬  nocarcinoma of the colon and the lung, and lymphoma.   Immunosuppressive drugs (e.g., cyclosporine) also  reduce GVH reactions. Note that although these drugs sup¬  press the allograft reaction, tolerance to the graft tissue  does not ensue. Therefore, most patients must take these  drugs for their entire lives.   1. Regarding tissue transplantation, which one of the following is the   (A) An allograft is a graft that transfers tissue or an organ from one  member of a species to a member of another species.   (B) The mother or father of the patient is typically the best donor  of a graft because they are two-haplotype matches.   (C) The ABO blood groups of the donor and recipient do not have  to be matched because they do not play a role in allograft  rejection.   (D) Even when a donor and a recipient are matched at both the  class I and class II MHC loci, rejection can occur and the  recipient should be given immunosuppressive drugs.   (E) If the same donor is the source of tissue for two grafts to a  recipient and the second graft is performed 1 month after the  first graft is rejected, then the second graft will not be rejected.   2. Regarding the MHC proteins and the genes that encode them,   (A) The genes encoding class II MHC proteins are highly polymor¬  phic, whereas the genes encoding class I MHC proteins are not.   (B) The genes encoding class I MHC proteins are located on a dif¬  ferent chromosome from the genes encoding class II MHC  proteins.   (C) The genes are codominant, and each person expresses class I  and class II MHC genes inherited from both mother and father.   (D) Class II MHC proteins are found on the surface of all cells,  whereas class I MHC proteins are found only on the surface of  phagocytes.    3. Regarding the graft-versus-host reaction, which one of the follow¬  ing is the most accurate?   (B) It is caused primarily by mature T cells in the graft.   (E) It occurs primarily when the haplotypes of the donor and  recipient are matched.   4. Listed below are transplants between individuals with various  genotypes and the outcome of these transplants. The genotypes  are designated A and B for simplicity. A person who is AA or  BB is homozygous, whereas a person who is AB is heterozygous.  Regarding outcomes X and Y, which one of the following is the  most accurate?   Questions on the topics discussed in this chapter can be found  in the Immunology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 733. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   Activation of Complement  Regulation of the Complement System  Biologic Effects of Complement   The complement system consists of approximately 20  proteins that are present in normal human (and other  animal) serum. The term complement refers to the ability  of these proteins to complement (i.e., augment) the effects  of other components of the immune system (e.g., anti¬  body). Complement is an important component of our  innate host defenses.   There are three main effects of complement: (1) lysis of  cells such as bacteria, allografts, and tumor cells; (2) gen¬  eration of mediators that participate in inflammation and  attract neutrophils; and (3) opsonization (i.e., enhance¬  ment of phagocytosis). Complement proteins are synthe¬  sized mainly by the liver.   Several complement components are proenzymes, which  must be cleaved to form active enzymes. Activation of the  complement system can be initiated either by antigen-  antibody complexes or by a variety of nonimmunologic  molecules (e.g., endotoxin).   Sequential activation of complement components  (Figure 63-1) occurs via one of three pathways: the classic  pathway, the lectin pathway, and the alternative pathway  (see later). Of these pathways, the lectin and the alterna¬  tive pathways are more important the first time we are  infected by a microorganism because the antibody required  to trigger the classic pathway is not present. The lectin  pathway and the alternative pathway are, therefore, partici¬  pants in the innate arm of the immune system.    All three pathways lead to the production of C3b, the  central molecule of the complement cascade. The presence  of C3b on the surface of a microbe marks it as foreign and  targets it for destruction. C3b has two important functions:  (1) It combines with other complement components to  generate C5 convertase, the enzyme that leads to the pro¬  duction of the membrane attack complex; and (2) it opso¬  nizes bacteria because phagocytes have receptors for C3b  on their surface.   (1) In the classic pathway, antigen-antibody complexes 1  activate Cl 2 to form a protease, which cleaves C2 and C4 to  form a C4b,2b complex. The latter is C3 convertase, which  cleaves C3 molecules into two fragments, C3a and C3b. C3a,  an anaphylatoxin, is discussed later. C3b forms a complex  with C4b,2b, producing a new enzyme, C5 convertase  (C4b,2b,3b), which cleaves C5 to form C5a and C5b. C5a is  an anaphylatoxin and a chemotactic factor (see later). C5b  binds to C6 and C7 to form a complex that interacts with C8  and C9 to produce the membrane attack complex  (C5b,6,7,8,9), which causes cytolysis. Note that the “b” frag¬  ment continues in the main pathway, whereas the “a” frag¬  ment is split off and has other activities.    1 Only IgM and IgG fix complement. One molecule of IgM can activate  complement; however, activation by IgG requires two cross-linked IgG  molecules. Cl is bound to a site located in the Fc region of the heavy  chain. Of the IgGs, only IgGl, IgG2, and IgG3 subclasses fix complement;  IgG4 does not.   2 Cl is composed of three proteins, Clq, Clr, and CIs. Clq is an aggregate  of 18 polypeptides that binds to the Fc portion of IgG and IgM. It is multi¬  valent and can cross-link several immunoglobulin molecules. Cl is a pro¬  enzyme that is cleaved to form an active protease. Calcium is required for  the activation of Cl.    FIGURE 63-1 The classic and alternative pathways of the complement system indicates that proteolytic cleavage of the molecule at the  tip of the arrow has occurred; a line over a complex indicates that it is enzymatically active. Note that all small fragments are labeled "a," and all  large fragments are labeled "b." Hence the C3 convertase is depicted as C4b,2b. Note that proteases associated with the mannan-binding lectin  cleave C4 as well as C2.    (2) In the lectin pathway, mannan-binding lectin (MBL)  (also known as mannose-binding protein) binds to the  surface of microbes bearing mannan (a polymer of the  sugar, mannose). This activates proteases associated with  MBL that cleave C2 and C4 components of complement  and activate the classic pathway. Note that this process  bypasses the antibody-requiring step and so is protective  early in infection before antibody is formed.   (3) In the alternative pathway, many unrelated cell sur¬  face substances (e.g., bacterial lipopolysaccharides [endo¬  toxin], fungal cell walls, and viral envelopes) can initiate  the process by binding C3(H 2 0) and factor B. This com¬  plex is cleaved by a protease, factor D, to produce C3b, Bb.  This acts as a C3 convertase to generate more C3b.   The first regulatory step in the classic pathway is at the level  of the antibody itself. The complement-binding site on the  heavy chain of IgM and IgG is unavailable to the Cl com¬  ponent of complement if antigen is not bound to these  antibodies. This means that complement is not activated by  IgM and IgG despite being present in the blood at all times.  However, when antigen binds to its specific antibody, a  conformational shift occurs and the Cl component can  bind and initiate the cascade.    (1) Cl inhibitor is an important regulator of the classic  pathway. It inactivates the protease activity of Cl. Activa¬  tion of the classic pathway proceeds past this point by  generating sufficient Cl to overwhelm the inhibitor.   (2) Regulation of the alternative pathway is mediated by  the binding of factor H to C3b and cleavage of this complex  by factor I, a protease. This reduces the amount of C5 con-  vertase available. The alternative pathway can proceed past  this regulatory point if sufficient C3b attaches to cell mem¬  branes. Attachment of C3b to cell membranes protects it  from degradation by factors H and I. Another component  that enhances activation of the alternative pathway is pro¬  perdin, which protects C3b and stabilizes the C3  convertase.   (3) Protection of human cells from lysis by the mem¬  brane attack complex of complement is mediated by decay-  accelerating factor (DAF, CD55)—a glycoprotein located  on the surface of human cells. DAF acts by binding to C3b  and C4b and limiting the formation of C3 convertase and  C5 convertase. This prevents the formation of the mem¬  brane attack complex.   Microbes, such as bacteria and viruses, are phagocytized  much better in the presence of C3b because there are C3b  receptors on the surface of many phagocytes.   C5a and the C5,6,7 complex attract neutrophils. They  migrate especially well toward C5a. C5a also enhances the  adhesiveness of neutrophils to the endothelium.   C3a, C4a, and C5a cause degranulation of mast cells with  release of mediators (e.g., histamine), leading to increased  vascular permeability and smooth muscle contraction,  especially contraction of the bronchioles leading to bron-  chospasm. Anaphylatoxins can also bind directly to smooth  muscle cells of the bronchioles and cause bronchospasm.  C5a is, by far, the most potent of these anaphylatoxins.  Anaphylaxis caused by these complement components is  less common than anaphylaxis caused by type I (IgE-  mediated) hypersensitivity (see Chapter 65).   Insertion of the C5b,6,7,8,9 membrane attack complex  (MAC) into the cell membrane forms a “pore” in the  membrane. This opening in the membrane results in the  killing (lysis) of many types of cells, including erythrocytes,    bacteria, and tumor cells. Gram-negative bacteria, espe¬  cially Neisseria species are very susceptible. Cytolysis is  not an enzymatic process; rather, it appears that insertion  of the complex results in disruption of the membrane and  the entry of water and electrolytes into the cell.   The binding of C3b to its receptors on the surface of acti¬  vated B cells greatly enhances antibody production com¬  pared with that by B cells that are activated by antigen  alone. The clinical importance of this is that patients who  are deficient in C3b produce significantly less antibody  than do those with normal amounts of C3b. The low con¬  centration of both antibody and C3b significantly impairs  host defenses, resulting in multiple, severe pyogenic  infections.    (1) Inherited (or acquired) deficiency of some com¬  plement components, especially C5-C8, greatly enhances  susceptibility to Neisseria bacteremia and other infec¬  tions. A deficiency of MBL also predisposes to severe  Neisseria infections. A deficiency of C3 leads to severe,  recurrent pyogenic sinus and respiratory tract  infections.   (2) Inherited deficiency of Cl esterase inhibitor  results in angioedema. When the amount of inhibitor is  reduced, an overproduction of esterase occurs. This leads  to an increase in anaphylatoxins, which cause capillary  permeability and edema.   (3) Acquired deficiency of decay-accelerating factor on  the surface of cells results in an increase in complement-  mediated hemolysis. Clinically, this appears as the disorder  paroxysmal nocturnal hemoglobinuria (see Chapter 68).   (4) In transfusion mismatches (e.g., when type A blood is  given by mistake to a person who has type B blood), anti¬  body to the A antigen in the recipient binds to A antigen on  the donor red cells, complement is activated, and large  amounts of anaphylatoxins and membrane attack complexes  are generated. The anaphylatoxins cause shock, and the  membrane attack complexes cause red cell hemolysis.   (5) Immune complexes bind complement, and thus  complement levels are low in immune complex diseases  (e.g., acute glomerulonephritis and systemic lupus erythe¬  matosus). Binding (activating) complement attracts poly¬  morphonuclear leukocytes, which release enzymes that  damage tissue.   (6) Patients with severe liver disease (e.g., alcoholic cir¬  rhosis or chronic hepatitis B), who have lost significant  liver function and therefore cannot synthesize sufficient  complement proteins, are predisposed to infections caused  by pyogenic bacteria.   1. Regarding the complement pathway, which one of the following is  the most accurate?   (B) C3a is a decay-accelerating factor and causes the rapid decay  and death of bacteria.   (C) In general, gram-positive bacteria are more likely to be killed  by complement than gram-negative bacteria.   (D) The membrane attack complex is formed as a result of activa¬  tion of the classic pathway but not by activation of the alterna¬  tive pathway.   (E) The first time a person is exposed to a microorganism, the  alternative pathway of complement is more likely to be acti¬  vated than the classic pathway.   2. Of the following complement components, which one is the most  important opsonin?   3. Of the following complement components, which one is the most  potent in attracting neutrophils to the site of infection (i.e., acting  as a chemokine)?   4. Of the following, which one is the most important function of the  complex formed by complement components C5b,6,7,8,9?    5. A deficiency of which one of the following complement compo¬  nents predisposes to bacteremia caused by members of the genus  Neisseria ?   6. Your patient is a 20-year-old woman who complains of swellings  on her arms and legs and a feeling of fullness in her throat that  makes it difficult to breath. The swellings are not red, hot, or ten¬  der. You suspect she may have angioedema caused by a comple¬  ment abnormality. Of the following, which one is the most likely  explanation?   Questions on the topics discussed in this chapter can be  found in the Immunology section of Part XIII: USMLE  (National Board) Practice Questions starting on page 733.  Also see Part XIV: USMLE (National Board) Practice Exami¬  nation starting on page 751.   Enzyme-Linked Immunosorbent Assay (ELISA)  Immunofluorescence (Fluorescent Antibody)  Complement Fixation  Neutralization Tests  Immune Complexes   Reactions of antigens and antibodies are highly specific. An  antibody will react only with the antigen that induced it or  with a closely related antigen. Because of the great specificity,  reactions between antigens and antibodies are suitable for  identifying one by using the other. This is the basis of serologic  reactions. However, cross-reactions between related antigens  can occur, and these can limit the usefulness of the test.   The ABO Blood Groups & Transfusion Reactions  Rh Blood Type & Flemolytic Disease of the Newborn    The results of many immunologic tests are expressed as  a titer, which is defined as the highest dilution of the speci¬  men (e.g., serum) that gives a positive reaction in the test.  Note that a patient’s serum with an antibody titer of, for  example, 1/64 contains more antibodies (i.e., is a higher  titer) than a serum with a titer of, for example, 1/4.   Table 64-1 describes the medical importance of sero¬  logic (antibody-based) tests. Their major uses are in the  diagnosis of infectious diseases, in the diagnosis of   • When the organism cannot be cultured (e.g., syphilis and hepatitis A, B, and C).   • When the organism is too dangerous to culture (e.g., rickettsial diseases).   • When culture techniques are not readily available (e.g., HIV, EBV).   • When the organism takes too long to grow (e.g., Mycoplasma).   One problem with this approach is that it takes time for antibodies to form (e.g., 7-10 days in the primary response). For this reason, acute and con¬  valescent serum samples are taken, and a fourfold or greater rise in antibody titer is required to make a diagnosis. By this time, the patient has often  recovered and the diagnosis becomes a retrospective one. If a test is available that can detect IgM antibody in the patient's serum, it can be used to  make a diagnosis of current infection. In certain infectious diseases, an arbitrary IgG antibody titer of sufficient magnitude is used to make a diagnosis.   • Antibodies against various normal body components are used (e.g., antibody against DNA in systemic lupus erythematosus, antibody against  human IgG [rheumatoid factor] in rheumatoid arthritis).   • Known antibodies are used to determine ABO and Rh blood types.   • Known antibodies are used to determine class I and class II FILA proteins prior to transplantation, although DNA sequencing is also being used.   EBV = Epstein-Barr virus; HIV - human immunodeficiency virus; HLA - human leukocyte antigen.    autoimmune diseases, and in the typing of blood and tis¬  sues prior to transplantation.   Microorganisms and other cells possess a variety of  antigens and thus induce antisera containing many differ¬  ent antibodies (i.e., the antisera are polyclonal). Monoclo¬  nal antibodies excel in the identification of antigens because  cross-reacting antibodies are absent (i.e., monoclonal anti¬  bodies are highly specific).   Many types of diagnostic tests are performed in the immu¬  nology laboratory. Most of these tests can be designed to  determine the presence of either antigen or antibody. To do  this, one of the components, either antigen or antibody, is  known and the other is unknown. For example, with a known  antigen such as influenza virus, a test can determine whether  antibody to the virus is present in the patient’s serum. Alter¬  natively, with a known antibody, such as antibody to herpes  simplex virus, a test can determine whether viral antigens are  present in cells taken from the patient’s lesions.   In this test, the antigen is particulate (e.g., bacteria and red  blood cells) 1 or is an inert particle (latex beads) coated with  an antigen. Antibody, because it is divalent or multivalent,  cross-links the antigenically multivalent particles and forms  a latticework, and clumping (agglutination) can be seen. This  reaction can be done in a small cup or tube or with a drop on  a slide. One very commonly used agglutination test is the test  that determines a person’s ABO blood group (Figure 64-1;  see the section on blood groups at the end of this chapter).    FIGURE 64~1 Agglutination test to determine ABO blood  type. On the slide at the bottom of the figure, a drop of the patient's  blood was mixed with antiserum against either type A (left) or type B  (right) blood cells. Agglutination (clumping) has occurred in the  drop on the left containing the type A antiserum but not in the drop  containing the type B antiserum, indicating that the patient is type A  (i.e., has A antigen on the red cells). The slide at the top shows that  the red cells (circles) are cross-linked by the antibodies ("Y" shapes) in  the drop on the left but not in the drop on the right. If agglutination  had occurred in the right side as well, it would indicate that the  patient was producing B antigen as well as A and was type AB.    1 When red cells are used, the reaction is called hemagglutination.   FIGURE 64~2 Precipitin curve. In the presence of a constant  amount of antibody, the amount of immune precipitate formed is plot¬  ted as a function of increasing amounts of antigen. In the top part of the  figure, the binding of antigen (a) and antibody (Y) in the three zones is  depicted. In the zones of antibody excess and antigen excess, a lattice is  not formed and precipitation does not occur, whereas in the equiva¬  lence zone, a lattice forms and precipitation is maximal. (Reproduced with  permission from Stites DP, Terr A, ParslowT, eds. Basic & Clinical Immunology. 9th ed.  Originally published by Appleton & Lange. Copyright 1997 McGraw-Hill.)   In this test, the antigen is in solution. The antibody cross¬  links antigen molecules in variable proportions, and aggre¬  gates (precipitates) form. In the zone of equivalence,  optimal proportions of antigen and antibody combine; the  maximal amount of precipitates forms, and the supernatant  contains neither an excess of antibody nor an excess of  antigen (Figure 64-2). In the zone of antibody excess,  there is too much antibody for efficient lattice formation,  and precipitation is less than maximal. 2 In the zone of anti¬  gen excess, all antibody has combined, but precipitation is  reduced because many antigen-antibody complexes are too  small to precipitate (i.e., they are “soluble”).   Precipitin tests can be done in solution or in semisolid  medium (agar).   The concept of precipitation in solution is used clinically to  measure the amount of immunoglobulins (IgM, IgG, etc.)    2 The term “prozone” refers to the failure of a precipitate or flocculate to  form because too much antibody is present. For example, a false-negative  serologic test for syphilis (VDRL) is occasionally reported because the  antibody titer is too high. Dilution of the serum yields a positive result.    FIGURE 64-3 Double diffusion in agar. Antigen is placed in the well on the left, and antibody is placed in the well on the right. The anti¬  gen and antibody diffuse through the agar and form a precipitate in the zone of equivalence. Close to the antigen-containing well is the zone  of antigen excess, and close to the antibody-containing well is the zone of antibody excess. No precipitate forms in the zones of antigen and  antibody excess.    in the blood plasma. The lab test used is called nephelometry,  in which the amount of precipitate formed is measured  by optical density of the precipitate. In the test, antibody  specific for IgM, IgG, IgA, or IgE is reacted with the  patient’s serum and the optical density measured. This  value is then compared with a standard curve, which dis¬  plays the optical density caused by known amounts of the  immunoglobulins.   This is done as either single or double diffusion. It can also  be done in the presence of an electric field.    antigen diffuses with time, precipitation rings form  depending on the antigen concentration. The greater the  amount of antigen in the well, the farther the ring will be  from the well. By calibrating the method, such radial  immunodiffusion is used to measure IgG, IgM, comple¬  ment components, and other substances in serum. (IgE  cannot be measured because its concentration is too low.)   Double Diffusion —In double diffusion, antigen and  antibody are placed in different wells in agar and allowed  to diffuse and form concentration gradients. Where opti¬  mal proportions (see zone of equivalence, earlier) occur,  lines of precipitate form (Figure 64-3). This method  (Ouchterlony) indicates whether antigens are identical,  related but not identical, or not related (Figure 64-4).   As = Antiserum in wells  1,2, 3, 4 = Antigens in wells    FIGURE 64-4 Double-diffusion (Ouchterlony) precipitin reactions. In these Ouchterlony reactions, wells are cut into an agar plate and  various antigens and antisera are placed in the wells. The antigens and antibodies diffuse toward each other within the agar, and a line of pre¬  cipitate forms in the zone of equivalence. Close to the antigen-containing well, a zone of antigen excess exists and no precipitate forms; close to  the antibody-containing well, a zone of antibody excess exists and no precipitate forms. A and B are unrelated antigens (i.e., they have no epit¬  opes in common). B and C are related antigens (i.e., they have some epitopes in common but some that are different). For example, chicken  lysozyme (well B) and duck lysozyme (well C) share some epitopes because they are both lysozymes but have unique epitopes as well because  they are from different species. The line of identity between B and C is caused by the reaction of the anti-B antibody with the shared epitopes  on antigens B and C.The spur pointing toward well 4 is caused by the reaction of some of the anti-B antibody with the unique epitopes on anti¬  gen B in well 3. These lines of partial identity occur because antibody to B (chicken lysozyme) is polyclonal and has some immunoglobulins that  react with the epitopes common to chicken and duck lysozyme and other immunoglobulins that react only with the epitopes unique to  chicken lysozyme. (Reproduced with permission from Brooks GF et al. Medical Microbiology. 19th ed. Originally published by Appleton & Lange. Copyright 1991  McGraw-Hill.)   FIGURE 64-5 Immunoelectrophoresis. A: Human serum  placed in the central well is electrophoresed, and the proteins  migrate to different regions (orange ellipses). Antiserum to human  serum is then placed in the elongated trough (gray areas). B: Human  serum proteins and antibodies diffuse into agar. C: Precipitate arcs  (orange lines) form in the agar. (Reproduced with permission from Stites  DP, Terr A, ParslowT, eds. Basic & Clinical Immunology. 9th ed. Originally published by  Appleton & Lange. Copyright 1997 McGraw-Hill.)   Precipitation in Agar with an Electric Field  Immunoelectrophoresis —A serum sample is placed in a  well in agar on a glass slide (Figure 64-5). A current is  passed through the agar, and the proteins move in the elec¬  tric field according to their charge and size. Then a trough  is cut into the agar and filled with antibody. As the antigen  and antibody diffuse toward each other, they form a series  of arcs of precipitate. This permits the serum proteins to be  characterized in terms of their presence, absence, or  unusual pattern (e.g., human myeloma protein).    Counter-lmmunoelectrophoresis —This method relies  on movement of antigen toward the cathode and of anti¬  body toward the anode during the passage of electric cur¬  rent through agar. The meeting of the antigen and antibody  is greatly accelerated by this method and is made visible in  30 to 60 minutes. This has been applied to the detection of  bacterial and fungal polysaccharide antigens in cerebrospi¬  nal fluid.   This method is used for the quantitation of antigens or  haptens that can be radioactively labeled. It is based on the  competition for specific antibody between the labeled  (known) and the unlabeled (unknown) concentration of  material. The complexes that form between the antigen and  antibody can then be separated and the amount of radioac¬  tivity measured. The more unlabeled antigen is present, the  less radioactivity there is in the complex. The concentra¬  tion of the unknown (unlabeled) antigen or hapten is deter¬  mined by comparison with the effect of standards. RIA is a  highly sensitive method and is commonly used to assay  hormones or drugs in serum. The radioallergosorbent test  (RAST) is a specialized RIA that is used to measure the  amount of serum IgE antibody that reacts with a known  allergen (antigen).   This method can be used for the quantitation of either  antigens or antibodies in patient specimens. It is based on  covalently linking an enzyme to a known antigen or anti¬  body, reacting the enzyme-linked material with the patient’s  specimen, and then assaying for enzyme activity by adding  the substrate of the enzyme. The method is nearly as sensi¬  tive as RIA yet requires no special equipment or radioactive  labels (Figure 64-6).    FIGURE 64~6 Enzyme -linked immunosorbent assay (ELISA). The term enzyme-linked refers to the covalent binding (linking) of an  enzyme to antibody to human IgG. If the patient has antibodies to the microbial or viral antigen, those antibodies will bind to the microbial or  viral antigens. The antibody to human IgG linked to the enzyme will then bind to the patient's antibodies. Then when the substrate of the  enzyme is added, the substrate changes color, indicating that the patient's serum contained antibodies.    For measurement of antibody, known antigens are fixed to  a surface (e.g., the bottom of small wells on a plastic plate),  incubated with dilutions of the patient’s serum, washed, and  then reincubated with antibody to human IgG labeled with  an enzyme (e.g., horseradish peroxidase). Enzyme activity is  measured by adding the substrate for the enzyme and esti¬  mating the color reaction in a spectrophotometer. The  amount of antibody bound is proportional to the enzyme  activity. The titer of antibody in the patient’s serum is the  highest dilution of serum that gives a positive color reaction.   Fluorescent dyes (e.g., fluorescein and rhodamine) can be  covalently attached to antibody molecules and made visible  by ultraviolet (UV) light in the fluorescence microscope.  Such “labeled” antibody can be used to identify antigens  (e.g., on the surface of bacteria such as streptococci and  treponemes, in cells in histologic section, or in other speci¬  mens) (Figure 64-7). The immunofluorescence reaction is  direct when known labeled antibody interacts directly with  unknown antigen and indirect when a two-stage process is  used. For example, known antigen is attached to a slide, the  patient’s serum (unlabeled) is added, and the preparation is  washed; if the patient’s serum contains antibody against the  antigen, it will remain fixed to it on the slide and can be  detected on addition of a fluorescent dye-labeled antibody  to human IgG and examination by UV microscopy. The  indirect test is often more sensitive than direct immuno¬  fluorescence, because more labeled antibody adheres per  antigenic site. Furthermore, the labeled antiglobulin  becomes a “universal reagent” (i.e., it is independent of the  nature of the antigen used because the antibody to IgG is  reactive with all human IgG).   The complement system consists of 20 or more plasma  proteins that interact with one another and with cell   FIGURE 64-7 Fluorescent antibody test. A: In the direct fluo¬  rescent antibody test, the fluorescent dye is attached directly to the  antibody that is interacting with the antigen (dark triangles) on the  surface of the cell. B: In the indirect fluorescent antibody test, the flu¬  orescent dye is attached to antibody made against human IgG.    membranes. Each protein component must be activated  sequentially under appropriate conditions for the reaction  to progress. Antigen-antibody complexes are among the  activators, and the complement fixation test can be used to  identify one of them if the other is known.   The reaction consists of the following two steps  (Figure 64-8): (1) Antigen and antibody (one known and  the other unknown; e.g., use a known antigen and deter¬  mine whether a patient’s serum contains antibodies to that  antigen) are mixed, and a measured amount of comple¬  ment (usually from guinea pig) is added. If the antigen and  antibody match, they will combine and use up (“fix”) the  complement. (2) An indicator system, consisting of “sensi¬  tized” red blood cells (i.e., red blood cells plus anti-red  blood cell antibody), is added. If the antibody matched the  antigen in the first step, complement was fixed and less (or  none) is available to attach to the sensitized red blood cells.  The red blood cells remain unhemolyzed (i.e., the test is  positive) because the patient’s serum had antibodies to  that antigen. If the antibody did not match the antigen in  the first step, complement is free to attach to the sensitized  red blood cells and they are lysed (i.e., the test is  negative).   Complement must be carefully standardized, and the  patient’s serum must be heated to 56°C for 30 minutes to  inactivate any human complement activity. The antigen  must be quantitated. The result is expressed as the highest  dilution of serum that gives positive results. Controls to  determine whether antigen or antibody alone fixes comple¬  ment are needed to make the test results valid. If antigen or  antibody alone fixes complement, it is said to be  anticomplementary.   These use the ability of antibodies to block the effect of toxins  or the infectivity of viruses. They can be used in cell culture  (e.g., inhibition of cytopathic effect and plaque-reduction  assays) or in host animals (e.g., mouse protection tests).   Immune complexes in tissue can be stained with fluores¬  cent complement. Immune complexes in serum can be  detected by binding to Clq or by attachment to certain  (e.g., Raji lymphoblastoid) cells in culture.   Many viruses clump red blood cells from one species or  another (active hemagglutination). This can be inhibited by  antibody specifically directed against the virus (hemagglu¬  tination inhibition) and can be used to measure the titer of  such antibody. Red blood cells also can absorb many anti¬  gens and, when mixed with matching antibodies, will  clump (this is known as passive hemagglutination, because  the red cells are passive carriers of the antigen).    FIGURE 64~8 Complement fixation. Left: Positive reaction (i.e., the patient's serum contains antibody). If a known antigen is mixed with  the patient's serum containing antibody against that antigen, then complement (solid circles) will be fixed. Because no complement is left over,  the sensitized red cells are not lysed. Right: Negative reaction. If a known antigen is mixed with the patient's serum that does not contain anti¬  body against that antigen, complement (solid circles) is not fixed. Complement is left over and the sensitized red cells are lysed. Ab, antibody;  Ag, antigen.   Some patients with certain diseases (e.g., hemolytic disease  of the newborn [Rh incompatibility] and drug-related hemo¬  lytic anemias) become sensitized but do not exhibit symp¬  toms of disease. In these patients, antibodies against the red  cells are formed and bind to the red cell surface but do not  cause hemolysis. These cell-bound antibodies can be detected  by the direct antiglobulin (Coombs) test, in which antiserum  against human immunoglobulin is used to agglutinate the  patient’s red cells. In some cases, antibody against the red  cells is not bound to the red cells but is in the serum and the  indirect antiglobulin test for antibodies in the patient’s serum  should be performed. In the indirect Coombs test, the  patient’s serum is mixed with normal red cells, and antise¬  rum to human immunoglobulins is added. If antibodies are  present in the patient’s serum, agglutination occurs.   This test is typically used to determine whether a positive  result in a screening immunologic test is a true-positive or a  false-positive result. For example, patients who are positive  in the screening ELISA for human immunodeficiency virus  (HIV) infection or for Lyme disease should have a Western  blot test performed. Figure 64-9 illustrates a Western blot  test for the presence of HIV antibodies in the patient’s  serum. In this test, HIV proteins are separated electropho-  retically in a gel, resulting in discrete bands of viral protein.  These proteins are then transferred from the gel (i.e.,  blotted) onto filter paper, and the person’s serum is added.    If antibodies are present, they bind to the viral proteins  (primarily gp41 and p24) and can be detected by adding  antibody to human IgG labeled with either radioactivity or  an enzyme such as horseradish peroxidase, which produces  a visible color change when the enzyme substrate is added.   This test is commonly used to measure the number of  the various types of immunologically active blood cells  (Figure 64-10). For example, it is used in HIV-infected  patients to determine the number of CD4-positive T cells.  In this test, the patient’s cells are labeled with monoclonal  antibody to the protein specific to the cell of interest (e.g.,  CD4 protein if the number of helper T cells is to be deter¬  mined). The monoclonal antibody is tagged with a fluores¬  cent dye, such as fluorescein or rhodamine. Single cells are  passed through a laser light beam, and the number of cells  that fluoresce is counted by use of a machine called a  fluorescence-activated cell sorter (FACS).   Many different blood group systems exist in humans. Each  system consists of a gene locus specifying antigens on the  erythrocyte surface. The two most important blood group¬  ings, ABO and Rh, are described next.    Patient’s serum  is added, and  HIV antibodies  bind to the  viral proteins    Enzyme-labeled  antibody to human  IgG is added. The  enzyme substrate  is then added, and  colored bands appear  at the location of the  viral proteins    FIGURE 64-9 Western blot (immunoblot test). In this test, microbial or viral proteins are separated on an acrylamide gel and then trans¬  ferred (blotted) onto paper. The patient's serum then interacts with the separated proteins. If antibodies are present in the patient's serum, they  bind to the proteins. The patient's antibodies are then detected by using labeled antibody to human IgG.   All human erythrocytes contain alloantigens (i.e., antigens  that vary among individual members of a species) of the  ABO group. A person’s ABO blood group is a very impor¬  tant determinant of the success of both blood transfusions  and organ transplants.    The A and B genes encode enzymes that add specific  sugars to the end of a polysaccharide chain on the surface  of many cells, including red cells (Figure 64-11). People  who inherit neither gene are type O. The genes are codomi¬  nant, so people who inherit both genes are type AB. People   Deflection plates cause  cells with a positive charge  to move into a different  tube from cells with a  negative charge    FIGURE 64-10 Flow cytometry. At the top of the figure, a cell  has interacted with monoclonal antibody labeled with a fluorescent  dye. As the cell passes down the tube, ultraviolet light causes the dye  to fluoresce and a sensor counts the cell. Farther down the tube, an  electrical charge can be put on the cell, which allows it to be  deflected into a test tube and subjected to additional analysis.    FIGURE 64-11 ABO blood groups. Structures of the terminal  sugars that determine ABO blood groups are shown. Blood group 0  cells have H antigen on their surface; blood group A cells have  A/-acetylgalactosamine added to the end of the H antigen; and blood  group B cells have galactosamine added to the end of the H antigen.  (Reproduced with permission from Stites DP, Stobo JD, Wells JV, eds. Basic & Clinical  immunology. 6th ed. Originally published by Appleton & Lange. Copyright 1987  McGraw-Hill.)    who are either homozygous AA or heterozygous AO are  type A, and, similarly, people who are either homozygous  BB or heterozygous BO are type B.   The A and B antigens are carbohydrates that differ by a  single sugar. Despite this small difference, A and B antigens  do not cross-react. Erythrocytes have three terminal sugars  in common on their surface: N-acetylglucosamine, galac¬  tose, and fucose. These three sugars form the H antigen  (See Figure 64-11). People who are blood group O have  only the H antigen on the surface of their red cells. People  who are blood group A have N -acetylgalactosamine added  to the galactose of the H antigen, whereas people who are  blood group B have galactose added to the galactose of the  H antigen.   There are four combinations of the A and B antigens,  called A, B, AB, and O (Table 64-2). A person’s blood group  is determined by mixing the person’s blood with antiserum  against A antigen on one area on a slide and with antiserum  against B antigen on another area (see Figure 64-1). If agglu¬  tination occurs only with A antiserum, the blood group is A;  if it occurs only with B antiserum, the blood group is B; if it  occurs with both A and B antisera, the blood group is AB;  and if it occurs with neither A nor B antisera, the blood  group is O. In the United States, the approximate percent of  each blood group is: Type O: 45%, Type A: 40%, Type B:  11%, Type AB: 4%.   The plasma contains antibody against the absent anti¬  gens (i.e., people with blood group A have antibodies to B  in their plasma). These antibodies are formed against  cross-reacting bacterial antigens, are first detectable at 3 to  6 months of age, and are of the IgM class. ABO antibodies  are IgM because they form against bacterial polysaccha¬  rides which induce a T-cell independent response consist¬  ing of IgM antibody.   Individuals are tolerant to their own blood group anti¬  gens, and therefore a person with blood group A does not  form antibodies to A antigen. The end result is that antigen  and corresponding antibody do not coexist in the same  person’s blood.   Transfusion reactions occur when incompatible donor  red blood cells are transfused (e.g., if group A blood was  transfused into a group B person [because anti-A antibody  is present]). The red cell-antibody complex activates com¬  plement, and a reaction consisting of shock caused by large  amounts of C3a and C5a (anaphylatoxins) and hemolysis  caused by C5, C6, C7, C8, and C9 (membrane attack com¬  plex) occurs (Figure 64-12).    FIGURE 64-12 Transfusion reaction. Top panel: Red blood  cells bearing A antigen are transfused into a person who is type B  and therefore has antibodies to A antigen. Middle panel: Anti-A  antibodies bind to A antigen on the red cells causing agglutination  of red cells that can block movement of blood through capillaries  causing anoxia to tissue. Bottom panel: Complement is activated by  the antigen-antibody complexes and the membrane attack complex  lyses the red cells, causing hemolysis and anemia. (Reproduced with per-  mission from Cowan MK, Talaro KP, eds. Microbiology: A Systems Approach. New York:  McGraw-Hill; 2009.)   To avoid antigen-antibody reactions that would result  in transfusion reactions, all blood for transfusions must be  carefully matched (i.e., erythrocytes are typed for their  surface antigens by specific sera). As shown in Table 64-2,  persons with group O blood have no A or B antigens on  their red cells and so are universal donors (i.e., they can  give blood to people in all four groups) (Table 64-3). Note    TABLE 64-3 Compatibility of Blood Transfusions  Between ABO Blood Groups 1 ^    ’Yes indicates that a blood transfusion from a donor with that blood group to a  recipient with that blood group is compatible (i.e., that no hemolysis will occur). No  indicates that the transfusion is incompatible and that hemolysis of the donor's cells  will occur.    that type O blood has A and B antibodies. Therefore when  type O blood is given to a person with type A, B, or AB  blood, you might expect a reaction to occur. A clinically  detectable reaction does not occur because the donor anti¬  body is rapidly diluted below a significant level. Persons  with group AB blood have neither A nor B antibody and  thus are universal recipients.   In addition to red blood cells, the A and B antigens  appear on the cells of many tissues. Furthermore, these  antigens can be secreted in saliva and other body fluids.  Secretion is controlled by a secretor gene. Approximately  85% of people carry the dominant form of the gene, which  allows secretion to occur.   ABO blood group differences can lead to neonatal jaun¬  dice and anemia, but the effects on the fetus are usually less  severe than those seen in Rh incompatibility (see next sec¬  tion). For example, mothers with blood group O have anti¬  bodies against both A and B antigens. These IgG antibodies  can pass the placenta and, if the fetus is blood group A or  B, cause lysis of fetal red cells. Mothers with either blood  group A or B have a lower risk of having a neonate with  jaundice because these mothers produce antibodies to  either B or A antigens, respectively, that are primarily IgM,  and IgM does not pass the placenta.   About 85% of humans have erythrocytes that express the  Rh(D) antigen on their surface. They are said to be    'No indicates that hemolysis of the newborn's red cells will not occur and that  hemolytic disease will therefore not occur. Yes indicates that hemolysis of the new¬  born's red cells is likely to occur and that symptoms of hemolytic disease will there¬  fore probably occur.   Rh-positive. The remaining 15% are Rh-negative, that is,  they lack the gene for the Rh(D) protein.   The Rh status of parents is clinically important because  a specific combination can result in hemolytic disease of  the newborn (erythroblastosis fetalis). When an Rh-  negative woman has an Rh-positive fetus (the D gene  being inherited from the father), the Rh(D) antigen on the  fetal red blood cells will stimulate the development of anti¬  bodies in the mother (Table 64-4). This occurs most often  when the Rh(D) erythrocytes of the fetus leak into the  maternal circulation during delivery of the first Rh(D)  child (Figure 64-13).    FIGURE 64~1 3 Hemolytic disease of the newborn (erythroblastosis fetalis). Left panel: Fetal red cells (RBCs) bearing the Rh antigen  enter the mother's blood when the placenta separates during the birth of the first Rh-positive child. IgG antibodies to Rh antigen are then pro¬  duced by the mother. Center panel: During a second pregnancy with an Rh-positive fetus, IgG antibodies pass from the mother into the fetus  via the placenta. The antibodies bind to the fetal red cells, complement is activated, and the membrane attack complex lyses the fetal red cells.  Right panel: Anemia and jaundice occur in the fetus/newborn. As a result of the anemia, large numbers of erythroblasts are produced by the  bone marrow and are seen in the blood of the newborn. {Reproduced with permission from Cowan MK,Talaro KP, eds. Microbiology: A Systems Approach. New York:  McGraw-Hill: 2009.)    Subsequent Rh(D) pregnancies are likely to be affected  by the mothers anti-D IgG antibody, and hemolytic disease  of the newborn (erythroblastosis fetalis) often results. This  disease results from the passage of maternal IgG anti-  Rh(D) antibodies through the placenta to the fetus, with  subsequent lysis of the fetal erythrocytes. The direct anti¬  globulin (Coombs) test is typically positive (see earlier  description of the Coombs test).   The problem can be prevented by administration of  high-titer Rh(D) immune globulins (Rho-Gam) to an  Rh(D) mother at 28 weeks of gestation and immediately  upon the delivery of an Rh(D) child. These antibodies  promptly attach to Rh(D) erythrocytes and prevent their  acting as sensitizing antigen. This prophylaxis is widely  practiced and effective.   1. Which one of the following laboratory tests would be the best  to determine the number of CD4-positive cells in the blood of a  patient infected with HIV?   2. You have just received a lab report that says your patient is  positive for IgM antibody to Borrelia burgdorferi in an enzyme-  linked immunosorbent assay (ELISA). This supports your clini¬  cal impression that the patient has Lyme disease. Which one of  the following best describes how the ELISA was performed? (For  brevity, the wash steps have been left out.)   (A) The patients serum was reacted with antibody to human mu  heavy chain. Then Borrelia antigens labeled with an enzyme  were added. Then the enzyme substrate was added, and a color  change was observed.   (B) The patients serum was reacted with Borrelia antigens. Then  antibody to human mu heavy chain labeled with an enzyme  was added. Then the enzyme substrate was added, and a color  change was observed.   (C) Borrelia antigens were reacted with antibody to human mu  heavy chain. Then the patient’s serum labeled with an enzyme  was added. Then the enzyme substrate was added, and a color  change was observed.   (D) Borrelia antigens were reacted with antibody to human mu  heavy chain labeled with an enzyme. Then the patients serum  was added. Then the enzyme substrate was added, and a color  change was observed.   3. Regarding ABO blood groups, which one of the following is the  most accurate?   (A) People who are blood group O have the O antigen on the sur¬  face of their red cells.   (B) The A and B blood group antigens are located on the surface of  red cells but not on the surface of other cells.    (C) The differences between the A and B blood group antigens are  dependent on the presence of different D-amino acids on the  cell surface.   (D) People who are blood group O do not have antibodies to A and  B blood group antigens and thus can be given both type A and  type B blood.   (E) The genes that determine ABO blood groups are codominant,  so a person who is blood group AB is expressing both genes  that encode the enzymes that synthesize the A and the B blood  group antigens.   4. Regarding hemolytic disease of the newborn (erythroblastosis  fetalis), which one of the following is the most accurate?   (A) Maternal red cells are the source of the antigen that induces the  antibody.   (B) It typically occurs when the father is Rh-positive and the  mother is Rh-negative.   (C) Maternal IgM anti-Rh antibody enters the fetus and causes  damage to the fetal red cells.   (D) Symptomatic disease is more likely to occur in the first child  than in the subsequent children.   (E) Administration of Rh antigen to the newborn can prevent  symptomatic disease if given early enough.   5. You think your patient has secondary syphilis, and you order a  VDRL serological test. The lab reports that the test is negative. If  this is a false-negative result due to the “prozone” phenomenon,  which one of the following is the most likely explanation?   (A) The patients serum has too much antibody, and the reaction is  in the zone of antibody excess.   (B) The patient’s serum has too much antigen, and the reaction is  in the zone of antigen-excess phase.   (C) The patient’s serum has too little antibody, and the reaction is  in the zone of antibody-deficient phase.   (D) The patient’s serum has too little antigen, and the reaction is in  the zone of antigen-deficient phase.   (E) The patient’s serum has an amount of antibody that puts it in  the zone of equivalence.   6. As part of a murder investigation, the blood group of the victim  was determined by analyzing the antibodies in her serum. (Unfor¬  tunately, the red cells of the victim were lost by the crime squad,  so they had to use her serum.) In this test, red cells known to be  either O, A, B, or AB were mixed with her serum and agglutina¬  tion observed. Based on the results in the following table, what is  the blood group of the victim?   (E) A laboratory error has occurred, and the test should be repeated   Questions on the topics discussed in this chapter can be found  in the Immunology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 733. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   Hypersensitivity is the term used when an immune response  results in exaggerated or inappropriate reactions harmful to  the host. Generally speaking, hypersensitivity reactions  occur in response to external stimuli (antigens) whereas  autoimmune reactions (see Chapter 66) occur in response to  internal stimuli (antigens). The term allergy is often equated  with hypersensitivity but more accurately should be limited  to the IgE-mediated reactions discussed later in the section  “Type I: Immediate (Anaphylactic) Hypersensitivity.”    The clinical manifestations of these reactions are typical  in a given individual and occur on contact with the specific  antigen to which the individual is hypersensitive. The first  contact of the individual with the antigen sensitizes (i.e.,  induces the antibody), and subsequent contacts elicit the  allergic response.   Hypersensitivity reactions can be subdivided into four  main types. Types I, II, and III are antibody-mediated,  whereas type IV is cell-mediated (Table 65-1). Type I reac¬  tions are mediated by IgE, whereas types II and III are  mediated by IgG. The immunologic reactions are   Antigen (allergen) induces IgE antibody that binds to mast cells and basophils. When exposed  to the allergen again, the allergen cross-links the bound IgE on those cells. This causes  degranulation and release of mediators (e.g., histamine).   Antigens on a cell surface combine with IgG antibody. This leads to complement-mediated lysis  of the cells (e.g., transfusion or Rh reactions) or autoimmune hemolytic anemia.   Antigen-antibody immune complexes are deposited in tissues, complement is activated, and  polymorphonuclear cells are attracted to the site. They release lysosomal enzymes, causing  tissue damage.   T lymphocytes activated/sensitized by an antigen release lymphokines upon second contact  with the same antigen. The lymphokines induce inflammation and activate macrophages,  which, in turn, release various inflammatory mediators.   Systemic anaphylaxis, urticaria (hives), asthma, hay fever, allergic rhinitis, allergic conjunctivitis,  food allergies (e.g., nuts, shellfish, eggs), drug allergies especially penicillin, eczema (atopic  dermatitis), bee venom, latex gloves, angioedema   Hemolytic anemia, neutropenia, thrombocytopenia, ABO transfusion reactions, Rh  incompatibility (erythroblastosis fetalis, hemolytic disease of the newborn), rheumatic  fever, Goodpasture's syndrome   Contact dermatitis, poison oak/ivy, tuberculin skin test reaction, drug rash, Stevens-Johnson  syndrome, toxic epidermal necrolysis, erythema multiforme    summarized in Table 65-1. The clinical manifestations of  the hypersensitivity reactions are described in Table 65-2.   An immediate hypersensitivity reaction occurs when an  antigen (allergen) binds to IgE on the surface of mast cells  with the consequent release of several mediators (see list of  mediators that follows) (Figure 65-1). The process begins  when an antigen induces the formation of IgE antibody,  which binds firmly by its Fc portion to receptors on the  surface of basophils and mast cells. Reexposure to the same  antigen results in cross-linking of the cell-bound IgE,  degranulation, and release of pharmacologically active  mediators within minutes (immediate phase). Cyclic  nucleotides and calcium play essential roles in release of the  mediators. 1 Symptoms such as edema and erythema  (“wheal and flare”) and itching appear rapidly because  these mediators (e.g., histamine) are preformed.   The late phase of IgE-mediated inflammation occurs  approximately 6 hours after exposure to the antigen and is  due to mediators (e.g., leukotrienes [SRS-A]) that are syn¬  thesized after the cell degranulates. These mediators cause  an influx of inflammatory cells, such as neutrophils and  eosinophils, and symptoms such as erythema and indura¬  tion occur. For example, eosinophils play a major role in  the late-phase reaction in asthma.   Complement is not involved with either the immediate or  late reactions because IgE does not activate complement.   Note that the allergens involved in hypersensitivity reac¬  tions are substances, such as pollens, animal danders, foods  (nuts, shellfish), and various drugs, to which most people do    1 An increase in cyclic guanosine monophosphate (GMP) within these  cells increases mediator release, whereas an increase in cyclic adenosine  monophosphate (AMP) decreases the release. Therefore, drugs that  increase intracellular cyclic AMP, such as epinephrine, are used to treat  type I reactions. Epinephrine also has sympathomimetic activity, which  is useful in treating type I reactions.    not exhibit clinical symptoms. However, some individuals  respond to those substances by producing large amounts of  IgE and, as a result, manifest various allergic symptoms. The  increased IgE is the result of increased class switching to IgE  in B cells caused by large amounts of interleukin (IL)-4 pro¬  duced by Th-2 cells. Nonallergic individuals respond to the  same antigen by producing IgG, which does not cause the  release of mediators from mast cells and basophils. (There  are no receptors for IgG on those cells.) There is a genetic  predisposition to immediate hypersensitivity reactions,  which is discussed in the “Atopy” section later.   The clinical manifestations of type I hypersensitivity can  appear in various forms (e.g., urticaria [also known as  hives], eczema, rhinitis and conjunctivitis [also known as  hay fever], and asthma). Which clinical manifestation  occurs depends in large part on the route of entry of the  allergen and on the location of the mast cells bearing the  IgE specific for the allergen. For example, some individuals  exposed to pollens in the air get hay fever, whereas others  who ingest allergens in food get diarrhea. Furthermore,  people who respond to an allergen with urticaria have the  allergen-specific IgE on mast cells in the skin, whereas  those who respond with rhinitis have the allergen-specific  mast cells in the nose.   The most severe form of type I hypersensitivity is sys¬  temic anaphylaxis, in which severe bronchoconstriction  and hypotension (shock) can be life-threatening. A sense of  doom and dizziness can occur. Other symptoms include  wheezing due to bronchoconstriction, hoarseness due to  laryngeal edema, pruritis, and urticaria. Tachycardia,  arrhythmia, cyanosis, and cardiac arrest can occur.   The most common causes of anaphylaxis are foods  such as peanuts and shellfish, bee venom, and drugs such  as penicillin. It is the proteins in peanuts, shellfish, and  bee venom that cross-link adjacent IgEs and trigger the  release of histamine and other mediators from mast cells  and basophils. Drugs such as penicillin are haptens that  need to bind to human proteins to cross-link adjacent  IgEs (see Chapter 57).   Of particular interest to medical personnel are type I  hypersensitivity reactions to the wearing of latex rubber    gloves, which include urticaria, asthma, and even systemic  anaphylaxis. Table 65-3 summarizes some of the important  clinical aspects of immediate hypersensitivities.   No single mediator accounts for all the manifestations of  type I hypersensitivity reactions. Some important media¬  tors and their effects are as follows:   (1) Histamine occurs in granules of tissue mast cells  and basophils in a preformed state. Its release causes  vasodilation, increased capillary permeability, and  smooth muscle contraction. Clinically, disorders such as  allergic rhinitis (hay fever), urticaria, and angioedema  can occur. The bronchospasm so prominent in acute ana¬  phylaxis results, in part, from histamine release. Antihis¬  tamine drugs block histamine receptor sites and can be  relatively effective in allergic rhinitis but not in asthma  (see later).   consists of several leukotrienes, which do not exist in a  preformed state but are produced during anaphylactic  reactions. This accounts for the slow onset of the effect of  SRS-A. Leukotrienes are formed from arachidonic acid by  the lipoxygenase pathway and cause increased vascular    permeability and smooth muscle contraction. They are the  principal mediators in the bronchoconstriction of asthma  and are not influenced by antihistamines.   (3) Eosinophil chemotactic factor of anaphylaxis  (ECF-A) is a tetrapeptide that exists preformed in mast cell  granules. When released during anaphylaxis, it attracts  eosinophils that are prominent in immediate allergic reac¬  tions. The role of eosinophils in type I hypersensitivity  reactions is uncertain, but they do release histaminase and  arylsulfatase, which degrade two important mediators, his¬  tamine and SRS-A, respectively. Eosinophils may therefore  reduce the severity of the type I response.   (4) Serotonin (hydroxytryptamine) is preformed in  mast cells and blood platelets. When released during ana¬  phylaxis, it causes capillary dilation, increased vascular  permeability, and smooth muscle contraction but is of  minor importance in human anaphylaxis.   (5) Prostaglandins and thromboxanes are related to  leukotrienes. They are derived from arachidonic acid via  the cyclooxygenase pathway. Prostaglandins cause dilation  and increased permeability of capillaries and bronchocon¬  striction. Thromboxanes aggregate platelets.   Pollens, house dust (feces of dust  mite), animal danders, many  occupational airborne allergens    (6) Platelet-activating factor (PAF) is a phospholipid  produced by mast cells that can cause bronchoconstriction,  hypotension, and vascular permeability.   The aforementioned mediators are active only for a few  minutes after release; they are enzymatically inactivated  and resynthesized slowly. Manifestations of anaphylaxis  vary among species because mediators are released at dif¬  ferent rates in different amounts, and tissues vary in their  sensitivity to them. For example, the respiratory tract  (bronchospasm, laryngeal edema) is a principal shock  organ in humans, but the liver (hepatic veins) plays that  role in dogs.   In allergic airway disease (asthma), the airway hyper¬  activity appears to be caused by IL-13. IL-13 is made by  Th-2 cells and binds to a receptor that shares a chain  with the IL-4 receptor. IL-13 does not increase the  amount of IgE. Lebrikizumab, a monoclonal antibody  against IL-13, reduces symptoms in some patients with  severe asthma.   In contrast to anaphylactic reactions, which are IgE-  mediated, anaphylactoid reactions, which appear clinically  similar to anaphylactic ones, are not IgE-mediated. In ana¬  phylactoid reactions, the inciting agents, usually drugs or  iodinated contrast media, directly induce the mast cells and  basophils to release their mediators without the involve¬  ment of IgE.   Atopic disorders, such as hay fever, asthma, eczema, and  urticaria, are immediate-hypersensitivity reactions that  exhibit a strong familial predisposition and are associated  with elevated IgE levels. Several processes seem likely to  play a role in atopy, for example, failure of regulation at the  T-cell level (e.g., increased production of IL-4 leads to  increased IgE synthesis), enhanced uptake and presenta¬  tion of environmental antigens, and hyperreactivity of tar¬  get tissues. Target tissues often contain large numbers of  Th-2 cells, and these are thought to play a major role in the  pathogenesis of atopic reactions.   It is estimated that up to 40% of people in the United  States have experienced an atopic disorder at some time  in their lives. The incidence of allergic diseases, such as  asthma, is increasing markedly in the developed coun¬  tries of North America and Europe. One hypothesis that  might explain this increase is that the parasite burden is  low in those countries. IgE evolved as a host defense  against parasites. In regions where the parasite burden is  high, IgE is used for host defense against those organ¬  isms. But in developed regions where the parasite burden  is low, IgE is available to cause allergic diseases. This is  called the “hygiene” hypothesis, which states that people  who live in countries with a high parasite burden have  fewer allergic diseases, whereas those who live in coun¬  tries with a low parasite burden have more allergic  diseases.    The symptoms of these atopic disorders are induced by  exposure to the specific allergens. These antigens are typi¬  cally found in the environment (e.g., pollens released by  plants and dust mite feces often found in bedding and car¬  pet) or in foods (e.g., shellfish, eggs, and nuts). Exposure of  nonatopic individuals to these substances does not elicit an  allergic reaction. Many sufferers give immediate-type reac¬  tions to skin tests (injection, patch, or scratch) containing  the offending antigen.   Atopic hypersensitivity is transferable by serum (i.e., it  is antibody-mediated), not by lymphoid cells. In the past,  this observation was used for diagnosis in the passive cuta¬  neous anaphylaxis (Prausnitz-Kiistner) reaction, which  consists of taking serum from the patient and injecting it  into the skin of a normal person. Some hours later, the test  antigen, injected into the “sensitized” site, will yield an  immediate wheal-and-flare reaction. This test is now  impractical because of the danger of transmitting certain  viral infections. Radioallergosorbent tests (RAST) permit  the identification of specific IgE against potentially offend¬  ing allergens if suitable specific antigens for in vitro tests  are available.   There is evidence that initiation of the atopic response  occurs when proteases in allergens, such as fungal aller¬  gens, pollens, and dust mite feces, cleave fibrinogen. The  resulting cleavage products then activate Toll-like receptors  (TLR-4) on the surface of macrophages and airway-lining  cells to activate the atopic response.   Several genes associated with atopy have been identi¬  fied. Mutations in the gene encoding the alpha chain of the  IL-4 receptor strongly predispose to atopy. These mutations  enhance the effectiveness of IL-4, resulting in an increased  amount of IgE synthesis by B cells. Other genes identified  include the gene for IL-4 itself, the gene for the receptor for  the epsilon heavy chain, and several class II major histo¬  compatibility complex (MHC) genes.   Drugs, particularly antimicrobial agents such as penicil¬  lin, are now among the most common causes of hypersen¬  sitivity reactions. Usually it is not the intact drug that  induces antibody formation. Rather, a metabolic product  of the drug, which acts as a hapten and binds to a body  protein, does so. The resulting IgE antibody can react with  the hapten or the intact drug to give rise to type I  hypersensitivity. 2   When reexposed to the drug, the person may exhibit a  drug rash, fever, or local or systemic anaphylaxis of variable  severity. Reactions to very small amounts of the drug can  occur (e.g., in a skin test with the hapten). A clinically use¬  ful example is the skin test using penicilloyl polylysine to  reveal an allergy to penicillin.    2 Some drugs are involved in cytotoxic hypersensitivity reactions (type II)  and in serum sickness (type III).   Major manifestations of anaphylaxis occur when large  amounts of mediators are suddenly released as a result of a  massive dose of antigen abruptly combining with IgE on  many mast cells. This is systemic anaphylaxis, which is  potentially fatal. Desensitization can prevent systemic  anaphylaxis.   Acute desensitization involves the administration of  very small amounts of antigen at 15-minute intervals.  Antigen-IgE complexes form on a small scale, and not  enough mediator is released to produce a major reaction.  This permits the administration of a drug or foreign protein  to a hypersensitive person, but the hypersensitive state  returns because IgE continues to be made.   Chronic desensitization involves the long-term weekly  administration of the antigen to which the person is hyper¬  sensitive. This stimulates the production of IgA- and IgG-  blocking antibodies, which can prevent subsequent antigen  from reaching IgE on mast cells, thus preventing a reaction.  It also induces regulatory T cells to produce IL-10, which  reduces the synthesis of IgE.   Treatment of anaphylactic reactions includes drugs to coun¬  teract the action of mediators, maintenance of an airway,  and support of respiratory and cardiac function. Epineph¬  rine, antihistamines, corticosteroids, or cromolyn sodium,  either singly or in combination, should be given. Cromolyn  sodium prevents release of mediators (e.g., histamine) from  mast cell granules. Prevention relies on identification of the  allergen by a skin test and avoidance of that allergen.   There are several approaches to the treatment of asthma.  Inhaled (3-adrenergic bronchodilators, such as albuterol,  are commonly used. Corticosteroids, such as prednisone,  are also effective. Aminophylline, a bronchodilator, is effec¬  tive but not commonly used. A monoclonal anti-IgE anti¬  body (omalizumab, Xolair) is indicated for patients with  severe asthma whose symptoms are not controlled by cor¬  ticosteroids. For the prevention of asthma, leukotriene  receptor inhibitors, such as montelukast (Singulair), and  cromolyn sodium are effective.   The treatment of allergic rhinitis typically involves anti¬  histamines along with nasal decongestants. For allergic    conjunctivitis, eye drops containing antihistamines or  vasoconstrictors are effective. Avoidance of the inciting  allergens, such as pollens, is helpful in prophylaxis. Desen¬  sitization can also be helpful.   Cytotoxic hypersensitivity occurs when antibody  directed at antigens of the cell membrane activates  complement (Figure 65-2). This generates a membrane  attack complex (see Chapter 63), which damages the cell  membrane. The antibody (IgG or IgM) attaches to the  antigen via its Fab region and acts as a bridge to comple¬  ment via its Fc region. As a result, there is complement-  mediated lysis as in hemolytic anemias, ABO transfusion  reactions, or Rh hemolytic disease. In addition to caus¬  ing lysis, complement activation attracts phagocytes to  the site, with consequent release of enzymes that damage  cell membranes.   Drugs (e.g., penicillins, phenacetin, quinidine) can  attach to surface proteins on red blood cells and initiate  antibody formation. Such autoimmune antibodies (IgG)  then interact with the red blood cell surface and result in  hemolysis. The direct antiglobulin (Coombs) test is typi¬  cally positive (see Chapter 64).   Other drugs (e.g., quinine) can attach to platelets and  induce autoantibodies that lyse the platelets, producing  thrombocytopenia and, as a consequence, a bleeding ten¬  dency. Others (e.g., hydralazine) may modify host tissue  and induce the production of autoantibodies directed at  cell DNA. As a result, disease manifestations resembling  those of systemic lupus erythematosus occur.   Certain infections (e.g., Mycoplasma pneumoniae infec¬  tion) can induce antibodies that cross-react with red cell  antigens, resulting in hemolytic anemia. In rheumatic fever,  antibodies against the group A streptococci cross-react  with cardiac tissue. In Goodpasture’s syndrome, antibody  to basement membranes of the kidneys and lungs bind to  those membranes and activate complement. Severe damage  to the membranes is caused by proteases released from  leukocytes attracted to the site by complement component  C5a (see page 544).   Immune complex hypersensitivity occurs when antigen-  antibody complexes induce an inflammatory response in  tissues (Figure 65-3). Normally, immune complexes are  promptly removed by the reticuloendothelial system, but  occasionally they persist and are deposited in tissues, result¬  ing in several disorders. In persistent microbial or viral infec¬  tions, immune complexes may be deposited in organs (e.g.,  the kidneys), resulting in damage. In autoimmune disorders,  “self” antigens may elicit antibodies that bind to organ anti¬  gens or deposit in organs as complexes, especially in joints  (arthritis), kidneys (nephritis), or blood vessels (vasculitis).   Wherever immune complexes are deposited, they acti¬  vate the complement system. Polymorphonuclear cells are  attracted to the site, and inflammation and tissue injury  occur. Two typical type III hypersensitivity reactions are  the Arthus reaction and serum sickness.   Arthus reaction is the name given to the inflammation  caused by the deposition of immune complexes at a localized  site. It is named for Dr. Arthus, who first described the  inflammatory response that occurs under the following con¬  ditions. If animals are given an antigen repeatedly until they  have high levels of IgG antibody 3 and that antigen is then  injected subcutaneously or intradermally, intense edema and  hemorrhage develop, reaching a peak in 3 to 6 hours.   Antigen, antibody, and complement are deposited in  vessel walls; polymorphonuclear cell infiltration and intra¬  vascular clumping of platelets then occur. These reactions  can lead to vascular occlusion and necrosis.   A clinical manifestation of the Arthus reaction is hyper¬  sensitivity pneumonitis (allergic alveolitis) associated with  the inhalation of thermophilic actinomycetes (“farmer’s  lung”) growing in plant material such as hay. There are  many other occupation-related examples of hypersensitiv¬  ity pneumonitis, such as “cheese-worker’s lung,” “wood¬  worker’s lung,” and “wheat-miller’s lung.” Most of these are  caused by the inhalation of some microorganism, either  bacterium or fungus, growing on the starting material. An  Arthus reaction can also occur at the site of tetanus immu¬  nizations if they are given at the same site with too short an  interval between immunizations. (The minimum interval  is usually 5 years.)   In contrast to the Arthus reaction, which is localized  inflammation, serum sickness is a systemic inflammatory    3 Much more antibody is typically needed to elicit an Arthus reaction  than an anaphylactic reaction.    response to the presence of immune complexes deposited  in many areas of the body. After the injection of foreign  serum (i.e., serum from another animal such as a horse [or,  more commonly these days, exposure to certain drugs]),  the antigen is excreted slowly. During this time, antibody  production starts. The simultaneous presence of antigen  and antibody leads to the formation of immune complexes,  which may circulate or be deposited at various sites.   Typical serum sickness results in fever, urticaria, arthral¬  gia, lymphadenopathy, splenomegaly, and eosinophilia a  few days to 2 weeks after injection of the foreign serum or  drug. Although it takes several days for symptoms to  appear, serum sickness is classified as an immediate reac¬  tion because symptoms occur promptly after immune  complexes form. Symptoms improve as the immune system  removes the antigen and subside when the antigen is elimi¬  nated. Nowadays, serum sickness is caused more com¬  monly by drugs (e.g., penicillin) than by foreign serum  because foreign serum is used so infrequently. A maculo-  papular drug-induced rash to penicillins, such as ampicil-  lin, is quite common. Use of antithymocyte globulin  (thymoglobulin), which is made in horses, to provide  immunosuppression in transplant patients may cause  serum sickness. Note also that diphtheria antitoxin made in  horses is known to cause serum sickness.   Many clinical disorders associated with immune complexes  have been described, although the antigen that initiates the  disease is often in doubt. Several representative examples  are described next.   Acute poststreptococcal glomerulonephritis is a well-  accepted immune complex disease. Its onset follows  several weeks after a group A (3-hemolytic streptococcal  infection, particularly of the skin, and often with neph-  ritogenic serotypes of Streptococcus pyogenes. Typically,  the complement level is low, suggesting an antigen-  antibody reaction. Lumpy deposits of immunoglobulin  and C3 are seen along glomerular basement membranes  by immunofluorescence, suggesting the presence of  antigen-antibody complexes. It is assumed that strepto¬  coccal antigen-antibody complexes, after being depos¬  ited on glomeruli, fix complement and attract neutrophils,  which start the inflammatory process.   Similar lesions with “lumpy” deposits containing immu¬  noglobulin and C3 occur in infective endocarditis, serum  sickness, and certain viral infections (e.g., hepatitis B and  dengue hemorrhagic fever). Lesions containing immune  complexes also occur in autoimmune diseases (e.g., the  nephritis of systemic lupus erythematosus, in which the  “lumpy” deposits contain DNA as the antigen) (see page xx).   Immune complexes  are deposited on  blood vessel wall,  complement is  activated, and C3a  and C5a are  released    Neutrophils are  attracted by C5a;  they release  enzymes that  destroy the  endothelium and  red cells escape  from within the  blood vessels    IgA nephropathy is one of the most common forms of  immune complex glomerulonephritis worldwide. This dis¬  ease is characterized by deposits of IgA on the glomeruli.  The cause is unknown; no infectious agent has been associ¬  ated with this disease. The course of the disease varies  widely. Some patients are asymptomatic, some have mild  symptoms, and others progress rapidly to kidney failure.  Diagnosis is made by doing renal biopsy and demonstrat¬  ing IgA deposits by immunohistologic testing.   Rheumatoid arthritis is a chronic inflammatory autoim¬  mune disease of the joints seen commonly in young  women. It is a systemic disease involving not only the  joints but other organs as well, most often the lung and  pericardium. Serum and synovial fluid of patients contain  “rheumatoid factor” (i.e., IgM and IgG antibodies that  bind to the Fc fragment of normal human IgG). Deposits  of immune complexes (containing the normal IgG and  rheumatoid factor) on synovial membranes and in blood  vessels activate complement and attract polymorphonu¬  clear cells, causing inflammation. Patients have high titers  of rheumatoid factor and low titers of complement in  serum especially during periods when their disease is most  active (see page 573).   Systemic lupus erythematosus is a chronic inflammatory  autoimmune disease that affects several organs, especially  the skin of the face, the joints, and the kidneys. Antibodies  are formed against DNA and other components of the  nucleus of cells. These antibodies form immune complexes  that activate complement. Complement activation pro¬  duces C5a, which attracts neutrophils that release enzymes,  thereby damaging tissue (see pages 544 and 573).   Vasculitis is an inflammation of the walls of blood vessels,  including large, medium, and small arteries and veins. Several  diseases that manifest with vasculitis are caused by immune  complexes, such as polyarteritis nodosa, Henoch-Schonlein  purpura (IgA vasculitis), cryoglobulin-related vasculitis, and  the vasculitis that occurs in systemic lupus erythematosus.  An important example of a cryoglobulin-related vasculitis  occurs in hepatitis C virus infection. In contrast, some dis¬  eases, such as giant cell arteritis (GCA) are caused by T cells  infiltrating the arterial wall. The most common form of GCA  is temporal arteritis that involves the temporal artery.   The symptoms and signs of vasculitis vary depending  on the organ affected. Nonspecific findings include fever,  weight loss, arthralgia, myalgia, and abdominal pain. Some  findings often associated with vasculitis are palpable pur¬  pura and mononeuritis multiplex which often manifests as  foot or wrist drop. A complete description of the diseases in  which vasculitis occurs is beyond the scope of this book.   Delayed hypersensitivity is a function of T lymphocytes,  not antibody (Figure 65-4). It can be transferred by immu-  nologically committed (sensitized) T cells, not by serum.  The response is “delayed” (i.e., it starts hours [or days] after  contact with the antigen and often lasts for days).   In certain contact hypersensitivities, such as poison oak,  the pruritic, vesicular skin rash is caused by CD8-positive  cytotoxic T cells that attack skin cells that display the plant  oil as a foreign antigen. In the tuberculin skin test, the  indurated skin rash is caused by CD4-positive helper  T cells and macrophages that are attracted to the injection  site. Table 65-4 describes some of the important clinical  aspects of delayed hypersensitivities.   FIGURE 65-4 Delayed (cell-mediated) hypersensitivity. The  macrophage ingests the antigen, processes it, and presents an  epitope on its surface in association with class II major histocom¬  patibility complex (MHC) protein. The helper T (Th-1) cell is  activated and produces gamma interferon, which activates macro¬  phages. These two types of cells mediate delayed hypersensitivity.  TCR, T-cell receptor.   This manifestation of cell-mediated hypersensitivity occurs  after sensitization with simple chemicals (e.g., nickel, form¬  aldehyde), plant materials (e.g., urushiol in poison ivy and  poison oak), topically applied drugs (e.g., sulfonamides,  neomycin), some cosmetics, soaps, and other substances.  Neomycin in topical antibacterial ointment is a very com¬  mon cause.   In all cases, the small molecules acting as haptens enter  the skin, attach to body proteins, and become complete  antigens. It is thought that these normal skin proteins to  which the immune system is tolerant now can act as a car¬  rier protein, because the hapten alters the protein enough  that the immune system recognizes it as foreign. Cell-  mediated hypersensitivity is induced, particularly in the  skin. Upon a later skin contact with the offending agent, the  sensitized person develops contact dermatitis characterized  by erythema, itching, vesicles, eczema, or necrosis of skin  within 12 to 48 hours caused by the attack of cytotoxic  T cells. Patch testing on a small area of skin can sometimes  identify the offending antigen. Subsequent avoidance of the  material will prevent recurrences.   Delayed hypersensitivity to antigens of microorganisms  occurs in many infectious diseases and has been used as an  aid in diagnosis. It is typified by the tuberculin reaction.  When a patient previously exposed to Mycobacterium  tuberculosis is injected with a small amount of tuberculin  (purified protein derivative [PPD]) intradermally, there is  little reaction in the first few hours. Gradually, however,  induration and redness develop and reach a peak in 48 to  72 hours. A positive skin test indicates that the person has  been infected with the agent, but it does not confirm the  presence of current disease. However, if the skin test con¬  verts from negative to positive, it suggests that the patient  has been recently infected. Infected persons do not always  have a positive skin test, because overwhelming infection,  disorders that suppress cell-mediated immunity (e.g., uremia,  measles, sarcoidosis, lymphoma, and acquired immuno¬  deficiency syndrome [AIDS]), or the administration of  immunosuppressive drugs (e.g., corticosteroids, antineo¬  plastics) may cause anergy.   A positive skin test response assists in diagnosis and  provides support for chemoprophylaxis or chemotherapy.  In leprosy, a positive lepromin test indicates the presence of  tuberculoid leprosy with competent cell-mediated immu¬  nity, whereas a negative lepromin test suggests the presence  of lepromatous leprosy with impaired cell-mediated immu¬  nity. In systemic mycotic infections (e.g., coccidioidomyco¬  sis and histoplasmosis), a positive skin test with the specific  antigen indicates exposure to the organism. Cell-mediated  hypersensitivity develops in many viral infections; how¬  ever, serologic tests are more specific than skin tests both  for diagnosis and for assessment of immunity. In protozoan  and helminthic infections, skin tests may be positive, but  they are generally not as useful as specific serologic tests.   Erythema multiforme, Stevens-Johnson syndrome, and  toxic epidermal necrolysis are related skin diseases caused  primarily by cytotoxic T-cell attack on skin cells (keratino-  cytes). The most common triggers are herpes simplex  virus-1, M. pneumoniae, and a variety of drugs, including   Oil of poison oak or poison ivy, topical drugs,  soaps, heavy metals (in jewelry)   (Reproduced with permission from Goldsmith LA, Katz SI et al (eds). Fitzpatrick's   Dermatology in General Medicine. 8th ed. New York: McGraw-Hill, 2012. Copyright ©   sulfonamides and penicillins. Several human leukocyte  antigen (HLA) alleles predispose to these diseases, espe¬  cially HLA-DQ3 and HLA-B12.   The clinical manifestations of these diseases are charac¬  terized by a continuum of symptoms that differ in severity  and anatomic location. Erythema multiforme minor is  characterized by relatively few, localized target lesions on  the skin, often involving the extremities (Figure 65-5), with  minimal involvement of mucous membranes. They begin  to heal in 7 days but may recur. In contrast, erythema mul¬  tiforme major has more extensive lesions on the skin and  involves the mucous membranes, often of the mouth and  conjunctivae.   Stevens-Johnson syndrome (SJS) has more extensive  blistering lesions, often on the face and trunk with signifi¬  cant lesions on the mucous membranes. In SJS, less than  10% of the body surface is involved; in toxic epidermal  necrolysis (TEN), more than 10% of the body surface is  involved. TEN is a life-threatening disease, and treatment  in a burn unit is recommended.   1. Your patient has episodes of eye tearing, “blood-shot” eyes, and  runny nose, which you think may be due to an allergy to some  plant pollen. You refer the patient to an allergist, who performs    skin tests with various allergens. A wheal-and-flare reaction is  seen on the patients back at the site where several pollens were  injected. What is the most likely sequence of events that produced  the wheal-and-flare reaction?   (A) Allergen binds to IgE on the surface of B cells and IL-4 is  released.   (B) Allergen binds to IgE on the surface of mast cells and histamine  is released.   (C) Allergen binds to IgE in the plasma, which activates comple¬  ment to produce C3b.   (D) Allergen binds to IgE in the plasma, and the allergen-IgE com¬  plex binds to the surface of macrophages and IL-1 is released.   2. One important test to determine whether your patient has been  exposed to M. tuberculosis , the organism that causes tuberculosis,  is to do a PPD skin test. In this test, PPD extracted from the organ¬  ism is injected intradermally. Of the following, which one is most  likely to occur at the site of a positive PPD?   (C) Histamine and leukotrienes are liberated from mast cells at  the site.   (D) Immune complexes consisting of PPD and IgG are deposited at  the site.   3. Your patient is a 77-year-old man with enterococcal endocardi¬  tis who was treated with penicillin G and gentamicin. Five days  later, fever and a diffuse maculopapular rash developed. There is  no urticaria, hypotension, or respiratory compromise. Urinalysis  revealed proteinuria and granular casts. You suspect he may have  serum sickness. Which one of the following immunopathogenic  mechanisms is most likely to be the cause?   (B) One of the drugs activated CD4-positive T cells and  macrophages.   (C) One of the drugs activated the alternative pathway of  complement.   (D) One of the drugs cross-linked IgE on the mast cells and caused  the release of histamine.   4. Of the following diseases, which one is most likely to be caused by  a delayed hypersensitivity reaction?   5. Atopic individuals (i.e., those with a hereditary predisposition  to immediate hypersensitivity reactions) produce an increased  amount of IgE. Of the following, which one is the most likely  explanation for the increased production of IgE?   (D) Large amounts of gamma interferon are produced by Th-1 cells.   (E) Large amounts of C3a are produced by the alternative pathway  of complement.   6. Of the following four types of hypersensitivity reactions, which  one causes the hemolysis that occurs in hemolytic disease of the  newborn (erythroblastosis fetalis)?   Questions on the topics discussed in this chapter can be  found in the Immunology section of Part XIII: USMLE  (National Board) Practice Questions starting on page 733.  Also see Part XIV: USMLE (National Board) Practice Exami¬  nation starting on page 751.   Tolerance is specific immunologic unresponsiveness (i.e.,  an immune response to a certain antigen [or epitope] does  not occur, although the immune system is otherwise func¬  tioning normally). In general, antigens that are present  during embryonic life are considered “self” and do not  stimulate an immunologic response (i.e., we are tolerant to  those antigens). The lack of an immune response in the  fetus is caused by the deletion of self-reactive T-cell pre¬  cursors in the thymus (Figure 66-1). On the other hand,  antigens that are not present during the process of    maturation (i.e., that are encountered first when the body is  immunologically mature) are considered “nonself” and  usually elicit an immunologic response. Although both  B cells and T cells participate in tolerance, it is T-cell toler¬  ance that plays the primary role.   The main process by which T lymphocytes acquire the abil¬  ity to distinguish self from nonself occurs in the fetal thy¬  mus (see Chapter 58). This process, called clonal deletion,  involves the killing of T cells (“negative selection”) that   Two immature T cells  (A and B) with different  antigen receptors   Binding of self antigen  to T-cell A in thymus but  not to T-cell B   Death of self-reacting  T-cell A; survival of T-  cell B that reacts against  foreign antigen   FIGURE 66-2 Clonal anergy outside the thymus. A: B7 protein on the antigen-presenting ceil interacts with CD28 on the helperT cell,  and full activation of the helperT cell occurs. B: B7 protein on the antigen-presenting cell is not produced; therefore, CD28 on the helperT cell  does not get a costimulatory signal. Anergy of the helper T cell occurs despite interaction of the T-cell receptor (TCR) with the antigen.    react against antigens (primarily self major histocompati¬  bility complex [MHC] proteins) present in the fetus at that  time. (Note that exogenous substances injected into the  fetus early in development are treated as self.) The self¬  reactive cells die by a process of programmed cell death  called apoptosis. Tolerance to self acquired within the  thymus is called central tolerance, whereas tolerance  acquired outside the thymus is called peripheral  tolerance.   For negative selection and clonal deletion to be efficient,  the thymic epithelial cells must display a vast repertoire of  “self” proteins. A transcriptional regulator called the  autoimmune regulator (AIRE) enhances the synthesis of this  array of self proteins. Mutations in the gene encoding the  AIRE protein result in the development of an autoimmune  disease called autoimmune polyendocrinopathy. The AIRE  transcription factor also functions in the peripheral lym¬  phoid organs such as the spleen and lymph nodes, where it  contributes to peripheral tolerance.   Peripheral tolerance is necessary because some antigens  are not expressed in the thymus and therefore some self¬  reactive T cells are not killed in the thymus. There are sev¬  eral mechanisms involved in peripheral tolerance: Some  self-reactive T cells are killed, some are not activated, and  others are suppressed by regulatory T cells producing  inhibitory cytokines. Clonal anergy is the term used to  describe self-reactive T cells that are not activated because  proper costimulation does not occur (Figure 66-2). Clonal  ignorance refers to self-reactive T cells that ignore self  antigens. These self-reactive T cells are either kept ignorant  by physical separation from the target antigens (e.g., the  blood-brain barrier) or ignore self antigens because the  antigens are present in such small amounts.   Although T cells that are clonally anergic are nonreac¬  tive, they can become reactive and initiate an autoimmune  disease if conditions change later in life. The mechanism of  clonal anergy involves the inappropriate presentation of  antigen, leading to a failure of interleukin-2 (IL-2) produc¬  tion. Inappropriate presentation is due to a failure of  “costimulatory signals” (e.g., sufficient amounts of IL-1  might not be made, or cell surface proteins, such as CD28    on the T cell and B7 on the B cell, might not interact prop¬  erly, leading to a failure of signal transduction by ras pro¬  teins). For example, the inhibitory protein CTLA-4 on the  surface of the T cells may displace CD28 and interact with  B7, resulting in a failure of T-cell activation. Furthermore,  B7 is an inducible protein, and failure to induce it in suffi¬  cient amounts can lead to anergy. In addition, the costimu¬  latory proteins, CD40 on the B cell and CD40L on the  helper T cell, may fail to interact properly.   The failure of costimulatory signals most often occurs  when there is an insufficient inflammatory response at the  site of infection. The presence of microbes typically stimu¬  lates the production of proinflammatory cytokines such as  tumor necrosis factor (TNF) and IL-1. However, if the  inflammatory response is insufficient (i.e., if the adjuvant  effect of the cytokines is inadequate, the T cells will die  instead of being activated).   B cells also become tolerant to self by two mechanisms: (1)  clonal deletion, probably while the B-cell precursors are in  the bone marrow, and (2) clonal anergy of B cells in the  periphery. However, tolerance in B cells is less complete  than in T cells, an observation supported by the finding that  most autoimmune diseases are mediated by antibodies.   B cells bearing an antigen receptor for a self protein can  escape clonal deletion (apoptosis) by a process called recep¬  tor editing. In this process, a new, different light chain is  produced that changes the specificity of the receptor so that  it no longer recognizes a self protein. This reduces the risk  of autoimmune diseases and increases the repertoire of B  cells that can react against foreign proteins. It is estimated  that as many as 50% of self-reactive B cells undergo receptor  editing. T cells do not undergo receptor editing.   Whether an antigen will induce tolerance rather than an  immunologic response is largely determined by the  following:    The immunologic maturity of the host (e.g., neonatal  animals are immunologically immature and do not respond  well to foreign antigens; for instance, neonates will accept  allografts that would be rejected by mature animals).   The structure and dose of the antigen (e.g., a very sim¬  ple molecule induces tolerance more readily than a com¬  plex one, and very high or very low doses of antigen may  result in tolerance instead of an immune response). Puri¬  fied polysaccharides or amino acid copolymers injected in  very large doses result in “immune paralysis”—a lack of  response.   Other aspects of the induction or maintenance of toler¬  ance are as follows:   (1) T cells become tolerant more readily and remain  tolerant longer than B cells.   (3) Administration of immunosuppressive drugs enhances  tolerance (e.g., in patients who have received organ  transplants).   (4) Tolerance is maintained best if the antigen to which  the immune system is tolerant continues to be present.    However, in certain circumstances, tolerance may be lost  and immune reactions to host antigens may develop,  resulting in autoimmune diseases.   The most important step in the production of autoim¬  mune disease is the activation of self-reactive helper  (CD4) T cells. These self-reactive Th-1 or Th-2 cells can  induce either cell-mediated or antibody-mediated autoim¬  mune reactions, respectively. As described in Table 66-1,  most autoimmune diseases are antibody-mediated.   Many autoimmune diseases exhibit a marked familial inci¬  dence, which suggests a genetic predisposition to these  disorders. There is a strong association of some diseases  with certain human leukocyte antigen (HLA) specificities,  especially the class II genes. For example, rheumatoid  arthritis occurs predominantly in individuals carrying the  HLA-DR4 gene. Ankylosing spondylitis is 100 times more  likely to occur in people who carry HLA-B27, a class I gene,  than in those who do not carry that gene.   There are two hypotheses offered to explain the rela¬  tionship between certain HLA genes and autoimmune  diseases. One is that those genes encode class I or class II  MHC proteins that present autoantigens with greater effi¬  ciency than do the MHC proteins that are not associated  with autoimmune diseases. The other hypothesis is that  autoreactive T cells escape negative selection in the thymus   ’These diseases involve a significant cell-mediated in addition to an antibody-mediated response.    because they bind poorly to those class I or class II MHC  proteins on the surface of the thymic epithelium.   It should be noted, however, that whether a person  develops an autoimmune disease or not is clearly multi¬  factorial, because people with HLA genes known to pre¬  dispose to certain autoimmune diseases nevertheless do  not develop the disease (e.g., many people carrying the  HLA-DR4 gene do not develop rheumatoid arthritis).  That is to say, HLA genes appear to be necessary but not  sufficient to cause autoimmune diseases. In general, class  II MHC-related diseases (e.g., rheumatoid arthritis,  Graves’ disease [hyperthyroidism], and systemic lupus  erythematosus) occur more commonly in women,  whereas class I MHC-related diseases (e.g., ankylosing  spondylitis and Reiter’s syndrome) occur more com¬  monly in men.   Approximately 90% of all autoimmune diseases occur in  women. Although the explanation for this markedly  unequal gender ratio is unclear, there is some evidence  from animal models that estrogen can alter the B-cell rep¬  ertoire and enhance the formation of antibody to DNA.  Clinically, the observation that systemic lupus erythemato¬  sus either appears or exacerbates during pregnancy (or  immediately postpartum) supports the idea that hormones  play an important role in predisposing women to autoim¬  mune diseases.   There are several environmental agents that trigger autoim¬  mune diseases, most of which are either bacteria or viruses.  For example, pharyngitis caused by Streptococcus pyogenes  predisposes to rheumatic fever. Other examples are  described in Table 66-2. It is speculative at this time, but  members of the normal flora of the bowel are thought to  play a role in the genesis of inflammatory bowel diseases,  such as Crohn’s disease and ulcerative colitis.   Certain infections cause autoimmune diseases in ani¬  mals (e.g., Coxsackie virus infection in mice causes type 1  diabetes) but have not been established as a cause in  humans. Other environmental triggers include certain  drugs such as procainamide, which causes systemic lupus  erythematosus, and certain heavy metals such as gold and  mercury, which cause autoimmune diseases in experimen¬  tal animals.   There are two main mechanisms by which environmen¬  tal factors could trigger autoimmune diseases. One is  molecular mimicry, which proposes that infectious agents  possess antigens that elicit an immune response that cross-  reacts with components of human cells. The other is that  tissue injury releases intracellular (sequestered) antigens  that elicit an immune response. These mechanisms are  described in more detail in the next section.    'Other viruses, such as Epstein-Barr virus, human herpes virus-6, influenza A virus,  and measles virus, are also implicated as the possible cause of multiple sclerosis. No  virus has definitely been shown to be the environmental trigger at this time.  2 Coxsackie virus infects and kills cardiac myocytes, causing the acute symptoms, but  the late phase is caused by the attack of cytotoxic T cells on the myocytes.   3 Causes diabetes mellitus in mice, but it is uncertain whether it is a cause in humans.   In summary, the current model is that autoimmune  diseases occur in people (1) with a genetic predisposition  that is determined by their MHC genes and (2) who are  exposed to an environmental agent that triggers a cross¬  reacting immune response against some component of  normal tissue. Furthermore, because autoimmune diseases  increase in number with advancing age, another possible  factor is a decline in the number of regulatory T cells,  which allows any surviving autoreactive T cells to prolifer¬  ate and cause disease.   Various bacteria and viruses are implicated as the source of  cross-reacting antigens that trigger the activation of autore¬  active T cells or B cells. For example, Reiter’s syndrome  occurs following infections with Shigella or Chlamydia, and  Guillain-Barre syndrome occurs following infections with  Campylobacter. The concept of molecular mimicry is used to  explain these phenomena (i.e., the environmental trigger  resembles [mimics] a component of the body sufficiently that  an immune attack is directed against the cross-reacting body  component). One of the best-characterized examples of  molecular mimicry is the relationship between the M protein  of S. pyogenes and the myosin of cardiac muscle. Antibodies  against certain M proteins cross-react with cardiac myosin,  leading to the heart damage seen in rheumatic fever.    Additional evidence supporting the molecular mimicry  hypothesis includes the finding that there are identical  amino acid sequences in certain viral proteins and certain  human proteins. For example, there is an identical six-  amino acid sequence in the hepatitis B viral polymerase  and the human myelin basic protein.   Drugs can bind to normal proteins and make them immu¬  nogenic. Procainamide-induced systemic lupus erythema¬  tosus is an example of this mechanism.   Certain tissues (e.g., sperm, central nervous system, and  the lens and uveal tract of the eye) are sequestered so that  their antigens are not exposed to the immune system.  These are known as immunologically privileged sites.  When such antigens enter the circulation accidentally (e.g.,  after damage), they elicit both humoral and cellular  responses, producing aspermatogenesis, encephalitis, or  endophthalmitis, respectively. Sperm, in particular, must  be in a sequestered, immunologically privileged site,  because they develop after immunologic maturity has been  reached and yet are normally not subject to immune attack.   Intracellular antigens, such as DNA, histones, and mito¬  chondrial enzymes, are normally sequestered from the  immune system. However, bacterial or viral infection may  damage cells and cause the release of these sequestered  antigens, which then elicit an immune response. Once  autoantibodies are formed, subsequent release of seques¬  tered antigens results in the formation of immune com¬  plexes and the symptoms of the autoimmune disease. In  addition to infection, radiation and chemicals can also  damage cells and release sequestered intracellular compo¬  nents. For example, sunlight is known to exacerbate the  skin rash in patients with systemic lupus erythematosus. It  is thought that ultraviolet (UV) radiation damages cells,  which releases the normally sequestered DNA and histones  that are the major antigens in this disease.   Epitope spreading is the term used to describe the new  exposure of sequestered autoantigens as a result of dam¬  age to cells caused by viral infection. These newly exposed  autoantigens stimulate autoreactive T cells, and autoimmune  disease results. In an animal model, a multiple sclerosis-like  disease was caused by infection with an encephalomyelitis  virus. Note that the self-reactive T cells were directed  against cellular antigens rather than the antigens of the  virus.   Regulatory T cells (Tr) suppress the proinflammatory  effects of other T cells. Tr cells are characterized as CD4  positive, CD25 positive, and FoxP3 positive. An important    function of Tr cells is to produce IL-10, which inhibits pro-  inflammatory Th-1 cells. Patients with a mutation in the  FoxP3 gene have an increase in autoimmune diseases, such  as systemic lupus erythematosus, because they have lost the  function of their regulatory T cells.   Table 66-1 describes several important autoimmune dis¬  eases according to the type of immune response causing the  disease and the target affected by the autoimmune response.  Some examples of autoimmune disease are described in  more detail next.   (1) Allergic encephalitis—A clinically important exam¬  ple of allergic encephalitis occurs when people are injected  with rabies vaccine made in rabbit brains. The immune  response against the foreign myelin protein in the vaccine  cross-reacts with human myelin, leading to inflammation  of the brain. Although rare, this is a serious disease, and  rabies vaccine made in rabbit brain is no longer used in the  United States (see Chapter 39). Allergic encephalitis can  also occur following certain viral infections (e.g., measles  or influenza) or following immunizations against these  infections. These reactions are rare, and the basis for the  autoimmune reaction is uncertain. Allergic encephalitis  can be reproduced in the laboratory by injecting myelin  basic protein into a rodent’s brain, which initiates a cell-  mediated response leading to demyelination.   (2) Multiple sclerosis—In this disease, autoreactive  T cells and activated macrophages cause demyelination of  the white matter of the brain. The trigger that stimulates  the autoreactive T cells is thought to be a viral infection.  There is molecular evidence that the polymerase of  Epstein-Barr virus may be the trigger. People with certain  alleles in the HLA-DR region have an increased risk of  contracting multiple sclerosis.   The clinical findings in multiple sclerosis typically wax  and wane and affect both sensory and motor functions.  Magnetic resonance imaging (MRI) of the brain reveals  plaques in the white matter. Oligoclonal bands of IgG are  found in the spinal fluid of most patients. Immunosup¬  pressive drugs (e.g., prednisone, methotrexate, or beta  interferon) are effective in reducing the severity of some of  the symptoms.   (3) Chronic thyroiditis—When animals are injected  with thyroid gland material, they develop humoral and  cell-mediated immunity against thyroid antigens and  chronic thyroiditis. Humans with Hashimoto’s chronic thy¬  roiditis have antibodies to thyroglobulin, suggesting that  these antibodies may provoke an inflammatory process  that leads to fibrosis of the gland.   (4) Hemolytic anemias, thrombocytopenias, and  granulocytopenias—Various forms of these disorders have    been attributed to the attachment of autoantibodies to cell  surfaces and subsequent cell destruction. Pernicious ane¬  mia is caused by antibodies to intrinsic factor, a protein  secreted by parietal cells of the stomach that facilitates the  absorption of vitamin B 12 . Idiopathic throm-bocytopenic  purpura is caused by antibodies directed against platelets.  Platelets coated with antibody are either destroyed in the  spleen or lysed by the membrane attack complex of  complement.   Several drugs, acting as haptens, bind to the platelet  membrane and form a “neoantigen” that induces the cyto¬  toxic antibody that results in platelet destruction. Penicil¬  lins, cephalothin, tetracyclines, sulfonamides, isoniazid,  and rifampin, as well as drugs that are not antimicrobials,  can have this effect. Autoimmune hemolytic anemia caused  by penicillins and cephalosporins is due to the same  mechanism.   this disease, autoreactive T cells destroy the islet cells of the  pancreas. The main antigen against which the T-cell attack  is directed is the islet cell enzyme, glutamic acid decarbox¬  ylase. Infection with Coxsackie virus B4 has been shown to  be a trigger of IDDM in mice, but it is yet to be established  as a cause in human diabetes. There is a six-amino acid  sequence in common between a Coxsackie virus protein  and glutamic acid decarboxylase. Antibodies against vari¬  ous antigens of the beta cells also are produced, but the  major damage is T-cell mediated.   (6) Insulin-resistant diabetes, myasthenia gravis, and  hyperthyroidism (Graves’ disease)—In these diseases,  antibodies to receptors play a pathogenic role. In insulin-  resistant diabetes, antibodies to insulin receptors have been  demonstrated that interfere with insulin binding. In myas¬  thenia gravis, which is characterized by severe muscular  weakness, antibodies to acetylcholine receptors of neuro¬  muscular junctions are found in the serum. Muscular  weakness also occurs in Lambert-Eaton syndrome, in  which antibodies form against the proteins in calcium  channels. Some patients with Graves’ disease have circulat¬  ing antibodies to thyrotropin receptors, which, when they  bind to the receptors, resemble thyrotropin in activity and  stimulate the thyroid to produce more thyroxine.   (7) Guillain-Barre syndrome—This disease is the most  common cause of acute paralysis in the United States. It  follows a variety of infectious diseases such as viral illnesses  (e.g., upper respiratory tract infections, human immunode¬  ficiency virus [HIV] infection, and mononucleosis caused  by Epstein-Barr virus and cytomegalovirus) and diarrhea  caused by Campylobacter jejuni. Infection with C. jejuni,  either symptomatic or asymptomatic, is considered to be  the most common antecedent to Guillain-Barre syndrome.  Antibodies against membrane gangliosides are formed, com¬  plement is activated, and the membrane attack complex  destroys the myelin sheath, resulting in a demyelinating  polyneuropathy. The main symptoms are those of a rapidly    progressing ascending paralysis. The treatment involves  either intravenous immunoglobulins or plasmapheresis.   (8) Pemphigus—Pemphigus is a skin disease character¬  ized by bullae (blisters). It is caused by autoantibodies  against desmoglein, a protein in the desmosomes that  forms the tight junctions between epithelial cells in the  skin. When the tight junctions are disrupted, fluid fills the  spaces between cells and forms the bullae. One form of  pemphigus, pemphigus foliaceus, is endemic in rural areas  of South America, which lends support to the idea that  infection with an endemic pathogen is the environmental  trigger for this disease.   (9) Reactive arthritis—Reactive arthritis is an acute  inflammation of the joints that follows infection with vari¬  ous bacteria, but the joints are sterile (i.e., the inflammation  is a “reaction” to the presence of bacterial antigen elsewhere  in the body). Reactive arthritis is associated with enteric  infections caused by Shigella, Campylobacter, Salmonella,  and Yersinia and with urethritis caused by Chlamydia  trachomatis. The arthritis is usually oligoarticular and asym¬  metric. The bacterial infection precedes the arthritis by a  few weeks. People who are HLA-B27 positive are at higher  risk for reactive arthritis. Antibiotics directed against the  organism have no effect. Anti-inflammatory agents are  typically used. (Reiter’s syndrome includes a reactive arthri¬  tis, but the syndrome affects multiple organs and is  described in the next section.)   (10) Celiac disease—Celiac disease (also known as  celiac sprue and gluten enteropathy) is characterized by  diarrhea, painful abdominal distention, fatty stools, and  failure to thrive. Symptoms are induced by ingestion of  gliadin, a protein found primarily in wheat, barley, and rye  grains. Gliadin is the antigen that stimulates a cytotoxic  T-cell attack on enterocytes, resulting in villous atrophy.  Patients who carry HLA haplotype DR3-DQ2 are predis¬  posed to celiac disease. Most patients have antibodies to  tissue transglutaminase. A gluten-free diet typically results  in marked improvement.   (11) Inflammatory bowel disease (Crohn’s disease and  ulcerative colitis)—These diseases are characterized by  diarrhea, often bloody, and crampy lower abdominal pain.  These symptoms arise from chronic inflammation, primar¬  ily in the ileum in Crohn’s disease and in the colon in ulcer¬  ative colitis. It is thought that the chronic inflammation is  caused by an abnormal immune response to the presence of  normal flora of the bowel. There is evidence that a type of  helper T cell called Th-17 and IL-23 are involved in the  pathogenesis of these diseases. Natalizumab, a monoclonal  antibody against a-integrin, is effective in inducing remis¬  sion in active Crohn’s disease.   (12) IgA nephropathy—This disease is one of the most  common types of glomerulonephritis and is characterized  primarily by hematuria, but proteinuria and progression to  end-stage renal disease can occur. Immune complexes con¬  taining IgA are found lining the glomeruli. Symptoms are    temporally related to viral infections, especially pharyngi¬  tis, but no specific virus has been identified. No treatment  regimen is clearly effective. Fish oil has been tried, with  variable results.   (13) Psoriasis—Psoriasis is a chronic autoimmune skin  disease characterized by raised erythematous plaques with  silvery scales, often on the elbows or knees. Skin lesions are  the most common manifestation, but psoriatic arthritis  also occurs.   The inflammatory infiltrate in the skin lesions consist of  dendritic cells, macrophages, and T cells. Individuals with  the class I MHC protein, HLA-Cw6, are predisposed to  psoriasis. The environmental trigger is unknown.   There are many treatment modalities. Topical corticoste¬  roids and UV phototherapy with psoralen are two common  modes. Methotrexate, cyclosporine, and TNF inhi-bitors  such as etanercept and infliximab are also used. There is  evidence that monoclonal antibody against either IL-17 or  the IL-17 receptor is also effective.   (1) Systemic lupus erythematosus—In this disease,  autoantibodies are formed against DNA, histones, nucleo¬  lar proteins, and other components of the cell nucleus.  Antibodies against double-stranded DNA are the hallmark  of systemic lupus erythematosus. The disease affects pri¬  marily women between the ages of 20 and 60 years. Indi¬  viduals with HLA-DR2 or -DR3 genes are predisposed to  systemic lupus erythematosus. The agent that induces these  autoantibodies in most patients is unknown. However, two  drugs, procainamide and hydralazine, are known to cause  systemic lupus erythematosus.   Most of the clinical findings are caused by immune com¬  plexes that activate complement and, as a consequence, dam¬  age tissue. For example, the characteristic rash on the cheeks  is the result of a vasculitis caused by immune complex depo¬  sition. The arthritis and glomerulonephritis commonly seen  in systemic lupus erythematosus are also caused by immune  complexes. The immune complexes found on the glomeru¬  lus contain antibodies (IgG, IgM, or IgA) and the C3 compo¬  nent of complement but not fibrinogen. However, the  anemia, leukopenia, and thrombocytopenia are caused by  cytotoxic antibodies rather than immune complexes.   The diagnosis of systemic lupus erythematosus is sup¬  ported by detecting antinuclear antibodies (ANAs) with  fluorescent antibody tests and anti-double-stranded DNA  antibodies with enzyme-linked immunosorbent assay  (ELISA). Antibodies to several other nuclear components  are also detected, as is a reduced level of complement.  Treatment of systemic lupus erythematosus varies depend¬  ing on the severity of the disease and the organs affected.  Aspirin, nonsteroidal anti-inflammatory drugs, and corti¬  costeroids are commonly used.    (2) Rheumatoid arthritis—In this disease, autoanti¬  body is formed against IgG. This autoantibody is called  rheumatoid factor and is of the IgM class. Rheumatoid  factor is associated with rheumatoid arthritis but is not  specific for it. Rheumatoid arthritis affects primarily  women between the ages of 30 and 50 years. People with  HLA-DR4 genes are predisposed to rheumatoid arthritis.  The agent that induces rheumatoid factor is unknown.  Within the inflamed joints, the synovial membrane is infil¬  trated with T cells, plasma cells, and macrophages, and the  synovial fluid contains high levels of macrophage-pro¬  duced inflammatory cytokines such as TNF, IL-1, and IL-6.   The main clinical finding is inflammation of the small  joints of the hands and feet. Other organs, such as the  pleura, pericardium, and skin, can also be involved. Most of  the clinical findings are caused by immune complexes that  activate complement and, as a consequence, damage tissue.  The diagnosis of rheumatoid arthritis is supported by  detecting rheumatoid factors in the serum. Detection of  antibody to citrullinated peptide in the serum also supports  the diagnosis.   Treatment of rheumatoid arthritis typically involves  aspirin, nonsteroidal anti-inflammatory drugs, immuno¬  suppressive drugs (especially methotrexate), or corticoste¬  roids. Anticytokine therapy consisting of a fusion protein  of TNF receptor and the Fc fragment of human IgG (etan¬  ercept, Enbrel) is also available. The soluble TNF receptor  neutralizes TNF, which is an important inflammatory  mediator in rheumatoid arthritis. Etanercept is particularly  effective in combination with methotrexate in reducing the  severity of joint inflammation in patients with persistently  active rheumatoid arthritis. The monoclonal antibodies  infliximab (Remicade) and adalimumab (Humira) are use¬  ful for the treatment of rheumatoid arthritis. These anti¬  bodies neutralize TNF, thereby decreasing the joint  inflammation. Table 62-2 describes infliximab and other  monoclonal antibodies that have different clinical uses.   Patients who have an inadequate response to these anti-  TNF drugs have demonstrated significant improvement  with abatacept (Orencia). Abatacept is CTLA-4-IG, a  fusion protein composed of CTLA-4 and a fragment of the  Fc domain of human IgG. CTLA-4 binds strongly to B7,  which displaces CD28 from its binding to B7. This results  in a reduction of the helper T-cell activity and the inflam¬  matory response. Tocilizumab (Actemra), an antibody to  IL-6 receptor, is also used in the treatment of rheumatoid  arthritis.   (3) Rheumatic fever—Group A streptococcal infections  regularly precede the development of rheumatic fever.  Antibodies against the M protein of group A streptococci  that cross-react with myosin in cardiac muscle and proteins  in joint and brain tissue are involved in the pathogenesis of  rheumatic fever.   (4) Reiter’s syndrome—This syndrome is characterized  by the triad of arthritis, conjunctivitis, and urethritis.    Cultures of the affected areas do not reveal a causative  agent. Infection by one of the intestinal pathogens (e.g.,  Shigella, Salmonella, Yersinia, and Campylobacter), as well  as other organisms such as Chlamydia, predisposes to the  disease. Most patients are men who are HLA-B27 positive.  The pathogenesis of the disease is unclear, but immune  complexes may play a role.   (5) Goodpasture’s syndrome—In this syndrome, auto¬  antibodies are formed against the collagen in basement  membranes of the kidneys and lungs. Goodpasture’s syn¬  drome (GS) affects primarily young men, and those with  HLA-DR2 genes are at risk for this disease. The agent that  induces these autoantibodies is unknown, but GS often fol¬  lows a viral infection.   The main clinical findings are hematuria, proteinuria,  and pulmonary hemorrhage. The clinical findings are  caused by cytotoxic antibodies that activate complement.  As a consequence, C5a is produced, neutrophils are  attracted to the site, and enzymes are released by the neu¬  trophils that damage the kidney and lung tissue. The diag¬  nosis of GS is supported by detecting antibody and  complement bound to basement membranes in fluorescent  antibody test. Because this is a rapidly progressive, often  fatal disease, treatment, including plasma exchange to  remove the antibodies and the use of immunosuppressive  drugs, must be instituted promptly.   (6) Wegener’s granulomatosis—The main pathologic  finding in this disease is a necrotizing granulomatous vas¬  culitis that primarily affects the upper and lower respiratory  tracts and the kidneys. Common clinical findings include  sinusitis, otitis media, cough, sputum production, and    arthritis. Glomerulonephritis is one of the main features of  this disease. The diagnosis is supported by finding antineu¬  trophil cytoplasmic antibodies (ANCAs) in the patient’s  serum. Immunosuppressive therapy with cyclophospha¬  mide and prednisone is effective. This disease is also called  granulomatosis with polyangiitis.   (7) Other collagen vascular diseases—Other diseases  in this category include ankylosing spondylitis, which is  very common in people carrying the HLA-B27 gene,  polymyositis-dermatomyositis, scleroderma, polyarteritis  nodosa, and Sjogren’s syndrome.   The conceptual basis for the treatment of autoimmune  diseases is to reduce the patient’s immune response or  inflammatory response sufficiently to eliminate the symp¬  toms (Table 66-3). Corticosteroids, such as prednisone, are  the mainstay of treatment, to which antimetabolites, such  as methotrexate and azathioprine can be added. The anti¬  metabolites inhibit DNA synthesis in the immune cells.  Immunosuppressive therapy must be given cautiously  because of the risk of opportunistic infections.   Other approaches to therapy that do not involve sys¬  temic suppression of the immune system include antibody  to TNF and soluble receptor for TNF that acts as a decoy.  Both infliximab and adalimumab (antibody to TNF) as  well as etanercept (TNF receptor) have been shown to ame¬  liorate the joint inflammation of rheumatoid arthritis and  the skin lesions of psoriasis. However, these anti-TNF  therapies increase the risk of infections, such as activating   1. Inhibits synthesis of inflammatory mediators  such as IL-1 and TNF   Binds to CD20 on surface of B cells resulting in  apoptosis of B cells    IL-1= lnterleukin-1;TNF=Tumor necrosis factor; CTLA= Cytotoxic T-cell associated;  PML= Progressive multifocal leukoencephalopathy; HBV= Hepatitis B virus.    latent tuberculosis, serious infections caused by Legionella  and Listeria, and skin and soft tissue infections caused by  pyogenic bacteria. These drugs increase the risk of activat¬  ing latent fungal infections such as histoplasmosis as well.   The costimulation inhibitor, abatacept, consists of  CTLA-4 protein fused to the Fc region of IgG. CTLA-4  binds to B7 on the APC preventing it from binding to  CD28 on the CD-4 positive helper T cell. Binding of B7 to  CD28 is the second signal required to activate the T cell.  The binding of CTLA-4 to B7 therefore inhibits the activa¬  tion of T cells and ameliorates the symptoms of the autoim¬  mune disease.   Rituximab is a monoclonal antibody against CD20, a  protein located on the surface of B cells but not plasma  cells. Rituximab causes the death of B cells either by com¬  plement-mediated killing or by the attack of natural killer  cells. Rituximab is useful in the treatment of both autoim¬  mune diseases and B-cell cancers such as lymphomas and  leukemias.   Certain antibody-mediated autoimmune diseases, such  as Guillain-Barre syndrome and myasthenia gravis, can be  treated either with plasmapheresis, which removes autoim¬  mune antibodies, or with high doses of IgG pooled from  healthy donors. One hypothesis regarding the mode of  action of high-dose intravenous IgG is that it binds to Fc  receptors on the surface of neutrophils and blocks the  attachment of immune complexes that activate the neutro¬  phils. Another hypothesis is that excess IgG saturates the  FcRn receptor on the surface of vascular endothelial cells,  which accelerates the catabolism of IgG, thereby reducing  the level of autoimmune antibodies.   (A) Clonal deletion occurs with T cells but not with B cells.   (B) Tolerance to certain self antigens occurs by negative selection  of immature T cells in the thymus.   (C) The presence of B7 on the surface of the antigen-presenting cell  is one of the essential steps required to establish tolerance.   (D) Tolerance is easier to establish in adults than in newborns  because more self-reactive T cells have undergone apoptosis in  adults than in newborns.   (E) Once tolerance is established to an antigen, it is permanent  (i.e., that individual cannot react against that antigen even  though the antigen is no longer present).    2. Antibodies against normal components of the body typically  occur in autoimmune diseases. In which one of the following sets  of two diseases do antibodies against DNA occur in one disease  and antibodies against IgG occur in the other disease?   3. Regarding the pathogenesis of autoimmune diseases, which one of  the following is the most accurate?   (B) In myasthenia gravis, antibodies are formed against acetylcho¬  line at the neuromuscular junction.   (C) In Goodpasture’s syndrome, antibodies are formed against the  synovial membrane in the large weight-bearing joints.   (D) In autoimmune hemolytic anemia, the red cells are destroyed  by tumor necrosis factor produced by activated macrophages.   (E) In Graves’ disease, antibodies bind to the receptor for thyroid-  stimulating hormone, which stimulates the thyroid to produce  excess thyroxine.   4. Your patient is a 25-year-old woman with a fever, a malar facial  rash, alopecia, and ulcerations on two fingertips. Urinalysis shows  proteinuria. You suspect she has systemic lupus erythematosus.  Which one of the following is the most likely explanation for her  proteinuria?   (B) An IgE-mediated response releases histamine and leukotrienes  that damage the tubules.   (C) A delayed hypersensitivity response consisting of macrophages  and CD4-positive T cells damages the glomeruli.   (D) Immune complexes are trapped by glomeruli and activate  complement, then C5a attracts neutrophils that damage the  glomeruli.   Questions on the topics discussed in this chapter can be  found in the Immunology section of Part XIII: USMLE  (National Board) Practice Questions starting on page 733.  Also see Part XIV: USMLE (National Board) Practice Exami¬  nation starting on page 751.   Animals carrying a chemically or virally induced malig¬  nant tumor can develop an immune response to that tumor  and cause its regression. In the course of neoplastic trans¬  formation, new antigens, called tumor-associated anti¬  gens (TAAs), develop at the cell surface, and the host  recognizes such cells as “nonself.” An immune response  then causes the tumor to regress.   In chemically induced tumors in experimental ani¬  mals, TAAs are highly specific (i.e., cells of one tumor will  have different TAAs from those on cells of another tumor  even when they arise within the same animal). In con¬  trast, virally induced tumors possess TAAs that cross-  react with one another if induced by the same virus. TAAs  on tumor cells induced by different viruses do not  cross-react.   Cell-mediated reactions attack these nonself tumor cells  and limit their proliferation. Such immune responses prob¬  ably act as a surveillance system to detect and eliminate  newly arising clones of neoplastic cells. In general, the  immune response against tumor cells is weak and can be  overcome experimentally by a large dose of tumor cells.  Some tumor cells can escape surveillance by “modulation”  (i.e., internalizing the surface antigen so that it no longer  presents a target for immune attack).   The cell-mediated immune responses that affect tumor  cells in vitro include natural killer (NK) cells, which act  without antibody; killer (K) cells, which mediate anti-  body-dependent cytolysis (antibody-dependent cellular    cytotoxicity); cytotoxic T cells; and activated macro¬  phages. Whether these immune responses function to  prevent or control tumors in vivo is unknown.   Tumor antigens can stimulate the development of spe¬  cific antibodies as well. Some of these antibodies are  cytotoxic, but others, called blocking antibodies, enhance  tumor growth, perhaps by blocking recognition of tumor  antigens by the host. Spontaneously arising human  tumors may have new cell surface antigens against which  the host develops both cytotoxic antibodies and cell-  mediated immune responses. Enhancement of these  responses can contain the growth of some tumors. For  example, the administration of BCG vaccine (bacillus  Calmette-Guerin, a bovine mycobacterium) into surface  melanomas can lead to their partial regression. Immuno-  modulators, such as interleukins and interferons, are also  being tested in such settings. One interleukin, tumor  necrosis factor-a (cachectin), is experimentally effective  against a variety of solid tumors (see Chapter 58). In addi¬  tion, lymphocytes activated by interleukin-2 (lymphokine-  activated killer [LAK] cells) may be useful in cancer  immunotherapy.   Another approach to cancer immunotherapy involves  the use of tumor-infiltrating lymphocytes (TIL). The  basis for this approach is the observation that some can¬  cers are infiltrated by lymphocytes (NK cells and cyto¬  toxic T cells) that seem likely to be trying to destroy the  cancer cells. These lymphocytes are recovered from the  surgically removed cancer, grown in cell culture until  large numbers of cells are obtained, activated with inter-  leukin-2, and returned to the patient in the expectation  that the TIL will “home in” specifically on the cancer cells  and kill them.   Certain approaches to cancer immunotherapy are clinically  effective. For example, monoclonal antibodies directed  against CTLA-4 and PD-1 (see “Costimulation Is Required  to Activate T Cells” section in Chapter 58) are effective in  enhancing the immune response against cancer cells.  CTLA-4 and PD-1 on T cells are inhibitors of the costimu¬  latory response, and antibody against these proteins blocks  their inhibitory effect. This enhances the immune response  against the tumor. This approach is called checkpoint  immunotherapy.   Another successful approach has produced sustained  remission in patients with acute lymphoblastic leukemia.  These remissions are induced by infusion of chimeric  antigen-receptor modified T cells that target CD-19 on the  surface of the leukemic B cells. Chimeric antigen-receptor  modified T cells have produced a sustained remission in  patients with multiple myeloma as well.   Some human tumors contain antigens that normally occur  in fetal but not in adult human cells.   (1) Carcinoembryonic antigen circulates at elevated  levels in the serum of many patients with carcinoma of the  colon, pancreas, breast, or liver. It is found in fetal gut, liver,  and pancreas and in very small amounts in normal sera.  Detection of this antigen (by radioimmunoassay) is not  helpful in diagnosis but may be helpful in the management  of such tumors. If the level declines after surgery, it suggests  that the tumor is not spreading. Conversely, a rise in the  level of carcinoembryonic antigen in patients with resected  carcinoma of the colon suggests recurrence or spread of the  tumor.   (2) Alpha fetoprotein is present at elevated levels in the  sera of hepatoma patients and is used as a marker for this  disease. It is produced by fetal liver and is found in small    amounts in some normal sera. It is, however, nonspecific;  it occurs in several other malignant and nonmalignant  diseases.   Monoclonal antibodies directed against new surface  antigens on malignant cells (e.g., B-cell lymphomas) can be  useful in diagnosis. Monoclonal antibodies coupled to  toxins, such as diphtheria toxin or ricin, a product of the  Ricinus plant, can kill tumor cells in vitro and someday may  be useful for cancer therapy.   1. Regarding tumor immunity, which one of the following is the   (A) Both cytotoxic T cells and cytotoxic antibodies attack human  cancer cells.   (B) An elevated level of alpha-fetoprotein is a marker for carci¬  noma of the lung.   (C) A declining level of carcinoembryonic antigen (CEA) is an  indication that the patient’s colon cancer has recurred.   (D) Cancer cells induced by chemicals have new antigens on the  surface but cancer cells induced by viruses do not.   (E) Natural killer (NK) cells do not participate in the cell-mediated  response to cancer cells because they do not have an antigen-  specific receptor on their surface.   Questions on the topics discussed in this chapter can be found  in the Immunology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 733. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   Combined B-Cell & T-Cell Deficiencies  Complement Deficiencies  Phagocyte Deficiencies  Pattern-Recognition Receptor Deficiency   Immunodeficiency can occur in any of the four major com¬  ponents of the immune system: (1) B cells (antibody), (2)  T cells, (3) complement, and (4) phagocytes. The deficiencies  can be either congenital or acquired (Table 68-1). Clini¬  cally, recurrent or opportunistic infections are commonly  seen. Recurrent infections with pyogenic bacteria (e.g.,  staphylococci) indicate a B-cell deficiency, whereas  recurrent infections with certain fungi, viruses, or proto¬  zoa indicate a T-cell deficiency.   Very low levels of all immunoglobulins (IgG, IgA, IgM,  IgD, and IgE) and a virtual absence of B cells are found in  young boys; female carriers are immunologically normal.  Pre-B cells are present, but they fail to differentiate into  B cells. This failure is caused by a mutation in the gene  encoding tyrosine kinase, an important signal transduc¬  tion protein. Cell-mediated immunity is relatively normal.    Clinically, recurrent pyogenic bacterial infections (e.g.,  otitis media, sinusitis, and pneumonia caused by Streptococcus  pneumoniae and Haemophilus influenzae ) occur in infants  at about 6 months of age, when maternal antibody is no  longer present in sufficient amount to be protective. Treat¬  ment with pooled gamma globulin reduces the number of  infections.   IgA deficiency is the most common of these; IgG and IgM   deficiencies are rarer. Patients with a deficiency of IgA typi¬  cally have recurrent sinus and lung infections. (However,  some individuals with IgA deficiency do not have frequent  infections, possibly because their IgG and IgM levels confer  protection.) The cause of IgA deficiency maybe a failure of  heavy chain gene switching because the amounts of IgG  and IgM are normal. Patients with a deficiency of IgA  should not be treated with gamma globulin preparations,  because these patients may form antibodies against the  foreign IgA and, by cross-reaction, deplete their already  low level of IgA.   Patients with selective IgM deficiency or deficiency of  one or more of the IgG subclasses also have recurrent  sinopulmonary infections caused by pyogenic bacteria  such as S. pneumoniae, H. influenzae, or Staphylococcus  aureus.   Recurrent bacterial infections,  especially of respiratory tract,  caused by pyogenic bacteria such as  pneumococci   Recurrent infections, especially of  the sinuses and lung, caused by  pyogenic bacteria   tive recombinases, defective kinases,  absence of class II MHC proteins, or  ADA or PNP deficiency    ADA = adenosine deaminase; MHC = major histocompatibility complex; NADPH = nicotinamide adenine dinucleotide phosphate hydrogen; PNP = purine nucleoside phosphorylase.   Severe viral, fungal, or protozoal infections occur in  affected infants early in life as a result of a profound deficit  of T cells. Pneumonia caused by Pneumocystis jiroveci and  thrush caused by Candida albicans are two common infec¬  tions in these patients. Antibody production may be  decreased or normal. If decreased, severe pyogenic bacte¬  rial infections can occur.   Both the thymus and the parathyroids fail to develop  properly as a result of a defect in the third and fourth pha¬  ryngeal pouches. The most common presenting symptom  is tetany due to hypocalcemia caused by hypoparathyroid¬  ism. Other congenital abnormalities are common. A trans¬  plant of fetal thymus may reconstitute T-cell immunity. A  thymus from a child older than 14 weeks should not be  used, because a graft-versus-host reaction may occur.   In this disease, the skin and mucous membranes of children  are infected with C. albicans, which in immunocompetent    individuals is a nonpathogenic member of the normal flora.  These children have a T-cell deficiency specifically for this  organism; other T-cell and B-cell functions are normal.  Treatment consists primarily of antifungal drugs.   In this syndrome, severe, recurrent pyogenic bacterial  infections resembling those seen in X-linked hypogam¬  maglobulinemia begin early in life. Patients have a high  concentration of IgM but very little IgG, IgA, and IgE.  They have normal numbers of T cells and B cells.  Although the main manifestations of this syndrome are  alterations in antibodies, the mutation is in the gene  encoding the CD40 ligand (CD40L) in the CD4-positive  helper T cells. As a result, the helper T cells have a defect  in the surface protein CD40L that interacts with CD40 on  the B-cell surface. The failure to properly interact with  CD40 results in an inability of the B cell to switch from  the production of IgM to the other classes of antibodies.  Treatment with pooled gamma globulin results in fewer  infections.   Patients with a deficiency of interleukin (IL)-12 receptor  have disseminated mycobacterial infections. The absence  of the receptor prevents IL-12 from initiating a Th-1  response, which is required to limit mycobacterial  infections.   Recurrent infections caused by bacteria, viruses, fungi, and  protozoa occur in early infancy (3 months of age) because  both B cells and T cells are defective. In some children,  the B and T cells are absent; in others, the number of cells  is normal but they do not function properly. Immuno¬  globulin levels are very low, and tonsils and lymph nodes  are absent. Note that innate immunity is normal.   Pneumocystis pneumonia is the most common presenting  infection in these infants. Infections caused by C. albicans  and viruses such as varicella-zoster virus, cytomegalovirus,  and respiratory syncytial virus are common and often fatal.   This is a group of inherited diseases, all of which are due  to a defect in the differentiation of an early stem cell. There  are two types: X-linked and autosomal; the X-linked form  constitutes about 75% of cases. Some patients with X-linked  SCID have a defect in the IL-2 receptor on T cells. They  lack the y chain of the IL-2 receptor that is essential for the  development of T cells. This is the most common form of  SCID in the United States. The chain of the IL-2 receptor is  shared with the IL-7 receptor, so SCID can also result from  a failure of IL-7 to stimulate the progression of stem cells  into T cells and B cells.   Some patients with the autosomal form have a mutation  in the gene encoding a tyrosine kinase called ZAP-70 that  plays a role in signal transduction in T cells. Another auto¬  somal form has mutations in the gene for a different kinase  called Janus kinase 3. Other SCID patients with the autoso¬  mal form have a mutation in the RAG-1 or RAG-2 genes  that encode the recombinase enzymes that catalyze the  recombination of the DNA required to generate the T-cell  antigen receptor and the IgM monomer on the B cell that  acts as the antigen receptor.   Because immunity is so profoundly depressed, these chil¬  dren must be protected from exposure to microorganisms,  usually by being enclosed in a plastic “bubble”. Live, attenu¬  ated viral vaccines should not be given. Bone marrow trans¬  plantation may restore immunity. It is interesting that because  infants with SCID do not reject allografts, bone marrow  transplants do not require immunosuppressive drugs.   Patients with a hereditary absence of adenosine deam¬  inase (ADA) and purine nucleoside phosphorylase  (PNP) can have a severe deficiency of B cells and T cells,  causing SCID, although some have only mild dysfunction.  The absence of these enzymes results in an accumulation of  deoxyadenosine triphosphate (dATP), an inhibitor of    ribonucleotide reductase, and a consequent decrease in the  deoxynucleoside triphosphate precursors of DNA. This  reduces the formation of B-cell and T-cell precursors in the  bone marrow. Bone marrow transplantation can be helpful.  Injections of ADA conjugated to polyethylene glycol reduce  the number and severity of infections. Several patients with  ADA deficiency have benefited from gene therapy. A retro¬  viral vector carrying a normal copy of the ADA gene was  allowed to infect the patient’s bone marrow cells. The ADA  gene functioned within some of these cells, and the patient’s  immune status improved.   Patients with bare lymphocyte syndrome exhibit the  signs and symptoms of SCID and are especially susceptible  to viral infections. These patients have defective class I or  class II major histocompatibility complex (MHC) pro¬  teins or both. Mutations resulting in the inability to synthe¬  size a transcription factor required for the synthesis of the  mRNA for class II MHC proteins are an important cause of  the failure to produce those proteins. Mutations in the gene  encoding the TAP protein have been identified as one cause  of the inability to display antigens on class I MHC proteins.  (The TAP transporter protein is described on page 508.)   Recurrent pyogenic infections, eczema, and bleeding caused  by thrombocytopenia characterize this syndrome. These  symptoms typically appear during the first year of life. It is  an X-linked disease and thus occurs only in male infants.  The most important defect is the inability to mount an  IgM response to the capsular polysaccharides of bacteria,  such as pneumococci. IgG levels and IgA levels are normal,  but cell-mediated immunity is variable. The defect appears  to be in the ability of T cells to provide help to B cells. The  mutant gene encodes a protein involved in actin filament  assembly. Bone marrow transplantation may be helpful.   In this disease, ataxia (staggering), telangiectasia (enlarged  small blood vessels of the conjunctivas and skin), and  recurrent infections appear by 2 years of age. It is an auto¬  somal recessive disease caused by mutations in the genes  that encode DNA repair enzymes. Lymphopenia and IgA  deficiency commonly occur. Treatment designed to correct  the immunodeficiency has not been successful.   This is an uncommon autosomal dominant disease caused  by a deficiency of Cl inhibitor. In the absence of inhibitor,  Cl continues to act on C4 to generate C4a and subse¬  quently additional vasoactive components such as C3a and  C5a. This leads to capillary permeability and edema in  several organs. Laryngeal edema can be fatal. Steroid drugs,  such as oxymetholone and danazol, can be useful in  increasing the concentration of Cl inhibitor.   Patients with deficiencies in Cl, C3, or C5 or the later com¬  ponents C6, C7, or C8 have an increased susceptibility to  bacterial infections. Patients with C3 deficiency are partic¬  ularly susceptible to sepsis with pyogenic bacteria such as  S. aureus. Those with reduced levels of C6, C7, or C8 are  especially prone to bacteremia with Neisseria meningitidis  or Neisseria gonorrhoeae.   Patients with C2 and C4 deficiencies have diseases resem¬  bling systemic lupus erythematosus or other autoimmune  diseases. C2 deficiency is the most common complement  defect and is frequently asymptomatic.   This rare disease is characterized by episodes of brownish  urine (hemoglobinuria), particularly upon arising. The  hemoglobinuria is due to complement-mediated hemoly¬  sis. This occurs especially at night because the lower oxy¬  gen concentration (and low pH) in the blood during sleep  increases the susceptibility of the red cells to lyse. Hemo¬  lysis occurs because there is a reduced amount of decay-  accelerating factor (DAF) bound to the surface of red  blood cells, leading to an increased activation of comple¬  ment (see Chapter 63). These patients have a defect in the  gene for the molecules that anchor DAF and other pro¬  teins to the cell membrane. There is no specific treatment.  Iron can be given for the anemia, and prednisone can be  helpful.   Patients with this disease are very susceptible to opportu¬  nistic infections with certain bacteria and fungi (e.g.,  S. aureus); enteric gram-negative rods, especially Serratia and  Burkholderia; and Aspergillus fumigatus. Recurrent infections  with catalase-positive bacteria, such as staphylococci,  are common in these patients, whereas infections with  catalase-negative bacteria, such as streptococci, are rare.  Viral, mycobacterial, and protozoal infections are not a  major concern. In 60% to 80% of cases, this is an X-linked  disease that appears by the age of 2 years. (In the remaining  patients, the disease is autosomal.)   CGD is due to a defect in the intracellular microbici¬  dal activity of neutrophils as a result of a lack of NADPH  oxidase activity (or similar enzymes). As a result, no  hydrogen peroxide or superoxides are produced (i.e., no  oxidative burst occurs), and the organisms, although  ingested, are not killed. B-cell and T-cell functions are usu¬  ally normal.   In the laboratory, diagnosis can be confirmed by the  nitroblue tetrazolium (NBT) dye reduction test or by the  dichlorofluorescein (DCF) test. In the NBT test, normal    neutrophils will turn the dye a blue color while the neutro¬  phils of a patient with CGD fail to produce the blue color.   The DCF test is the more informative of the two because  the analysis is done by flow cytometry, which provides  information regarding the oxidative ability of single cells.  For example, in the mothers of boys with CGD who are  carriers, half of their neutrophils show normal oxidative  activity because the X chromosome carrying the mutant  gene has been inactivated, whereas the other half show no  oxidative activity because the X chromosome carrying the  normal gene has been inactivated.   Prompt, aggressive treatment of infection with the  appropriate antibiotics is important. Chemoprophylaxis  using trimethoprim-sulfamethoxazole can reduce the  number of infections. Gamma interferon significantly  reduces the frequency of recurrent infections, probably  because it increases phagocytosis by macrophages.   The name chronic granulomatous disease arises from the  widespread granulomas seen in these patients, even in the  absence of clinically apparent infection. These granulomas  can become large enough to cause obstruction of the stom¬  ach, esophagus, or bladder. The cause of these granulomas  is unknown.   In this autosomal recessive disease, recurrent pyogenic infec¬  tions, caused primarily by staphylococci and streptococci,  occur. This is due to the failure of the lysosomes of neutro¬  phils to fuse with phagosomes. The degradative enzymes in  the lysosomes are, therefore, not available to kill the ingested  organisms. Large granular inclusions composed of abnormal  lysosomes are seen. In addition, the neutrophils do not func¬  tion correctly during chemotaxis as a result of faulty micro¬  tubules. The mutant gene in this disease encodes a  cytoplasmic protein involved in protein transport. Peroxide  and superoxide formation is normal, as are B-cell and T-cell  functions. Treatment involves antimicrobial drugs. There is  no useful therapy for the phagocyte defect.   The main immunologic defect is a failure to produce  gamma interferon by helper T cells, which reduces the  ability of macrophages to kill bacteria. This leads to an  increase in Th-2 cells and, as a consequence, a high IgE  level. The increased IgE causes histamine release, which  blocks certain aspects of the inflammatory response, hence  the cold abscesses. Histamine also inhibits neutrophil che¬  motaxis, another feature of this syndrome. Treatment con¬  sists of antimicrobial drugs.    1 “Cold” refers to the absence of inflammation of the lesions (i.e., the  lesions are not warm and red).   Patients with this syndrome have severe pyogenic infec¬  tions early in life because they have defective adhesion  (LFA-1) proteins on the surface of their phagocytes. This  is an autosomal recessive disease in which there is a muta¬  tion in the gene encoding the (3 chain of an integrin that  mediates adhesion. As a result, neutrophils adhere poorly  to endothelial cell surfaces, and phagocytosis of the bacte¬  ria is inadequate.   In this autosomal dominant disease, patients have a very low  neutrophil count (less than 200/pL) for 3 to 6 days of a 21 -day  cycle. During the neutropenic stage, patients are susceptible  to life-threatening bacterial infections, but when neutrophil  counts are normal, patients are not susceptible. Mutations in  the gene encoding neutrophil elastase have been identified in  these patients, but it is unclear how these contribute to the  cyclic nature of the disease. It is hypothesized that irregular  production of granulocyte colony-stimulating factor may  play a role in the cyclic aspect of the disease.   Deficiency of myeloperoxidase (either reduced amount or  reduced function) is quite common but has little clinical  importance. Surprisingly, most patients with this defi¬  ciency do not have a significant increase in infectious dis¬  eases. Myeloperoxidase catalyzes the production of  hypochlorite, an important microbicidal agent, so an  increase in infections would be expected. However, other  intracellular killing mechanisms are intact and must be suf¬  ficient to kill the ingested microbes.   Patients with this deficiency have severe infections with  atypical mycobacteria or with bacillus Calmette-Guerin  (BCG), the attenuated mycobacterium in the BCG vaccine.  They have a mutation in the gene encoding either the  ligand-binding portion or the signal-transducing portion  of the receptor for interferon-gamma. As a result,    macrophages are not activated, and severe mycobacterial  infections occur. Defects in the production of IL-12 or in  the receptor for IL-12 cause the same clinical picture.   Mutations in the genes encoding the pattern-recognition  receptors (PRRs) on the surface of and within the cells of  the innate immune system result in susceptibility to severe  infections (Table 68-2). For more information on PRRs, see  the “Innate Immunity” section in Chapter 57.   Deficiency of Toll-like receptor-5 (TLR-5) results in a failure to  recognize flagellin on bacteria and a marked susceptibility to  Legionella infections. This deficiency is quite common. Defi¬  ciency of mannose-binding ligand (MBL) is also common. It  results in a failure to activate complement, leading to severe  infections by gram-negative bacteria such as Neisseria.   NOD receptors in the cytoplasm recognize the peptidogly-  can of gram-positive and gram-negative bacteria. Defi¬  ciency of NOD-2 results in a defect in gut immunity that is  involved in the pathogenesis of Crohn’s disease. RIG heli-  case receptors recognize viral double-stranded RNAs syn¬  thesized during replication in the cytoplasm. Deficiency of  these receptors results in a reduced interferon response to  various viruses (i.e., influenza virus).   Patients present with recurrent infections caused by pyo¬  genic bacteria (e.g., sinusitis and pneumonia caused by  pyogenic bacteria such as S. pneumoniae and H. influenzae).  The infections usually occur in persons between the ages of  15 and 35 years. The number of B cells is usually normal,   Reduced interferon response and susceptibility to  various viruses such as influenza virus    but the ability to synthesize IgG (and other immunoglobulins)  is greatly reduced. Cell-mediated immunity is usually  normal. The cause of the failure to produce IgG is  unknown but appears to be due to defective T-cell signaling.  Intravenous gamma globulin given monthly reduces the  number of infections.   Severe malnutrition can reduce the supply of amino acids  and thereby reduce the synthesis of IgG. This predisposes  to infection by pyogenic bacteria.   Patients with acquired immunodeficiency syndrome  (AIDS) present with opportunistic infections caused by  certain bacteria, viruses, fungi, and protozoa (e.g., Myco¬  bacterium avium-intracellulare, herpesviruses, C. albicans,  and P. jiroveci). This is due to greatly reduced helper T-cell  numbers caused by infection with the retrovirus human  immunodeficiency virus (HIV; see Chapter 45). This virus  specifically infects and kills cells bearing the CD4 protein  as a surface receptor. The response to specific immuniza¬  tions is poor; this is attributed to the loss of helper T-cell  activity. AIDS patients also have a high incidence of tumors  such as lymphomas, which may be the result of a failure of  immune surveillance. See Chapter 45 for information on  treatment and prevention.   Patients with measles have a transient suppression of  delayed hypersensitivity as manifested by a loss of PPD skin  test reactivity. Quiescent tuberculosis can become active. In  these patients, T-cell function is altered but immunoglobu¬  lins are normal.   Liver failure caused by alcoholic cirrhosis or by chronic  hepatitis B or hepatitis C can reduce the synthesis of com¬  plement proteins by the liver to a level that severe pyogenic  infections can occur.   Severe malnutrition can reduce the supply of amino acids  and thereby reduce the synthesis of complement proteins  by the liver. This predisposes to infection by pyogenic  bacteria.   Patients with neutropenia present with severe infections  caused by pyogenic bacteria such as S. aureus and    S. pneumoniae and enteric gram-negative rods. Neutrophil  counts below 500/pL predispose to these infections. Com¬  mon causes of neutropenia include cytotoxic drugs, such as  those used in cancer chemotherapy; leukemia, in which the  bone marrow is “crowded out” by leukemic cells; and auto¬  immune destruction of the neutrophils. Ciprofloxacin is  used to try to prevent infections in neutropenic patients.   The predominant finding in patients with chronic fatigue  syndrome (CFS) is persistent, debilitating fatigue that has  lasted for at least 6 months and is not relieved by bed rest.  Because fatigue is a nonspecific symptom, all other causes  of fatigue, including physical (e.g., cancer, autoimmune  disease, and infection) and psychiatric (e.g., depression  and neurosis), as well as prolonged use of drugs (e.g.,  tranquilizers), must be ruled out. The cause of CFS is  unknown; attempts to isolate a causative organism from  these patients have failed. A proposed relationship  between CFS and chronic Epstein-Barr virus infection  remains unsubstantiated.   There is a similarity between the symptoms of CFS and  the symptoms that occur when alpha interferon or IL-2 is  administered to patients. Abnormalities in various compo¬  nents of the immune system have been reported (e.g., loss of  delayed hypersensitivity reactivity in skin tests and increased  levels of cytotoxic T cells), but no definitive findings have  emerged. There is no specific laboratory test for CFS. The  approach to therapy involves treating the symptoms. Treat¬  ment with various antimicrobial drugs, such as acyclovir,  ketoconazole, and gamma globulin, had no effect.   1. Your patient is a 2-year-old boy who has had several episodes of  pustules and lymphadenitis caused by Staphylococcus aureus. His  immunoglobulins and complement levels are normal. A nitroblue  tetrazolium test reveals defective cells. Which one of the following  cells is the most likely to be defective?   2. Your patient is a 25-year-old woman who has had several seri¬  ous episodes of bacterial pneumonia in the last 5 months. She has  not had frequent or unusual infections prior to the onset of these  pneumonias. Which one of the following is the most likely immu¬  nodeficiency that predisposes her to these infections?   (A) She is likely to have a defect in her cytotoxic T cells.   (B) She is likely to have a reduced level of immunoglobulins.   (C) She is likely to have a mutation in the gene that encodes the  C3a portion of complement.   (D) She is likely to have a mutation in one of the genes that encode  the class I MHC proteins.   (A) There is very little IgG produced, but IgM and IgA levels are  normal.   (C) The number of B cells is normal, but they cannot differentiate  into plasma cells.   (D) There is a mutation in the gene for tyrosine kinase, an impor¬  tant enzyme in the signal transduction pathway in B cells.   4. Which one of the following is the most accurate description of the  defect in chronic granulomatous disease?   (D) There is a mutant protein kinase in a signal transduction  pathway.   (E) There is a mutation in the gene that encodes a class II MHC  protein.   5. Which one of the following immunodeficiencies is most likely to  predispose to both pyogenic bacterial infections and viral infec¬  tions in a young child?   6. Regarding immunodeficiency diseases, which one of the following  is most accurate?   (A) Patients who have a deficiency of IgA have a high incidence of  pyogenic infections of the sinuses and lungs.   (B) Common variable hypogammaglobulinemia typically occurs  in boys under the age of 6 months and results from a virtual  absence of B cells.    (C) In Wiskott-Aldrich syndrome, the combination of antibody  deficiency and complement deficiency leads to disseminated  viral and fungal infections.   (D) Patients with DiGeorges syndrome (congenital thymic aplasia)  have a reduced number of both T cells and B cells and have  severe infections caused by pyogenic bacteria.   (E) Patients who cannot produce one or more of the late-acting  complement components, such as C6, C7, C8, or C9, have epi¬  sodes of angioedema, including laryngeal edema that can be  fatal.   Questions on the topics discussed in this chapter can be found  in the Immunology section of Part XIII: USMLE (National  Board) Practice Questions starting on page 733. Also see Part  XIV: USMLE (National Board) Practice Examination starting  on page 751.   Ectoparasites are organisms that are found either on the  skin or only in the superficial layers of the skin. Ecto is a  prefix meaning “outer.” Virtually all ectoparasites are  arthropods; that is, they are invertebrates with a chitinous  exoskeleton.   The ectoparasites that cause human disease fall into two  main categories: insects (six-legged arthropods) and arach¬  nids (eight-legged arthropods). The ectoparasites discussed  in this chapter include insects such as lice, flies, and bed¬  bugs and arachnids such as mites, ticks, and spiders.   Many arthropods are vectors that transmit the organ¬  isms that cause important infectious diseases. A well-  known example is the Ixodes tick that transmits Borrelia  burgdorferi, the cause of Lyme disease. Table XII-3    describes the medically important vectors. However, in this  chapter, the arthropods are discussed not as vectors, but as  the cause of the disease itself. Table 69-1 summarizes the  common features of diseases caused by the medically  important ectoparasites that are described in this chapter.  Ectoparasites of minor medical importance are briefly  described at the end of the chapter.   Pediculosis is caused by two species of lice: Pediculus  humanus and Phthirus pubis. P. humanus has two subspe¬  cies: Pediculus humanus capitus (head louse), which   Pruritic, painful, and erythematous nodule; larva may be seen emerging  from nodule    primarily affects the scalp, and Pediculus humanus corporis  (body louse), which primarily affects the trunk. Phthirus  pubis (pubic louse) primarily affects the genital area, but  the axilla and eyebrows can be involved as well.   Note that the body louse is the vector for several human  pathogens, notably Rickettsia prowazekii, the cause of epi¬  demic typhus, whereas the head louse and the pubic louse  are not vectors of human disease.   Lice are easily visible, being roughly 2 to 4 mm long. They  have six legs armed with claws by which they attach to the  hair and skin (Figure 69-1). Pediculus has an elongated   gated abdomen of Pediculus corporis. In contrast, the pubic louse has  a short "crab-like" abdomen. (Source: Dr. F. Collins, Public Health Image Library,  Centers for Disease Control and Prevention.)    body, whereas Phthirus has a short body and resembles a  crab, and hence its nickname, the crab louse (Figure 69-2).  People infected with Phthirus are said to have “crabs”.   Nits are the eggs of the louse and are typically found  attached to the hair shaft (Figure 69-3). They are white and  can be seen with the naked eye. Nits of the body louse are  often attached to the fibers of clothing.   Head lice are transmitted primarily by fomites such as hats,  combs, and towels. These are especially common in school  children. Body lice live primarily on clothing and are trans¬  mitted either by clothing or by personal contact. Body lice  leave the clothing when they require a blood meal. Pubic  lice are transmitted primarily by sexual contact.   Widespread infestations of body lice occur when per¬  sonal hygiene is poor (e.g., during wartime or in crowded  refugee camps).   rounded "crab-like" abdomen of Phthirus pubis. In contrast, the body  louse has an elongated abdomen. (Source: Public Health Image Library,  Centers for Disease Control and Prevention. Original content provider is the World  Health Organization.)   FIGURE 69-3 Pediculus capitis —egg case (nit). Arrow points  to an egg case (also known as a nit) attached to hair shaft. Note  embryo within egg. (Source: Dr. D. Juranek, Public Health Image Library, Centers  for Disease Control and Prevention.)   Adult lice feed on blood and, in the process, inject saliva  into the skin, which induces a hypersensitivity reaction  and, as a consequence, pruritus.   Pruritus is the main symptom. Excoriations may result  from scratching, and secondary bacterial infections may  occur. In pediculosis capitis, the adult lice are often difficult  to see, but the nits are easily visualized. In pediculosis cor¬  poris, the adult lice are primarily in the clothing rather than  on the body. In pediculosis pubis, the adult lice and nits can  be seen attached to the pubic hair.   The laboratory is not involved in diagnosis. Nits fluoresce  under ultraviolet light of a Wood’s lamp, which can be used  to screen the hair of large numbers of people.   Permethrin (Nix, RID) is the treatment of choice, as it is  both pediculicidal and ovicidal. However, resistance to  permethrin is increasing. Ivermectin (Sklice) is also effec¬  tive, and resistance has not been reported. Nits are removed  using a fine-toothed (nit) comb. Patients with body lice  often do not need to be treated, but the clothing should be  either discarded or treated.   Children should not share articles of clothing. Many  schools have a policy that children cannot attend school  until they are nit-free, but the need for this exclusionary  approach is under review. The personal items of affected  individuals, such as towels, combs, hair brushes, clothing,    and bedding, should be treated. Sexual partners of those  infested with pubic lice should be treated and tested for  other sexually transmitted diseases.   Myiasis is caused by the larva of many species of flies, but  the one best known is the botfly, Dermatobia hominis. Fly  larvae are also known as maggots. Note that maggots are  occasionally used to debride nonhealing wounds, but these  maggots do not cause myiasis.   The flies that cause myiasis are found worldwide and infest  many animals as well as humans. Human infestation occurs  most often in tropical areas. Dermatobia is common in  Central and South America.   The precise route of transmission varies depending on the  species of fly. In one scenario, the adult fly deposits its egg  in a wound and the egg hatches to produce the larva. In  another, the fly deposits the egg in the nostrils, the con¬  junctiva, or on the lips. In yet another, the fly deposits the  egg on unbroken skin and the larva invades the skin.   Dermatobia is especially interesting in that it deposits its  egg on a mosquito. When the mosquito bites a human, the  warmth of the skin induces the egg to hatch and the larva  enters the skin at the site of the mosquito bite.   The presence of the larva in tissue induces an inflammatory  response.   The characteristic lesion is a painful, erythematous papule  resembling a furuncle (Figure 69-4). The lesion may also  be pruritic. The larva can often be seen within a central  pore. Some patients report a sense of movement within the  lesion. A history of travel to tropical regions is commonly  elicited. Cutaneous myiasis is the most common form but  ocular, intestinal, genitourinary, and cerebral forms occur.   The laboratory is not involved in diagnosis except when  identification of the larva is needed.   Surgical removal of the larva is the most common mode of  treatment. If the larva is visible, manual extraction can be  performed. If the larva is not visible, the central pore can be   FIGURE 69-4 Dermatobia hominis —myiasis. Botfly larva  emerging from center of erythematous nodular skin lesion. Insert  shows intact larva. (Reproduced with permission from Goldsmith LA, Katz SI et  al (eds): Fitzpatrick's Dermatology in General Medicine, 8th ed. New York: McGraw-Hill,  2012. Copyright © 2012 by The McGraw-Hill Companies, Inc.)    covered with petroleum jelly, thus causing anoxia in the  larva. This induces the larva to migrate to the surface.   Prevention involves limiting exposure to flies, especially in  tropical areas. General measures, such as wearing clothing  that covers the extremities, mosquito netting, and insect  repellant, are recommended.   FIGURE 69~6 Bedbug bites—several urticarial wheals  surrounded by erythema on the patient's back. (Reproduced with permission  from Goldsmith LA, Katz SI et al (eds). Fitzpatrick's Dermatology in General Medicine.  8th ed. New York: McGraw Hill, 2012. Copyright © 2012 by The McGraw-Hill  Companies, Inc.)    histamine-related hypersensitivity reaction to proteins in  the bug saliva (Figure 69-6). Some individuals show little  reaction. The bite of a bedbug is not known to transmit any  human disease. Calamine lotion can be used to relieve the  itching. Malathion or lindane can be used to treat mat¬  tresses and beds.   Cimex lectularius is the most common bedbug found in the  United States. It has an oval, brownish body and is about 5  mm long (Figure 69-5). Bedbugs reside in mattresses and  in the crevices of wooden beds. At night, they emerge to  take a blood meal from sleeping humans. The main  symptom of a bedbug bite is a pruritic wheal caused by a   FIGURE 69-5 Cimex lenticularis —bedbug. Bedbug in the pro¬  cess of ingesting blood from skin. They are wingless, reddish-brown,  and about 5 mm long. (Source: Public Health Image Library, Centers for Disease  Control and Prevention.)   The adult female Sarcoptes mite is approximately 0.4 mm  in length, with a rounded body and eight short legs  (Figure 69-7). It is found worldwide, and it is estimated  that several hundred million people are affected around  the globe.   It is transmitted by personal contact or by fomites such as  clothing, especially under unhygienic conditions (e.g., in  the homeless and during wartime). It is not a vector for  other human pathogens.   The pruritic lesions result from a delayed hypersensitivity  reaction to the feces of the mite. The mite is located within  the stratum corneum of the epidermis.   The typical lesions in immunocompetent people are either  tracks or papules that are very pruritic (Figure 69-8). The  most common sites are the hands, wrists, axillary folds, and   FIGURE 69~7 Sarcoptesscabiei —"itch"mite. Long arrow points  to the mouth. Short arrow points to one of the eight legs. This is a ven¬  tral view. (Source: Public Health Image Library, Centers for Disease Control and Pre¬  vention; donated by the World Health Organization, Geneva, Switzerland.)    genitals. Areas of the body where clothing is tight, such as  along the belt line, are often involved. The itching is typi¬  cally worse at night.   In immunocompromised individuals, an extensive  crusted dermatitis (Norwegian scabies) can occur. These  patients may be infested with thousands of mites. Excoria¬  tions may become infected with Staphylococcus aureus or  Streptococcus pyogenes, resulting in pyoderma.   Microscopic examination of skin scrapings reveals the  mites, their eggs, or fecal pellets.   that point to linear track-like lesions on the hand. (Reproduced with  permission from Wolff K, Johnson R. Fitzpatrick's Color Atlas & Synopsis of Clinical  Dermatology. 6th ed. New York: McGraw-Hill, 2009. Copyright © 2009 by The  McGraw-Hill Companies, Inc.)   Permethrin (Elimite) is the drug of choice. Topical steroids  are used to relieve the itching.   Prevention involves treatment of close contacts of the  patient and treating or discarding fomites such as clothing  and towels.   Tick paralysis is caused by many species of ticks, the most  common of which in the United States are Dermacentor  species. Ticks are vectors for several human diseases,  including Lyme disease and Rocky Mountain spotted fever,  but in this chapter, we discuss only tick paralysis, which is  caused by a toxin produced by the tick itself.   Female ticks require a blood meal for maturation of their  eggs, and hence it is the female that causes tick paralysis as  well as serves as the vector of diseases. Ticks are commonly  found in grassy woodland areas and are attracted by carbon  dioxide and warmth from humans. A tick attaches to  human skin by means of its proboscis.   Dermacentor andersoni, the wood tick, is more common  in the western United States, whereas Dermacentor variabilis,  the dog tick, is more common in the eastern states  (Figure 69-9). Both species can cause tick paralysis. Presently  in the United States, there are no cases of paralysis caused   FIGURE 69-9 Dermacentor tick. This tick causes tick paralysis  and is the vector that transmits Rickettsia rickettsiae, the cause of  Rocky Mountain spotted fever. (Source: Dr. Christopher Paddock, Public  Health Image Library, Centers for Disease Control and Prevention.)    by Ixodes ticks; however, such cases are reported in other  countries, especially Australia.   Paralysis is mediated by a neurotoxin that blocks acetyl  choline release at the neuromuscular junction—an action  similar to that of botulinum toxin. The toxin is made in the  salivary gland of the tick. The tick must remain attached for  at least 4 days prior to the onset of symptoms.   An ascending paralysis resembling Guillain-Barre syn¬  drome occurs. Ataxia is an early presenting symptom. The  paralysis is symmetrical and can ascend from the legs to the  head within several hours. Respiratory failure and death  can occur. Recovery typically occurs within 24 hours of  removal of the tick.   The tick is often found at the hairline at the back of the  neck or near the ear. Children younger than 8 years are  most often affected.   Tick bites can be prevented by application of insect repel-  lant and wearing clothes that cover the extremities. Search¬  ing for and removing ticks promptly is an important  preventive measure.   Two species of spiders cause most of the significant disease  in the United States, namely the black widow spider ( Latro-  dectus mactans ) and the brown recluse spider (Loxosceles  reclusa). The black widow spider is about 1 cm in length  with a characteristic orange-red hourglass on its ventral  surface (Figure 69-10). The brown recluse spider is also  about 1 cm in length, but has a characteristic violin-shaped  pattern on its dorsal surface (Figure 69-11). It is also called  the “fiddleback” spider.   The bite of the black widow spider causes neurologic symp¬  toms primarily. Within an hour after the bite, pain and  numbness spread from the site. Severe pain and spasms in  the extremities and abdominal pain occur. Fever, chills,  sweats, vomiting, and other constitutional symptoms can  occur. In contrast to the bite of the brown recluse spider,  tissue necrosis does not occur. Most patients recover in   FIGURE 69-10 Latrodectus mactans (black widow spider).  Note red "hourglass" on ventral surface. (Source: Dr. Paula Smith, Public  Health Image Library, Centers for Disease Control and Prevention.)    several days, but some, mainly children, die. Antiserum, if  available, to the venom of the black widow should be given  in severe cases. The antiserum is made in horses so testing  for hypersensitivity to horse serum should be performed.   The bite of the brown recluse spider causes tissue necrosis  symptoms primarily. The necrosis is due to proteolytic  enzymes in the venom. Pain and pruritus at the site of the  bite occur early, followed by vesicles and then hemorrhagic  bullae (Figure 69-12). The lesion ulcerates, becomes  necrotic, and may not heal for weeks to months. Skin graft¬  ing may be required. Antiserum to the venom of the brown  recluse spider is not available in the United States.   FIGURE 69-11 Loxosceles reclusa (brown recluse spider). Note  "violin" shape on dorsal surface of thorax. (Source: Dr. Andrew J. Brooks,  Public Health Image Library, Centers for Disease Control and Prevention.)   Dr. M.A. Parsons and donated by Dr. G. Rosenfeld, Head Hospital Vital, of Department  of Physiopathology, Brazil; Public Health Image Library, Centers for Disease Control  and Prevention.)   Demodex mites are also known as hair follicle or eyelash  mites. They cause folliculitis, especially on the eyelashes  (blepharitis) and on the face. They block the follicles, caus¬  ing an inflammatory response and loss of eyelashes. Dry  eyes and chalazions can occur. They are implicated as a  cause of rosacea-like lesions on the face.   These mites are very small. As many as 25 mites have  been found in one hair follicle. The diagnosis is made by  observing the mite with a slit-lamp biomicroscope. Treat¬  ment involves careful debridement of the affected areas  plus application of tea tree oil ointment.    skin at the time of the bite. The larvae are not blood¬  sucking but obtain nutrients from dissolved skin cells. They  are found in vegetation in hot, humid regions, such as  southeastern states of the United States.   Dermatophagoides mites are also known as house dust  mites. They feed on exfoliated human skin cells. They do  not cause disease in humans directly, but proteins in their  feces are powerful allergens for some people. Small parti¬  cles in house dust can become airborne, be inhaled, and  induce asthma and atopic dermatitis.   1. Your patient is a homeless person with several papules on his  hands that are very pruritic. One lesion is a linear track. You sus¬  pect the patient may have scabies. Which one of the following is  most likely to be seen?   (C) The nymph form of a tick is seen in the lesions.   (D) Examination of a skin scraping in the microscope reveals a mite.   2. Regarding the patient in Question 1, which one of the following is  the best drug to treat the infection?   3. Your patient has recently returned from a trip to Central America  that included a 2-week trek in the tropical rainforest. She now has  a raised erythematous lesion on her leg that is quite painful. A  7-day course of cephalexin has had no effect. Which one of the  following is the most likely cause?   4. Regarding pediculosis, which one of the following is most accurate?   (A) Nits are the eggs of the louse and are typically found attached  to the hair shaft.   (B) Praziquantel is the drug of choice for pediculosis caused by  both Pediculus and Phthirus.   (C) To visualize the organism, a skin sample should be examined  using the lOx objective in a light microscope.   (D) The lesions caused by the body louse are pruritic, but the  lesions caused by the pubic louse form a painful necrotic black  eschar.    Trombicula mites are also known as harvest mites or chig-  gers. The bite of the larva causes papules accompanied by  intense itching. The pruritic papules result from an allergic  response to proteins in the saliva that are injected into the   Brief summaries of the organisms described in this chapter  begin on page 689. Please consult these summaries for a rapid  review of the essential material.   Bone and joint infections are serious infections because  destruction of bone or cartilage can lead to significant dis¬  ability. Osteomyelitis and infectious arthritis are caused  primarily by bacteria or fungi. In these diseases, the organ¬  isms directly infect the bone and joint. In contrast, immune  complex arthritis, reactive arthritis, and rheumatic fever  are caused by immune reactions to either bacteria or  viruses, and organisms are not found in the joints.   The clinical diagnosis of infectious arthritis often involves an  analysis of joint fluid. Radiologic studies of joints and bone  contribute important information. Microbiologic diagnosis of  osteomyelitis and infectious arthritis is typically made by cultur¬  ing either a bone specimen or joint fluid. Antimicrobial therapy  is typically given for long periods (i.e., weeks to months).   Osteomyelitis is an infection of the bone. The term osteo  refers to bone, and myelo refers to the bone marrow. Osteo¬  myelitis is classified as either acute or chronic.   The most common mode by which organisms reach the  bone is by hematogenous spread (i.e., either bacteremia or  fungemia) from a distant site. Acute bacterial osteomyelitis  often arises from a pyogenic skin infection such as a boil,  but many sources are undetected. Mycobacterial and fungal  osteomyelitis often arise from the initial site of infection in  the lung.   In children, hematogenous spread tends to result in  osteomyelitis located at the end of long bones (at the  metaphyses) that are richly endowed with blood vessels. In  adults, hematogenous spread results most commonly in  vertebral osteomyelitis and discitis, not osteomyelitis of the  long bones.   Osteomyelitis also occurs by direct extension from  an infected contiguous site such as a skin or soft tissue  infection. It also can occur following trauma that  results in an open fracture and direct contamination of  the bone.   Chronic osteomyelitis tends to occur in the lower  extremity, especially in diabetics who often have vascular  insufficiency. They are predisposed to skin and soft tissue  infections that extend into the bone.   FIGURE 70-1 Vertebral osteomyelitis. Arrow indicates site of  vertebral lesion. (Reproduced with permission from McKean SC et al. Principles  and Practice of Hospital Medicine. New York: McGraw-Hill, 2012. Copyright © 2012 by  The McGraw-Hill Companies, Inc.)   The most characteristic clinical manifestations are bone  pain and localized tenderness at the site of infection. Most  patients also have constitutional symptoms such as fever,  night sweats, and fatigue. Limited range of motion of an  affected extremity is seen. In vertebral osteomyelitis, the  lumbar region is affected more often than the cervical or  thoracic regions (Figure 70-1).   In acute osteomyelitis, the symptoms occur abruptly  and progress rapidly, whereas in chronic osteomyelitis,  the course is more indolent. In chronic osteomyelitis,  necrosis of the bone occurs, and a sequestrum (an avascu¬  lar piece of infected bone) can form at the site of the lesion  (Figure 70-2). Relapses tend to occur in chronic osteomy¬  elitis more than in acute osteomyelitis, and surgical  debridement, especially to remove sequestra, is important  to minimize the risk of relapse.   The most common bacterial cause of acute osteomyeli¬  tis in both children and adults is Staphylococcus aureus  (Table 70-1). Flowever, vertebral osteomyelitis in adults  may be caused by Mycobacterium tuberculosis (Pott’s  disease). Osteomyelitis in patients with hip or knee  prostheses is likely to be caused by Staphylococcus   FIGURE 70-2 Chronic osteomyelitis. A. White arrow points to  draining fistula at site of chronic osteomyelitis. B. White arrow points  to necrotic bone caused by chronic osteomyelitis. (Reproduced with per¬  mission from Kemp WL, Burns DK, Brown TG. Chapter 19. Pathology of the Bones and  Joints. In: Kemp WL, Burns DK, Brown TG. eds. Pathology: The Big Picture. New York, NY:  McGraw-Hill; 2008.)   Diabetics often have arterial insufficiency in the lower  extremities leading to chronic infections and skin ulcers.  These infections can extend into the adjacent bone causing  osteomyelitis. Staphylococcus aureus and skin anaerobes  such as P. acnes are often involved.   When osteomyelitis occurs in an intravenous drug  user, it is most often caused by S. aureus; however, gram¬  negative rods, such as Pseudomonas and Serratia, and  yeasts, such as Candida species, are also important causes.  Osteomyelitis following a puncture wound of the foot  through a sneaker is often caused by Pseudomonas aerugi¬  nosa, and osteomyelitis associated with a cat bite is likely    to be caused by Pasteurella multocida. Patients with sickle  cell anemia are predisposed to osteomyelitis cause by Sal¬  monella species.   Fungal osteomyelitis is most often caused by either Coc-  cidioides immitis or Histoplasma capsulatum. Living in  areas where these fungi are endemic is an important predis¬  posing factor. Viruses, protozoa, and helminths do not  cause osteomyelitis.   A microbiologic diagnosis of acute osteomyelitis is most  consistently made by culture of a specimen of the bone  lesion. Blood cultures are positive in approximately half of  cases.   The typical radiologic finding in acute osteomyelitis is a  defect in the bone accompanied by periosteal elevation  (Figure 70-3). Early in the disease, X-rays and even com¬  puted tomography (CT) scans may be negative. Magnetic  resonance imaging (MRI) scans are the most sensitive  radiologic tests for diagnosis of osteomyelitis.   Empiric therapy for acute osteomyelitis should include  drugs that are bactericidal, penetrate well into bone, and  include coverage for S. aureus. Vancomycin, nafcillin, or  cephalexin administered parenterally can be used. Vanco¬  mycin is often used until the culture results and the sensi¬  tivity of the organism are known. If methicillin-resistant  S. aureus (MRSA) is the cause then either vancomycin,  daptomycin, or linezolid can be used. If gram-negative rods  are the cause, then either ceftriaxone, ceftazidime, or cefip-  ime can be used. The duration of therapy ranges from 3 to   FIGURE 70-3 Periosteal elevation (arrow) in acute osteo¬  myelitis of the tibia. (Reproduced with permission from Longo DL et al (eds).  Harrison's Principles of Internal Medicine. 18th ed. New York: McGraw-Hill, 2012.  Copyright © 2012 by The McGraw-Hill Companies, Inc.)   6 weeks or longer. Surgical debridement of chronic osteo¬  myelitis lesions is often necessary.   There is no vaccine effective against the common causes of  osteomyelitis, and chemoprophylaxis is typically not  employed. Generally speaking, prophylactic antibiotics are  not recommended prior to dental procedures to prevent  prosthetic joint infection. Proper foot care in diabetics can  prevent osteomyelitis.   Infectious (septic) arthritis is an infection of the joints. The  terms infectious and septic are used to distinguish these  infections from immune-mediated arthritis, such as rheu¬  matoid arthritis. Bacteria, especially S. aureus, cause the  vast majority of cases of infectious (septic) arthritis. Mono¬  articular involvement of a large weight-bearing joint, such  as the hip or knee, is the most common presentation.   Analysis of synovial fluid aspirated from a swollen joint  plays an important role in the diagnosis of arthritis.    Table 70-2 shows the findings in the fluid aspirated from  an infected joint compared to normal synovial fluid. Syno¬  vial fluid from an infected joint may appear cloudy, has at  least 20,000 neutrophils/pL, and has a low glucose concen¬  tration. Analysis of the fluid from the joints of those with  rheumatoid arthritis and those who have a traumatic injury  to the joint is included for comparison.   Organisms typically reach the joint via the bloodstream  from a skin site. Less frequently, organisms enter the joints  through penetrating trauma, medical procedures such as  arthroscopy, or a contiguous osteomyelitis.   Patients with long-standing rheumatoid arthritis and  those with prosthetic hips and knees are predisposed to  infectious arthritis.   The acute onset of an inflamed joint, typically a large weight¬  bearing joint such as the hip or knee, is the typical manifesta¬  tion (Figure 70-4). Fever is often present. On physical  examination, the affected joint is red, warm, and swollen, and  a joint effusion is typically present. Reluctance to use a joint,  especially in a child, may be a sign of infectious arthritis.   The most common cause of infectious arthritis overall is  S. aureus (Table 70-3). In young sexually active adults,  Neisseria gonorrhoeae is the most common cause. Patients  with a prosthetic hip or knee joint are predisposed to infec¬  tious arthritis caused by S. epidermidis. Staphylococcus aureus  and P. aeruginosa are the most common causes in intrave¬  nous drug users. Borrelia burgdorferi, the cause of Lyme  disease, should also be mentioned as the cause of inflamed  joints that resemble those seen in infectious arthritis. How¬  ever, in Lyme disease, the arthritis is immune-mediated and  organisms are not recovered from the affected joints.   Visualization of the organisms in the Gram stain of joint  fluid is used to guide empiric therapy. A microbiologic   inflamed left knee. (Used with permission from the clinical slide collection  on Rheumatic Diseases, 1991, 1995. Used by permission of the American  College of Rheumatology.)   diagnosis of infectious arthritis is typically made by culture  of a specimen of the joint fluid. Blood cultures are positive  in less than 30% of cases.   The typical radiologic finding in infectious arthritis is  soft tissue swelling. Evidence of joint destruction can be  seen if the infection progresses.   Untreated infectious arthritis can lead to joint destruction  and loss of mobility, so prompt antibiotic treatment is  required for optimal recovery. Empiric therapy for infectious    arthritis should include drugs such as vancomycin, nafcillin,  or cefazolin that are bactericidal against S. aureus. Ceftriax¬  one should be used if there is evidence that N. gonorrhoeae is  the cause. Removal of joint fluid via arthrocentesis and/or  surgical drainage is an important adjunct to antibiotics.   There is no vaccine effective against the common causes of  infectious arthritis, and chemoprophylaxis is typically not  employed.   Viral arthritis is often called immune complex arthritis  because the virus does not infect the joint but rather, the  virus forms immune complexes with antiviral antibody  that is deposited in joints and elicits an inflammatory  response.   The clinical features of viral arthritis consist of either  arthralgia (painful joints but without visible inflammation)  or frank arthritis in which inflammation is apparent. Most  cases of viral arthritis are of short duration and resolve  spontaneously, but chronic arthritis may occur. The small  joints of the hands are most often affected, but large joints  can also be involved.   Viral arthritis occurs during the course of infection  by several viruses. Rubella virus, either from the natural  infection or from the immunization, is a well-recognized  cause. Parvovirus B19 is an important cause in that the  lesions resemble those of rheumatoid arthritis. The joint  lesions of chronic hepatitis C also resemble rheumatoid  arthritis. Arthralgia and arthritis occur in the prodromal  period of hepatitis B infection. Several arboviruses also  cause severe arthralgia, the most common of which is  dengue virus. There is no antiviral treatment for viral  arthritis.   Reactive arthritis is the term used to describe arthritis  that occurs following infection by several bacteria that  infect the gastrointestinal or genitourinary tract. The bac¬  teria do not infect the joints. Rather, the arthritis is a  result of the immune response to the bacterial infection.  People who are HLA-B27 positive are predisposed to reac¬  tive arthritis. The bacteria commonly associated with this  arthritis are Campylobacter, Shigella, Salmonella, Yersinia,  and Chlamydia.   The main clinical manifestation is an asymmetric  arthritis of the knee or ankle accompanied by fever. It  typically resolves within a few days or weeks, but chronic  arthritis may occur. Recurrences are common. Culture of  synovial fluid is negative. Reactive arthritis accompanied  by conjunctivitis and urethritis is called Reiter’s    syndrome. Nonsteroidal anti-inflammatory drugs are  considered first-line therapy. Antibiotics have no effect on  reactive arthritis.   Rheumatic fever is an immune-mediated, poststreptococ¬  cal disease that affects the joints, heart, brain, and skin. It  follows pharyngitis caused by Streptococcus pyogenes  (group A Streptococcus). It typically occurs in children  ages 5 to 15 years. It is rare in the United States today  probably because streptococcal pharyngitis is treated  promptly.   Rheumatic fever typically begins with a migratory polyar¬  thritis involving the large joints approximately 2 to 3 weeks  after the pharyngitis. Carditis often occurs and is the main,  life-threatening component of rheumatic fever. The carditis  is a pancarditis (i.e., endocarditis, myocarditis, and pericar¬  ditis occur, often resulting in congestive heart failure). The  mitral valve is most frequently involved. Chorea consisting  of involuntary athetoid movements also occurs but is a rare  manifestation. Skin involvement consists of erythema mar¬  ginatum and subcutaneous nodules.   There is no diagnostic test for rheumatic fever. Table 70-4  shows the Jones criteria that are used as a guideline to estab¬  lish the diagnosis. Two major manifestations or one major  plus two minor manifestations suggest the diagnosis. In  addition, laboratory evidence of prior infection by S. pyogenes  is needed. This consists of either (1) a positive throat culture  or positive rapid streptococcal antigen test or (2) a rising  anti-streptolysin O antibody titer.   The drug of choice is aspirin to reduce the inflamma¬  tion. Antibiotics such as penicillin G have no effect on the  course of the disease but can be given to reduce carriage of  streptococci in the pharynx.   Prevention of rheumatic fever involves prompt diagno¬  sis and treatment of strep throat with penicillin G or oral  penicillin V. In patients with residual heart disease, preven¬  tion of additional damage to heart valves by preventing  subsequent episodes of streptococcal pharyngitis is very  important. This is achieved by monthly administration of  benzathine penicillin G, a depot preparation. This should  continue until the patient is at least 20 years old or for  10 years after the last attack.    TABLE 70-4 Jones Guidelines for the Diagnosis of  Acute Rheumatic Fever   Cardiac infections are severe, life-threatening infections  in many cases. The heart valves (endocardium), myocar¬  dium, and pericardium can all be infected. In addition,  infection of cardiac devices (pacemakers, defibrillators)  is becoming more frequently diagnosed with their  increase in use. Diagnosis of cardiac infection can be  challenging and usually requires a combination of micro¬  biologic testing and cardiac imaging. Treatment often  requires antimicrobial therapy but may also require sur¬  gical management for cure.   An electrocardiogram (ECG) measures electrical activity in  the heart using noninvasive monitoring with leads attached  to the skin. Cardiac infections can cause disease-specific  ECG changes, which can assist in diagnosis.   Echocardiography uses Doppler ultrasound to visualize  structures and flow of blood through the heart. The test is  very helpful in diagnosing most types of cardiac infections.  There are two types of echocardiograms, a transthoracic  echocardiogram (TTE), where the probe is placed on the  chest wall, and a transesophageal echocardiogram (TEE),  where the probe is inserted into the esophagus. The TEE  often produces higher-quality images, particularly of aor¬  tic and mitral valves, since the TEE probe is closer to the  heart itself.   Infection of the heart valves is thought to result from the  colonization of damaged valvular endothelium by circu¬  lating pathogens. Endothelial damage may result from  turbulent blood flow around the valve (congenital or  rheumatic heart disease), direct injury from foreign  bodies (e.g., intravenous catheters), or repeated intrave¬  nous injections of particles in intravenous drug users.  Organisms enter the bloodstream most often at the site  of dental surgery, indwelling intravenous catheters, or  intravenous drug use. Adhesion of bacteria to the dam¬  aged endothelium is enhanced by their ability to produce  a glycocalyx.   Once the infection has begun, a combination of  organisms and thrombus organize to form a vegetation  (Figure 71-1). Destruction of the valve occurs at differ¬  ent rates depending on the virulence of the organism. As  the valve is destroyed, symptoms of valvular regurgita¬  tion can develop. Organisms can spread to surrounding  myocardium, resulting in abscess formation and destruc¬  tion of the electrical conduction system.   As the vegetation on the valve enlarges, fragments can  spread via the bloodstream (emboli), resulting in cata¬  strophic effects, such as cerebrovascular accidents (CVA)  and metastatic infections. Prolonged infection as seen in  subacute endocarditis can result in antigen-antibody com¬  plex formation. Deposition of these complexes can result in  other clinical manifestations as described in the next sec¬  tion. Artificial materials within the heart, such as   FIGURE 71-1 Endocarditis. Note vegetations on mitral valve.  Black arrows point to vegetations. (Reproduced with permission from Longo  DL et al (eds). Harrison's Principles of Internal Medicine. 18th ed. New York: McGraw-  Hill, 2012, p 1052. Copyright © 2012 by The McGraw-Hill Companies, Inc.)   prosthetic heart valves, pacemakers, and defibrillators,  serve as potential sites for infection.   The clinical manifestations of infective endocarditis  can include any of the following listed below. Depend¬  ing on the virulence of the infecting pathogen, the time  course of illness may be days (acute endocarditis;  caused by, e.g.. Staphylococcus aureus) or weeks to  months (subacute endocarditis; caused by, e.g., viridans  group streptococci).   • Consequences of destruction of heart valves and  associated structures: new murmur, heart failure,  atrioventricular (AV) block (PR prolongation seen  on ECG; Figure 71-2)   • Left-sided endocarditis: CVAs or brain abscess  (Figure 71-3) (new focal neurologic deficits),  splenic or renal infarcts (abdominal or flank  pain), and emboli to others sites manifesting   (Reproduced with permission from McKean SC et al. Principles and Practice of Hospi¬  tal Medicine. New York: McGraw-Hill, 2012. Copyright© 2012 by The McGraw-Hill  Companies, Inc.)   FIGURE 71-3 Brain abscess. Red arrow points to a characteristic  ring-enhancing lesion. The blue arrows point to two additional  abscesses. (Reproduced with permission from Ropper AH, Samuels MA .Adamsand  Victor's Principles of Neurology. 9th ed. New York: McGraw-Hill, 2009. Copyright © 2009  by The McGraw-Hill Companies, Inc.)   as splinter hemorrhages (Figure 71-4), Jane¬  way lesions (Figure 71-5), retinal hemorrhages  (Figure 71-6), and conjunctival hemorrhages  (Figure 71-7).   RP et al: The Color Atlas of Family Medicine, New York: McGraw-Hill, 2009.)   lesion. (Reproduced with permission from Wolff K, Johnson R (eds). Fitzpatrick's  Color Atlas & Synopsis of Clinical Dermatology. 7th ed. New York: McGraw-Hill, 2013.  Copyright© 2013 by The McGraw-Hill Companies, Inc.)   FIGURE 71-6 Retinal hemorrhages. Blue arrow points to a  retinal hemorrhage. (Reproduced with permission from Usatine, RP et al: The Color  Atlas of Family Medicine, New York: McGraw-Hill, 2009. Courtesy of Paul D. Comeau, MD.)   FIGURE 71-7 Conjunctival hemorrhages. Blue arrow points to  one of several conjunctival hemorrhages. (Reproduced with permission  from Haidar SM, O'Gara PT. Chapter 86. Infective Endocarditis. In: FusterV, Walsh RA  Harrington RA. eds. Hurst's The Heart, 13e. New York, NY: McGraw-Hill; 2011.).   • Right-sided endocarditis: septic pulmonary  emboli (cough, shortness of breath, chest pain,  hemoptysis).   • Antigen-antibody deposition from uncon¬  trolled infection: Osier’s nodes (Figure 71-8),  Roth’s spots (Figure 71-9), glomerulonephritis  (hematuria), and/or arthritis.   FIGURE 71-8 Osier's node in pulp of big toe. Red arrow points to  an Osier's node. Note also Janeway lesions on sole of foot. Blue arrow  points to a Janeway lesion. (Reproduced with permission from Usatine RP et al. The  Color Atlas of Family Medicine. New York: McGraw-Hill, 2009. Courtesy of David A. Kasper  DO, MBA.)   FIGURE 71-9 Roth's spots. Note the central white spots char¬  acteristic of Roth's spots (red arrow). (Reproduced with permission from  Usatine RP et al. The Color Atlas of Family Medicine. New York: McGraw-Hill, 2009.  Courtesy of Paul D. Comeau, MD.)    such as Staphylococcus epidermidis, and S. aureus are the  most common pathogens. Other less common pathogens  that grow relatively well in routine culture media include  the (1-hemolytic streptococci, Streptococcus pneumoniae,  HACEK organisms ( Haemophilus aphrophilus and Hae¬  mophilus paraphrophilus, (now called Aggrigatibacter  aphrophilus and Aggrigatibacter paraphrophilus), Actinoba-  cillus actinomycetemcomitans, (now called Aggrigatibacter  actinomycetemcomitans, Cardiobacterium hominis,  Eikenella corrodens, and K ingella kingae), and Candida  species.   Pathogens that do not grow in routine blood culture  media and require specialized testing for diagnosis include  Bartonella species, Coxiella burnetii. Brucella species, and  Tropheryma whipplei. These are cited as pathogens that  cause “culture-negative” endocarditis. The most frequent  cause of “culture-negative endocarditis” is the use of anti¬  microbials prior to obtaining blood cultures.   Bacteria are, by far, the most common causes of endocarditis,  but yeasts such as Candida species are involved as well. The  modern classification of pathogens causing endocarditis is  divided into native valve versus prosthetic valve, with sub¬  classifications within each group (Table 71-1). Among  patients with native valve endocarditis who present from the  community, viridans group streptococci are the most com¬  mon pathogens, whereas S. aureus is most common in  patients who have had exposure to the health care setting or  intravenous drug use. Other important pathogens in native  valve endocarditis are Streptococcus bovis (which is associ¬  ated with colorectal cancer) and Enterococcus species.   In patients who have prosthetic valves, pacemakers, or  defibrillators in place, coagulase-negative staphylococci   A definitive diagnosis of endocarditis requires direct patho¬  logic examination and microbiologic analysis of the heart  valve. Because in most cases the heart valve tissue is not  available for evaluation, most clinicians use a combination  of blood cultures and echocardiographic findings to make  the diagnosis of infective endocarditis. The Modified Duke  Criteria are the most frequently used criteria for making  the diagnosis of endocarditis (Table 71-2) and help guide  clinicians to make an accurate diagnosis.   Infecting pathogens are most commonly recovered  through blood cultures. To maximize sensitivity of the test,  it is recommended to obtain three sets of blood cultures  over at least an hour. Whenever possible, blood cultures  should be obtained prior to administering antibiotics. In  some rare cases of endocarditis due to organisms that do  not grow easily in blood culture media ( Bartonella species),  serology can be used to help make the diagnosis.   Evaluation of valves for infection is best accomplished  through echocardiography. A transthoracic echocardio¬  gram (TTE) has reduced sensitivity when compared with a  transesophageal echocardiogram (TEE) to assess for vege¬  tations and myocardial abscesses but is a less invasive test.  Not only can echocardiogram identify new vegetations on  valves, which are evidence of infection, but it can also  assess the degree of valvular damage and complications  such as perivalvular abscesses (Figure 71-10). ECG can be  used to detect damage to the conducting system. The most  common finding is PR prolongation in patients with aortic  valve endocarditis and associated perivalvular abscess (see  Figure 71-2).   Without treatment, endocarditis is always fatal, so prompt  effective therapy is essential. Bactericidal drugs should be    TABLE 71 -2 Modified Duke Criteria for the  Diagnosis of Infective Endocarditis   • Microorganism demonstrated by culture or histology in a vegeta¬  tion or in a vegetation that has embolized or in an intracardiac  abscess   • Pathologic lesions, vegetation or intracardiac abscess, confirmed  by histology showing active endocarditis   • Two major criteria OR one major criterion and three minor criteria  OR five minor criteria   • Positive blood cultures of typical organism for infective endocarditis  from 2 separate blood cultures or persistently positive blood culture  or single culture positive for culture or serology consistent with  Coxiella burnetii infection   • Vascular phenomena (arterial emboli, septic pulmonary infarcts,  mycotic aneurysm, etc.)   FIGURE 71~10 Transesophageal echocardiography in  endocarditis. Segmented abscess cavity (labeled A) between the left  atrium (labeled LA) and the aortic root (labeled AO). Red arrow  indicates the wall that segments the abscess. RA, right atrium; RVOT,  right ventricular outflow tract. (Reproduced with permission from DeMaria  AN, Blanchard DG. Chapter 18. Echocardiography. In: FusterV, Walsh RA, Harrington  RA. eds. Hurst's The Heart, 13e. New York, NY: McGraw-Hill; 2011.)    used. The treatment for endocarditis always includes anti¬  microbial therapy, and in some cases, surgical removal of  the infected valve is indicated as well. Empiric therapy for  endocarditis is recommended in cases where the patient  has hemodynamic instability, severe disease, evidence of  embolic disease, or large vegetations. Empiric antimicro¬  bial coverage should be active against methicillin-resistant  S. aureus, viridans group streptococci, enterococci, and  HACEK organisms. Common empiric regimens include  vancomycin plus either ceftriaxone or gentamicin. Specific  antimicrobial therapy should be instituted when the results  of blood cultures and antibiotic susceptibility tests are  known. Antimicrobial therapy for endocarditis is usually  required for 4 to 6 weeks.   Surgical therapy is either indicated or should be strongly  considered in patients with severe congestive heart failure,  perivalvular abscesses, infections refractory to medical  management, and embolic events with large vegetations.   In patients with prior endocarditis, a prosthetic heart  valve, or select types of congenital heart disease, antibi¬  otic prophylaxis is recommend prior to certain proce¬  dures. Guidelines support giving antibiotics, such as  amoxicillin, to these high-risk patients at the time of  invasive dental procedures (not for routine cleanings),  surgery involving respiratory mucosa, or surgery involv¬  ing infected tissues.   Infection of the myocardium most frequently occurs fol¬  lowing hematogenous spread of virus or other pathogen to  the heart muscle, although direct spread from adjacent  structures can occur. Infection and inflammation of myo¬  cardium may result in cardiac dysfunction, leading to heart  failure.   Patients with myocarditis present with signs and symptoms  of heart failure. Depending on the pathogen, the pace of  disease progression may be over days or weeks. Patients  may have signs and symptoms of a systemic infection as  well (fever, constitutional symptoms). Those with associ¬  ated pericarditis often have chest pain.   Viral pathogens are thought to be the predominant cause of  infectious myocarditis, although many cases are idiopathic.  Coxsackie viruses are the most common cause, although    cytomegalovirus, Epstein-Barr virus, parvovirus B19, and  influenza virus have been implicated. Other pathogens  include Trypanosoma cruzi, the agent of Chagas’ disease,  and Trichinella spiralis.   A definitive diagnosis requires cardiac muscle biopsy  revealing myocardial inflammation and necrosis. How¬  ever, most cases are presumptively diagnosed in a patient  presenting with heart failure, who has (often global)  cardiac dysfunction on echocardiogram and elevated  cardiac enzymes. The ECG may be abnormal and may  show ST changes mimicking an acute myocardial  infarction.   There is no known treatment for most causes of myocardi¬  tis, and supportive care is most often given. Patients may  ultimately require heart transplant.   Pericarditis refers to inflammation of the pericardium,  which can be due to infection, autoimmune diseases,  trauma, or malignancy.   Pathogens reach the pericardium by either hematoge¬  nous spread through the blood or direct spread from  adjacent intrathoracic structures or, rarely, directly from  infected myocardium. Inflammation of the pericardium  can result in the formation of pericardial effusion. Peri¬  cardial effusions can result in cardiac tamponade.  Inflammation can also result in a constrictive physiol¬  ogy. Certain infections causing pericarditis may also be  associated with a concomitant myocarditis (see previous  “Myocarditis” section).   Chest pain is the most common manifestation of peri¬  carditis. Pain often worsens with inspiration or cough¬  ing. Sitting up and leaning forward often improve the  pain associated with pericarditis. Patients may have  fever and constitutional symptoms. On exam, a friction  rub (often consisting of three phases) may be heard  when performing auscultation of the heart. This exam  finding is very specific for pericarditis. Severe infection    may result in cardiac tamponade or constrictive cardiac  physiology. These patients present with acute or sub¬  acute/chronic onset of symptoms of heart failure,  respectively.   Viruses, bacteria, mycobacteria, and fungi have all been  reported to cause pericarditis. Among viral infections,  Coxsackie virus and echovirus are most common, although  human immunodeficiency virus and cytomegalovirus can  cause pericarditis as well. Among bacteria, S. aureus and S.  pneumoniae are most common. Mycobacterium tuberculo¬  sis is one of the most common infectious causes of pericar¬  ditis worldwide. Clinical presentation is often subacute and  may result in a constrictive pattern. Several fungi such as  Histoplasma capsulatum and Coccidioides immitis can cause  pericarditis, which clinically presents similarly to tubercu¬  lous pericarditis.   Culture of pericardial fluid or pericardial tissue may reveal  causative bacteria. Viruses are rarely isolated. Additional  diagnostic tests that can help make the diagnosis include  ECG that reveals changes in the PR and ST segments. If a  significant pericardial effusion is present, the ECG may  have reduced amplitude in all leads. An echocardiogram  and/or cardiac magnetic resonance imaging will often  reveal a pericardial effusion and/or pericardial thickening  (Figure 71-11). In addition, chest X-ray may show an   FIGURE 71~11 Magnetic resonance imaging of thorax show¬  ing pericardial thickening (two black arrows) in patient with constric¬  tive pericarditis. (Reproduced with permission from Sokolow M, Mcllroy MB.  Clincial Cardiology, 6th ed. Originally published by Appleton & Lange. Copyright ©  1993 by the McGraw-Hill Companies, Inc. Courtesy of C. Higgins.)   FIGURE 71-12 Chest X-ray of a patient with pericardial  effusion. Red arrow indicates left border of dilated pericardial sac  containing effusion fluid. (Reproduced with permission from Kabbani SS,  LeWinter M, in Crawford MH et al (eds). Cardiology. London: Mosby, 2001.)    enlarged cardiac silhouette (Figure 71-12), and cardiac  enzymes can be elevated. Recovery of a pathogen often  requires a pericardiocentesis or pericardial biopsy.   Treatment for infectious pericarditis is dependent on the  pathogen. Most viral etiologies are treated with symptom¬  atic management and supportive care, whereas bacterial,  mycobacterial, and fungal infections will require directed  antimicrobial therapy. In patients with constrictive pericar¬  ditis and tamponade, pericardiocentesis can be life-saving.  Untreated bacterial pericarditis is rapidly fatal.   Immunization against S. pneumoniae may be effective.  Treatment of early or latent stages of infections (e.g., tuber¬  culosis) may prevent development of pericarditis in some  cases.   Central nervous system (CNS) infections are often life-  threatening and can have severe sequelae. These infections  cause inflammation and edema within the unyielding cra¬  nium, resulting in damage to brain tissue and loss of func¬  tion. The most common causes of CNS infections are  bacteria and viruses, but fungi, protozoa, and helminths  also cause these infections.   In addition to the history and physical examination,  clinical diagnosis of CNS infections requires a spinal  fluid analysis combined with neuroimaging using either  magnetic resonance imaging (MRI) or computed  tomography (CT) scan. Microbiologic diagnosis of bac¬  terial infections frequently is made using Gram stain  and culture of spinal fluid and blood. Polymerase chain  reaction (PCR) assays and serologic tests are also use¬  ful. Antimicrobial therapy requires that the antibiotics  be bactericidal and that they penetrate the blood-brain  barrier. Some CNS infections, such as a brain abscess,  often require surgical drainage.   Examination of cerebrospinal fluid (CSF) is critical in mak¬  ing the diagnosis of CNS infections. CSF is obtained by  performing a lumber puncture at the L3-L4 interspace.  During the process, the CSF pressure is measured and fluid  obtained for analysis of cells (both number and cell type,  i.e., neutrophils or lymphocytes), protein, and glucose. The  results of CSF analysis in acute bacterial meningitis, acute  viral meningitis, and subacute meningitis are described in  Table 72-1.   Although CSF analysis is a very important step in the  diagnosis of many CNS infections, a lumbar puncture  should not be performed if there are signs of increased  intracranial pressure, such as papilledema or focal neuro¬  logic signs, because herniation of the brainstem and death  may occur. A CT scan should be performed prior to the  lumbar puncture to determine whether a mass lesion,  such as a brain abscess or cancer, is present. If a mass  lesion is seen, a lumbar puncture should not be  performed.    CSF = cerebrospinal fluid; Lymphs = lymphocytes; Polys = polymorphonuclear leukocytes; TB = tuberculosis.   Meningitis is an infection of the meninges, the membranes  that line the brain and spinal cord (Figure 72-1). Meningitis  can be categorized as acute, subacute, or chronic depending  on the speed of onset of the initial presentation and the rate  of progression of the illness. Acute meningitis is caused by  either pyogenic bacteria, such as Streptococcus pneumoniae  and Neisseria meningitidis, or viruses, such as Coxsackie  virus and herpes simplex virus type 2. Viral meningitis is  often called aseptic meningitis because routine cultures for  bacterial pathogens are negative. Subacute meningitis is  caused by Mycobacterium tuberculosis and fungi, such as  Cryp-tococcus. The causative organisms are often found in  the spinal fluid located in the subarachnoid space.    meninges. Direct spread via adjacent infections, such as  otitis media and sinusitis; via neurosurgery, such as a shunt  to relieve hydrocephalus; or via trauma, such as a fracture  of the cribriform plate, occurs less frequently. The impor¬  tance of hematogenous spread is emphasized by the success  of the conjugate vaccines against S. pneumoniae, N. menin¬  gitidis, and Haemophilus influenzae type B that induce cir¬  culating IgG antibodies that neutralize the bacteria in the  blood.   Acute bacterial meningitis begins with nasopharygeal  colonization followed by local invasion, entry into the  bloodstream, and invasion of the meninges (Figure 72-2).  This is followed by an inflammatory response that causes  many of the clinical manifestations, especially the edema  resulting in increased intracranial pressure leading to head¬  ache. Cerebral vasculitis and infarction can also occur.   Flematogenous spread (i.e., bacteremia or viremia) is the  most common route by which organisms reach the   FIGURE 72-1 Purulent meningitis. Note film of greenish pus in  the subarachnoid space covering the brain.The dura is reflected back  and held by forceps. (Source: Centers for Disease Control and Prevention.)   Early symptoms include the classic triad of fever, head¬  ache, and stiff neck (nuchal rigidity). Altered mental  status also commonly occurs. If untreated, meningitis  may progress to vomiting, seizures, photophobia, and  focal neurologic deficits. Different pathogens can pres¬  ent with different rates of clinical progression, from  acute onset and rapid progression (hours to days) to  subacute or chronic onset and slow progression (days to  weeks). Neisseria meningitidis infection can be associ¬  ated with disseminated disease (meningococcemia) and  result in petechial rash and ultimately purpura fulmi-  nans (Figure 72-3).   The most common bacterial cause of acute meningitis  overall is S. pneumoniae. However, Streptococcus agalactiae  (group B Streptococcus) predominates in neonates, and  N. meningitidis is common in teenagers and young adults  (Table 72-2). Haemophilus influenzae type B used to be an  important cause in young children, but the widespread use  of the conjugate polysaccharide vaccine has greatly  decreased its incidence. Listeria monocytogenes is reason¬  ably common in the very young, the very old, and in immu¬  nocompromised patients. Less common pathogens include  Borrelia burgdorferi (Lyme disease) and Treponema  pallidum (syphilis).   The most common viral causes of acute meningitis are  enteroviruses such as Coxsackie virus and echovirus.  Enteroviral meningitis occurs primarily in young chil¬  dren, and the peak incidence is in the summer and fall  seasons. Herpes simplex virus type 2 (HSV-2) is also a  common cause of meningitis. Note that HSV-2 typically  causes meningitis, whereas herpes simplex virus type 1   from Longo DL et a! (eds). Harrison's Principles of Internal Medicine. 18th ed. New York: McGraw-Hill, 2012. Copyright © 2012 by The McGraw-Hill Companies, Inc.)    (HSV-1) causes encephalitis. Primary genital infections  with HSV-2 are more likely to result in meningitis than  recurrent HSV-2 infections. Primary and reactivation  varicella-zoster virus (VZV) infection can also be associ¬  ated with meningitis. Although arboviruses typically  cause encephalitis, arboviruses such as West Nile virus  (WNV) and St. Louis encephalitis virus can also cause  meningitis. Mumps virus used to be a common cause of  meningitis, but widespread use of the mumps vaccine has  greatly reduced its incidence.   The most common causes of subacute and chronic menin¬  gitis are M. tuberculosis and fungi such as Cryptococcus,  Coccidioides, and Histoplasma. Cryptococcal meningitis  occurs most commonly in immunocompromised patients,  such as those with acquired immunodeficiency syndrome  (AIDS).   A microbiologic diagnosis of acute bacterial meningitis is  typically made by Gram stain and culture of CSF. Analysis  of spinal fluid can distinguish between acute bacterial men¬  ingitis and viral meningitis (see Table 72-1). While they  both tend to have elevated white blood cells (WBCs) and  protein in CSF, bacterial infections tend to be neutrophil  predominant, whereas viral infections are lymphocyte pre¬  dominant. Bacterial infections are associated with low glu¬  cose concentrations in CSF, whereas viral infections have  normal glucose levels.   Subacute and chronic meningitis tend to be lymphocyte  predominant with very high protein levels and low glucose.  Viral infections are often diagnosed by using PCR assay for  viral DNA or RNA in cerebral spinal fluid or by serologic  tests for specific antibody. Gram stain and bacteriologic  cultures of CSF are negative in viral meningitis. Fungal  infections can be diagnosed by culture or by serologic tests.   meningitidis. (Reproduced with permission from Wolff K, Johnson R (eds): Fitzpat¬  rick's Color Atlas & Synopsis of Clinical Dermatology, 6th ed. New York: McGraw-Hill,  2009. Copyright © 2009 by The McGraw-Hill Companies, Inc.)    In the case of Cryptococcus , the India ink test and the cryp-  tococcal antigen test are also useful.   Empiric therapy for acute bacterial meningitis must include  drugs with excellent penetration into the CSF, are bactericidal,  and are active against the most common pathogens. In older  children and adults, ceftriaxone or cefotaxime plus vancomy¬  cin is a common empiric regimen. Ampicillin should be  added if Listeria is a likely cause. Empiric therapy for neonatal  bacterial meningitis includes ampicillin plus either ceftriaxone  or cefotaxime, with or without gentamicin. Acyclovir is used  for the treatment of HSV and VZV infection.   Prevention strategies include both immunization and che¬  moprophylaxis. Several vaccines are effective in preventing  bacterial meningitis, namely the conjugate vaccines against  S. pneumoniae, N. meningitidis, and H. influenzae type B.  The immunogen in these vaccines is the capsular polysac¬  charide of the organism. The current pneumococcal vaccine  (Prevnar 13) protects against the 13 most common sero¬  types. The current meningococcal vaccine (Menactra) pro¬  tects against four common serotypes (A, C, Y, and W-135);    however, it does not contain the type B polysaccharide. In  2014, a vaccine against type B meningococcus containing  Factor H binding protein (fHbp) as the immunogen was  approved. In 2015, a second vaccine against type B meningo¬  cocci containing four surface proteins (fHbp, NadA, NHBA,  and PorA) was approved, The current H. influenzae vaccine  protects only against the type B serotype.   Chemoprophylaxis against S. agalactiae (group B Strep¬  tococcus) is aimed at reducing vaginal carriage in the  mother. If vaginal or rectal cultures are positive at 35 to  37 weeks of gestation, then ampicillin should be given.  Chemoprophylaxis is also used to reduce nasopharyngeal  carriage of N. meningitidis and H. influenzae type B. Close  contacts of patients with meningitis caused by these organ¬  isms should receive either ciprofloxacin for Neisseria or  rifampin for Haemophilus.   Encephalitis is an infection of the brain parenchyma pre¬  dominantly caused by viruses. Sometimes both the brain  and the meninges are involved, a condition called  meningoencephalitis.   The mode of acquisition of the viruses that cause encepha¬  litis varies (Table 72-3). Neonates acquire HSV-2 during  passage through the birth canal. HSV-2 then reaches the  brain by hematogenous spread. Mothers with visible vesic¬  ular lesions are much more likely to have newborns with  serious HSV-2 infections than mothers who are asymptom¬  atic shedders of HSV-2 because the amount of virus present  is significantly greater in the former.   In contrast, HSV-1 probably reaches the temporal lobe  by travel down sensory neurons following activation of  latent infection in the trigeminal ganglion (Figure 72-4).  Rabies virus also reaches the brain by axonal travel from  the site of the animal bite.   Arboviruses, such as WNV, are acquired primarily by  mosquito bite and then travel to the brain via the blood¬  stream. The incidence of arboviral encephalitis peaks in the  summer and early fall because that is when mosquitoes are  most active.   In United States, dogs  and cats are uncom¬  mon reservoirs. Bats  are the most common  reservoir; raccoons are  common reservoirs  east of the Mississippi.   West Nile virus, East¬  ern and Western  equine encephali¬  tis viruses, St. Louis  encephalitis virus   West Nile virus is the  most common arbovi¬  ral infection in the  United States.   FIGURE 72~4 Encephalitis caused by herpes simplex virus-1.  Note destruction of temporal lobe on left side of image. (Used with per-  mission from Dr. John Mills, Monash University, Melbourne, Australia, and Dr. Kim  Erlich, University of California School of Medicine, San Francisco, CA.)   VZV can cause encephalitis during the primary infec¬  tion (varicella is also known as chickenpox) or during the  reactivation infection (zoster is also known as shingles).  VZV also causes a postinfectious encephalomyelitis involv¬  ing the brain and spinal cord after resolution of the primary  infection. Cytomegalovirus (CMV) causes encephalitis  primarily in immunocompromised individuals such as  AIDS patients and those receiving drugs to prevent trans¬  plant rejection. Encephalitis caused by Epstein-Barr virus  (EBV) is a rare complication of infectious mononucleosis.   Postinfection encephalitis typically follows an infection  or an immunization by several weeks. It is a demyelinating  disease caused by an immune attack on neurons, primarily  those of the white matter.   Note that the lesions in encephalitis are inflammatory  (contain WBCs, especially lymphocytes), whereas the  lesions of an encephalopathy show degenerating neurons  but no inflammation and do not contain WBCs. Encepha¬  lopathy is discussed later in a separate section.   The most characteristic clinical manifestations of encepha¬  litis include fever, headache, and altered mental status, as  well as seizures and focal neurologic deficits.   Rabies encephalitis has two clinical manifestations.  Most cases of rabies (80%) present with hyperactivity, agi¬  tation, delirium, hydrophobia, and seizures (called furious  rabies). The other 20% of cases have paralytic symptoms in  which an ascending paralysis without hyperactivity is the  predominant feature (called dumb rabies). Coma and death  are the final common pathway in both forms.   Viruses are the main cause of encephalitis; however, the  cause of at least half of the cases of encephalitis is unknown.  Approximately 15% are caused by HSV-1. Encephalitis  caused by HSV-1 and HSV-2 is very important because  HSV-1 and HSV-2 are the most common causes for which  antiviral drugs are available, namely acyclovir. About 5%  are caused by arboviruses such as WNV. Rabies virus is a  rare cause in the United States but occurs more frequently  in countries where immunization of dogs is not a common  practice. VZV, CMV, and EBV also cause encephalitis.   WNV is the most common arboviral cause of encepha¬  litis in the United States. Most WNV infections (80%) are  asymptomatic. Most of the remaining 20% develop an  acute febrile “flulike” illness. Less than 1% develop CNS  disease, of which half have encephalitis. Other arboviruses  that cause encephalitis with some frequency are St. Louis  encephalitis virus, the LaCrosse strain of California  encephalitis virus, and Eastern and Western equine enceph¬  alitis viruses (EEE and WEE, respectively). They are all  transmitted by either Culex or Aedes mosquitoes.   Postinfection encephalitis follows immunization or  infection caused most often by VZV, measles, and  influenza.   In contrast to meningitis, CSF findings in encephalitis are  more variable. A mild elevation in CSF lymphocytes can be  seen along with an elevation of protein and a normal  glucose. A normal CSF pattern can also be seen in  encephalitis.   PCR-based testing of CSF is commonly used to deter¬  mine a specific etiology, such as with HSV and VZV. WNV  encephalitis is often diagnosed by finding WNV-specific  IgM in the spinal fluid.   Rabies can be diagnosed by direct fluorescent antibody  staining of a biopsy of skin from the nape of the neck. A  PCR assay using CSF, saliva, or tissue is also available. The  PCR assay has the advantage of identifying the animal res¬  ervoir and the geographic location of the virus because the  base sequence of the RNA genome varies in accord with  those two features.    Radiographic findings can be useful as well. In particu¬  lar, in HSV encephalitis, temporal lobe abnormalities are  frequently seen.   Intravenous acyclovir is the treatment of choice for HSV-1,  HSV-2, and VZV encephalitis. There is no antiviral therapy  for arboviral or rabies encephalitis.   Prevention of rabies includes both preexposure (before  the bite) and postexposure (after the bite) prophylaxis.  Preexposure prophylaxis with the killed vaccine should be  given to veterinarians and others at risk of exposure. Post¬  exposure prophylaxis consists of both the killed vaccine  and the hyperimmune globulins that contain a high titer  of anti-rabies virus antibodies. They are inoculated at dif¬  ferent sites so the antibodies do not neutralize the virus in  the vaccine. This is an important example of passive-  active immunization. There is no vaccine for HSV-1,  HSV-2, and WNV.   To reduce the transmission of HSV-2 to the neonate,  pregnant women with active lesions late in pregnancy  should receive acyclovir and should be considered for  Cesarean section.   A brain abscess is a localized, walled-off collection of pus  surrounded by a fibrous capsule within the brain paren¬  chyma. Bacteria are the most common cause of brain  abscesses, but fungi and protozoa are also involved. Viruses  do not cause brain abscess.   Brain abscess is a recognized complication of head and neck  pyogenic infections, such as sinusitis, otitis media, and den¬  tal infections. Sinusitis predisposes to lesions in the frontal  lobe, whereas otitis media predisposes to lesions in the tem¬  poral lobe. Hematogenous spread from an infected site, such  as with infective endocarditis, also occurs. Table 72-4 cor¬  relates various predisposing conditions with the organisms  likely to cause brain abscess.   With increasing use of immunosuppressive drugs,  indwelling intravenous catheters, and hyperalimentation,  fungal brain abscesses have become more common. Immu¬  nocompromised patients, especially those with AIDS, also  have brain abscesses caused by Toxoplasma gondii.   Headache alone is the most common symptom of brain  abscess and thus, when headache alone occurs early in the   Aerobic and anaerobic streptococci, gram¬  negative anaerobes such as Bacteroides,  Prevotella, and Fusobacterium    disease, the diagnosis of brain abscess can often be  missed. As the lesion progresses, patients may develop  fever, behavioral changes, focal neurologic deficits, and  seizures.   Streptococci, both aerobic and anaerobic, are most com¬  monly isolated from bacterial brain abscesses. They are  typically of oropharyngeal origin, such as Streptococcus  anginosus and viridans group streptococci. They are typi¬  cally seen in mixed infections with oral anaerobes such as  Prevotella, Fusobacterium, and Bacteroides. Nocardia aster-  oides also causes brain abscesses. Monomicrobial infections  with Staphylococcus aureus are often associated with infec¬  tive endocarditis.   Fungal abscesses occur primarily in immunocompromised  patients. Aspergillus fumigatus can occur in neutropenic  patients, rhinocerebral mucormycosis (caused by Mucor  and Rhizopus species) in diabetic patients with ketoacido¬  sis, and cryptococcal infection in patients with HIV/AIDS.  Candida species are also involved.   FIGURE 72-5 Brain abscess. Red arrow points to a characteristic  ring-enhancing lesion. The blue arrows point to two additional  abscesses. (Reproduced with permission from Ropper AH, Samuels MA. A dams and  Victor's Principles of Neurology. 9th ed. New York: McGraw-Hill, 2009. Copyright © 2009 by  The McGraw-Hill Companies, Inc.)   MRI is an important diagnostic modality, often revealing a  “ring-enhancing” lesion (Figure 72-5). A microbiologic  diagnosis requires obtaining pus from the abscess and per¬  forming a culture for aerobic and anaerobic bacteria, and  fungi. In bacterial brain abscesses, the Gram stain frequently  reveals several types of bacteria indicting a mixed infection.  Aspiration of pus from the lesion is both diagnostic and  therapeutic, having the effect of draining the abscess.   A microbiologic diagnosis of Toxoplasma infection is  usually made by identifying specific radiographic findings  in an at-risk host (e.g., HIV/AIDS) with a positive Toxo¬  plasma IgG and a response to specific anti-protozoal ther¬  apy. A PCR-based assay for Toxoplasma nucleic acid is also  available.   Toxoplasma gondii is the main protozoal cause of brain  abscess. It is an important cause in immunocompromised  patients, especially those with AIDS, patients receiving  cancer chemotherapy, or patients on immunosuppressive  drugs used to enhance transplant survival. Toxoplasma  gondii can be transmitted by solid organ transplant, espe¬  cially heart transplants, as well as by the more common  modes of transmission, namely ingestion of raw meat con¬  taining cysts or by exposure to cat feces containing oocytes.  Transplacental transmission of T. gondii can cause intracra¬  nial calcifications in the fetus.   Empiric antimicrobial therapy for bacterial brain abscesses  consists of a third-generation cephalosporin, such as ceftri¬  axone or cefotaxime, plus metronidazole. The latter is cov¬  erage for the anaerobic bacteria. Vancomycin should be  added if the patient has undergone a neurosurgical proce¬  dure. Treatment of bacterial and fungal brain abscesses may  require aspiration of pus from the abscess in addition to  antibacterial or antifungal drugs.   Brain abscess caused by Nocardia can be treated with  trimethoprim-sulfamethoxazole. Brain abscess caused by    Aspergillus can be treated with voriconazole. Treatment of  Toxoplasma brain abscess includes a combination of pyri¬  methamine and sulfadiazine.   There are no vaccines to prevent brain abscesses. Early  treatment of odontogenic and sinus infections may prevent  these complications. Tight control of blood glucose may  prevent rhinocerebral mucormycosis in diabetics. Treat¬  ment of AIDS patients with antiretroviral therapy may  prevent Toxoplasma brain abscess, and when the CD4  count is <100 cells/pL, primary prophylaxis with trime¬  thoprim-sulfamethoxazole is recommended in patients  who are positive for Toxoplasma IgG.   Subdural empyema is a collection of pus on the inner sur¬  face of the dura mater, whereas epidural empyema is a col¬  lection of pus on the outer surface. They can occur adjacent  to the dura of either the brain or spinal cord.   Sinusitis and otitis media are common predisposing fac¬  tors, and the bacteria causing these empyemas are those that  cause sinusitis and otitis media, namely, aerobic and anaero¬  bic streptococci, staphylococci, enteric gram-negative rods  such as Escherichia coli, and anaerobic gram-negative rods  such as Prevotella. Mixed infections are common.   The clinical features include fever plus symptoms of  increased intracranial pressure, such as headache, vomit¬  ing, focal neurologic deficits, and altered mental status.  MRI with gadolinium enhancement reveals a mass adjacent  to the dura. Microbiologic diagnosis involves aspirating  pus from the lesion and performing a Gram stain and cul¬  ture. Treatment involves surgical drainage of the pus com¬  bined with antibiotics appropriate for the bacteria isolated  from the aspirated pus.   Encephalopathy refers to altered brain function in the absence  of inflammation. In general, patients with encephalopathy do  not have fever, headache, seizures, focal neurologic signs, and  an increased WBC count in the blood and spinal fluid,  whereas patients with encephalitis often do. Common mani¬  festations of encephalopathy include confusion, personality  changes, disorientation, aphasia, delirium, and dementia.   There are several infection-related causes of encepha¬  lopathy (see later), but most causes are noninfectious (e.g.,  alcohol, drugs, lead, uremia, or liver failure).   • Progressive multifocal leukoencephalopathy  (PML). PML is caused by JC virus and occurs in    immunocompromised patients, notably AIDS  patients. Infection with JC virus occurs early in  life and remains latent until the immune system is  compromised. PML has occurred in multiple scle¬  rosis patients being treated with natalizumab and  in transplant recipients being treated with myco-  phenolate. Microbiologic diagnosis is made by  detecting JC virus DNA using PCR assay on brain  specimens or spinal fluid. There is no antiviral drug  therapy and no vaccine. Additional information  can be found in Chapter 44.   • HIV encephalopathy including AIDS dementia.  Another CNS disease that is seen in HIV-infected  individuals is encephalopathy caused by HIV itself.  It can vary from mild symptoms such as memory  problems and apathy to more serious disease such  as profound memory loss and psychosis (AIDS  dementia). AIDS dementia is more likely to occur  when CD4 counts are below 200/pL and when the  viral load in the CSF is high.   • Creutzfeldt-Jakob disease (CJD) and kuru. CJD  is one of the human transmissible spongiform  encephalopathies. The term “spongiform” refers  to the spongy, Swiss cheese-like appearance of the  brain of patients with CJD. CJD is caused by prions,  a misfolded protein in which the normal alpha-  helical configuration has changed to a beta-pleated  sheet, thereby altering the function of the protein  and leading to death of neurons. Additional infor¬  mation on prions can be found in Chapter 44.   CJD occurs sporadically worldwide at a rate of  about one case per million population. CJD has  been transmitted iatrogenically by corneal trans¬  plant, intracerebral electrodes, and dura mater  grafts. CJD does not have any relationship to the  ingestion of any food, unlike variant CJD, which is  discussed later.   The main clinical findings in CJD are demen¬  tia and myoclonus. The progression is gradual but  inexorable, resulting in coma and death. Defini¬  tive diagnosis is made by observing spongiform  changes in brain biopsy followed by histochemi-  cal staining with anti-prion antibodies. There is no  drug treatment for CJD and no vaccine.   Variant CJD is acquired by the ingestion of  prion-containing beef. It is declining as a result of  the ban on the addition of animal products to cattle  feed.   Kuru is a spongiform encephalopathy found in  the Fore tribe in New Guinea. It is now very rare  because the eating rituals that transmitted the  agent are no longer practiced.   • Reye’s syndrome. Reyes syndrome is a postinfec-  tious disease consisting of encephalopathy plus  liver failure. It occurs primarily following influ¬  enza B and varicella infections in children and    is associated with aspirin use. The role of aspirin  in pathogenesis is uncertain but a toxic effect on  mitochondria has been proposed.   After the child has recovered from the viral  infection, Reyes syndrome begins with prominent  vomiting followed by encephalopathic changes  such as lethargy and combative behavior progress¬  ing to coma and death. Cerebral edema is marked.  Fatty degeneration of the liver occurs, and liver  enzymes such as transaminases are elevated. Blood  ammonia levels are elevated.    Treatment should be instituted promptly. If  coma occurs, treatment is less effective. Antiviral  drugs are not effective. Supportive measures such  as cooling blanket, ventilator to provide respiratory  support, control of intracranial pressure, hemodi¬  alysis, and fluid and electrolyte balance are used.  Vaccines against varicella and influenza and public  health campaigns to reduce aspirin use in febrile  children have greatly reduced the incidence of this  disease. Acetaminophen should be used to reduce  fever in children.   Infections with a variety of agents can occur in any part of  the gastrointestinal (GI) tract from the mouth to the anal  canal. Infections can range in severity from self-limited to  life-threatening, particularly if infection spreads from the  gut to other parts of the body. Infections are typically  caused by the ingestion of exogenous pathogens in suffi¬  cient quantities to evade host defenses and then cause dis¬  ease by multiplication, toxin production, or invasion  through the gastrointestinal mucosa to reach the blood¬  stream and other tissues. In other cases, members of the  normal flora of the GI tract can cause disease.   Inflammation caused by infection, typically by fungi such  as Candida or viruses such as herpes simplex virus, causes  the symptoms of esophagitis. Most cases occur in immuno¬  compromised patients, especially those with reduced cell-  mediated immunity. The extent of damage to the esophagus  is typically related to the severity of symptoms.   Odynophagia (pain on swallowing) and dysphagia (diffi¬  culty in swallowing) are the key clinical manifestations of  esophagitis.   Candida is the most common etiology, particularly among  human immunodeficiency virus (HIV)-infected patients  and other immunocompromised hosts (Figure 73-1). Less  common pathogens include herpesviruses such as cyto¬  megalovirus and herpes simplex virus. Noninfectious  causes also occur, such as acid reflux from the stomach and  medication-induced disease (e.g., doxycycline).   Diagnosis may be empiric after a trial of fluconazole  results in improvement for presumed Candida esophagitis.   FIGURE 73-1 Candida esophagitis. Note the many whitish  lesions on the esophageal mucosa seen on endoscopy. (Reproduced  with permission from McKean SC et al. Principles and Practice of Hospital Medicine.  New York: McGraw-Hill, 2012. Copyright © 2012 by The McGraw-Hill Companies, Inc.)    If an empiric course of fluconazole does not work, then  endoscopy for visualization and biopsy could be helpful,  particularly in immunocompromised hosts. Biopsy sam¬  ples should be analyzed by using pathologic and microbio¬  logic tests.   In a typical patient (e.g., HIV-infected patient) presenting  with odynophagia and retrosternal pain, an empiric diag¬  nosis of esophageal candidiasis is made and fluconazole  therapy instituted. If there is no effect on symptoms and if  Candida resistance is not suspected, then further diagnos¬  tics as outlined earlier may identify a specific organism that  could be targeted for treatment.   One option to prevent recurrent esophageal candidiasis is by  using fluconazole prophylaxis. However, this is not generally  advised given the high risk of selecting for fluconazole-  resistant Candida. Immune restoration in HIV-infected  patients may decrease the incidence of esophageal and oro¬  pharyngeal candidiasis.   Gastritis refers to inflammation of the mucosa of the  stomach. It may be erosive or nonerosive, depending on  histologic and endoscopic findings. A break in the gastric  and adjacent duodenal mucosa defines peptic ulcer  disease.   The mechanism by which one of the main pathogens,  Helicobacter pylori causes peptic ulcer disease has been  largely elucidated. Following attachment to the gastric  mucosa, H. pylori causes direct mucosal damage by the  combination of ammonia production (from the action of  the organism’s urease on urea) and the host inflammatory  response. The ability of the organism to survive is  enhanced by the neutralization of the stomach’s acid by  the ammonia produced.   Patients with gastritis typically complain of dyspepsia  (epigastric pain, burning), nausea, and vomiting. In the  case of peptic ulcer disease, epigastric pain is the pri¬  mary symptom. Some patients may report alleviation of  pain with food, particularly those with duodenal  ulcers. Gastrointestinal bleeding is a complication of  peptic ulcer disease. Some patients with gastritis may  be asymptomatic.   Infectious and noninfectious etiologies are possible. Among  infectious causes, H. pylori is the most important. Viruses  such as cytomegalovirus and fungi such as Mucor may  rarely cause ulcer disease as well, particularly among  immuncompromised patients. Following ingestion of raw  fish, larvae of Anisakis species may become embedded in  the gastric mucosa and cause severe abdominal pain.  Mycobacteria (tuberculosis and nontuberculosis mycobac¬  teria), Giardia, and Strongyloides may also cause gastritis.  Noninfectious causes such as alcohol and medications (e.g.,  nonsteroidal anti-inflammatory drugs) are also  implicated.   Upper endoscopy with gastric biopsy is the definitive diag¬  nostic strategy. If abnormal findings are detected, patho¬  logic analysis and further directed testing may be  performed. For the most common infectious cause of pep¬  tic ulcer disease, H. py/on-associated ulcers can be con¬  firmed using a urease test on the biopsy specimen or using  noninvasive tests such as the urea breath test or stool anti¬  gen test.   Treatment is directed at the underlying pathogen, taking  the host immune status into consideration. For H. pylori,  combination therapy with two antibiotics, such as ampi-  cillin and clarithromycin, plus a proton pump inhibitor,  such as omeprazole, or bismuth is used with varying  success.   It is useful to think of diarrhea as acute (lasting <2 weeks)  or chronic (persisting >4 weeks). We will focus on acute  diarrhea in this chapter because most of the etiologies are  infectious in nature. We can further categorize acute diar¬  rhea as noninflammatory (watery, nonbloody) or inflam¬  matory (bloody). Bloody diarrhea is also known as  dysentery. For example, bloody diarrhea caused by Shi¬  gella is often called bacillary dysentery. Table 73-1  describes the important features of watery and bloody  diarrhea. Table 73-2 lists the important organisms that  cause either watery or bloody diarrhea.   Diarrhea must be calibrated against the patient’s normal  bowel movements but is usually considered to be greater  than three to five bowel movements per day. Most of the  infectious agents that cause diarrhea act at the small intes¬  tine (where the majority of fluid normally gets absorbed) or  the colon.    Acute diarrhea is very common. There are approxi¬  mately 179 million cases each year in the United States.  The most common cause of acute diarrhea in the  United States is norovirus. It is an especially prominent  cause of outbreaks of diarrhea. The most common  cause of fatal diarrhea is hospital-associated Clostrid¬  ium difficile.   Pathogens or their associated toxins disrupt the normal  absorption and secretory processes in the small intestines.  Acute diarrhea is usually caused by preformed exotoxins in  food or by the infectious agents in the intestinal tract (via  either enterotoxin and cytotoxin production or mucosal  invasion). Pathogens that produce preformed exotoxins  include Staphylococcus aureus, Bacillus cereus, and Clos¬  tridium perfringens. Other pathogens that cause nonin¬  flammatory acute diarrhea by enterotoxin production  include enterotoxigenic Escherichia coli (ETEC) and Vibrio  cholerae. Chapter 7 describes the mechanism of action of  these toxins.    TABLE 73-2 Important Organisms That Typically  Cause Either Watery or Bloody Diarrhea   Pathogens that cause acute inflammatory diarrhea  include Salmonella, Shigella, Campylobacter (via mucosal  invasion), Shiga toxin-producing E. coli (STEC) such as  E. coli 0157:H7, and C. difficile (via cytotoxin production).  Antibiotic use predisposes to pseudomembranous colitis  caused by C. difficile. Chapter 18 provides additional infor¬  mation on these enteric gram-negative rods, and Chapter 17  discusses C. difficile.   There are several host factors that predispose to diar¬  rheal illness. Patients taking proton-pump inhibitors are at  risk because gastric acid levels are reduced. Recent travel to  developing countries and antibiotic treatment are also asso¬  ciated with an increased incidence of diarrhea. Immuno-  suppressed patients have more frequent and more severe  diarrheal illness.   Table 73-3 describes the clinical presentation caused by  important GI tract pathogens. Patients complain of diar¬  rhea accompanied by urgency, abdominal bloating, and  cramping. In the case of acute inflammatory diarrhea, there  is also blood or pus seen in the stool, and patients can be  febrile. If vomiting is a major feature of the clinical presen¬  tation, this suggests S. aureus food poisoning or viral gas¬  troenteritis. If symptoms begin within 6 hours after  ingestion of suspected contaminated food, then preformed  toxin of S. aureus or B. cereus should be suspected. On  physical examination, patients may also show signs of  dehydration with tachycardia and orthostatic changes in  blood pressure.   Children infected with STEC often have bloody diar¬  rhea and may progress to hemolytic-uremic syndrome  (HUS). HUS occurs when Shiga toxin produced by STEC  enters the blood stream. The symptoms of HUS include  hemolytic anemia, thrombocytopenia, and renal failure.  Distorted red blood cells called schistocytes can be seen in  blood smears. The use of ciprofloxacin increases the risk of  HUS. Ingestion of undercooked hamburger or contami¬  nated produce or contact with animals at petting zoos pre¬  disposes to disease caused by STEC.   Most cases of mild, acute watery diarrhea of short duration  are caused by viruses. These include norovirus, rotavirus,  and less commonly, adenovirus and astrovirus. Outbreaks  of norovirus infection commonly occur in closed popula¬  tions, such as nursing homes, hospitals, cruise ships, and  dormitories. Rotavirus is a common cause of diarrhea in  children but the incidence is declining due to increased use  of the rotavirus vaccine. ETEC and Listeria monocytogenes  are important bacterial causes. Vibrio cholerae causes  severe, life-threatening watery diarrhea.   Most cases of severe diarrhea, however, are caused by  bacteria. Pathogens such as Salmonella, Shigella,    TABLE 73-3 Clinical Presentation, Diagnosis, and Treatment of Diarrhea Caused by Important Gastrointestinal  Tract Pathogens   Usually within 6 hours of consump¬  tion of infected food (dairy prod¬  ucts, mayonnaise, meat products);  recovery in 1-2 days   Usually within 6 hours of  consumption of infected food  (reheated rice)   Suspect Listeria when rou¬  tine stool cultures do  not show a pathogen,  particularly in an out¬  break setting   Acquired by ingestion of unpas¬  teurized soft cheese, deli meats  and raw vegetables. Can grow at  refrigerator temperature   Severe, watery diarrhea  with rapid fluid and  volume loss. Vomiting  in early disease   Supportive care (e.g.,  aggressive fluid repletion,  electrolytes). Antibiotics  (e.g., ciprofloxacin) in  severe disease   Suspect cholera if watery diarrhea  associated with rapid and severe  volume loss, or in an outbreak  setting   Abdominal cramps,  flatulence, diarrhea  (acute or chronic);  stools are fatty, foul¬  smelling, and may  float   Stool ova and parasite  analysis may reveal cysts  or trophozoites. Stool  antigen test increasingly  used   Nitazoxanide for severe  diarrhea. Antiretroviral  therapy to restore  immune system in AIDS  patients   Cause of large communitywide  outbreaks from contaminated  water supply; important cause of  diarrhea in AIDS patients   2. Acute inflammatory diarrhea (stools can be bloody; can be febrile)   Stool culture grows E. coli  that does not ferment  sorbitol. Need special  test to identify toxin-  producing strains   None. Antibiotics many  increase risk of  hemolytic-uremic  syndrome, especially in  children   Acquired by ingestion of under¬  cooked ground beef, fruits and  vegetables contaminated with  cattle manure   Traditionally associated with anti¬  microbial drug use; increasingly,  community-acquired cases in  patients without traditional risk  factors    TABLE 73-3 Clinical Presentation, Diagnosis, and Treatment of Diarrhea Caused by Important Gastrointestinal  Tract Pathogens ( Continued)   Diarrhea with blood or  pus usually; abdomi¬  nal cramps; can be  febrile   Person-to-person spread can occur;  humans are the reservoir; not  found in animals   Acquired by ingestion of under¬  cooked eggs, unpasteurized dairy  products, raw vegetables, or  undercooked poultry. Also by  exposure to pet snakes and  turtles   Acquired by ingestion of unpas¬  teurized dairy products, or under¬  cooked poultry. Associated with  Guillain-Barre syndrome   Bloody diarrhea, fever,  and abdominal pain.  Syndrome is called  amebic dysentery   Stool ova and parasite  analysis may reveal cysts  or trophozoites;  serology   Metronidazole or tinidazole  to eliminate tissue tro¬  phozoites, plus a luminal  agent such as  paromomycin    Campylobacter, STEC, and C. difficile are implicated in this  category. In the United States, Salmonella and Campylo¬  bacter are the most common bacterial causes. Diarrhea  caused by these bacteria is typically bloody.   Protozoa, such as Giardia, Entamoeba histolytica,  Cryptosporidium, Cyclospora, and microsporidia, are less  common causes of diarrhea but are suspected in certain  scenarios (e.g., in returning travelers or immunocom¬  promised patients). Of these protozoa, Giardia is the  most common cause of diarrhea in the United States.  Giardiasis typically occurs in young children in day care,  in men who have sex with men, and in hikers who drink  untreated ambient water. In HIV-infected patients with  very low CD4 counts, Cryptosporidium causes prolonged  diarrhea and may cause extraintestinal disease involving  the biliary and respiratory tracts. Entamoeba histolytica  causes amebic dysentery characterized by bloody  diarrhea.   Diagnosis is generally focused on deciding who and  when to test (i.e., determining when a test result may  potentially impact the outcome). Because many causes  of acute diarrhea are self-limited, this is an important  issue. In general, we seek a diagnosis in cases of severe  watery diarrhea, in cases of bloody diarrhea, if the    patient is febrile, or if the patient is elderly or immuno¬  compromised. Routine stool cultures will identify Sal¬  monella, Shigella, and Campylobacter. If diarrhea is  bloody, a special culture (e.g., MacConkey-sorbitol agar)  is specifically set up to detect STEC. The basis for the  special culture is that STEC strains typically do not fer¬  ment sorbitol. The definitive laboratory diagnosis of an  STEC strain is made by either polymerase chain reaction  (PCR) test or immunoassay for the Shiga toxin.   Also, if bloody diarrhea is associated with antibiotic use,  laboratory tests for the presence of the C. difficile toxin in  the stool should be done. Colonoscopy may reveal the char¬  acteristic yellowish plaques seen in pseudomembranous  colitis (Figure 73-2).   Rotavirus infection can be diagnosed by testing the stool  for rotaviral antigen. A PCR test for norovirus RNA in the  stool can be used to diagnose infection by that virus and is  especially useful in outbreak situations.   Sending stool samples for analysis of ova and para¬  sites (O&P) is generally not cost effective, except in  immunocompromised patients, patients with a history  of recent foreign travel, or when diarrhea is associated  with community waterborne outbreaks. Stools for O&P  are usually sent on three consecutive days given that  some parasites only intermittently shed eggs or cysts in  the stool.    There are two rotavirus vaccines available (see  Chapter 40). Both contain live virus and are given orally.  One is a live, attenuated vaccine (Rotarix), which contains  the single most common rotavirus serotype (Gl) causing  disease in the United States. The other is a live reassortant  vaccine (Rotateq), which contains five rotavirus strains. An  increased risk of intussusception has been reported with  both vaccines. Patients with a history of intussusception  should not receive either vaccine.    FIGURE 73-2 Pseudomembranous colitis caused by Clostrid¬  ium difficile. Note yellowish pseudomembranes seen on colonoscopy.  (Reproduced with permission from Longo DL et al (eds): Harrison's Principles of Inter¬  nal Medicine, 18th ed. New York: McGraw-Hill, 2012. Copyright © 2012 by The  McGraw-Hill Companies, Inc.)   The most important treatment modality in diarrhea is  hydration. Oral rehydration solution containing water, salt,  and sugar has been life-saving in many parts of the world.  In general, for most cases of community-acquired diarrhea,  empiric treatment with antimicrobials is not beneficial.   The drug of choice for mild or moderate colitis caused  by C. difficile is metronidazole. Severe infections caused by  C. difficile should be treated with oral vancomycin. Severe  cases of shigellosis (bacillary dysentery) should be treated  with ciprofloxacin. Severe cases of Campylobacter infection  should be treated with azithromycin. Listeria gastroenteri¬  tis does not require antibiotics in immunocompetent, non¬  pregnant patients.There is no antiviral drug available for  either norovirus or rotavirus. Giardia infection should be  treated with tinidazole.   Most of the preventative strategies are directed at travelers  to developing countries. They are advised to avoid poten¬  tially contaminated water sources as well as fresh fruit and  vegetables if not washed in boiled water. One current  approach is to provide the traveler with a supply of antimi¬  crobials, such as ciprofloxacin, to be taken in the event of a  diarrheal episode. Probiotics may be useful to prevent  antibiotic-associated colitis caused by C. difficile.   Appendicitis is inflammation of the vestigial vermiform  appendix. It is one of the most common causes of acute  abdomen requiring surgical exploration.   Obstruction of the appendix by one of a variety of causes  (e.g., fecaliths, infection such as parasites, tumor) leads to  an increase in luminal and intramural pressure. Bacterial  overgrowth is accompanied by inflammation. If there is  necrosis, perforation followed by diffuse peritonitis caused  by bacteria of the normal colonic flora (e.g., E. coli and  Bacteroides) may occur.   Clinical manifestations include abdominal pain (especially  periumbilical pain migrating to the right lower quadrant),  anorexia, nausea, and vomiting. Low grade fever and mild  leukocytosis may be present. Initial symptoms may be  missed because they may be nonspecific (e.g., indigestion).  A standard abdominal computed tomography (CT) scan  with contrast is often used when appendicitis is suspected.   Early in the course of the disease, the predominant organ¬  isms are anaerobic. In late disease, mixed organisms pre¬  dominate. Escherichia coli, Peptostreptococcus, Bacteroides  fragilis, and Pseudomonas are commonly isolated. Yersinia,  Campylobacter, and Salmonella can cause an acute ileitis  and mesenteric adenitis that can mimic appendicitis.   Clinical manifestations combined with imaging, using  either CT scan or ultrasonography, are typically used to  make a decision as to whether a patient should be taken to  the operating room.   Surgery is the definitive treatment for appendicitis, usually  in concert with perioperative antibiotics. Laparascopic    appendectomy is preferred to open appendectomy, as the  laparascopic approach has a reduced risk of surgical wound  site infections, a reduced risk of bowel obstruction, and a  faster recovery time. A course of antibiotics alone (without  surgery) is sometimes used, but there is an increased risk of  recurrent appendicitis.   Diverticulitis is inflammation of a sac-like protrusion of the  colonic wall, usually in the sigmoid colon (Figure 73-3).  Perforation of the diverticulum with consequent abscess  formation or peritonitis may occur.   Colonic diverticula may occur following years of a diet  deficient in fiber.   There is a range of symptoms depending on the degree of  perforation. Patients usually present with dull, aching, left  lower quadrant abdominal pain. This is often accompanied  by a low-grade fever, leukocytosis, nausea, and vomiting.  Diarrhea or constipation may be present. If perforation  occurs, patients can present with generalized peritoneal  signs with diffuse abdominal pain and shock.   FIGURE 73-3 Diverticulosis. Three black arrows indicate  diverticula. A diverticulum is an outpouching from the colonic wall.  Diverticulitis is an inflammation of the diverticulum. (Reproduced with  permission from Marcin P, Matta EJ, Elsayes KM. Gastrointestinal Imaging. In:  Elsayes KM, Oldham SA. eds. Introduction to Diagnostic Radiology. New York, NY:  McGraw-Hill; 2015.)   Bowel flora such as anaerobes, that is, B. fragilis, and those  in the Enterobacteriaceae family, such as E. coli, are typi¬  cally involved.   Abdominal CT will show colonic diverticula and associated  wall thickening, fat infiltration, abscesses, and extraluminal  air or contrast medium.   Oral antibiotics with excellent anaerobic activity (e.g.,  amoxicillin with clavulanate or a combination of cipro¬  floxacin plus metronidazole) are used in mild cases. In  more serious cases requiring hospitalization, intravenous  fluids and antibiotics are given with bowel rest as needed. If  there is an associated abdominal abscess or signs of perito¬  nitis, surgical evaluation must be undertaken.   Enteric fever is a clinical syndrome comprised of constitu¬  tional symptoms, such as fever and headache, and nausea,  vomiting, and abdominal pain. Although enteric fever can  be caused by several Salmonella species, “typhoid fever”  refers to enteric fever caused by the Salmonella typhi. S.  typhi is also known as Salmonella enterica serotype Typhi.  Typhoid fever is a significant global health problem.   Following the consumption of contaminated food, Salmo¬  nella bacteria enter through the intestinal mucosal epithe¬  lium by transcytosis. The microbes then replicate in the  macrophages of Peyer patches, mesenteric lymph nodes,  and the spleen. Bacteremia then occurs with dissemination  to lungs, gallbladder, kidneys, or central nervous system.   Humans are the only reservoir for S. typhi, so contami¬  nation of food or water by human feces should be  suspected.   A prodromal phase is characterized by constitutional  symptoms such as malaise, together with abdominal pain,  constipation, and headache. Fever increases over the next  several days. During the second week of disease, a typical  transient rash of pink maculopapular lesions (rose spots)    may be seen. Splenomegaly occurs more commonly than  hepatomegaly, but both may occur. Relative bradycardia  and leukopenia are often observed. Diarrhea is  uncommon.   The chronic carrier state occurs in approximately 3% of  patients with typhoid fever. The organisms typically reside  in the gallbladder and are excreted in the stool, serving as a  source of infection for others.   Salmonella typhi and other Salmonella species, such as Sal¬  monella paratyphi A and S. paratyphi B, cause typhoid  fever.   A history of travel to endemic areas, together with a compat¬  ible clinical presentation, is often used initially. Any fever in  a returning traveler should prompt blood cultures and a  clinical suspicion for enteric fever. Early in the disease, blood  cultures are typically positive and stool cultures are often  negative. Later in the disease and in the carrier state, stool  cultures are positive and blood cultures are negative. Stool  cultures are positive at this stage because bile from an  infected gallbladder carries organisms into the stool.   Oral or intravenous ciprofloxacin is often used. Intrave¬  nous ceftriaxone is another effective treatment modality.  Ciprofloxacin for 4 weeks can also be used to eliminate the  carrier state. Cholecystectomy should be considered for  those chronic carriers who do not respond to antimicrobial  therapy.   Hygienic measures to protect the food and water supply  from human fecal contamination are an important public  health intervention. Immunization may not always be  effective but can be considered in epidemic outbreaks, for  travelers to endemic countries, and for household contacts  of typhoid carriers.   Two vaccines against typhoid fever are available in the  United States, both providing approximately 50% to 80%  protection. The vaccine containing the Vi capsular polysac¬  charide of S. typhi has the advantage of being administered  once, intramuscularly. The other vaccine contains live  attenuated S. typhi organisms and is administered orally. It  has the advantage of stimulating gut immunity (IgA)  thereby interrupting transmission.   Infections in the pelvic organs and surrounding structures  comprise a heterogenous group of diseases. They primarily  affect sexually active women and men. Many of the patho¬  gens implicated are sexually transmitted, so an important  facet of management is partner notification and treatment,  as well as patient education regarding safe sexual practices.  Among sexual transmitted infections, major syndromes that  will be discussed are genital ulcer disease, vaginitis, cervici¬  tis, pelvic inflammatory disease, urethritis, and prostatitis.   Genital ulcer disease manifests as a breach in the skin or  mucosa of the genitalia, usually caused by a sexually trans¬  mitted infection. Of these infections, herpes simplex virus  type 2 (HSV-2) is the most common etiology in most geo¬  graphic areas, followed by syphilis and chancroid. The most  important noninfectious cause is Behcet’s disease.   The mechanisms by which ulcers are produced by patho¬  gens are incompletely understood, and there are different  mechanisms of injury depending on the pathogen. In chan¬  croid, a cytotoxin secreted by Haemophilus ducreyi may be  important in epithelial cell injury.   Although the various lesions may have a characteristic  appearance, it is important to note that local epidemiology  is an important consideration because lesions may appear    in an atypical fashion. The appearance of the ulcer, whether  it is painful, and the nature of the associated lymphade-  nopathy may be clues in the etiology of the ulcer.  Figure 74-1 shows several vesicles on the shaft of the penis  of a patient with genital herpes. The vesicular lesions are  typically painful. The vesicles can then progress to form  shallow ulcers. Figure 74-2 shows the chancre of primary  syphilis. It is a painless lesion with a shallow base and a  firm, rolled edge. Table 74-1 describes the important clini¬  cal features of genital ulcer lesions, their diagnostic proce¬  dures, and treatment.   Common infectious etiologies of genital ulcer disease  include HSV-2 (causing genital herpes), Treponema palli¬  dum (causing primary syphilis), and H. ducreyi (causing  chancroid). Less common pathogens include Chlamydia  trachomatis serovars Ll-3 (causing lymphogranuloma  venereum) and Klebsiella granulomatis (causing granuloma  inguinale, also known as donovanosis).   A thorough sexual and medical history, followed by the  physical examination, are important for diagnosis.  Although clinical characteristics can be very helpful, there  is often overlap in presentation, and there may also be mul¬  tiple syndromes copresenting. Therefore, diagnostic testing  is highly recommended. These typically include the follow¬  ing tests at the initial visit: direct fluorescent antibody  (DFA) test for HSV antigens, viral culture for HSV, or  nucleic acid amplification methods for HSV DNA using a  specimen taken from the base of the ulcer, and syphilis  serologic testing (e.g., rapid plasmin reagin, RPR) using a   FIGURE 74-1 Genital herpes caused by herpes simplex virus-2.  Note group of vesicles on shaft of penis. (Reproduced with permission from  Wolff K, Johnson R, Saavedra A (eds). Fitzpatrick's Color Atlas & Synopsis of Clinical Der¬  matology, 7th ed. New York: McGraw-Hill, 2013 Copyright © 2013 by The McGraw-Hill  Companies, Inc.)   serum sample. Testing for other sexually transmitted dis¬  eases including human immunodeficiency virus (HIV) is  also important because there is often cotransmission of  multiple pathogens (see Table 74-1).   FIGURE 74-2 Chancre of primary syphilis caused by  Treponema pallidum. Note shallow ulcer with clean base and rolled  edge. (Reproduced with permission from Goldsmith LA, et al. Fitzpatrick's Derma¬  tology in General Medicine. 8th ed. New York: McGraw-Hill, 2012. Copyright © 2012  by The McGraw-Hill Companies, Inc.)   The drug of choice for genital herpes is acyclovir or one of  its derivatives, famciclovir or valacyclovir. Primary and  secondary syphilis are treated with a long-acting penicillin,  benzathine penicillin G. The drug of choice for chancroid  is azithromycin, whereas for lymphogranuloma venereum,  it is doxycycline (see Table 74-1).   Empiric treatment is often used before diagnostic tests  return. As for most sexually transmitted infections, treat¬  ment that involves one dose and that is observed is pre¬  ferred if possible.   Consistent use of condoms is an important measure that can  prevent genital ulcers. In some cases, primary prevention of  HSV infection can be undertaken by treatment of the nega¬  tive partner in serodiscordant couples with acyclovir or one  of its derivatives. Prophylaxis with these drugs can be effec¬  tive in preventing recurrences of HSV outbreaks in patients  who have had frequent occurrences, especially among those  who are immunosuppressed. Partner notification and treat¬  ment are important prevention strategies as well.   Vaginitis is inflammation of the vagina that can result in  discharge, itching, and pain. Common causes of vaginitis  are candidiasis, trichomoniasis, and bacterial vaginosis.  Noninfectious causes include lichen planus and certain  medications (e.g., oral contraceptives).   The use of antibiotics that inhibit the normal flora of the  vagina, especially lactobacilli, predisposes to Candida vagini¬  tis. Candida is a member of the normal flora of many women.  The pathogenesis of bacterial vaginosis is uncertain, but it  does not appear to be a sexually transmitted disease. Tricho¬  moniasis, on the other hand, is a sexually transmitted disease.   Patients are usually prompted to seek medical attention  because of an abnormal vaginal discharge. This may be  accompanied by pruritus, pain (including dyspareunia),  and symptoms of vaginal irritation. Figure 74-3 depicts the  white, “cottage cheese” appearance of vaginal candidiasis.  Figure 74-4 shows the “strawberry” cervix of trichomonia¬  sis. There are red, punctate lesions on the cervix, and frothy  exudate can be seen at the cervical os. The vaginal dis¬  charge in bacterial vaginosis is thin and grayish and has an  unpleasant odor, often described as “fishy.” Table 74-2  describes the important clinical features of vaginitis, its  diagnostic procedures, and its treatment.   Direct fluorescent antibody  (DFA) testing and/or viral  culture; sample taken from  the base of the ulcer; Tzanck  smear shows multinucle-  ated giant cells; NAAT may  be useful   Single (usually),  indurated ulcer  with a clean  base; self¬  resolves and  may not be  observed   Serologic screening with non-  treponemal test (e.g., rapid  plasma reagin [RPR] or  Venereal Disease Research  Laboratory [VDRL]);  treponemal test (e.g.,  fluorescent treponemal  antibody absorption [FTA-  ABS]) for confirmation; dark  field examination of fluid  from the lesion if possible   Difficult to diagnose; special  culture media, if available,  or polymerase chain reac¬  tion (PCR)   Characteristic  appearance of  lymph nodes is  key feature; may  be bilateral, large,  and painful; pres¬  ents with fluctu¬  ant "buboes" and  sinus tracts   Marked, beefy red,  vascular ulcer  with granuloma¬  tous appearance  and rolled edges   Difficult to diagnose; PCR if  available; may also see dark-  staining oval organisms on  Giemsa staining (Donovan  bodies) of biopsy specimen   FIGURE 74~3 Vaginal candidiasis caused by Candida albicans.  Note areas of whitish, "cottage cheese-like" exudate on cervical  mucosa. (Source: Centers for Disease Control and Prevention.)   Candida albicans is the most common cause of vaginal  candidiasis. Trichomonas vaginalis is the cause of trichomo¬  niasis. Overgrowth of bacteria such as Gardnerella vaginalis  is implicated in bacterial vaginosis, but anaerobes such as  Mobiluncus are also involved.   A patient’s complaint of vaginal discharge should prompt a  careful history, including time of last menstrual period,  medications, and sexual activity. The physical examination  should include a microscopic examination of the vaginal  discharge itself on a glass slide using a drop of 0.9% saline  solution (to look for motile trichomonads or clue cells),  followed by a drop of 10% potassium hydroxide (to look for  Candida). If microscopy is negative, Candida culture and  Trichomonas nucleic acid amplification test (NAAT) are  performed to increase sensitivity.   FIGURE 74-4 Trichomoniasis caused by Trichomonas vagina¬  lis. Note frothy discharge and punctate "strawberry" lesions on cervix.  (Reproduced with permission from Usatine, RP et al: The Color Atlas of Family  Medicine, New York: McGraw-Hill, 2009. Courtesy of Richard P. Usatine, MD.)    Trichomonads are shown in Figure 74-5. Figure 74-6  shows clue cells as large, vaginal epithelial cells dotted with  bacteria. A Gram stain of clue cells reveals many gram-  variable rods on the surface of the epithelial cells.  Figure 74-7 shows the appearance of the yeasts and pseu-  dohyphae of Candida. Cultures for Gardnerella are not  done because at least 50% of asymptomatic women carry  the organism. See Table 74-2 for additional information.   Metronidazole is the drug of choice for both bacterial vagi¬  nosis and trichomoniasis. For candidiasis, either oral flucon¬  azole or vaginally administered miconazole or butoconazole  is the drug of choice (see Table 74-2). Trichomonas vaginalis  is a sexually transmitted infection, so a one-time treatment  regimen of patient and partner is preferred. Following treat¬  ment of trichomoniasis, the CDC recommends retesting  with NAAT to evaluate for reinfection, regardless of whether  the partner was treated.   Cervicitis is inflammation of the uterine cervix. Acute cervi¬  citis is usually due to a sexually transmitted infection caused  by either C. trachomatis or Neisseria gonorrhoeae or both.   FIGURE 74-5 Trichomonas vaginalis in vaginal discharge. Note  trichomonads (arrows) mounted in saline and visualized in light micro¬  scope. (Reproduced with permission from Usatine, RP et al: The Color Atlas of  Family Medicine, New York: McGraw-Hill, 2009. Courtesy of Richard P. Usatine, MD.)   A large proportion of women with cervicitis are asymptom¬  atic. In many cases, cervicitis is detected on speculum exami¬  nation (Figure 74-8) and/or following routine screening  for C. trachomatis and N. gonorrhoeae. Women who have   FIGURE 74-6 Clue cells in bacterial vaginosis. Note that the  lower epithelial cell is a "clue cell" because its surface is covered with bac¬  teria. The upper epithelial cell is not a "clue cell" because its surface has  few bacteria. {Reproduced with permission from Usatine, RP et al. The Color Atlas of  Family Medicine, New York: McGraw-Hill, 2009. Courtesy of EJ. Mayeaux, Jr., MD.)   FIGURE 74-7 Candida visualized in KOH preparation. Note yeast  cells (red arrow) and pseudohyphae (blue arrow). (Reproduced with  permission from Goldsmith LA et al. Fitzpatrick's Dermatology in General Medicine. 8th ed.  New York: McGraw-Hiil, 2012. Copyright © 2012 by The McGraw-Hill Companies, Inc.)    concomitant urethral infection may have dysuria. On physi¬  cal examination, increased friability of the cervical tissue  after a swab is inserted may be a clue to the diagnosis.   The usual pathogens are C. trachomatis serovars D-K and/  or N. gonorrhoeae. Other less common etiologies include  HSV and T. vaginalis.   A clinical diagnosis may be made based on increased fria¬  bility of the cervix, with or without mucopurulent dis¬  charge. To make a laboratory diagnosis, nucleic acid  amplification testing (NAAT) for C. trachomatis and N.  gonorrhoeae is routinely performed in many centers. If  NAAT testing is not available, then Gram stain and culture  may be performed.   If there is clinical evidence of cervicitis, empiric treatment  for both C. trachomatis and N. gonorrhoeae (ceftriaxone  intramuscularly plus azithromycin orally) is recommended,  particularly if follow-up of test results by the patient is   Could be clear and  mucoid, espe¬  cially at midcycle  estrogen surge;  during preg¬  nancy, secre¬  tions may be  thicker and  white   Clue cells are epithe¬  lial cells covered  with gram-variable  bacteria (see   Yeasts and pseudohy¬  phae seen in KOH  prep (10% potas¬  sium hydroxide)   Fluconazole  (orally), micon¬  azole (vaginal  suppository),  butoconazole  (vaginal cream)  are single-dose  options   (Reproduced with permission from Knoop KJ, Stack LB, Storrow AB, Thurman RJ.   The Atlas of Emergency Medicine, 3rd ed. © 2009 by McGraw-Hill, Inc., New York.   uncertain. Sex partners of patients with a confirmed diag¬  nosis should also be notified and treated.   Consistent use of condoms is an important measure that  can prevent sexually transmitted diseases. Partner notifica¬  tion and treatment are important prevention strategies as  well.   Pelvic inflammatory disease (PID) is a polymicrobial infec¬  tion of upper genital tract structures, namely the uterus,  fallopian tubes, and ovaries.   When the endocervical canal barrier is compromised, vagi¬  nal bacteria can ascend into the normally sterile space of  the upper genital tract (uterus, fallopian tubes, and ova¬  ries). A sexually transmitted infection affecting the cervix    (e.g., N. gonorrhoeae and C. trachomatis ) can initiate the  process, permitting the anaerobic bacteria of the vagina to  ascend.   Having multiple sex partners increases the risk of PID.  Multiple episodes of PID lead to scarring of the fallopian  tubes and an increased risk of ectopic pregnancy and steril¬  ity. PID is especially common in adolescent and young  adult women.   Patients can present with a range of symptoms, from  lower back pain to fever, chills, lower abdominal pain, and  cervical and adnexal tenderness. The abrupt onset of  abdominal pain associated with menses is a common  finding in PID. On physical exam, tenderness on motion  of the cervix and an abnormal vaginal discharge are  important diagnostic signs.   PID is primarily associated with N. gonorrhoeae and  C. trachomatis, together with enteric gram-negative rods  and anaerobes.   Because it is often difficult to diagnose PID precisely (given  the nonspecific findings) and because the consequences of  not treating PID can be grave, many opt to treat with mini¬  mum diagnostic criteria such as uterine, adnexal, or cervi¬  cal motion tenderness. Fever, the presence of leukocytes on  cervical or vaginal discharge, elevated C-reactive protein,  and laboratory evidence of cervical infection with N. gonor¬  rhoeae or C. trachomatis, such as nucleic acid amplification  tests (NAATS) for these organisms can increase the speci¬  ficity of the diagnosis.   If symptoms are mild, women can be treated as outpatients  with cefoxitin or ceftriaxone (one dose) plus doxycycline  (14 days). Metronidazole is sometimes added to the regimen.  In the inpatient setting, intravenous therapy is preferred.  Cefoxitin or cefotetan with doxycycline, or clindamycin plus  gentamicin are initial options with oral antibiotics only after  24 hours of improvement of the patient.   Urethritis is inflammation of the urethra. It is usually  caused by a sexually transmitted infection, particularly in  sexually active men. A noninfectious cause is Reiters syn¬  drome, an autoimmune disease that includes urethritis,  uveitis, and reactive arthritis.   FIGURE 74-9 Urethral discharge in gonorrhea. Note thick  purulent urethral discharge. (Reproduced with permission from Goldsmith LA  et al. Fitzpatrick's Dermatology in General Medicine. 8th ed. New York: McGraw-Hill,  2012. Copyright © 2012 by The McGraw-Hill Companies, Inc.)   Dysuria is a common presenting complaint. Discharge  from the urethra (Figure 74-9), pruritus, and burning are  also common complaints.   Neisseria gonorrhoeae and C. trachomatis are the most  common organisms implicated. Other organisms include  Mycoplasma genitalium and T. vaginalis.   Nucleic acid amplification testing (NAAT) for C. trachomatis  and N. gonorrhoeae is routinely performed in many centers.  Some centers also perform NAAT testing for M. genitalium.   If there is clinical evidence of urethritis such as a purulent  urethral discharge, empiric treatment for both N. gonor¬  rhoeae and C. trachomatis (ceftriaxone intramuscularly  plus azithromycin orally) is recommended.   Consistent use of condoms is an important measure that can  prevent sexually transmitted diseases. Partner notification  and treatment are important prevention strategies as well.   Acute bacterial prostatitis is characterized by the presence  of typical irritative voiding symptoms (urinary frequency,  hesitancy, feeling of incomplete voiding, dribbling), fever,  pyuria, and positive urine cultures. Chronic bacterial pros¬  tatitis is characterized by the same voiding symptoms, but  fever and pyuria are typically absent. Prostatitis is also dis¬  cussed in Chapter 78 on Urinary Tract Infections.   Bacteria ascend the urethra, and then reflux into the pros¬  tatic ducts where infection occurs.   Patients appear ill in acute prostatitis with fevers, chills,  irritative voiding symptoms, and pelvic or perineal pain.  Physical examination may reveal a very tender and enlarged  prostate. Symptoms in chronic bacterial prostatitis may be  more subtle. Patients may present with recurrent urinary  tract infections, but only prolonged treatment of prostatitis  will result in a cure.   Generally, gram-negative rods that reflect the range of  organisms that cause cystitis in men are involved. These  organisms include the Enterobacteriaceae (e.g., Escherichia  coli, Klebsiella and Proteus species) as well as Pseudomonas.  In sexually active men, N. gonorrhoeae and C. trachomatis  can cause prostatitis, especially in association with urethri¬  tis and epididymitis.   A patient with symptoms of prostatitis who has an edema¬  tous and tender prostate on examination is considered to  have acute bacterial prostatitis. Culture of urine is done to  determine the causative organism. Culture of prostatic  fluid is not done in acute prostatitis because prostatic mas¬  sage should not be done during the acute phase. Prostatic  massage may be useful in chronic prostatitis.   Trimethoprim-sulfamethoxazole or a fluoroquinolone such  as ciprofloxacin can be used as empiric therapy until cul¬  ture results return. These agents exhibit good penetration  into the prostate. Therapy is prolonged, usually given for 4  to 6 weeks.   Infections of the upper respiratory tract are a common  ambulatory care complaint, resulting in a large proportion  of office visits. Although the vast majority of infections are  viral and are self-limited, some may require hospitalization,  particularly in the pediatric population. Bacterial etiologies  of some of the common upper respiratory tract infections  may be primary or superinfections of the original viral  processes and are amenable to treatment (Table 75-1).   Otitis media is an infection of the middle ear caused by  either viruses or bacteria. Otitis media can be either acute  or chronic. The information in this chapter refers to acute  otitis media.   Any process that leads to eustachian tube obstruction can  result in fluid retention and concomitant infection of the  middle ear. The most common predisposing factors are  upper respiratory tract infections and seasonal allergic rhi¬  nitis. Otitis media is very common in children under the  age of 3 years because they have a small opening of the  eustachian tube that is easily blocked by the inflammation  caused by a viral infection or an allergic response.   Patients present with ear pain and pressure, often accompa¬  nied by an upper respiratory tract infection. In infants, the  ear pain may manifest as ear pulling. Patients may also  complain of decreased hearing and fever. On examination,  the tympanic membrane is erythematous (Figure 75-1A  and B) with a loss of the light reflex and decreased mobility.  In some cases, the tympanic membrane may bulge and  then rupture.   Both bacteria and viruses cause otitis media. Among bacte¬  ria, Streptococcus pneumoniae is the most common cause.  Nontypeable strains of Haemophilus influenzae and Morax-  ella catarrhalis are also common causes. Among viruses,   FIGURE 75-1 A: Normal tympanic membrane in 6-year-old child. B: Otitis media in 3-year-old child. Note bulging tympanic membrane   and loss of light reflex. (Reproduced with permission Tintinalli JE et al. Tintinalli's Emergency Medicine: A Comprehensive Study Guide. 7th ed. New York: McGraw-Hill, 2009.  Copyright © 2009 by The McGraw-Hill Companies, Inc. Courtesy of Dr. Shelagh Cofer, Department of Otolaryngology, Mayo Clinic.)   Otitis media is usually diagnosed clinically. If the mem¬  brane ruptures, a sample of the exudate can be analyzed by  Gram stain and culture. If indicated, tympanocentesis can  be done to relieve pressure before the drum ruptures and to  obtain a specimen for culture.   Amoxicillin orally is usually the drug of choice together  with nasal decongestants to open the eustachian tube. In  cases of bacterial resistance, amoxicillin-clavulanate (Aug-  mentin) may be used.   Recurrent episodes of otitis media can be suppressed by  prophylactic antibiotics such as amoxicillin or sulfisoxa-  zole. Ventilating tubes may be inserted as a strategy to  prevent recurrent infections. The conjugate pneumococcal  vaccine is effective in preventing invasive pneumococcal  disease but is less effective in preventing otitis media.   Sinusitis is inflammation of the paranasal sinuses. It can be  either acute or chronic. Acute infections are considered  those with symptoms lasting less than 4 weeks. The infor¬  mation in this chapter refers to acute sinusitis.   Impaired mucociliary clearance caused by viral infection or  allergic rhinitis can obstruct the orifice of the sinus. Mucus  then accumulates in the sinus cavity. Stasis can lead to bac¬  terial overgrowth and superinfection. Sinusitis frequently  involves the maxillary sinus because the ostium of that  sinus is located superior to most of the sinus and drainage  of mucus has to occur against gravity. Drainage of the other  sinuses is aided by gravity.   Clinical manifestations include purulent nasal discharge,  nasal congestion, facial or sinus pain, decreased sense of  smell, and fever. Headache and malodorous breath may be  present.   Many cases begin with a viral upper respiratory tract infec¬  tion. Bacterial superinfection can then occur. In the case of  acute bacterial sinusitis, common organisms are S. pneu¬  moniae, H. influenzae, and M. catarrhalis, as in the case of  acute otitis media. Staphylococcus aureus also causes sinus¬  itis but less commonly. In immunocompromised patients  and diabetics, sinusitis caused by fungi such as Aspergillus  or Mucor may occur.   Sinusitis is often diagnosed based on a typical constellation  of symptoms and clinical findings. Computed tomography  scan of the sinuses is a very sensitive modality for indicat¬  ing inflammatory processes of the sinus. However, in the    FIGURE 75-2 Sinusitis. Arrow points to opacified maxillary  sinus seen in CT scan of head. (Reproduced with permission from Brunicardi  FC, Andersen D, Billiar T, et al. Schwartz's Principles of Surgery, 8th ed. New York,  McGraw-Hill, 2004.)   absence of bony destruction, these are nonspecific findings  for diagnosing clinically significant sinusitis requiring anti¬  biotic therapy (Figure 75-2).   If symptoms are severe, antibiotics are given in concert with  intranasal corticosteroids, as well as nasal decongestants.  Amoxicillin is the drug of choice, but if resistance is a con¬  cern, then amoxicillin-clavulanate (Augmentin) is used. In  mild cases, antibiotics are not normally used unless the  symptoms have lasted for longer than 10 to 14 days.   There is no convincing evidence that the pneumococcal  vaccine and the H. influenzae type B vaccine have a signifi¬  cant effect in reducing sinusitis caused by these  organisms.   Pharyngitis is inflammation of the throat caused primarily  by viruses. Approximately 10% of cases of pharyngitis are  caused by Streptococcus pyogenes (group A Streptococcus  [GAS]). Streptococcal pharyngitis (strep throat) is impor¬  tant because poststreptococcal immune sequellae, such as  rheumatic fever, may occur.   Patients will complain of sore throat that is worse when swal¬  lowing. Fever may also be present. Typical symptoms associ¬  ated with an upper respiratory tract infection (rhinorrhea,    FIGURE 75~3 Pharyngitis caused by Streptococcus pyogenes.  Note inflamed pharynx, tonsils, and palatal petechiae. (The white cir¬  cles and curved lines are an artifact of the lighting during photogra¬  phy.) (Source: Dr. Heinz F. Eichenwald, Public Health Image Library, Centers for  Disease Control and Prevention.)   sinus tenderness, ear pain, cough) may accompany the sore  throat. On examination, an inflamed pharynx, tonsils, and  palate are typically seen. A grayish exudate is often present  on the tonsils. Tender, anterior cervical lymphadenopathy  may be present. Petechiae on the palate may also be a diag¬  nostic clue for GAS (Figure 75-3).   Streptococcus pyogenes (GAS) is the most important bacte¬  rial cause. Group C and G streptococci also cause pharyn¬  gitis but are not antecedents to rheumatic fever. Pharyngitis  caused by Neisseria gonorrhoeae is likely to be the result of  sexual activity and, if it occurs in children, is considered as  a sign of child abuse. Mycoplasma pneumoniae. Chlamydia  pneumoniae, and Arcanobacterium haemolyticum also  cause pharyngitis. In certain countries where the diphthe¬  ria vaccine is not widely used, Corynebacterium diphtheriae  is a significant cause of pharyngitis, often accompanied by  a pseudomembrane. Fusobacterium necrophorum, a gram¬  negative anaerobe, can cause pharyngitis accompanied by  septic thrombophlebitis (Lemierre’s syndrome). Note that  although S. pneumoniae and H. influenzae colonize the  oropharynx, they do not cause pharyngitis.   Most cases of pharyngitis are caused by respiratory viruses,  such as adenovirus, influenza A and B viruses, parainflu¬  enza virus, rhinovirus, and coronavirus. Other viral causes  include Coxsackie virus (herpangina), Epstein-Barr virus  (infectious mononucleosis), and herpes simplex virus,  especially type 1. Human immunodeficiency virus causes  an acute retroviral syndrome that includes pharyngitis as  one of its components.   The main strategy in diagnostics is to establish whether  there is infection with GAS. This is because GAS is treat¬  able, and timely intervention may prevent complications  such as acute rheumatic fever. The Centor criteria are crite¬  ria that may be used to aid in the diagnosis of GAS. These  criteria include tonsillar exudates, tender anterior cervical  adenopathy, fever, and absence of cough. Rapid antigen  detection tests for GAS and throat culture are often used to  confirm the diagnosis.   It can be difficult to distinguish between a bacterial  pharyngitis and a viral pharyngitis when examining the  throat. Figure 75-4 shows extensive exudates on the tonsils  in pharyngitis caused by Epstein-Barr virus. A throat  culture is the most reliable method of determining whether  S. pyogenes is the cause.   If GAS is diagnosed, treatment with penicillin G,  penicillin V, or amoxicillin is undertaken. In penicillin-  allergic patients, erythromycin or cephalexin can be used.  The overuse of antibiotics, such as penicillin and macro-  lides, in cases of pharyngitis continues to be a problem.  Although only 10% of sore throats are bacterial, data show  that 60% of sore throats are treated with antibiotics, result¬  ing in unnecessary expense, adverse effects, and the selec¬  tion of resistant bacteria.   There is a vaccine against C. diphtheriae and influenza  virus but not against pharyngitis caused by S. pyogenes or  any other bacterial or viral cause. Long-term carriers of  GAS should not be treated because there is no evidence   FIGURE 75-4 Pharyngitis caused by Epstein-Barr virus.  Note several whitish exudates on tonsils (red arrow). (Reproduced  with permission from Caglar D, Kwun R. Chapter 117. The Mouth and Throat. In:  Tintinalli JE, et al. ( eds. Tintinalli's Emergency Medicine: A Comprehensive Study  Guide, 7E. New York, NY: McGraw-Hill; 2011.)    that such treatment prevents spread of the organism to  close contacts or the development of complications such as  acute rheumatic fever. Note that children who have rheu¬  matic heart disease should receive penicillin orally for  many years to prevent infection by S. pyogenes, which could  cause a flare of their rheumatic heart disease.   The common cold is a viral infection of the upper respira¬  tory tract, including some or all of the following structures:  the nose, throat, sinuses, eustachian tubes, trachea, and  larynx.   The viruses that cause the common cold are transmitted  primarily by aerosols generated by sneezing, or by direct  contact. Direct contact involves either hand-to-hand con¬  tact or hand-to-surface contact. The hand then transfers  the virus to the recipient’s nose, mouth, or eyes. The non-  enveloped viruses such as rhinoviruses and adenoviruses  are particularly stable in the environment and are often  transmitted by hand-to-surface contact.   The common cold and other respiratory infections  such as influenza occur more often in the winter months  than in the summer months in both the Northern and  Southern hemispheres. The reason for this seasonality is  uncertain.   Clinical manifestations include nasal congestion, decreased  sense of smell, rhinorrhea (watery nasal discharge without  purulence), and sneezing. Patients also complain of general  malaise and sore throat. In some cases, headache may also  be reported.   Rhinoviruses (more than 100 serotypes) are the most com¬  mon etiology (up to 50%). Coronaviruses, adenoviruses,  and enteroviruses such as Coxsackie viruses are other  causes. Viruses such as parainfluenza virus and respiratory  syncytial virus are also possible causes of the common cold,  although they primarily cause other diseases (croup and  bronchiolitis, respectively).   The common cold is usually diagnosed clinically. Ery¬  thematous and edematous nasal mucosa is seen on physical  examination. Conjunctival and pharyngeal injection may  also be seen. (Injection in this context means hyperemia of  small blood vessels.)   Generally, symptomatic therapy is offered. It is controver¬  sial whether zinc salts may be helpful. Zinc acetate in doses  greater than 75 mg/d may reduce the duration of symp¬  toms. Other strategies include oral decongestants and buff¬  ered hypertonic saline nasal irrigation. If used for more  than a few days, nasal sprays may be associated with  rebound congestion after stopping.   Although many vitamins and herbal therapies (e.g.,  echinacea) have been evaluated, there has been no con¬  clusive evidence that any one therapy is helpful. Vitamin  C taken prophylactically may be helpful in a population  of cold weather athletes. However, when vitamin C was  tested in the general population (rather than athletes),  its ability to prevent colds was marginal. Handwashing  may prevent the transmission of respiratory viruses.  There is no vaccine against any virus that causes the  common cold.   Croup is an inflammation of the larynx, trachea, and large  bronchi (laryngotracheobronchitis).   Inspiratory stridor is the key finding, together with a bark¬  ing cough and a hoarse voice. Symptoms may begin in a  subtle fashion with nasal irritation and congestion and then  rapidly progress to stridor over a day.   Parainfluenza viruses, especially type 1, are the most com¬  mon cause. Respiratory syncytial virus and influenza virus  account for 1% to 10% of cases.   Patients with moderate to severe symptoms may be given  corticosteroids such as dexamethasone, with or without  epinephrine. There is no antiviral drug therapy.   FIGURE 75~5 Croup .This X-ray shows the "steeple sign" of  croup in a 12-year-old child. The red arrow points to the upper tip of  the steeple. The steeple represents the constriction caused by an  inflamed larynx and trachea. (Reproduced with permission from Stone CK,  Humphries RL (eds). Current Diagnosis and Treatment of Emergency Medicine. 7th ed.  New York: McGraw-Hill. Copyright © 2011 by The McGraw-Hill Companies, Inc.)   Clinical manifestations include hoarseness and the inability  to speak (aphonia). Laryngitis maybe accompanied or pre¬  ceded by an upper respiratory infection.   Parainfluenza viruses and rhinoviruses are the most com¬  mon causes of laryngitis. Other respiratory viruses such as  influenza virus, adenovirus, and coronavirus have been  isolated from patients. Bacteria such as S. pyogenes,  M. catarrhalis, and H. influenzae have also been isolated.   There is no vaccine against parainfluenza virus and rhino-  viruses. There is no convincing evidence that the influenza  virus vaccine and the H. influenzae type B vaccine have  reduced the number of cases of laryngitis.   Patients present with rapidly worsening sore throat and  odynophagia (pain on swallowing) or dysphagia (difficulty  in swallowing). Pain may be out of proportion to physical  examination findings. Airway obstruction can occur in  severe cases. Epiglottitis in young children should be  treated as a medical emergency.   Haemophilus influenzae type B is, by far, the most com¬  mon cause, although the widespread use of the vaccine  against H. influenzae type B has greatly reduced the inci¬  dence of epiglottitis. Less common pathogens include  other H. influenzae types, S. pneumoniae, S. pyogenes,  and S. aureus.   Diagnosis is made by visualization of the epiglottis. If indi¬  rect laryngoscopy (done primarily in children) is per¬  formed, a swollen and erythematous “cherry-red” epiglottis  may be visualized. On lateral plain X-rays, an enlarged  epiglottis may be seen as a “thumb” sign (Figure 75-6).   Treatment involves intravenous ceftriaxone. Some centers  add corticosteroids to reduce inflammation, but its effects  are undocumented. An adequate airway must be  maintained.   arrow) in a lateral view of X-ray of neck. (Reproduced with permission from  Longo DL et al (eds). Harrison's Principles of Internal Medicine. 18th ed. New York:  McGraw-Hill, 2012. Copyright © 2012 by The McGraw-Hill Companies, Inc.)   Prevention includes immunization against H. influenzae  type B and S. pneumoniae. Rifampin prophylaxis should be  given to close household contacts to reduce oropharyngeal  carriage.   Lower respiratory tract infections are an important cause of  morbidity and mortality worldwide in children and in  adults. Community-acquired pneumonia, for example, is  the most deadly infectious disease in the United States.  This chapter takes an anatomic approach to lower respira¬  tory tract infections, moving from the large bronchi (bron¬  chitis) down to the very small bronchioles (bronchiolitis)  and then into the alveoli where pneumonia occurs.   Bronchitis is a self-limited inflammation of the bronchi.  Acute bronchitis must be distinguished from chronic bron¬  chitis in which patients have a cough for more than 3  months. The information in this chapter refers to acute  bronchitis.   The coughing so characteristic of bronchitis is an attempt  to clear the mucus produced by the inflammatory response  to viral infection. Bronchitis occurs more often in the win¬  ter months than in the summer. Smoking predisposes to  bronchitis (and pneumonia) by damaging the cilia in the  bronchi, leading to an inability to clear mucus from the  respiratory tract.   Cough is the most prominent symptom of bronchitis. Initially,  bronchitis presents with the symptoms of an upper respiratory  infection, namely, nasal congestion, scratchy sore throat, and  perhaps a low-grade fever. Physical examination typically    reveals expiratory wheezes. However, if cough persists for  more than 5 days and pneumonia has been ruled out, acute  bronchitis should be suspected. Bronchitis is self-limited and  usually resolves in 1 to 2 weeks. However, cough may persist  for several more weeks due to airway hyperreactivity.   Respiratory viruses are the most common pathogens  (influenza A and B, parainfluenza virus, coronavirus, rhi-  novirus, respiratory syncytial virus [RSV], and human  metapneumovirus). Bacterial pathogens are not thought to  play a significant role in acute bronchitis.   The diagnosis is primarily made clinically. Cough, with or  without sputum production, which may persist for more  than 5 days, is the typical presentation. Patients are usually  afebrile but may have a low-grade fever. Sputum cultures  are typically not done. In patients with chronic cardiorespi¬  ratory disease, a rapid antigen test for influenza virus may  be useful because oseltamivir (Tamiflu) can shorten the  duration and intensity of symptoms.   Because treatment of both upper respiratory infections  and acute bronchitis is largely supportive, these distinc¬  tions may have less clinical significance. What may be more  important clinically is to distinguish acute bronchitis (usu¬  ally viral) from pneumonia (mainly bacterial; see section  on Pneumonia), which does require antimicrobial therapy.  A chest radiograph may be performed to determine  whether pneumonia is present.   Treatment involves reassurance and symptom relief with  agents such as nonsteroidal anti-inflammatory drugs and/or    a bronchodilator such as ipratropium. If influenza is diag¬  nosed, oseltamivir (Tamiflu) may reduce the length and  severity of symptoms. Antibiotics should be used only in  those for whom a bacterial etiology has been clearly  demonstrated.   Influenza vaccine can prevent bronchitis and pneumonia  caused by influenza A and B viruses. The neuraminidase  inhibitor oseltamivir (Tamiflu) should be given to unim¬  munized individuals with chronic cardiorespiratory dis¬  ease. Handwashing is recommended to reduce the carriage  of respiratory viruses.   Bronchiolitis is inflammation of the bronchioles—the small  airways less than 2 mm in diameter. The focus in this sec¬  tion will be on bronchiolitis among infants and young  children where the etiology is primarily infectious.   Particularly among children under 2 years of age, viruses  can directly damage the epithelial cells of the terminal  bronchioles, causing inflammation and obstruction of the  small airways. Prematurity is an important predisposing  factor.   Usually children initially have symptoms consistent with an  upper respiratory tract infection and then are noticed to  have increased respiratory distress. Children under 2 years  old in particular may have tachypnea, wheezing, nasal flar¬  ing, and chest retractions. In severe cases, hypoxia, apnea,  and respiratory failure may ensue. In most cases, recovery  occurs in 1 to 2 weeks.   RSV is the most common pathogen. Other etiologies  include influenza virus, parainfluenza virus, adenovirus,  coronavirus, rhinovirus, and human metapneumovirus. In  children, viruses are the main etiology of bronchiolitis.  Bacteria are not thought to be involved. In adults, the  causes are more varied and range from viruses, to inhaled  toxic chemicals in the workplace, to idiopathic causes.  Bronchiolitis caused by RSV occurs primarily in the winter  months.   The diagnosis is primarily clinical. Upper respiratory tract  infection symptoms followed by lower respiratory tract    symptoms and signs (e.g., nasal flaring, wheezing) in a  young child during the fall and winter would be very sug¬  gestive of bronchiolitis. Chest radiograph typically shows  hyperinflation of the lungs. An enzyme immunoassay  (EIA) for RSV antigen in respiratory secretions is available  for diagnosis in hospitalized patients. A polymerase chain  reaction (PCR) assay that detects the RNA of RSV is also  available.   Because this is a self-limited disease in most cases, general  supportive measures are adequate in most cases. Patients  with moderate or severe respiratory distress will require  hospitalization. Ribavirin, delivered by aerosol into the  lungs, is approved for severe disease caused by RSV, but its  use is limited to hospitalized infants. Inhaled bronchodila-  tors (albuterol or epinephrine) may be useful.   Handwashing to minimize transmission of pathogens is an  important strategy. Infection control procedures should be  instituted in hospitalized patients to prevent the spread of  viruses to others.   Palivizumab is a humanized monoclonal antibody  against the RSV F (fusion) envelope protein that may be  used in certain populations to decrease the risk of disease  caused by RSV. These populations include children with  bronchopulmonary dysplasia and congenital heart disease  and prematurely born infants. An annual influenza vaccine  in everyone older than 6 months of age is recommended.  There is no viral vaccine against RSV.   Pneumonia is an inflammation of the lung affecting the  alveoli. We consider whether pneumonia is community  acquired versus hospital acquired to help us determine the  spectrum of potential pathogens that differs based on set¬  ting. More importantly, because empiric therapy is often  given in pneumonia, therapeutic interventions differ based  on the different populations.   The focus in this section will be on community-acquired  pneumonia which is a leading cause of death both in the  United States and worldwide. Hospital-acquired pneumo¬  nia, also known as nosocomial pneumonia, is pneumonia  that occurs 48 hours or more after admission to the hospi¬  tal and was not present at the time of admission. The term  “health care-associated” pneumonia is also used.   The alveoli of the lungs are continually exposed to microbes  from the environment via the upper respiratory tract.    Our host defenses usually keep these potential pathogens  in check. However, disease can occur when there is a par¬  ticularly virulent organism, when there is a large burden of  organisms inhaled from the environment or aspirated from  the oropharynx, or when there is a defect in host  immunity.   Predisposing factors to pneumonia include the extremes  of age (the very young and very old), chronic obstructive  pulmonary disease (COPD) and chronic bronchitis, diabe¬  tes mellitus, cystic fibrosis, and congestive heart failure.  Injection drug users who overdose, alcoholics, and those  with seizure disorders have a high risk of pneumonia  because they can aspirate organisms into the lung when  unconscious. People exposed to water aerosols, especially  from air conditioners, are at risk for pneumonia caused by  Legionella. Hospitalized patients in the intensive care unit  are at risk for ventilator-associated pneumonia caused by  gram-negative rods such as Escherichia coli, Pseudomonas,  and Acinetobacter.   Symptoms include cough that may be productive of spu¬  tum, fever, chills, chest pain, and shortness of breath.  “Rusty” sputum is a well-known finding in pneumococcal  pneumonia. Sputum that has a “currant jelly” appearance  occurs in pneumonia caused by Klebsiella because the  organism is heavily encapsulated. Physical examination  findings include tachypnea, rales, and rhonchi. If the lung  is consolidated, dullness to percussion may be detected.   Patients who are intubated and who acquire a nosoco¬  mial pneumonia may only have fever as a presenting sign,  which may be accompanied by increased respiratory secre¬  tions or increased oxygen requirements. Pneumonia may  be complicated by an infected pleural effusion or a pleural  empyema. A pleural empyema is a walled-off collection of  pus in the pleural space.   Streptococcus pneumoniae is the most common cause of  community-acquired pneumonia. Other common bacterial  pathogens include Klebsiella pneumoniae and Haemophilus  influenzae. Note that it is the nontypeable strains of H.  influenzae rather than the type B strain that cause pneumo¬  nia in elderly patients with COPD. Community-acquired,  methicillin-resistant S. aureus (CA-MRSA) is increasingly  found. Mycoplasma pneumoniae, Legionella species, and  Chlamydophila pneumoniae are other pathogens reported.  Infection with Mycobacterium tuberculosis can also mani¬  fest as a pneumonia. In approximately 30% of adults with  community-acquired pneumonia, no pathogen, neither  bacteria nor virus, is isolated.   Table 76-1 shows the important causes of community-  acquired pneumonia as a function of age. Note that the  causes of pneumonia in a neonate are those acquired    during passage through the birth canal. The main cause of  pneumonia in an infant, Chlamydia trachomatis, is also  acquired during passage through the birth canal but is a  less aggressive pathogen so its onset in delayed. Note that  M. pneumoniae is the most common cause in young adults.   Table 76-2 shows the typical causes of community-  acquired pneumonia as a function of various predisposing  factors. In certain patient populations, Pseudomonas  aeruginosa and other gram-negative organisms and Staph¬  ylococcus aureus may be important pathogens causing  pneumonia. For example, P. aeruginosa, Stenotroph-  omonas, and Burkholderia cause pneumonia in cystic  fibrosis patients, and S. aureus is a well-recognized cause  of pneumonia in patients with influenza. People exposed  to certain animals have an increased risk of pneumonia;  for example, those exposed to psittacine birds such as par¬  rots are at risk for psittacosis caused by Chlamydophila  psittaci, and those exposed to the placentas of pregnant  sheep are at risk for Q fever caused by Coxiella burnetii.  People exposed to the spores of the anthrax bacillus in  sheep wool may get “woolsorter’s disease,” a pneumonia  caused by Bacillus anthracis.   Common pathogens for hospital-acquired pneumonia  include gram-negative rods such as E. coli, K. pneumoniae,  P. aeruginosa, Enterobacter species, Serratia marcescens,  Acinetobacter species, and gram-positive cocci, especially  S. aureus.   The most common viral cause of pneumonia is influ¬  enza virus. However, other viral pathogens such as RSV,  parainfluenza virus, adenovirus, human metapneumovi¬  rus, and SARS coronavirus can also cause pneumonia. In    TABLE 76-2 Predisposing Factors Associated With  Typical Pathogens Causing Community-Acquired  Pneumonia   Travel to or reside in Central Valley of  California, Arizona, or New Mexico    patients with reduced cell-mediated immunity, herpesvi¬  ruses, such as herpes simplex virus, varicella-zoster virus,  and cytomegalovirus, can cause life-threatening pneumo¬  nia. In certain geographical areas, such as the rural south¬  western part of the United States, outbreaks of pneumonia  caused by hantavirus have occurred.   Fungi such as Coccidioides and Histoplasma also cause  pneumonia. Pneumocystis jiroveci causes pneumonia, espe¬  cially in patients with acquired immunodeficiency syn¬  drome (AIDS) with low CD4 counts.   The “gold standard” for a diagnosis of pneumonia is an  infiltrate on a plain chest radiograph (Figure 76-1). Clini¬  cal data may help, but ultimately the chest radiograph is the  most important diagnostic tool. Sputum analysis for Gram  stain and culture and blood cultures may be helpful in the  hospitalized patient but are only optional in an outpatient  setting because therapy is largely empiric for community-  acquired pneumonia. Specimens for sputum and blood  cultures should be obtained before starting antibiotics. In  pneumonia caused by one of the encapsulated pyogenic  bacteria, such as S. pneumoniae, the white blood cell count   FIGURE 76-1 Lobar pneumonia caused by Streptococcus  pneumoniae. Arrow points to area of consolidation in right lung.  (Reproduced with permission from McKean SC et al. Principles and Practice of Hospi¬  tal Medicine. New York: McGraw-Hill, 2012. Copyright © 2012 by The McGraw-Hill  Companies, Inc.)   is frequently elevated and the number of neutrophils is  often increased. The urinary antigen test for pneumococcal  polysaccharide is also useful.   It is important that sputum ( not saliva ) be sent to the lab  for Gram stain and culture. If the specimen contains many  neutrophils and few epithelial cells, then the specimen is  likely to be sputum and will be analyzed. If, however, the  specimen contains many epithelial cells and few neutro¬  phils, then the specimen is saliva and will be rejected by the  lab. An “induced sputum” sample produced by using nebu¬  lized hypertonic saline increases the probability of obtain¬  ing a good sputum sample.   Pneumonia caused by M. tuberculosis is diagnosed by  acid-fast stain of sputum and culture on mycobacterial  medium. A PCR assay done directly on sputum is avail¬  able also. Pneumonia caused by Legionella pneumophila is  often diagnosed by urinary antigen. PCR tests for various  respiratory pathogens such as M. pneumoniae, C. pneu¬  moniae, influenza virus, and RSV are useful in special  circumstances. The cold agglutinin test is no longer rec¬  ommended for the diagnosis of pneumonia caused by  Mycoplasma.   Treatment for community-acquired pneumonia is largely  empiric because microbiologic diagnostic strategies are  generally insensitive. Outpatients are generally treated with    a macrolide such as azithromycin, a tetracycline such as  doxycycline, or a respiratory quinolone such as levofloxa-  cin. Amoxicillin-clavulanate with or without a macrolide  can also be used. Inpatients may be prescribed ceftriaxone  plus a macrolide or respiratory quinolone monotherapy.   If influenza is occurring in the community, then oselta-  mivir should be prescribed. Note that influenza can predis¬  pose to bacterial pneumonia, especially caused by S. aureus  and S. pneumoniae, so antibiotics may be needed even in a  person with established influenza.   Patients with suspected hospital-acquired pneumonia  may be given broader spectrum agents such as a carbape-  nem depending on the local epidemiology, given that many  hospital-acquired infections are multidrug resistant.  Prompt initiation of antibiotics is important because mor¬  bidity and mortality increase after a delay of more than 8  hours. Drainage of an empyema or infected pleural fluid  should be performed.   The influenza vaccine is effective in decreasing the likeli¬  hood of pneumonia. The pneumococcal polysaccharide  (nonconjugate) vaccine available for older adults is impor¬  tant in decreasing bacteremia associated with pneumonia  and mortality but not the likelihood of pneumonia itself.  Because it is the nontypeable strains of H. influenzae,  rather than the type B strain, that are the most common  cause of pneumonia, the conjugate vaccine against type B  is not an important source of protection. Smoking cessa¬  tion and treatment of alcohol abuse may also decrease  pneumonia risk.   Lung abscess is a necrotic process within the lung paren¬  chyma that frequently results in a cavity with an air-fluid  level.   Patients may aspirate oropharyngeal bacteria into the  lower airways and alveoli. This usually occurs when the  patient is in the recumbent position and cannot clear  secretions. For example, aspiration can occur when a  person is unconscious from drug overdose, excess alco¬  hol intake, or the anesthesia that accompanies surgery.  Poor oral hygiene is a common predisposing factor. A  pneumonitis may first occur, but this can progress to  necrosis in a week or so. A lung abscess caused by S.  aureus may infect the lung via the bloodstream from a  distant site of infection such as right-sided endocarditis  in an intravenous drug user.   Patients present with symptoms typical of pneumonia with  fever and productive cough. The sputum is often foul  smelling, indicating the presence of anaerobes. These  symptoms may be indolent and progress over a period of  weeks. Systemic symptoms such as night sweats, fatigue,  and weight loss may also be present.   The most common organisms are anaerobes or mixed  aerobes and anaerobes that are part of the oral flora.  Anaerobes commonly involved include Peptostreptococ-  cus species, Prevotella species, and Fusobacterium  nucleatum. Aerobes include Streptococcus milleri and S.  aureus. (Clinicians often use the term “aerobe” rather  than facultative to describe bacteria that are not  anaerobic.)   A chest radiograph shows a pulmonary infiltrate with a  cavity, often with an air-fluid level (Figure 76-2). An air-  fluid level occurs when the abscess erodes a bronchus and  some of the pus in the abscess is coughed up and replaced  by air. Pleural fluid, if present, and blood cultures may pro¬  vide microbiologic data, but anaerobes may be difficult to  identify.   FIGURE 76~2 Lung abscess. Arrow points to air-fluid interface  within the abscess. (Reproduced with permission from McKean SC et al.  Principles and Practice of Hospital Medicine. New York: McGraw-Hill, 2012. Copyright  © 2012 by The McGraw-Hill Companies, Inc.)   Clindamycin or ampicillin-sulbactam are typical treatment  options. Duration of therapy is usually 4 to 6 weeks.  Patients who do not respond to antibiotics will require sur¬  gical drainage.   There is no vaccine against the organisms that cause lung  abscess. Preventive measures include good dental hygiene  and avoidance of unconsciousness caused by drug overdose  and alcohol abuse.   Skin and soft tissue infections are some of the most com¬  mon infectious diagnoses and result in hundreds of thou¬  sands of medical office and emergency room visits each  year. These infections often occur following a break in  normal skin integrity from either trauma or skin disease  (e.g., atopic dermatitis). The vast majority of these infec¬  tions are caused by Staphylococcus aureus and Streptococcus  pyogenes (Table 77-1). Hematogenous seeding of organ¬  isms into the skin can occur but is uncommon. Normal  histology of the skin can be seen in Figure 77-1.   There are two modes of acquisition of impetigo, primary  infection that occurs in otherwise normal skin or second¬  ary impetigo, which occurs following a break in normal  skin integrity. Bacteria invade into the epidermal layer and  cause local damage. Bullous impetigo occurs when strains  of S. aureus secrete exfoliative toxin, a protease that  degrades desmoglein, resulting in loss of adhesion of the  superficial epidermis. This is the same toxin that causes  staphylococcal scalded skin syndrome.   There are three clinical variants of impetigo: (1) classic  impetigo, (2) bullous impetigo, and (3) ecthyma. Classic  impetigo begins as papules that progress to vesicles    surrounded by erythema. Subsequently, the fluid-filled  lesions enlarge and break down to form thick, adherent  crusts with a characteristic golden “honey-colored”  appearance (Figure 77-2). Bullous impetigo is similar to  classic impetigo, but bullae form (Figure 77-3) via the  mechanism described earlier. Ecthyma is an ulcerating  form of impetigo where the lesion penetrates through  the epidermis into the dermis (Figure 77-4). Some  strains of S. pyogenes that cause impetigo have been  associated with poststreptococcal glomerulonephritis,  and providers should be aware of this potential compli¬  cation. Rheumatic fever is a less common sequel to  streptococcal skin infections.   Staphylococcus aureus and S. pyogenes are the two main  pathogens that cause impetigo. In neutropenic patients, a  clinical syndrome termed ecthyma gangrenosum is due to  disseminated Pseudomonas aeruginosa infection. Its cuta¬  neous findings are a result of hematogenous seeding of  dermal vessels with bacteria, resulting in thrombosis, isch¬  emia, and focal skin necrosis. This is not a superficial skin  infection.   The diagnosis of impetigo is made clinically in most cases.  Culture of bullous fluid or pus can be considered when  patients do not respond to standard treatment.   Antibacterial therapy should be directed against both  S. aureus and S. pyogenes. Topical therapy with mupirocin  or retapamulin is preferred when only a few lesions are    FIGURE 77~1 Layers of the skin and subcutaneous tissue. Note sebaceous glands (purple coils) and hair follicles with protruding hair.  (Reproduced with permission from Mescher AL. Chapter 18. Skin. In: Mescher AL. eds . Junqueira's Basic Histology: Text & Atlas, 13e. New York, NY: McGraw-Hill; 2013.)   FIGURE 77-2 Classic nonbullous impetigo. Note lesions with  a "honey-colored" crust around the nose and mouth. (Reproduced with  permission from Wolff K, Johnson R (eds). Fitzpatrick's Color Atlas & Synopsis of  Clinical Dermatology. 6th ed. New York: McGraw-Hill, 2009. Copyright © 2009 by The  McGraw-Hill Companies, Inc.)   (Reproduced with permission from Ma OJ, Cline DM, Tintinalli JE et al. Emergency  Medicine Manual. 6th ed. © 2004, McGraw-Hill, New York.)   (Reproduced with permission from Wolff K, Johnson R (eds). Fitzpatrick's Color Atlas  & Synopsis of Clinical Dermatology. 6th ed. New York: McGraw-Hill, 2009. Copyright <i  2009 by The McGraw-Hill Companies, Inc.)    present. In patients with widespread disease, a systemic  antimicrobial is preferred. If concern for methicillin-  resistant S. aureus (MRSA) exists, clindamycin is recom¬  mended; otherwise, cephalexin or dicloxacillin would be  appropriate.   Handwashing and covering draining lesions should be used  to prevent spread of bacteria.   These infections occur following a break in normal skin  integrity. Cellulitis and erysipelas both involve the dermis,  but erysipelas involves the upper dermis and superficial  lymphatics, whereas cellulitis involves the deeper dermis  and subcutaneous fat.   clearly demarcated border on right cheek, across the nose to the left  cheek. (Reproduced with permission from Longo DL et al (eds). Harrison's Principles  of Internal Medicine. 18th ed. New York: McGraw-Hill, 2012. Copyright © 2012 by The  McGraw-Hill Companies, Inc.)   Cellulitis and erysipelas both manifest with erythema,  swelling, and pain in the affected region plus or minus  fever. However, erysipelas lesions are raised above the level  of surrounding skin, and there is a clear line of demarcation  between involved and uninvolved tissue (Figure 77-5). In  contrast, the lesions of cellulitis are not significantly raised  and have an irregular line of demarcation (Figure 77-6).   FIGURE 77-6 Cellulitis. Note diffuse inflammation on right leg  including the skin over the ankle and dorsum of the foot. (Reproduced  with permission from Knoop KJ et al. The Atlas of Emergency Medicine. 3rd ed. Copy¬  right ©The McGraw-Hill Companies, Inc. All rights reserved. Photo contributor: Law¬  rence B. Stack, MD.)   Beta-hemolytic streptococci are the most common patho¬  gens to cause these infections, with S. pyogenes and Strepto¬  coccus agalactiae being some of the most common species.  Staphylococcus aureus can also cause this type of infection.  Other less common pathogens are listed in Table 77-2 by  exposure risk.   Diagnosis is made clinically because it is difficult to obtain  cultures from the skin in the absence of pus. Sometimes  patients will have bacteremia.   Empiric therapy should be focused on beta-hemolytic strepto¬  cocci and S. aureus. A systemic oral agent can be used for mild  infection (cephalexin, dicloxaciUin, or clindamycin), but for  severe infection, hospitalization and the administration of  intravenous antibiotics (cefazolin or vancomycin) is recom¬  mended. Dalbavancin may be useful in serious cases of cellu¬  litis caused by S. aureus, both MRSA and MSSA. Intravenous  dalbavancin is administered once weekly in contrast to intra¬  venous vancomycin which is administered twice daily.   In patients with recurrent cellulitis, a strategy of chronic  suppressive antibiotics may effectively prevent subsequent  infections.    TABLE 77-2 Causes of Skin and Soft Tissue  Infections and Their Associated Risk Factors   Staphylococcus aureus, enteric gram¬  negative rods such as Serratia and  Pseudomonas, Clostridium botulinum   Bacteria and purulent material accumulate in hair follicles  in the epidermal layer of the skin.   Folliculitis presents with pinpoint erythema around indi¬  vidual hair follicles. A small amount of purulence may be  seen (Figure 77-7). This can be seen in an isolated body  area or throughout the skin.   Staphylococcus aureus is the most common cause of fol¬  liculitis. Pseudomonas aeruginosa can also cause follicu¬  litis and is associated with the use of unchlorinated hot  tubs. Rarely, Candida and certain dermatophytes can  cause folliculitis.   Diagnosis is made clinically, but if purulent material is  present, it can be cultured.   Folliculitis often resolves on its own, and treatment is not  needed. Warm compresses or topical antibiotics can be  considered in select cases.   FIGURE 77-7 Folliculitis. Note the multiple, small pustules  on the chin and neck. (Reproduced with permission from Wolff K, Gold¬  smith LA, Katz SI et al (eds). Fitzpatrick's Dermatology in General Medicine, 7th ed.  New York: McGraw-Hill, 2008, p. 1699. Copyright © 2008 by The McGraw-Hill  Companies, Inc.)   Handwashing and covering draining lesions should be used  to prevent spread of bacteria. Avoiding unchlorinated hot  tubs is recommended.   A skin abscess is an infection of the dermis and deeper lay¬  ers of skin that contains purulent material.   Abscesses occur when pathogens enter a break in the skin  following trauma or when they spread from infected hair  follicles (Figure 77-8). When a single follicle is infected  and tracks down into the dermis, it is termed a furuncle  (“boil”), and when multiple infected hair follicles coalesce,  it is termed a carbuncle. Occasionally an abscess may  develop following hematogenous dissemination of an  infection.   A furuncle consists of a central pustule usually surrounded  by an area of erythema, warmth, and tenderness with  underlying fluctuance. Patients may have multiple furun¬  cles. A carbuncle is a larger, more serious lesion than a  furuncle. It is composed of several adjacent furuncles that  have coalesced into an inflamed, indurated lesion that typi¬  cally extends deep into subcutaneous tissue. Carbuncles are  often found on the nape of the neck, where a shirt collar  rubs in people with poor hygiene (Figure 77-9). Patients  may have signs and symptoms of systemic infection, and   FIGURE 77-8 Abscess. Note localized area of inflammation  containing a central core of yellowish pus (arrow) on medial aspect  of foot. This lesion occurred at the site of a sewing needle injury.  (Reproduced with permission from Wolff K, Johnson R (eds). Fitzpatrick's Color Atlas  & Synopsis of Clinical Dermatology. 6th ed. New York: McGraw-Hill, 2009. Copyright ©  2009 by The McGraw-Hill Companies, Inc.)   FIGURE 77-9 Carbuncle. Note multiple furuncles that  have coalesced to form a large area of inflammatory lesion.  (Reproduced with permission from Goldsmith LA et al. Fitzpatrick's Dermatol¬  ogy in General Medicine. 8th ed. New York: McGraw-Hill, 2012. Copyright ©  2012 by The McGraw-Hill Companies, Inc.)   Staphylococcus aureus is, by far, the most common  cause of skin abscesses (more than 75% of cases). Beta-  hemolytic streptococci are also capable of causing these  types of infections. Occasionally Mycobacterium tuber¬  culosis, nontuberculous mycobacteria, and fungi such  as Coccidioides, Candida, and Cryptococcus can cause  abscesses.   Gram stain and culture of purulent material obtained  from the abscess allows for diagnosis. Radiographic  imaging such as ultrasound or computed tomography  (CT) may help further define the size and extent of an  abscess.   The primary treatment for abscesses is incision and  drainage. In select situations, the addition of antibiotics  may be beneficial. Antibiotics should be considered when  the patient has signs and symptoms of systemic infection,  a rapidly progressive or severe infection, infection in a  hard-to-drain area of the body, extremes of age, immu¬  nocompromised state, or failure to resolve with previ¬  ous incision and drainage. When antibiotics are  indicated, the patient should be treated with an empiric  antibiotic regimen that has activity against MRSA. Such  oral antibiotic regimens include clindamycin, trime¬  thoprim-sulfamethoxazole, or doxycycline (Table 77-3).    TABLE 77-3 Antibiotics Active and Inactive Against  Methicillin-Resistant Staphylococcus aureus (MRSA)    Empiric intravenous regimens include vancomycin or dap-  tomycin. If antibiotic susceptibilities demonstrate that  methicillin-susceptible S. aureus (MSSA) is the pathogen,  oral regimens can include cephalexin or dicloxacillin,  whereas intravenous antibiotics would include nafcillin or  cefazolin.   Handwashing and covering draining lesions should be used  to prevent spread of bacteria.   Necrotizing fasciitis is a necrotizing infection of the  deep structures of the skin including the underlying  fascia. In myonecrosis, the underlying muscle becomes  necrotic.   Break in the skin caused by trauma or surgery allows for  passage of organisms to deeper structures. Infection in the  fascial layer results in thrombosis of the vascular supply  and adjacent nerve tissue. Destruction of these vital struc¬  tures manifests as necrosis and anesthesia of the more  superficial layers of skin.   FIGURE 77~10 Necrotizing fasciitis. Note two large hemor¬  rhagic bullae surrounded by dusky red inflamed tissue. (Reproduced  with permission from Knoop K, Stack L, Storrow A, Thurman RJ. Atlas of Emergency  Medicine, 3rd ed. © 2010, McGraw-Hill, New York. Photo contributed by Lawrence B.  Stack, MD.)    skin changes often spread and progress very quickly and  are followed by evidence of skin hypoperfusion, blue-  gray coloring, bullae, and anesthesia (Figure 77-10).  Crepitus may be felt. Patients often demonstrate signs  and symptoms of systemic infection progressing to  severe sepsis.   There are two classifications of necrotizing fasciitis—type I,  which is polymicrobial, and type II, which is monomicro-  bial. Type I infection is often due to both aerobic and  anaerobic bacteria and is more common following intra¬  abdominal surgery, in diabetics, and in intravenous drug  users, and can be seen in the male perineum, a disease  called Fournier’s gangrene (Figure 77-11). Type II infec¬  tion is most often due to S. pyogenes but can also be caused  by Vibrio vulnificus following trauma in brackish water,  Aeromonas species following trauma in fresh water, Clos¬  tridium perfringens from soil-contaminated wounds caused  by motor vehicle/motorcycle accidents or shrapnel, and  community-acquired MRSA.   Early symptoms of necrotizing fasciitis are skin ery¬  thema, warmth, and tenderness. Patients may have pain  out of proportion of the examination findings. These   Gram stain and culture from debrided tissue can assist  in making a microbiologic diagnosis. Radiographic  imaging may be useful. Plain films may demonstrate   FIGURE 77-11 Fournier's gangrene. Note gangrene of the  genitalia and skin of lower abdomen. (Reproduced with permission from  Knoop KJ et al. The Atlas of Emergency Medicine. 3rd ed. New York: McGraw-Hill,  2010. Copyright © 2010 by The McGraw-Hill Companies, Inc.)    presence of gas in tissues, and a CT scan may reveal  enhancement in the fascial plane. Magnetic resonance  imaging is the most sensitive approach but is limited in  specificity.   Necrotizing soft tissue infections are a medical emer¬  gency, and treatment requires a combination of surgical  debridement of infected tissue and antibiotic therapy.  Antibiotic therapy should be directed at S. pyogenes,  MRSA, and anaerobic and aerobic gram-negative rods.  A common empiric regimen would include clindamycin  plus vancomycin plus piperacillin-tazobactam.   Handwashing and covering draining lesions should be used  to prevent spread of bacteria.   Urinary tract infections are a group of common diseases  that occur predominantly by ascension of normal enteric  flora through the urethra into the bladder. These infections  more frequently affect women due to anatomic differences  including a shorter urethra. Diagnosis is made by identify¬  ing related clinical symptoms in combination with an  abnormal urinalysis and growth on urine culture. Antibiot¬  ics are often effective therapy, although antibiotic resistance  is increasing.   Urine microscopy is the use of a microscope to look at  urine. In patients with urinary tract infections, one can often  find pyuria (elevated white blood cells [WBCs] in urine) and  hematuria (red blood cells in urine), and sometimes bacteria  can be seen. The presence of WBC casts indicates pyelone¬  phritis rather than cystitis. A urine sample that has abundant  squamous epithelial cells suggests that it is contaminated and  the results of the culture are not reliable.   Urine dipsticks use different chemicals reagents on a  strip that is dipped in urine to diagnose urinary tract dis¬  eases. Certain dipstick test results are suggestive of infec¬  tion, namely positive leukocyte esterase, positive nitrite,  and positive hemoglobin. The positive nitrite occurs from  the conversion of nitrate to nitrite by Enterobacteriaceae.   Urine culture allows identification of the organism  causing infection. Urine in the bladder is normally sterile.  Because contamination of samples can occur as urine  passes through the outer third of the urethra, a numeric    threshold of colony-forming units (CFUs) per milliliter has  been established to confirm infection. In samples obtained  from a midstream void, >1 x 10 5 CFU/mL is consistent  with infection. In samples collected via catheterization,  >1 x 10 2 CFU/mL is consistent with infection. Either a  voided midstream urine specimen or a specimen obtained  by bladder catheterization can be used for urine culture.   Cystitis is an infection of the bladder. The term “cysto”  refers to bladder, and “itis” refers to inflammation. Uncom¬  plicated cystitis is defined as cystitis in otherwise healthy  women, whereas complicated cystitis is defined as cystitis  in all other groups such as men, pregnant women, diabet¬  ics, those with anatomic and neurologic problems, and  those with recurrent urinary tract infections.   Bacteria (rarely fungi) reach the bladder via ascension  through the urethra. This is much more common in  women due to the short urethra and close approximation of  the urethra to the vagina and anus. Preceding infection, the  vagina, which is normally colonized by Lactobacillus spe¬  cies, will become colonized by enteric organisms such as  Escherichia coli instead. Escherichia coli are able to adhere  to the urethral and bladder mucosa via pili. Once bacteria  enter the bladder, they are able to reproduce and cause an  inflammatory response, resulting in the symptoms of  infection.    Medical conditions that cause abnormal emptying of  bladder increase risk for urinary tract infections. These  include anatomic abnormalities such as cystoceles, neuro¬  logic disorders such as spinal cord injuries and multiple  sclerosis, and the presence of foreign bodies such as  indwelling Foley catheters. In infants less than 3 months of  age, uncircumcised boys are at higher risk for urinary tract  infections than girls. However, after infancy, girls are at  higher risk for infection than all boys.   The most common clinical manifestations of cystitis  include dysuria (pain with urination); frequent, low-  volume urination; suprapubic tenderness; and gross hema¬  turia. Men may experience some penile discharge. Most  patients with cystitis do not have fever or other systemic  symptoms of infection, and when they are present, an  upper urinary tract infection (pyelonephritis) should be  considered.   Escherichia coli is by far the most common cause of uri¬  nary tract infections, especially cystitis. Other enteric  gram-negative rods such as Klebsiella species and Proteus  species are regular culprits. Pseudomonas aeruginosa can  cause urinary tract infection, but this is most common in  health care-associated infections, patients with anatomic/  neurologic abnormalities afflicting their urinary tract, or  heavily antibiotic-experienced patients. Gram-positive  pathogens include Enterococcus species and Staphylococcus  saprophyticus. Staphylococcus saprophyticus is common in  young women. Candida species can cause infection in  patients who have extensive prior antibiotic use and  indwelling Foley catheters. Rarely, viruses such as adenovi¬  rus, BK virus, and cytomegalovirus can cause a hemor¬  rhagic cystitis. These viruses almost exclusively cause  cystitis in immunocompromised hosts such as those who  have undergone stem cell transplants.   The diagnosis of cystitis requires identifying a combination  of pyuria (by seeing WBCs on microscopy or positive leu¬  kocyte esterase on urine dipstick) often accompanied by a  positive nitrite test and evidence of red blood cells in the  urine, plus positive urine cultures plus clinical symptoms  consistent with infection. Either a voided midstream urine  specimen or a specimen obtained by bladder catheteriza¬  tion can be used for urine culture.   Treatment of cystitis requires antibiotic therapy. Empiric  therapy is directed against E. coli in cases of uncompli¬  cated cystitis and is accomplished with either    trimethoprim-sulfamethoxazole or nitrofurantoin.  Empiric therapy for complicated cystitis is usually with a  fluoroquinolone (ciprofloxacin or levofloxacin). Symp¬  tomatic relief of the dysuria can be accomplished using  phenazopyridine.   There is no known method for primary prevention of  cystitis. However, prevention of cystitis in patients with  a history of recurrent cystitis may be accomplished with  several strategies. These include ways to enhance growth  of the normal vaginal flora ( Lactobacillus species) to  prevent colonization with enteric gram-negative rods,  such as E. coli, intravaginal estrogen in postmenopausal  women and avoidance of spermicide as a form of  contraception. In women who frequently have cystitis  following sexual intercourse, postcoital antibiotics can  be beneficial.   Pyelonephritis is an infection of the kidney(s). “Pyelo”  refers to the renal pelvis, and “nephritis” means inflamma¬  tion of the kidney. Uncomplicated pyelonephritis is defined  as pyelonephritis in otherwise healthy women, whereas  complicated pyelonephritis is pyelonephritis in all other  patients.   Pyelonephritis may occur either by ascension of bacteria  from the urethra to the bladder and then to the kidney(s)  or, less commonly, through hematogenous spread from  other sites of infection such as endocarditis. Kidney stones  predispose to pyelonephritis (Figure 78-1). Urinary tract  infections in children can be associated with anatomic  abnormalities, and additional workup for diseases such as  vesicoureteral reflex should be considered.   Patients with pyelonephritis typically present with fever,  flank pain, nausea, and vomiting. They may or may not  have signs and symptoms of lower tract infection (dysuria,  frequency, hematuria, suprapubic tenderness).   Escherichia coli is the most common pathogen causing  pyelonephritis. Other enteric gram-negative rods such as  Klebsiella and Proteus species are also involved. Pseudo¬  monas aeruginosa can cause pyelonephritis, but this typi¬  cally occurs in health care-associated infections, patients  with anatomic/neurologic abnormalities afflicting their  urinary tract, or heavily antibiotic-experienced patients.   FIGURE 78~1 Pyelonephritis. Note enlarged right kidney  (left side of image) caused by a stone at the ureteropelvic junction.  (Reproduced with permission from McKean SC et al. Principles and Practice of Hospi¬  tal Medicine. New York: McGraw-Hill, 2012. Copyright © 2012 by The McGraw-Hill  Companies, Inc.)   Patients with recurrent Proteus pyelonephritis should  be evaluated for struvite stones. Infection of the kidney  following hematogenous spread of infection can occur  with essentially any organism but is seen most commonly  with Staphylococcus aureus. Hematogenous spread also  occurs with Mycobacterium tuberculosis and can been  seen in disseminated fungal infection as well.   Urine test findings are similar to those seen in cystitis, but  urinary WBC casts can be seen (Figure 78-2). Blood  WBC counts are frequently elevated, and occasionally  blood cultures can be positive. Ultrasound and computed  tomography scans can reveal inflammation and can   FIGURE 78-2 White blood cell casts. Note cylindrical-shaped  casts containing round, retractile white blood cells (arrow). (Used with  permission from Longo DL et al [eds]. Harrison's Principles of Internal Medicine, 18th ed.  New York: McGraw-Hill, 2009. Copyright © 2012 by The McGraw-Hill Companies, Inc.)    occasionally reveal obstruction or perinephric abscess  (see Figure 78-1). Radiographic imaging is not routinely  recommended in patients who respond quickly to antibi¬  otics and in whom there is no clinical concern for associ¬  ated nephrolithiasis or obstruction. Patients with renal  tuberculosis may have pyuria in the absence of positive  cultures (sterile pyuria) because M. tuberculosis does not  grow in routine culture media.   Antibiotics that are able to obtain high concentrations in  the renal parenchyma and have activity against common  pathogens are required to treat pyelonephritis. Empiric  regimens for community-onset infection include a fluoro¬  quinolone (ciprofloxacin or levofloxacin) or a third-  generation cephalosporin such as ceftriaxone. Patients with  heavy exposure to prior antibiotics, anatomic abnormali¬  ties, or exposure to the health care setting should be treated  with antibiotics with reliable activity against Pseudomonas,  such as cefepime, piperacillin, or meropenem. Antibiotic  therapy should be narrowed once antibiotic susceptibilities  become available.   Patients who have bladder dysfunction that predisposes  them to pyelonephritis may require frequent catheteriza¬  tion to allow proper urinary tract drainage. Pregnant  women with asymptomatic bacteriuria (see next section)  may benefit from antibiotic therapy to prevent  pyelonephritis.   Asymptomatic bacteriuria is when bacteria colonize the  urinary bladder in the absence of signs or symptoms of  upper or lower urinary tract infection. It is defined as the  presence of >1 x 10 5 CFU/mL of a single bacterial species  on two successive urine cultures in a patient without uri¬  nary tract symptoms.   Asymptomatic bacteriuria is common in many populations  including persons with diabetes, patients with anatomic  and neurologic abnormalities of the urinary tract, patients  with indwelling Foley catheters, and elderly patients. The  bacteria reach the bladder via ascension through the ure¬  thra, not from hematogenous dissemination.   Patients with asymptomatic bacteriuria have no signs or  symptoms of upper or lower tract infection.   The same organisms that commonly cause cystitis also  cause asymptomatic bacteriuria. Asymptomatic candiduria  can occur as well.   The diagnosis of asymptomatic bacteriuria requires the iden¬  tification of positive urine cultures. Patients have pyuria  present in about 50% cases of asymptomatic bacteriuria.   Treatment of asymptomatic bacteriuria is indicated in select  populations who have been identified to be at risk for subse¬  quent severe infection from presence of bacteriuria. These  high-risk groups include (1) pregnant women, (2) adults  scheduled to undergo urinary tract procedures that could  cause mucosal bleeding and translocation of bacteria into the  blood, and (3) neutropenic patients.   Prostatitis is inflammation of the prostate, most often  caused by bacterial infection. Prostatitis is also discussed in  Chapter 74 on Pelvic Infections.   Infection most frequently occurs via the urethra then into  to the prostatic ducts. However, hematogenous seeding of  the prostate can occur as well. Microabscesses may develop  within the prostate (Figure 78-3).   Acute prostatitis may present with acute onset of fever,  dysuria, urinary frequency, and severe pain with palpation  of the prostate. Patients may be very ill and can present  with severe sepsis. In contrast, chronic prostatitis presents  with more subacute onset of dysuria, frequency, urinary  hesitancy, and pelvic discomfort.   In younger patients, Neisseria gonorrhoeae and Chlamydia  trachomatis are the most common causes of prostatitis.   FIGURE 78-3 Prostatitis. Note yellow-green areas of pus  (arrow) forming multiple abscesses in prostate gland. (Reproduced with  permission from Kemp WL, Burns DK, Brown TG. Chapter 17. Pathology of the Male  and Female Reproductive Tract and Breast. In: Kemp WL, Burns DK, Brown TG. eds.  Pathology: The Big Picture. New York, NY: McGraw-Hill; 2008.)    However, in older patients, enteric bacteria, such as E. coli,  are the predominant pathogens. When hematologic seed¬  ing occurs, S. aureus is a common cause.   The diagnosis of acute bacterial prostatitis is often con¬  firmed by the finding of an acutely tender prostate on  digital rectal exam. Recovery of an organism, when pos¬  sible, is from urine or blood cultures. Prostatic massage is  contraindicated in acute prostatitis. However, in chronic  prostatitis, prostate massage following collection of pros¬  tatic secretion is recommended to obtain a microbiologic  diagnosis.   Antimicrobial therapy with excellent penetration to the  prostatic tissues is recommended for treatment of prostati¬  tis. Fluoroquinolones (e.g., ciprofloxacin or levofloxacin)  and trimethoprim-sulfamethoxazole both achieve high  levels in the prostate and are good options. Antibiotic sus¬  ceptibility testing should be used to guide treatment of  infecting pathogens.   Prompt treatment of acute prostatitis may reduce the risk of  development of chronic prostatitis.   Diseases —Abscesses of many organs, endocarditis, osteomy¬  elitis, septic arthritis, and impetigo. Also hospital-acquired  pneumonia, surgical wound infections, and sepsis. Also exo¬  toxin-mediated diseases such as gastroenteritis (food poison¬  ing), toxic shock syndrome, and scalded skin syndrome. It is  one of the most common causes of human infections.   Characteristics —Gram-positive cocci in clusters. Coagulase-  positive. Catalase-positive. Most isolates produce (3-lactamase.   Pathogenesis —Abscess containing pus is the most common  lesion. Three exotoxins are also made. Toxic shock syndrome  toxin is a superantigen and causes toxic shock syndrome by  stimulating many helper T cells to release large amounts of  lymphokines, especially IL-2. Enterotoxin, which causes food  poisoning, is also a superantigen. Food poisoning has a short  incubation period because it is preformed in food. Scalded  skin syndrome toxin is a protease that cleaves desmoglein in  tight junctions in the skin. Protein A is an important viru¬  lence factor because it binds to the heavy chain of IgG and  prevents the activation of complement. Predisposing factors  to infection include breaks in the skin, foreign bodies such as  sutures, neutrophil levels below 500/pL, intravenous drug use  (predisposes to right-sided endocarditis), and tampon use  (predisposes to toxic shock syndrome).   Laboratory Diagnosis —Gram-stained smear and culture.  Yellow or gold colonies on blood agar; colonies often  (3-hemolytic. Staphylococcus aureus is coagulase-positive;  Staphylococcus epidermidis is coagulase-negative. Serologic tests  not useful.   Treatment —Penicillin G for sensitive isolates; (3-lactamase-  resistant penicillins such as nafcillin for resistant isolates;    vancomycin for isolates resistant to nafcillin. About 85% are  resistant to penicillin G. Plasmid-encoded (3-lactamase medi¬  ates most resistance. Resistance to nafcillin is caused by  changes in binding proteins. Some isolates are tolerant to  penicillin. Rare vancomycin-resistant strains have emerged.   Prevention —Cefazolin is used to prevent surgical wound infec¬  tions. No vaccine is available. Handwashing reduces spread.   Diseases —Endocarditis on prosthetic heart valves, prosthetic  hip infection, intravascular catheter infection, cerebrospinal  fluid shunt infection, neonatal sepsis.   Habitat and Transmission —Normal flora of the human skin  and mucous membranes. It is probably the patients own  strains that cause infection, but transmission from person to  person via hands may occur.   Pathogenesis —Glycocalyx-producing strains adhere well to  foreign bodies such as prosthetic implants and catheters. It is a  low-virulence organism that causes disease primarily in immu¬  nocompromised patients and in those with implants. It is a  major cause of hospital-acquired infections. Unlike S. aureus,  no exotoxins have been identified.   Laboratory Diagnosis —Gram-stained smear and culture.  Whitish, nonhemolytic colonies on blood agar. It is coagu¬  lase-negative. Staphylococcus epidermidis is sensitive to  novobiocin, whereas the other coagulase-negative staphylo¬  coccus, Staphylococcus saprophyticus, is resistant. Serologic  tests are not useful.   Gram-positive cocci in clusters. Coagulase-negative. Resistant  to novobiocin in contrast to S. epidermidis, which is sensitive.  Causes community-acquired urinary tract infections in young  women (but Escherichia coli is a much more common cause).   Diseases —Suppurative (pus-producing) diseases (e.g., phar¬  yngitis and cellulitis); nonsuppurative (immunologic) diseases  (e.g., rheumatic fever and acute glomerulonephritis).   Characteristics —Gram-positive cocci in chains. [3-1 Iemolytic  colonies. Catalase-negative. Bacitracin-sensitive. (3-Hemolytic  streptococci are subdivided into group A, B, etc., by differ¬  ences in the antigenicity of their cell wall carbohydrate.   Habitat and Transmission —Habitat is the human throat and  skin. Transmission is via respiratory droplets.   Pathogenesis —For suppurative infections, hyaluronidase  (“spreading factor”) mediates subcutaneous spread seen in  cellulitis; erythrogenic toxin (a superantigen) causes the rash  of scarlet fever; M protein impedes phagocytosis. For nonsup¬  purative (immunologic) diseases, rheumatic fever is caused by  immunologic cross-reaction between bacterial antigen and  human heart and joint tissue (i.e., antibody against strepto¬  coccal M protein reacts with myosin in cardiac muscle), and  acute glomerulonephritis is caused by immune complexes  formed between streptococcal antigens and antibody to those  antigens. The immune complexes are trapped by glomeruli,  complement is activated, neutrophils are attracted to the site  by C5a, and proteases produced by neutrophils damage  glomeruli.   Laboratory Diagnosis —The diagnosis of suppurative  infections (e.g., cellulitis) differs from immunologic diseases  (e.g., rheumatic fever). For suppurative infections, use  Gram-stained smear and culture. (3-1 Iemolytic colonies on  blood agar. (Hemolysis due to streptolysins O and S.) If  isolate is sensitive to bacitracin, it is identified as Streptococcus  pyogenes. Rapid ELISA tests for group A streptococcal anti¬  gens in throat swabs are available. Assay for antibody in  patient’s serum is not done for suppurative infections. If  rheumatic fever is suspected, patient’s antistreptolysin O  (ASO) antibody titer is tested to determine whether previ¬  ous exposure to S. pyogenes has occurred. If acute glo¬  merulonephritis is suspected, antibody to streptococcal  DNase B is used as evidence of a previous skin infection by  S. pyogenes.   Prevention —Penicillin is used in patients with rheumatic  fever to prevent recurrent S. pyogenes pharyngitis. This  prevents additional damage to heart valves. There is no  vaccine.   Characteristics —Gram-positive cocci in chains. (3-Hemolytic  colonies. Catalase-negative. Bacitracin-resistant. (3-Hemolytic  streptococci are subdivided into group A, B, etc., by differ¬  ences in the antigenicity of their cell wall carbohydrate.   Pathogenesis —Pyogenic organism. No exotoxins identified.  Predisposing factors to neonatal infection include rupture of  membranes more than 18 hours before delivery, labor prior to  37 weeks (infant is premature), absence of maternal antibody,  and heavy colonization of the genital tract by the organism.   Laboratory Diagnosis —Gram-stained smear and culture.  (3-Hemolytic (narrow zone) colonies on blood agar that are  resistant to bacitracin. Organisms hydrolyze hippurate and are  CAMP test-positive.   Prevention —No vaccine. Ampicillin should be given to  mothers if prolonged rupture of membranes occurs, if mother  has a fever, or if the neonate is premature.   Diseases —Urinary tract and biliary tract infections are most  frequent. Endocarditis rare but life-threatening.   Habitat and Transmission —Habitat is the human colon;  urethra and female genital tract can be colonized. May enter  bloodstream during gastrointestinal (GI) or genitourinary  tract procedures. May infect other sites (e.g., endocarditis).   Laboratory Diagnosis —Gram-stained smear and culture,  a-, (3-, or nonhemolytic colonies on blood agar. Grows in 6.5%  NaCl and hydrolyzes esculin in the presence of 40% bile. Sero¬  logic tests not useful.   Treatment —Penicillin or vancomycin plus an aminoglyco¬  side such as gentamicin is bactericidal. Organism is resistant  to either drug given individually, but given together they have  a synergistic effect. Aminoglycoside alone is ineffective  because it cannot penetrate. Penicillin or vancomycin weak¬  ens the cell wall, allowing the aminoglycoside to penetrate.  Vancomycin-resistant enterococci (VRE) are important causes  of nosocomial (hospital-acquired) infections. Linezolid can be  used to treat VRE.   Prevention —Penicillin and gentamicin should be given to  patients with damaged heart valves prior to intestinal or uri¬  nary tract procedures. No vaccine is available.   Diseases —The most common diseases are pneumonia and  meningitis in adults and otitis media and sinusitis in children.   Characteristics —Gram-positive “lancet-shaped” cocci in  pairs (diplococci) or short chains. a-Hemolytic colonies. Cat¬  alase-negative. Growth is inhibited by optochin in contrast to  viridans streptococci, which are resistant. Colonies are bile-  soluble. Prominent polysaccharide capsule. Eighty-five sero¬  types based on antigenicity of polysaccharide capsule. One of  the three classical encapsulated pyogenic bacteria ( Neisseria  meningitidis and Haemophilus influenzae are the other two).   Habitat and Transmission —Habitat is the human upper  respiratory tract. Transmission is via respiratory droplets.   Pathogenesis —Induces inflammatory response. No known  exotoxins. Polysaccharide capsule retards phagocytosis. Anti¬  polysaccharide antibody opsonizes the organism and provides  type-specific immunity. IgA protease degrades secretory IgA  on respiratory mucosa, allowing colonization. Viral respira¬  tory infection predisposes to pneumococcal pneumonia by  damaging mucociliary elevator; splenectomy predisposes to  sepsis. Skull fracture with spinal fluid leakage from nose pre¬  disposes to meningitis.   Laboratory Diagnosis —Gram-stained smear and culture.  a-Hemolytic colonies on blood agar. Growth inhibited by bile  and optochin. Quellung reaction occurs (swelling of capsule  with type-specific antiserum). Serologic tests for antibody not  useful. Tests for capsular antigen in spinal fluid and C polysac¬  charide in urine can be diagnostic.   Treatment — Penicillin G. Low-level and high-level resistance  to penicillin is caused by alterations in penicillin-binding pro¬  teins. No (5-lactamase is made.   Prevention —Two vaccines are available. The one used in  adults contains capsular polysaccharide of the 23 serotypes  that cause bacteremia most frequently. The other, which is  used primarily in children under the age of 2 years, contains  capsular polysaccharide of 13 serotypes coupled to carrier  protein (diphtheria toxoid). Oral penicillin is used in immu¬  nocompromised children.   Diseases —Endocarditis is the most important disease. Also  brain abscess, especially in mixed infections with mouth  anaerobes. Streptococcus mutans implicated in dental caries.   Characteristics —Gram-positive cocci in chains. a-Hemolytic  colonies. Catalase-negative. Growth is resistant to optochin in  contrast to pneumococci, which are inhibited. Colonies are  not dissolved by bile.   Pathogenesis —Bacteremia from dental procedures spreads  organism to damaged heart valves. Organism is protected  from host defenses within vegetations. No known toxins. Gly-  cocalyx composed of polysaccharide enhances adhesion to  heart valves.   Laboratory Diagnosis —Gram-stained smear and culture.  a-Hemolytic colonies on blood agar. Growth not inhibited by  bile or optochin, in contrast to pneumococci. Viridans strep¬  tococci are classified into species by using various biochemical  tests. Serologic tests not useful.   Prevention —Penicillin to prevent endocarditis in patients  with damaged or prosthetic heart valves who undergo dental  procedures.   Habitat and Transmission —Habitat is the human upper  respiratory tract; transmission is via respiratory droplets.   Pathogenesis —After colonizing the upper respiratory tract, the  organism reaches the meninges via the bloodstream. Endotoxin  in cell wall causes symptoms of septic shock seen in meningococ¬  cemia. No known exotoxins; IgA protease produced. Capsule is  antiphagocytic. Deficiency in late complement components pre¬  disposes to recurrent meningococcal infections.   Laboratory Diagnosis —Gram-stained smear and culture. Oxi¬  dase-positive colonies on chocolate agar. Ferments maltose in  contrast to gonococci, which do not. Serologic tests not useful.   Prevention — The vaccines against groups A, C, Y, and  W-135 meningococci contain the polysaccharide capsule as  the immunogen. The vaccine against group B meningococci  contains Factor H binding protein as the immunogen. The  polysaccharide vaccine exists in two forms: the conjugate vac¬  cine contains the polysaccharides coupled to a carrier protein  such as diphtheria toxoid, and nonconjugate vaccine contains  only the polysaccharides. Rifampin or ciprofloxacin given to  close contacts to decrease oropharyngeal carriage.   Habitat and Transmission —Habitat is the human genital  tract. Transmission in adults is by sexual contact. Transmis¬  sion to neonates is during birth.   Pathogenesis —Organism invades mucous membranes and  causes inflammation. Endotoxin present but weaker than that  of meningococcus, so less severe disease when bacteremia  occurs. No exotoxins identified. IgA protease and pili are viru¬  lence factors.   Laboratory Diagnosis —Gram-stained smear and culture.  Organism visible intracellularly within neutrophils in urethral  exudate. Oxidase-positive colonies on Thayer-Martin  medium. Gonococci do not ferment maltose, whereas menin¬  gococci do. Serologic tests not useful. Nucleic acid amplifica¬  tion tests (NAAT) are used as a screening test in urogenital  infections.   Treatment —Ceftriaxone for uncomplicated cases. Azithromy¬  cin or doxycycline added for urethritis caused by Chlamydia  trachomatis. High-level resistance to penicillin is caused by  plasmid-encoded penicillinase. Low-level resistance to  penicillin is caused by reduced permeability and altered bind¬  ing proteins.   Characteristics —Aerobic, gram-positive, spore-forming  rods. Capsule composed of poly-D-glutamate. Bacillus anthracis  is the only medically important organism that has a capsule  composed of amino acids rather than polysaccharides.   Habitat and Transmission —Habitat is soil. Transmission is  by contact with infected animals or inhalation of spores from  animal hair and wool.   Pathogenesis —Anthrax toxin consists of three proteins:  edema factor, which is an adenylate cyclase; lethal factor,  which kills cells by inhibiting a signal transduction protein  involved in cell division; and protective antigen, which medi¬  ates the entry of the other two components into the cell. The  capsule is antiphagocytic.   Laboratory Diagnosis —Gram-stained smear plus aerobic  culture on blood agar. Bacillus anthracis is nonmotile, in con¬  trast to other Bacillus species. Rise in antibody titer in indirect  hemagglutination test is diagnostic.    Prevention —Vaccine consisting of protective antigen is given  to individuals in high-risk occupations.   Habitat and Transmission —Habitat is grains, such as rice.  Spores survive boiling during preparation of rice, then germi¬  nate when rice is held at warm temperature.   Pathogenesis —Two enterotoxins are produced: one acts like  cholera toxin (i.e., cyclic AMP is increased within entero-  cytes); the other acts like staphylococcal enterotoxin (i.e., it is  a superantigen).   Characteristics —Anaerobic, gram-positive, spore-forming  rods. Spore is at one end (“terminal spore”), so organism looks  like a tennis racket.   Pathogenesis —Spores germinate under anaerobic condi¬  tions in the wound. Organism produces exotoxin, which  blocks release of inhibitory neurotransmitters (glycine and  y-aminobutyric acid [GABA]) from spinal neurons. Excit¬  atory neurons are unopposed, and extreme muscle spasm  (tetanus, spastic paralysis) results. “Lock-jaw” and “risus sar-  donicus” are two examples of the muscle spasms. Tetanus  toxin (tetanospasmin) is a protease that cleaves proteins  involved in the release of neurotransmitters.   Treatment —Hyperimmune human globulin to neutralize  toxin. Also penicillin G and spasmolytic drugs (e.g., Valium).  No significant resistance to penicillin.   Prevention —Toxoid vaccine (toxoid is formaldehyde-treated  toxin). Usually given to children in combination with diph¬  theria toxoid and acellular pertussis vaccine (DTaP). If patient  is injured and has not been immunized, give hyperimmune  globulin plus toxoid (passive-active immunization). Debride  wound. Give tetanus toxoid booster every 10 years.    Habitat and Transmission —Habitat is the soil. Organism and  botulinum toxin transmitted in improperly preserved food.   Pathogenesis —Botulinum toxin is a protease that cleaves  proteins involved in the release of acetylcholine at the myo¬  neural junction, causing flaccid paralysis. Failure to sterilize  food during preservation allows spores to survive. Spores  germinate in anaerobic environment and produce toxin. The  toxin is heat-labile; therefore, foods eaten without proper  cooking are usually implicated.   Laboratory Diagnosis —Presence of toxin in patient’s serum  or stool or in food. Detection of toxin involves either antitoxin  in serologic tests or production of the disease in mice. Sero¬  logic tests for antibody in the patient are not useful.   Treatment —Antitoxin to types A, B, and E made in horses.  Respiratory support may be required.   Prevention —Observing proper food preservation techniques,  cooking all home-canned food, and discarding bulging cans.   Habitat and Transmission —Habitat is soil and human  colon. Myonecrosis results from contamination of wound  with soil or feces. Food poisoning is transmitted by ingestion  of contaminated food.   Pathogenesis —Gas gangrene in wounds is caused by germina¬  tion of spores under anaerobic conditions and the production of  several cytotoxic factors, especially alpha toxin, a lecithinase that  cleaves cell membranes. Gas in tissue (C0 2 and H 2 ) is produced  by organisms anaerobic metabolism. Food poisoning is caused  by production of enterotoxin within the gut. Enterotoxin acts as  a superantigen, similar to that of S. aureus.   Laboratory Diagnosis —Gram-stained smear plus anaerobic  culture. Spores not usually seen in clinical specimens; the  organism is growing, and nutrients are not restricted. Produc¬  tion of lecithinase is detected on egg yolk agar and identified  by enzyme inhibition with specific antiserum. Serologic tests  not useful.   Treatment —Penicillin G plus debridement of the wound in  gas gangrene (no significant resistance to penicillin). Only  symptomatic treatment needed in food poisoning.    Habitat and Transmission —Habitat is the human colon.  Transmission is fecal-oral.   Pathogenesis —Antibiotics suppress normal flora of colon,  allowing C. difficile to overgrow and produce large amounts of  exotoxins. Exotoxins A and B inhibit GTPases, causing inhibi¬  tion of signal transduction and depolymerization of actin fila¬  ments. This leads to apoptosis and death of enterocytes. The  pseudomembranes seen in the colon are the visual result of  the death of enterocytes.   Laboratory Diagnosis —Exotoxin in the stool is typically  detected by using known antibody to the toxin in an ELISA  test or by polymerase chain reaction (PCR) assay. Exotoxin in  stool can also be detected by cytopathic effect on cultured  cells. Identified by neutralization of cytopathic effect with  known antibody.   Treatment —Metronidazole. Vancomycin, although effective,  should not be used because it may select for vancomycin-  resistant enterococci.   Characteristics —Club-shaped gram-positive rods arranged  in V or L shape. Granules stain metachromatically. Aerobic,  non-spore-forming organism.   Pathogenesis —Organism secretes an exotoxin that inhibits  protein synthesis by adding ADP-ribose to elongation factor-2  (EF-2). Toxin has two components: subunit A, which has the  ADP-ribosylating activity, and subunit B, which binds the  toxin to cell surface receptors. Pseudomembrane in throat  caused by death of mucosal epithelial cells.   Treatment —Antitoxin made in horses neutralizes the toxin.  Penicillin G kills the organism. No significant resistance to  penicillin.   Prevention —Toxoid vaccine (toxoid is formaldehyde-treated  toxin), usually given to children in combination with tetanus  toxoid and acellular pertussis vaccine (DTaP).    Habitat and Transmission —Organism colonizes the GI and  female genital tracts; in nature, it is widespread in animals,  plants, and soil. Transmission is across the placenta or by con¬  tact during delivery. Outbreaks of sepsis in neonates and gas¬  troenteritis in the general population are related to ingestion  of unpasteurized milk products (e.g., cheese).   Pathogenesis —Listeriolysin is an exotoxin that degrades cell  membranes. Reduced cell-mediated immunity and immuno¬  logic immaturity as in neonates predispose to disease. Intra¬  cellular pathogen that moves from cell-to-cell via “actin  rockets.”   Laboratory Diagnosis —Gram-stained smear and culture.  Small, (3-hemolytic colonies on blood agar. Tumbling motility.  Serologic tests not useful.   Prevention —Pregnant women and immunocompromised  patients should not ingest unpasteurized milk products or raw  vegetables. Trimethoprim-sulfamethoxazole given to immu¬  nocompromised patients to prevent Pneumocystis pneumonia  also can prevent listeriosis. No vaccine is available.   Facultative gram-variable rod. Involved in bacterial vaginosis,  along with Mobiluncus species, which are anaerobic. See “clue  cells,” which are vaginal epithelial cells covered with G. vaginalis  cells. Positive “whiff” test found in bacterial vaginosis.   Habitat and Transmission —Habitat is the human colon; it  colonizes the vagina and urethra. From the urethra, it ascends  and causes UTI. Acquired during birth in neonatal meningitis  and by the fecal-oral route in diarrhea.   Pathogenesis —Endotoxin in cell wall causes septic shock.  Two enterotoxins are produced by enterotoxigenic E.coli  (ETEC) strains. The heat-labile toxin (LT) stimulates adenyl¬  ate cyclase by ADP-ribosylation. Increased cyclic AMP causes  outflow of chloride ions and water, resulting in diarrhea. The  heat-stable toxin (ST) causes diarrhea, perhaps by stimulating  guanylate cyclase. Virulence factors include pili for attach¬  ment to mucosal surfaces and a capsule that impedes phago¬  cytosis. Shiga toxin (verotoxin) is an enterotoxin produced by  E. coli strains (STEC) with the 0157:H7 serotype. It causes  bloody diarrhea and hemolytic-uremic syndrome associated    with eating undercooked meat. Shiga toxin (verotoxin) inhib¬  its protein synthesis by removing adenine from the 28S rRNA  of human ribosomes.   Predisposing factors to UTI in women include the proximity  of the anus to the vagina and urethra, as well as a short urethra.  This leads to colonization of the urethra and vagina by the fecal  flora. Abnormalities (e.g., strictures, valves, and stones) predis¬  pose as well. Indwelling urinary catheters and intravenous lines  predispose to UTI and sepsis, respectively. Colonization of the  vagina leads to neonatal meningitis acquired during birth. The  main virulence factor for neonatal meningitis is the K1 capsular  polysaccharide.   Laboratory Diagnosis —Gram-stained smear and culture.  Lactose-fermenting colonies on eosin-methylene blue (EMB)  or MacConkey’s agar. Green sheen on EMB agar. Triple sugar  iron (TSI) agar shows acid slant and acid butt with gas but no  H 2 S. Differentiate from other lactose-positive organisms by  biochemical reactions. For epidemiologic studies, type organ¬  ism by O and H antigens by using known antisera. Serologic  tests for antibodies in patient’s serum not useful.   Treatment —Ampicillin or sulfonamides for UTIs. Third-  generation cephalosporins for meningitis and sepsis. Rehydration  is effective in traveler’s diarrhea; trimethoprim-sulfamethoxazole  may shorten duration of symptoms. Antibiotic resistance  mediated by plasmid-encoded enzymes (e.g., (3-lactamase and  aminoglycoside-modifying enzymes).   Prevention —Prevention of UTI involves limiting the fre¬  quency and duration of urinary catheterization. Prevention of  sepsis involves promptly removing or switching sites of intra¬  venous lines. Traveler’s diarrhea is prevented by eating only  cooked food and drinking boiled water in certain countries.  Prophylactic doxycycline or Pep to-Bismol may prevent trav¬  eler’s diarrhea. There is no vaccine that prevents any of the  diseases caused by E. coli.   Habitat and Transmission —Habitat is the human colon only,  in contrast to other salmonellae, which are found in the colon  of animals as well. Transmission is by the fecal-oral route.   Pathogenesis —Infects the cells of the reticuloendothelial  system, especially in the liver and spleen. Endotoxin in cell  wall causes fever. Capsule (Vi antigen) is a virulence factor. No  exotoxins known. Decreased stomach acid resulting from  ingestion of antacids or gastrectomy predisposes to Salmonella  infections. Chronic carrier state established in gallbladder.  Organism excreted in bile results in fecal-oral spread to  others.   Laboratory Diagnosis —Gram-stained smear and culture.  Non-lactose-fermenting colonies on EMB or MacConkey’s agar.    TSI agar shows alkaline slant and acid butt, with no gas and a  small amount of H 2 S. Biochemical and serologic reactions  used to identify species. Identity can be determined by using  known antisera against O, H, and Vi antigens in agglutination  test. Widal test detects agglutinating antibodies to O and H  antigens in patient’s serum, but its use is limited.   Treatment —Most effective drug is ceftriaxone. Ampicillin  and trimethoprim-sulfamethoxazole can be used in patients  who are not severely ill. Resistance to chloramphenicol and  ampicillin is mediated by plasmid-encoded acetylating  enzymes and (3-lactamase, respectively.   Prevention —Public health measures (e.g., sewage disposal,  chlorination of the water supply, stool cultures for food han¬  dlers, and handwashing prior to food handling). Two vaccines  are in common use; one vaccine contains purified Vi polysac¬  charide capsule as the immunogen and the other contains live,  attenuated S. typhi as the immunogen.   Characteristics —Facultative gram-negative rods. Non¬  lactose-fermenting. Produces H 2 S. Motile, in contrast to  Shigella. More than 1500 serotypes.   Habitat and Transmission —Habitat is the enteric tract of  humans and animals (e.g., chickens and domestic livestock).  Transmission is by the fecal-oral route.   Pathogenesis —Invades the mucosa of the small and large  intestines. Can enter blood, causing sepsis. Infectious dose is  at least 100,000 organisms, much greater than the infectious  dose of Shigella. Infectious dose is high because organism is  inactivated by stomach acid. Endotoxin in cell wall; no exo¬  toxin. Predisposing factors include lowered stomach acidity  from either antacids or gastrectomy. Sickle cell anemia predis¬  poses to Salmonella osteomyelitis.   Laboratory Diagnosis —Gram-stained smear and culture.  Non-lactose-fermenting colonies on EMB or MacConkey’s  agar. TSI agar shows alkaline slant and acid butt, with gas and  H 2 S. Biochemical and serologic reactions used to identify spe¬  cies. Can identify the organism by using known antisera in  agglutination assay. Widal test detects antibodies in patient’s  serum to the O and H antigens of the organism but is not  widely used.   Treatment —Antibiotics usually not recommended for  uncomplicated enterocolitis. Ceftriaxone or other drugs are  used for sepsis, depending on sensitivity tests. Resistance to  ampicillin and chloramphenicol is mediated by plasmid-  encoded (3-lactamases and acetylating enzymes, respectively.   Prevention —Public health measures (e.g., sewage disposal,  chlorination of the water supply, stool cultures for food    handlers, and handwashing prior to food handling). Do not  eat raw eggs or meat. No vaccine is available.   Characteristics —Facultative gram-negative rods. Non¬  lactose-fermenting. Nonmotile, in contrast to Salmonella.   Habitat and Transmission —Habitat is the human colon  only; unlike Salmonella enterica, there are no animal carriers  for Shigella. Transmission is by the fecal-oral route.   Pathogenesis —Invades the mucosa of the ileum and colon  but does not penetrate farther; therefore, sepsis is rare. Endo¬  toxin in cell wall. Infectious dose is much lower (1-10  organisms) than that of Salmonella. The infectious dose of  Shigella is low because it is resistant to stomach acid. Children  in mental institutions and day care centers experience out¬  breaks of shigellosis. No chronic carrier state.   Laboratory Diagnosis —Gram-stained smear and culture.  Non-lactose-fermenting colonies on EMB or MacConkey’s  agar. TSI agar shows an alkaline slant with an acid butt and no  gas or H 2 S. Identified by biochemical reactions or by serology  with anti-0 antibody in agglutination test. Serologic tests for  antibodies in the patient’s serum are not done.   Treatment —In most cases, fluid and electrolyte replacement  only. In severe cases, ciprofloxacin. Resistance is mediated by  plasmid-encoded enzymes (e.g., (3-lactamase, which degrades  ampicillin, and a mutant pteroate synthetase, which reduces  sensitivity to sulfonamides).   Prevention —Public health measures (e.g., sewage disposal,  chlorination of the water supply, stool cultures for food han¬  dlers, and handwashing prior to food handling). Prophylactic  drugs not used. No vaccine is available.   Habitat and Transmission —Habitat is the human colon and  shellfish. Transmission is by the fecal-oral route.   Pathogenesis —Massive, watery diarrhea caused by entero -  toxin that activates adenylate cyclase by adding ADP-ribose to  the stimulatory G protein. Increase in cyclic AMP causes  outflow of chloride ions and water. Toxin has two compo¬  nents: subunit A, which has the ADP-ribosylating activity;  and subunit B, which binds the toxin to cell surface receptors.  Organism produces mucinase, which enhances attachment to  the intestinal mucosa. Role of endotoxin is unclear. Infectious  dose is high (>10 7 organisms). Carrier state rare.    Laboratory Diagnosis —Gram-stained smear and culture.  (During epidemics, cultures not necessary.) Agglutination of  the isolate with known antisera confirms the identification.   Treatment —Treatment of choice is fluid and electrolyte  replacement. Tetracycline is not necessary but shortens dura¬  tion and reduces carriage.   Prevention —Public health measures (e.g., sewage disposal,  chlorination of the water supply, stool cultures for food han¬  dlers, and handwashing prior to food handling). Vaccine  containing killed cells has limited effectiveness. Tetracycline  used for close contacts.   Comma-shaped gram-negative rod found in warm sea water.  Causes watery diarrhea. Acquired by eating contaminated raw  seafood. Outbreaks have occurred on cruise ships in Caribbean.  Diarrhea is mediated by enterotoxin similar to cholera toxin.   Comma-shaped gram-negative rod found in warm sea water.  Causes cellulitis and life-threatening sepsis with hemorrhagic  bullae. Acquired either by trauma to skin, especially in shellfish  handlers, or by ingestion of raw shellfish, especially in patients  who are immunocompromised or have reduced complement  caused by liver damage.   Habitat and Transmission —Habitat is human and animal  feces. Transmission is by the fecal-oral route.   Laboratory Diagnosis —Gram-stained smear plus culture on  special agar (e.g., Skirrows agar) at 42°C in high-C0 2 , low-0 2  atmosphere. Serologic tests not useful.   Prevention —Public health measures (e.g., sewage disposal,  chlorination of the water supply, stool cultures for food han¬  dlers, and handwashing prior to food handling). No preven¬  tive vaccine or drug is available.   Pathogenesis —Organisms synthesize urease, which pro¬  duces ammonia that damages gastric mucosa. Ammonia also  neutralizes acid pH in stomach, which allows the organism to  live in gastric mucosa.   Habitat and Transmission —Habitat is the human upper  respiratory and enteric tracts. Organism is transmitted to the  lungs by aspiration from upper respiratory tract and by inha¬  lation of respiratory droplets. It is transmitted to the urinary  tract by ascending spread of fecal flora.   Pathogenesis —Endotoxin causes fever and shock associated  with sepsis. No exotoxin known. Organism has large capsule,  which impedes phagocytosis. Chronic pulmonary disease pre¬  disposes to pneumonia; catheterization predisposes to UTE   Laboratory Diagnosis —Gram-stained smear and culture.  Characteristic mucoid colonies are a consequence of the  organisms abundant polysaccharide capsule. Lactose-fermenting  colonies on MacConkey’s agar. Differentiated from Enterobacter  and Serratia by biochemical reactions.   Treatment —Cephalosporins alone or with aminoglycosides,  but antibiotic sensitivity testing must be done. Resistance is  mediated by plasmid-encoded enzymes, especially (5-lactamase.   Enteric gram-negative rod similar to K. pneumoniae. Causes  hospital-acquired pneumonia, UTI, and sepsis. Highly  antibiotic-resistant.   Enteric gram-negative rod similar to K. pneumoniae. Causes  hospital-acquired pneumonia, UTI, and sepsis. Red-pigmented  colonies. Highly antibiotic-resistant.   Characteristics —Facultative gram-negative rods. Non¬  lactose-fermenting. Highly motile. Produce urease, as do  Morganella and Providencia species (see later). Antigens of  OX strains of P. vulgaris cross-react with many rickettsiae.   Habitat and Transmission —Habitat is the human colon and  the environment (soil and water). Transmission to urinary  tract is by ascending spread of fecal flora.   Pathogenesis —Endotoxin causes fever and shock associ¬  ated with sepsis. No exotoxins known. Urease is a virulence  factor because it degrades urea to produce ammonia, which  raises the pH. This leads to “struvite” stones, which can  obstruct urine flow, damage urinary epithelium, and serve  as a nidus for recurrent infection by trapping bacteria  within the stone. Organism is highly motile, which may  facilitate entry into the bladder. Predisposing factors are  colonization of the vagina, urinary catheters, and abnor¬  malities of the urinary tract such as strictures, valves, and  stones.   Laboratory Diagnosis —Gram-stained smear and culture.  “Swarming” (spreading) effect over blood agar plate as a con¬  sequence of the organism’s active motility. Non-lactose-  fermenting colonies on EMB or MacConkey’s agar. TSI agar  shows an alkaline slant and acid butt with H 2 S. Organism  produces urease, whereas Salmonella, which can appear simi¬  lar on TSI agar, does not. Serologic tests not useful. Proteus  mirabilis is indole-negative, whereas P. vulgaris, M. morganii,  and Providencia species are indole-positive.   Treatment —Trimethoprim-sulfamethoxazole or ampicil-  lin is often used for uncomplicated UTIs, but a third-  generation cephalosporin should be used for serious  infections. The indole-negative species P. mirabilis is more  likely to be sensitive to antibiotics such as ampicillin than  are the indole-positive species. Antibiotic sensitivities  should be tested. Resistance is mediated by plasmid-  encoded enzymes.   Enteric gram-negative rod similar to Proteus species. Causes  UTIs and sepsis. Highly motile and produces urease. Indole¬  positive and more resistant to antibiotics than P. mirabilis.   Enteric gram-negative rod similar to Proteus species. Causes  UTIs and sepsis. Highly motile and produces urease. Indole¬  positive and more resistant to antibiotics than P. mirabilis.   Diseases —Wound infection, UTI, pneumonia, and sepsis.  One of the most important causes of nosocomial infections,  especially in burn patients and those with cystic fibrosis.  Causes endocarditis in intravenous drug users.   Characteristics —Aerobic gram-negative rods. Non-lactose-  fermenting. Pyocyanin (blue-green) pigment produced. Oxi¬  dase-positive, which distinguishes it from members of the  Enterobacteriaceae family.   Habitat and Transmission —Habitat is environmental water  sources (e.g., in hospital respirators and humidifiers). Also  inhabits the skin, upper respiratory tract, and colon of about  10% of people. Transmission is via water aerosols, aspiration,  and fecal contamination.   Pathogenesis —Endotoxin is responsible for fever and shock  associated with sepsis. Produces exotoxin A, which acts like  diphtheria toxin (inactivates EF-2). Pili and capsule are viru¬  lence factors that mediate attachment and inhibit phagocyto¬  sis, respectively. Glycocalyx-producing strains predominate in  chronic infections in cystic fibrosis patients. Strains with type  III secretion systems are more virulent than those without.  Severe burns and neutropenia are important predisposing  factors.   Laboratory Diagnosis —Gram-stained smear and culture.  Non-lactose-fermenting colonies on EMB or MacConkey’s  agar. TSI agar shows an alkaline slant and an alkaline butt  because the sugars are not fermented. Oxidase-positive. Sero¬  logic tests not useful.   Treatment —Antibiotics must be chosen on the basis of  antibiotic sensitivities because resistance is common. Anti-  pseudomonal penicillin and aminoglycoside are often used.  Resistance is mediated by a variety of plasmid-encoded  enzymes (e.g., (3-lactamases and acetylating enzymes).   Prevention —Disinfection of water-related equipment in the  hospital, handwashing, and prompt removal of urinary and  intravenous catheters. There is no vaccine.   Gram-negative rod resembling P. aeruginosa. Important cause  of chronic infections in patients with cystic fibrosis. Formerly  called Pseudomonas cepacia.   Gram-negative rod resembling P. aeruginosa. Important cause  of chronic infections in patients with cystic fibrosis. Formerly  called Pseudomonas maltophilia.    Habitat and Transmission —Habitat is the human colon,  where it is the predominant anaerobe. Transmission occurs by  spread from the colon to the blood or peritoneum.   Pathogenesis —Lipopolysaccharide in cell wall is chemically  different from and less potent than typical endotoxin. No exo¬  toxins known. Capsule is antiphagocytic and promotes abscess  formation. Predisposing factors to infection include bowel  surgery and penetrating abdominal wounds.   Laboratory Diagnosis —Gram-stained smear plus anaerobic  culture. Identification based on biochemical reactions and gas  chromatography. Serologic tests not useful.   Treatment —Metronidazole, clindamycin, and cefoxitin are  all effective. Abscesses should be surgically drained. Resis¬  tance to penicillin G, some cephalosporins, and aminoglyco¬  sides is common. Plasmid-encoded (3-lactamase mediates  resistance to penicillin.   Prevention —In bowel surgery, perioperative cefoxitin can  reduce the frequency of postoperative infections. No vaccine  is available.   Anaerobic gram-negative rod resembling B.fragilis. Member of  normal flora found primarily above the diaphragm (e.g., mouth)  in contrast to B. fragilis, which is found below (e.g., colon).  Often involved in brain and lung abscesses. Formerly called  Bacteroides melaninogenicus.   Anaerobic gram-negative rod with pointed ends. Member of the  normal human flora in mouth, colon, and female genital tract.  Causes brain, lung, abdominal, and pelvic abscesses, typically in  combination with other anaerobes and facultative bacteria.   Diseases —Sinusitis, otitis media, and pneumonia are com¬  mon. Epiglottitis is uncommon, but H. influenzae is the most  important cause. Haemophilus influenzae used to be a leading  cause of meningitis, but the vaccine has greatly reduced the  number of cases.   Characteristics —Small gram-negative (coccobacillary) rods.  Requires factors X (hemin) and V (NAD) for growth. Of the  six capsular polysaccharide types, type b causes 95% of inva¬  sive disease. Type b capsule is polyribitol phosphate.   Habitat and Transmission —Habitat is the upper respiratory  tract. Transmission is via respiratory droplets.    strains cause mucosal infections but not invasive infections.  IgA protease is produced. Most cases of meningitis occur in  children younger than 2 years of age, because maternal anti¬  body has waned and the immune response of the child to  capsular polysaccharides can be inadequate. No exotoxins  identified.   Laboratory Diagnosis —Gram-stained smear plus culture on  chocolate agar. Growth requires both factors X and V. Deter¬  mine serotype by using antiserum in various tests (e.g., latex  agglutination). Capsular antigen can be detected in serum or  cerebrospinal fluid. Serologic test for antibodies in patient’s  serum not useful.   Treatment —Ceftriaxone is the treatment of choice for men¬  ingitis. Approximately 25% of strains produce (3-lactamase.   Prevention —Vaccine containing the type b capsular polysac¬  charide conjugated to diphtheria toxoid or other protein is  given between 2 and 18 months of age. Rifampin can prevent  meningitis in close contacts.   Pathogenesis —Pertussis toxin stimulates adenylate cyclase  by adding ADP-ribose onto the inhibitory G protein. Toxin  has two components: subunit A, which has the ADP-ribosyl-  ating activity, and subunit B, which binds the toxin to cell  surface receptors. Pertussis toxin causes lymphocytosis in the  blood by inhibiting chemokine receptors. Inhibition of these  receptors prevents lymphocytes from entering tissue, resulting  in large numbers being retained in the blood. Inhibition of  chemokine receptors occurs because pertussis toxin ADP-  ribosylates the inhibitory G protein which prevents signal  transduction within the cell. In addition, extracellular adenyl¬  ate cyclase is produced, which can inhibit killing by phago¬  cytes. Tracheal cytotoxin damages ciliated epithelium of  respiratory tract.   Laboratory Diagnosis —Gram-stained smear plus culture on  Bordet-Gengou agar. Identified by biochemical reactions and  slide agglutination with known antisera. PCR tests, if avail¬  able, are both sensitive and specific. Serologic tests for anti¬  body in patient’s serum not useful.   Prevention —The acellular vaccine containing pertussis tox¬  oid and four other purified proteins is recommended rather  than the killed vaccine, which contains whole organisms. Usu¬  ally given to children in combination with diphtheria and  tetanus toxoids (DTaP). Azithromycin is useful in unimmu¬  nized people who are known to be exposed.   Characteristics —Gram-negative rods, but stain poorly with  standard Gram stain. Require increased iron and cysteine for  growth in culture. Sixteen serogroups; most cases caused by  serogroup 1.   Habitat and Transmission —Habitat is environmental water  sources. Transmission is via aerosol from the water source.  Person-to-person transmission does not occur.   Pathogenesis —Aside from endotoxin, no toxins, enzymes,  or virulence factors are known. Predisposing factors include  being older than 55 years of age, smoking, and having a high  alcohol intake. Immunosuppressed patients (e.g., renal trans¬  plant recipients) are highly susceptible. The organism repli¬  cates intracellularly; therefore, cell-mediated immunity is an  important host defense. Smoking damages alveolar macro¬  phages, which explains why it predisposes to pneumonia.   Laboratory Diagnosis —Microscopy with silver impregna¬  tion stain or fluorescent antibody. Culture on charcoal yeast  extract agar containing increased amounts of iron and cyste¬  ine. Urinary antigen provides rapid diagnosis for serogroup 1  bacteria only. Diagnosis can be made serologically by detect¬  ing rise in antibody titer in patient’s serum.   Treatment —Azithromycin or erythromycin. Rifampin can  be added in severe cases.   Acinetobacter baumannii is a small coccobacillary gram¬  negative rod that causes pneumonia and urinary tract infections.  Disease occurs primarily in immunocompromised hospitalized  patients associated with respiratory therapy equipment (ventila¬  tor-associated pneumonia) and indwelling catheters. It is highly  antibiotic-resistant. Imipenem and colistin are effective in sus¬  ceptible strains. There is no vaccine.   Habitat and Transmission —Reservoir is domestic livestock.  Transmission is via unpasteurized milk and cheese or direct  contact with the infected animal.   Pathogenesis —Organisms localize in reticuloendothelial  cells, especially the liver and spleen. Able to survive and    replicate intracellularly. No exotoxins. Predisposing factors  include consuming unpasteurized dairy products and work¬  ing in an abattoir.   Laboratory Diagnosis —Gram-stained smear plus culture on  blood agar plate. Identified by biochemical reactions and by  agglutination with known antiserum. Diagnosis may be made  serologically by detecting antibodies in patient’s serum.   Habitat and Transmission —Reservoir is many species of  wild animals, especially rabbits, deer, and rodents. Transmis¬  sion is by ticks (e.g., Dermacentor ), aerosols, contact, and  ingestion.   Laboratory Diagnosis —Culture is rarely done because spe¬  cial media are required and there is a high risk of infection of  laboratory personnel. Diagnosis is usually made by serologic  tests that detect antibodies in patient’s serum.   Habitat and Transmission —Reservoir is the mouth of many  animals, especially cats and dogs. Transmission is by animal  bites.   Prevention —Ampicillin should be given to individuals with  cat bites. There is no vaccine.    Habitat and Transmission —Reservoir is wild rodents (e.g.,  rats, prairie dogs, and squirrels). Transmission is by flea bite.   Pathogenesis —Virulence factors include endotoxin, an exo¬  toxin, two antigens (V and W), and an envelope (capsular)  antigen that protects against phagocytosis. V and W proteins  allow organism to grow within cells. Bubo is a swollen inflamed  lymph node, usually located in the region of the flea bite.   Treatment —Streptomycin either alone or in combination  with doxycycline. Strict quarantine for 72 hours.   Prevention —Control rodent population and avoid contact  with dead rodents. Killed vaccine is available for persons in  high-risk occupations. Close contacts should be given  tetracycline.   Laboratory Diagnosis —Diagnosis of CSD usually made by  serologic tests. Biopsy of BA lesion shows pleomorphic rods  using Warthin-Starry stain.   Characteristics —Aerobic, acid-fast rods. High lipid content  of cell wall, which prevents dyes used in Gram stain from  staining organism. Lipids include mycolic acids and wax D.  Grows very slowly, which requires that drugs be present for  long periods (months). Produces catalase, which is required to  activate isoniazid to the active drug.   Habitat and Transmission —Habitat is the human lungs.  Transmission is via respiratory droplets produced by  coughing.    Pathogenesis —Granulomas and caseation mediated by cel¬  lular immunity (i.e., macrophages and CD4-positive T cells  [delayed hypersensitivity]). Cord factor (trehalose mycolate)  correlates with virulence. No exotoxins or endotoxin. Sup¬  pression of cell-mediated immunity increases risk of reactiva¬  tion and dissemination.   Laboratory Diagnosis —Acid-fast rods seen with Ziehl-  Neelsen (or Kinyoun) stain. Slow-growing (3-6 weeks) colony  on Lowenstein-Jensen medium. Organisms produce niacin  and are catalase-positive. Serologic tests for antibody in  patient’s serum not useful.   Skin Test —Purified protein derivative (PPD) skin test is posi¬  tive if induration measuring fO mm or more appears 48 hours  after inoculation. Induration is caused by a delayed hypersen¬  sitivity response. Positive skin test indicates that the person  has been infected but not necessarily that the person has the  disease tuberculosis.   Treatment —Long-term therapy (6-9 months) with three  drugs, isoniazid, rifampin, and pyrazinamide. A fourth drug,  ethambutol, is used in severe cases (e.g., meningitis), in immu¬  nocompromised patients (e.g., those with AIDS), and where the  chance of isoniazid-resistant organisms is high, as in Southeast  Asians. Most patients become noninfectious within 2 weeks of  adequate therapy. Treatment of latent (asymptomatic) infec¬  tions consists of isoniazid taken for 6 to 9 months or isoniazid  plus rifapentine for 3 months. Multidrug-resistant (MDR)  strains have emerged and require other drug combinations.   Prevention —Bacillus Calmette-Guerin (BCG) vaccine con¬  taining live, attenuated Mycobacterium bovis organisms may  prevent or limit extent of disease but does not prevent infec¬  tion with M. tuberculosis. Vaccine used rarely in the United  States but widely used in parts of Europe and Asia.   These mycobacteria are called atypical because they differ from  M. tuberculosis in various ways. The most important difference  is that the atypicals are found in the environment, whereas M.  tuberculosis is found only in humans. The atypicals are also  called “Mycobacteria other than M. tuberculosis” or MOTTS.   The atypicals are subdivided into slow growers and rapid  growers based on whether they form colonies in more than or  less than 7 days. (Pigment production by the slow growers need  not concern us here.)   (1 ) Mycobacterium avium-intracellulare complex (MAC)  causes tuberculosis-like disease, especially in immunocompro¬  mised patients, such as those with AIDS. It is highly  antibiotic-resistant.   (2) Mycobacterium kansasii also causes tuberculosis-like dis¬  ease but is less antibiotic-resistant than MAC.   (3) Mycobacterium marinum causes “swimming pool granu¬  loma or fish tank granuloma,” which is a skin lesion at the site  of an abrasion acquired in a swimming pool or an aquarium.    (4) Mycobacterium scrofulaceum causes scrofula, which  manifests as swollen, nontender cervical lymph nodes (cervical  adenitis).   The important rapid grower is Mycobacterium fortuitum-  chelonei complex, which causes infections of prosthetic joints  and indwelling catheters. It also causes skin and soft tissue  infections at the site of puncture wounds. The organisms are  usually resistant to most antituberculosis drugs.   Characteristics —Aerobic, acid-fast rods. Cannot be cultured  in vitro. Optimal growth at less than body temperature, so  lesions are on cooler parts of the body, such as skin, nose, and  superficial nerves.   Habitat and Transmission —Humans are the main reservoir.  Also found in armadillos. Most important mode of transmis¬  sion is nasal secretions of patients with the lepromatous form.  Patients with the lepromatous form are more likely to transmit  than those with the tuberculoid form because they have much  higher numbers of organisms than those with tuberculoid  leprosy. Prolonged exposure is usually necessary.   Pathogenesis —Lesions usually occur in the cooler parts of  the body (e.g., skin and peripheral nerves). In tuberculoid  leprosy, destructive lesions are due to the cell-mediated  response to the organism. Damage to fingers is due to burns  and other trauma, because nerve damage causes loss of sensa¬  tion. In lepromatous leprosy, the cell-mediated response to  M. leprae is lost, and large numbers of organisms appear in the  lesions and blood. No toxins or virulence factors are known.   Laboratory Diagnosis —Acid-fast rods are abundant in lep¬  romatous leprosy, but few are found in the tuberculoid form.  Cultures and serologic tests not done. Lepromin skin test is  positive in the tuberculoid but not in the lepromatous form. A  serologic test for IgM against phenolic glycolipid-1 is useful in  the diagnosis of lepromatous leprosy.   Treatment —Dapsone plus rifampin for the tuberculoid form.  Clofazimine is added to that regimen for the lepromatous  form or if the organism is resistant to dapsone. Treatment is  for at least 2 years.    Habitat and Transmission —Habitat is human mouth, espe¬  cially anaerobic crevices around the teeth. Transmission into  tissues occurs during dental disease or trauma. Organism also  aspirated into lungs, causing thoracic actinomycosis. Retained  intrauterine device (IUD) predisposes to pelvic  actinomycosis.   Habitat and Transmission —Habitat is the soil. Transmission  is via airborne particles, which are inhaled into the lungs.   Laboratory Diagnosis —Gram-stained smear and modified  Ziehl-Neelsen stain. Aerobic culture on blood agar plate. No  serologic tests.   Characteristics —Smallest free-living organisms. Not seen on  Gram-stained smear because they have no cell wall, so dyes  are not retained. Penicillins and cephalosporins are not effec¬  tive because there is no cell wall (peptidoglycan). The only  bacteria with cholesterol in cell membrane. Can be cultured  in vitro.   Pathogenesis —No exotoxins produced. No endotoxin  because there is no cell wall. Produces hydrogen peroxide,  which may damage the respiratory tract.    to grow, which is too long to be clinically useful. Positive  cold-agglutinin test is presumptive evidence. Complement  fixation test for antibodies to Mycoplasma pneumoniae is  more specific.   Habitat and Transmission —Habitat is the human genital  tract. Transmission is by sexual contact and from mother to  fetus across the placenta.   Pathogenesis —Organism multiplies at site of inoculation  and then spreads widely via the bloodstream. Many fea¬  tures of syphilis are attributed to blood vessel involvement  causing vasculitis. Primary (chancre) and secondary lesions  heal spontaneously. Tertiary lesions consist of gummas  (granulomas in bone, muscle, and skin), aortitis, or central  nervous system inflammation. No toxins or virulence fac¬  tors known.   Laboratory Diagnosis —Seen by dark field microscopy or  immunofluorescence. Serologic tests important: VDRL and  RPR are nontreponemal (nonspecific) tests used for screening;  FTA-ABS is the most widely used specific test for Treponema  pallidum. Antigen in VDRLand RPR is beef heart cardiolipin;  antigen in FTA-ABS is killed T. pallidum. VDRL declines with  treatment, whereas FTA-ABS remains positive for life.   Treatment —Penicillin is effective in the treatment of all  stages of syphilis. In primary and secondary syphilis, use ben¬  zathine penicillin G (a depot preparation) because T. pallidum  grows slowly, so drug must be present for a long time. There  is no resistance.   Habitat and Transmission —The main reservoir is the white¬  footed mouse. Transmitted by the bite of ixodid ticks, espe¬  cially in three areas in the United States: Northeast (e.g.,  Connecticut), Midwest (e.g., Wisconsin), and West Coast  (e.g., California). Eighty percent of cases are in the northeastern    states of Connecticut, New York, and New Jersey. Very small  nymph stage of ixodid tick (deer tick) is most common vector.  Tick must feed for at least 24 hours to deliver an infectious  dose of B. burgdorferi.   Pathogenesis —Organism invades skin, causing a rash called  erythema migrans. It then spreads via the bloodstream to  involve primarily the heart, joints, and central nervous sys¬  tem. No toxins or virulence factors identified.   Laboratory Diagnosis —Diagnosis usually made serologi¬  cally (i.e., by detecting IgM antibody). Confirm positive sero¬  logic test with Western blot assay.   Prevention —Vaccine containing outer membrane protein of  the organism was available but has been withdrawn. Avoid  tick bite. Can give doxycycline or amoxicillin to people who  are bitten by a tick in endemic areas.   Characteristics —Spirochetes that can be seen on dark field  microscopy but not light microscopy. Can be cultured in vitro.   Habitat and Transmission —Habitat is wild and domestic  animals. Transmission is via animal urine. In the United  States, transmission is chiefly via dog, livestock, and rat urine.   Pathogenesis —Two phases: an initial bacteremic phase and  a subsequent immunopathologic phase with meningitis. No  toxins or virulence factors known.   Laboratory Diagnosis —Dark field microscopy and culture  in vitro are available but not usually done. Diagnosis usually  made by serologic testing for antibodies in patient’s serum.   Causes relapsing fever. Transmitted by human body louse.  Organism well-known for its rapid antigenic changes, which  account for the relapsing nature of disease. Antigenic changes  are due to programmed rearrangements of bacterial DNA  encoding surface proteins.   Diseases —Nongonococcal urethritis, cervicitis, inclusion  conjunctivitis, lymphogranuloma venereum, and trachoma.  Also pneumonia in infants.    Characteristics —Obligate intracellular parasites. Not seen  on Gram-stained smear. Exists as inactive elementary body  extracellularly and as metabolically active, dividing reticulate  body intracellularly.   Habitat and Transmission —Habitat is the human genital  tract and eyes. Transmission is by sexual contact and during  passage of neonate through birth canal. Transmission in tra¬  choma is chiefly by hand-to-eye contact.   Laboratory Diagnosis —Nucleic acid amplification test  (NAAT) using the patients urine is used to diagnose chla¬  mydial sexually transmitted disease. Gram stain of urethral  exudates that show neutrophils but no gram-negative diplo-  cocci (gonococci) is presumptive evidence for chlamydial  infection. Cytoplasmic inclusions seen on Giemsa-stained or  fluorescent antibody-stained smear of exudate. Organism  grows in cell culture and embryonated eggs, but these are not  often used.   Habitat and Transmission —Habitat is birds, both psittacine  and others. Transmission is via aerosol of dried bird feces.   Laboratory Diagnosis —Diagnosis usually made by testing  for antibodies in patient’s serum. Cytoplasmic inclusion seen  by Giemsa or fluorescent antibody staining. Organism can be  isolated from sputum, but this is rarely done.   Characteristics —Obligate intracellular parasites. Not seen  well on Gram-stained smear. Antigens cross-react with OX  strains of Proteus vulgaris (Weil-Felix reaction).   Habitat and Transmission — Dermacentor (dog) ticks are  both the vector and the main reservoir. Transmission is via  tick bite. Dogs and rodents can be reservoirs as well.   Laboratory Diagnosis —Diagnosis made by detecting anti¬  body in serologic tests such as the ELISA test. Weil-Felix test  is no longer used. Stain and culture rarely done.   Prevention —Protective clothing and prompt removal of  ticks. Tetracycline effective in exposed persons. No vaccine is  available.   Habitat and Transmission —Humans are the reservoir, and  transmission is via the bite of the human body louse.   Prevention —A killed vaccine is used in the military but is  not available for civilian use.   Habitat and Transmission —Habitat is domestic livestock.  Transmission is by inhalation of aerosols of urine, feces, amni-  otic fluid, or placental tissue. The only rickettsia not transmit¬  ted to humans by an arthropod.   Laboratory Diagnosis —Diagnosis usually made by serologic  tests. Weil-Felix test is negative. Stain and culture rarely done.   Member of Rickettsia family. Causes human granulocytic ana-  plasmosis. Transmitted from reservoir (rodents, dogs) to  humans by ticks, especially Ixodes, the deer tick. Endemic in  northeastern and northcentral states (e.g., Connecticut and  Wisconsin). Forms morulae in cytoplasm of granulocytes.  (A morula is a “mulberry-shaped” inclusion body composed of  many A. phagocytophilum cells.) Doxycycline is the drug of  choice. There is no vaccine.   Member of Rickettsia family. Causes human monocytic ehrlichi¬  osis. Transmitted from dog reservoir to humans by ticks, espe¬  cially Dermacentor, the dog tick. Endemic in southern states  (e.g., Arkansas). Forms morulae in cytoplasm of monocytes.  (A morula is a “mulberry-shaped” inclusion body composed of  many E. chaffeensis cells.) Doxycycline is the drug of choice.  There is no vaccine.   Only the most important of the minor bacterial pathogens are  summarized in this section.   Gram-negative rod that is a member of the normal flora in the  human mouth. It causes skin and bone infections associated  with human bites and “clenched fist” injuries.   Small gram-negative rod. Causes chancroid. Sexually transmit¬  ted disease with painful ulcer on genitals (in contrast to syphilis,  which is painless). To grow in culture, it requires factor X  (heme) but not factor V (in contrast to H. influenzae, which  requires both).   Small coccobacillary gram-negative rod that resembles the cocci  of the genus Neisseria. Causes otitis media and sinusitis primar¬  ily in children. Also causes bronchitis and pneumonia, primarily  in older people with chronic obstructive pulmonary disease. It is  found only in humans and is transmitted by respiratory aerosol.   Gram-negative rods. Causes enterocolitis similar to that caused  by Shigella and Salmonella. Also causes mesenteric adenitis,  which can mimic appendicitis. Found in domestic animals and  transmitted to humans by fecal contamination of food.   Diseases —Herpes labialis (fever blisters or cold sores), kera¬  titis, encephalitis.   Characteristics —Enveloped virus with icosahedral nucleo-  capsid and linear double-stranded DNA. No virion poly¬  merase. One serotype; cross-reaction with herpes simplex  virus (HSV) type 2 occurs. HSV-1 can be distinguished from  HSV-2 by using monoclonal antibody against glycoprotein G.  No herpes group-specific antigen.   Pathogenesis —Initial vesicular lesions occur in the mouth  or on the face. The virus then travels up the axon and becomes  latent in sensory (trigeminal) ganglia. Recurrences occur in  skin innervated by affected sensory nerve and are induced by  fever, sunlight, stress, etc. Dissemination to internal organs  occurs in patients with depressed cell-mediated immunity  with life-threatening consequences. HSV-1 encephalitis often  affects the temporal lobe.    Laboratory Diagnosis —Virus causes cytopathic effect (CPE)  in cell culture. It is identified by antibody neutralization or  fluorescent antibody test. Tzanck smear of cells from the base  of the vesicle reveals multinucleated giant cells with intranu¬  clear inclusions. These giant cells are not specific for HSV-1;  they are seen in the vesicular lesions caused by HSV-2 and  varicella-zoster virus as well. A rise in antibody titer can be  used to diagnose a primary infection but not recurrences.  HSV encephalitis can be diagnosed using a PCR assay to  detect HSV-1 DNA in spinal fluid.   Treatment —Acyclovir for encephalitis and disseminated  disease. Acyclovir has no effect on the latent state of the virus.  Trifluridine or acyclovir for keratitis. Primary infections and  localized recurrences are self-limited.   Prevention —Recurrences can be prevented by avoiding the  specific inciting agent such as intense sunlight. Acyclovir,  valacyclovir, or famciclovir is used to reduce recurrences. No  vaccine is available.    Characteristics —Enveloped virus with icosahedral nucleo-  capsid and linear double-stranded DNA. No virion polymerase.  One serotype; cross-reaction with HSV-1 occurs. HSV-2 can be  distinguished from HSV-1 by using monoclonal antibody  against glycoprotein G. No herpes group-specific antigen.   Pathogenesis —Initial vesicular lesions occur on genitals.  The virus then travels up the axon and becomes latent in sen¬  sory (lumbar or sacral) ganglion cells. Recurrences are less  severe than the primary infection. HSV-2 infections in neo¬  nate can be life-threatening because neonates have reduced  cell-mediated immunity. Asymptomatic shedding of HSV-2 in  the female genital tract is an important contributing factor to  neonatal infections.   Laboratory Diagnosis —Virus causes CPE in cell culture.  Identify by antibody neutralization or fluorescent antibody  test. Tzanck smear reveals multinucleated giant cells but is not  specific for HSV-2. A rise in antibody titer can be used to  diagnose a primary infection but not recurrences.   Treatment —Acyclovir is useful in the treatment of primary  and recurrent genital infections as well as neonatal infections.  It has no effect on the latent state.   Prevention —Primary disease can be prevented by protection  from exposure to vesicular lesions. Recurrences can be  reduced by the long-term use of oral acyclovir, valacyclovir, or  famciclovir. Neonatal infection can be prevented by delivering  the child by cesarean section if the mother has visible vesicu¬  lar lesions in the birth canal. There is no vaccine.   Characteristics —Enveloped virus with icosahedral nucleo-  capsid and linear double-stranded DNA. No virion poly¬  merase. One serotype.   Transmission —Varicella is transmitted primarily by respira¬  tory droplets. Zoster is not transmitted; it is caused by a reac¬  tivation of latent virus.   Pathogenesis —Initial infection is in the oropharynx. It  spreads via the blood to the internal organs such as the liver  and then to the skin. After the acute episode of varicella, the  virus remains latent in the sensory ganglia and can reactivate  to cause zoster years later, especially in older and immuno¬  compromised individuals.   Laboratory Diagnosis —Virus causes CPE in cell culture and  can be identified by fluorescent antibody test. Multinucleated  giant cells seen in smears from the base of the vesicle. Intranu¬  clear inclusions seen in infected cells. A fourfold or greater rise  in antibody titer in convalescent-phase serum is diagnostic.    Treatment —No antiviral therapy is indicated for varicella or  zoster in the immunocompetent patient. In the immunocom¬  promised patient, acyclovir can prevent dissemination.   Prevention —Both the varicella vaccine and the zoster vac¬  cine contain live, attenuated varicella-zoster virus. Immuno¬  compromised patients exposed to the virus should receive  passive immunization with varicella-zoster immune globulin  (VZIG) and acyclovir to prevent disseminated disease.   Diseases —Most common cause of congenital abnormalities  in the United States. Cytomegalic inclusion body disease in  infants. Mononucleosis in transfusion recipients. Pneumonia  and hepatitis in immunocompromised patients. Retinitis and  enteritis, especially in AIDS patients.   Characteristics —Enveloped virus with icosahedral nucleo-  capsid and linear double-stranded DNA. No virion poly¬  merase. One serotype.   Transmission —Virus is found in many human body fluids,  including blood, saliva, semen, cervical mucus, breast milk,  and urine. It is transmitted via these fluids, across the pla¬  centa, or by organ transplantation.   Pathogenesis —Initial infection usually in the oropharynx. In  fetal infections, the virus spreads to many organs (e.g., central  nervous system and kidneys). In adults, lymphocytes are fre¬  quently involved. A latent state occurs in monocytes. Dissemi¬  nated infection in immunocompromised patients can result from  either a primary infection or reactivation of a latent infection.   Laboratory Diagnosis —The virus causes CPE in cell culture  and can be identified by fluorescent antibody test. “Owl’s eye”  nuclear inclusions are seen. A fourfold or greater rise in anti¬  body titer in convalescent-phase serum is diagnostic.   Treatment —Ganciclovir is used to treat pneumonia and reti¬  nitis. Acyclovir is ineffective.   Prevention —No vaccine is available. Ganciclovir suppresses  retinitis. Do not transfuse CMV antibody-positive blood into  newborns or antibody-negative immunocompromised patients.   Characteristics —Enveloped virus with icosahedral nucleo-  capsid and linear double-stranded DNA. No virion poly¬  merase. One serotype.   Pathogenesis —In infectious mononucleosis, infection  begins in the pharyngeal epithelium, spreads to the cervical  lymph nodes, and then travels via the blood to the liver and  spleen. EBV establishes latency in B lymphocytes. In Burkitt’s    lymphoma, oncogenesis is a function of the translocation of  the c-myc oncogene to a site adjacent to an immunoglobulin  gene promoter. This enhances synthesis of the c-myc protein,  a potent oncoprotein.   Laboratory Diagnosis —The virus is rarely isolated. In infec¬  tious mononucleosis, lymphocytosis, including atypical lym¬  phocytes, occurs. Heterophil antibody is typically positive  (Monospot test). Heterophil antibody agglutinates sheep or  horse red blood cells. A significant rise in EBV-specific anti¬  body to viral capsid antigen is diagnostic.   Causes Kaposi’s sarcoma, especially in AIDS patients. Transmit¬  ted sexually. Diagnosis made by pathologic examination of  lesion biopsy. See spindle cells and extravasated red blood cells.  Purple color of lesions due to collections of venous blood.   Treatment consists of either surgical excision, radiation, che¬  motherapy or immunomodulatory drugs, such as alpha inter¬  feron. In early HIV-associated KS, highly active antiretroviral  drugs (HAART) can be effective. Note that antiherpesvirus  drugs, such as acyclovir are not effective. HAART also can pre¬  vent KS. There is no vaccine against HHV-8.   Disease —Smallpox. The disease smallpox has been eradi¬  cated by use of the vaccine. The last known case was in 1977  in Somalia.   Transmission —By respiratory droplets or direct contact with  the virus from skin lesions.   Pathogenesis —The virus infects the mucosal cells of the  upper respiratory tract, then spreads to the local lymph nodes  and by viremia to the liver and spleen and later the skin. Skin  lesions progress in the following order: macule, papule, vesi¬  cle, pustule, crust.   Laboratory Diagnosis —Virus identified by CPE in cell cul¬  ture or “pocks” on chorioallantoic membrane. Electron  microscopy reveals typical particles; cytoplasmic inclusions  seen in light microscopy. Viral antigens in the vesicle fluid can  be detected by precipitin tests. A fourfold or greater rise in  antibody titer in the convalescent-phase serum is diagnostic.   Prevention —Vaccine contains live, attenuated vaccinia virus.  Vaccine is no longer used except by the military, because the  disease has been eradicated.   Causes molluscum contagiosum. See pinkish, papular skin  lesions with an umbilicated center. Lesions usually on the face,  especially around the eyes. Transmitted by direct contact. Diag¬  nosis made clinically; laboratory is not involved. There is no  established antiviral therapy and no vaccine. Cidofovir may be  useful in the treatment of the extensive lesions that occur in  immunocompromised patients.   Characteristics —Nonenveloped virus with icosahedral  nucleocapsid and linear double-stranded DNA. No virion  polymerase. There are 41 serotypes, some associated with  specific diseases.   Pathogenesis —Virus preferentially infects epithelium of  respiratory tract and eyes. After acute infection, persistent,  low-grade virus production without symptoms can occur in  the pharynx.   Laboratory Diagnosis —Virus causes CPE in cell culture and  can be identified by fluorescent antibody or complement fixa¬  tion test. Antibody titer rise in convalescent-phase serum is  diagnostic.   Prevention —Live vaccine against types 3, 4, and 7 is used in  the military to prevent pneumonia.   Diseases —Papillomas (warts); condylomata acuminata (gen¬  ital warts); associated with carcinoma of the cervix and penis.   Characteristics —Nonenveloped virus with icosahedral  nucleocapsid and circular double-stranded DNA. No virion  polymerase. There are at least 60 types, which are determined  by DNA sequence not by antigenicity. Many types infect the  epithelium and cause papillomas at specific body sites.   Pathogenesis —Two early viral genes, E6 and E7, encode  proteins that inhibit the activity of proteins encoded by tumor  suppressor genes (e.g., the p53 gene and the retinoblastoma  gene, respectively).    Laboratory Diagnosis —Diagnosis is made clinically by  finding koilocytes in the lesions. DNA hybridization tests are  available. Virus isolation and serologic tests are not done.   Treatment —Treatment varies according to the site of lesions:  liquid nitrogen is used for skin lesions, podophyllin for genital  lesions, and salicylic acid for plantar lesions.   Prevention —Three vaccines are available: one contains  the capsid proteins of nine HPV types (6, 11, 16, and 18 and  five others) and another contains only the capsid proteins  of the four most common HPV types (6, 11, 16, and 18)  that cause cancer. The third vaccine contains the capsid  proteins of two types (16 and 18) that are the most com¬  mon cause of cervical cancer.   Diseases —Slapped cheek syndrome (erythema infectiosum),  aplastic anemia, arthritis, and hydrops fetalis.   Characteristics —Nonenveloped virus with icosahedral sym¬  metry and single-stranded DNA genome. Virion contains no  polymerase. There is one serotype.   Pathogenesis —Virus preferentially infects erythroblasts,  causing aplastic anemia in patients with hereditary anemias;  immune complexes cause rash and arthritis. Virus can infect  fetus and cause severe anemia, leading to congestive heart  failure and edema (hydrops fetalis). Maternal antibody pro¬  tects fetus from infection.   Disease —Influenza. Influenza A virus is the main cause of  worldwide epidemics (pandemics). A pandemic caused by  a swine-origin strain of H1N1 influenza A virus began in  2009.   Characteristics —Enveloped virus with a helical nucleocap-  sid and segmented, single-stranded RNA of negative polarity.  RNA polymerase in virion. The two major antigens are the  hemagglutinin (HA) and the neuraminidase (NA) on separate  surface spikes. Antigenic shift in these proteins as a result of  reassortment of RNA segments accounts for the epidemics of  influenza caused by influenza A virus. Influenza A viruses of  animals are the source of the new RNA segments. Antigenic  drift due to mutations also contributes. The virus has  many serotypes because of these antigenic shifts and drifts.    The antigenicity of the internal nucleocapsid protein deter¬  mines whether the virus is an A, B, or C influenza virus.   Laboratory Diagnosis —A rapid ELISA test to detect influ¬  enza viral antigen in respiratory secretions is often used. Virus  grows in cell culture and embryonated eggs and can be  detected by hemadsorption or hemagglutination. It is identi¬  fied by hemagglutination inhibition or complement fixation.  A fourfold or greater antibody titer rise in convalescent-phase  serum is diagnostic.   Treatment —The neuraminidase inhibitor, oseltamivir  (Tamiflu), is the drug of choice. Zanamivir, another neur¬  aminidase inhibitor, is also available. Amantadine and riman¬  tadine are no longer used due to widespread resistance.   Prevention —Two types of vaccines are available: (1) a killed  (subunit) vaccine containing purified HA and NA; and (2) a  vaccine containing a live, temperature-sensitive mutant of  influenza virus. The virus in the live vaccine replicates in cool  nasal passages, where it induces secretory IgA, but not in  warm lower respiratory tract. Both vaccines contain the  strains of influenza A and B virus currently causing disease.  The killed vaccine is not a good immunogen and must be  given annually. The vaccine against “standard” influenza con¬  tains either two A strains (H1N1 and H3N2) and one B strain  or those two A strains and two B strains. The vaccine against  “swine” influenza contains only the novel H1N1 strain of  swine origin. Most of these vaccines are made in eggs, so any¬  one who has had a severe anaphylactic response to egg pro¬  teins should not receive the egg-derived vaccine. In 2013, a  vaccine not made in eggs became available. Oseltamivir  (Tamiflu) can be used for prophylaxis in unimmunized people  who have been exposed.   Characteristics —Enveloped virus with a helical nucleocap¬  sid and one piece of single-stranded, negative-polarity RNA.  RNA polymerase in virion. It has a single serotype.   Pathogenesis —Initial site of infection is the upper respira¬  tory tract. Virus spreads to local lymph nodes and then via the  blood to other organs, including the skin. Giant cell pneumo¬  nia and encephalitis can occur. The maculopapular rash is due  to cell-mediated immune attack by cytotoxic T cells on virus-  infected vascular endothelial cells in the skin.   Prevention —Vaccine contains live, attenuated virus. Usually  given in combination with mumps and rubella vaccines.   Disease —Mumps. Sterility due to bilateral orchitis is a rare  complication.   Characteristics —Enveloped virus with a helical nucleocap-  sid and one piece of single-stranded, negative-polarity RNA.  RNA polymerase in virion. It has a single serotype.   Pathogenesis —The initial site of infection is the upper respi¬  ratory tract. The virus spreads to local lymph nodes and then  via the bloodstream to other organs, especially the parotid  glands, testes, ovaries, meninges, and pancreas.   Laboratory Diagnosis —The virus can be isolated in cell  culture and detected by hemadsorption. Diagnosis can also be  made serologically. PCR assay is available.   Prevention —Vaccine contains live, attenuated virus. Usually  given in combination with measles and rubella vaccines.   Disease —Rubella. Congenital rubella syndrome is character¬  ized by congenital malformations, especially affecting the  cardiovascular and central nervous systems, and by prolonged  virus excretion. The incidence of congenital rubella has been  greatly reduced by the widespread use of the vaccine.   Characteristics —Enveloped virus with an icosahedral  nucleocapsid and one piece of single-stranded positive-polarity  RNA. No polymerase in virion. It has a single serotype.   Pathogenesis —The initial site of infection is the nasophar¬  ynx, from which it spreads to local lymph nodes. It then dis¬  seminates to the skin via the bloodstream. The rash is  attributed to both viral replication and immune injury. Dur¬  ing maternal infection, the virus replicates in the placenta and  then spreads to fetal tissue. If infection occurs during the first  trimester, a high frequency of congenital malformations  occurs. Maternal antibody protects against fetal infection.   Laboratory Diagnosis —Virus is detected by PCR assay. To  determine whether an adult woman is immune, a single  serum specimen to detect IgG antibody in the hemagglutina¬  tion inhibition test is used. To detect whether recent infection  has occurred, either a single serum specimen for IgM anti¬  body or a set of acute- and convalescent-phase sera for IgG  antibody can be used.    Prevention —Vaccine contains live, attenuated virus. Usually  given in combination with measles and mumps vaccine.   Diseases —Bronchiolitis in infants, croup in young children,  and the common cold in adults.   Characteristics —Enveloped virus with helical nucleocapsid  and one piece of single-stranded, negative-polarity RNA.  RNA polymerase in virion. Unlike influenza viruses, the anti¬  genicity of its hemagglutinin and neuraminidase is stable.  There are four serotypes.   Laboratory Diagnosis —Isolation of the virus in cell culture  is detected by hemadsorption. Immunofluorescence is used  for identification. A fourfold or greater rise in antibody titer  can also be used for diagnosis. PCR assay is available.   Diseases —Most important cause of bronchiolitis and  pneumonia in infants. Also causes otitis media in older  children.   Characteristics —Enveloped virus with a helical nucleocap¬  sid and one piece of single-stranded, negative-polarity RNA.  RNA polymerase in virion. Unlike other paramyxoviruses, it  has only a fusion protein in its surface spikes. It has no hemag¬  glutinin. It has two serotypes.   Pathogenesis —Infection involves primarily the lower respi¬  ratory tract in infants without systemic spread. Immune  response probably contributes to pathogenesis. Multinucle-  ated giant cells caused by the viral fusion protein are a  hallmark.   Laboratory Diagnosis —Enzyme immunoassay (rapid anti¬  gen test) detects respiratory syncytial virus (RSV) antigens in  respiratory secretions. Isolation in cell culture. Multinucleated  giant cells visible. Immunofluorescence is used for identifica¬  tion. Serology is not useful for diagnosis in infants. PCR assay  is available.   Prevention —Passive immunization with palivizumab  (monoclonal antibody) or immune globulins in infants who  have been exposed is effective. Handwashing and the use of  gloves may prevent nosocomial outbreaks in the newborn  nursery.   Characteristics —Enveloped virus with helical nucleocapsid  and one piece of single-stranded, positive-polarity RNA. No  virion polymerase. There are two serotypes.   Pathogenesis —Infection is typically limited to the mucosal  cells of the respiratory tract. At least 50% of infections are  asymptomatic. Immunity is brief and reinfection occurs.   Laboratory Diagnosis —The diagnosis primarily a clinical  one. Antibody-based and PCR-based tests are available but  not often done.   Characteristics —Bullet-shaped enveloped virus with a heli¬  cal nucleocapsid and one piece of single-stranded, negative-  polarity RNA. RNA polymerase in virion. The virus has a  single serotype.   Transmission —Main reservoir is wild animals such as  skunks, raccoons, and bats. Transmission to humans is usually  by animal bite, but the virus is also transmitted by aerosols of  bat saliva. In the United States, dogs are infrequently involved  because canine immunization is so common, but in develop¬  ing countries they are often involved.   Pathogenesis —Viral receptor is the acetylcholine receptor.  Replication of virus at the site of the bite, followed by axonal  transport up the nerve to the central nervous system. After  replicating in the brain, the virus migrates peripherally to the  salivary glands, where it enters the saliva. When the animal is  in the agitated state as a result of encephalitis, virus in the  saliva can be transmitted via a bite.   Laboratory Diagnosis —Tissue can be stained with fluores¬  cent antibody or with various dyes to detect cytoplasmic  inclusions called Negri bodies. The virus can be grown in cell  culture, but the process takes too long to be useful in deter¬  mining whether a person should receive the vaccine. Serologic  testing is useful only to make the diagnosis in the clinically ill  patient. Antibody does not form quickly enough to help in the  decision whether or not to immunize the person who has  been bitten. Serologic testing is also used to evaluate the anti¬  body response to the vaccine given before exposure to those  in high-risk occupations. PCR assay can provide rapid  diagnosis.    Prevention —Preexposure prevention of rabies consists of the  vaccine only. Postexposure prevention consists of (1) washing  the wound; (2) giving rabies immune globulins (passive  immunization), mostly into the wound; and (3) giving the  inactivated vaccine (active immunization) made in human  cell culture. The decision to give the immune serum and the  vaccine depends on the circumstances. Prevention of rabies in  dogs and cats by using a killed vaccine has reduced human  rabies significantly.   Disease —Adult T-cell leukemia/lymphoma and human  T-cell lymphotropic virus (HTLV)-associated myelopathy  (also known as tropical spastic paraparesis or chronic progres¬  sive myelopathy).   Characteristics —HTLV is a member of the retrovirus family.  It causes malignant transformation of CD4-positive T cells (in  contrast to human immunodeficiency virus [HIV], which  kills those cells). HTLV has three structural genes common to  all retroviruses, namely, gag, pol, and env, plus two regulatory  genes, tax and rex. The Tax protein is required for malignant  transformation. It activates the synthesis of IL-2 (which is  T-cell growth factor) and of the IL-2 receptor. IL-2 promotes  rapid T-cell growth, which predisposes to malignant  transformation.   Transmission —HTLV is transmitted primarily by intrave¬  nous drug use, sex, and breast feeding. Transmission by  donated blood has greatly decreased in the United States  because donated blood that has antibodies to HTLV is dis¬  carded. HTLV infection is endemic in certain geographic  areas, namely, the Caribbean region including southern  Florida, eastern South America, western Africa, and southern  Japan.   Pathogenesis —HTLV induces malignant transformation of  CD4-positive T lymphocytes by activating IL-2 synthesis as  described previously. It also causes HTLV-associated myelop¬  athy (HAM), which is a demyelinating disease of the brain and  spinal cord caused either by an autoimmune cross-reaction in  which the immune response against HTLV damages the neu¬  rons or by cytotoxic T cells that kill HTLV-infected neurons.   Laboratory Diagnosis —Anti-HTLV antibodies can be  detected in the patients serum using the ELISA test. Western  blot assay is used to confirm a positive ELISA result. PCR  assay can detect the presence of HTLV RNA or DNA within  infected cells.   Treatment and Prevention —No specific antiviral treatment  for HTLV infection, and no antiviral drug will cure latent  infections by HTLV. No vaccine against HTLV. Preventive  measures include discarding donated blood if anti-HTLV  antibodies are present, using condoms to prevent sexual  transmission, and encouraging women with HTLV antibodies  to refrain from breast feeding.   Member of the Filovirus family. Causes Ebola hemorrhagic  fever, which has a very high mortality rate. Animal reservoir  and mode of transmission to humans are unknown. Human-to-  human transmission, especially in hospital setting, is by blood  and other body fluids. Diagnosis is usually a clinical one, but  serologic tests and PCR tests are available. In electron micro¬  scope, seen as long “thread-like” viruses. Culturing the virus is  very dangerous and should be done only in special laboratories.  There is no antiviral therapy and no vaccine.   Diseases —Paralytic poliomyelitis and aseptic meningitis.  Poliomyelitis has been eradicated in the Western Hemisphere  and in many other countries.   Characteristics —Naked nucleocapsid virus with single-  stranded, positive-polarity RNA. Genome RNA acts as mRNA  and is translated into one large polypeptide, which is cleaved  by virus-encoded protease to form functional viral proteins.  No virion polymerase. There are three serotypes.   Pathogenesis —The virus replicates in the pharynx and the  GI tract. It can spread to the local lymph nodes and then  through the bloodstream to the central nervous system. Most  infections are asymptomatic or very mild. Aseptic meningitis  is more frequent than paralytic polio. Paralysis is the result of  death of motor neurons, especially anterior horn cells in the  spinal cord. Pathogenesis of postpolio syndrome is unknown.   Laboratory Diagnosis —Recovery of the virus from spinal  fluid indicates infection of the central nervous system. Isola¬  tion of the virus from stools indicates infection but not neces¬  sarily disease. It can be found in the GI tract of asymptomatic  carriers. The virus can be detected in cell culture by CPE and  identified by neutralization with type-specific antiserum. A  significant rise in antibody titer in convalescent-phase serum  is also diagnostic.   Prevention —Disease can be prevented by both the inacti¬  vated (Salk) vaccine and the live, attenuated (Sabin) vaccine;  both induce humoral antibody that neutralizes the virus in the  bloodstream. However, only the oral vaccine induces intesti¬  nal IgA, which interrupts the chain of transmission by pre¬  venting GI tract infection. For that reason and because it  induces immunity of longer duration and is orally adminis¬  tered rather than injected, the Sabin vaccine has been the  preferred vaccine for many years. However, there have been a  few vaccine-associated cases of paralytic polio caused by    poliovirus in the vaccine that reverted to virulence. In view of  this, the current recommendation in the United States is to  use the killed vaccine.   Diseases —Aseptic meningitis, herpangina, pleurodynia,  myocarditis, pericarditis, and hand, foot, and mouth disease  are the most important diseases. Also Coxsackie virus B4 may  cause juvenile diabetes, as it will do so in mice.   Characteristics —Naked nucleocapsid virus with single-  stranded, positive-polarity RNA. No virion polymerase.  Group A and B viruses are defined by their different pathoge¬  nicity in mice. There are multiple serotypes in each group.   Pathogenesis —The initial site of infection is the orophar¬  ynx, but the main site is the GI tract. The virus spreads  through the bloodstream to various organs.   Laboratory Diagnosis —The virus can be detected by CPE in  cell culture and identified by neutralization. A significant rise  in antibody titer in convalescent-phase serum is diagnostic.   Characteristics —Naked nucleocapsid viruses with single-  stranded, positive-polarity RNA. No virion polymerase. There  are more than 100 serotypes, which explains why the common  cold is so common. Rhinoviruses are destroyed by stomach  acid and therefore do not replicate in the GI tract, in contrast  to other picornaviruses such as poliovirus, Coxsackie virus,  and echovirus, which are resistant to stomach acid.   Pathogenesis —Infection is limited to the mucosa of the  upper respiratory tract and conjunctiva. The virus replicates  best at the low temperatures of the nose and less well at 37°C,  which explains its failure to infect the lower respiratory tract.   Laboratory Diagnosis —Laboratory tests are rarely used clini¬  cally The virus can be recovered from nose or throat washings  by growth in cell culture. Serologic tests are not useful.   Characteristics —Nonenveloped virus with icosahedral nucleo¬  capsid and one piece of single-stranded, positive-polarity   Pathogenesis —Infection is typically limited to the mucosal  cells of the intestinal tract. Many infections are asymptomatic.  Immunity is brief and reinfection occurs.   Laboratory Diagnosis —The diagnosis primarily a clinical  one. A PCR-based test is available but not often done.   Characteristics —Naked double-layered capsid with 11 seg¬  ments of double-stranded RNA. RNA polymerase in virion.  Rotavirus is resistant to stomach acid and hence can reach the  small intestine. There are at least six serotypes.   Pathogenesis —Rotavirus infection is limited to the GI tract,  especially the small intestine.   Laboratory Diagnosis —Detection of rotavirus in the stool  by ELISA. Isolation of the virus is not done from clinical  specimens.   Prevention —There are two rotavirus vaccines. One is a live  attenuated vaccine that contains the single most common  rotavirus serotype (Gl), and the other is a live reassortant vac¬  cine that contains five rotavirus strains.   Transmission —Fecal-oral route. In contrast to hepatitis B  virus (HBV) and hepatitis C virus (HCV), blood-borne trans¬  mission of hepatitis A virus (HAV) is uncommon because  viremia is brief and of low titer.   Pathogenesis —The virus replicates in the GI tract and then  spreads to the liver during a brief viremic period. The virus is  not cytopathic for the hepatocyte. Hepatocellular injury is  caused by immune attack by cytotoxic T cells.    Laboratory Diagnosis —The most useful test to diagnose  acute infection is IgM antibody. Isolation of the virus from  clinical specimens is not done.   Diseases —Hepatitis B; implicated as a cause of hepatocellu¬  lar carcinoma.   Characteristics —Enveloped virus with incomplete circular  double-stranded DNA (i.e., one strand has about one-third  missing and the other strand is “nicked” [not covalently  bonded]). DNA polymerase in virion. HBV-encoded DNA  polymerase acts as a reverse transcriptase by using viral  mRNA as the template for the synthesis of progeny genome  DNA. There are three important antigens: the surface antigen,  the core antigen, and the e antigen. Another protein, HBx,  inactivates p53 tumor suppressor protein, a process involved  in causing hepatocellular carcinoma. In the patient’s serum,  long rods and spherical forms composed solely of HBsAg  predominate. HBV has one serotype based on the surface  antigen.   Pathogenesis —Hepatocellular injury due to immune attack  by cytotoxic (CD8) T cells. Chronic carrier state occurs in 5%  of adult infections but in 90% of neonatal infections because  neonates have poor cytotoxic T-cell activity. Chronic carrier  state can lead to chronic hepatitis, cirrhosis, and hepatocellu¬  lar carcinoma. Hepatocellular carcinoma maybe related to the  integration of part of the viral DNA into hepatocyte DNA and  subsequent synthesis of HBx protein. Antigen-antibody com¬  plexes cause arthritis, rash, and glomerulonephritis.   Laboratory Diagnosis —HBV has not been grown in cell  culture. Three serologic tests are commonly used: surface  antigen (HBsAg), surface antibody (HBsAb), and core anti¬  body (HBcAb). Detection of HbsAg for more than 6 months  indicates a chronic carrier state. The presence of e antigen  indicates a chronic carrier who is making infectious virus. The  presence of e antigen is an important indicator of transmissi-  bility. An HBV-infected person who has neither detectable  HBs antigen nor HBs antibody is said to be in the “window”  phase. Diagnosis of this patient is made by detecting HB core  antibody. See Chapter 41 for a discussion of the results of  these tests.   Treatment —No treatment is given for acute hepatitis B. For  chronic hepatitis B, a reverse transcriptase inhibitor, such as  tenofovir or entecavir, can reduce the inflammation associated  with chronic hepatitis B but does not cure the carrier state. A  combination of tenofovir and emtricitabine is also effective.    Prevention —There are three main approaches: (1) vaccine  that contains HBsAg as the immunogen; (2) hyperimmune  serum globulins obtained from donors with high titers of  HBsAb; and (3) education of chronic carriers regarding pre¬  cautions. Passive-active immunization using both vaccine  and immune globulins can prevent infection in neonates and  those with needle-stick injuries.   Disease —Hepatitis C; associated with hepatocellular carci¬  noma. HCV is the most prevalent bloodborne pathogen in the  United States.   Pathogenesis —Hepatocellular injury caused by cytotoxic  T cells. HCV replication itself does not kill cells (i.e., does not  cause a cytopathic effect). More than 50% of infections result  in the chronic carrier state. The chronic carrier state predis¬  poses to chronic hepatitis and to hepatocellular carcinoma.   Laboratory Diagnosis —Serologic testing detects antibody to  HCV. A PCR-based assay for “viral load” can be used to evalu¬  ate whether active infection is present.   Treatment —Treatment of acute hepatitis C with pegylated  interferon alpha significantly reduce the number of patients  who become chronic carriers. Treatment of chronic hepatitis C  with a combination of drugs from three classes: an RNA poly¬  merase inhibitor such as sofosbuvir, an NS5A inhibitor such as  ledipasvir, and a protease inhibitor such as paritaprevir.   Prevention —Posttransfusion hepatitis can be prevented by  discarding donated blood if antibody to HCV is detected.  There is no vaccine, and hyperimmune globulins are not  available.   Characteristics —Defective virus that uses hepatitis B surface  antigen as its protein coat. Hepatitis D virus (HDV) can repli¬  cate only in cells already infected with HBV (i.e., HBV is a  helper virus for HDV). Genome is one piece of single-stranded,  negative-polarity, circular RNA. No polymerase in virion.  HDV has one serotype (because HBV has only one serotype).   Laboratory Diagnosis —Serologic testing detects either delta  antigen or antibody to delta antigen.    Treatment —Pegylated alpha interferon mitigates symptoms  but does not eradicate the carrier state.   Prevention —Prevention of HBV infection by using the HBV  vaccine and the HBV hyperimmune globulins will prevent  HDV infection also.   Causes outbreaks of hepatitis, primarily in developing coun¬  tries. Similar to hepatitis A virus in the following ways: trans¬  mitted by fecal-oral route, no chronic carrier state, no cirrhosis,  and no hepatocellular carcinoma. No antiviral therapy and no  vaccine.   All arboviruses are transmitted by arthropods (arthropod-  borne) such as mosquitoes and ticks from the wild animal res¬  ervoir to humans.   Characteristics —Enveloped virus with icosahedral nucleo-  capsid and single-stranded, positive-polarity RNA. No poly¬  merase in virion.   Laboratory Diagnosis —Virus isolation from blood, spinal  fluid, or brain. Also antibodies in patients serum. PCR test is  available.   Member of the togavirus family. Causes encephalitis along the  East Coast of the United States. Encephalitis is severe but  uncommon. Transmitted to humans (and horses) by mosqui¬  toes from small wild birds, such as sparrows. Humans and  horses are “dead-end” hosts because viremia is low. There is no  antiviral therapy and no vaccine for humans.   The transmission of these encephalitis viruses is similar (i.e., they  are transmitted to humans by mosquitoes from small wild birds).  However, they differ in details (i.e., they belong to different  virus families and cause disease in different geographic areas).  Please consult Chapter 42 in the text for specific information.   Member of the flavivirus family. Causes yellow fever in the  tropical areas of Africa and South America. “Jungle” yellow  fever is transmitted from monkeys to humans by mosquitoes.  “Urban” yellow fever is transmitted from human to human by  Aedes mosquitoes (i.e., humans are the reservoir in the urban  form). Humans are not a “dead-end” host because viremia is  high. There is no antiviral therapy. There is a live, attenuated  vaccine for humans.   Member of the flavivirus family. Causes dengue fever in the  Caribbean region and other tropical areas. Dengue is the most  common insect-borne viral disease in the world. Transmitted by  Aedes mosquitoes from one human to another. A monkey reser¬  voir is suspected. Second episodes may result in dengue hemor¬  rhagic fever, a life-threatening complication. There is no  antiviral therapy and no vaccine.   Member of the togavirus family. Causes chickungunya fever in  the Caribbean region and other tropical areas. Transmitted by  Aedes mosquitoes from one human to another. There is no anti¬  viral therapy and no vaccine.   HTLV causes adult T-cell leukemia. Oncogenesis is a function  of the tax gene of HTLV. The Tax protein activates synthesis of  IL-2, also known as T-cell growth factor, and the receptor for  IL-2. For further information, see summary of HTLV in  Chapter 39, “RNA Enveloped Viruses.”   HCV causes hepatocellular carcinoma in chronic carriers of  HCV. The mechanism of oncogenesis by HCV is unclear. It  appears to be a consequence of the rapid cell division that  occurs in an effort to replace the killed hepatocytes. An onco¬  gene has not been identified in the HCV genome. For further  information, see summary of HCV in Chapter 41, “Hepatitis  Viruses.”   HPV primarily causes carcinoma of the cervix, penis, and anus.  Oncogenesis is a function of the E6 and E7 genes of HPV. The    E6 and E7 proteins inactivate the tumor suppressor proteins,  p53 and RB, respectively. For further information, see summary  of HPV in Chapter 38, “DNA Nonenveloped Viruses.”   EBV primarily causes Burkitt’s lymphoma and nasopharyngeal  carcinoma. Oncogenesis is a function of the translocation of the  c-myc oncogene to a site adjacent to an immunoglobulin gene  promoter. This enhances synthesis of the c-myc protein, a  potent oncoprotein. For further information, see summary of  EBV in Chapter 37, “DNA Enveloped Viruses.”   Human herpesvirus 8 (HHV-8) causes Kaposi’s sarcoma. Onco¬  genesis is primarily a function of an early protein analogous to  the E7 protein of HPV that inactivates the tumor suppressor  protein RB. For further information, see summary of HHV-8 in  Chapter 37, “DNA Enveloped Viruses.”   HBV causes hepatocellular carcinoma in chronic carriers of  HBV. Oncogenesis is primarily a function of the Hbx protein that  inactivates the tumor suppressor protein p53. For further infor¬  mation, see summary of HBV in Chapter 41, “Hepatitis Viruses.”   Merkel cell polyomavirus (MCPV) causes carcinoma of Merkel  cells in the skin, often on sun-exposed areas such as the face and  neck. MCPV is a nonenveloped virus with a double-stranded  DNA genome. The T antigen protein of MCPV inhibits tumor  suppressor proteins, p53 and RB. Infection with MCPV is wide¬  spread, but the carcinoma is rare. The carcinoma cells do not  produce virus, so transmission from patients with the carci¬  noma to others does not occur. Diagnosis is made by micro¬  scopic analysis of surgical specimens. There is no virus-based  assay. There is no antiviral drug or vaccine.   Member of the papovavirus family. Causes progressive multifo¬  cal leukoencephalopathy (PML). Infection with JC virus is  widespread, but PML occurs only in immunocompromised  patients, such as those with AIDS. Invariably fatal. No antiviral  therapy and no vaccine.   Diseases —Creutzfeldt-Jakob disease (CJD), variant CJD,  and kuru. These are transmissible spongiform encepha¬  lopathies. There is a hereditary form of CJD called Gerstmann-  Straussler-Scheinker (GSS) syndrome.    Characteristics —Prions are composed of protein only. They  have no detectable nucleic acid and are highly resistant to ultra¬  violet (UV) light, formaldehyde, and heat. They are encoded by  a cellular gene. The pathogenic form increases in amount by  inducing conformational change in normal form. Normal con¬  formation is alpha helix; abnormal is beta-pleated sheet. In GSS  syndrome, a mutation occurs that enhances the probability of  the conformational change to the beta-pleated sheet form.   Transmission —In most cases of CJD, mode of transmission  is unknown. CJD has been transmitted by pituitary extracts,  brain electrodes, and corneal transplants. Kuru was transmit¬  ted by ingestion or inoculation of human brain tissue. Variant  CJD probably is transmitted by ingestion of cow brain tissue  in undercooked food.   Pathogenesis —Aggregation of prion filaments within neurons  occurs and vacuoles within neurons cause spongiform changes  in brain. No inflammation or immune response occurs.   Laboratory Diagnosis —Brain biopsy shows spongiform  changes. No serologic tests are useful. Prions cannot be grown  in culture.   Characteristics —Enveloped virus with two copies (diploid)  of a single-stranded, positive-polarity RNA genome. RNA-  dependent DNA polymerase (reverse transcriptase) makes a  DNA copy of the genome, which integrates into host cell  DNA. Precursor polypeptides must be cleaved by virus-  encoded protease to produce functional viral proteins. The tat  gene encodes a protein that activates viral transcription. Anti¬  genicity of the gpl20 protein changes rapidly; therefore, there  are many serotypes.   Transmission —Transfer of body fluids (e.g., blood and  semen). Also transplacental and perinatal transmission.   Pathogenesis —Two receptors are required for HIV to enter  cells. One receptor is CD4 protein found primarily on helper  T cells. HIV infects and kills helper T cells, which predisposes  to opportunistic infections. Other cells bearing CD4 proteins  on the surface (e.g., astrocytes) are infected also. The other  receptor for HIV is a chemokine receptor such as CCR5. The  NEF protein is an important virulence factor. It reduces class  I MHC protein synthesis, thereby reducing the ability of cyto¬  toxic T cells to kill HIV-infected cells. Cytotoxic T cells are the  main host defense against HIV.   Laboratory Diagnosis —HIV can be isolated from blood or  semen, but this procedure is not routinely available. Diagnosis  is usually made by detecting antibody with ELISA as screening    test and Western blot as confirmatory test. Determine the  “viral load” (i.e., the amount of HIV RNA in the plasma) using  PCR-based assays. A high viral load predicts a more rapid pro¬  gression to AIDS than a low viral load. PCR-based assays can  also detect viral RNA in the patients blood, which is useful to  detect early infections before antibody is detectable.   Treatment —Highly active antiretroviral therapy (HAART)  consists of several drugs combined into various regimens. Each  regimen has emtricitabine and tenofovir, to which efavirenz,  raltegravir, rilpivirine, or a combination of two protease inhibi¬  tors (either ritonavir plus atazanavir or ritonavir plus darunavir)  is added. Clinical improvement occurs, but the virus persists.   Nucleoside analogues, such as zidovudine, lamivudine,  emtricitabine, tenofovir, and others inhibit HIV replication by  inhibiting reverse transcriptase. Nonnucleoside inhibitors of  reverse transcriptase, such as efavirenz, nevirapine, and others,  are used also. Protease inhibitors (e.g., indinavir, ritonavir, and  others) prevent cleavage of precursor polypeptides. Integrase  inhibitors, such as raltegravir, dolutegravir, and elvitegravir,  block the integration of HIV DNA into host cell DNA by inhib¬  iting the integrase of HIV. Enfuvirtide, a “fusion inhibitor” that  blocks entry of HIV, and maraviroc, which inhibits binding of  the gpl20 envelope protein of HIV to the cell co-receptor CCR-  5, are also useful. Treatment of the opportunistic infection  depends on the organism.   Prevention —Screening of blood prior to transfusion for the  presence of antibody. “Safe sex,” including the use of condoms.  A regimen of zidovudine with or without a protease inhibitor  can be given to HIV-infected mothers and their newborns. A  combination of tenofovir, emtricitabine and raltegravir is one  of the regimens given after a needle-stick injury. A combina¬  tion of tenofovir and emtricitabine can be used for preexpo¬  sure prophylaxis in individuals at high risk of infection. There  is no vaccine.   Only the most important of the minor viral pathogens are sum¬  marized in this section.   Member of the bunyavirus family. Causes hantavirus pulmo¬  nary syndrome. Sin Nombre virus (SNV) is a robovirus (i.e., it  is rodent-borne). Deer mice are the reservoir, and the virus is  acquired by inhalation of dried urine and feces. Diagnosis is  made by detecting viral RNA in lung tissue or by serologic tests.  No antiviral therapy and no vaccine.   Member of the flavivirus family. Causes outbreaks of encephali¬  tis in Asian countries. Transmitted to humans by mosquitoes  from the reservoir hosts, birds, and pigs. No antiviral therapy.  An inactivated vaccine is available.   Characteristics —These fungi are molds that use keratin as  a nutritional source. Not dimorphic. Habitat of most der¬  matophytes that cause human disease is human skin, with  the exception of Microsporum canis, which infects dogs and  cats also.   Pathogenesis —These fungi grow only in the superficial  keratinized layer of the skin. They do not invade underlying  tissue. The lesions are due to the inflammatory response to the  fungi. Frequency of infection is enhanced by moisture and  warmth (e.g., inside shoes). An important host defense is pro¬  vided by the fatty acids produced by sebaceous glands. The  “id” reaction is a hypersensitivity response in one skin loca¬  tion (e.g., fingers) to the presence of the organism in another  (e.g., feet).   Laboratory Diagnosis —Skin scales should be examined  microscopically in a KOH preparation for the presence of  hyphae. The organism is identified by the appearance of its  mycelium and its asexual spores on Sabouraud’s agar. Sero¬  logic tests are not useful.   Skin Test — Trichophyton antigen can be used to determine  the competence of a patients cell-mediated immunity. Not  used for diagnosis of tinea.   Treatment —Topical agents, such as miconazole, clotrima¬  zole, or tolnaftate, are used. Undecylenic acid is effective  against tinea pedis. Griseofulvin is the treatment of choice for  tinea unguium and tinea capitis.   Characteristics —Thermally dimorphic. Mold in the soil,  yeast in the body at 37°C. Habitat is soil or vegetation.   Transmission —Mold spores enter skin in puncture wounds  caused by rose thorns and other sharp objects in the garden.   Characteristics —Thermally dimorphic (i.e., a yeast at body  temperature and a mold in the soil at ambient temperature).  The mold grows preferentially in soil enriched with bird drop¬  pings. Endemic in Ohio and Mississippi River Valley areas.   Pathogenesis —Microconidia enter the lung and differenti¬  ate into yeast cells. The yeast cells are ingested by alveolar  macrophages and multiply within them. An immune response  is mounted, and granulomas form. Most infections are con¬  tained at this level, but suppression of cell-mediated immunity  can lead to disseminated disease.   Laboratory Diagnosis —Sputum or tissue can be examined  microscopically and cultured on Sabouraud’s agar. Yeasts vis¬  ible within macrophages. The presence of tuberculate chla-  mydospores in culture at 25°C is diagnostic. A rise in antibody  titer is useful for diagnosis, but cross-reaction with other  fungi (e.g., Coccidioides) occurs.   Skin Test —Histoplasmin, a mycelial extract, is the antigen.  Useful for epidemiologic purposes to determine the incidence  of infection. A positive result indicates only that infection has  occurred; it cannot be used to diagnose active disease. Because  skin testing can induce antibodies, serologic tests must be  done first.   Treatment —Amphotericin B or itraconazole for dissemi¬  nated disease; itraconazole for pulmonary disease.   Prevention —No vaccine is available. Itraconazole can be  used for chronic suppression in AIDS patients.   Characteristics —Thermally dimorphic. At 37°C in the body,  it forms spherules containing endospores. At 25°C, either in  the soil or on agar in the laboratory, it grows as a mold. The    cells at the tip of the hyphae differentiate into asexual spores  (arthrospores). Natural habitat is the soil of arid regions (e.g.,  San Joaquin Valley in California and parts of Arizona and  New Mexico).   Pathogenesis —Arthrospores differentiate into spherules in  the lungs. Spherules rupture, releasing endospores that form  new spherules, thereby disseminating the infection within  the body. A cell-mediated immune response contains the  infection in most people, but those who have reduced cell-  mediated immunity are at high risk for disseminated  disease.   Laboratory Diagnosis —Sputum or tissue should be exam¬  ined microscopically for spherules and cultured on Sab-  ouraud’s agar. A rise in IgM (using precipitin test) antibodies  indicates recent infection. A rising titer of IgG antibodies  (using complement-fixation test) indicates dissemination; a  decreasing titer indicates a response to therapy.   Skin Test —Either coccidioidin, a mycelial extract, or spher-  ulin, an extract of spherules, is the antigen. Useful in deter¬  mining whether the patient has been infected. A positive  test indicates prior infection but not necessarily active  disease.   Treatment —Amphotericin B or itraconazole for dissemi¬  nated disease; ketoconazole for limited pulmonary disease.   Characteristics —Thermally dimorphic. Mold in the soil,  yeast in the body at 37°C. The yeast form has a single, broad-  based bud and a thick, refractile wall. Natural habitat is rich  soil (e.g., near beaver dams), especially in the upper midwest -  ern region of the United States.   Pathogenesis —Inhaled conidia differentiate into yeasts,  which initially cause abscesses followed by formation of  granulomas. Dissemination is rare, but when it occurs, skin  and bone are most commonly involved.   Laboratory Diagnosis —Sputum or skin lesions examined  microscopically for yeasts with a broad-based bud. Culture on  Sabourauds agar also. Serologic tests are not useful.   Characteristics —Thermally dimorphic. Mold in the soil,  yeast in the body at 37°C. The yeast form has multiple buds  (resembles the steering wheel of a ship).   Laboratory Diagnosis —Yeasts with multiple buds visible in  pus or tissues. Culture on Sabourauds agar shows typical  morphology.   Characteristics —Candida albicans is a yeast when part of the  normal flora of mucous membranes but forms pseudohyphae  and hyphae when it invades tissue. The yeast form produces  germ tubes when incubated in serum at 37°C. Not thermally  dimorphic.   Transmission —Part of the normal flora of skin, mucous  membranes, and GI tract. No person-to-person transmission.   Pathogenesis —Opportunistic pathogen. Predisposing fac¬  tors include reduced cell-mediated immunity, altered skin and  mucous membrane, suppression of normal flora by antibiot¬  ics, and presence of foreign bodies. Thrush is most common  in infants, immunosuppressed patients, and persons receiving  antibiotic therapy. Skin lesions occur frequently on moisture-  damaged skin. Disseminated infections, such as endocarditis  and endophthalmitis, occur in immunosuppressed patients  and intravenous drug users. Chronic mucocutaneous candi¬  diasis occurs in children with a T-cell defect in immunity to  Candida.   Laboratory Diagnosis —Microscopic examination of tissue  reveals yeasts and pseudohyphae. If only yeasts are found,  colonization is suggested. The yeast is gram-positive. Forms  colonies of yeasts on Sabourauds agar. Germ tube formation  and production of chlamydospores distinguish C. albicans  from virtually all other species of Candida. Serologic tests not  useful.    Skin Test —Used to determine competency of cell-mediated  immunity rather than to diagnose candidal disease.   Treatment —Skin infections can be treated with topical anti¬  fungal agents such as nystatin or clotrimazole. Oral thrush is  treated with fluconazole. Esophageal thrush can be treated with  fluconazole or caspofungin. Vaginitis can be treated with either  intravaginal clotrimazole or oral fluconazole. Disseminated  disease can be treated with either amphotericin B or flucon¬  azole. Chronic mucocutaneous candidiasis can be controlled  with fluconazole.   Prevention —Predisposing factors should be reduced or  eliminated. Oral thrush can be prevented by using clotrima¬  zole troches or nystatin “swish and swallow.” Fluconazole is  used to prevent disseminated infection in immunocompro¬  mised patients. There is no vaccine.   Pathogenesis —Organisms cause influenza-like syndrome or  pneumonia. They spread via the bloodstream to the menin¬  ges. Reduced cell-mediated immunity predisposes to severe  disease, but some cases of cryptococcal meningitis occur in  immunocompetent people who inhale a large dose of  organisms.   Laboratory Diagnosis —Visualization of the encapsulated  yeast in India ink preparations of spinal fluid. Culture of sputum  or spinal fluid on Sabourauds agar produces colonies of yeasts.  Cryptococcal antigen test (CRAG) is a latex agglutination test  that detects polysaccharide capsular antigen in spinal fluid.   Prevention —Cryptococcal meningitis can be prevented in  AIDS patients by using oral fluconazole. There is no vaccine.   Diseases —Invasive aspergillosis is the major disease. Allergic  bronchopulmonary aspergillosis and aspergilloma (fungus  ball) are important also.    invades blood vessels, causing thrombosis and infarction. A  person with a lung cavity (e.g., from tuberculosis) may  develop a “fungal ball” (aspergilloma). An allergic (hyper¬  sensitive) person (e.g., one with asthma) is predisposed to  allergic bronchopulmonary aspergillosis mediated by IgE  antibody.   Laboratory Diagnosis —Septate hyphae invading tissue are  visible microscopically. Invasion distinguishes disease from  colonization. Forms characteristic mycelium when cultured  on Sabourauds agar. See chains of conidia radiating from a  central stalk. Serologic tests detect IgG precipitins in patients  with aspergillomas and IgE antibodies in patients with allergic  bronchopulmonary aspergillosis.   Treatment —Amphotericin B or voriconazole for invasive  aspergillosis. Some lesions (e.g., fungus balls) can be surgi¬  cally removed. Corticosteroids plus itraconazole are recom¬  mended for allergic bronchopulmonary aspergillosis.   Characteristics —Molds with nonseptate hyphae that typi¬  cally branch at a 90-degree angle (wide-angle branching). Not  dimorphic. Habitat is the soil.   Pathogenesis —Opportunistic pathogens. They cause dis¬  ease primarily in ketoacidotic diabetic and leukemic patients.  The sinuses and surrounding tissue are typically involved.  Hyphae invade the mucosa and progress into underlying tis¬  sue and vessels, leading to necrosis and infarction.   Prevention —No vaccine or prophylactic drug is available.  Control of underlying disease (e.g., diabetes) tends to prevent  mucormycosis.   Although there is molecular evidence that Pneumocystis  jiroveci is a fungus, it is described in these brief summaries in  the section on protozoa that cause blood and tissue infections  (see page 683).   Characteristics —Intestinal protozoan. Motile ameba (tro¬  phozoite); forms cysts with four nuclei. Life cycle: Humans  ingest cysts, which form trophozoites in small intestine. Tro¬  phozoites pass to the colon and multiply. Cysts form in the  colon, which then pass in the feces.   Pathogenesis —Trophozoites invade colon epithelium and  produce flask-shaped ulcer. Can spread to liver and cause  amebic abscess.   Laboratory Diagnosis —Trophozoites or cysts visible in  stool. Serologic testing (indirect hemagglutination test) posi¬  tive with invasive (e.g., liver) disease.   Treatment —Metronidazole or tinidazole for symptomatic  disease. Iodoquinol or paromomycin for asymptomatic cyst  carriers.   Characteristics —Intestinal protozoan. Pear-shaped, flagel¬  lated trophozoite, forms cyst with four nuclei. Life cycle:  Humans ingest cysts, which form trophozoites in duodenum.  Trophozoites form cysts that are passed in feces.   Pathogenesis —Trophozoites attach to wall but do not  invade. They interfere with absorption of fat and protein.    and merozoites form, microgametes and macrogametes are  produced; they unite to form a zygote and then an oocyst.   Pathogenesis —Trophozoites attach to wall of small intestine  but do not invade.   Pathogenesis —Trophozoites attach to wall of vagina and  cause inflammation and discharge.   Characteristics —Protozoan that infects red blood cells and  tissue (e.g., liver, kidney, and brain). Life cycle: Sexual cycle  consists of gametogony (production of gametes) in humans  and sporogony (production of sporozoites) in mosquitoes;  asexual cycle (schizogony) occurs in humans. Sporozoites in  saliva of female Anopheles mosquito enter the human blood¬  stream and rapidly invade hepatocytes (exoerythrocytic  phase). There they multiply and form merozoites ( P. vivax and  P. ovale also form hypnozoites, a latent form). Merozoites leave  the hepatocytes and infect red cells (erythrocytic phase). There  they form schizonts that release more merozoites, which infect  other red cells in a synchronous pattern (3 days for P. malariae;  2 days for the others). Some merozoites become male and  female gametocytes, which, when ingested by female    Anopheles, release male and female gametes. These unite to  produce a zygote, which forms an oocyst containing many  sporozoites. These are released and migrate to salivary glands.   Transmission and Epidemiology —Transmitted by female  Anopheles mosquitoes. Occurs primarily in the tropical areas  of Asia, Africa, and Latin America.   Pathogenesis —Merozoites destroy red cells, resulting in  anemia. Cyclic fever pattern is due to periodic release of  merozoites. Plasmodium falciparum can infect red cells of all  ages and cause aggregates of red cells that occlude capillaries.  This can cause tissue anoxia, especially in the brain (cerebral  malaria) and the kidney (blackwater fever). Hypnozoites can  cause relapses.   Laboratory Diagnosis —Organisms visible in blood smear.  Thick smear is used to detect presence of organism and thin  smear to speciate.   Treatment —Chloroquine if sensitive. For chloroquine-resis¬  tant P. falciparum, use Coartem or Malarone. Primaquine for  hypnozoites of P. vivax and P. ovale. In severe cases, use paren¬  teral artesunate or quinidine.   Prevention —Chloroquine in areas where organisms are sen¬  sitive. For those in areas with a high risk of chloroquine resis¬  tance, Malarone, mefloquine, or doxycycline. Primaquine to  prevent relapses of P. vivax or P. ovale. Protection from bites.  Control mosquitoes by using insecticides and by draining  water from breeding areas.   Characteristics —Tissue protozoan. Life cycle: Cysts in cat  feces or in meat are ingested by humans and differentiate in the  gut into forms that invade the gut wall. They infect macro¬  phages and form trophozoites (tachyzoites) that multiply rap¬  idly, kill cells, and infect other cells. Cysts containing bradyzoites  form later. Cat ingests cysts in raw meat, and bradyzoites  excyst, multiply, and form male and female gametocytes. These  fuse to form oocysts in cat gut, which are excreted in cat feces.   Transmission and Epidemiology —Transmitted by ingestion  of cysts in raw meat and in food contaminated with cat feces.  Also by passage of trophozoites transplacentally from mother to  fetus. Infection of fetus occurs only when mother is infected  during pregnancy and when she is infected for the first time (i.e.,  she has no protective antibody). Cat is definitive host; humans  and other mammals are intermediate hosts. Occurs worldwide.   Pathogenesis —Trophozoites infect many organs, especially  brain, eyes, and liver. Cysts persist in tissue, enlarge, and cause  symptoms. Severe disease in patients with deficient cell-  mediated immunity (e.g., encephalitis in AIDS patients).   Laboratory Diagnosis —Serologic tests for IgM and IgG anti¬  bodies are usually used. Trophozoites or cysts visible in tissue.   Prevention —Meat should be cooked. Pregnant women  should not handle cats, cat litter boxes, or raw meat. Trime¬  thoprim-sulfamethoxazole is used to prevent Toxoplasma  encephalitis in HIV-infected patients.   Treatment —Trimethoprim-sulfamethoxazole is drug of  choice. Pentamidine is an alternative drug.   Characteristics —Blood and tissue protozoan. Life cycle: Try-  pomastigotes in blood of reservoir host are ingested by redu-  viid bug and form epimastigotes and then trypomastigotes in  the gut. When the bug bites, it defecates and feces containing  trypomastigotes contaminate the wound. Organisms enter the  blood and form amastigotes within cells; these then become  trypomastigotes.   Transmission and Epidemiology —Transmitted by reduviid  bugs. Humans and many animals are reservoirs. Occurs in  rural Latin America.   Pathogenesis —Amastigotes kill cells, especially cardiac  muscle, leading to myocarditis. Also neuronal damage, lead¬  ing to megacolon and megaesophagus.   Laboratory Diagnosis —Trypomastigotes visible in blood,  but bone marrow biopsy, culture in vitro, xenodiagnosis, or  serologic tests may be required.   Treatment —Nifurtimox or benznidazole for acute disease.  No effective drug for chronic disease.   Prevention —Protection from bite. Insect control. Blood for  transfusion should not be used if antibodies to T. cruzi are  present.   Characteristics —Blood and tissue protozoan. Life cycle: Try-  pomastigotes in blood of human or animal reservoir are  ingested by tsetse fly. They differentiate in the gut to form  epimastigotes and then metacyclic trypomastigotes in salivary  glands. When fly bites, trypomastigotes enter the blood.  Repeated variation of surface antigen occurs, which allows the  organism to evade the immune response.   Transmission and Epidemiology —Transmitted by tsetse  flies. Trypanosoma gambiense has a human reservoir and  occurs primarily in west Africa. Trypanosoma rhodesiense has  an animal reservoir (especially wild antelope) and occurs pri¬  marily in east Africa.   Laboratory Diagnosis —Trypomastigotes visible in blood in  early stages and in cerebrospinal fluid in late stages. Serologic  tests useful.   Characteristics —Blood and tissue protozoan. Life cycle:  Human macrophages containing amastigotes are ingested by  sandfly. Amastigotes differentiate in fly gut to promastigotes,  which migrate to pharynx. When sandfly bites a human, pro¬  mastigotes enter blood macrophages and form amastigotes.  These can infect other reticuloendothelial cells, especially in  spleen and liver.   Transmission and Epidemiology —Transmitted by sandflies  (.Phlebotomus or Lutzomyia). Animal reservoir (chiefly dogs,  small carnivores, and rodents) in Africa, Middle East, and  parts of China. Human reservoir in India.   Pathogenesis —Amastigotes kill reticuloendothelial cells,  especially in liver, spleen, and bone marrow.   Leishmania tropica and L. mexicana cause cutaneous leishmani¬  asis; L. braziliensis causes mucocutaneous leishmaniasis.    Leishmania tropica occurs primarily in the Middle East, Asia,  and India, whereas L. mexicana and L. braziliensis occur in Cen¬  tral and South America. All are transmitted by sandflies. Forest  rodents are the main reservoir. Diagnosis is made by observing  amastigotes in smear of skin lesion. Treatment is sodium stibo¬  gluconate. No specific means of prevention.   Ameba that causes meningoencephalitis. Also causes keratitis  in contact lens wearers. Life cycle includes trophozoite and  cyst stages. Found in freshwater lakes and soil. Transmitted via  trauma to skin or eyes. Disease occurs primarily in immuno¬  compromised patients. Diagnosis made by finding ameba in  spinal fluid. Treatment with pentamidine, ketoconazole, or  flucytosine may be effective. No specific means of  prevention.   Ameba that causes meningoencephalitis. Found in freshwater  lakes and soil. Life cycle includes trophozoite and cyst stages.  Transmitted while swimming or diving in contaminated lake.  Disease occurs primarily in healthy individuals. Diagnosis  made by finding ameba in spinal fluid. Treatment with ampho¬  tericin B may be effective. No specific means of prevention.   Sporozoan that causes babesiosis. Endemic in rodents along the  northeast coast of the United States. Transmitted by Ixodes ticks  to humans. Infects red blood cells, causing them to lyse, and  anemia results. Asplenic patients have severe disease. Diagnosis  is made by observing organism in “Maltese cross” tetrad pattern  in red blood cells. Treat with combination of atovaquone and  azithromycin for mild disease. Use a combination of quinine  and clindamycin for serious disease. No specific means of  prevention.   Only ciliated protozoan to cause human disease. Causes diar¬  rhea. Acquired by fecal-oral transmission from domestic ani¬  mals, especially pigs. Diagnosis is made by finding trophozoites  or cysts in feces. Treat with tetracycline. No specific means of  prevention.   Coccidian protozoan. Causes diarrhea, especially in immuno¬  compromised (e.g., AIDS) patients. Acquired by fecal-oral  transmission. No evidence for animal reservoir. Diagnosis is  made by finding oocytes in acid-fast stain of feces. Treat with  trimethoprim-sulfamethoxazole. No specific means of  prevention.   Coccidian protozoan. Causes diarrhea, especially in immuno¬  compromised (e.g., AIDS) patients. Acquired by fecal-oral  transmission from either human or animal sources. Diagnosis  is made by finding oocytes in acid-fast stain of feces. Treat  with trimethoprim-sulfamethoxazole. No specific means of  prevention.   Group of spore-forming, obligate intracellular protozoa. Two  important species are Enterocytozoon bieneusi and Septata  intestinalis. Cause diarrhea, especially in immunocompromised  (e.g., AIDS) patients. Acquired by fecal-oral transmission from  human sources. Diagnosis is made by finding spores within cells  in feces or intestinal biopsy specimens. Treat with albendazole.  No specific means of prevention.   Characteristics —Cestode (fish tapeworm). Scolex has two  elongated sucking grooves; no circular suckers or hooks.  Gravid uterus forms a rosette. Oval eggs have an operculum at  one end. Life cycle: Humans ingest undercooked fish contain¬  ing sparganum larvae. Larvae attach to gut wall and become  adults containing gravid proglottids. Eggs are passed in feces.  In fresh water, eggs hatch and the embryos are eaten by cope-  pods. When these are eaten by freshwater fish, larvae form in  the fish muscle.   Transmission and Epidemiology —Transmitted by eating  raw or undercooked freshwater fish. Humans are definitive  hosts; copepods are the first and fish the second intermediate  hosts, respectively. Occurs worldwide but endemic in Scandi¬  navia, Japan, and northcentral United States.   Characteristics —Cestode (dog tapeworm). Scolex has four  suckers and a double circle of hooks. Adult worm has only  three proglottids. Life cycle: Dogs are infected when they  ingest the entrails of sheep (e.g., liver) containing hydatid  cysts. The adult worms develop in the gut, and eggs are passed  in the feces. Eggs are ingested by sheep (and humans) and  hatch hexacanth larvae in the gut that migrate in the blood to    various organs, especially the liver and brain. Larvae form  large, unilocular hydatid cysts containing many protoscoleces  and daughter cysts.   Transmission and Epidemiology —Transmitted by ingestion  of eggs in food contaminated with dog feces. Dogs are main  definitive hosts; sheep are intermediate hosts; humans are  dead-end hosts. Endemic in sheep-raising areas (e.g., Mediter¬  ranean, Middle East, some western states of the United States).   Pathogenesis —Hydatid cyst is a space-occupying lesion.  Also, if cyst ruptures, antigens in fluid can cause anaphylaxis.   Characteristics —Cestode (beef tapeworm). Scolex has four  suckers but no hooks. Gravid proglottids have 15 to 20 uterine  branches. Life cycle: Humans ingest undercooked beef con¬  taining cysticerci. Larvae attach to gut wall and become adult  worms with gravid proglottids. Terminal proglottids detach,  pass in feces, and are eaten by cattle. In the gut, oncosphere  embryos hatch, burrow into blood vessels, and migrate to  skeletal muscles, where they develop into cysticerci.   Transmission and Epidemiology —Transmitted by eating  raw or undercooked beef. Humans are definitive hosts; cattle  are intermediate hosts. Occurs worldwide but endemic in  areas of Asia, Latin America, and Eastern Europe.   Pathogenesis —Tapeworm in gut causes little damage. In  contrast to Taenia solium, cysticercosis does not occur.   Characteristics —Cestode (pork tapeworm). Scolex has four  suckers and a circle of hooks. Gravid proglottids have 5 to 10  uterine branches. Life cycle: Humans ingest undercooked  pork containing cysticerci. Larvae attach to gut wall and  develop into adult worms with gravid proglottids. Terminal  proglottids detach, pass in feces, and are eaten by pigs. In gut,  oncosphere (hexacanth) embryos burrow into blood vessels  and migrate to skeletal muscle, where they develop into cysti¬  cerci. If humans eat T. solium eggs in food contaminated with    human feces, the oncospheres burrow into blood vessels and  disseminate to organs (e.g., brain, eyes), where they encyst to  form cysticerci.   Transmission and Epidemiology —Taeniasis acquired by  eating raw or undercooked pork. Cysticercosis acquired only  by ingesting eggs in fecally contaminated food or water.  Humans are definitive hosts; pigs or humans are intermediate  hosts. Occurs worldwide but endemic in areas of Asia, Latin  America, and southern Europe.   Pathogenesis —Tapeworm in gut causes little damage. Cysti¬  cerci can expand and cause symptoms of mass lesions, espe¬  cially in brain.   Hymenolepis nana infection is the most common tapeworm in  the United States. Infection is usually asymptomatic. It is  endemic in the southeastern states, mostly in children. It is  called the dwarf tapeworm because of its small size. It is also  different from other tapeworms because the eggs are directly  infectious for humans without the need for an intermediate  animal host. Diagnosis is made by finding eggs in feces. Treat  with praziquantel. No specific means of prevention.   Characteristics —Trematode (blood fluke). Adults exist as  two sexes but are attached to each other. Eggs are distin¬  guished by spines: S. mansoni has large lateral spine; S. japonicum  has small lateral spine; S. haematobium has terminal spine.  Life cycle: Humans are infected by cercariae penetrating skin.  Cercariae form larvae that penetrate blood vessels and are car¬  ried to the liver, where they become adults. The adult flukes  migrate retrograde in the portal vein to reach the mesenteric  venules (S. mansoni and S. japonicum) or urinary bladder  venules (S. haematobium ). Eggs penetrate the gut or bladder  wall, are excreted, and hatch in fresh water. The ciliated larvae  (miracidia) penetrate snails and multiply through generations  to produce many free-swimming cercariae.    are intermediate hosts. Endemic in tropical areas: S. mansoni  in Africa and Latin America, S. haematobium in Africa and  Middle East, S. japonicum in Asia.   Pathogenesis —Eggs in tissue induce inflammation, granulo¬  mas, fibrosis, and obstruction, especially in liver and spleen.  Schistosoma mansoni damages the colon (inferior mesenteric  venules), S. japonicum damages the small intestine (superior  mesenteric venules), and S. haematobium damages the blad¬  der. Bladder damage predisposes to carcinoma.   Prevention —Proper disposal of human waste. Swimming in  endemic areas should be avoided.   Characteristics —Trematode (liver fluke). Life cycle: Humans  ingest undercooked fish containing encysted larvae (metacer-  cariae). In duodenum, immature flukes enter biliary duct,  become adults, and release eggs that are passed in feces. Eggs  are eaten by snails; the eggs hatch and form miracidia. These  multiply through generations (rediae) and then produce many  free-swimming cercariae, which encyst under scales of fish  and are eaten by humans.   Transmission and Epidemiology —Transmitted by eating  raw or undercooked freshwater fish. Humans are definitive  hosts; snails and fish are first and second intermediate hosts,  respectively. Endemic in Asia.   Characteristics —Trematode (lung fluke). Life cycle: Humans  ingest undercooked freshwater crab meat containing encysted  larvae (metacercariae). In gut, immature flukes enter perito¬  neal cavity, burrow through diaphragm into lung parenchyma,  and become adults. Eggs enter bronchioles and are coughed  up or swallowed. In fresh water, eggs hatch, releasing mira¬  cidia that enter snails, multiply through generations (rediae),  and then form many cercariae that infect and encyst in crabs.   Transmission and Epidemiology —Transmitted by eating  raw or undercooked crab meat. Humans are definitive hosts;  snails and crabs are first and second intermediate hosts,  respectively. Endemic in Asia and India.   Characteristics —Intestinal nematode. Life cycle: Filariform  larvae penetrate skin, enter the blood, and migrate to the  lungs. They enter alveoli, pass up the trachea, then are swal¬  lowed. They become adults in small intestine and attach to  walls via teeth ( Ancylostoma ) or cutting plates (Necator). Eggs  are passed in feces and form noninfectious rhabditiform lar¬  vae and then infectious filariform larvae in soil.   Characteristics —Intestinal nematode. Life cycle: Humans  ingest eggs, which form larvae in gut. Larvae migrate through  the blood to the lungs, where they enter the alveoli, pass up  the trachea, and are swallowed. In the gut, they become adults  and lay eggs that are passed in the feces. They embryonate  (i.e., become infective) in soil.   Transmission and Epidemiology —Transmitted by food  contaminated with soil containing eggs. Humans are the only  hosts. Endemic in the tropics.   Characteristics —Intestinal nematode. Life cycle: Humans  ingest eggs, which develop into adults in gut. At night, females  migrate from the anus and lay many eggs on skin and in envi¬  ronment. Embryo within egg becomes an infective larva  within 4 to 6 hours. Reinfection is common.   Characteristics —Intestinal nematode. Life cycle: Filariform  larvae penetrate skin, enter the blood, and migrate to the  lungs. They move into alveoli and up the trachea and are swal¬  lowed. They become adults and enter the mucosa, where  females produce eggs that hatch in the colon into noninfec¬  tious, rhabditiform larvae that are usually passed in feces.  Occasionally, rhabditiform larvae molt in the gut to form  infectious, filariform larvae that can enter the blood and  migrate to the lung (autoinfection). The noninfectious larvae  passed in feces form infectious filariform larvae in the soil.  These larvae can either penetrate the skin or form adults.  Adult worms in soil can undergo several entire life cycles  there. This free-living cycle can be interrupted when filari¬  form larvae contact the skin.   Pathogenesis —Little effect in immunocompetent persons.  In immunocompromised persons, massive superinfection can  occur, accompanied by secondary bacterial infections.   Treatment —Ivermectin is the drug of choice. Thiabendazole  is an alternative.    encysted larvae, which mature into adults in small intestine.  Female worms release larvae that enter blood and migrate to  skeletal muscle or brain, where they encyst.   Transmission and Epidemiology —Transmitted by ingestion  of raw or undercooked meat, usually pork. Reservoir hosts are  primarily pigs and rats. Humans are dead-end hosts. Occurs  worldwide but endemic in Eastern Europe and west Africa.   Treatment —Thiabendazole effective early against adult  worms. For severe symptoms, steroids plus mebendazole can  be tried.   Characteristics —Intestinal nematode. Life cycle: Humans  ingest eggs, which develop into adults in gut. Eggs are passed  in feces into soil, where they embryonate (i.e., become  infectious).   Transmission and Epidemiology —Transmitted by food or  water contaminated with soil containing eggs. Humans are the  only hosts. Occurs worldwide, especially in the tropics.   Characteristics —Tissue nematode. Life cycle: Humans ingest  copepods containing infective larvae in drinking water. Lar¬  vae are released in gut, migrate to body cavity, mature, and  mate. Fertilized female migrates to subcutaneous tissue and  forms a papule, which ulcerates. Motile larvae are released  into water, where they are eaten by copepods and form infec¬  tive larvae.   Transmission and Epidemiology —Transmitted by cope¬  pods in drinking water. Humans are major definitive hosts.  Many domestic animals are reservoir hosts. Endemic in tropi¬  cal Africa, Middle East, and India.   Characteristics —Tissue nematode. Life cycle: Bite of deer fly  (mango fly) deposits infective larvae, which crawl into the  skin and develop into adults that migrate subcutaneously.  Females produce microfilariae, which enter the blood. These  are ingested by deer flies, in which the infective larvae are  formed.   Characteristics —Tissue nematodes. Life cycle: Bite of female  blackfly deposits larvae in subcutaneous tissue, where they  mature into adult worms within skin nodules. Females pro¬  duce microfilariae, which migrate in interstitial fluids and are  ingested by blackflies, in which the infective larvae are  formed.   Transmission and Epidemiology —Transmitted by female  blackflies. Humans are the only definitive hosts. No animal  reservoir. Endemic along rivers of tropical Africa and Central  America.   Treatment —Ivermectin affects microfilariae, not adult  worms. Suramin for adult worms.    Characteristics —Tissue nematodes. Life cycle: Bite of female  mosquito deposits infective larvae that penetrate bite wound,  form adults, and produce microfilariae. These circulate in the  blood, chiefly at night, and are ingested by mosquitoes, in  which the infective larvae are formed.   Transmission and Epidemiology —Transmitted by female  mosquitoes of several genera, especially Anopheles and Culex,  depending on geography. Humans are the only definitive  hosts. Endemic in many tropical areas.   Pathogenesis —Adult worms cause inflammation that blocks  lymphatic vessels (elephantiasis). Chronic, repeated infection  required for symptoms to occur.   Characteristics —Nematode larvae cause disease. Life cycle  in humans: Toxocara eggs are passed in dog feces and ingested  by humans. They hatch into larvae in small intestine; larvae  enter the blood and migrate to organs, especially liver, brain,  and eyes, where they are trapped and die.    Transmission and Epidemiology —Transmitted by ingestion  of eggs in food or water contaminated with dog feces. Dogs  are definitive hosts. Humans are dead-end hosts.   The filariform larvae of A. caninum (dog hookworm) and A. bra¬  ziliense (cat hookworm) cause cutaneous larva migrans. The lar¬  vae in the soil burrow through the skin, then migrate within the  subcutaneous tissue, causing a pruritic rash called “creeping erup¬  tion.” These organisms cannot complete their life cycle in humans.  The diagnosis is made clinically. Thiabendazole is effective.   The larvae of A. simplex cause anisakiasis. They are ingested in  raw seafood, such as sashimi and sushi, and migrate into the  submucosa of the intestinal tract. Acute infection resembles  appendicitis. Diagnosis is not dependent on the clinical labora¬  tory. There is no effective drug therapy. Larvae can be removed  when visualized during gastroscopy. Prevention consists of not  eating raw fish.   Characteristics —Lice are easily visible. Pediculus humanus  has an elongated body, whereas P. pubis has a short body  resembling a crab. Nits are the eggs of the louse, often attached  to the hair shaft or clothing.   Transmission —Hair and body lice are transmitted from  human to human by contact, especially fomites such as hats  and combs. Pubic lice are transmitted by sexual contact.   Pathogenesis —Itching is caused by a hypersensitivity  response to saliva of the louse.   Characteristics —Fly larvae (maggots) cause the disease, not  the adult flies.   Transmission — Dermatobia deposits its egg on a mosquito,  and when the mosquito bites, the eggs are then deposited on  the skin. The warmth of the skin causes the egg to hatch, and  the larva enters the skin at the site of the mosquito bite.   Characteristics —Round body with eight short legs. Too  small to be seen with naked eye.   Pathogenesis —Itching is caused by a hypersensitivity  response to feces of the mite.   Transmission —Ticks reside in grassy areas and attach to  human skin.   Characteristics —Black widow spiders have an orange-red  hourglass on their ventral surface.   Pathogenesis —Neurotoxin causes pain in extremities and  abdomen. Numbness, fever, and vomiting also occur.   Characteristics —Brown recluse spiders have a violin-shaped  pattern on their dorsal surface.    These brief clinical case vignettes are typical presentations of common infectious diseases. Learning the most likely causative  organisms of these classic cases will help you answer the USMLE questions and improve your diagnostic skills. These cases are  presented in random order similar to the way they are on the USMLE. The important features of the case are written in   A 22-year-old woman has a severe sore throat. Findings on physical examination include an inflamed throat, swollen cervical  lymph nodes, and an enlarged spleen. Her heterophile agglutinin test (Monospot test) is positive.   Diagnosis: Infectious mononucleosis caused by Epstein-Barr virus. Other viruses and bacteria, especially Streptococcus pyo¬  genes, can cause pharyngitis and cervical lymphadenopathy, but an enlarged spleen and a positive Monospot test make infec¬  tious mononucleosis the most likely diagnosis. See page 296 for additional information.   A 5-year-old boy with diabetic ketoacidosis has ptosis of his right eyelid, periorbital swelling, and a black, necrotic skin lesion  under his eye. Biopsy of the skin lesion shows nonseptate hyphae with wide-angle branching.   Diagnosis: Mucormycosis caused by Mucor or Rhizopus species. Diabetic ketoacidosis and renal acidosis predispose to mucor¬  mycosis. Fungal spores are inhaled into the sinuses, resulting in lesions on the face. See page 419 for additional information.   A 40-year-old man complains of watery, foul-smelling diarrhea and flatulence for the past 2 weeks. He drank untreated water  on a camping trip about a month ago. See pear-shaped flagellated trophozoites in stool.   Diagnosis: Giardiasis caused by Giardia lamblia. Of the protozoa that are common causes of diarrhea, Giardia and Cryptospo¬  ridium cause watery diarrhea, whereas Entamoeba causes bloody diarrhea. See page 428 for additional information on Giardia,  page 429 for additional information on Cryptosporidium, and page 424 for additional information on Entamoeba.   A 35-year-old man who is human immunodeficiency virus (HIV) antibody positive has had a persistent headache and a low-  grade fever (temperature, 100°F) for the past 2 weeks. See budding yeasts with a wide capsule in India ink preparation of  spinal fluid.   Diagnosis: Meningitis caused by Cryptococcus neoformans. The latex agglutination test, which detects the capsular polysac¬  charide antigen of Cryptococcus in the spinal fluid, is a more sensitive and specific test than is the test with India ink. See  page xx for additional information. If acid-fast rods are seen in spinal fluid, think Mycobacterium tuberculosis. See page 417  for additional information.   A 12-year-old boy has a painful arm that he thought he had injured while pitching in a Little League baseball game. The pain  has gotten worse over a 2-week period, and he now has a temperature of 100°F. X-ray of the humerus reveals raised periosteum.  Aspirate of lesion reveals gram-positive cocci in clusters.   Diagnosis: Osteomyelitis caused by Staphylococcus aureus. This organism is the most common cause of osteomyelitis in  children. Osteomyelitis in prosthetic joints is often caused by Staphylococcus epidermidis. See page 109 for additional informa¬  tion on staphylococci.   A 50-year-old woman receiving chemotherapy via a subclavian catheter for acute leukemia has the sudden onset of blindness in  her right eye. Her total white blood cell (WBC) count is 120/pL. Blood cultures grew budding yeasts that formed germ tubes.  Diagnosis: Endophthalmitis (infection inside the eye) caused by Candida albicans. A catheter-related infection gave rise to an  embolus containing the organism, which traveled through the bloodstream to reach the eye. C. albicans is a member of the  normal flora of the skin and enters through a break in the skin at the catheter site. See page 414 for additional information.   If the blood culture grew colonies of gram-positive cocci in clusters that were coagulase-negative, think S. epidermidis, another  member of the skin flora that is also a common cause of catheter-associated infections. See page 115 for additional information.   A 60-year-old man has had a nonproductive cough and fever (temperature, 101°F) for 1 week. He received a kidney transplant  6 weeks ago and has had one episode of rejection that required increased prednisone. There was no response to erythromycin,  indicating that Legionella and Mycoplasma are unlikely causes. See owl’s-eye inclusion bodies within the nucleus of infected  cells in bronchoalveolar lavage fluid.   Diagnosis: Cytomegalovirus (CMV) pneumonia. These intranuclear inclusions are typical findings in CMV infections. Immu¬  nosuppression predisposes to disseminated CMV infections. See page 295 for additional information.   A 45-year-old woman complains that her right arm has become increasingly weak during the past few days. This morning, she  had a generalized seizure. She recently finished a course of cancer chemotherapy. Magnetic resonance imaging (MRI) of the  brain reveals a lesion resembling an abscess. Brain biopsy shows gram-positive rods in long filaments. Organism is weakly  acid-fast.   Diagnosis: Brain abscess caused by Nocardia asteroides. Nocardia asteroides initially infects the lung, where it may or may not  cause symptoms in immunocompetent people. Dissemination to the brain is common in immunocompromised patients. See  page 195 for additional information.   A 20-year-old man has a severe headache and vomiting that began yesterday. He is now confused. On examination, his tem¬  perature is 39°C and his neck is stiff. Spinal fluid reveals no bacteria on Gram stain, 25 lymphs, normal protein, and normal  glucose. Culture of the spinal fluid on blood agar shows no bacterial colonies.   Diagnosis: Viral meningitis, which is most often caused by Coxsackie virus. Can isolate the virus from spinal fluid. See  page 336 for additional information.   A 60-year-old man with a history of tuberculosis now has a cough productive of bloody sputum. Chest X-ray reveals a round  opaque mass within a cavity in his left upper lobe. Culture of the sputum grew an organism with septate hyphae that had  straight, parallel walls. The hyphae exhibited low-angle branching.   Diagnosis: “Fungus ball” caused by Aspergillus fumigatus. Fungal spores are inhaled into the lung, where they grow within a  preexisting cavity caused by infection with M. tuberculosis. See page 418 for additional information.   A 3-month-old girl has watery, nonbloody diarrhea. Stool culture reveals only normal enteric flora.   Diagnosis: Think rotavirus, the most common cause of diarrhea in infants. The enzyme-linked immunosorbent assay  (ELISA) test for rotavirus antigen in the stool is positive, which confirms the diagnosis. See page 339 for additional  information.   A 30-year-old woman has a painless ulcer on her tongue. She is HIV antibody positive and has a CD4 count of 25. Her serum  is nonreactive in the VDRL test. Biopsy of the lesion revealed yeasts within macrophages.   Diagnosis: Disseminated histoplasmosis caused by Histoplasma capsulatum. Patients with a low CD4 count have severely  reduced cell-mediated immunity, which predisposes to disseminated disease caused by this dimorphic fungus. A negative  VDRL test indicates the ulcer was not caused by Treponema pallidum. See page 409 for additional information on  Histoplasma.   A 20-year-old man has a swollen, red, hot, tender ankle, accompanied by a temperature of 100°F for the past 2 days. There is  no history of trauma. See gram-negative diplococci in joint fluid aspirate. Organism is oxidase-positive.   Diagnosis: Arthritis caused by Neisseria gonorrhoeae, the most common cause of infectious arthritis in sexually active  adults. Sugar fermentation tests were used to identify the organism as N. gonorrhoeae. See page 132 for additional  information.   A 40-year-old woman has blurred vision and slurred speech. She is afebrile. She is famous in her neighborhood for her home-  canned vegetables and fruits.   Diagnosis: Botulism caused by Clostridium botulinum. Botulinum toxin causes a descending paralysis that starts with the  cranial nerves, typically appearing initially as diplopia. The toxin is a protease that cleaves the proteins involved in the release  of acetylcholine at the neuromuscular junction. Treat with antiserum immediately. Confirm diagnosis with mouse protec¬  tion test or ELISA test using a sample of food suspected of containing the toxin. See page 139 for additional information.  Wound botulism occurs in heroin users (e.g., users of black tar heroin), especially in those who “skin pop.” Bacterial spores in  the heroin germinate in the anaerobic conditions in necrotic skin tissue.   A neonate was born with a small head (microcephaly), jaundice, and hepatosplenomegaly. Urine contained multinucleated  giant cells with intranuclear inclusions.   Diagnosis: Cytomegalovirus infection acquired in utero. Cytomegalovirus is the leading cause of congenital abnormalities.  For fetal infection to occur, the mother must be infected for the first time during pregnancy. She therefore would have no  preexisting antibodies to neutralize the virus prior to its infecting the placenta and the fetus. See page 295 for additional  information.   A 14-year-old girl has a rapidly spreading, painful, erythematous rash on her leg. The rash is warm and tender, and her tem¬  perature is 38°C. Gram-positive cocci in chains were seen in an aspirate from the lesion. Culture of the aspirate on blood agar  grew colonies surrounded by clear (beta) hemolysis. Growth of the organism was inhibited by bacitracin.   Diagnosis: Cellulitis caused by S. pyogenes. The rapid spread of cellulitis caused by S. pyogenes is due to hyaluronidase (spread¬  ing factor) that degrades hyaluronic acid in subcutaneous tissue. Acute glomerulonephritis (AGN) can follow skin infections  caused by S. pyogenes. AGN is an immunologic disease caused by antigen-antibody complexes. See page 120 for additional  information.   A 4-year-old boy wakes up at night because his anal area is itching. See worm eggs in “Scotch tape” preparation.   Diagnosis: Pinworm infection (enterobiasis) caused by Enterobius vermicularis. Pinworm infection is the most common hel¬  minth disease in the United States. See page 470 for additional information.   A 25-year-old woman has a painful, inflamed swollen hand. She was bitten by a cat about 8 hours ago. See small gram-negative  rods in the exudate from lesion.   Diagnosis: Cellulitis caused by Pasteurella multocida. Organism is normal flora in cat’s mouth. See page 180 for additional  information.   A 7-year-old girl has bloody diarrhea and fever (temperature, 38°C), but no nausea or vomiting. Only lactose-fermenting colo¬  nies are seen on EMB agar.   Diagnosis: Think either Campylobacter jejuni or enterohemorrhagic strains of Escherichia coli ( E. coli 0157:H7). If Campylo¬  bacter is the cause, see colonies on Campylobacter agar containing curved gram-negative rods, and the colonies on EMB agar  are likely to be nonpathogenic E. coli. If E. coli 0157:147 is the cause, the organism in the lactose-fermenting colonies on EMB  agar is unable to ferment sorbitol. The absence of non-lactose-fermenting colonies indicates that Shigella and Salmonella are not  the cause. See page 161 for additional information on Campylobacter and page 154 for additional information on E. coli 0157:147.   A 15-year-old girl has had a nonproductive cough and temperature of 100°F for the past 5 days. The symptoms came on gradu¬  ally. Lung examination shows few scattered rales. Chest X-ray shows patchy infiltrate in left lower lobe but no consolidation.   Diagnosis: Atypical pneumonia caused by Mycoplasma pneumoniae. This organism is the most common cause of atypical  pneumonia in teenagers and young adults. In the cold agglutinin test, antibodies in the patient’s serum agglutinate human red  blood cells in the cold (4°C). These antibodies do not react with Mycoplasma. If sputum is available, a PCR test can confirm  Mycoplasma infection. See page 197 for additional information.   A 45-year-old man sustained a skull fracture in an automobile accident. The following day, he noted clear fluid dripping from  his nose, but he did not notify the hospital personnel. The following day, he spiked a fever to 39°C and complained of a severe  headache. Nuchal rigidity was found on physical examination. Spinal fluid analysis revealed a WBC count of 5200/pL, 90% of  which were neutrophils. Gram stain showed gram-positive diplococci.   Diagnosis: Meningitis caused by Streptococcus pneumoniae. Patients with a fracture of the cribriform plate who leak spinal  fluid into the nose are predisposed to meningitis by this organism. Pneumococci can colonize the nasal mucosa and enter the  subarachnoid space through the fractured cribriform plate. See page 123 for additional information.   A 7-year-old girl was well until about 3 weeks ago, when she began complaining of being “tired all the time.” On examination,  her temperature is 38°C and there is tenderness below the right knee. Llemoglobin: 10.2; WBC: 9600 with increased neutro¬  phils. A sickle cell prep shows a moderate sickling tendency. Gram-negative rods grew in the blood culture.   Diagnosis: Osteomyelitis caused by Salmonella species. Sickle cell anemia predisposes to osteomyelitis caused by Salmo¬  nella species. The abnormally shaped sickle cells are trapped in the small capillaries of the bone and cause microinfarcts. These  microinfarcts enhance the likelihood of infection by Salmonella. See page 155 for additional information.   A 3-month-old boy has a persistent cough and severe wheezing for the past 2 days. On physical examination, his temperature  is 39°C and coarse rhonchi are heard bilaterally. Chest X-ray shows interstitial infiltrates bilaterally. Diagnosis was made by    Diagnosis: Think pneumonia caused by respiratory syncytial virus (RSV), the most common cause of pneumonia and bron¬  chiolitis in infants. RSV causes giant cells (syncytia) that can be seen in respiratory secretions and in cell culture. See  page 321 for additional information.   A 34-year-old man was in his usual state of health until last night, when he felt feverish, had a shaking chill, and became short  of breath at rest. Temperature 39°C, blood pressure 110/60, pulse 104, respirations 18. Scattered rales were heard in both bases.  A new murmur consistent with tricuspid insufficiency was heard. Needle tracks were seen on both forearms. Gram-positive  cocci in clusters grew in blood culture.   Diagnosis: Acute endocarditis caused by S. aureus. This organism is the most common cause of acute endocarditis in intrave¬  nous drug users. The valves on the right side of the heart are often involved. See page 109 for additional information.   A 2-week-old infant was well on discharge from the hospital 10 days ago and remained so until last night, when he appeared  drowsy and flushed. His skin felt hot to the touch. On physical examination, the infant was very difficult to arouse, but there  were no other positive findings. His temperature was 40°C. Blood culture grew gram-positive cocci in chains. A narrow zone  of clear (beta) hemolysis was seen around the colonies. Hippurate hydrolysis test was positive.   Diagnosis: Neonatal sepsis caused by Streptococcus agalactiae (group B streptococci). Group B streptococci are the most com¬  mon cause of neonatal sepsis. Think E. coli if gram-negative rods are seen or Listeria monocytogenes if gram-positive rods are  seen. See page 121 for additional information on group B streptococci, page 153 for additional information on E. coli, and  page 144 for additional information on L. monocytogenes.   A 70-year-old woman had a hip replacement because of severe degenerative joint disease. She did well until a year later, when  a fall resulted in a fracture of the femur and the prosthesis had to be replaced. Three weeks later, bloody fluid began draining  from the wound site. The patient was afebrile, and the physical examination was otherwise unremarkable. Two days later,  because of increasing drainage, the wound was debrided and pus was obtained. Gram stain of the pus was negative, but an   Diagnosis: Prosthetic joint infection caused by Mycobacterium fortuitum-chelonei complex. Think S. epidermidis if gram¬  positive cocci in clusters are seen. See page 190 for additional information on M. fortuitum-chelonei complex and page 115 for  additional information on S. epidermidis.   An 80-year-old man complains of a painful rash on his left forehead. The rash is vesicular and only on that side. He is being  treated with chemotherapy for leukemia. Smear of material from the base of the vesicle reveals multinucleated giant cells with  intranuclear inclusions.   Diagnosis: Herpes zoster (shingles) caused by varicella-zoster virus. The rash of zoster follows the dermatome of the neuron  that was latently infected. Herpes simplex virus type 1 can cause a similar picture. These viruses can be distinguished using  fluorescent antibody assay. See page 293 for additional information.   A 55-year-old woman has an inflamed ulcer on her right hand and several tender nodules on the inner aspect of her right arm.  She is an avid gardener and especially enjoys pruning her roses. Biopsy of the lesion reveals budding yeasts.   Diagnosis: Sporotrichosis caused by Sporothrix schenckii. The organism is a mold in the soil and a yeast in the body (i.e., it is  dimorphic). Infection occurs when spores produced by the mold form are introduced into the skin by a penetrating injury.  See page 405 for additional information.   A 15-year-old boy sustained a broken tooth in a fist fight several weeks ago. He now has an inflamed area on the skin over the  broken tooth, in the center of which is a draining sinus tract. Gram stain of the drainage fluid reveals filamentous gram¬  positive rods.    Diagnosis: Actinomycosis caused by Actinomyces israelii. See “sulfur granules” in the sinus tract. These granules are particles  composed of interwoven filaments of bacteria. See page 194 for additional information.   A 24-year-old woman experienced the sudden onset of high fever, myalgias, vomiting, and diarrhea. Her vital signs were as  follows: temperature 40°C, blood pressure 70/30, pulse 140, respirations 30. A sunburn-like rash appeared over most of her  body. Blood cultures and stool cultures are negative. She is recovering from a surgical procedure on her maxillary sinus, and  the bleeding was being staunched with nasal tampons. Gram-positive cocci in clusters were seen in blood adherent to the nasal  tampon.   Diagnosis: Toxic shock syndrome caused by S. aureus. Toxic shock syndrome toxin is a superantigen that stimulates the  release of large amounts of cytokines from many helper T cells. See page 114 for additional information.   An 8-year-old girl has a pruritic rash on her chest. Lesions are round or oval with an inflamed border and central clearing. The  lesions contain both papules and vesicles. See hyphae in KOH prep of scrapings from the lesion.   Diagnosis: Tinea corporis (ringworm) caused by one of the dermatophytes, especially species of Microsporum, Trichophyton,  or Epidermophyton. Dermatophytes use keratin as a nutrient source, so lesions are limited to the skin. See page 404 for addi¬  tional information.   A 25-year-old woman has a papular rash on her trunk, arms, and palms. She says the rash does not itch. Vaginal examination  reveals two flat, moist, slightly raised lesions on the labia. Material from a labial lesion examined in a dark field microscope  revealed spirochetes.   Diagnosis: Secondary syphilis caused by T. pallidum. The rash on the palms coupled with the vaginal lesions (condylomata  lata) is compatible with secondary syphilis. Serologic tests, such as the nonspecific test (VDRL) and the specific test (FTA-  ABS), were positive. See page 201 for additional information.   A 5-year-old girl complains of an earache for the past 2 days. On examination, she has a temperature of 39°C, the right external  canal contained dried blood, the drum was perforated, and a small amount of purulent fluid was seen. Gram stain of the pus  revealed gram-positive diplococci. Colonies formed green (alpha) hemolysis on blood agar. Growth was inhibited by  optochin.   Diagnosis: Otitis media caused by S. pneumoniae. Think Haemophilus influenzae if small gram-negative rods are seen. These  organisms colonize the oropharynx and enter the middle ear via the eustachian tube. See page 123 for additional information  on S. pneumoniae and page 171 for additional information on H. influenzae.   A 25-year-old woman was well until the sudden onset of high fever (temperature, 40°C) accompanied by several purple skin  lesions (ecchymoses, purpura). The lesions are scattered over the body, are irregularly shaped, and are not raised. Her blood  pressure is 60/10, and her pulse rate is 140. Blood culture grew gram-negative diplococci.   Diagnosis: Meningococcemia caused by Neisseria meningitidis. The endotoxin (lipopolysaccharide, or LPS) of the organism  triggers release of interleukin-1, tumor necrosis factor, and nitric oxide from macrophages. These cause the high fever and low  blood pressure. The purpuric lesions are a manifestation of disseminated intravascular coagulation (DIC). Endotoxin acti¬  vates the coagulation cascade, causing DIC. Lipid A is the toxic part of LPS. See page 130 for additional information.   A 40-year-old woman was well until 2 days ago, when she experienced the sudden onset of fever, shaking chills, and profuse  sweating. Today, she also complains of headache and abdominal pain but no nausea, vomiting, or diarrhea. She does not have  a stiff neck, rash, or altered mental status. Travel history reveals she returned from an extended trip to several countries in  central Africa 1 week ago. Blood smear reveals ring-shaped trophozoites within red blood cells.    Diagnosis: Malaria caused by Plasmodium species. If banana-shaped gametocytes seen in the blood smear, think Plasmodium  falciparum. Plasmodium falciparum is the species that causes the life-threatening complications of malaria, such as cerebral  malaria. The fever and chills experienced by the patient coincide with the release of merozoites from infected red blood cells  and occur in either a tertian or quartan pattern. See page 433 for additional information.   A 35-year-old man is seen in the emergency room (ER) complaining of severe headache and vomiting that began last night.  His temperature is 40°C. While in the ER, he is increasingly combative and has a grand mal seizure. He is “foaming at the  mouth” and cannot drink any liquids. Analysis of his spinal fluid reveals no abnormality, and no organisms are seen in the  Gram stain. Two days later, despite supportive measures, he dies. Pathologic examination of the brain reveals eosinophilic  inclusion bodies in the cytoplasm of neurons.   Diagnosis: Rabies (an encephalitis) caused by rabies virus. The inclusions are Negri bodies. Diagnosis can be confirmed by  using fluorescent antibody assays. The patient was a farm worker who was bitten by a bat about a month prior to the onset of  symptoms. Note the long incubation period, which can be as long as 6 months. People bitten by a bat (or any wild animal)  should receive rabies immunization consisting of the inactivated vaccine plus rabies immune globulins (passive-active immu¬  nization). See page 326 for additional information.   A 70-year-old man was admitted to the hospital after suffering extensive third-degree burns. Three days later, he spiked a fever,  and there was pus on the dressing that had a blue-green color. Gram stain of the pus revealed gram-negative rods.  Diagnosis: Wound (burn) infection caused by Pseudomonas aeruginosa. The blue-green color is caused by pyocyanin, a pig¬  ment produced by the organism. See page 165 for additional information.   A 65-year-old woman reports that she has had several episodes of confusion and memory loss during the past few weeks. On  examination, she is afebrile but has a staggering gait and myoclonus can be elicited. Over the next several months, her condition  deteriorates and death ensues. On autopsy, microscopic examination of the brain reveals many vacuoles but no viral inclusion  bodies.   Diagnosis: Creutzfeldt-Jakob disease (CJD) caused by prions. CJD is a spongiform encephalopathy. The vacuoles give the  brain a sponge-like appearance. See page 374 for additional information.   A 20-year-old man complains of several episodes of blood in his urine. He has no dysuria or urethral discharge. He is not sexu¬  ally active. He is a college student but was born and raised in Egypt. Physical examination reveals no penile lesions. Urinalysis  shows many red cells, no white cells, and several large eggs with terminal spines.   Diagnosis: Schistosomiasis caused by Schistosoma haematobium. Schistosome eggs in venules of the bladder damage the blad¬  der epithelium and cause bleeding. The eggs are excreted in the urine. See page 462 for additional information.   A 35-year-old man complains of night sweats, chills, and fatigue at varying intervals during the past 2 months. These episodes  began while he was traveling in Latin America. When questioned, he says that cheeses, especially the unpasteurized varieties,  are some of his favorite foods. On examination, his temperature is 39°C, and his liver and spleen are palpable. His hematocrit  is 30%, and his WBC count is 5000. Blood culture grew small gram-negative rods.   Diagnosis: Brucellosis caused by Brucella species. Domestic animals such as cows and goats are the main reservoir for  Brucella, and it is often transmitted in unpasteurized dairy products. This patient could also have typhoid fever caused by  Salmonella typhi, but S. typhi is only a human pathogen (i.e., there is no animal reservoir). See page 177 for additional infor¬  mation on Brucella species and page 155 for additional information on S. typhi.   A 6-year-old girl has a rash on her face that appeared yesterday. The rash is erythematous and located over the malar emi¬  nences bilaterally. The rash is macular; there are no papules, vesicles, or pustules. A few days prior to the appearance of the  rash, she had a runny nose and anorexia.    Diagnosis: Slapped cheek syndrome caused by parvovirus B19. This virus also causes aplastic anemia because it prefer¬  entially infects and kills erythroblasts. It also infects the fetus, causing hydrops fetalis, and causes an immune complex-  mediated arthritis, especially in adult women. See page 308 for additional information.   A 20-year-old man fell off his motorcycle and suffered a compound fracture of the femur. The fracture was surgically reduced  and the wound debrided. Forty-eight hours later, he spiked a fever (temperature, 40°C), and the wound area became necrotic.  Crepitus was felt, and a foul-smelling odor was perceived originating from the wound. Marked anemia and a WBC count of  22,800 were found. Gram stain of the exudate showed large gram-positive rods. Colonies grew on blood agar incubated  anaerobically but not aerobically.   Diagnosis: Gas gangrene (myonecrosis) caused by Clostridium perfringens. The main virulence factor produced by this organ¬  ism is an exotoxin that is a lecithinase. It causes necrosis of tissue and lysis of red blood cells (causing hemolytic anemia). The  spores of the organism are in the soil and enter at the wound site. A foul-smelling exudate is characteristic of infections caused  by anaerobic bacteria. See page 140 for additional information.   A 30-year-old woman complains of a burning feeling in her mouth and pain on swallowing. Sexual history reveals she is a  commercial sex worker and has had unprotected vaginal, oral, and anal intercourse with multiple partners. On examination,  whitish lesions are seen on the tongue, palate, and pharynx. No vesicles are seen. The test for HIV antibody is positive, and her  CD4 count is 65. Gram stain of material from the lesions reveals budding yeasts and pseudohyphae.   Diagnosis: Thrush caused by C. albicans. This organism forms pseudohyphae when it invades tissue. The absence of vesicles  indicates that her symptoms are not caused by herpes simplex virus type 2. See page 414 for additional information.   You’re a physician at a refugee camp in sub-Saharan Africa, when an outbreak of diarrhea occurs. Massive amounts of watery  stool, without blood, are produced by the patients. Curved gram-negative rods are seen in a Gram stain of the stool.  Diagnosis: Cholera caused by Vibrio cholerae. There are three genera of curved gram-negative rods: Vibrio, Campylobacter,  and Helicobacter. V. cholerae causes watery, nonbloody diarrhea, whereas C. jejuni typically causes bloody diarrhea.  Helicobacter pylori causes gastritis and peptic ulcer, not diarrhea. Enterotoxigenic E. coli causes watery diarrhea by producing  an exotoxin that has the same mode of action as does the exotoxin produced by V. cholerae. However, E. coli is a straight gram¬  negative rod, not a curved one. If an outbreak of bloody diarrhea had occurred in the refugee camp, then Shigella dysenteriae  would be the most likely cause. See the following pages for additional information: Vibrio, page 159; Campylobacter, page 161;  Helicobacter, page 162; Escherichia, page 153; and Shigella, page 158.   A 40-year-old man with low-grade fever and night sweats for the past 4 weeks now has increasing fatigue and shortness of  breath. He says he has difficulty climbing the one flight of stairs to his apartment. Pertinent past history includes rheumatic  fever when he was 15 years old and the extraction of two wisdom teeth about 3 weeks before his symptoms began. No che¬  moprophylaxis was given at the time of the extractions. There is no history of intravenous drug use. His temperature is  38.5°C, and a loud holosystolic murmur can be heard over the precordium. His spleen is palpable. He is anemic, and his  WBC count is 13,500. Blood cultures grow gram-positive cocci in chains that produce green (alpha) hemolysis on blood  agar. Growth is not inhibited by optochin.   Diagnosis: Subacute bacterial endocarditis caused by one of the viridans group streptococci, such as Streptococcus sanguinis.  The laboratory findings are also compatible with Enterococcus faecalis, but the history of dental surgery makes the viridans  group streptococci more likely to be the cause. Endocarditis caused by E. faecalis is associated with gastrointestinal or genito¬  urinary tract surgery. See page 121 for additional information on both viridans group streptococci and E. faecalis.   A 60-year-old woman is asymptomatic but has a lung nodule seen on chest X-ray. Pertinent past history includes her cigarette  smoking (2 packs per day for 40 years) and her occupation as an archaeologist, digging primarily in Arizona and New Mexico.  Because of concern that the nodule may be malignant, it was surgically removed. Pathologic examination revealed large  (25 pm) round structures with thick walls and many round spores inside. No malignant cells were seen.    Diagnosis: Coccidioidomycosis caused by Coccidioides immitis. These structures are spherules, which are pathognomonic for  this disease. The mold form of the organism is found in the soil of the southwestern United States, and the organism is acquired  by inhalation of arthrospores produced by the mold. The inhaled arthrospores form spherules in the lung. Coccidioides immitis  is dimorphic and forms spherules at 37°C. See page 407 for additional information.   A 20-year-old woman in her 30th week of pregnancy had an ultrasound examination that revealed a growth-retarded fetus  with a large head (indicating hydrocephalus) and calcifications within the brain. Umbilical blood was cultured, and crescent¬  shaped trophozoites were grown.   Diagnosis: Toxoplasmosis caused by Toxoplasma gondii. Detection of IgM antibody in the Sabin-Feldman dye test can also be  used to make a diagnosis. The main reservoir is domestic cats. Domestic farm animals, such as cattle, acquire the organism by  accidentally eating cat feces. Pregnant women should not be exposed to cat litter or eat undercooked meat. See page 438 for  additional information.   A 10-day-old neonate has several vesicles on the scalp and around the eyes. The child is otherwise well, afebrile, and feeding  normally. A Giemsa-stained smear of material from the base of a vesicle revealed multinucleated giant cells with intranuclear  inclusions.   Diagnosis: Neonatal infection caused by herpes simplex virus type 2. Infection is acquired during passage through the birth  canal. Life-threatening encephalitis and disseminated infection of the neonate also occur. See page 292 for additional  information.   A 40-year-old woman has just had a grand mal seizure. There is a history of headaches for the past week and one episode of  vertigo but no previous seizures. She is afebrile. She is a native of Honduras but has lived in the United States for the past 5  years. MRI reveals a mass in the parietal lobe. Surgical removal of the mass reveals a larva within a cystlike sac.   Diagnosis: Cysticercosis caused by the larva of Taenia solium. Infection is acquired by ingesting the tapeworm eggs, not by  ingesting undercooked pork. This clinical picture can also be caused by a brain abscess, a granuloma such as a tuberculoma,  or a brain tumor. See page 454 for additional information.   A 1-week-old neonate has a yellowish exudate in the corners of both eyes. The child is otherwise well, afebrile, and feeding  normally. Gram stain of the exudate reveals no gram-negative diplococci. A Giemsa-stained smear of the exudate reveals a   Diagnosis: Conjunctivitis caused by Chlamydia trachomatis. Confirm the diagnosis with direct fluorescent antibody test.  Infection is acquired during passage through the birth canal. The inclusion contains large numbers of the intracellular repli¬  cating forms called reticulate bodies. See page 210 for additional information.    Many questions on the USMLE can be answered by  knowing the meaning of the epidemiologic information  provided in the case description. In order to do this, the  student should know the reservoir of the organism, its  mode of transmission, and the meaning of factors such as  travel, occupation, and exposure to pets, farm animals, or  wild animals. Knowledge of the microbes that typically  cause disease in individuals with specific immunodefi¬  ciencies will also be helpful.   In addition to being useful for the USMLE, this infor¬  mation will prove valuable to make the diagnosis of infec¬  tious diseases on the wards and in your clinical practice.   Table XII-1. Farm Animals and Household Pets as Res¬  ervoirs of Medically Important Organisms   Table XII-6. Occupations and Avocations That Increase  Exposure to Medically Important Organisms   Table XII-7. Hospital-Related Events That Predispose to  Infection by Medically Important Organisms   Table XII-8. Organisms That Commonly Cause Disease  in Patients with Immunodeficiencies or Reduced Host  Defenses   Table XII-9. Important Factors That Predispose to  Infections by Specific Organisms   Table XII-10. Maternal Infections That Pose Significant  Risk to the Fetus or Neonate    TABLE XI1—1 Farm Animals and Household Pets as Reservoirs of Medically Important Organisms    3 lngestion of eggs in human feces, not ingestion of pork, results in cysticercosis.    TABLE XII-2 Wild Animals as Reservoirs of Medically Important Organisms ( Continued)   Inhale spores in aerosol of soil con¬  taminated by starling guano   3. Midwestern states in the Ohio and Mississippi River val¬  leys (e.g., Missouri and Illinois)    TABLE XII-6 Occupations and Avocations That Increase Exposure to Medically Important Organisms    TABLE XII-7 Hospital-Related Events That Predispose to Infection by Medically Important Organisms   1. Escherichia coli primarily, but also other enteric gram¬  negative rods (e.g., Proteus, Serratia, and Pseudomonas)    AIDS = acquired immunodeficiency syndrome; HBV = hepatitis B virus; HCV = hepatitis C virus; HIV = human immunodeficiency virus.    TABLE XII—8 Organisms That Commonly Cause Disease in Patients with Immunodeficiencies or Reduced Host  Defenses    TABLE XII— 8 Organisms That Commonly Cause Disease in Patients with Immunodeficiencies or Reduced Host  Defenses (Continued)    AIDS = acquired immunodeficiency syndrome; HIV = human immunodeficiency virus; IgA = immunoglobulin A; MAI = Mycobacterium avium-intracellulare complex.    TABLE XII-9 Important Factors That Predispose to Infections by Specific Organisms   Abnormally shaped red cells block blood  vessels in bone and trap bacteria   Abnormally shaped red cells block blood  vessels in spleen causing infarction of  spleen   These bacteria are normal flora in the  mouth and enter the blood at the site of  the surgical wound   Skin flora enter blood stream at site of  catheter or skin wound   Skin flora enter blood stream at site of  catheter or skin wound    TABLE XII—10 Maternal Infections That Pose Significant Risk to the Fetus or Neonate   Most are perinatal but transplacental and via breast milk also occurs   Neonatal HBV infection greatly increases the risk of chronic carrier state   Neonatal HCV infection greatly increases the risk of chronic carrier state   Benign tumors except HPV 16 and 18  cause carcinoma of cervix   Scattered pustules and inflamed  tendons, especially of wrists and  fingers (tenosynovitis)   Systemic fungi (e.g., Coccidioi-  des ) and Mycobacteria (e.g.,  Mycobacterium tuberculosis  and Mycobacterium leprae)   See Koplik's spots on buccal mucosa;  rash caused by cytotoxicT-cell attack  on virus-infected cells   A sign of disseminated intravascular  coagulation (DIC) that occurs in sepsis;  can enlarge to form purpuric (ecchy-  motic) lesions   Rickettsia infect and kill vascular endo¬  thelium, resulting in hemorrhage into  skin   Diffuse, macular, red (scarlet) rash;  also strawberry tongue and cir-  cumoral pallor   Caused by strains of S. pyogenes that  produces erythrogenic toxin that is a  superantigen   ’Description of certain important skin lesions: Macule is a flat, erythematous lesion. Papule is a raised, erythematous lesion with no visible fluid inside; resembles a mosquito  bite. Vesicle is a raised, erythematous lesion with yellowish fluid (resembling plasma) inside; approximately the same size as a papule. Pustule is a raised, erythematous lesion  with cloudy fluid (pus) inside; typically larger than a papule or vesicle.    These practice questions are presented in the format used by the  United States Medical Licensing Examination (USMLE) Step 1.  Note that in the computerized version of the USMLE, all ques¬  tions are of the “ONE-BEST-ANSWER” type. There are no  questions of the “EXCEPT” or “LEAST ACCURATE” type in  which you are asked to determine the one wrong answer. Nev¬  ertheless, for studying purposes, the EXCEPT or LEAST ACCU¬  RATE type of questions are excellent learning tools because they  provide you with several correct statements and only one incor¬  rect statement rather than several incorrect ones. In view of this  learning advantage, many practice questions in Part XIII of this  book are of the EXCEPT or LEAST ACCURATE type. However,  in Part XIV, the questions in the USMLE Practice Examination    are presented in the ONE-BEST-ANSWER format, and no  EXCEPT type questions are used.   After the questions regarding the specific content areas (i.e.,  bacteriology, virology, mycology, parasitology, and immunol¬  ogy), there are two additional sections, one containing questions  in an extended matching format and the other containing ques¬  tions based on infectious disease cases. The questions in the  computerized version of the USMLE have 4 to 10 answer  choices. Although the format of the questions in the extended  matching section of this book is different from the format used  in the USMLE, the questions in this section are designed to be a  highly time-effective way of transmitting the important  information.    DIRECTIONS (Questions 1-39): Select the ONE lettered answer that  is BEST in each question.   1. Each of the following statements concerning the surface struc¬  tures of bacteria is correct EXCEPT:   (C) Both gram-negative rods and cocci have lipopolysaccharide  (“endotoxin”) in their cell wall.   (D) Bacterial flagella are nonantigenic in humans because they  closely resemble human flagella in chemical composition.   (A) It has a backbone composed of alternating units of muramic  acid and acetylglucosamine.   3. Each of the following statements concerning bacterial spores is  correct EXCEPT:   (A) Their survival ability is based on their enhanced metabolic  activity.   (C) They can be killed by being heated to 121°C for 15 minutes.    4. Which one of the statements is the MOST accurate com¬  parison of human, bacterial, and fungal cells?   (A) Human cells undergo mitosis, whereas neither bacteria  nor fungi do.   (B) Human and fungal cells have a similar cell wall, in con¬  trast to bacteria, whose cell wall contains peptidoglycan.   (C) Human and bacterial cells have plasmids, whereas fun¬  gal cells do not.   (D) Human and fungal cells have similar ribosomes,  whereas bacterial ribosomes are significantly different.   5. Which statement is MOST accurate regarding the drug  depicted in the diagram?   6. Each of the following statements regarding the selective action  of antibiotics on bacteria is correct EXCEPT:   (A) Chloramphenicol affects the large subunit of the bacterial  ribosome, which is different from the large subunit of the  human ribosome.   (B) Isoniazid affects the DNA polymerase of bacteria but not  that of human cells.   (C) Sulfonamides affect folic acid synthesis in bacteria, a path¬  way that does not occur in human cells.   (D) Penicillins affect bacteria rather than human cells because  bacteria have a cell wall, whereas human cells do not.   7. Each of the following statements concerning endotoxins is cor¬  rect EXCEPT:   (A) They are less toxic (i.e., less active on a weight basis) than  exo toxins.   (C) They bind to specific cell receptors, whereas exotoxins do  not.   (D) They are part of the bacterial cell wall, whereas exotoxins are  not.   (D) Modulation of host cell receptors in response to the toxin   9. Which one of the following processes involves a sex pilus?   10. Each of the following statements concerning the normal flora is  correct EXCEPT:   (A) The most common organism found on the skin is Staphylo¬  coccus epidermidis.   (B) Escherichia coli is a prominent member of the normal flora of  the throat.   (C) The major site where Bacteroidesfragilis is found is the colon.   (D) One of the most common sites where Staphylococcus aureus  is found is the nose.   11. Each of the following statements concerning the mechanism of  action of antimicrobial drugs is correct EXCEPT:   (B) Quinolones, such as ciprofloxacin, act by inhibiting the DNA  gyrase of bacteria.   (C) Erythromycin is a bactericidal drug that disrupts cell mem¬  branes by a detergent-like action.   (D) Aminoglycosides such as streptomycin are bactericidal drugs  that inhibit protein synthesis.   12. Each of the following statements concerning the resistance of  bacteria to antimicrobial drugs is correct EXCEPT:   (A) Resistance to chloramphenicol is known to be due to an  enzyme that acetylates the drug.   (B) Resistance to penicillin is known to be due to reduced affin¬  ity of transpeptidases.    (C) Resistance to penicillin is known to be due to cleavage by  (3-lactamase.   (D) Resistance to tetracycline is known to be due to an enzyme  that hydrolyzes the ester linkage.   13. Of the following choices, the MOST important function of anti¬  body in host defenses against bacteria is:   14. Which of the following events is MOST likely to be due to bac¬  terial conjugation?   (A) A strain of Corynebacterium diphtheriae produces a toxin  encoded by a prophage.   (B) A strain of Pseudomonas aeruginosa produces (3-lactamase  encoded by a plasmid similar to a plasmid of another gram¬  negative organism.   (C) An encapsulated strain of Streptococcus pneumoniae acquires  the gene for capsule formation from an extract of DNA from  another encapsulated strain.   (D) A gene encoding resistance to gentamicin in the Escherichia  coli chromosome appears in the genome of a bacteriophage  that has infected E. coli.   15. Which one of the following BEST describes the mode of action  of endotoxin?   16. The identification of bacteria by serologic tests is based on the  presence of specific antigens. Which one of the following bac¬  terial components is LEAST likely to contain useful antigens?   17. Each of the following statements concerning bacterial spores is  correct EXCEPT:   (A) Spores are formed under adverse environmental conditions  such as the absence of a carbon source.   (C) Spores are metabolically inactive and contain dipicolinic  acid, a calcium chelator.   (D) Spores are formed primarily by organisms of the genus  Neisseria.   18. Each of the following statements concerning the mechanism of  action of antibacterial drugs is correct EXCEPT:   (A) Cephalosporins are bactericidal drugs that inhibit the trans¬  peptidase reaction and prevent cell wall synthesis.   (B) Tetracyclines are bacteriostatic drugs that inhibit protein  synthesis by blocking tRNA binding.   (C) Aminoglycosides are bacteriostatic drugs that inhibit pro¬  tein synthesis by activating ribonuclease, which degrades  mRNA.   (D) Erythromycin is a bacteriostatic drug that inhibits protein  synthesis by blocking translocation of the polypeptide.    19. Each of the following is a typical property of obligate anaerobes  EXCEPT:   20. Each of the following statements concerning the Gram stain is  correct EXCEPT:   (A) Escherichia coli stains pink because it has a thin peptidogly-  can layer.   (B) Streptococcus pyogenes stains blue because it has a thick pep-  tidoglycan layer.   (C) Mycobacterium tuberculosis stains blue because it has a thick  lipid layer.   (D) Mycoplasma pneumoniae is not visible in the Gram stain  because it does not have a cell wall.   21. Each of the following statements concerning the killing of bac¬  teria is correct EXCEPT:   22. In the Gram stain, the decolorization of gram-negative bacteria  by acetone-alcohol is MOST closely related to:   23. Chemical modification of benzylpenicillin (penicillin G) has  resulted in several beneficial changes in the clinical use of this  drug. Which one of the following is NOT one of those beneficial  changes?   24. Each of the following statements concerning resistance to anti¬  biotics is correct EXCEPT:   (A) Resistance to aminoglycosides can be due to phosphorylat-  ing enzymes encoded by R plasmids.   (B) Resistance to sulfonamides can be due to enzymes that  hydrolyze the five-membered ring structure.   (C) Resistance to penicillins can be due to alterations in binding  proteins in the cell membrane.   (D) Resistance to cephalosporins can be due to cleavage of the  (3-lactam ring.   25. The effects of endotoxin include each of the following EXCEPT:   26. Bacterial surface structures that show antigenic diversity  include each of the following EXCEPT:    27. The effects of antibody on bacteria include each of the following  EXCEPT:   (A) When treated chemically, some exotoxins lose their toxicity  and can be used as immunogens in vaccines.   (B) Some exotoxins are capable of causing disease in purified  form, free of any bacteria.   (C) Some exotoxins act in the gastrointestinal tract to cause  diarrhea.   29. Each of the following statements concerning bacterial and  human cells is correct EXCEPT:   (A) Bacteria are prokaryotic (i.e., they have one molecule of  DNA, are haploid, and have no nuclear membrane), whereas  human cells are eukaryotic (i.e., they have multiple chromo¬  somes, are diploid, and have a nuclear membrane).   (B) Bacteria derive their energy by oxidative phosphorylation  within mitochondria in a manner similar to human cells.   (C) Bacterial and human ribosomes are of different sizes and  chemical compositions.   (A) An intact (3-lactam ring of penicillin is required for its  activity.   (B) The structure of penicillin resembles that of a dipeptide of  alanine, which is a component of peptidoglycan.   (C) Penicillin is a bacteriostatic drug because autolytic enzymes  are not activated.   31. Each of the following statements concerning the mechanisms of  resistance to antimicrobial drugs is correct EXCEPT:   (A) R factors are plasmids that carry the genes for enzymes that  modify one or more drugs.   (B) Resistance to some drugs is due to a chromosomal mutation  that alters the receptor for the drug.   (C) Resistance to some drugs is due to transposon genes that  code for enzymes that inactivate the drugs.   (A) The toxicity of endotoxins is due to the lipid portion of the  molecule.   (D) The antigenicity of somatic (O) antigen is due to repeating  oligosaccharides.    39. Each of the following statements concerning cholera toxin is  correct EXCEPT:   (B) Exotoxins are more easily inactivated by heat than are  endotoxins.   (C) Exotoxins are less toxic than the same amount of endotoxins.   34. Each of the following statements concerning the killing of bac¬  teria is correct EXCEPT:   (A) A 70% solution of ethanol kills more effectively than absolute  (100%) ethanol.   (B) An autoclave uses steam under pressure to reach the killing  temperature of 121°C.   (C) The pasteurization of milk kills pathogens but allows many  organisms and spores to survive.   (D) Iodine kills by causing the formation of thymine dimers in  bacterial DNA.   (B) Binding of cholera toxin to the mucosal epithelium occurs  via interaction of the B subunit of the toxin with a ganglio-  side in the cell membrane.   (C) Cholera toxin acts by adding ADP-ribose to a G protein.   (D) Cholera toxin activates the enzyme adenylate cyclase in the  enterocyte.   DIRECTIONS (Questions 40-51): Select the ONE lettered option that  is MOST closely associated with the numbered items. Each lettered  option may be selected once, more than once, or not at all.   (D) The portion of the molecule required for activity is labeled B.   36. Each of the following statements concerning the normal flora is   (A) The normal flora of the colon consists predominantly of  anaerobic bacteria.   (B) The presence of the normal flora prevents certain pathogens  from colonizing the upper respiratory tract.   (C) Fungi (e.g., yeasts) are not members of the normal flora.   (D) Organisms of the normal flora are permanent residents of  the body surfaces.   (A) Some gram-positive cocci contain teichoic acid external to  the peptidoglycan.   (B) Some gram-positive rods produce spores that are resistant to  boiling.   (C) Some gram-negative rods contain lipid A in their outer cell  membrane.   38. Each of the following statements concerning the normal flora is   (A) Streptococcus mutans is found in the mouth and contributes  to the formation of dental caries.   (C) Bacteroidesfragilis is found in greater numbers than Escherichia  coli in the colon.   (D) Candida albicans is part of the normal flora of both men and  women.   42. Inhibit(s) protein synthesis by binding to the 30S ribosomal  subunit   44. During an outbreak of gastrointestinal disease caused by an  Escherichia coli strain sensitive to ampicillin, tetracycline, and  chloramphenicol, a stool sample from one patient yields E. coli  with the same serotype resistant to the three antibiotics.   45. A mutant cell line lacking a functional thymidine kinase gene  was exposed to a preparation of DNA from normal cells; under  appropriate growth conditions, a colony of cells was isolated  that makes thymidine kinase.   46. A retrovirus without an oncogene does not induce leukemia in  mice; after repeated passages through mice, viruses recovered  from a tumor were highly oncogenic and contained a new gene.   50. Stimulates the production of cyclic AMP by adding ADP-ribose  to a G protein    DIRECTIONS (Questions 52-136): Select the ONE lettered answer  that is BEST in each question.   52. An outbreak of sepsis caused by Staphylococcus aureus has  occurred in the newborn nursery. You are called upon to inves¬  tigate. According to your knowledge of the normal flora, what  is the MOST likely source of the organism?   53. Each of the statements about the classification of streptococci is  correct EXCEPT:   (A) Pneumococci ( Streptococcus pneumoniae) are a-hemolytic  and can be serotyped on the basis of their polysaccharide  capsules.   (B) Enterococci are group D streptococci and can be classified by  their ability to grow in 6.5% sodium chloride.   (C) Although pneumococci and the viridans streptococci are  a-hemolytic, they can be differentiated by the bile solubility  test and their susceptibility to optochin.   (D) Viridans streptococci are identified by Lancefield grouping,  which is based on the C carbohydrate in the cell wall.   54. Each of the following agents is a recognized cause of diarrhea  EXCEPT:   55. Each of the following organisms is an important cause of uri¬  nary tract infections EXCEPT:   56. Your patient is a 30-year-old woman with nonbloody diarrhea  for the past 14 hours. Which one of the following organisms is  LEAST likely to cause this illness?   57. Each of the following statements concerning Mycobacterium  tuberculosis is correct EXCEPT:   (A) After being stained with carbolfuchsin, M. tuberculosis  resists decolorization with acid alcohol.   (B) Mycobacterium tuberculosis has a large amount of mycolic  acid in its cell wall.    (C) Mycobacterium tuberculosis appears as a red rod in Gram-  stained specimens.   (D) Mycobacterium tuberculosis appears as a red rod in acid-fast  stained specimens.   58. A 50-year-old homeless alcoholic has a fever and is coughing up  1 cup of green, foul-smelling sputum per day. You suspect that  he may have a lung abscess. Which one of the following pairs of  organisms is MOST likely to be the cause?   59. Which one of the following diseases is BEST diagnosed by sero¬  logic means?   60. Your patient has subacute bacterial endocarditis caused by a  member of the viridans group of streptococci. Which one of the  following sites is MOST likely to be the source of the organism?   61. A culture of skin lesions from a patient with pyoderma  (impetigo) shows numerous colonies surrounded by a zone  of (3-hemolysis on a blood agar plate. A Gram-stained smear  shows gram-positive cocci. If you found the catalase test to  be negative, which one of the following organisms would you  MOST probably have isolated?   62. The coagulase test, in which the bacteria cause plasma to clot, is  used to distinguish:   63. Which one of the following is a virulence factor for Staphylococ¬  cus aureus ?   (A) A heat-labile toxin that inhibits glycine release at the inter-  nuncial neuron   (D) Protein A that binds to the Fc portion of IgG    64. Which one of the following host defense mechanisms is  the MOST important for preventing dysentery caused by  Salmonella ?   66. Five hours after eating reheated rice at a restaurant, a 24-year-  old woman and her husband both developed nausea, vomit¬  ing, and diarrhea. Which one of the following organisms is the  MOST likely to be involved?   67. Which one of the following bacteria has the LOWEST 50%  infectious dose (ID 50 )?   70. Which one of the following organisms principally infects vascu¬  lar endothelial cells and as a result often causes a petechial rash?   71. Which one of the following statements MOST accurately  depicts the ability of the organism to be cultured in the  laboratory?   (A) Treponema pallidum from a chancre can be grown on a spe¬  cial artificial medium supplemented with cholesterol.   (B) Mycobacterium leprae can be grown in the armadillo and the  mouse footpad but not on any artificial media.   (C) Mycobacterium tuberculosis can be grown on enriched artifi¬  cial media and produces visible colonies in 48 to 96 hours.   (D) Atypical mycobacteria are found widely in soil and water  but cannot be cultured on artificial media in the  laboratory.    72. Each of the following statements concerning chlamydiae is cor¬  rect EXCEPT:   (A) Chlamydiae are strict intracellular parasites because they  cannot synthesize sufficient adenosine triphosphate (ATP).   (B) Chlamydiae possess both DNA and RNA and are bounded  by a cell wall.   (C) Chlamydia trachomatis has multiple serotypes that can cause  different diseases.   73. For which one of the following bacterial vaccines are toxic side  effects an important concern?   74. Each of the following statements concerning Staphylococcus  aureus is correct EXCEPT:   (A) Gram-positive cocci in grapelike clusters are seen on Gram-  stained smear.   75. Your patient is a 70-year-old man who underwent bowel surgery  for colon cancer 3 days ago. He now has a fever and abdomi¬  nal pain. You are concerned that he may have peritonitis. Which  one of the following pairs of organisms is MOST likely to be the  cause?   76. A 65-year-old man develops dysuria and hematuria. A Gram  stain of a urine sample shows gram-negative rods. Culture of  the urine on EMB agar reveals lactose-negative colonies with¬  out evidence of swarming motility. Which one of the following  organisms is MOST likely to be the cause of his urinary tract  infection?   IT. A 25-year-old man complains of a urethral discharge. You per¬  form a Gram stain on a specimen of the discharge and see neu¬  trophils but no bacteria. Of the organisms listed, the one MOST  likely to cause the discharge is:   78. Two hours after a delicious Thanksgiving dinner of barley soup,  roast turkey, stuffing, sweet potato, green beans, cranberry sauce,  and pumpkin pie topped with whipped cream, the Smith family  of four experience vomiting and diarrhea. Which one of the fol¬  lowing organisms is MOST likely to cause these symptoms?    79. Your patient has a brain abscess that was detected 1 month after  a dental extraction. Which one of the following organisms is  MOST likely to be involved?   80. The MOST important contribution of the capsule of Streptococ¬  cus pneumoniae to virulence is:   81. The MOST important way the host counteracts the function of  the pneumococcal polysaccharide capsule is via:   82. The pathogenesis of which one of the following organisms is  MOST likely to involve invasion of the intestinal mucosa?   83. Which one of the following organisms that infects the gastroin¬  testinal tract is the MOST frequent cause of bacteremia?   84. A 30-year-old woman with systemic lupus erythematosus is  found to have a positive serologic test for syphilis (VDRL test).  She denies having had sexual contact with a partner who had  symptoms of a venereal disease. The next best step would be to:   (A) Reassure her that the test is a false-positive reaction related  to her autoimmune disorder   (D) Perform a fluorescent treponemal antibody-absorbed (FTA-  ABS) test on a specimen of her serum   (B) Treponema pallidum cannot be grown on conventional labo¬  ratory media.   (C) Treponemes are members of the normal flora of the human  oropharynx.   (D) Patients infected with T. pallidum produce antibodies that  react with beef heart cardiolipin.   86. Each of the following statements concerning clostridia is cor¬  rect EXCEPT:   (A) Pathogenic clostridia are found both in the soil and in the  normal flora of the colon.   (B) Antibiotic-associated (pseudomembranous) colitis is due to  a toxin produced by Clostridium difficile.    (C) Anaerobic conditions at the wound site are not required to  cause tetanus, because spores will form in the presence of  oxygen.   (D) Botulism, which is caused by ingesting preformed toxin, can  be prevented by boiling food prior to eating.   87. Each of the following statements concerning Bacteroides fragilis  is correct EXCEPT:   (A) Bacteroides fragilis is a gram-negative rod that is part of the  normal flora of the colon.   (B) Bacteroides fragilis forms endospores, which allow it to sur¬  vive in the soil.   (C) The capsule of B. fragilis is an important virulence factor.   (A) Staphylococcus aureus is differentiated from Staphylococcus  epidermidis by the production of coagulase.   (C) The majority of clinical isolates of S. aureus produce penicil¬  linase; therefore, penicillin G should not be used for antibi¬  otic therapy for S. aureus infections.   (D) Scalded skin syndrome caused by S. aureus is due to enzy¬  matic degradation of epidermal desmosomes by catalase.   89. Acute glomerulonephritis is a nonsuppurative complication  that follows infection by which one of the following organisms?   90. Each of the following statements concerning gram-negative  rods is correct EXCEPT:   (A) Escherichia coli is part of the normal flora of the colon; there¬  fore, it does not cause diarrhea.   (B) Escherichia coli ferments lactose, whereas the enteric patho¬  gens Shigella and Salmonella do not.   (C) Klebsiella pneumoniae , although a cause of pneumonia, is  part of the normal flora of the colon.   (D) Proteus species are highly motile organisms that are found in  the human colon and cause urinary tract infections.   91. A 70-year-old man is found to have a hard mass in his prostate,  which is suspected to be a carcinoma. Twenty-four hours after  surgical removal of the mass, he develops fever to 39°C and has  several shaking chills. Of the organisms listed, which one is  LEAST likely to be involved?   92. Five days ago a 65-year-old woman with a lower urinary tract  infection began taking ampicillin. She now has a fever and  severe diarrhea. Of the organisms listed, which one is MOST  likely to be the cause of the diarrhea?    93. The pathogenesis of which one of the following diseases does  NOT involve an exotoxin?   94. Regarding the effect of benzylpenicillin (penicillin G) on bacte¬  ria, which one of the following organisms is LEAST likely to be  resistant?   95. Which one of the following organisms is MOST likely to be the  cause of pneumonia in an immunocompetent young adult?   96. Each of the following statements concerning chlamydial genital  tract infections is correct EXCEPT:   (A) Infection can be diagnosed by finding antichlamydial anti¬  body in a serum specimen.   (C) Symptomatic infections can be associated with urethral or cervi¬  cal discharge containing many polymorphonuclear leukocytes.   97. Which one of the following illnesses is NOT a zoonosis?   98. Which one of the following is NOT a characteristic of the Staph¬  ylococcus associated with toxic shock syndrome?   (C) Appearance of the organism in grapelike clusters on Gram-  stained smear   99. Which one of the following is NOT an important characteristic  of either Neisseria gonorrhoeae or Neisseria meningitidis?   100. Which one of the following is NOT an important characteristic  of Streptococcus pyogenes?   101. Each of the following is associated with the Lancefield group B  streptococci (S. agalactiae ) EXCEPT:   (B) Vaginal carriage in 5% to 25% of normal women of child¬  bearing age    102. Three organisms, Streptococcus pneumoniae, Neisseria men¬  ingitidis , and Haemophilus influenzae , cause the vast majority  of cases of bacterial meningitis. What is the MOST important  pathogenic component they share?   103. Diarrhea caused by which one of the following agents is charac¬  terized by the presence of fecal leukocytes?   104. Each of the following statements concerning Chlamydia tracho¬  matis is correct EXCEPT:   105. Each of the following statements concerning Actinomyces and  Nocardia is correct EXCEPT:   (A) Actinomysis israelii is an anaerobic rod found as part of the  normal flora in the mouth.   (D) Infections are usually diagnosed by detecting a significant  rise in antibody titer.   107. Tissue-degrading enzymes play an important role in the patho¬  genesis of several bacteria. Which one of the following is NOT  involved in tissue or cell damage?   108. The soil is the natural habitat for certain microorganisms of  medical importance. Which one of the following is LEAST  likely to reside there?   109. Which one of the following organisms is the MOST frequent  bacterial cause of pharyngitis?    110. Several pathogens are transmitted either during gestation or at  birth. Which one of the following is LEAST likely to be trans¬  mitted at these times?   (A) Some strains of Escherichia coli produce an enterotoxin that  causes diarrhea.   (C) Diphtheria is caused by an exotoxin that inhibits protein  synthesis by inactivating an elongation factor.   (D) Botulism is caused by a toxin that hydrolyzes lecithin (leci-  thinase), thereby destroying nerve cells.   112. Each of the following statements concerning the VDRL test for  syphilis is correct EXCEPT:   (C) False-positive results are more frequent than with the fluo¬  rescent treponemal antibody-absorbed (FTA-ABS) test.   113. Each of the following statements concerning the fluorescent  treponemal antibody-absorbed (FTA-ABS) test for syphilis is  correct EXCEPT:   114. Each of the following statements concerning Corynebacterium  diphtheriae is correct EXCEPT:   (A) Corynebacterium diphtheriae is a gram-positive rod that does  not form spores.   (B) Toxin production is dependent on the organism’s being  lysogenized by a bacteriophage.   (C) Diphtheria toxoid should not be given to children younger  than 3 years because the incidence of complications is too  high.   (A) Pseudomonas aeruginosa causes wound infections that are  characterized by blue-green pus as a result of pyocyanin  production.   (B) In unimmunized individuals, invasive disease caused by  Haemophilus influenzae is most often due to strains possess¬  ing a type b polysaccharide capsule.   (C) Legionella pneumophila infection is acquired by inhalation of  aerosols from environmental water sources.   (D) Whooping cough, which is caused by Bordetella pertussis, is  on the rise because changing antigenicity of the organism has  made the vaccine relatively ineffective.    116. Each of the following statements concerning enterotoxins is  correct EXCEPT:   (A) Enterotoxins typically cause bloody diarrhea with leukocytes  in the stool.   (B) Staphylococcus aureus produces an enterotoxin that causes  vomiting and diarrhea.   (C) Vibrio cholerae causes cholera by producing an enterotoxin  that increases adenylate cyclase activity within the  enterocyte.   117. Each of the following statements concerning plague is correct  EXCEPT:   (A) Plague is caused by a gram-negative rod that can be cultured  on blood agar.   (B) Plague is transmitted from the animal reservoir to humans  by flea bite.   (D) Plague is of concern in many underdeveloped countries but  has not occurred in the United States since 1968.   (C) Brucellae infect reticuloendothelial cells in the liver, spleen,  and bone marrow.   (D) Brucellae are obligate intracellular parasites that are usually  identified by growth in human cell culture.   119. Each of the following statements concerning epidemic typhus is  correct EXCEPT:   (B) The Weil-Felix test can aid in diagnosis of the disease.   (D) The causative organism is transmitted from rodents to  humans by a tick.   120. Which one of the following organisms causes diarrhea by pro¬  ducing an enterotoxin that increases adenylate cyclase activity  within enterocytes?   121. Each of the following statements concerning Rocky Mountain  spotted fever is correct EXCEPT:   (B) Headache, fever, and rash are characteristic features of the  disease.   122. Each of the following statements concerning Clostridium per-  fringens is correct EXCEPT:   (C) It produces an exotoxin that degrades lecithin and causes  necrosis and hemolysis.   (D) It is a gram-negative rod that does not ferment lactose.    123. Each of the following statements concerning Clostridium tetani  is correct EXCEPT:   (B) Pathogenesis is due to the production of an exotoxin that  blocks inhibitory neurotransmitters.   (C) It is a facultative organism; it will grow on a blood agar plate  in the presence of room air.   (A) Species of Treponema are part of the normal flora of the  mouth.   (B) Species of Borrelia cause a tick-borne disease called relapsing  fever.   (C) The species of Leptospira that cause leptospirosis grow pri¬  marily in humans and are usually transmitted by human-to-  human contact.   (A) Infection in men is more frequently symptomatic than in  women.   (B) A presumptive diagnosis can be made by finding gram¬  negative kidney bean-shaped diplococci within neutrophils  in a urethral discharge.   (C) The definitive diagnosis can be made by detecting antibodies  to Neisseria gonorrhoeae in the patients serum.   (D) Gonococcal conjunctivitis of the newborn rarely occurs in  the United States, because silver nitrate or erythromycin is  commonly used as prophylaxis.   126. Each of the following statements concerning Mycobacterium  tuberculosis is correct EXCEPT:   (A) Some strains of M. tuberculosis isolated from patients exhibit  multiple drug resistance (i.e., they are resistant to both iso-  niazid and rifampin).   (B) Mycobacterium tuberculosis contains a small amount of lipid  in its cell wall and therefore stains poorly with the Gram  stain.   (C) Mycobacterium tuberculosis grows slowly, often requiring 3  to 6 weeks before colonies appear.   (D) The antigen in the tuberculin skin test is a protein extracted  from the organism.   127. Which one of the following statements concerning immuniza¬  tion against diseases caused by clostridia is CORRECT?   (A) Antitoxin against tetanus protects against botulism as well,  because the two toxins share antigenic sites.   (B) Vaccines containing alpha toxin (lecithinase) are effective in  protecting against gas gangrene.   (C) The toxoid vaccine against Clostridium difficile infection  should be administered to immunocompromised patients.   (D) Immunization with tetanus toxoid induces effective protec¬  tion against tetanus toxin.    129. Which one of the following statements concerning Legionella  pneumophila is CORRECT?   (A) It is part of the normal flora of the colon.   (D) It causes atypical pneumonia, especially in those with  reduced cell-mediated immunity.   130. Each of the following statements concerning wound infections  caused by Clostridium perfringens is correct EXCEPT:   131. Each of the following statements concerning infection with  Chlamydia psittaci is correct EXCEPT:   (A) Chylamydia psittaci can be isolated by growth in cell culture  and will not grow in blood agar.   (C) The infection is more readily diagnosed by serologic tests  than by isolation of the organism.   (D) The infection is more commonly acquired from a nonhuman  source than from another human.   132. Ticks are vectors for the transmission of each of the following  diseases EXCEPT:   133. Each of the following statements concerning pneumonia caused  by Mycoplasma pneumoniae is correct EXCEPT:   (A) Pneumonia caused by M. pneumoniae is associated with a  rise in the titer of cold agglutinins.   (B) Pneumonia caused by M. pneumoniae occurs primarily in  immunocompetent individuals.   (D) Mycoplasma pneumoniae cannot be cultured in vitro because  it has no cell wall.   135. Each of the following statements concerning Q fever is correct  EXCEPT:   136. Each of the following statements concerning Mycobacterium  leprae is correct EXCEPT:   (A) In lepromatous leprosy, large numbers of organisms are usu¬  ally seen in acid-fast-stained smears.   (B) The organism will grow on bacteriologic media in 3 to 6  weeks.    (C) Prolonged therapy (9 months or longer) is required to pre¬  vent recurrence.   (D) Loss of sensation due to nerve damage is often seen in  leprosy.    DIRECTIONS (Questions 137-158): Select the ONE lettered option  that is MOST closely associated with the numbered items. Each lettered  option may be selected once, more than once, or not at all.   143. Expanding, bull’s eye-shaped red rash in a 6-year-old boy after  a camping trip   146. Causes a pseudomembrane in the throat, which can cause  respiratory tract obstruction   148. Is frequently implicated in nosocomial infections, is an impor¬  tant cause of community-acquired pneumonia in adults, and  has a thick, mucoid capsule   156. Oxidase-positive gram-negative rod that is an important cause  of wound and burn infections   158. Anaerobic gram-negative rod that is an important cause of  peritonitis   DIRECTIONS (Questions 159-192): Select the ONE lettered answer  that is BEST in each question.   159. Viruses enter cells by adsorbing to specific sites on the outer  membrane of cells. Each of the following statements regarding  this event is correct EXCEPT:   (B) The interaction determines whether the purified genome of  a virus is infectious.   (D) If the sites are occupied, interference with virus infection  occurs.   160. Many viruses mature by budding through the outer membrane  of the host cell. Each of the following statements regarding  these viruses is correct EXCEPT:   (B) Some new viral antigens appear on the surface of the host  cell.   161. Biochemical analysis of a virus reveals the genome to be composed  of eight unequally sized pieces of single-stranded RNA, each of  which is complementary to viral mRNA in infected cells. Which  one of the following statements is UNLIKELY to be correct?   (A) Different proteins are encoded by each segment of the viral  genome.   (B) The virus particle contains a virus-encoded enzyme that can  copy the genome into its complement.   (D) The virus can acquire new antigens via reassortment of its  RNA segments.   162. Latency is an outcome particularly characteristic of which one  of the following virus groups?   163. Each of the following statements concerning viral serotypes is  correct EXCEPT:   (A) In naked nucleocapsid viruses, the serotype is usually deter¬  mined by the outer capsid proteins.   (B) In enveloped viruses, the serotype is usually determined by  the outer envelope proteins, especially the spike proteins.   (D) Some viruses have an RNA polymerase that determines the  serotype.   164. The ability of a virus to produce disease can result from a vari¬  ety of mechanisms. Which one of the following mechanisms is  LEAST likely?   (C) Immune response to virus-induced antigens on the surface  of infected cells    165. Which one of the following forms of immunity to viruses would  be LEAST likely to be lifelong?   166. Which one of the following statements concerning alpha, beta,  and gamma interferons is LEAST accurate?   (A) Interferons inhibit a broad range of viruses, not just the virus  that induced the interferon.   (C) Interferons induce the synthesis of a protein kinase that  phosphorylates an elongation factor, thereby inactivating  protein synthesis.   (D) Interferons induce the synthesis of a ribonuclease that  degrades viral mRNA.   167. You have isolated a virus from the stool of a patient with diar¬  rhea and shown that its genome is composed of multiple pieces  of double-stranded RNA. Which one of the following is LEAST  LIKELY to be true?   168. A temperate bacteriophage has been induced from a new patho¬  genic strain of Escherichia coli that produces a toxin. Which one  of the following is the MOST convincing way to show that the  phage encodes the toxin?   (A) Carry out conjugation of the pathogenic strain with a non-  pathogenic strain.   (A) The virion carries an RNA-directed DNA polymerase  encoded by the viral genome.   (B) The viral genome consists of three segments of double-  stranded RNA.   (C) The virion is enveloped and enters cells via an interaction  with specific receptors on the host cell.   (D) During infection, the virus synthesizes a DNA copy of its  RNA, and this DNA becomes covalently integrated into host  cell DNA.   170. A stock of virus particles has been found by electron micros¬  copy to contain 10 8 particles/mL, but a plaque assay reveals only  10 5 plaque-forming units/mL. The BEST interpretation of these  results is that:   (B) A nonpermissive cell line was used for the plaque assay.   171. Reasonable mechanisms for viral persistence in infected indi¬  viduals include all of the following EXCEPT:    (C) Integration of a provirus into the genome of the host   (C) They participate in active transport of nutrients across the  viral envelope membrane.   173. Each of the following statements concerning viral vaccines is  correct EXCEPT:   (A) In live, attenuated vaccines, the virus has lost its ability to  cause disease but has retained its ability to induce neutraliz¬  ing antibody.   (B) In live, attenuated vaccines, the possibility of reversion to  virulence is of concern.   (D) With inactivated vaccines, protective immunity is due mainly  to the production of IgG.   174. The major barrier to the control of rhinovirus upper respiratory  infections by immunization is:   (A) The poor local and systemic immune response to these viruses   175. The feature of the influenza virus genome that contributes  MOST to the antigenic variation of the virus is:   (A) A high G + C content, which augments binding to nucleo-  proteins   176. Which one of the following is the BEST explanation for the  selective action of acyclovir (acycloguanosine) in herpes sim¬  plex virus (HSV)-infected cells?   (A) Acyclovir binds specifically to viral receptors only on the  surface of the HSV-infected cell.   (B) Acyclovir is phosphorylated by a virus-encoded phosphoki-  nase only within HSV-infected cells.   (C) Acyclovir selectively inhibits the RNA polymerase in the  HSV virion.   (D) Acyclovir specifically blocks the matrix protein of HSV,  thereby preventing release of progeny HSV.   (A) Interferon inhibits the growth of both DNA and RNA viruses.   (C) Interferon made by cells of one species acts more effectively  in the cells of that species than in the cells of other species.   178. Each of the following statements concerning the viruses that  infect humans is correct EXCEPT:   (A) Only viruses with a negative polarity RNA genome have a  polymerase in the virion.   (B) The purified nucleic acid of some viruses is infectious, but at  a lower efficiency than the intact virions.    (C) Some viruses contain lipoprotein envelopes derived from the  plasma membrane of the host cell.   (D) The nucleic acid of some viruses is single-stranded DNA and  that of others is double-stranded RNA.   179. Which one of the following statements about virion structure  and assembly is CORRECT?   (A) Most viruses acquire surface glycoproteins by budding  through the nuclear membrane.   (C) The symmetry of virus particles prevents inclusion of any  nonstructural proteins, such as enzymes.   (D) Enveloped viruses use a matrix protein to mediate interac¬  tions between viral glycoproteins in the plasma membrane  and structural proteins in the nucleocapsid.   (B) The proteins on the surface of the virus mediate the entry of  the virus into host cells.   (C) Neutralizing antibody is directed against proteins on the  surface of the virus.   (C) Viruses must degrade host cell DNA in order to obtain  nucleotides.   (D) Enveloped viruses require host cell membranes to obtain  their envelopes.   182. Each of the following statements concerning lysogeny is correct  EXCEPT:   183. Each of the following viruses possesses an outer envelope of  lipoprotein EXCEPT:   184. Which one of the following viruses possesses a genome of  single-stranded RNA that is infectious when purified?   185. Each of the following viruses possesses an RNA polymerase in  the virion EXCEPT:    186. Each of the following viruses possesses a DNA polymerase in  the virion EXCEPT:   187. Each of the following viruses possesses double-stranded nucleic  acid as its genome EXCEPT:   (A) They are defective viruses that are missing the DNA coding  for the matrix protein.   (B) They consist of RNA without a protein or lipoprotein outer  coat.   (D) They require an RNA polymerase in the particle for replica¬  tion to occur.   189. Each of the following statements about both measles virus and  rubella virus is correct EXCEPT:   190. Each of the following statements about both influenza virus and  rabies virus is correct EXCEPT:   (D) They do not integrate their genome into host cell DNA.   192. Each of the following statements about human immunodefi¬  ciency virus (HIV) is correct EXCEPT:   (C) A DNA copy of the HIV genome integrates into host cell  DNA.   DIRECTIONS (Questions 193-211): Select the one lettered option  that is MOST CLOSELY associated with the numbered items. Each let¬  tered option may be selected once, more than once, or not at all.   202. Part of the genome DNA is synthesized by the virion polymerase.   203. The translation product of viral mRNA is a polyprotein that is  cleaved to form virion structural proteins.   208. Enveloped RNA virus that has the surface antigen of another  virus   209. Enveloped RNA virus that is the most common cause of non-A,  non-B hepatitis   210. Nonenveloped RNA virus that is transmitted by the fecal-oral  route   211. Purified surface protein of this virus is the immunogen in a  vaccine    DIRECTIONS (Questions 212-275): Select the ONE lettered answer  that is BEST in each question.   213. Each of the following pathogens is likely to establish chronic or  latent infection EXCEPT:   214. Each of the following statements regarding poliovirus and its  vaccine is correct EXCEPT:   (B) Pathogenesis by poliovirus primarily involves the death of  sensory neurons.   (C) The live, attenuated vaccine contains all three serotypes of  poliovirus.   (D) An unimmunized adult traveling to countries where there is  a known risk of being infected with poliovirus should receive  the inactivated vaccine.   215. Which one of the following strategies is MOST likely to induce  lasting intestinal mucosal immunity to poliovirus?   216. Each of the following clinical syndromes is associated with  infection by picornaviruses EXCEPT:   217. Each of the following statements concerning rubella vaccine is  correct EXCEPT:   (A) The vaccine prevents reinfection, thereby limiting the spread  of virulent virus.   (C) The vaccine induces antibodies that prevent dissemination  of the virus by neutralizing it during the viremic stage.   (D) The incidence of both childhood rubella and congenital  rubella syndrome has decreased significantly since the  advent of the vaccine.   218. Each of the following statements concerning the rabies vaccine  for use in humans is correct EXCEPT:   (B) If your patient is bitten by a wild animal (e.g., a skunk) the  rabies vaccine should be given.   (C) When the vaccine is used for postexposure prophylaxis,  rabies immune globulin should also be given.   (D) The virus in the vaccine is grown in human cell cultures, thus  decreasing the risk of allergic encephalomyelitis.    219. Each of the following statements concerning influenza is  correct EXCEPT:   (A) Major epidemics of the disease are caused by influenza A  viruses rather than influenza B and C viruses.   (B) Likely sources of new antigens for influenza A viruses are the  viruses that cause influenza in animals.   (C) Major antigenic changes (shifts) of viral surface proteins are  seen primarily in influenza A viruses rather than in influenza B  and C viruses.   (D) The antigenic changes that occur with antigenic drift are due  to reassortment of the multiple pieces of the influenza virus  genome.   220. Each of the following statements concerning the prevention and  treatment of influenza is correct EXCEPT:   (A) The inactivated influenza vaccine contains HjNj virus, whereas  the live, attenuated influenza vaccine contains H 3 N 2 virus.   (B) The vaccine is recommended to be given each year because  the antigenicity of the virus drifts.   (C) Oseltamivir (Tamiflu) is effective against both influenza A  and influenza B viruses.   (D) The main antigen in the vaccine that induces protective anti¬  body is the hemagglutinin.   221. A 6-month-old child develops a persistent cough and a fever.  Physical examination and chest X-ray suggest pneumonia.  Which one of the following organisms is LEAST likely to cause  this infection?   222. A 45-year-old man was attacked by a bobcat and bitten repeat¬  edly about the face and neck. The animal was shot by a com¬  panion and brought back to the public health authorities. Once  you decide to immunize against rabies virus, how would you  proceed?   (D) Use active immunization and follow this with hyperimmune  serum if adequate antibody titers are not obtained in the  patients serum   223. Each of the following statements concerning mumps is correct  EXCEPT:   (A) Mumps virus is a paramyxovirus and hence has a single-  stranded RNA genome.   (C) Mumps orchitis in children prior to puberty often causes  sterility.   (D) During mumps, the virus spreads through the bloodstream  (viremia) to various internal organs.    225. The principal reservoir for the antigenic shift variants of influ¬  enza virus appears to be:   226. The role of an infectious agent in the pathogenesis of kuru was  BEST demonstrated by which one of the following observations?   (A) A 16-fold rise in antibody titer to the agent was observed.   (C) Electron micrographs of the brains of infected individuals  demonstrated intracellular structures resembling paramyxo¬  virus nucleocapsids.   227. A 64-year-old man with chronic lymphatic leukemia develops  progressive deterioration of mental and neuromuscular func¬  tion. At autopsy the brain shows enlarged oligodendrocytes  whose nuclei contain naked, icosahedral virus particles. The  MOST likely diagnosis is:   228. A 20-year-old man, who for many years had received daily  injections of growth hormone prepared from human pitu¬  itary glands, develops ataxia, slurred speech, and dementia. At  autopsy the brain shows widespread neuronal degeneration, a  spongy appearance due to many vacuoles between the cells, no  inflammation, and no evidence of virus particles. The MOST  likely diagnosis is:   229. A 24-year-old woman has had fever and a sore throat for the  past week. Moderately severe pharyngitis and bilateral cervical  lymphadenopathy are seen on physical examination. Which one  of the following viruses is LEAST likely to cause this picture?   230. Scrapie and kuru possess all of the following characteristics  EXCEPT:   (C) High titers of measles antibody are found in cerebrospinal  fluid.    232. The slow virus disease that MOST clearly has immunosuppres¬  sion as an important factor in its pathogenesis is:   233. You think your patient may be in the “window period” of  hepatitis B virus (HBV) infection because his blood tests for  HBs antigen and anti-HBs antibody are negative. Which one of  the following additional tests is MOST useful to establish that  he has been infected with HBV and is in the “window period.”   234. Which one of the following is the MOST reasonable explanation  for the ability of hepatitis B virus to cause chronic infection?   235. The routine screening of transfused blood has greatly reduced  the problem of post-transfusion hepatitis. For which one of the  following viruses has screening eliminated a large number of  cases of post-transfusion hepatitis?   236. A 35-year-old man addicted to intravenous drugs has been a  carrier of HBs antigen for 10 years. He suddenly develops acute  fulminant hepatitis and dies within 10 days. Which one of the fol¬  lowing laboratory tests would contribute MOST to a diagnosis?   237. Which one of the following is the BEST evidence on which to  base a decisive diagnosis of acute mumps disease?   238. Varicella-zoster virus and herpes simplex virus share many  characteristics. Which one of the following characteristics is  NOT shared?   239. Herpes simplex virus and cytomegalovirus share many features.  Which one of the following features is LEAST likely to be shared?    240. The eradication of smallpox was facilitated by several features  of the virus. Which one of the following contributed LEAST to  eradication?   241. Which one of the following statements concerning infectious  mononucleosis is the MOST accurate?   (A) Acyclovir reduces the number of recurrent disease episodes  by eradicating latendy infected cells.   (B) Genital herpes can be transmitted in the absence of apparent  lesions.   (C) Multinucleated giant cells with intranuclear inclusions are  found in the lesions.   (D) Initial disease episodes are generally more severe than recur¬  rent episodes.   243. There are several influenza vaccines administered in the United  States. Regarding these vaccines, which one of the following  statements is LEAST accurate?   (A) One of the vaccines contains purified peptide subunits of  neuraminidase produced in yeast.   (B) One of the vaccines is an inactivated vaccine consisting of  formaldehyde-treated influenza virions.   (C) One of the vaccines contains a temperature-sensitive mutant  of influenza virus that replicates in the nose but not in the  lungs.   (D) Influenza vaccines contain influenza A and B strains but not  C strains.   244. Which of the following is the MOST common lower respiratory  pathogen in infants?   245. Which of the following conditions is LEAST likely to be caused  by adenoviruses?   246. Regarding the serologic diagnosis of infectious mononucleosis,  which one of the following is CORRECT?   (A) A heterophil antibody is formed that reacts with a capsid  protein of Epstein-Barr virus.   (B) A heterophil antibody is formed that agglutinates sheep or  horse red blood cells.   (C) A heterophil antigen occurs that cross-reacts with Proteus  0X19 strains.    247. Herpes simplex virus type 1 (HSV-1) is distinct from HSV-2 in  several different ways. Which one of the following is the LEAST  accurate statement?   (A) HSV-1 causes lesions above the umbilicus more frequently  than HSV-2 does.   (B) Infection by HSV-1 is not associated with any tumors in  humans.   (C) Antiserum to HSV-1 neutralizes HSV-1 much more effec¬  tively than HSV-2.   (D) HSV-1 causes frequent recurrences, whereas HSV-2 infec¬  tion rarely recurs.   248. Which one of the following statements about the src gene and  src protein of Rous sarcoma virus is INCORRECT?   (A) The src protein inactivates a protein encoded by p53, a tumor  suppressor gene.   (B) The src protein is a protein kinase that preferentially phos-  phorylates tyrosine in cellular proteins.   (C) The src protein is required to maintain neoplastic transfor¬  mation of infected cells.   (D) The viral src gene is derived from a cellular gene found in  many vertebrate species.   249. Each of the following statements supports the idea that cel¬  lular proto-oncogenes participate in human carcinogenesis  EXCEPT:   (A) The c-abl gene is rearranged on the Philadelphia chromo¬  some in myeloid leukemias and encodes a protein with  increased tyrosine kinase activity.   (B) The N -myc gene is amplified as much as 100-fold in many  advanced cases of neuroblastoma.   (C) The receptor for platelet-derived growth factor is a trans¬  membrane protein that exhibits tyrosine kinase activity.   (D) The c-Ha-ras gene is mutated at specific codons in several  types of human cancer.   250. Each of the following statements concerning human immuno¬  deficiency virus (HIV) is correct EXCEPT:   (A) Screening tests for antibodies are useful to prevent transmis¬  sion of HIV through transfused blood.   (B) The opportunistic infections seen in AIDS are primarily the  result of a loss of cell-mediated immunity.   (D) The presence of circulating antibodies that neutralize HIV is  evidence that an individual is protected against HIV-induced  disease.   251. Which one of the following statements concerning viral menin¬  gitis and viral encephalitis is CORRECT?   (A) Herpes simplex virus type 2 is the leading cause of viral  meningitis.   (B) Herpes simplex virus type 1 is an important cause of viral  encephalitis.   (C) The spinal fluid protein is usually decreased in viral  meningitis.   (D) The diagnosis of viral meningitis can be made by using the  India ink stain on a sample of spinal fluid.   (A) Coxsackie viruses are enteroviruses and can replicate in both  the respiratory and gastrointestinal tracts.   (B) Influenza viruses have multiple serotypes based on hemag¬  glutinin and neuraminidase proteins located on the envelope  surface.   (C) Flaviviruses are RNA enveloped viruses that replicate in ani¬  mals as well as humans.   (D) Adenoviruses are RNA enveloped viruses that are an impor¬  tant cause of sexually transmitted disease.   253. Which one of the following statements concerning the preven¬  tion of viral disease is CORRECT?   (A) Adenovirus vaccine contains purified penton fibers and is  usually given to children in conjunction with polio vaccine.   (B) Coxsackie virus vaccine contains live virus that induces IgA,  which prevents reinfection by homologous serotypes.   (C) Flavivirus immunization consists of hyperimmune serum  plus a vaccine consisting of subunits containing the surface  glycoprotein.   (D) One of the influenza virus vaccines contains killed virus that  induces neutralizing antibody directed against the  hemagglutinin.   254. Each of the following statements concerning hepatitis C virus  (HCV) and hepatitis D virus (HDV) is correct EXCEPT:   (B) HDV is a defective virus that can replicate only in a cell that  is also infected with hepatitis B virus.   (D) People infected with HCV commonly become chronic carri¬  ers of HCV and are predisposed to hepatocellular  carcinoma.   255. Each of the following statements concerning measles virus is  correct EXCEPT:   (A) Measles virus is an enveloped virus with a single-stranded  RNA genome.   (C) The initial site of measles virus replication is the upper respi¬  ratory tract, from which it spreads via the blood to the skin.   (D) Latent infection by measles virus can be explained by the  integration of provirus into the host cell DNA.   256. Each of the following statements concerning measles vaccine is  correct EXCEPT:   (B) The vaccine should not be given at the same time as the  mumps vaccine because the immune system cannot respond  to two viral antigens given at the same time.   (D) The vaccine should not be given prior to 15 months of age  because maternal antibodies can prevent an immune  response.   257. Each of the following statements concerning rubella is correct  EXCEPT:   (A) Congenital abnormalities occur primarily when a pregnant  woman is infected during the first trimester.   (B) Women who say that they have never had rubella can, nev¬  ertheless, have neutralizing antibody in their serum.    (C) In a 6-year-old child, rubella is a mild, self-limited disease  with few complications.   (D) Acyclovir is effective in the treatment of congenital rubella  syndrome.   258. Each of the following statements concerning rabies and rabies  virus is correct EXCEPT:   (A) The virus has a lipoprotein envelope and single-stranded  RNA as its genome.   (C) In the United States, dogs are the most common reservoir.   (D) The incubation period is usually long (several weeks) rather  than short (several days).   259. Each of the following statements concerning arboviruses is  correct EXCEPT:   (A) The pathogenesis of dengue hemorrhagic shock syndrome is  associated with the heterotypic anamnestic response.   (B) Wild birds are the reservoir for encephalitis viruses but not  for yellow fever virus.   (C) Ticks are the main mode of transmission for both encephali¬  tis viruses and yellow fever virus.   (D) There is a live, attenuated vaccine that effectively prevents  yellow fever.   260. Each of the following statements concerning rhinoviruses is  correct EXCEPT:   (A) Rhinoviruses are picornaviruses (i.e., small, nonenveloped  viruses with an RNA genome).   (B) Rhinoviruses are an important cause of lower respiratory  tract infections, especially in patients with chronic obstruc¬  tive pulmonary disease.   (C) Rhinoviruses do not infect the gastrointestinal tract because  they are inactivated by the acid pH in the stomach.   (D) There is no vaccine against rhinoviruses because they have  too many antigenic types.   261. Each of the following statements concerning herpes simplex  virus type 2 (HSV-2) is correct EXCEPT:   (A) Primary infection with HSV-2 does not confer immunity to  primary infection with HSV-1.   (C) HSV-2 can cause alterations of the cell membrane, leading to  cell fusion and the formation of multinucleated giant cells.   (D) Recurrent disease episodes due to reactivation of latent  HSV-2 are usually more severe than the primary episode.   262. Each of the following statements concerning Epstein-Barr virus  is correct EXCEPT:   (B) The earlier in life primary infection is acquired, the more  likely the typical picture of infectious mononucleosis will be  manifest.   (C) Latently infected lymphocytes regularly persist following an  acute episode of infection.   (D) Infection confers immunity against second episodes of infec¬  tious mononucleosis.   263. Each of the following statements regarding rotaviruses is correct  EXCEPT:   (A) The rotavirus vaccine contains recombinant RNA polymer¬  ase as the immunogen.   (B) Rotaviruses are a leading cause of diarrhea in young  children.   (D) Rotaviruses belong to the reovirus family, which have a  double-stranded, segmented RNA genome.   264. Each of the following statements concerning the antigenicity of  influenza A virus is correct EXCEPT:   (A) Antigenic shifts, which represent major changes in anti¬  genicity, occur infrequently and are due to the reassortment  of segments of the viral genome.   (C) The worldwide epidemics caused by influenza A virus are  due to antigenic shifts.   (D) The protein involved in antigenic drift is primarily the inter¬  nal ribonucleoprotein.   (A) Adenoviruses are composed of a double-stranded DNA  genome and a capsid without an envelope.   (D) Adenoviruses are implicated as a cause of tumors in animals  but not humans.   266. Each of the following statements concerning the prevention of  viral respiratory tract disease is correct EXCEPT:   (A) To prevent disease caused by adenoviruses, a live enteric-  coated vaccine that causes asymptomatic enteric infection is  used in the military.   (B) To prevent disease caused by influenza A virus, an inacti¬  vated vaccine is available for the civilian population.   (D) To prevent disease caused by rhinoviruses, a vaccine contain¬  ing purified capsid proteins is used.   (A) Exogenous stimuli can cause reactivation of herpesvirus  replication in latently-infected cells.   (B) During latency, antiviral antibody is not demonstrable in the  sera of infected individuals.   (C) Reactivation of latent herpesviruses is more common in  patients with impaired cell-mediated immunity than in  immunocompetent patients.   268. Each of the following statements concerning rhinoviruses is  correct EXCEPT:   (A) Rhinoviruses are the most common cause of the common  cold.   (B) Rhinoviruses grow better at 33°C than at 37°C; hence they  tend to cause disease in the upper respiratory tract rather  than the lower respiratory tract.   (C) Rhinoviruses are members of the picornaviruses family and  hence resemble poliovirus in their structure and replication.   (D) The immunity provided by the rhinovirus vaccine is excel¬  lent because there is only one serotype.   269. Which one of the following statements concerning poliovirus  infection is CORRECT?    (C) A skin test is available to determine prior exposure to the  virus.   270. Each of the following statements concerning yellow fever is correct  EXCEPT:   (A) Yellow fever virus is transmitted by the Aedes aegypti mos¬  quito in the urban form of yellow fever.   (B) Infection by yellow fever virus causes significant damage to  hepatocytes.   (C) Nonhuman primates in the jungle are a major reservoir of  yellow fever virus.   271. Which one of the following statements concerning mumps is  CORRECT?   (A) Although the salivary glands are the most obvious sites of  infection, the testes, ovaries, and pancreas can be involved as  well.   (B) Because there is no vaccine against mumps, passive immuni¬  zation is the only means of preventing the disease.   (C) The diagnosis of mumps is made on clinical grounds because  the virus cannot be grown in cell culture and serologic tests  are inaccurate.   (D) Second episodes of mumps can occur because there are two  serotypes of the virus and protection is type-specific.   272. Many of the oncogenic retroviruses carry oncogenes closely  related to normal cellular genes, called proto-oncogenes. Which  one of the following statements concerning proto-oncogenes is  INCORRECT?   (A) Several proto-oncogenes have been found in mutant form in  human cancers that lack evidence for viral etiology.   (B) Several viral oncogenes and their progenitor proto-oncogenes  encode protein kinases specific for tyrosine.   (C) Some proto-oncogenes encode cellular growth factors and  receptors for growth factors.   (A) The CD4 protein on the T-cell surface is one of the receptors  for the virus.   (B) There is appreciable antigenic diversity in the envelope gly¬  coprotein of the virus.   (C) One of the viral genes codes for a protein that augments the  activity of the viral transcriptional promoter.   (D) A major problem with testing for antibody to the virus is its  cross-reactivity with human T-cell lymphoma virus type I.   274. Each of the following statements concerning human immuno¬  deficiency virus (HIV) is correct EXCEPT:   (A) Patients infected with HIV typically form antibodies against  both the envelope glycoproteins (gpl20 and gp41) and the  internal group-specific antigen (p24).   (B) HIV probably arose as an endogenous virus of humans  because HIV proviral DNA is found in the DNA of certain  normal human cells.   (C) Transmission of HIV occurs primarily by the transfer of  blood or semen in adults, and neonates can be infected at the  time of delivery.   (D) The Western blot test is more specific for HIV infection than  the ELISA is.    275. Each of the following statements concerning hepatitis A virus  (HAV) is correct EXCEPT:   (A) The hepatitis A vaccine contains inactivated HAV as the  immunogen.   (C) The diagnosis of hepatitis A is usually made by isolating  HAV in cell culture.   (D) Gamma globulin is used to prevent hepatitis A in exposed  persons.   DIRECTIONS (Questions 276-294): Select the ONE lettered option  that is MOST closely associated with the numbered items. Each lettered  option may be selected once, more than once, or not at all.   290. It is implicated as the cause of carcinoma of the cervix.   292. It is an important cause of pneumonia in immunocompromised  patients.   293. Donated blood containing antibody to this RNA virus should  not be used for transfusion.   DIRECTIONS (Questions 295-317): Select the ONE lettered answer  that is BEST in each question.   295. Which one of the following fungi is MOST likely to be found  within reticuloendothelial cells?   296. Your patient is a woman with a vaginal discharge. You suspect,  on clinical grounds, that it may be due to Candida albicans.   (A) A Gram stain of the discharge should reveal budding yeasts.   (B) Culture of the discharge on Sabouraud’s agar should produce  a white mycelium with aerial conidia.   (C) The clinical laboratory can use germ tube formation to iden¬  tify the isolate as C. albicans.   (D) Antibiotics predispose to Candida vaginitis by killing the  normal flora lactobacilli that keep the vaginal pH low.    297. You have made a clinical diagnosis of meningitis in a  50-year-old immunocompromised woman. A latex agglu¬  tination test on the spinal fluid for capsular polysaccharide  antigen is positive. Of the following organisms, which one  is the MOST likely cause?   298. Fungi often colonize lesions due to other causes. Which  one of the following is LEAST likely to be present as a  colonizer?   299. Your patient complains of an “itching rash” on her abdomen.  On examination, you find that the lesions are red, circular,  with a vesiculated border and a healing central area. You  suspect tinea corporis. Of the following choices, the MOST  appropriate laboratory procedure to make the diagnosis is a:   300. Each of the following statements concerning Cryptococcus  neoformans is correct EXCEPT:   (A) Its natural habitat is the soil, especially associated with  pigeon feces.   (B) Pathogenesis is related primarily to the production of  exotoxin A.   301. A woman who pricked her finger while pruning some rose  bushes develops a local pustule that progresses to an ulcer.  Several nodules then develop along the local lymphatic  drainage. The MOST likely agent is:   303. Fungal cells that reproduce by budding are seen in the  infected tissues of patients with:   306. Which one of the following is NOT a characteristic of  histoplasmosis?   307. Each of the following statements concerning mucormyco¬  sis is correct EXCEPT:   (A) The fungi that cause mucormycosis are transmitted by  airborne asexual spores.   (B) Tissue sections from a patient with mucormycosis show  budding yeasts.   (C) Hyphae typically invade blood vessels and cause necro¬  sis of tissue.   (D) Ketoacidosis in diabetic patients is a predisposing fac¬  tor to mucormycosis.   (C) Thermally dimorphic fungi exist as yeasts at 37°C and  as molds at 25°C.   (D) Both yeasts and molds have a cell wall made of  peptidoglycan.   (A) Yeasts have chitin in their cell walls and ergosterol in  their cell membranes.   (C) Yeasts have eukaryotic nuclei and contain mitochon¬  dria in their cytoplasm.   310. Each of the following statements concerning fungi and  protozoa is correct EXCEPT:   (C) Both fungi and protozoa use flagella as their organ of  motility.   311. You suspect that your patients disease may be caused  by Cryptococcus neoformans. Which one of the follow¬  ing findings would be MOST useful in establishing the  diagnosis?   (A) A positive heterophil agglutination test for the presence  of antigen   (B) A history of recent travel in the Mississippi River valley  area   (A) Candida albicans is a budding yeast that forms pseudo-  hyphae when it invades tissue.   (D) Impaired cell-mediated immunity is an important pre¬  disposing factor to disease.   313. Each of the following statements concerning Coccidioides  immitis is correct EXCEPT:   (A) The mycelial phase of the organism grows primarily in  the soil, which is its natural habitat.   (D) Most infections are symptomatic and require treatment  with amphotericin B.   314. Each of the following statements concerning Histoplasma  capsulatum is correct EXCEPT:   (A) The natural habitat of H. capsulatum is the soil, where  it grows as a mold.   (B) Histoplasma capsulatum is transmitted by airborne  conidia, and its initial site of infection is the lung.   (C) Within the body, H. capsulatum grows primarily intra-  cellularly within macrophages.   (D) Passive immunity in the form of high titer antibodies  should be given to those known to be exposed.   315. Each of the following statements concerning infection  caused by Coccidioides immitis is correct EXCEPT:   (C) More than 50% of clinical isolates are resistant to  amphotericin B.   316. Each of the following statements concerning Blastomyces  dermatitidis is correct EXCEPT:   (A) Blastomyces dermatitidis grows as a mold in the soil in  North America.   (B) Blastomyces dermatitidis is a dimorphic fungus that  forms yeast cells in tissue.   (C) Blastomyces dermatitidis infection is commonly diag¬  nosed by serologic tests because it does not grow in  culture.   317. Aspergillus fumigatus can be involved in a variety of clini¬  cal conditions. Which one of the following is LEAST likely  to occur?   DIRECTIONS (Questions 318-325): Select the ONE lettered option  that is MOST closely associated with the numbered items. Each lettered  option may be selected once, more than once, or not at all.   318. A budding yeast that is a member of the normal flora of the  vagina   319. A dimorphic organism that is transmitted by trauma to the skin   320. A dimorphic fungus that typically is acquired by inhalation of  asexual spores   322. A yeast acquired by inhalation that causes meningitis primarily  in immunocompromised patients   323. A mold that invades blood vessels primarily in patients with  diabetic ketoacidosis   324. A dimorphic fungus that is acquired by inhalation by people  living in certain areas of the southwestern states in the United  States   325. A dimorphic fungus that causes granulomatous skin lesions in  people living in many areas of North America    DIRECTIONS (Questions 326-352): Select the ONE lettered answer  that is BEST in each question.   326. Children at day care centers in the United States have a high rate  of infection with which one of the following?   327. The main anatomic location of Schistosoma mansoni adult  worms is:   328. In malaria, the form of plasmodia that is transmitted from mos¬  quito to human is the:   330. Each of the following parasites has an intermediate host as part  of its life cycle EXCEPT:   331. Each of the following parasites passes through the lung during  human infection EXCEPT:   333. Each of the following parasites is transmitted by mosquitoes  EXCEPT:   334. Pigs or dogs are the source of human infection by each of the  following parasites EXCEPT:   336. Laboratory diagnosis of a patient with a suspected liver abscess  due to Entamoeba histolytica should include:   337. Each of the following statements concerning Toxoplasma gondii  is correct EXCEPT:   (A) Toxoplasma gondii can be transmitted across the placenta to  the fetus.   (D) Toxoplasma gondii can be diagnosed by finding trophozoites  in the stool.   338. Each of the following statements concerning Giardia lamblia is  correct EXCEPT:   (A) Giardia lamblia has both a trophozoite and a cyst stage in its  life cycle.   (B) Giardia lamblia is transmitted by the fecal-oral route from  both human and animal sources.   (D) Giardia lamblia can be diagnosed by the string test in which  a weighted string is swallowed and passes into the upper GI  tract.   (C) Release of merozoites from red blood cells causes periodic  fever and chills.   (D) The principal site of gametocyte formation is the human  gastrointestinal tract.   341. Which one of the following agents can be used to prevent  malaria?   (B) Pneumocystis carinii can be diagnosed by seeing cysts in  tissue.   (D) Pneumocystis carinii symptomatic infections can be pre¬  vented by administering penicillin orally.   343. Each of the following statements concerning Trypanosoma  cruzi is correct EXCEPT:   (C) Trypanosoma cruzi can be diagnosed by seeing amastigotes  in a bone marrow aspirate.   (D) Trypanosoma cruzi typically affects heart muscle, leading to  cardiac failure.   344. Each of the following statements concerning sleeping sickness  is correct EXCEPT:   (C) Sleeping sickness can be diagnosed by finding eggs in the  stool.   345. Each of the following statements concerning kala-azar is correct  EXCEPT:   (D) Kala-azar can be diagnosed by finding amastigotes in bone  marrow.   346. Each of the following statements concerning Diphyllobothrium  latum is correct EXCEPT:   (C) Diphyllobothrium latum causes a megaloblastic anemia due  to vitamin B 12 deficiency.   (D) Diphyllobothrium latum is a tapeworm that has a scolex with  a circle of hooks.   (C) The disease is caused by a parasite whose adult form lives in  dogs’ intestines.   (A) Schistosoma haematobium is acquired by humans when cer-  cariae penetrate the skin.    349. Each of the following statements concerning hookworm infec¬  tion is correct EXCEPT:   (B) Hookworm infection is acquired by humans when filariform  larvae penetrate the skin.   (D) Hookworm infection can be diagnosed by finding the tro¬  phozoite in the stool.   (C) Both dogs and cats are intermediate hosts of A. lumbricoides.   351. Each of the following statements concerning Strongyloides  stercoralis is correct EXCEPT:   (B) Trichinosis is caused by a protozoan that has both a tropho¬  zoite and a cyst stage in its life cycle.   (C) Trichinosis can be diagnosed by seeing cysts in muscle  biopsy specimens.   DIRECTIONS (Questions 353-386): Select the ONE lettered  option that is MOST closely associated with the numbered items.  Each lettered option may be selected once, more than once, or not  at all.   357. Transmitted by the bite of a deer fly or mango fly   DIRECTIONS (Questions 387-474): Select the ONE lettered answer  that is BEST in each question.   387. Which category of hypersensitivity BEST describes hemolytic  disease of the newborn caused by Rh incompatibility?   388. The principal difference between cytotoxic (type II) and  immune complex (type III) hypersensitivity is:   (B) Whether the antibody reacts with the antigen on the cell or  reacts with antigen before it interacts with the cell   389. A child stung by a bee experiences respiratory distress within  minutes and lapses into unconsciousness. This reaction is prob¬  ably mediated by:   390. A patient with rheumatic fever develops a sore throat from  which (3-hemolytic streptococci are cultured. The patient is   374. A nematode (roundworm) acquired when filariform larvae  penetrate the skin   386. Has a scolex with suckers and a circle of hooks   started on treatment with penicillin, and the sore throat resolves  within several days. However, 7 days after initiation of penicillin  therapy, the patient develops a fever of 103°F, a generalized rash,  and proteinuria. This MOST probably resulted from:   391. A kidney biopsy specimen taken from a patient with acute glo¬  merulonephritis and stained with fluorescein-conjugated anti¬  human IgG antibody would probably show:   392. A patient with severe asthma gets no relief from antihistamines.  The symptoms are MOST likely to be caused by:    393. Hypersensitivity to penicillin and hypersensitivity to poison  oak are both:   394. A recipient of a two-haplotype MHC-matched kidney from a  relative still needs immunosuppression to prevent graft rejec¬  tion because:   396. What is the role of class II MHC proteins on donor cells in graft  rejection?   (A) They are the receptors for interleukin-2, which is produced  by macrophages when they attack the donor cells.   (B) They are recognized by helper T cells, which then activate  cytotoxic T cells to kill the donor cells.   (C) They induce the production of blocking antibodies that pro¬  tect the graft.   (A) Are rejected slowly as a result of minor histocompatibility  antigens   (D) Are not rejected if a kidney is grafted, but skin grafts are  rejected   398. Penicillin is a hapten in both humans and mice. To explore  the hapten-carrier relationship, a mouse was injected with  penicillin covalently bound to bovine serum albumin and, at  the same time, with egg albumin to which no penicillin was  bound. Of the following, which one will induce a secondary  response to penicillin when injected into the mouse 1 month  later? (An explanation of the answer to this question is given on  page 741.)   (B) Are different for two tumors of different histologic type even  if induced by the same carcinogen   401. Polyomavirus (a DNA virus) causes tumors in “nude mice”  (nude mice do not have a thymus because of a genetic defect)  but not in normal mice. The BEST interpretation is that:   (B) Natural killer cells can reject polyomavirus-induced tumors  without help from T lymphocytes.   (C) T lymphocytes play an important role in the rejection of  polyomavirus-induced tumors.   (D) B lymphocytes play no role in rejection of polyomavirus-  induced tumors.   402. C3 is cleaved to form C3a and C3b by C3 convertase. C3b is  involved in all of the following EXCEPT:   403. After binding to its specific antigen, a B lymphocyte may switch its:   404. Diversity is an important feature of the immune system. Which  one of the following statements about it is INCORRECT?   (A) Humans can make antibodies with about 10 8 different V H x  V L combinations.   (B) A single cell can synthesize IgM antibody then switch to IgA  antibody.   (C) The hematopoietic stem cell carries the genetic potential to  create more than 10 4 immunoglobulin genes.   (D) A single B lymphocyte can produce antibodies of many dif¬  ferent specificities, but a plasma cell is monospecific.   (B) The destruction of complement in serum by heating at 56°C  for 30 minutes   408. The classic complement pathway is initiated by interaction of  Cl with:   412. A positive tuberculin skin test (a delayed hypersensitivity reac¬  tion) indicates that:   414. A child disturbs a wasp nest, is stung repeatedly, and goes into  shock within minutes, manifesting respiratory failure and vas¬  cular collapse. This is MOST likely to be due to:   (A) Simultaneous insertion of V H genes adjacent to each C H gene   (B) Successive insertion of a V H gene adjacent to different C H  genes   416. Which one of the following pairs of genes is linked on a single  chromosome?   (A) V gene for lambda chain and C gene for kappa chain   (B) C gene for gamma chain and C gene for kappa chain   (C) V gene for lambda chain and V gene for heavy chain   (D) C gene for gamma chain and C gene for alpha chain   418. A primary immune response in an adult human requires  approximately how much time to produce detectable antibody  levels in the blood?    419. The membrane IgM and IgD on the surface of an individual  B cell:   420. During the maturation of a B lymphocyte, the first immuno¬  globulin heavy chain synthesized is the:   421. In the immune response to a hapten-protein conjugate, in  order to get anti-hapten antibodies, it is essential that:   422. In the determination of serum insulin levels by radioimmuno¬  assay, which one of the following is NOT needed?   423. Which one of the following sequences is appropriate for testing  a patient for antibody against the AIDS virus with the ELISA  procedure? (The assay is carried out in a plastic plate with an  incubation and a wash step after each addition except the final  one.)   (C) Enzyme-labeled antibody against human gamma globulin/  patients serum/HIV antigen/enzyme substrate   425. A woman had a high fever, hypotension, and a diffuse macular  rash. When all cultures showed no bacterial growth, a diagnosis  of toxic shock syndrome was made. Regarding the mechanism  by which the toxin causes this disease, which one of the follow¬  ing is LEAST accurate?   (B) The toxin binds to both the class II MHC protein and the  T-cell receptor.   (C) The toxin activates many CD4-positive T cells, and large  amounts of interleukins are released.   (D) The toxin has an A-B subunit structure—the B subunit binds  to a receptor, and the A subunit enters the cells and activates  them.    426. A patient with a central nervous system disorder is maintained  on the drug methyldopa. Hemolytic anemia develops, which  resolves shortly after the drug is withdrawn. This is MOST  probably an example of:   427. Which one of the following substances is NOT released by acti¬  vated helper T cells?   429. Two dissimilar inbred strains of mice, A and B, are crossed to  yield an F[ hybrid strain, AB. If a large dose of spleen cells from  an adult A mouse is injected into an adult AB mouse, which one  of the following is MOST likely to occur? (An explanation of the  answer to this question is given on page 741.)   (B) The spleen cells will survive and will have no effect in the  recipient.   (C) The spleen cells will induce a graft-versus-host reaction in  the recipient.   (D) The spleen cells will survive and induce tolerance of strain A  grafts in the recipient.   430. This question is based on the same strains of mice described in  the previous question. If adult AB spleen cells are injected into a  newborn B mouse, which one of the following is MOST likely to  occur? (An explanation of the answer to this question is given  on page 741.)   (B) The spleen cells will survive without any effect on the  recipient.   (C) The spleen cells will induce a graft-versus-host reaction in  the recipient.   (D) The spleen cells will survive and induce tolerance of strain A  grafts in the recipient.   (B) Are controlled by several genes in the major histocompatibil¬  ity complex   (D) Induce reactions that can cumulatively lead to a strong rejec¬  tion response   (A) They can be assayed by a cytotoxic test that uses antibody  and complement.   (B) One of their two polypeptide chains is a beta-2 microglobulin.   (C) They are encoded by at least three gene loci in the major  histocompatibility complex.   (D) They are found mainly on B cells, macrophages, and acti¬  vated T cells.    433. An antigen found in relatively high concentration in the plasma  of normal fetuses and a high proportion of patients with pro¬  gressive carcinoma of the colon is:   434. An antibody directed against the idiotypic determinants of a  human IgG antibody would react with:   (B) An IgM antibody produced by the same plasma cell that  produced the IgG   435. Which one of the following is NOT true of the gene segments  that combine to make up a heavy chain gene?   (C) V, D, and J segments combine to encode the antigen-binding site.   (D) A V segment and a J segment are preselected by an antigen  to make up the variable-region portion of the gene.   436. When immune complexes from the serum are deposited on  glomerular basement membrane, damage to the membrane is  caused mainly by:   437. If an individual was genetically unable to make J chains, which  immunoglobulin(s) would be affected?   439. The class of immunoglobulin present in highest concentration  in the blood of a human newborn is:   441. Cytotoxic T cells induced by infection with virus A will kill target  cells:   (B) Infected by virus A and identical at class I MHC loci of the  cytotoxic T cells    (C) Infected by virus A and identical at class II MHC loci of the  cytotoxic T cells   (D) Infected with a different virus and identical at class I MHC  loci of the cytotoxic cells   (E) Infected with a different virus and identical at class II MHC  loci of the cytotoxic cells   442. Antigen-presenting cells that activate helper T cells must  express which one of the following on their surfaces?   (A) It can be triggered by infectious agents in absence of  antibody.   (C) It cannot be initiated unless C3b fragments are already  present.   (D) It has the same terminal sequence of events as the classic  pathway.   445. In setting up a complement fixation test to detect antibody  in the patients serum, the reactants should be added in what  sequence? (Ag = antigen; C = complement; EA = antibody-  coated indicator erythrocytes.)   446. Proteins from two samples of animal blood, A and B, were tested  by the double-diffusion (Ouchterlony) test in agar against antibody  to bovine albumin. Which sample(s) contain horse blood? (An  explanation of the answer to this question is given on page 741.)   449. Which one of the following properties of antibodies is NOT  dependent on the structure of the heavy chain constant region?   450. In which one of the following situations would a graft-versus-  host reaction be MOST likely to occur? (Mouse strains A and  B are highly inbred; AB is an F t hybrid between strain A and  strain B.)   (A) Newborn strain A spleen cells injected into a strain B adult   (B) X-irradiated adult strain A spleen cells injected into a strain B  adult   (C) Adult strain A spleen cells injected into an x-irradiated strain  AB adult   (D) Adult strain AB spleen cells injected into a strain A  newborn   451. In a mixed-lymphocyte culture, lymphocytes from person X,  who is homozygous for the HLA-Dw7 allele, are irradiated and  then cultured with lymphocytes from person Z. It is found that  DNA synthesis is NOT stimulated. The proper conclusion to be  drawn is that:   452. A patient skin-tested with purified protein derivative (PPD) to  determine previous exposure to Mycobacterium tuberculosis  develops induration at the skin test site 48 hours later. Histo¬  logically, the reaction site would MOST probably show:   453. Hemolytic disease of the newborn caused by Rh blood group  incompatibility requires maternal antibody to enter the fetal  bloodstream. Therefore, the mediator of this disease is:   454. An Rh-negative woman married to a heterozygous Rh-positive  man has three children. The probability that all three of their  children are Rh-positive is:    455. Which one of the following statements BEST explains the  relationship between inflammation of the heart (carditis) and  infection with group A |3-hemolytic streptococci?   (C) Streptococcal antigens bind to IgE on the surface of heart  tissue and histamine is released.   (D) Streptococci are ingested by neutrophils that release prote¬  ases that damage heart tissue.   456. Your patient became ill 10 days ago with a viral disease.  Laboratory examination reveals that the patients antibodies  against this virus have a high ratio of IgM to IgG. What is your  conclusion?   (A) It is unlikely that the patient has encountered this organism  previously.   (D) It is likely that the patient has an autoimmune disease.   457. If you measure the ability of cytotoxic T cells from an HLA-B27  person to kill virus X-infected target cells, which one of the fol¬  lowing statements is CORRECT?   (B) Only virus X-infected cells of HLA-B27 type will be killed.   458. You have a patient who makes autoantibodies against his own  red blood cells, leading to hemolysis. Which one of the follow¬  ing mechanisms is MOST likely to explain the hemolysis?   (C) Interleukin-2 binds to its receptor on the red cells, which  results in lysis of the red cells.   (D) Complement is activated, and membrane attack complexes  lyse the red cells.   459. Your patient is a child who has no detectable T or B cells. This  immunodeficiency is most probably the result of a defect in   460. The role of dendritic cells during an antibody response is to:   461. The structural basis of blood group A and B antigen specificity  is:   462. Complement can enhance phagocytosis because of the presence  on macrophages and neutrophils of receptors for:   463. The main advantage of passive immunization over active  immunization is that:   464. On January 15, a patient developed an illness suggestive of  influenza, which lasted 1 week. On February 20, she had a  similar illness. She had no influenza immunization during this  period. Her hemagglutination inhibition titer to influenza A  virus was 10 on January 18, 40 on January 30, and 320 on  February 20. Which one of the following is the MOST appropri¬  ate interpretation?   (A) The patient was ill with influenza A on January 15.   (B) The patient was ill with influenza A on February 20.   465. An individual who is heterozygous for Gm allotypes contains  two allelic forms of IgG in serum, but individual lymphocytes  produce only one of the two forms. This phenomenon, known  as “allelic exclusion,” is consistent with:   (A) A rearrangement of a heavy chain gene on only one chromo¬  some in a lymphocyte   (B) Rearrangements of heavy chain genes on both chromosomes  in a lymphocyte   (C) A rearrangement of a light chain gene on only one chromo¬  some in a lymphocyte   (D) Rearrangements of light chain genes on both chromosomes  in a lymphocyte   (D) They are important in the skin test response to Mycobacte¬  rium tuberculosis.   467. Which one of the following is the BEST method of reducing the  effect of graft-versus-host disease in a bone marrow recipient?   468. Regarding Th-1 and Th-2 cells, which one of the following is  LEAST accurate?   (A) Th-1 cells produce gamma interferon and promote cell-  mediated immunity.   (C) Both Th-1 and Th-2 cells have both CD3 and CD4 proteins  on their outer cell membrane.   (D) Before naive Th cells differentiate into Th-1 or Th-2 cells,  they are double-positives (i.e., they produce both gamma  interferon and interleukin-4).    469. Each of the following statements concerning the variable  regions of heavy chains and the variable regions of light chains  in a given antibody molecule is correct EXCEPT:   (A) They are found on the surface of both B and T cells.   (C) They are involved in the presentation of antigen by  macrophages.   471. Which one of the following statements concerning immuno¬  globulin allotypes is CORRECT?   (A) Tolerance is not antigen-specific (i.e., paralysis of the  immune cells results in a failure to produce a response  against many antigens).   (B) Tolerance is more easily induced in T cells than in B cells.   (C) Tolerance is more easily induced in neonates than in adults.   (D) Tolerance is more easily induced by simple molecules than  by complex ones.   (A) The spleen cell component provides the ability to form  antibody.   (B) The myeloma cell component provides the ability to grow  indefinitely.   (C) The antibody produced by a hybridoma cell is IgM, because  heavy chain switching does not occur.   (D) The antibody produced by a hybridoma cell is homogeneous  (i.e., it is directed against a single epitope).   (B) A hapten cannot induce an antibody by itself; rather, it must  be bound to a carrier protein to be able to induce antibody.   (C) In both penicillin-induced anaphylaxis and poison ivy, the  allergens are haptens.   (D) Haptens must be processed by CD8 + cells to become  immunogenic.   DIRECTIONS (Questions 475-535): Select the ONE lettered option  that is MOST closely associated with the numbered items. Each lettered  option may be selected once, more than once, or not at all.   484. Typically takes 7 to 10 days for antibody to appear   The following double-immunodiffusion plate contains antibody pre¬  pared against whole human serum in the center well. Identify the  contents of each peripheral well from the following list (each well to be  used once). (An explanation of the answer to this question is given on  page 741.)   503. Can contain a polypeptide chain not synthesized by a B  lymphocyte   504. Found in highest concentration in the milk of lactating women   In this double-diffusion (Ouchterlony) assay, the center well contains  antibody against whole human serum. The peripheral (numbered)  wells each contain one of the following proteins:   517. Which protein is present in well No. 4? (An explanation of the  answer to this question is given on page 741.)   526. Caused by a defect in the ability of neutrophils to kill  microorganisms   527. Caused by a development defect that results in a profound loss  of T cells    528. Caused by a deficiency in an inhibitor of the Cl component of  complement   530. Caused by a virtual absence of both B and T cells   Explanation of question 398: Bovine serum albumin is the correct  answer because it activates helper T cells that are required to provide  the interleukins needed for a secondary response. Penicillin alone is  incorrect because it is a hapten and cannot activate helper T cells.  Choices B and C refer to egg albumin which can activate helper T cells  but not the ones that were activated by the initial stimulus that con¬  tained penicillin bound to bovine serum albumin.   Explanation of question 429: Spleen cells from the adult donor A will  recognize the B antigen on the recipient’s cells as foreign. Spleen cells  from the adult donor will contain mature CD4 and CD8 cells that will  attack the recipient cells, causing a graft-versus-host reaction; there¬  fore, answer C is correct. Because the recipient is tolerant to antigen A,  the donor A spleen cells will not be destroyed; therefore, answer A is  incorrect. Answer B is incorrect because although the donor cells will   DIRECTIONS (Questions 536-593): Each set of matching ques¬  tions in this section consists of a list of lettered options followed by  several numbered items. For each numbered item, select the ONE let¬  tered option that is MOST closely associated with it. Each lettered  option may be selected once, more than once, or not at all.    survive, they will have an effect on the recipient. Answer D is incorrect  because the recipient is already tolerant to antigen A.   Explanation of question 430: Because the donor AB spleen cells will  not see any foreign antigen in the recipient, no graft-versus-host reac¬  tion will occur; therefore, answer C is incorrect. The immune cells of  the newborn mouse do not have the capability to kill the donor cells;  therefore, answer A is incorrect. Answer D is more correct than  answer B because the donor cells will survive and induce tolerance to  antigen A in the newborn recipient.   Explanation of question 446: There is a line of identity between sam¬  ple A and bovine albumin; therefore, sample A is bovine albumin.  There is a line of identity between sample B and horse albumin; there¬  fore, sample B is horse albumin. The answer to the question is therefore  B. Note that there is a spur formed between the wells containing sample  A and horse albumin and between the wells containing sample B and  bovine albumin. The spur indicates partial identity between the two  proteins. Partial identity means that there are epitopes shared between  the two albumins but that, because they are from different species,  there are epitopes unique to each protein, also. A spur is formed by the  interaction of the subset of antibodies in the anti-bovine serum with  the unique epitopes in bovine albumin. The other lines are formed by  the interaction of the subset of antibodies in the anti-bovine serum  with the epitopes shared by the two albumins.   Explanation of questions 495-498: The center well contains antibody  against whole human serum; therefore, well D must contain whole  human serum because there are multiple lines representing some of the  many proteins in whole human serum. There is a line of identity  between well C and a protein in whole human serum and a line of  partial identity with that same protein and well A. This indicates that  well C contains human IgG and well A contains baboon IgG. The con¬  cept of partial identity is explained above in the discussion of ques¬  tion 446. There is a line of nonidentity between wells B and C;  therefore, well B contains human transferrin, a protein immunologi-  cally distinct from human IgG.   Explanation of questions 514-517: There is a line of identity between  wells 1 and 2; therefore, they contain human serum albumin (HSA).  Note that the line of immunoprecipitate is very close to well 2. This line  would not form if well 2 contained the high concentration of HSA  because it would be a zone of antigen excess and the line only forms in  a zone of equivalence. Therefore, well 2 contains the low concentration  and well 1 contains the high concentration of HSA. There is a line of  partial identity between wells 2 and 3; therefore, well 3 contains sheep  serum albumin (SSA). There is a line of nonidentity between wells 1  and 4 and wells 3 and 4; therefore, well 4 contains human transferrin,  which is immunologically distinct from HSA and SSA.   540. Anatomic location where Actinomyces israelii is most com¬  monly found   542. Its lipid component causes fever and shock by inducing tumor  necrosis factor (TNF)   543. Causes fever and shock by binding to the T-cell receptor   546. Inhibits protein synthesis by blocking formation of the initia¬  tion complex so that no polysomes form   551. The vaccine contains a single serotype of a capsular polysaccha¬  ride coupled to a protein carrier   559. Causes bloody diarrhea; does not ferment lactose and does not  produce H 2 S   569. Nonenveloped virus with a genome composed of single-  stranded, positive-polarity RNA   570. Enveloped virus with a genome composed of two identical  strands of positive-polarity RNA   571. Enveloped virus with a genome composed of double-stranded  DNA and has a DNA polymerase in the virion   572. Enveloped virus with a genome composed of segmented, negative-  polarity, single-stranded RNA   573. Nonenveloped virus with a genome composed of segmented  double-stranded RNA   575. Causes a painful vesicular rash along the course of a thoracic  nerve   585. Dimorphic fungus that enters the body through puncture  wounds in the skin   DIRECTIONS (Questions 594-654): Select the ONE lettered answer  that is BEST in each question.   CASE 1. Your patient is a 20-year-old woman with the sudden onset of  fever to 104°F and a severe headache. Physical examination reveals  nuchal rigidity. You suspect meningitis and do a spinal tap. Gram stain  of the spinal fluid reveals many neutrophils and many gram-negative  diplococci.   594. Of the following bacteria, which one is MOST likely to be the  cause?   595. Additional history reveals that she has had several serious infec¬  tions with this organism previously. On the basis of this, which  one of the following is the MOST likely predisposing factor?   (C) She is deficient in one of the late-acting complement  components.   CASE 2. Your patient is a 70-year-old man with a long history of smok¬  ing who now has a fever and a cough productive of greenish sputum.  You suspect pneumonia, and a chest X-ray confirms your suspicion.   596. If a Gram stain of the sputum reveals very small gram-negative  rods and there is no growth on a blood agar but colonies do  grow on chocolate agar supplemented with NAD and heme,  which one of the following bacteria is the MOST likely cause?   CASE 3. Your patient is a 50-year-old woman who returned yesterday  from a vacation in Peru, where there is an epidemic of cholera. She now  has multiple episodes of diarrhea.   (A) Watery diarrhea without blood, no polys in the stool, and  growth of curved gram-negative rods in the blood culture   (B) Watery diarrhea without blood, no polys in the stool, and no  organisms in the blood culture   (C) Bloody diarrhea, polys in the stool, and growth of curved  gram-negative rods in the blood culture   (D) Bloody diarrhea, polys in the stool, and no organisms in the  blood culture   CASE 4. Your patient is a 55-year-old man who is coughing up green¬  ish blood-streaked sputum. For the past 2 weeks, he has had fever and  night sweats. He thinks he has lost about 10 lb. On physical examina¬  tion, there are crackles in the apex of the right lung, and a chest X-ray  shows a cavity in that location.   (B) Culture of the sputum on blood agar shows no predominant  organism.    (C) Culture of the sputum on Lowenstein-Jensen medium shows  tan colonies after incubation for 4 weeks.   CASE 5. Your patient is a 5-year-old girl with bloody diarrhea and no  vomiting. There is no history of travel outside of San Francisco. Stool  culture grows both lactose-positive and lactose-negative colonies on  EMB agar.   599. Of the following organisms, which one is MOST likely to be the  cause?   CASE 6. Your patient is a 25-year-old woman with acute onset of pain  in her left lower quadrant. On pelvic examination, there is a cervical  exudate and tenderness in the left adnexa. You conclude that she has  pelvic inflammatory disease (PID) and order laboratory tests.   (A) Gram stain of the cervical exudate shows gram-negative  diplococci within polys.   (B) Culture of the cervical exudate on Thayer-Martin agar shows  oxidase-positive colonies.   CASE 7. Your patient is a 22-year-old man with fever, fatigue, and a  new diastolic murmur. You suspect endocarditis and do a blood  culture.   (A) If he had dental surgery recently, one of the most likely  organisms to grow would be a viridans group  streptococcus.   (B) If he is an intravenous drug user, one of the most likely  organisms to grow would be Candida albicans.   (C) If he had colon surgery recently, one of the most likely organ¬  isms to grow would be Enterococcus faecalis.   (D) If he has a prosthetic aortic valve, one of the most likely  organisms to grow would be Streptococcus agalactiae.   In fact, none of the above organisms grew in the blood culture. What  did grow was a gram-positive coccus arranged in clusters. When sub¬  cultured on blood agar, the colonies were surrounded by a zone of clear  hemolysis, and a coagulase test was positive.   602. In view of this, which one of the following is MOST accurate?   (B) He probably lives on a farm and has had contact with preg¬  nant sheep.   (D) He probably has been camping and was bitten by a tick.   CASE 8. Your patient is a 70-year-old woman who had a hysterectomy  for carcinoma of the uterus 3 days ago. She has an indwelling urinary  catheter in place and now has a fever to 39°C, and the urine in the  collection bottle is cloudy. A Gram stain of the urine specimen shows    many neutrophils and gram-positive cocci in chains. You also do a  urine culture.   603. Which one of the following is the MOST likely set of findings  on the urine culture?   CASE 9. Your patient is a 27-year-old woman who was treated with  oral ampicillin for cellulitis caused by Streptococcus pyogenes. Several  days later, she developed bloody diarrhea. You suspect that she may  have pseudomembranous colitis.   604. Regarding the causative organism of pseudomembranous coli¬  tis, which one of the following is the MOST accurate?   (A) It is an anaerobic gram-positive rod that produces  exo toxins.   (B) It is a comma-shaped gram-negative rod that grows best at  41°C.   (C) It is an obligate intracellular parasite that grows in cell cul¬  ture but not on blood agar.   (D) It is a yeast that forms germ tubes when incubated in human  serum at 37°C.   CASE 10. Your patient is a 10-year-old girl who has had pain in her left  arm for the past 5 days. On physical examination, her temperature is  38°C, and there is tenderness of the humerus near her deltoid. On  X-ray of the humerus, an area of raised periosteum and erosion of bone  is seen. You do a blood culture.   (A) Gram-negative rods that grow on EMB agar, forming purple  colonies and a green sheen   (B) Gram-positive cocci that grow on blood agar, causing a clear  zone of hemolysis and are coagulase-positive   (C) Gram-positive rods that grow only anaerobically and form a  double zone of hemolysis on blood agar   (D) Gram-negative diplococci that grow on blood agar, are  oxidase-positive, and ferment maltose   CASE 11. Your patient is a 30-year-old man who is HIV antibody  positive and has a history of Pneumocystis pneumonia 2 years ago. He  now has an ulcerating lesion on the side of his tongue. A Giemsa stain  of the biopsy specimen reveals budding yeasts within macrophages. A  culture of the specimen grows an organism that is a budding yeast at  37°C but produces hyphae at 25°C.   606. Of the following, which one is the MOST likely organism to  cause this infection?   CASE 12. Your patient is a 10-year-old boy who is receiving chemo¬  therapy for acute leukemia. He develops fever, headache, and a stiff  neck, and you make a presumptive diagnosis of meningitis and do a  lumbar puncture. A Gram stain reveals a small gram-positive rod,  and culture of the spinal fluid grows a (3-hemolytic colony on blood  agar.   CASE 13. Ms. Jones calls to say that she, her husband, and their child  have had nausea and vomiting for the past hour or so. Also, they have  had some non-bloody diarrhea. You ask when their last meal together  was, and she says they had a picnic lunch in the park about 3 hours ago.  They have no fever.   (A) Gram stain of the leftover food would show many gram¬  positive cocci in clusters.   (B) Gram stain of the stool would show many gram-negative  diplococci.   (C) KOH prep of the leftover food would show many budding  yeasts.   (D) Acid-fast stain of the stool would show many acid-fast rods.   CASE 14. Your patient is a 9-year-old boy who was sent home from  school because his teacher thought he was acting strangely. This morn¬  ing, he had a seizure and was rushed to the hospital. On physical  examination, his temperature is 40°C and he has no nuchal rigidity. A  computed tomography (CT) scan is normal. A lumbar puncture is  done, and the spinal fluid protein and glucose are normal. A Gram  stain of the spinal fluid reveals no organisms and no polys. He is treated  with various antibiotics but becomes comatose and dies 2 days later.  The blood culture and spinal fluid culture grow no bacteria or fungi.  On autopsy of the brain, eosinophilic inclusion bodies are seen in the  cytoplasm of neurons.   CASE 15. Your patient is a 20-year-old man who was in a fist fight and  suffered a broken jaw and lost two teeth. Several weeks later, he devel¬  oped an abscess at the site of the trauma that drained to the surface of  the skin, and yellowish granules were seen in the pus.   610. Regarding this disease, which one of the following is MOST  accurate?   (A) The causative organism is a gram-positive rod that forms  long filaments.   (B) The causative organism is a comma-shaped gram-negative  rod that produces an exotoxin which increases cyclic AMP.   (C) The causative organism cannot be seen in the Gram stain but  can be seen in an acid-fast stain.   (D) A combination of gram-negative cocci and spirochetes cause  this disease.   CASE 16. Your patient is a 25-year-old man who is HIV antibody  positive and has a CD4 count of 120 cells (normal, 1000-1500). He has  had a mild headache for the past week and vomited once yesterday. On  physical examination, he has a temperature of 38°C and mild nuchal  rigidity but no papilledema. The rest of the physical examination is  negative.    611. Of the following, which one is the MOST likely to be found  on examination of the spinal fluid?   CASE 17. Your patient is a 25-year-old woman with a sore throat  since yesterday. On physical examination, her throat is red, but no  exudate is seen. Two enlarged, tender cervical lymph nodes are  palpable. Her temperature is 101°F. A throat culture reveals no  (3-hemolytic colonies. After receiving this result, you do another  physical examination, which reveals an enlarged spleen. A hetero¬  phil antibody test finds that sheep red blood cells are agglutinated  by the patient’s serum.   612. Which one of the following is the MOST likely cause of  this disease?   CASE 18. Your patient is a 15-year-old boy with migratory poly¬  arthritis, fever, and a new, loud cardiac murmur. You make a  clinical diagnosis of rheumatic fever.   613. Which one of the following laboratory results is MOST  likely to be found in this patient?   (A) A blood culture is positive for Streptococcus pyogenes at  this time.   (B) A throat culture is positive for Streptococcus pyogenes at  this time.   (C) A Gram stain of the joint fluid shows gram-positive  cocci in chains at this time.   614. Which one of the following modes of pathogenesis is  MOST compatible with a diagnosis of rheumatic fever?   (A) Bacteria attach to joint and heart tissue via pili, invade,  and cause inflammation.   (B) Bacteria secrete exotoxins that circulate via the blood to  the joints and heart.   (C) Bacterial antigens induce antibodies that cross-react  with joint and heart tissue.   (D) Bacterial endotoxin induces interleukin-1 and tumor  necrosis factor, which cause inflammation in joint and  heart tissue.   615. Which one of the following approaches is MOST likely to  prevent endocarditis in patients with rheumatic fever?    CASE 19. Your patient is a 10-year-old girl who has leukemia and  is receiving chemotherapy through an indwelling venous catheter.  She now has a fever of 39°C but is otherwise asymptomatic. You  do a blood culture, and the laboratory reports growth of  Staphylococcus epidermidis.   616. Which one of the following results is LEAST likely to be  found by the clinical laboratory?   (A) Gram-positive cocci in clusters were seen on Gram  stain of the blood culture.   (B) Subculture of the blood culture onto blood agar  revealed nonhemolytic colonies.   CASE 20. Your patient is a 25-year-old woman with several pur¬  puric lesions indicative of bleeding into the skin. Her vital signs  are as follows: temperature, 38°C; blood pressure, 70/40; pulse,  140; respiratory rate, 24. You think she has septic shock and do a  blood culture.   617. Which one of the following organisms is LEAST likely to  be the cause of her septic shock?   618. Of the following mechanisms, which one is LEAST likely  to be involved with the pathogenesis of her septic shock?   CASE 21. Your patient is a 55-year-old man with severe cellulitis  of the right leg, high fever, and a teeth-chattering chill. He is a  fisherman who was working on his boat in the waters off the Texas  coast yesterday.   619. Which one of the following organisms is MOST likely to  be the cause of his disease?   CASE 22. Your patient is a 30-year-old woman with facial nerve  paralysis. She also has fever and headache but does not have a stiff  neck. On physical examination, she has a circular, erythematous,  macular rash on the back of her thigh. You suspect that she has  Lyme disease.   620. Of the following, which one is the MOST appropriate test  to order to confirm a diagnosis of Lyme disease?   (B) Stain for inclusion bodies within cells involved in the  rash    CASE 23. Your patient is a 60-year-old man with confusion for 2  months. He has no history of fever or stiff neck. On physical  examination, he was ataxic and his coordination was abnormal. A  diagnosis of tertiary syphilis was made by the laboratory.   621. Of the following tests, which one is the MOST appropriate  to make a diagnosis of tertiary syphilis?   (B) Stain for inclusion bodies in the lymphocytes in the  spinal fluid   (C) Test for antibody in the spinal fluid that reacts with  cardiolipin   CASE 24. Your patient is a 65-year-old man who had an adeno¬  carcinoma of the pancreas that was surgically removed. Several  blood transfusions were given, and he did well until 2 weeks later,  when fever, vomiting, and diarrhea began. Blood and stool cul¬  tures were negative, and the tests for Clostridium difficile and  hepatitis B surface antigen were negative. A liver biopsy revealed  intranuclear inclusion bodies.   CASE 25. Your patient is a 3-year-old girl with fever and pain in  her right ear. On physical examination, the drum is found to be  perforated, and a bloody exudate is seen. A Gram stain of the  exudate reveals gram-positive diplococci.   CASE 26. Your patient is a 70-year-old man with a fever of 40°C  and a very painful cellulitis of the right buttock. The skin appears  necrotic, and there are several fluid-filled bullae. Crepitus can be  felt, indicating gas in the tissue. A Gram stain of the exudate  reveals large gram-positive rods.   CASE 27. Your patient is a 45-year-old woman with a cadaveric  renal transplant that is being rejected despite immunosuppressive  therapy. She is now in renal failure with a blood pH of 7.32. This  morning, she awoke with a pain near her right eye. On physical  examination, her temperature is 38°C, and the skin near her eye is  necrotic. A biopsy specimen of the lesion contains nonseptate  hyphae invading the blood vessels.    CASE 28. Your patient is a 35-year-old man who is HIV antibody  positive and has a CD4 count of 85 cells. He recently had a seizure,  and a magnetic resonance imaging (MRI) scan indicates a lesion  in the temporal lobe. A brain biopsy specimen reveals multinucle-  ated giant cells with intranuclear inclusions.   CASE 29. Your patient is a 40-year-old woman with a severe attack  of diarrhea that began on the airplane while she was returning from  a vacation in the Middle East. She had had multiple episodes of  watery, nonbloody diarrhea and little vomiting. She is afebrile. A  stool culture reveals only lactose-fermenting colonies on EMB agar.   CASE 30. Your patient is a 20-year-old man with a sore throat for  the past 3 days. On physical examination, his temperature is 38°C,  the pharynx is red, and several tender submaxillary nodes are  palpable.   628. Of the following, which one is the MOST likely organism  to cause this infection?   You do a throat culture, and many small, translucent colonies that  are (3-hemolytic grow on blood agar. Gram stain of one of these  colonies reveals gram-positive cocci in chains.   629. Of the following, which one is the MOST likely organism  to cause this infection?   CASE 31. Your patient is a 55-year-old woman with a lymphoma  who is receiving chemotherapy via intravenous catheter. She sud¬  denly develops fever, shaking chills, and hypotension.   631. If a blood culture drawn from the patient described in case  31 grows a gram-negative rod, which one of the following  is the LEAST likely organism to cause this infection?    632. Of the following virulence factors, which one is the MOST  likely to cause the fever and hypotension seen in the patient  described in case 31?   CASE 32. Your patient is a 30-year-old woman who was part of a tour  group visiting a Central American country. The day before leaving,  several members of the group developed fever, abdominal cramps, and  bloody diarrhea.   633. Of the following, which one is the LEAST likely organism to  cause this infection?   A stool culture reveals no lactose-negative colonies on the EMB agar.   634. Which one of the following is the MOST likely organism to  cause this infection?   CASE 33. Your patient is a 78-year-old man who had an episode of  acute urinary retention and had to be catheterized. He then underwent  cystoscopy to determine the cause of the retention. Two days later, he  developed fever and suprapubic pain. Urinalysis revealed 50 white  blood cells and 10 red blood cells per high-power field. Culture of the  urine revealed a thin film of bacterial growth over the entire blood agar  plate, and the urease test was positive.   635. Which one of the following is the MOST likely organism to  cause this infection?   CASE 34. Your patient is a 40-year-old man with a depigmented lesion  on his chest that appeared about a month ago. The skin of the lesion is  thickened and has lost sensation. He has lived most of his life in rural  Louisiana.   636. Of the following tests, which one is the MOST appropriate to do  to reveal the cause of this disease?   (A) Perform a biopsy of the lesion and do an acid-fast stain   (C) Culture on blood agar anaerobically and do a Gram stain   CASE 35. Your patient is a 28-year-old man with third-degree burns  over a large area of his back and left leg. This morning, he spiked a fever  to 40°C and had two teeth-chattering chills. A blood culture grows a  gram-negative rod that is oxidase-positive and produces a blue-green  pigment.    637. Of the following, which one is the MOST likely organism to  cause this infection?   CASE 36. Your patient is a 32-year-old moving-van driver who lives in  St. Louis. He arrived in San Francisco about 10 days ago after picking  up furniture in Little Rock, Dallas, Albuquerque, and Phoenix. He now  has a persistent cough and fever to 101°F, and he feels poorly. On  physical examination, crackles are heard in the left lower lobe, and  chest X-ray reveals an infiltrate in that area.   (B) If dissemination to the bone occurs, this indicates a failure of  his cell-mediated immunity.   (D) The causative organism of this disease exists as a yeast in the  soil.   CASE 37. Your patient is a 25-year-old man with an ulcerated lesion on  his penis that is not painful. You suspect that it may be a chancre.   639. Which one of the following tests is the MOST appropriate to do  with the material from the lesion?   CASE 38. Your patient is a 6-year-old boy with papular and pustular  skin lesions on his face. A serous, “honey-colored” fluid exudes from  the lesions. You suspect impetigo. A Gram stain of the pus reveals many  neutrophils and gram-positive cocci in chains.   641. If you cultured the pus on blood agar, which one of the follow¬  ing would you be MOST likely to see?   (B) Small a-hemolytic colonies containing bacteria that are  resistant to optochin   (D) Small nonhemolytic colonies containing bacteria that grow  in 6.5% NaCl   CASE 39. Your patient is a 66-year-old woman being treated with che¬  motherapy for lymphoma. She develops fever to 38°C and a nonpro¬  ductive cough. A chest X-ray reveals an infiltrate. You treat her  empirically with an appropriate antibiotic. The following day, several  vesicles appear on her chest.    642. Which one of the following viruses is the MOST likely cause of  her disease?   CASE 40. Your patient is a 40-year-old woman with systemic lupus  erythematosus who is being treated with high-dose prednisone during  a flare of her disease. She develops a fever to 38°C and a cough produc¬  tive of a small amount of greenish sputum. On physical examination,  you hear coarse breath sounds in the left lower lobe. Chest X-ray  reveals an infiltrate in that region. Gram stain of the sputum reveals  long filaments of gram-positive rods.   643. Which one of the following organisms is the MOST likely cause  of this disease?   CASE 41. Your patient is a 10-year-old girl with acute leukemia who  responded well to her first round of chemotherapy but not to the most  recent one. In view of this, she had a bone marrow transplant and is on  an immunosuppressive regimen. She is markedly granulocytopenic.  Ten days after the transplant, she spikes a fever and coughs up bloody,  purulent sputum. Chest X-ray shows pneumonia. A wet mount of the  sputum shows septate hyphae with dichotomous (Y-shape) branching.   644. Which one of the following organisms is the MOST likely cause  of this disease?   CASE 42. Your patient is a 30-year-old man with acute onset of fever  to 40°C and a swollen, very tender right femoral node. His blood pres¬  sure is 90/50, and his pulse is 110. As you examine him, he has a teeth -  chattering shaking chill. He returned from a camping trip in the  Southern California desert 2 days ago.   645. Regarding this disease, which one of the following is MOST  accurate?   (A) An aspirate of the node will reveal a small gram-negative rod  with bipolar staining (appears like a “safety pin”).   (B) The organism was probably acquired by eating food con¬  taminated with rodent excrement.   (C) The aspirate of the node should be cultured on Lowenstein-  Jensen agar and an acid-fast stain performed.   (D) The organism causes disease primarily in people with  impaired cell-mediated immunity.   CASE 43. Your patient is a 62-year-old woman with a history of carci¬  noma of the sigmoid colon that was removed 5 days ago. The surgery  was complicated by the escape of bowel contents into the peritoneal  cavity. She now has fever and pain in the perineum and left buttock. On  physical examination, her temperature is 39°C, and myonecrosis with a  foul-smelling discharge is found. A Gram stain of the exudate reveals  gram-negative rods.    646. Of the following, which one is the MOST likely organism to  cause this infection?   CASE 44. Your patient is an 18-year-old woman with a swollen left  ankle. Two days ago, when the ankle began to swell, she thought she  had twisted it playing soccer. However, today she has a fever to 38°C,  and the ankle has become noticeably more swollen, warm, and red. Her  other joints are asymptomatic. You aspirate fluid from the joint.   647. Using the joint fluid, which one of the following procedures is  MOST likely to provide diagnostic information?   CASE 45. Your patient is a 6-year-old boy with a history of several  episodes of pneumonia. A sweat test revealed an increased amount of  chloride, indicating that he has cystic fibrosis. He now has a fever and  is coughing up a thick, greenish sputum. A Gram stain of the sputum  reveals gram-negative rods.   648. Of the following, which one is the MOST likely organism to  cause this infection?   CASE 46. Your patient is a 7-year-old boy with fever, two episodes of  vomiting, and a severe headache that began this morning. He has no  diarrhea. On physical examination, his temperature is 39°C, and  nuchal rigidity is found. Examination of the spinal fluid revealed a  white cell count of 800, of which 90% were lymphs, and a normal con¬  centration of both protein and glucose. A Gram stain of the spinal fluid  revealed no bacteria.   649. Of the following, which one is the MOST likely to cause this  infection?   CASE 47. Your patient is a 22-year-old man who has been on a low-  budget trip to India, where he ate many of the local foods. He has had  a low-grade fever, anorexia, and mild abdominal pain for about a  month. You suspect that he may have typhoid fever.   650. If he does have typhoid fever, which one of the following is the  LEAST likely laboratory finding?   (B) Culture of the stool grows lactose-negative colonies in EMB  agar.   (D) His serum contains antibodies that cause a positive Weil-  Felix reaction.    CASE 48. Your patient is a 30-year-old man who is HIV antibody  positive and has had two episodes of Pneumocystis pneumonia. He now  complains of pain in his mouth and difficulty swallowing. On physical  examination, you find several whitish plaques on his oropharyngeal  mucosa.   651. Regarding the most likely causative organism, which one of the  following statements is MOST accurate?   (A) It is a filamentous gram-positive rod that is part of the nor¬  mal flora in the mouth.   (B) It is an anaerobic gram-negative rod that is part of the nor¬  mal flora in the colon.   (C) It is a yeast that forms pseudohyphae when it invades tissue.   (D) It is a spirochete that grows only in cell culture.   CASE 49. Your patient is a 20-year-old woman with a rash that began  this morning. She has been feeling feverish and anorexic for the past  few days. On physical examination, there is a papular rash bilaterally  over the chest, abdomen, and upper extremities including the hands.  There are no vesicles and no petechiae. Cervical and axillary lymph  nodes were palpable. Her temperature was 38°C. White blood count  was 9000 with a normal differential.   652. Of the following organisms, which one is the MOST likely cause  of her disease?   CASE 50. Your patient is a 10-year-old boy who fell, abraded the skin  of his thigh, and developed cellulitis (i.e., the skin was red, hot, and  tender). Several days later, the infection was treated with a topical anti¬  biotic ointment, and the cellulitis gradually healed. However, 2 weeks  later, he told his mother that his urine was cloudy and reddish, and she  noted that his face was swollen. You suspect acute glomerulonephritis.    653. Regarding the causative organism, what is the MOST  likely appearance of a Gram stain of the exudate from the  skin infection?   654. What is the pathogenesis of the cloudy urine and facial  swelling?    This practice examination consists of two blocks, each  containing 40 microbiology and immunology questions.  You should be able to complete each block in 50 minutes.  The proportion of the questions devoted to bacteriology,  virology, mycology, parasitology, and immunology is  approximately that of the USMLE. As in the USMLE, the  questions are randomly assorted (i.e., they are not grouped  according to subject matter).   All of the questions have between 4 and 10 answer  choices. Each question has a single “BEST” answer; there  are no “EXCEPT” type questions. The answer choices are  listed either in alphabetical order or in order of the length  of the answer. The answer key is located at the end of each  block.   1. A 9-year-old girl was playing soccer when she began to  limp. She has a pain in her leg and points to her upper thigh  when asked where it hurts. Her temperature is 101°F. X-ray  of the femur reveals that the periosteum is eroded. You  order a blood culture. Which one of the following would be  the MOST likely blood culture findings?   (A) Gram-negative rods that grow on EMB agar, forming  purple colonies and a green sheen   (B) Gram-positive cocci that grow on blood agar, causing a  clear zone of hemolysis, and are coagulase-positive   (C) Gram-positive rods that grow only anaerobically and  form a double zone of hemolysis on blood agar   (D) Gram-negative diplococci that grow on chocolate agar,  are oxidase-positive, and ferment maltose   (E) Gram-positive cocci that grow on blood agar, causing  a green zone of hemolysis, and are not inhibited by  optochin and bile   2. Your summer research project is to study the viruses that  cause upper respiratory tract infections. You have isolated    a virus from a patient’s throat and find that its genome is  RNA. Furthermore, you find that the genome is the comple¬  ment of viral mRNA within the infected cell. Of the following,  which one is the MOST appropriate conclusion you could  draw?   (E) The genome RNA encodes a precursor polypeptide that  must be cleaved by a protease.   3. A 25-year-old man has a history of four episodes of boils in  the last year. Boils are abscesses caused by Staphylococcus  aureus. Which one of the following is MOST likely to be  the underlying immunologic factor that predisposes him to  multiple episodes of boils?   (A) A deficient amount of the C8 component of complement  in his plasma   (B) An inability of his macrophages to present antigen in  association with class I MHC proteins   (C) A failure to release granzymes from his cytotoxic T cells   4. You are reading an article that says that otitis media is com¬  monly caused by nonencapsulated strains of Haemophilus  influenzae. You are surprised that nonencapsulated strains  can cause this disease. Which one of the following BEST  explains why your surprise is justified?   (E) Nonencapsulated strains should be susceptible to killing  by cytotoxic T cells.   5. A 35-year-old man is HIV antibody positive and has a CD4  count of 50/mL (normal, 1000-1500). He has had a fever of  101°F for a few weeks and “feels tired all the time.” He has  no other symptoms, and findings on physical examination  are normal. Complete blood cell count, urinalysis, and chest  X-ray are normal. Blood, stool, and urine cultures show no  growth. A bone marrow biopsy reveals granulomas, and a  culture grows an organism that is a budding yeast at 37°C   6. A 70-year-old woman has sustained third-degree burns  over a significant area of her body. Despite appropriate burn  care in the hospital, she spiked a fever to 39°C, and the nurse  reports blue-green pus on the dressing covering the burned  area. Gram stain of the pus reveals gram-negative rods, and  antibiotic sensitivity tests show resistance to most antibiotics.  Which one of the following organisms is MOST likely to  cause this disease?   7. A 20-year-old woman has had several episodes of high fever,  shaking chills, and a severe headache. She has a hematocrit of  30%. She has recently returned from Africa, where she was a  Peace Corps volunteer. Which one of the following is MOST  likely to be seen in the blood smear sample from this patient?   8. Certain microorganisms, such as the protozoan Try¬  panosoma and the bacterium Neisseria gonorrhoeae, can  change their surface antigens quite frequently. This allows  the organisms to evade our host defenses. Which one of  the following BEST explains how this frequent change in  antigenicity occurs?   (A) It is due to the transposition of existing genes into an  active expression site.   (B) It is due to the acquisition of new fertility plasmids by  transduction.   (C) It is due to conjugation, during which the recipient  obtains new chromosomal genes.   (D) It is due to new mutations that occur at “hot spots” in  the genome.   9. A 60-year-old woman had an adenocarcinoma of the colon  that was surgically removed. Several blood transfusions  were given, and she did well until 3 weeks after surgery,  when fever, vomiting, and diarrhea began. Blood and stool  cultures were negative for bacteria, and the tests for Clostrid¬  ium difficile and hepatitis B surface antigen were negative.  A liver biopsy revealed intranuclear inclusion bodies. Which  one of the following is the MOST likely cause?   10. Which one of the immunoglobulins BEST fits the following  description: It is found in plasma as a dimer with a J chain.    As it passes through mucosal cells, it acquires a secretory  piece that protects it from degradation by proteases.   11. Mycobacterium tuberculosis (MTB) and Mycobacterium  avium-complex (MAC) are important causes of disease,  especially in immunocompromised patients. (MAC is also  known as Mycobacterium avium-intracellulare.) Regarding  MTB and MAC, which one of the following statements is  the MOST accurate?   (A) Cell-mediated immunity is the most important host  defense mechanism against MTB, whereas antibody-  mediated immunity is the most important host defense  mechanism against MAC.   (B) In the clinical laboratory, MAC can be distinguished from  MTB by the fact that MAC forms colonies in 7 days,  whereas MTB does not.   (C) Multidrug-resistant strains of MAC are much less com¬  mon than multidrug-resistant strains of MTB.   (D) MAC is found in the environment and is not transmitted  from person to person, whereas MTB is found in humans  and is transmitted from person to person.   12. In the laboratory, a virologist was studying the properties of  mutant viruses. When she infected cells with mutant virus #1,  no progeny viruses were produced. When she infected cells  with mutant virus #2, no progeny viruses were produced. But  when she infected cells with both mutant virus #1 and mutant  virus #2, progeny viruses of both virus #1 and virus #2 were  produced. Which one of the following is the term that BEST  describes this phenomenon?   13. Your patient has been treated for endocarditis with penicillin G  for the past 2 weeks. She now has a fever and maculopapular  erythematous rash over her chest and abdomen. A urinalysis  shows significant protein in the urine. If the fever, rash, and  proteinuria are immunologic in origin, which one of the fol¬  lowing is MOST likely to be involved?   14. Endotoxin is an important underlying cause of septic shock  and death, especially in hospitalized patients. Regard¬  ing endotoxin, which one of the following is the MOST  accurate?   (E) It can be treated with formaldehyde to form an effective  toxoid vaccine.   15. A 12-year-old girl had a seizure this morning and was  rushed to the hospital. On examination, her temperature    was 40°C, and she had no nuchal rigidity. Computed tomog¬  raphy (CT) scan revealed no abnormality. A spinal tap was  done, and the protein and glucose were normal. Gram stain  of the spinal fluid showed no organisms and no polys. She  was treated with various antibiotics but became comatose  and died 2 days later. The routine blood culture and spinal  fluid culture grew no organism. On autopsy of the brain,  eosinophilic inclusion bodies were seen in the cytoplasm  of neurons. Of the following, which one is the MOST likely  cause?   16. A 70-year-old woman presents with rapid onset of fever to  39°C and a cough productive of greenish sputum. She is not  hospitalized and not immunocompromised. A chest X-ray  reveals a left lower lobe infiltrate. Of the following, which set  of findings describes the MOST likely causative organism  found in the sputum culture?   17. Regarding the function of chemokines in host defenses,  which one of the following is the MOST accurate?   (A) Chemokines bind to the T-cell receptor outside of the  antigen-binding site and activate many T cells.   (B) Chemokines induce gene switching in B cells that  increases the amount of IgE synthesized, thereby predis¬  posing to allergies.   (C) Chemokines penetrate the membranes of target cells  during attack by cytotoxic T cells.   (D) Chemokines attract neutrophils to the site of bacterial  infection, thereby playing a role in the inflammatory  response.   18. Which one of the following answer choices consists of  bacteria, BOTH of which produce exotoxins that act by  ADP-ribosylation?   19. Regarding hepatitis C virus (HCV) and hepatitis D virus  (HDV), which one of the following is MOST accurate?   (B) More than half of HCV infections result in a chronic  carrier state.   (C) There is an effective vaccine against HCV but not  against HDV.   (D) Both HCV and HDV are defective RNA viruses and  require concurrent HBV infection to replicate.    20. Which one of the following is MOST likely to induce an  IgM antibody response without the participation of helper  T cells?   21. A 25-year-old pregnant woman in the third trimester  comes to the emergency room saying that about 12 hours  ago she began to feel feverish and weak. On examination,  she has a temperature of 40°C but no other pertinent find¬  ings. A blood culture grows small gram-positive rods that  cause (3-hemolysis on a blood agar plate incubated in room  air. Which one of the following bacteria is the MOST likely  cause?   22. Regarding the mode of action of antiviral drugs, which one  of the following is MOST accurate?   (A) Amantadine inhibits influenza A virus by inhibiting the  RNA polymerase carried by the virion.   (B) Foscarnet inhibits varicella-zoster virus by inhibiting  the RNA polymerase carried by the virion.   (C) Acyclovir action is greater in herpesvirus-infected cells  than in uninfected cells because herpesvirus-infected  cells contain an enzyme that phosphorylates acyclovir  very efficiently.   (D) Azidothymidine inhibits human immunodeficiency  virus (HIV) by inhibiting viral mRNA synthesis more  efficiently than cellular mRNA synthesis.   (E) Indinavir blocks HIV replication by inhibiting the pro¬  tease required for the envelope protein gpl20 to bind to  the CD8 protein on the surface of the T cell.   23. Which one of the following diseases is MOST likely to be  caused by a delayed hypersensitivity reaction?   24. Members of the genus Mycobacterium stain better with  the acid-fast stain than with the Gram stain. Which one of the  following is the BEST explanation for this finding?   (A) They lack a cell wall; therefore, they cannot adsorb the  crystal violet.   (B) They have a very thin cell wall that does not retain  the crystal violet.   (C) They have a thick polysaccharide capsule that prevents  entry of the iodine solution.   (D) They have a large amount of lipid in their cell wall that  prevents entry of the crystal violet.   25. A 50-year-old man with a cadaveric renal transplant is  rejecting the transplant despite immunosuppressive drugs.  He is now in renal failure with a blood pH of 7.31. Yesterday,  he developed a pain near his left eye that has become pro¬  gressively more severe. On examination, his temperature is    37.5°C, and the skin near his eye is swollen and necrotic.  Microscopic examination of a biopsy of the lesion reveals  non-septate hyphae with right-angle branching. Which one  of the following organisms is the MOST likely cause?   26. A 60-year-old woman had surgery for ovarian carcinoma  4 days ago and has an indwelling urinary catheter in place.  She now spikes a fever to 39°C and has cloudy urine in the  collection bottle. Gram stain of the urine shows many polys  and gram-positive cocci in chains. Which one of the follow¬  ing would be the MOST likely finding in the urine culture?   (A) a-Hemolytic colonies on the blood agar plate that are  optochin-sensitive   (B) (3-Hemolytic colonies on the blood agar plate that are  bacitracin-sensitive   (C) (3-Hemolytic colonies on the blood agar plate that  hydrolyze hippurate   (D) Nonhemolytic colonies on the blood agar plate that  grow in 6.5% sodium chloride   27. Your patient is a 40-year-old man with a history of confusion  for the past 2 days and a grand mal seizure that occurred this  morning. He is HIV antibody positive and has a CD4 count  of 100/mL. On examination, his temperature is 37.5°C, and  the findings of the remainder of the examination are within  normal limits. Magnetic resonance imaging (MRI) reveals  several “ring-enhancing” cavitary brain lesions. He has not  traveled outside of the United States, is employed as the  manager of a supermarket, is a strict vegetarian, and has  several household pets, namely, a dog, a cat, a parrot, and a  turtle. Which one of the following organisms is the MOST  likely cause?   28. The emergence of antibiotic-resistant bacteria, especially in  enteric gram-negative rods, is an extremely important phe¬  nomenon. The acquisition of resistance most commonly  occurs by a process that involves a sex pilus and the sub¬  sequent transfer of plasmids carrying one or more trans-  posons. Which one of the following is the name that BEST  describes this process?   29. Regarding the diagnosis, treatment, and prevention of HIV,  which one of the following is the MOST accurate?   (A) The drug zidovudine (AZT) is a “chain terminating”  drug; that is, it inhibits the growing polypeptide chain  by causing misreading of the viral mRNA.   (B) The drug lamivudine (3TC) acts by binding to the inte-  grase, which prevents integration of the viral DNA into  cellular DNA.    (C) In the screening test for HIV infection, the enzyme-  linked immunosorbent assay (ELISA) test detects the  presence of antibody to the p24 protein of HIV.   (D) A major limitation to our ability to produce a vaccine  against HIV is that there are many serologic types of the  viral p24 protein.   (A) They are typically polypeptides that are resistant to  proteolytic cleavage within the antigen-presenting  cell.   (B) They bind to class II MHC proteins but not to class I  MHC proteins.   (C) They cannot induce antibodies unless they are bound to  a carrier protein.   (D) They activate complement by binding to the Fc part of  the heavy chain of IgG.   31. Your patient is a 20-year-old man with a urethral discharge.  Gram stain of the pus reveals many neutrophils but no bac¬  teria. Which one of the following organisms is the MOST  likely cause?   32. Regarding host defenses against viruses, which one of the  following is MOST accurate?   (A) IgA exerts its main antiviral effect by enhancing the  cytopathic effect of natural killer cells—a process called  antibody-dependent cellular cytotoxicity.   (B) IgG plays a major role in neutralizing virus infectivity  during the primary infection.   (C) Complexes of virus and IgE are the cause of the inflam¬  matory arthritis seen in several viral infections, such as  hepatitis B and rubella.   (D) Alpha and beta interferons exert their antiviral action  by inducing a ribonuclease that degrades viral mRNA  and a protein kinase that inactivates protein  synthesis.   (E) Alpha and beta interferons exert their antiviral effect  against viruses with RNA genomes but not against  those with DNA genomes.   33. Allergic rhinitis is characterized by sneezing, rhinorrhea,  nasal congestion, and itching of the eyes and nose. Persons  with allergic rhinitis have “X” that binds to high-affinity  receptors on “Y.” On reexposure to antigen, the “Y” of patients  with allergic rhinitis degranulate, releasing “Z” and other  mediators. Which one of the following sets BEST describes  X, Y, and Z?   (A) X is IgE, Y is macrophages, and Z is tumor necrosis  factor.   (B) X is IgE, Y is basophils, and Z is histamine.   (C) X is IgG, Y is eosinophils, and Z is histamine.   (D) X is IgG, Y is neutrophils, and Z is tumor necrosis  factor.   (E) X is IgA, Y is eosinophils, and Z is interleukin-5.   34. An outbreak of postsurgical wound infections caused by  Staphylococcus aureus has occurred. The infection control  team was asked to determine whether the organism could    be carried by one of the operating room personnel. Using  your knowledge of normal flora, which one of the following  body sites is the MOST likely location for this organism?   35. A 35-year-old man who is HIV antibody positive and has  a CD4 count of 30 says, “I can’t remember the simplest  things.” You are concerned about dementia. An MRI indi¬  cates several widely scattered lesions in the brain. Over the  next 4 months, he develops visual field defects, becomes  paralyzed, and dies. Autopsy reveals that many neurons of  the brain have lost myelin and contain intranuclear inclu¬  sions. Electron microscopy reveals the inclusions contain  nonenveloped viruses. Which one of the following viruses  is the MOST likely cause?   36. A 75-year-old man with substernal chest pain was found  to have angina pectoris caused by syphilitic aortitis that  affected his coronary arteries. Of the following, which one  is the MOST likely way that the diagnosis of syphilis was  made?   37. A 22-year-old woman has an erythematous rash on the malar  eminences of her face that gets worse when she is out in the  sun. She has lost about 10 lb and feels tired much of the time.  She took her temperature a few times, and it was 99°F. Physical  examination was normal except for the rash. Laboratory tests  revealed a hemoglobin of 11 and a white blood cell count  of 5500. Urinalysis showed albumin in the urine but no red  cells, white cells, or bacteria. Which one of the following is  the MOST likely laboratory finding in this disease?   38. Regarding antimicrobial drugs that act by inhibiting nucleic  acid synthesis in bacteria, which one of the following is the  MOST accurate?   (A) Quinolones, such as ciprofloxacin, inhibit the RNA  polymerase in bacteria by acting as nucleic acid  analogues.   (B) Rifampin inhibits the RNA polymerase in bacteria by  binding to the enzyme and inhibiting messenger RNA  synthesis.   (C) Sulfonamides inhibit the DNA polymerase in bacteria  by causing chain termination of the elongating strand.   (D) Trimethoprim inhibits the DNA polymerase in bacteria  by preventing the unwinding of double-stranded DNA.    39. Regarding parvovirus B19, which one of the following is the   (A) Parvovirus B19 has a double-stranded DNA genome  but requires a DNA polymerase in the virion because it  replicates in the cytoplasm.   (C) Parvovirus B19 causes severe anemia because it prefer¬  entially infects erythrocyte precursors.   (D) Patients infected by parvovirus B19 can be diagnosed in  the laboratory using the cold agglutinin test.   (E) Patients with disseminated disease caused by parvovirus  B19 should be treated with acyclovir.   40. Which one of the following laboratory tests would be the   1. A 4-year-old girl has papular and pustular lesions on her  face. The lesions are exuding a honey-colored serous fluid.  You make a clinical diagnosis of impetigo. A Gram stain  of the exudate reveals gram-positive cocci in chains, and a  culture reveals |3-hemolytic colonies on blood agar. For  which one of the following sequelae is she MOST at risk?   2. The purified genome of certain RNA viruses can enter a  cell and elicit the production of progeny viruses (i.e., the  genome is infectious). Regarding these viruses, which one  of the following statements is MOST accurate?   (E) Their genome RNA has the same base sequence as mRNA.    3. A 77-year-old man with enterococcal endocarditis  needed to be treated with penicillin G but had a history  of a severe penicillin reaction. He was therefore skin  tested using penicilloyl-polylysine as the antigen. Which  one of the following is MOST likely to occur in a positive  skin test?   (D) The antigen activates CD8-positive T cells by binding to  class I MHC proteins.   (E) The antigen cross-links IgE on the mast cells and causes  the release of histamine.   4. Regarding the Gram stain, which one of the following is the  MOST accurate?   (A) After adding crystal violet and Grams iodine, both  gram-positive bacteria and gram-negative bacteria will  appear blue.   (B) If you forget to stain with the red dye (safranin or basic  fuchsin), both gram-positive bacteria and gram-negative  bacteria will appear blue.   (C) If you forget to heat-fix, both gram-positive bacteria  and gram-negative bacteria will appear blue.   (D) One reason why bacteria have a different color in this  stain is because the gram-positive bacteria have lipid  in their membrane, whereas gram-negative bacteria  do not.   5. A 35-year-old man with a CD4 count of 50 presents with  a skin nodule on his chest. The nodule is about 3 cm in  diameter and is not red, hot, or tender. He says it has been  slowly growing bigger for the past 3 weeks. You biopsy the  nodule, and the pathologist calls to say that the patient has  disseminated cryptococcosis. Which one of the following  is the BEST description of what the pathologist saw in the  biopsy specimen?   6. A 22-year-old woman complains of a persistent nonproduc¬  tive cough and a fever of 101°F that came on slowly over  the last 4 days. Physical examination reveals some rales in  the left lung base. A patchy infiltrate is seen on chest X-ray.  She works as a secretary in a law office and has not traveled  recently. She is not immunocompromised and has not been  hospitalized recently. A sample of her serum agglutinates  red blood cells at 4°C but not at 37°C. Which one of the  following BEST describes the organism that is the MOST  likely cause of her disease?   (E) A spirochete that has never been grown on blood agar    7. The mother of a 4-year-old child notes that her child is  sleeping poorly and scratching his anal area. You suspect  the child may have pinworms. Which one of the following  is the BEST method to make that diagnosis?   (C) They are killed by boiling at sea level but not at high  altitude.   (D) They are produced by anaerobes only in the presence of  oxygen.   (E) They contain endotoxin, which accounts for their  ability to cause disease.   9. A 22-year-old woman had fever to 100°F and anorexia for  the past 2 days, and this morning she appears jaundiced. On  examination, her liver is enlarged and tender. She has a total  bilirubin of 5 mg/dL (normal, <1) and elevated transami¬  nases. She received the complete course of the hepatitis B  vaccine 2 years ago but has not had the hepatitis A vaccine.  The results of her hepatitis serologies are as follows: HAV-  IgM negative, HAV-IgG positive, HBsAg negative, HBsAb  positive, HBcAb negative, HCV-Ab positive. Of the  following, which one is the MOST accurate?   (A) She probably has hepatitis A now, probably has not been  infected with hepatitis B virus (HBV), and probably had  hepatitis C in the past.   (B) She probably has hepatitis A now, probably has been  infected with HBV in the past, and probably had  hepatitis C in the past.   (C) She has been infected with hepatitis A virus (HAV) in  the past, probably has not been infected with HBV, and  probably has hepatitis C now.   (D) She has been infected with F1AV in the past, probably  has hepatitis B now, and probably had hepatitis C in the  past.   10. Regarding the function of the different classes of antibod¬  ies, which one of the following statements is the MOST  accurate?   (A) IgA acts as an antigen receptor on the surface of B  cells.   (B) IgG activates the alternative pathway of complement,  resulting in the production of C3a that degrades the  bacterial cell wall.   (C) IgG binds to the bacterial surface and makes the bacteria  more easily ingested by phagocytes.   (E) IgE blocks the binding of viruses to the gut mucosa.    11. A 6-year-old boy fell and sustained a deep wound from a  rusty nail that penetrated his thigh. His mother removed  the nail and cleaned the wound with soap and water. The  next morning, he had a temperature of 102°F, and his  thigh was very painful and swollen. In the emergency  room, crepitus (gas in the tissue) was noted. A Gram stain  of exudate from the wound area revealed large gram¬  positive rods. Which one of the following is the MOST  likely cause?   12. The two most common types of viral vaccines are killed vac¬  cines and live, attenuated vaccines. Regarding these vaccines,  which one of the following statements is the MOST accurate?   (A) Killed vaccines induce a longer-lasting response than  do live, attenuated vaccines.   (B) Killed vaccines are no longer used in this country  because they do not induce secretory IgA.   (C) Killed vaccines induce a broader range of immune  responses than do live, attenuated vaccines.   (D) Killed vaccines are safer to give to immunocompro¬  mised patients than are live, attenuated vaccines.   13. Regarding immediate (type I) and immune complex (type III)  hypersensitivities, which one of the following is the MOST  accurate?   (A) IgE is involved in both immediate and immune com¬  plex hypersensitivities.   (B) Complement is involved in both immediate and  immune complex hypersensitivities.   (C) Less antigen is typically needed to trigger an immediate  reaction than an immune complex reaction.   (D) Neutrophils play a more important role in immediate  reactions than in immune complex reactions.   14. Disease caused by which one of the following bacteria can  be prevented by a toxoid vaccine?   15. A 50-year-old woman has had a gradual onset of headaches  that have become increasingly more severe during the past  3 weeks. On examination, she is confused regarding time,  place, and person, and she is febrile to 39°C. Her spinal  fluid reveals a normal glucose, normal protein, and 17 cells,  all of which were lymphocytes. Gram stain of the spinal  fluid shows no organism. An MRI reveals a 2-cm radio-  lucent lesion in the temporal lobe. A biopsy of the brain    lesion was performed. A Giemsa stain of the tissue shows  multinucleated giant cells with intranuclear inclusion bod¬  ies. Which one of the following is the MOST likely caus¬  ative organism?   16. An 80-year-old man had a carcinoma of the colon removed  3 days ago. He was doing well until this morning, when he  spiked a fever to 39°C and complained of severe abdominal  pain. Examination revealed a “board-like” abdomen indica¬  tive of peritonitis. He was taken to the operating room, where  it was discovered that his anastomosis had broken down and  bowel contents had spilled into the peritoneal cavity. A foul¬  smelling exudate was observed. A Gram stain of the perito¬  neal exudate revealed many gram-negative rods. Which one  of the following sets of bacteria is the MOST likely cause of  this infection?   17. Regarding the primary and secondary antibody responses,  which one of the following statements is MOST accurate?   (A) The IgM made in the primary response is made primar¬  ily by memory B cells.   (B) The lag phase is shorter in the primary response than in  the secondary response.   (C) In the primary response, memory B cells are produced,  but memory T cells are not.   (D) Antigen must be processed and presented in the pri¬  mary response but not in the secondary response.   (E) The amount of IgG made in the secondary response is  greater than the amount made in the primary response.   18. A 70-year-old man who is receiving chemotherapy for  leukemia develops a fever to 40°C and has two episodes  of teeth-chattering chills, and his blood pressure drops to  80/20 mmHg. Of the following factors, which one is MOST  likely to be the cause of his fever, chills, and hypotension?   19. A 22-year-old woman presents with “the worst sore throat  I’ve ever had.” She also complains of fatigue and anorexia.  She is not immunocompromised and has not been hospi¬  talized recently. On examination, she is febrile to 38°C, the    pharynx is inflamed, and there are a few tender cervical  nodes bilaterally. There are no white lesions on the tongue  or pharynx. A throat culture grows a-hemolytic colonies  on blood agar that are optochin-resistant. Of the following,  which one is the MOST likely cause?   (A) C5a mediates chemotaxis and attracts neutrophils to  the site of infection.   (B) C5b plays an important role in the opsonization of  gram-negative bacteria.   (C) C3a is a decay-accelerating factor that causes the rapid  decay and death of bacteria.   (D) Cl binds to the surface of gram-positive bacteria, which  initiates the classic pathway.   (E) The membrane attack complex is produced in the classic  pathway but not in the alternative pathway.   21. A 65-year-old woman had symptoms of dementia. An MRI  revealed significant cortical atrophy. It was determined that  her intraventricular pressure was very high, and a ventric¬  uloperitoneal shunt (from the brain, tunneling under the  skin into the peritoneal cavity) was placed to relieve the  pressure. Three weeks later, she developed a fever to 38°C,  malaise, and anorexia but no other symptoms. Of the fol¬  lowing, which one BEST describes the MOST likely organ¬  ism causing her current symptoms?   (C) An acid-fast rod that does not grow on bacteriologic media   (D) An obligate intracellular parasite that forms a cytoplasmic  inclusion body   (E) A spirochete that induces an antibody that agglutinates  a lipid from a cows heart   22. Two mutants of poliovirus, one mutated at gene X and  the other mutated at gene Y, have been isolated. If a cell is  infected with each mutant alone, no virus is produced. If a  cell is infected with both mutants, which one of the follow¬  ing is MOST likely to occur?   (A) Complementation between the mutant gene products  may occur, and, if so, both X and Y progeny viruses will  be made.   (B) Phenotypic mixing may occur, and, if so, both X and Y  progeny viruses will be made.   (C) Reassortment of the genome segments may occur, and,  if so, both X and Y progeny viruses will be made.   (D) The genome may be transcribed into DNA, and, if so,  both X and Y viruses will be made.   23. A 40-year-old woman has a history of chronic inflamma¬  tion of the small joints of the hands bilaterally. You suspect  rheumatoid arthritis. Which one of the following statements   (A) It is caused by sensitized CD4-positive T lymphocytes  and macrophages invading the joints.   (B) It is caused by antibody against human IgG-forming  immune complexes within the joints.   (C) It is caused by the release of mediators from mast cells  when environmental agents cross-link adjacent IgEs  within the joints.   (D) It is caused by superantigens inducing the release of  large amounts of lymphokines from helper T cells  within the joints.   25. A 70-year-old man with leukemia initially responded to  chemotherapy but now is refractory. He therefore under¬  went a bone marrow transplant and is now receiving large  doses of cyclosporine A and prednisone. Three weeks after  the transplant, he became febrile to 39°C and began cough¬  ing up purulent sputum. A chest X-ray revealed pneumonia.  A Gram stain of the sputum did not reveal a predominant  organism, but a KOH prep of the sputum revealed septate  hyphae with parallel walls and low-angle branching. Of the  following organisms, which one is MOST likely to be the  cause of this pneumonia?   26. Your patient is a 20-year-old woman with severe diar¬  rhea that began yesterday. She has just returned from a  3-week trip to Peru, where she ate some raw shellfish at  the farewell party. She now has watery diarrhea, perhaps  20 bowel movements a day, and is feeling quite weak and  dizzy. Her stool is guaiac-negative, a test that determines  whether there is blood in the stool. A Gram stain of the  stool reveals curved gram-negative rods. Of the following  organisms, which one is MOST likely to be the cause of  her diarrhea?   27. A 50-year-old man has had low-grade, persistent head¬  aches for several months. In the last few days, nausea,  vomiting, and blurred vision have occurred. An MRI  reveals several cystlike lesions in the brain parenchyma.  The patient lived for many years on one of the small  Caribbean islands. On the basis of a positive serologic  test, a diagnosis of neurocysticercosis was made. Of the    following, which one is the MOST likely mode by which  this disease was acquired?   (D) Ingestion of the larvae of the organism in raw fish   (E) Ingestion of the eggs of the organism in contaminated food   (F) Penetration of the skin by the organism while walking  bare-footed   (G) Penetration of the skin by the organism while bathing  in fresh water   28. A 30-year-old woman with a previous history of rheumatic  fever now has a fever for the past 2 weeks. Physical exami¬  nation reveals a new heart murmur. You suspect endocar¬  ditis and do a blood culture, which grows a viridans group  streptococcus later identified as Streptococcus sanguinus.  Of the following body sites, which one is the MOST likely  source of this organism?   (A) Poliovirus remains latent within sensory ganglia, and  reactivation occurs primarily in immunocompromised  patients.   (B) When the live, attenuated virus in the oral vaccine rep¬  licates, revertant mutants can occur that can cause para¬  lytic polio.   (C) The widespread use of the killed vaccine in the coun¬  tries of North and South America has led to the virtual  elimination of paralytic polio in those areas.   (D) The current recommendation is to give the live, attenu¬  ated vaccine for the first three immunizations to pre¬  vent the child from acting as a reservoir, followed by  boosters using the killed vaccine.   (A) People with type O are called universal recipients  because they have antibodies against H substance but  not against A and B antigens.   (B) If the father is Rh-positive and the mother is Rh-negative,  hemolytic disease of the newborn only occurs when the  child is Rh-negative.   (C) People who are Rh-negative usually have antibodies to  the Rh antigen because they are exposed to cross¬  reacting antigen located on bacteria in the colon.   (D) If type A blood is transfused into a person with type B  blood, complement will be activated, and the mem¬  brane attack complex will cause lysis of the type A red  cells.   31. A 25-year-old man was in a motorcycle accident 3 days ago,  in which he sustained severe head trauma. He has had spi¬  nal fluid leaking from his nose since the accident and now  develops a severe headache. His temperature is 39°C, and  on examination you find nuchal rigidity. You do a lum¬  bar puncture and find that the spinal fluid is cloudy and  contains 5000 WBC/mL, 90% of which are polys. Of the    following, which one is the MOST likely result observed in  the laboratory analysis of the spinal fluid?   (B) A motile spirochete that formed beta-hemolytic colo¬  nies on blood agar   (C) Gram-positive cocci that formed alpha-hemolytic colo¬  nies on blood agar   (D) Gram-positive rods that grew only on chocolate agar  supplemented with X and V factors   (E) No organism was seen using Gram stain, but tissue  stains revealed cytoplasmic inclusion bodies   32. Regarding prions and prion-caused diseases, which one of  the following is MOST accurate?   (A) Prions are highly resistant to both ultraviolet light and  to boiling but are inactivated by hypochlorite.   (B) Prions are protein-containing particles surrounded by a  lipoprotein envelope with a DNA polymerase in the  envelope.   (C) The diagnosis of prion-caused diseases such as  Creutzfeldt-Jakob disease is typically made by observ¬  ing cytopathic effect in cell culture.   (D) Creutzfeldt-Jakob disease occurs primarily in children  younger than the age of 2 years because they cannot  mount an adequate immune response to the prion  protein.   33. A 2-year-old boy has had several infections of the sinuses  and lungs and is being evaluated to determine whether he  has chronic granulomatous disease. Regarding this disease,  which one of the following is the MOST accurate?   (B) The defect is primarily in antigen-presenting cells such  as macrophages.   (C) Pneumocystis jiroveci infections are common in patients  with this disease.   (D) The diagnosis is primarily made by ELISA, in which  antibody against the affected cell component is detected.   (C) Their life cycle consists of a metabolically inactive par¬  ticle in the extracellular phase.   (D) They can replicate only within cells because they lack  the ability to produce certain essential mRNAs.   (E) They replicate in the nucleus of infected cells, where  they form inclusions that are useful diagnostically.   35. Regarding human papillomavirus (HPV), which one of the  following is MOST accurate?   (A) Blood and blood products are an important mode of  transmission of HPV.   (B) HPV is an enveloped virus with a genome composed of  double-stranded RNA.   (C) Amantadine is a chain-terminating drug that inhibits  HPV replication by blocking DNA synthesis.   (D) HPV induces the formation of koilocytes in the skin that  are an important diagnostic feature of HPV infection.   (E) The P2 capsid protein of HPV activates the c-sarc onco¬  gene in human cells, which is the process by which  HPV predisposes to malignancy.   (D) Fleas are the principal mode of transmission of the  causative organism.   (E) The diagnosis in the clinical laboratory is typically  made by culturing the organism on chocolate agar.   (B) There is very little IgG, but IgM and IgA levels are normal.   (C) The number of B cells is normal, but they cannot dif¬  ferentiate into plasma cells.   (D) There is a defect in a tyrosine kinase, one of the  enzymes in the signal transduction pathway.   (E) Viral infections are more common in patients with this  disease than are pyogenic bacterial infections.   38. A 20-year-old woman presents with a history of vaginal dis¬  charge for the past 3 days. On pelvic examination, you see a  mucopurulent exudate at the cervical os, and there is tender¬  ness on palpation of the right fallopian tube. You do a Gram  stain and culture on the cervical discharge. The culture is done  on Thayer-Martin medium, which is a chocolate agar that con¬  tains antibiotics that inhibit the growth of normal flora. Of the  following, which findings are the MOST likely to be found?   (A) A Gram stain reveals many neutrophils and spirochetes,  and culture on Thayer-Martin medium reveals no colonies.   (B) A Gram stain reveals many neutrophils and gram-variable  rods, and culture on Thayer-Martin medium reveals  |3-hemolytic colonies.   (C) A Gram stain reveals many neutrophils and gram-negative  diplococci, and culture on Thayer-Martin medium reveals  oxidase-positive colonies.   (D) A Gram stain reveals many neutrophils but no gram¬  negative diplococci are seen, and culture on Thayer-  Martin medium reveals coagulase-positive colonies.   (A) The term viral load refers to the concentration of HIV  RNA in the patient’s blood plasma.   (B) Both zidovudine and lamivudine block HIV replication  by inhibiting cleavage of the precursor polypeptide by  the virion-encoded protease.   (C) The antigenicity of the GAG protein of HIV is highly  variable, which is a significant impediment to the devel¬  opment of a vaccine against HIV.   (D) The Western blot test for antibodies to HIV has more  false-positive results than the ELISA test.   40. Regarding Th-1 and Th-2 cells, which one of the following   (A) Th-1 cells produce gamma interferon and promote cell-  mediated immunity.   (B) Th-2 cells produce interleukin-12, which inhibits the  formation of Th-1 cells.   (C) Both Th-1 and Th-2 cells have class IIMHC proteins on  their outer cell membrane.   (D) Before they differentiate into Th-1 or Th-2 cells, naive  Th cells are double-positives (i.e., they produce both  gamma interferon and interleukin-4).    Note: Page numbers followed by/and t indicate figures and tables, respectively; those followed by b and n indicate boxes and notes,  respectively; those followed by s indicate summaries; and those in boldface indicate main discussions.    A-B subunit structure, of exotoxins, 39  Bacillus anthracis, 136  Corynebacterium diphtheriae , 143  Abacavir (ABC, Ziagen), 270 1, 271 1, 274,  3861, 387   Abatacept (Orencia), 507, 575  ABC (abacavir), 270 1, 271 1, 274, 3861, 387  Abdomen. See Intra-abdominal infections  Abiotrophia, 218, 2181  ABO blood groups, transfusion reactions  and, 552-554, 552/ 553/ 5531  ABPA (allergic bronchopulmonary  aspergillosis), 419  Abscess(es)  abdominal, 28   peptostreptococci causing, 121  viridans streptococci causing, 121  Arachnia propionica, 218  of brain, 610-612, 611/   Bacteroides fragilis, 167  Nocardia asteroides, clinical case, 692  peptostreptococci causing, 121  Toxoplasma gondii, in AIDS, 3851  viridans streptococci causing, 121  dental, Porphyromonas, 221  of jaw, 28   Staphylococcus aureus, 109, 110/  112-114  drainage of, 116  Abscess cultures, 65  Absidia, 419  Acanthamoeba, 425 1    Acanthamoeba castellanii, 450, 4501, 684s  predisposing factors for, 7061  Accelerated allograft reactions, 538  Acetylcholine, botulinum toxin and, 139  Acetylcholine receptor, rabies virus  attachment to, 326  Achromobacter, 218, 2181  Acid(s)   clinical case, 691, 695  Acid-fast staining, of Mycobacterium  tuberculosis, 188, 638  Acinetobacter, 175, 2181  Acinetobacter baumannii, 81  Acne, 221   Acquired immunity, 32, 52, 57, 581,  59b-60b, 489-490, 4891, 4901,  495-496, 4961  active, 57, 96, 495-496, 4961   bacterial vaccines and, 96-98 971, 98b  viral vaccines and, 280-281, 2801,   Antibody-mediated immunity  cell-mediated. See Cell-mediated  immunity (cellular immunity)  passive, 57, 96, 496, 4961   to viruses, 263, 281  primary response in, 57  secondary (anamnestic) response in, 57  to viruses, 263   classification of, 1061  as normal flora, 271  Actinomyces israelii, 194, 1951, 665s  clinical case, 695   Actinomycetes, 194, 194/ 1951, 665s  Actinomycosis, 194, 195/  clinical case, 695   Activated protein C (drotrecogin-alfa,  Xigris), for septic shock, 46  Active immunity, 57, 96, 280-281, 496,  4961   bacterial vaccines and, 96-98, 971, 98 b  viral vaccines and, 280-281, 2801, 283b  to viruses, 263, 281  Activities, increasing exposure to   inflammatory, 615-619, 6171  pathogens causing, 6171  noninflammatory, 615-619, 6171  pathogens causing, 616-618, 6171  Acute glomerulonephritis (AGN)  clinical case, 693  poststreptococcal, 121  Streptococcus pyogenes, 120-121  Acute osteomyelitis, 594-595, 595/   Acute sinusitis, 6291  Acute-phase response, 54, 495  Acyclovir (acycloguanosine, Zovirax),  270-273, 2701, 2711, 272/ 273/  2771, 2901, 293, 6241  for encephalitis, 610  for genital ulcer disease, 623  for HSV-1 and HSV-2 infections, 293  mechanism of action of, 291  prophylactic, for varicella-zoster virus,  295   ADCC (antibody-dependent cellular  cytotoxicity), 511, 526  Addisons disease, 5691  Adefovir (Hepsera), 2701, 2711, 275  Adenine deaminase (ADA), defective gene  for, gene therapy for  immunodeficiency resulting from,  245   Adenovirus(es), 2361, 2481, 250, 285, 2861,  304-305, 3121, 3341, 670s  animal, 370-371  clinical findings in, 3041, 305  complementarity in, 2391  diseases caused by, 304  genome replication in, 2371  immune evasion by, 257  mechanism, 2571  portal of entry for, 2541  shape and size of, 227/   Adenovirus vaccines, 2821, 305  Adenylate cyclases  Bacillus anthracis, 136  Bordetella pertussis, 173  stimulation of   by choleragen, 159  by heat-labile toxin, 153  Adhesins, 37  Adjuvants, 498   in cell-mediated immunity, 535  ADP-ribosylation, 39-40, 401, 411  catalysis by heat-labile toxin, 153  of elongation factor 2, by   Cache Valley virus transmission by, 392  California encephalitis virus  transmission by, 357  chikungunya virus transmission by,    Eastern equine encephalitis virus  transmission by, 356  Jamestown Canyon virus transmission  by, 393   La Crosse virus transmission by, 357  yellow fever virus transmission by, 358,  3581, 7031   Zika virus transmission by, 395  Aerobes, obligate, 16, 184  Aerobic growth, 16  Aeromonas, 218, 2181  Aeromonas hydrophila, 218  Aerosols (airborne droplets)   adenoviral transmission via, 305  Bordetella pertussis transmission by, 173  Chlamydia psittaci transmission by,   common cold, 632  Corynebacterium diphtheriae  transmission by, 143  Coxsackie viruses transmission by, 336  Cryptococcus neoformans transmission  by, 417, 7031  diseases caused by, 320  Histoplasma capsulatum transmission  by, 7031   Q fever transmission by, 212, 215  rabies virus transmission by, 326  respiratory syncytial virus transmission  by, 322   African sleeping sickness, 4251, 443-445  African trypanosomiasis, 4251, 443-445  Agammaglobulinemia, 7051  Bruton’s, 578, 5791  Agar, precipitation in, 548, 548/  with electric field, 549, 549/   Age. See also Children; Elderly persons;  Infant(s); Neonates  immune response and, 498  viral infections and, 262   Agglutination tests, 547, 547/  in cholera, 160  for Salmonella, 64, 152  for Shigella, 64, 152  AGN. See Acute glomerulonephritis  AIDS, 3731, 374. See also HIV (human  immunodeficiency virus)  cancer in, 583   clinical findings in, 384, 3851  cytomegalovirus infection in, 296  dementia and, 612  histoplasmosis in, 410  Kaposi’s sarcoma in, 299, 299/  opportunistic infections in, 583  Penicillium marneffei infection in, 420  Pneumocystis proved pneumonia in,   T-cell deficiencies in, 583  AIDS-related complex (ARC), 384  Airborne conidia, Aspergillus transmission  by, 418   Airborne droplets. See Aerosols  AIRE (autoimmune regulator), 502, 568  Alcaligenes, 218, 2181  Alcaligenes faecalis, 218  Alcohol, as antiseptic, 102  Alkalis, disinfection using, 103  Allelic exclusion, 530  Allergens, 558, 5591   atopic reactions and, 560  Allergic alveolitis, 562  Allergic bronchopulmonary aspergillosis  (ABPA), 419   Allergic encephalitis, 571  Allergic encephalomyelitis, 5691  Allergic reactions. See Hypersensitivity  Allergic rhinitis, 559, 561, 5591  treatment of, 561  Allergy, 557  Allograft(s), 538  fetus as, 539  rejection of, 538  Allograft reaction, 538  Allotypes, 527  Alpha fetoprotein, 577  Alpha hemolysis, on blood agar, caused by  Streptococci, 116, 118/    Alpha-chain disease, 47  Alpha-hemolytic streptococci, 116, 118/,  119. See also Streptococcus mutans;  Streptococcus pneumoniae;  Streptococcus sanguis, clinical case;  Viridans streptococci  Alphaviruses, 251  Altabax (retapamulin), 641  clinical uses of, 76 1, 79  mechanism of action of 79  Alternative pathway of complement  activation, 543, 543/   Alveolitis, allergic, 562  Amanita mushroom toxins, 399  Amantadine (Symmetrel), 269, 270f 271 1,  272 f 2771, 316  Amastigotes, 423  Leishmania donovani, 445, 446/  Trypanosoma cruzi, 441, 443/  Ambisense regions, 235  Amebas, 423-424, 450, 450f. See also  Sarcodina   Amebic abscess, 425  Amebomas, Entamoeba histolytica, 425  American trypanosomiasis, 425f  441-443   Amikacin, clinical uses of 76 1  p-Aminobenzoic acid (PABA), 79, 80/  Aminoglycosides. See also specific  aminoglycosides  clinical uses of, 75-77, 76 1   for Escherichia coli infections, 155  for group D streptococcal infections,  118   resistance to, 88f, 89f, 90  6-Aminopenicillanic acid, 72, 72/  Amniocentesis, rubella virus detection by,  325   activity against gram-negative rods, 72  chemoprophylactic use of 83f  clinical uses of, 73 1, 122, 630-632  for endocarditis prevention, 84, 831,  123   Amphotericin B (Fungizone), 75, 400  adverse effects of, 81, 400f  mechanism of action of, 81, 82/ 4001  structure of 81, 82/   activity against gram-negative rods, 72  chemoprophylactic use of, 832  clinical uses of Tit, 608   for Proteus mirabilis infections, 164  prophylactic, for neonatal sepsis, 123  pseudomembranous colitis due to, 141  resistance to, 882   Anaphylactic hypersensitivity, 557-565,  557t, 5582, 559/ 559f  atopy and, 560  desensitization and, 561  drug hypersensitivity and, 560  IgE in, 526   treatment and prevention of 561  Anaphylactic reactions (anaphylaxis)  basophils and mast cells in, 517  cutaneous, 560  to eggs, 281  penicillin causing, 73  systemic, 558, 5591  Anaphylactoid reactions, 560  Anaphylatoxin, complement and, 542, 544  Anaplasma phagocytophilum, 215, 668s,  7031   systemic lupus erythematosus, 573  Ancylostoma, 423, 469, 4701  host defenses against, 526  Ancylostoma braziliense, 485, 689s  Ancylostoma caninum, 469, 4711, 485,  485/ 689s   aplastic, parvovirus B19 and, 308  clinical case, 698  hemolytic, 5691, 571-572  in Mycoplasma pneumoniae  infections, 198  in hookworm infection, 475  in malaria, 434, 437  megaloblastic, Diphyllobothrium  causing, 458  pernicious, 5691, 572  sickle cell. See Sickle cell anemia  in typhoid fever, 157  Anergy, 416, 507  clonal, 568, 568/   hereditary, 5791, 580  Angiomatosis, bacillary, 218  Angiostrongylus cantonensis, 485-486  Anidulafungin, 75  Animal(s). See also specific animals  bites of, rabies virus transmission by,  326   as reservoirs, 36, 361, 7021-7031  for Campylobacter, 161  for cholera, 159  for Escherichia coli, 153  for Listeria monocytogenes, 144  for Q fever, 215  for Salmonella, 157  slow diseases of, 376  Animal retroviruses, 370  Animal tumor viruses, 3671, 370-371  DNA, 3671, 370-371  human cancer and, 369  RNA, 3671, 370  Anisakiasis, 469, 4701, 486  Anisakis, 469, 4701, 615  Anisakis simplex, 486, 689s  reservoirs for, 7031  Anopheles mosquitoes, 7031   Antagonistic drug interaction, 93, 93/  Antagonizers, of repressor proteins, viral  replication and, 239, 242/   black eschar of, 136/ 137, 707 1  cutaneous, 135-136, 136/  gastrointestinal, 135-136  pulmonary (inhalational), 136-137  Anthrax toxins, 36f 41 1, 43  Anthrax vaccine, 96 1, 97  Anti-ABO antibodies, preformed,  allograft rejection and, 538  Antibiogram, 91   Antibiotic(s). See also specific drugs and  drug types  broad-spectrum, 70  Candida and, 623, 625  Candida vaginitis and, 416, 623, 625  combination therapy using, 93, 93/  hemolytic-uremic syndrome and, 154  for methicillin-resistant Staphylococcus  aureus, 647 1  narrow-spectrum, 70  as predisposing factor for Clostridium  difficile, 7061   to artesunate, 437, 4371  of enteric gram-negative rods, 152  genetic basis of, 88-89, 94 b   mechanisms of, 87-88, 871  medically important bacteria  exhibiting, 88, 881  methicillin/nafcillin resistant   methicillin-resistant Staphylococcus  aureus and, 43, 90, 111  diseases caused by, 113-115  drugs effective against, 78, 79, 741  of Morganella morganii, 164  multidrug-resistant organisms and, 184,  189. See also specific organisms  of Mycobacterium tuberculosis, 184, 189  nafcillin-resistant Staphylococcus aureus  and, 111, 115   of Proteus vulgaris, 164  of Providencia rettgeri, 164  of Pseudomonas aeruginosa, 166  sensitivity testing and, 91-93, 94 b  (3-lactamase production and, 93  minimal bactericidal concentration  and, 92, 92/   specific mechanisms of, 89-91, 94 b  vancomycin-resistant enterococci and,  78, 90, 118, 123   vancomycin-resistant Staphylococcus  aureus and, 111, 115  Antibodies, 57, 491, 522-530, 525/ 569 1.  See also specific antibodies and  immunoglobulins  active induction of, 532  allelic exclusion and, 530  allotypes, 527   anti-ABO, preformed, allograft  rejection and, 538  autoimmune diseases and, 569  B-cell production of, 493  catalytic, 522, 530  classes of, 525-526. See also specific  immunoglobulins  switching of, 528/ 529  complement system regulation and,  543-544   functions of 490, 490/ 522, 5251, 532  genes for, 528-529, 528/  idiotypes, 528    reduced, 705f  seroconversion and, 267  serum, with known antigens,  identification of, 66  structure of, 522-525, 524/ 5251  titer of, 267, 267 n  to tumors, 576  Antibody binding, 532  Antibody-dependent cellular cytotoxicity  (ADCC), 511, 526   Antibody-mediated immunity, 32, 581,  490/ 531-532  antibody functions and, 532  B lymphocytes and, 490  diversity of, 490  fetal antibodies and, 532  memory of, 490  primary response in, 531, 531/  secondary response in, 532  simultaneous administration of  multiple antigens and, 532  specificity of, 490, 491-493, 493/  tests for evaluating, 533  Antifungal drugs, 81, 83, 400, 4001, 401b.   See also specific drugs  Antigen(s), 491, 493, 496-498, 497/  binding of, 524  Borrelia, 205   encoded by HLA genes, 536-537  of Enterobacteriaceae, 151  of influenza viruses, 314-315  interactions with antibodies. See  Antigen-antibody reactions  known, serum antibodies with,  identification with, 66  multivalent, 512   programmed rearrangements and, 19  protective, of Bacillus anthracis, 136-137  recognition by B cells, 492-493  sequestered, release of, autoimmune  diseases and, 571  surface antigen of HBV, 344  T-cell recognition of, 508-509  T-cell-independent, 491  tumor-specific transplantation, 366  viral    Antigen-antibody reactions, 497, 546-555  diagnostic tests and, 546-555, 5461  reactions involving red blood cell  antigens and, 551-555  types of, 547-551  Antigenic drift, of influenza virus,   in Neisseria gonorrhoeae, 128  in Trypanosoma brucei, 444  viral, 244  of HCV, 349  of HIV, 380   interaction with B cells and T cells, 515  Antiglobulin test, 551  Anti-inflammatory cytokines, 495  Antimicrobial drugs, 69 - 85 . See also   Antibiotic(s); Antibiotic resistance;  Antifungal drugs; specific  antimicrobial drugs  bactericidal activity of, 70, 71/ 84b  bacteriostatic activity of, 70, 71/ 84 b  chemoprophylaxis using, 83, 831, 85b,  131   alteration of cell membrane function  as, 81-82, 85b  antibacterial, 82-83, 82/  antifungal, 83   inhibition of nucleic acid synthesis  as, 79-81, 791, 85b  inhibition of protein synthesis as,  75-79, 761, 85b   selective toxicity of, 70, 711, 84b  stewardship, 69, 701  Antinuclear antibodies (ANAs), in   systemic lupus erythematosus, 573  Antioncogenes, 365  Antiretroviral drugs, 385-388, 3861  Antiretroviral therapy, 6171  Antiseptics, 101   antiretroviral, 385-388, 3861  chemoprophylactic use of, 277, 2771  early event inhibition by, 269-270, 2701,  272/ 277b   hepatitis B virus inhibitors, 275, 278b  hepatitis C virus inhibitors, 275, 278b  herpesvirus inhibitors, 270-274,  277b-278b   nonnucleoside, 274, 277b-278b  nucleoside, 270-274, 278b  integrase inhibitors, 276, 278b  limitations of, 2701, 2711  nucleic acid synthesis inhibition by,  270-276, 2711   protease inhibitors, 276, 278b  retrovirus inhibitors, 274-275, 278b  nonnucleoside, 275, 277b-278b  nucleoside, 274-275, 278b  viral protein synthesis inhibitors,  276-277, 278b   viral release inhibitors, 277, 278b  APCs (antigen-presenting cells), 491, 515 .  See also Dendritic cells;  Macrophage(s)   interaction with B cells and T cells, 515  Aplastic anemia, parvovirus B19 and, 308  clinical case, 698   APOBEC3G (apolipoprotein B RNA-  editing enzyme), 262, 379, 495  Apoptosis, 264, 502, 503/ 568  Appendicitis, 619-620  Aptivus (tipranavir), 2701, 276, 3861, 387  Aquatic vegetation, Fasciolopsis buski  transmission by, 467  Aqueous penicillin G, 72  Arachnia, 218, 2181   Arboviruses, 354 - 359 , 676s-677s. See also  specific arboviruses  causing diseases in United States,  356-357   causing diseases outside United States,  358-359, 3581  classification of, 354, 3551  clinical findings and epidemiology of,  356    Arenaviruses, 227f 235, 2371, 252  Arizona, 218, 2181  Arizona hinshawii, 218  Artemether/lumefantrine (Coartem),   Artemisinins, 437  Artesunate resistance, 437, 4371  Arthralgias, in Mycoplasma pneumoniae  infections, 198   Arthritis. See also Reactive arthritis;  Rheumatoid arthritis  gonococcal, 132  in hepatitis B, 346  in Lyme disease, 205  parvovirus B19 and, 308  in rheumatic fever, 121  rubella virus and, 325  Staphylococcus aureus and, 113  Staphylococcus epidermidis and, 115  Arthritis. See Reactive arthritis;   Rheumatoid arthritis  Arthropods, 585 - 591 , 5861. See also  Arachnids; Insects; specific  arthropods   Arthus reaction, 562  Ascariasis, 4701, 473-474  Ascaris, 423, 469, 4701  host defenses against, 526  Ascaris lumbricoides, 473 - 474 , 473/ 475/  687s   Aspergillus fumigatus, 418 - 419 , 681s  clinical case, 692  diseases caused by, 611  with immunodeficiencies or reduced  host defenses, 7051   Asymptomatic bacteriuria, 651-652  Asymptomatic infections, 32  Ataxia-telangiectasia, 580  Atazanavir (Reyataz), 270f 276, 386f  387   Attenuated viral vaccines, 280  Attenuated viruses, 256  Atypical mycobacteria. See Mycobacterium ,  atypical; Mycobacterium avium-  intracellulare (MAI) complex;  Mycobacterium fortuitum-chelonei  complex; Mycobacterium kansasii  Autoclaving, 103  Autografts, 538   complement deficiencies and, 581  enteric pathogen infections and, 149  environmental factors in, 570, 570f  genetic factors in, 569-570  hormonal factors in, 570  involving multiple organs, 573-574  involving primarily one type of cell or  organ, 571-573  mechanisms of, 570-571  treatment of, 574-575  Autoimmune polyendocrinopathy, 568  Autoimmune reactions, hepatitis C virus  infection and, 350   Autoimmune regulator (AIRE), 502, 568  Autoinfection, Strongyloides, 476  Autolytic enzymes, 71  Autosomal severe combined   immunodeficiency, 580  Avian influenza virus, 2551, 317  vaccine against, 317  Avibactam, 73-74, 89  Avidity, 497    Azidothymidine (zidovudine), 270f 271 1,  272/ 274-276, 2771, 387, 3861  Azithromycin, 78, 122, 133, 1711, 174-175,  198, 211, 6171, 623, 6241, 639   adverse effects of, 4001  clinical uses of, 81-82, 831, 138, 142  mechanism of action of, 82, 4001  structure of, 81-82, 82/   AZT (zidovudine), 2701, 2711, 272/  274-276, 2771, 387, 3861  Aztreonam, 74, 74/   antibody-mediated immunity and, 490  antigen presentation and, 508  clonal selection and, 514  combined B-cell and T-cell deficiency  and, congenital, 580, 5791  deficiency of   acquired, 582-583  congenital, 578, 5791  effector functions of, 514  enumeration of, 535  Epstein-Barr virus infection of, 296  functions of, 4911  interaction with T cells and   Babesia microti, 4251, 451, 4501, 684s  with immunodeficiencies or reduced  host defenses, 7061  vectors for, 7031  Babesiosis, 4251, 451  transmission of, 33, 451  Bacillary angiomatosis, 181, 181/ 218  Bacillary dysentery, 158,615  transmission of, 341, 351  Bacillus, 4, 4f, 135-137, 1351  classification of, 1061  shape of, 4/  spores of, 11-12   activities increasing exposure to, 7051  disease caused by, 135, 136/  environmental source of, 7041  exotoxins of, 401, 421, 43  pneumonia caused by, 637  properties of, 135-136, 136/  reservoirs for, 7021    skin lesions caused by, 7071  surface virulence factors of, 381  transmission of, 361, 136  Bacillus anthracis vaccine, 961, 97  Bacillus Calmette-Guerin (BCG), atypical,  582   Bacillus Calmette-Guerin (BCG) vaccine,  961, 97, 187-188  for cancer, 576  tuberculin skin test and, 186  Bacillus cereus, 137, 616, 6161, 6171, 656s  disease caused by, 137  toxins produced by, 421, 44  transmission of, 351, 1361, 137  Bacitracin   Bacteroides fragilis, 167  Salmonella, 157  Staphylococcus aureus, 114  Streptococcus bovis, 121  Streptococcus pneumoniae, 123  in typhoid fever, 620  Bacteria. See also specific bacteria  acid-fast. See Acid-fast bacteria  adherence to cell surfaces, 36-37, 49 b  anaerobic. See Anaerobic bacteria  autoimmune disorders and, 570, 5701  causing pharyngitis, 631-632  cell wall of, 4-9, 5/ 61, I2b-13b   drugs inhibiting synthesis of, 71-75  classification of, 24-25, 251  cytoplasm of, 9-11  cytoplasmic membrane of, 9  DNAof, 9-10, 10/ 13b   mutations and, 18-19  recombination and, 22  glycocalyx (slime layer) of, 61, 11  gram-negative. See Gram-negative  bacteria; specific gram-negative  bacteria    granules in cytoplasm of, 6f, 9  growth cycle of 15, 15/  ingestion of in phagocytosis, 55-56, 55/  intracellular survival of, 38  invasive, enzymes secreted by, 38  lysogenic conversion and, 239, 241/  lysogeny in, latency in human cells  versus, 241  mRNAof   pili of 6f 11  plasmids of, 6 t, 10  ribosomes of, 6t, 9  secretion systems of, 40  shape of, 4, 4/ 12b  size of, 4, 5 f 12 b  spores of, 6 t, 11-12, 12/ 12f, 13b  killing, 103  structure of 5, 5/ 6 t  structures outside cell wall of 11, 13b  types of infections caused by32  Bacterial endocarditis, 598-602   Cardiobacterium hominis, 219  enterococcal, 121, 123, 601  Enterococcus faecalis, 116  Kingella kingae, 220  prevention of 123  Serratia marcescens, 163  Staphylococcus aureus, 601, 692  Staphylococcus epidermidis, 109, 113,   Streptococcus agalactiae, 121  Streptococcus bovis, 116, 121, 601  subacute, clinical case, 698  viridans streptococci causing, 28, 116,  117/ 119-121, 123  Bacterial pharyngitis, 631-632  Bacterial vaccines, 96-98, 96t. See also  specific vaccines  antitoxins as, 98   transduction and, 20-21, 20 1, 22f  Bactericidal activity, 70, 71/ 84b  Bactericidal drugs, 92  Bacteriocins, 10   Bacteriologic approach to diagnosis, 61,  62-65, 67b. See also specific types  of cultures  Bacteriophages  CTX, 160   transduction and, 20-21, 20f, 22/  Bacteriostatic activity, 70, 71/ 84b  Bacteroides, 167-168  classification of 1061  diseases caused by, 155, 167  as normal flora, 26t, 271, 28, 29 1  properties of, 167  Bacteroides corrodens, 167  Bacteroides fragilis, 167, 619, 661s-662s  clinical findings in infections with, 167  drugs effective against, 78  identification of, 65  as normal flora, 27 1, 28, 29 1  oxygen requirement of, 106, 1071  properties of, 167   Balantidium coli, 4251, 451, 684s  Baraclude (entecavir), 2701, 2711, 275  Bare lymphocyte syndrome, 580  Bartonella bacilliformis, 218, 2181  Bartonella henselae, 180-181, 664s  diseases caused by, 331, 180  transmission of, 361, 7021  Bartonella quintana, 218, 2181  Base substitution, 18  Basidiospores, 398   bites of, rabies virus transmission by,  326, 7021  clinical case, 697  as coronavirus reservoirs, 7021  Baylisascaris procyonis, 486  BCG (Bacillus Calmette-Guerin), atypical,  582    BCG (Bacillus Calmette-Guerin) vaccine,  961, 97, 187-188  for cancer, 576  tuberculin skin test and, 186  bcr-abl oncogene, 364  Beavers, as Giardia lamblia reservoirs,  7031   Creutzfeldt-Jakob disease and, 375  Escherichia coli transmitted by, 153  Taenia saginata transmitted by, 457  Beef tapeworm, 455  Behcet’s disease, 622  Bejel, 203   Belatacept (Nulojix), 540  Benzathine penicillin G, 72, 72 f 122-123  Benzalkonium chloride, as antiseptic, 102  Benzodiazepines, for tetanus, 138  Benzylpenicillin, 72, 72 f 122-123  Beta hemolysis, clinical case, 693, 695  Beta interferon, 495  Beta-glucan, 75   group A. See Streptococcus pyogenes  group B. See Streptococcus agalactiae  group D, 118. See also Enterococcus  faecalis; Enterococcus faecium;  Streptococcus bovis  Lancefield groups of, 118  Beta-lactam drugs, 72. See also   Cephalosporins; Penicillin(s);  specific drugs  resistance to, 87 1  Beta-lactamases, 87-89  defenses against, 73  detection of, 93   for arboviruses, 356, 356f, 357  for Chlamydia psittaci, 36f, 703f  for Cryptococcus neoformans, 703f  for encephalitis viruses, 703f  for Histoplasma capsulatum, 703 1  for influenza virus, 703f  Birth canal, transmission within, 33, 34f  Bismuth, 615  Bites   arthropod. See Arthropods; specific  arthropods  human, 28, 65, 219  BK virus, 248f, 250, 309, 391-392  Black death. See Plague  Black eschar, of anthrax, 136/ 137, 707 1  Black flies, Onchocerca volvulus  transmission by, 483, 703f  Black sickness (kala-azar), in  leishmaniasis, 447  Black water fever in malaria, 434  Black widow spider, 586f, 590, 590 f, 690s  Bladder carcinoma, schistosomiasis and,  463   Chlamydia trachomatis, 210  Cytomegalovirus, 296  herpes simplex virus, 291  in onchocerciasis, 484  Blocking antibodies, 576  Blood   hepatitis C virus transmission via, 346  HIV transmission via, 381-382  human T-cell leukemia virus type 1  transmission by, 368  as viral portal of entry, 254f  Blood agar, 61-62, 63 1  Campylobacter and, 161    Clostridium perfringens and, 140  in diphtheria, 144  Enterobacteriaceae and, 151, 1511  Escherichia coli and, 155  in Listeria monocytogenes infections,  145   Blood and tissue protozoa, 433-447, 434f  Blood cultures, 62  Blood fluke, 462, 686s  Blood flukes. See Schistosoma;   universal donors for, 553, 553f  universal recipients for, 554, 553f  Blood types, matching, 553  Blood-brain barrier, 39  Blue pus, in Pseudomonas aeruginosa  infections, 165  Blueberry muffin lesion, 296  Blue-green pus, in Pseudomonas   Boceprevir (Victrelis), 270 1, 276, 278 b  Body lice, 585-587, 586/ 586f   Bordetella, classification of, 106f  Bordetella pertussis, 171t, 173-174, 662s  disease caused by, 1061, 173  exotoxin of, 40 1, 41, 4 It, 44  properties of, 173  Bordetella pertussis vaccine, 96 1, 97  Bordet-Gengou agar, 63f, 173  Borna virus, 392  Bornholm disease, 337  Borrelia, shape of, 4/   Borrelia burgdorferi, 199 1 , 203-205, 666s  activities increasing exposure to, 705f  disease caused by. See Lyme disease  geographical location of, 704f  properties of, 203  reservoirs for, 702f  skin lesions caused by, 7071  transmission of, 36f, 585, 703f  Borrelia hermsii, 205  Borrelia miyamotoi, 205-206  Borrelia recurrentis, 199 1, 205, 666s   Botulinum antitoxin, 98, 139  Botulinum toxin, 138-139  mechanism of action of, 42  synthesis of, 239  Botulism, 107, 139  clinical case, 693  infant, 139   Bradyrhizobium, 219  Bradyrhizobium enterica, 219  Bradyzoites, Toxoplasma gondii, 438  Brain, abscess of, 610-612, 611/  in endocarditis, 593, 593/   bacterial transmission by, 33, 34f  viral transmission by, 255, 255f  hepatitis B, 346  Brill-Zinsser disease, 214  Broad-spectrum antibiotics, 70  Bronchiolitis, 312 1, 636  parainfluenza virus, 322  respiratory syncytial virus, 322  Bronchitis   acute, 635-636  adenoviral, 305  Chlamydia pneumoniae, 210  coronavirus, 324  Moraxella catarrhalis, 220  parainfluenza virus, 322  respiratory syncytial virus, 322, 695  Bronchopulmonary aspergillosis, allergic,  418   Brucella, 177-178,663 s  classification of, 106f  clinical case, 697  disease caused by. See Brucellosis  intracellular survival of, 38  properties of, 177  reservoirs for, 702f  transmission of, 35f, 36f    Brucella abortus, 177  clinical findings in, 178  Brucella melitensis, 177  Brucellosis, 177-178  clinical case, 697  clinical findings in, 178  diagnosis of, 66  laboratory diagnosis of, 178  pathogenesis and epidemiology of,  177-178   Brutons agammaglobulinemia, 578, 579 1  Brutons tyrosine kinase, 513  BSE (bovine spongiform encephalopathy),  373, 376   in bubonic plague, 179  in chancroid, 220  Bubonic plague. See Plague  Bud(s)   Bunyamwera virus, 252  Bunyaviruses, 227/ 235, 252, 3551, 357  Burkholderia aeruginosa. See Pseudomonas  aeruginosa    C2 deficiency, 581  C3 deficiency, 579 1, 581, 7051  C4 deficiency, 581  C6 deficiency, 579 1, 581, 7051  C7 deficiency, 5791, 581, 7051  C8 deficiency, 579 1, 581, 7051  C9 deficiency, 7051  Cache Valley virus, 392  Cachectin, 519   California encephalitis (CE) virus, 249 1,  2871, 3561, 357, 676s  Calymmatobacterium granulomatis, 219  CAMP test, 122  Campylobacter, 161, 616, 618  classification of, 105, 1061  diseases caused by, 161   autoimmune diseases and, 149  frequency of, 149, 1491  Guillain-Barre syndrome following  infections with, 570, 572  identification of, 64  properties of, 161  reactive arthritis and, 572  Reiter’s syndrome and, 573-574  Campylobacter fetus, 161 n  Campylobacter intestinalis, 161  disease caused by, 161  Campylobacter jejuni, 106, 6161, 6171, 660s  cancer associated with, 47  clinical case, 694, 698  disease caused by, 161  clinical findings in, 161  epidemiology of, 161  laboratory diagnosis of, 161  pathogenesis of, 1591, 161  prevention of, 161  treatment of, 161    bacterial infections associated with, 47  cellular oncogenes in tumorigenesis  and, 363-365, 3641  herpesviruses and, 288-299  human papillomavirus and, 307  human T-cell leukemia virus type 1  and, 327-329   papillomaviruses and, 305  Salmonella septicemia in, 155  SV40 virus and, 336  tumor immunity and, 576-577  alpha fetoprotein and, 577  carcinoembryonic antigen and, 577  mechanism of, 576  tumor-associated antigens and, 576  vaccines against, 369  Cancer chemotherapy, 7051   pseudomembranous colitis and, 141  Cancidas (caspofungin), 75, 397, 4001  Candida, 623-625  diseases caused by, 594-595  drugs effective against, 75, 81-82  esophagitis caused by, 614  identification of, 62  with immunodeficiencies or reduced  host defenses, 7061   urinary tract infection due to, in  diabetes mellitus, 57  Candida albicans, 414-417, 4141, 6261,  626/ 680s-681s  in AIDS, 3851   chlamydospores of, 398, 398/  chronic mucocutaneous candidiasis due  to, 414, 416, 579, 5791  clinical case, 692, 698  differentiation from other Candida  species, 415, 415/ 416/  diseases caused by, 414, 5951, 601  hospital-related events predisposing to  infection by, 7051   pseudohyphae of, 398, 398/, 414  tetracyclines and, 76  thrush due to, in thymic aplasia, 579  transmission of, 34f, 7071  vaginal candidiasis and, 624, 624/  Candida esophagitis, 614-615, 614/  Candida glabrata, 415  Candida krusei, 415  Candida parapsilosis, 415, 416  Candida tropicalis, 415, 416  Candida vaginitis, 29, 623, 625  Candidemia, 414   Candidiasis, mucocutaneous, chronic,  414, 416, 579, 579 1  Candidiasis, vaginal, 623-625, 624/  Capnocytophaga canimorsus, 219  reservoirs for, 7021  Capnocytophaga gingivalis, 219   Cryptococcus neoformans, 418  of Enterobacteriaceae, 151  Capsular polysaccharide vaccines, 11, 97  Capsular swelling reaction, 65  Capsules, 61, 10-11, 36-37  as antiphagocytic factor, 37, 381  Bacillus anthracis, 135  Cryptococcus neoformans, 417, 417/  Escherichia coli, 153  Streptococcus pneumoniae, 123  Carbapenem, 639  Carbapenemases, 90  Carbapenems   Carbuncles, 646-647, 646 f 707 1  Staphylococcus aureus, 113, 114/  Carcinoembryonic antigen, 577  Carcinoma,  of bladder, 463  of cervix, 250  of liver, 346, 349   Cardiac arrhythmias, in Mycoplasma  pneumoniae infections, 198  Cardiac infections, 598-604  diagnostic testing, 598  myocarditis, 602-603  pericarditis, 603-604  Cardiac muscle, in Chagas’ disease,   in Haemophilus influenzae vaccine, 173  in Neisseria meningitidis vaccine, 131  in Streptococcus pneumoniae vaccine,  125   chronic, 258  in hepatitis B, 346  in hepatitis C, 350  Carrion’s disease, 218  Caseation necrosis, in Mycobacterium  tuberculosis infections, 186  Caspofungin (Cancidas), 75, 400, 4001  Cat(s)  bites of, 65   Pasteurella multocida transmission  by, 36t, 180, 702 1  rabies virus transmission by, 7021  civet, as coronavirus reservoirs, 7021  as definitive host, for Toxoplasma  gondii, 438   scratches of, Bartonella henselae  transmission by, 361, 180  tapeworm of, 461  toxoplasmosis and, 702 1  clinical case, 699  Catalase, 16   anaerobic growth inhibition by, 106  staphylococcal, 109  Catalytic antibodies, 522, 530  Cat-scratch disease (CSD), 33 1, 361,  180-181, 181/    for Bacillus anthracis, 7021  for Coxiella burnetii, 215  for Escherichia coli, 36t, 153, 7021  for Mycobacterium bovis, 36t  for prions, 7021  for Salmonella enteritidis, 36t  for Sporothrix schenckii, 7021  for Taenia saginata, 7021  for Toxoplasma gondii, 7021   CD3 proteins, on T cells, 504  CD4 protein, 233  CD4 T cells. See Helper T cells  CD8 T cells. See Cytotoxic T cells  CD28 protein, clonal anergy and, 568  CD40 protein   class switching and, 530  clonal anergy and, 568  in hyper-IgM syndrome, 579  CD40L protein, clonal anergy and, 568  CD55, 544   CE (California encephalitis) virus, 249f,  2871, 356 1 , 357, 676s  Cefazolin, 645   chemoprophylactic use of, 84, 831, 116  clinical uses of, 741  Cefepime, 741  Cefotaxime, 155, 163  Cefotetan, 627  Cefoxitin, 741, 1291, 627  Ceftaroline, 741  Ceftazidime, 741  Ceftriaxone, 627, 6291, 651  chemoprophylactic use of, 831  clinical uses of, 741, 125, 1291, 172, 634  Cefuroxime, 741  Celiac disease, 5691, 572  Cell(s)   invasion by bacteria, 38, 49b-50b  malignant transformation by tumor  viruses, 361-362, 3621, 365-367,  3661, 3671   role of tumor viruses in, 362-363  nucleic acids of, 2, 21  properties of, alteration of, in malignant  transformation, 361-362, 3621  replication of, 2, 2t  rigid, thick-walled, 25t  structure of, 1,21  virus infection in, 253-254  viruses compared with, 225, 225t  wall-less, 25t   Cell-mediated autoimmune diseases, 5691  Cell-mediated hypersensitivity, 5571,  563-565, 564/ 5641  clinically important, 564-565  Cell-mediated immunity (cellular   immunity), 32, 581, 490/ 534-535  adjuvants in, 535  CMV infection and, 295-296  constituents of, 534  diversity of, 490  to herpes simplex virus, 291  Legionella pneumophila and, 175  lipids in, 535  memory of, 490   Listeria monocytogenes and, 144-145  by measles virus infection, 319  in thymic aplasia, 7061  viral infections and, 264  T cells and, 490,511-512  tests for evaluating, 534-535  to Toxoplasma gondii, 438    to tumors, 576  VZV infection and, 294  Cellular oncogenes  over expression of, 364-365  role in tumorigenesis, 363-365, 3641  Cellulitis, 6421, 644-645, 7071  Haemophilus influenzae, 172  Pasteurella multocida, clinical case, 694  Pseudomonas, 165, 165/   Pseudomonas aeruginosa, 165, 165/  Staphylococcus aureus, 109  Streptococcus agalactiae, 120  Streptococcus pyogenes, 37, 116, 117/  120   clinical case, 691  Vibrio vulnificus, 161  Central nervous system (CNS)  infections of, 605-613  neonatal infections of, transmission of,  341   Nocardia asteroides brain abscess and,  clinical case, 692  slow diseases and, 372  Central tolerance, 568  Cephalexin, 741, 645, 647  Cephalosporins, 651. See also   (3-Lactam(s); specific drugs  clinical uses of, 73, 741, 155, 163-164,  198   generations of, 73, 741  hypersensitivity to, 73  mechanism of action of, 73, 73/ 741  resistance to, 89, 891  structure of, 73, 73/   Chlamydia trachomatis, 210  Neisseria gonorrhoeae, 132  nongonococcal, diagnosis of, 65  Cestodes (Cestoda), 423, 453-461, 4531,  685s-686s   of minor importance, 459-461  CF (complement fixation) test, 550, 551/  for Histoplasma capsulatum, 410  viral identification by, 267  CFS (chronic fatigue syndrome), 583  CGD (chronic granulomatous disease),  5791, 581, 7051  phagocytic defect and, 56-57  Staphylococcus aureus infection in, 112  Chagas’ disease, 4251, 441-443   Chain of transmission, 33  Chain termination, acyclovir and, 271  Chancre, 7071  in syphilis, 200-201, 622  Chancroid, 220, 622-623, 623/ 6241  drugs effective against, 79  Charcoal-yeast extract, 631, 175  Chediak-Higashi syndrome, 56, 581  Checkpoint immunotherapy, 577  Cheese   Brucella melitensis in, 178  Listeria monocytogenes in, 144  Chemokine(s), 516, 518, 5171  inflammatory response and, 54  Chemokine receptors, HIV replication  and, 381   antiviral, 211, 2111, 322  of malaria, 437-438  Chemotactic factors, 518  Chemotaxis, 11   for Campylobacter jejuni, 7021  for influenza virus, 7031  for Salmonella enteritidis, 7021  Chickenpox. See Varicella  Chiggers, 591   gastroenteritis in, adenoviral, 305  Haemophilus aegyptius infection in, 220  Haemophilus influenzae infection in,  171-172   hemolytic-uremic syndrome in, 155  infectious mononucleosis in, 297  meningitis in, Haemophilus influenzae,  171-172, 1711  otitis media in, 629, 630/  parainfluenza virus infection in, 322  pertussis in, 173  reducing case of HIV in, 388  respiratory syncytial virus infection in,  321   rotavirus infection in, 339  Salmonella infection in, 156  shigellosis in, 158   Chimeric monoclonal antibodies, 523 b  Chitin, fungal, 397  Chlamydia, 208-211, 666s-667s  classification of, 105, 1061  genital infections with, number of cases  of, 106, 1061  Gram stain and, 81  as obligate intracellular parasite, 31  properties of, 208-209, 2081, 209/  Reiter’s syndrome and, 570, 5701, 572,  573-574   Chlamydia pneumoniae, 631, 667 s  diseases caused by, 208-210, 2081  drugs effective against, 77  properties of, 209  reclassification of, 208  Chlamydia psittaci, 667s   disease caused by, 208, 2081, 210. See  also Psittacosis  properties of, 208  reclassification of, 208  reservoirs for, 7031  transmission of, 361  Chlamydia trachomatis, 208, 6241,  626-628, 666s-667s  cervicitis, 625  clinical case, 699  diseases caused by, 114, 208  epidemiology of, 209  genital ulcer disease, 622  Gram stain and, 81  identification of, 65-66  laboratory diagnosis of, 210-211  Neisseria gonorrhoeae coinfection and,  210   pelvic inflammatory disease, 627  prevention of, 211  properties of, 208-209  prostatitis caused by, 652  reactive arthritis and, 572  transmission of, 341, 209-210, 7071  treatment of, 211   Candida albicans, 416  Chloramphenicol, 75  clinical uses of, 761, 77  “gray baby” syndrome and, 78  mechanism of action of, 761, 77-78  resistance to, 891, 90  structure of, 77-78, 78/   Chlorhexidine, as antiseptic, 102, 115  Chlorine, as disinfectant, 102  Chloroquine, 437  resistance to, 434, 436-437, 4371  Chocolate agar, 62-64, 631  Cholera, 159-160, 1591  clinical case, 698  epidemics of, 159  transmission of, 341  vaccine against, 98  Cholera toxin (choleragen), 411  ADP-ribosylation by, 39-40, 411  mechanism of action of, 411  synthesis of, 239  Cholera vaccine, 961, 160  Choleragen. See Cholera toxin  Cholesterol, in Mycoplasma pneumoniae  bacterial membrane, 197  Chorioretinitis, Toxoplasma gondii, 438  Chromobacterium violaceum, 219  Chromomycosis, 405  Chromosomal antibiotic resistance, 88  Chronic allograft rejection, 538  Chronic bacterial prostatitis, 628  Chronic carrier state, 621  bacterial, 32, 48  viral, 258   Chronic fatigue syndrome (CFS), 583  Chronic granulomatous disease (CGD),  5791, 581, 7051  phagocytic defect and, 56-57  Staphylococcus aureus infection in, 112  Chronic mucocutaneous candidiasis  (CMC), 414, 416, 579, 5791  Chronic myelogenous leukemia, 364  Chronic osteomyelitis, 593, 594/   Chronic progressive myelopathy, 327-328  Chronic wasting disease (CWD), 376  Chryseobacterium, 219  Chryseobacterium meningosepticum, 219  Chrysops, Loa loa transmission by,    Cidofovir (HPMPC, hydroxyphospho-  nylmethoxypropylcytosine,  Vistide), 2701, 2711, 273-274  Ciliary elevator, 52  Ciliates (Ciliata), 423  Cimex lectularius, 5861, 588, 588/  Ciprofloxacin, 791, 80, 80/ 831, 6171, 628,  650, 651   for Escherichia coli infections, 155  for Neisseria meningitidis, 131  clinical uses of  for anthrax, 137   CJD (Creutzfeldt-Jakob disease), 229, 258,  374-375, 3741, 612  clinical case, 697   iatrogenic transmission of, 373, 375  variant, 373, 3741, 375-376  Clades, of HIV, 379  Cladosporium, 405  Cladosporium werneckii, 405  Clarithromycin, 615   Class I major histocompatibility complex  (MHC) proteins, 491, 506, 537  antigens presented by, 508-509  in bare lymphocyte syndrome, 580  helper T-cell activation and, 506  Class II major histocompatibility complex  (MHC) proteins, 491, 515, 537  allograft acceptance or rejection and,  538   antigens presented by, 508-509  in bare lymphocyte syndrome, 580  helper T-cell activation and, 506  macrophage display of, 502  Class switching, 529-530, 529/   Clear hemolysis, clinical case, 693, 695  Clenched fist injuries, Eikenella corrodens  infections and, 219  “Clenched-fist” injuries, 28  Clindamycin, 627, 640, 644-645, 647  clinical uses of, 761, 115, 122  mechanism of action of, 761, 78  pseudomembranous colitis due to, 78  Clonal anergy, 568, 568/   Clonal deletion, 502, 567  Clonal ignorance, 568  Clonal selection  B cells and, 514  T cells and, 514  Clonorchiasis, 466  Clonorchis, 4631   Clonorchis sinensis, 4631, 465 f 466, 686s  Clostridium, 135, 135f, 137-142, 138/  138f   disease caused by, 29, 42-43, 141, 141/  drugs effective against, 81  exo toxins of, 40 1, 42, 421  normal flora suppression and, 54  overgrowth of, 29, 78  predisposing factors for, 7061  probiotics and, 84  transmission of, 1381, 141  Clostridium histolyticum  disease caused by, 140  oxygen requirement of, 106, 1071  Clostridium novyi, disease caused by, 140  Clostridium perfringens, 107, 1381, 140/   diseases caused by, 140, 140/  drugs effective against, 78  environmental source of, 7041  exotoxins of, 401, 421, 43  identification of, 65  as normal flora, 291    oxygen requirement of, 106  predisposing factors for, 7061  transmission of, 351  Clostridium septicum, disease caused by,  140   Clostridium tetani, 137-139, 1381, 656s  disease caused by, 137-139, 139/ See  also Tetanus   CNS (central nervous system)  infections of, 605-613  neonatal infections of, transmission of,  341   Coartem (artemether/lumefantrine), 4371  Cocci, 4, 4 f. See also Gram-negative  bacteria, cocci; Gram-positive  bacteria, cocci  Coccidioides   host defense against, 57  pneumonia caused by, 638  skin lesions caused by, 7081  transmission and geographic location  of, 3991    environmental source of, 7041  geographical location of, 7041  transmission of, 341  Coccidioidomycosis, 407-409, 408/  clinical case, 699  Cocco-bacillary rod, 172/   Codominant gene expression, 536  Cold agglutinin(s), in Mycoplasma  pneumoniae infections, 197  Cold agglutinin test, 66  clinical case, 694   cord, Bradyrhizobium causing, 219  hemorrhagic, Escherichia coli 0157:H7,  transmission of, 351  pseudomembranous,   antibiotic-associated  Clostridium difficile and, 27, 42  pseudomembranes in, 39, 141, 141/  ulcerative, 572   normal flora of, 271, 28, 291, 29b-30£>,  119, 140, 167, 219-221  antibiotic suppression of,   Colonic diverticula, 620  Colonization, 26-27, 32  Colonization resistance, 27  Colorado tick fever (CTF) virus, 3561, 357  Comensals, 26. See also Normal flora  Common cold, 251, 255, 257, 6291,  632-633  adenoviral, 304  coronavirus, 3121, 323  Coxsackie virus, 336  immunity to, 263  organisms causing, 632-633  parainfluenza virus, 322, 632-633  respiratory syncytial virus, 322,   activation of, 522, 542-543, 543/  transfusion mismatches and, 544  alternative pathway of, 543  anaphylotoxin and, 544  antibody production and, 544  biological effects of, 544  chemotaxis and, 544  classic pathway of, 542  clinical aspects of, 544  cytolysis and, 542, 544  deficiency of, 7051  acquired, 583  congenital, 5791, 580-581  late-acting components, Neisseria  meningitidis, 131  lectin pathway of, 543  membrane attack complex of, 542  opsonization and, 542, 544  regulation of, 543-544  Complement fixation (CF) test, 550,   for Histoplasma capsulatum, 410  viral identification by, 267  Complementarity, in viral genome  replication, 237, 239 1  Complementation, viral, 245, 245/  Complementation test, 245  Conditional lethal mutations, 19, 244  Condylomata acuminata, 307, 307/  treatment of, 276   Congenital infections. See also Fetus;  Pregnancy; Transplacental  transmission  Toxoplasma gondii, 438  Congenital rubella syndrome, 325  Conidia, Aspergillus, 418  Conjugate vaccines, 497   adenoviral, 304-305  allergic, 561, 5591  Chlamydia trachomatis, 114, 208  clinical case, 699  neonatal, 341, 211  gonococcal, 114  Haemophilus aegyptius, 220  Haemophilus influenzae, 114, 171  hemorrhagic   Coxsackie virus, 336  enterovirus, 70, 337  Neisseria gonorrhoeae, 128, 129/  adult, 132   polypeptide chains, 523-524, 528  Contact hypersensitivity, 564  Contact inhibition, loss of, malignant  transformation and, 362  Contact lenses, as predisposing factor for  Pseudomonas aeruginosa and  Acanthamoeba castellani, 7061  Contagious infections, 32  Convalescence period, 48  Coombs test, 551   Core antigen, hepatitis B, 344  Coreceptor antagonists, for HIV, 3861, 388  Corneal infections  herpes simplex virus-1 causing, 291  Pseudomonas aeruginosa, 166  Coronaviruses, 251, 3121, 323-324, 629 1,  673s   diagnosis of disease caused by, 62  disease caused by, 142-143, 1421, 143/  diagnosis of, 143-144  exotoxin of, 401, 41, 421   beta phage required, 142  mechanism of action of, 41, 42/  host defense against, 581  metachromatic granules of, 9  properties of, 142, 143/  Corynebacterium diphtheriae vaccine, 961,  97   Corynebacterium jeikeium, 219  Corynebacterium minutissimum, 219  Costimulatory signals, T-cell activation  and, 506-508, 507/   disease caused by, 212, 2131, 214  reservoirs for, 7021  transmission of, 361, 212  Coxiella burnetii, pneumonia caused by,  215, 637   Coxiella burnetii vaccine, 961, 98, 215  Coxsackie viruses, 251, 286, 2861, 3121,  3331, 3341, 336-337, 674s  clinical case, 692  meningitis and, 606  in myocarditis, 602  proteases of, 2391  skin lesions caused by, 7081  CPE (cytopathic effect), 232-233, 253  viral identification by, 266, 325  C-polysaccharide, pneumococcal, 123  Crab lice, 586, 586/ 5861  Crab meat, Paragonimus westermani  transmission by, 466  Cranberry juice, prophylactic, for    Cranial neuropathies, aseptic, 205  C-reactive protein (CRP)  inflammatory response and, 54  Streptococcus pneumoniae and, 123-124  Creutzfeldt-Jakob disease (CJD), 229, 258,  374-375, 374 1, 612  clinical case, 697   iatrogenic transmission of, 373, 375  variant, 373, 374f, 375-376  Cribriform plate, fracture of, with spinal  fluid leakage, clinical case, 694  Crixivan (indinavir), 2701, 276, 278 b,   Crohn’s disease, 572  Cross-matching, 539  Cross-presentation, 506  Cross-reacting antibody, dengue virus,   Croup, 312 1, 322-323, 629 1, 633, 633/  CRP (C-reactive protein)  inflammatory response and, 54  Streptococcus pneumoniae and, 123-124  Crustaceans. See also specific crustaceans  Dracunculus medinensis transmission  by, 484   Cryptosporidium hominis, 425 1 , 429-431,  430/ 431/ 6161, 6171, 682s  in AIDS, 3851  life cycle of, 430/  transmission of, 351  Crystal violet, 103  CSD (cat-scratch disease), 331, 361,  180-181, 180/    C-substance, pneumococcal, 123  CTF (Colorado tick fever) virus, 3561, 357  CTLA-4 (Cytotoxic T lymphocyte  antigen-4), 507-508, 508/  for cancer, 577  clonal anergy and, 507, 568  CTLA-4-IG, for rheumatoid arthritis, 573  CTX bacteriophage, 160  Cubicin (daptomycin)   West Nile virus transmission by, 357  Wuchereria bancrofti transmission by,  481, 7031   Cache Valley virus transmission by, 392  Eastern equine encephalitis virus  transmission by, 356  Cutaneous anaphylaxis, 560  Cutaneous anthrax, 135, 136, 136/  Cutaneous diphtheria, 144  Cutaneous larva migrans, 469, 4701, 485,  485/ 7071   Cutaneous leishmaniasis, 4251, 447  Cutaneous mycoses, 404-405, 4041  CWD (chronic wasting disease), 376  CXCR4, 380-381  Cyclic neutropenia, 582  Cycloserine, mechanism of action of, 75  Cyclospora cayetanensis, 4251, 451, 684s  Cyclosporiasis, 4251  Cyclosporine, graft rejection and,   Entamoeba coli, 425  Giardia lamblia, 428, 429/  hydatid, Echinococcus granulosus, 459  liver, 459  protozoan, 423   Toxoplasma gondii, ingestion of, 438  Cystic fibrosis, Pseudomonas aeruginosa  and, 165-166, 7061  Cysticerci, 454, 457, 461, 455/  Cysticercosis, 454-455  clinical case, 699  Cystitis, 649-650  definition of, 155  diagnosis of, 64  Escherichia coli causing, 155  hemorrhagic, adenoviral, 304-305  Staphylococcus saprophyticus, 113   affecting polymorphonuclear  leukocytes, 518-519  affecting stem cells, 519  anti-inflammatory, 495  functions of, 517, 5171  production of   by CD4 T-cell subsets, 504, 5061  endotoxins and, 46  by macrophages, 515, 5141, 518  proinflammatory, 54, 495  T-cell tolerance and, 567-568  superantigen release of, clinical case,  696   Cytomegalic inclusion disease, 295  Cytomegalovirus (CMV), 2371, 2481, 250,  289, 295-296, 296/ 3121, 669s  in AIDS, 3851   congenital abnormalities caused by,  clinical case, 693  diseases caused by, 295  esophagitis caused by, 615  features of, 2891, 295  heterophile-negative mononucleosis  caused by, 296  immune evasion by, 257  mechanism, 2571   multinucleated giant cells and, 289  portal of entry for, 2541  transmission of, 341, 2551, 295, 7071  treatment of, 277, 296  Cytopathic effect (CPE), 232-233, 253  viral identification by, 266, 325  Cytoplasm of bacteria, 9-11  Cytoplasmic inclusions, chlamydial, 210  clinical case, 699    Cytotoxic T cells (CD8 T cells, cytotoxic T  lymphocytes), 263, 491, 504-506,  511, 534   activation of, 506  allograft rejection and, 538  delayed hypersensitivity and, 564-565,  564f   immunity to viruses and, 263  regulatory T cells and, 511-512  Cytotoxic T lymphocyte antigen-4  (CTLA-4), 507-508, 508/  for cancer, 577  clonal anergy and, 507, 568  Cytotoxicity tests, Clostridium difficile  and, 142  Cy to toxins, 42   clinical uses of, 81, 115, 123  mechanism of action of, 81  Darkfield microscopy, spirochetes on, 202  Darunavir (Prezista), 270f 274-276, 386f  387   Dasabuvir, 275, 350  DCF (dichlorofluorescein) test, 581  ddl (didanosine), 270f 271 1, 274, 386f  387   D-dimers, 46  Dead-end hosts  of arboviruses, 355  of Francisella tularensis, 178  Death phase, of bacterial growth cycle, 15  Decay-accelerating factor (DAF), 544  deficiency of, 544, 581  Deer   Deer flies, Loa loa transmission by, 484  Defective interfering particles, 244  Defective viruses, 229  replication of, 244  Defensins, 53, 495  a-Defensins    Dehydration, in cholera, 160  Delavirdine (Rescriptor), 270 1, 271 1, 275,  386f, 387   Delayed hypersensitivity reactions, 491,  5571, 558f 563-565, 564/ 564 1  tests for ability to develop, 535  tests for presence of, 535  Delayed hypersensitivity skin test, for  fungal infections, 398  Deletion mutants, viral, 244  Delivery, Listeria monocytogenes  transmission during, 144  Deltavirus (hepatitis D virus), 249f, 252,  254f, 342 1, 351-352, 351 f 676s  Demodex mites, 591  Dendritic cells, 491, 515  follicular, 515   inflammatory response and, 55  Dengue hemorrhagic fever, 358-359  Dengue virus, 249f, 251, 358-359, 3581,  677s   geographical location of, 7041  portal of entry for, 254f  proteases of, 2391  reservoir for, 255f  vectors for, 703f   brain abscesses following, 121  endocarditis following, 121  prevention of, 84, 123  as predisposing factor for viridans  group streptococci, 121, 706f  Deoxyribonucleic acid. See DNA  Dermacentor ticks, 5861, 589-590, 589/  690s   Anaplasma phagocytophilum  transmission by, 7031  Ehrlichia chaffeensis transmission by,  361, 7031   rickettsial disease transmission by, 361,  213, 357, 7031  tick paralysis caused by, 589  Dematiaceous fungi, 405  Dermatobia hominis, 5861, 587-588, 588/  689s-690s   DGIs (disseminated gonococcal  infections), 132, 7081  DHPG (ganciclovir), 2701, 2711, 272/  273-274, 2771  Diabetes mellitus, 7061  Coxsackie virus and, 337  diseases associated with, 57  host defense defect in, 57  insulin-dependent, 5691, 572  insulin-resistant, 5691, 572  Pseudomonas aeruginosa infection in,  163   Diagnosis. See also Laboratory diagnosis  Koch’s postulates and, 48-49  Diaminodiphenylsulfone (dapsone), 192  Diapedesis, 55   Balantidium coli, 4251, 451  Campylobacter, 1501, 161  compared to watery, 6161  Entamoeba histolytica, 424  Escherichia coli, 153-154  organisms causing, 6171  Salmonella enterica, 155  Shiga toxin and, 44  Shigella, 158. See also Shigellosis  transmission of, 351  clinical presentation, 6171  Clostridium difficile, 141  Clostridium perfringens, 140  transmission of, 351    Cryptosporidium parvum  in AIDS, 3851  transmission of, 351  cytomegalovirus, in AIDS, 3851  diagnosis, 6171  enterovirus, 71  Escherichia, 150 1  Escherichia coli, 1501  bloody, 153-154  enteropathic, 1501  prevention of, 155  transmission of, 351  treatment of 155  watery, 153-154  Giardia lamblia, 428  in AIDS, 3851  transmission of, 351  Isospora belli, 451-452  Listeria monocytogenes, transmission of,  351   Norovirus, transmission of 351, 338  organisms causing, 64, 616, 6161, 6171  clinical presentation, 6171  diagnosis, 6171  rotaviral, 251  Salmonella, 1501, 156-157  in AIDS, 3851  transmission of, 351  Salmonella enteritidis, 351  Shigella, 1501, 158  in AIDS, 3851  transmission of, 351  treatment, 6171  Vibrio cholerae, 1501, 159  transmission of, 351  Vibrio parahaemolyticus, transmission  of, 351   watery, 32, 615-619  in cholera, 160  clinical case, 698  compared to bloody, 6161  Cryptosporidium parvum, 430  Cyclospora cayetanesis, 451  Escherichia coli, 153-154  in giardiasis, 428  heat-labile enterotoxin and, 43  organisms causing, 6161  rotavirus, clinical case, 693  Vibrio parahaemolyticus, 160  DIC (disseminated intravascular  coagulation)  endotoxins and, 45, 451  in meningococcemia, clinical case,    Didanosine (ddl, dideoxyinosine, Videx),  2701, 2711, 274, 3861, 387  Didehydrodideoxythymidine (stavudine),  2701, 2711, 274, 3861, 387  Dideoxyinosine (didanosine), 2701, 2711,  274, 3861, 387   Dideoxythiacytidine (lamivudine), 2701,  2711, 274, 2771, 3861, 387  Dificid (fidaxomicin), 81, 142  DiGeorge’s syndrome, 579, 5791, 7061  Dihydrofolate reductase, inhibition by  trimethoprim, 80, 791  Dihydroxypropoxymethylguanine   Dimorphic fungi, 398, 405, 407, 408/  Blastomyces dermatitidis as, 411  Coccidioides immitis as, 407  Histoplasma capsulatum as, 409  Paracoccidioides brasiliensis as, 412  Diphosphatidylglycerol (cardiolipin), 66,  202   Diphtheria, 142-144, 1421, 143/  diagnosis of, 62, 143-144  pseudomembranes in, 39  Diphtheria antitoxin, 98, 144  Diphtheria toxin, 41, 411   ADP-ribosylation by, 39-40, 40/ 41, 421  mechanism of action of, 41, 42/  synthesis of, 239  Diphtheria toxoid, 97, 971, 144  Diphtheria toxoid-tetanus toxoid-acellular  pertussis (DTaP) vaccine, 971, 144,  174   Diphtheria vaccine, 961, 97  Diphtheroids, 142, 145  as normal flora, 29  Diphyllobothriasis, 458-459  Diphyllobothrium latum, 4541, 458-459,  685s, 7031  Dipicolinic acid, 12  Diplococci, 4   gram-negative, clinical case, 693  pneumococcal, 123  Diploid cells, 18  Diploid viruses, 235  HIV as, 378   Direct immunofluorescence, 550  Directly observed therapy (DOT), for  tuberculosis, 189  Disinfection, 100-101, 104b    chemical agents for, 101-102  clinical uses of, 1011  definition of, 100  rate of killing and, 101-103  Disseminated gonococcal infections  (DGIs), 132, 7081   synthesis of, drugs inhibiting, 79  damage to, mutations caused by, 18-19  fungal, synthesis of, drugs inhibiting,   between bacterial cells, 19-22, 201  within bacterial cells, 19, 20/  transposons and, 10-11, 10/  viral, integration into cell DNA, 240  DNA polymerase   herpes simplex virus, 291  DNA probes, for Chlamydia trachomatis  diagnosis, 65  DNA rearrangement, in   immunoglobulins, 528-529, 529/  DNA sequencing, for HLA typing, 539  DNA synthesis, RNA-dependent, of  hepatitis B virus, 345  DNA vaccines, 281  DNA viruses. See Virus(es), DNA  DNase, of Streptococcus pyogenes, 120  Dog(s)  bites of, 65   Capnocytophaga canimorsus  infections due to, 219  Capnocytophaga canimorsus  transmission by, 7021  Pasteurella multocida infections due  to, 180   for Echinococcus granulosus, 7021  for Ehrlichia chaffeensis, 361, 7031  for Leptospira interrogans, 361, 7021  for Microsporum canis, 702 1  for Rickettsia rickettsii, 36 1, 7031  for Toxocara canis, 702 1  tapeworm of, 461  Dog tapeworm, 459, 460/   Rocky Mountain spotted fever  transmission by, 213  Dolutegravir, 276, 278 b, 388  Domestic animals. See also specific  animals and organisms  as reservoirs, 36 1   for Escherichia coli infections, 155  Dracunculiasis, 484  Dracunculus, 469, 4701, 471 1  Dracunculus medinensis, 484, 688s  Drinking water. See Water, transmission  by   botulism and, 139  Candida albicans and, 414, 416  Erwinia and, 220  Pseudomonas aeruginosa and, 166  Staphylococcus aureus and, 706f  tetanus and, 138  Drug interaction, 93/  antagonistic, 93, 93/  synergistic, 80, 93  Drug rashes, 560, 562  Drug-resistant mutants, viral, 244  d4T (stavudine), 2701, 271 1, 274, 3861,    DTaP (diphtheria toxoid-tetanus toxoid-  acellular pertussis) vaccine, 97 1,  144, 174   Dumb rabies, 610  Dwarf tapeworm, 460  Dyes, as antiseptics, 103  Dysentery, 615. See also Diarrhea, bloody  Entamoeba histolytica, amebic  dysentery causing, 425  Shigella, bacillary dysentery causing,  34f 78, 1501, 154, 158  Dyspepsia, gastritis and, 615  Dysphagia, in esophagitis, 614   e antigen, hepatitis B, 344  E6 gene, of papillomaviruses, 305  E7 gene, of papillomaviruses, 305  Early genes, malignant transformation  and, 365   Early proteins, viral, 236  Eastern equine encephalitis (EEE) virus,  356, 676s   Ebola virus, 2491, 251, 329, 583, 674s  diagnosis, 330  disease caused by, 329  genome replication in, 2371  geographical location of, 7041  hemorrhagic fever, 329-330  immune evasion by, 256-257  pathogenesis by, 256  properties, 329-330  replication, 330  skin lesions caused by, 7081  transmission, 330  treatment, 330-331  types, 330   EBV. See Epstein-Barr virus  EBV-specific antibody tests, 298  ECF-A (eosinophil chemotactic factor of  anaphylaxis), 559   Echoviruses, 251, 2861, 3331, 3341, 337  Eclipse period, of viral growth curve, 232  ECM (erythema chronicum migrans), in  Lyme disease, 204, 204/ 7071   Ecthyma gangrenosum, Pseudomonas  aeruginosa, 166, 166/  Ectoparasites, 585-591, 5861, 689s-690s  arachnids as, 5861, 588-590  disease caused by, 585, 5861  insects as, 585-588, 5861  medical importance of, 591  Ectopic pregnancy   Chlamydia trachomatis, 210  Neisseria gonorrhoeae, 132  Eczema, in hypersensitivity reactions, 558,  5591   periorbital in trichinosis, 481  Edema factor, of Bacillus anthracis, 136  Edwardsiella, 219   epidemiology of, 3561  proteases of, 2391  EF-2 (elongation factor 2), ADP-  ribosylation of, by  Corynebacterium diphtheriae,   anaphylactic reaction to, 281  as reservoir, for Salmonella, 361, 157  schistosome,  clinical case, 697  tapeworm, 458   Clostridium perfringens and, 140  Ehrlichia chaffeensis, 215, 668s, 7031  transmission of, 361  Ehrlichia equi. See Anaplasma  phagocytophilum  Ehrlichiosis, transmission of, 33  Eikenella corrodens, 219, 668s  as normal flora, 28  elPV (enhanced polio vaccine), 335  Elderly persons   vaccines recommended for, 97, 125, 291  Electrocardiogram (ECG), for cardiac  infections, 598   immunosorbent assay  Elite controllers, 383  Elk, chronic wasting disease in, 376  Elongation factor 2 (EF-2), ADP-  ribosylation of, by  Corynebacterium diphtheriae,   Elvitegravir, 2701, 276, 388  EMB (eosin-methylene blue) agar, 63 1, 64,  1501, 151-152, 155, 157-158, 163  Embolic phenomena, in endocarditis, 599  Emtricitabine (Emtriva, FTC), 270 1, 271 1,  274, 386, 3861   EN (erythema nodosum), 6881  in coccidioidomycosis, 409  in histoplasmosis, 410  in leprosy, 192   arbovirus, 356. See also California  encephalitis; Colorado tick fever  virus; Eastern equine encephalitis;  St. Louis equine encephalitis; West  Nile virus; Western equine  encephalitis  enterovirus 71, 337  herpes simplex virus-1, 290, 292, 609,  609/   Encephalitis viruses. See also specific  encephalitis viruses  reservoirs for, 2551, 6831  vectors for, 6831  Encephalitozoon intestinalis, 452  Encephalopathy, 612-613  HIV and, 612   Blastomyces dermatitidis as, 411  Coccidioides immitis as, 408  Histoplasma capsulatum as, 409  Endemic infections, 32  Endocarditis, 598-602   bacterial. See Bacterial endocarditis  Candida species, 601  Candida albicans, 414, 416  clinical manifestations of, 599-600  diagnosis of, 601-602, 6021  etiology, 6011  left-sided, 599   pathophysiology of, 598-599  prevention, 602  right-sided, 600  Staphylococcus aureus, 109  treatment, 601-602  vegetations in, 598, 599/  viridans streptococci, 121  Endogenous pyrogen. See Interleukin-1  Endometritis   Endotoxin(s), 8, 32, 44-46, 50 b. See also  Lipopolysaccharide (LPS); specific  endotoxins   effects of, 44-46, 451, 461  Escherichia coli, 154  exotoxins versus, 391  in gram-negative bacterial cell wall, 6  mechanism of action of, 44, 45/  Neisseria, 128  Neisseria gonorrhoeae, 132  Neisseria meningitidis, 131  Pseudomonas aeruginosa, 166  toll-like receptors and, 494-495  Yersinia pestis, 179   Endotoxin-induced septic shock, tumor  necrosis factor-a and, 519  End-stage hosts, humans as, for  Trichinella spiralis, 480  Endurant (rilpivirine), 2701, 2711, 275,  3861   4261, 427/ 428/ 6161, 6171, 682s  diseases caused by, 424  Entecavir (Baraclude), 2701, 2711, 275, 347  Enteric fever, 620-621  Enterobacter, 162-163  antibiotic-resistant, 90  classification of, 1061  identification of, 64  laboratory diagnosis of, 1511  as normal flora, 28  in public water supply, 152  Enterobacter cloacae, 162-163, 660s  Klebsiella pneumoniae differentiated  from, 152   carbapenemase-producing, drugs  effective against, 81  common features of, 149  laboratory diagnosis of, 151-152, 1511  nonfermenting gram-negative rods  distinguished from, 149  pathogenesis by, 151  public health and, 152  Enterobiasis, 4701, 470-471  clinical case, 694  Enterobius, 469, 4701, 470  Enterobius vermicularis, 470, 4711, 472/  687s   clinical case, 694  Enterococcus, 601  classification of, 1061  diseases caused by, 121  as normal flora, 281  vancomycin-resistant, 90, 123  Enterococcus faecalis, 654s   antibiotic-resistant, 88, 881, 90  clinical case, 698  diseases caused by, 116  drugs effective against, 78, 81  hospital-related events predisposing to  infection by, 7051  identification of, 64  as normal flora, 261, 28, 281  pathogenesis by, 1191  Enterococcus faecium, 81, 118    Enterocolitis, 615-619, 6161  Arizona hinshawii, 218  Campylobacter, 161  in children, Salmonella septicemia in,  156   diagnosis of, 64  Edwardsiella, 219  organisms causing, 64  Salmonella, 1501, 155-156  clinical findings in, 157  Shigella. See Shigellosis  Yersinia, 1501  Yersinia enterocolitica, 222  Enterocytozoon bieneusi, 4251,   Enterotoxins. See also specific enterotoxins  Bacillus cereus, 137  Clostridium perfringens, 140-141  Escherichia coli, 153-154  Staphylococcus aureus, 112  Vibrio cholerae, 159   See also specific enteroviruses  Enterovirus 68, 337  Enterovirus 70, 337  Enterovirus 71, 337  Enterovirus 72. See Hepatitis A virus  Entry inhibitors, for HIV, 3861, 388  env gene   Envelope, viral, 226, 228-229, 230b  Envelope glycoproteins, type-specific, of  HIV, 379-380  Enveloped viruses   DNA, 285, 2861, 288-301, 668s-670s.  See also specific enveloped DNA  viruses   icosahedral, 249  helical, 251  icosahedral, 251  RNA, 285-286, 2861, 311-331,  671s-674s. See also specific  enveloped RNA viruses  Environment, as fungal habitat, 398  Environmental agents, autoimmune  disorders and, 570, 5701  Enzyme(s)  autolytic, 71   Enzyme-linked immunosorbent assay  (ELISA), 66, 340, 549-550, 549/  for Chlamydia trachomatis diagnosis, 65  Clostridium difficile and, 142  viral detection by, of HIV, 385  viral identification by, 267  Eosin-methylene blue (EMB) agar, 631, 64,  151-152, 155, 157-158, 163  Eosinophil(s), 516-517  Th-2 cells and, 511  Eosinophil chemotactic factor of  anaphylaxis (ECF-A), 559  Eosinophilia  in filariasis, 481   nematode infections causing, 469  in hookworm infection, 477  in Strongyloides infection, 478  in trichinosis, 481   cytoplasm of neurons, clinical  case, 697  Eotaxin, 517  Epidemic(s), 32  of cholera, 159  of influenza, 315  Epidemic encephalitis, Japanese   encephalitis virus and, 393  Epidemic myalgia, 337  Epidermophyton, 404, 4041  skin lesions caused by, 7081  Epidural empyema, 612  Epiglottitis, 171, 629 1, 634, 634 f  Epithelial cells, of mucous membranes   B-cell presentation of, 493  immunogenicity and, 498  Epitope spreading, autoimmune diseases  and, 571   Epstein-Barr virus (EBV), 237 1, 248 1, 250,  296-299, 3121, 603, 632, 632/,   amphotericin B and, 81, 400  synthesis of, inhibition by azoles, 81  Ergotism, 399  Ehrlichia chaffeensis, 215  Erwinia, 220   Erythema chronicum migrans (ECM), in  Lyme disease, 204, 204/ 7071  Erythema gangrenosum, 7071  Erythema infectiosum, 250, 308  Erythema multiforme, 198, 564-565, 565/  hepatitis B virus, 292  hepatitis C virus, 292  herpes simplex virus, 290, 292  major. See Stevens-Johnson syndrome  Mycoplasma pneumoniae, 198, 293  sulfonamides causing, 79  Erythema nodosum (EN), 7081  in coccidioidomycosis, 409  in histoplasmosis, 410  in leprosy, 192   Erythroblastosis fetalis, 554, 554/  Erythrogenic toxin, 43, 239  of Streptococcus pyogenes, 120  Erythromycin   chemoprophylactic use of, 831  clinical uses of, 761, 78, 122, 125, 144,  161, 198, 2081, 211  mechanism of action of, 761, 78  resistance to, 891, 90    diseases caused by, 149, 1491, 1501  laboratory diagnosis of, 1511  Escherichia coli, 148/ 153-155, 1541, 658s  antibiotic-resistant, 88, 90  causing appendicitis, 619  causing diverticulitis, 620  clinical case, 694-695, 698  colicins produced by, 10  curli of, 37   Enterohemorrhagic Escherichia  coli; Escherichia coli 0157  enteropathic, 64, 153  enterotoxigenic, 153-154  exotoxins of, 32, 401, 411  heat-labile, 43  heat-stable, 43  mechanism of action of 411  flagella of, 11  genetic material of, 18  growth of 15, 16   hospital-related events predisposing to  infection by, 7051  identification of, 62, 64  injectosomes of, 40  as normal flora, 261, 271, 28  oxygen requirement of, 106, 1071  pathogenicity islands of, 38, 47, 48/  predisposing conditions for infections  by, 581   properties of, 153  prostatitis caused by, 652  Salmonella distinguished from, 16, 153  Shigella distinguished from, 16, 153  surface virulence factors of, 36-37,   toxigenic, identification of, 64  transmission of, 341, 351, 7071  urinary tract infection due to, in  diabetes mellitus, 57  uropathic, 154   water quality testing and, 152  Escherichia coli 0157, 153-154, 616  clinical case, 694  exotoxin of, 421  identification of, 64  reservoirs for, 7021  Shiga toxin of 44  transmission of, 351, 361  Escherichia coli toxin, ADP-ribosylation  by, 39-41, 421   Candida albicans, 414, 416  cytomegalovirus, oral, 3851  herpes simplex virus-1, 292  oral, 3851   clinical uses of, 82, 188  mechanism of action of, 82  resistance to, 91  Ethanol, as antiseptic, 102  Ethoxynaphthamidopenicillin. See  Nafcillin   387, 3861  Eubacterium, 220  as normal flora, 281  Eukaryotes. See also Fungi; Molds;  Yeast(s); specific fungi  as diploid cells, 18  DNA transfer between cells in, 201,  21-22   Exotoxin(s), 32, 39-44, 401, 50b. See also  specific exotoxins  acute diarrhea caused by, 616  of Bacillus anthracis, 136  Bacillus cereus, 616  Clostridium botulinum, 139  Clostridium difficile, 141-142  Clostridium perfringens, 616  Clostridium tetani, 138  Corynebacterium diphtheriae, 143  endotoxins versus, 391  Escherichia coli, 153-154, 616  of gram-negative bacteria, 43-44, 44/  of gram-positive bacteria, 41-43  main location of symptoms of diseases  caused by, 421   mechanisms of action of, 40-41, 421  Staphylococcus aureus, 112, 616  Streptococcus pyogenes, 120  structure of, 39-40, 40/  synthesis of, lysogeny and, 239  toxicity of, 39  Vibrio cholerae, 159, 616  Exotoxin B, of Streptococcus pyogenes, 120  Exponential phase, of bacterial growth  cycle, 15, 15/   Exudative lesions, Mycobacterium  tuberculosis, 186  Eye, Loa loa in, 484  Eye infections. See Conjunctivitis,   Facial nerve palsy, in Lyme disease, 205  Factor H binding protein, 131  Factor V, Haemophilus influenzae growth  on laboratory media and, 172  Factor X, Haemophilus influenzae growth  on laboratory media and, 172  Facultative anaerobes, 16  Facultative parasites, 31  Famciclovir (Famvir), 270f 271 1, 273, 623,  6241   Farm animals. See specific animals  FAS protein, activation of, 264  Fasciola hepatica, 462, 467  Fasciolopsis buski, 462, 467  Fas-Fas ligand (FasL) interaction, 511  Fat, malabsorption of, in giardiasis, 428  Fatal familial insomnia, 3741, 375  Favus, 404   Fecal-oral transmission  of adenoviruses, 304-305  of Campylobacter, 161  of Coxsackie viruses, 336  of Cryptosporidium parvum, 430  of echoviruses, 337  of Entamoeba histolytica, 425  of Giardia lamblia, 428  of hepatitis A virus, 343  of poliovirus, 334  of rotavirus, 339  of Shigella, 159  Fermentation, of sugars, 16  Fertility (F) plasmid, 19-20, 21/   cytomegalic inclusion disease in, 295  hydrops fetalis in, 308-309  clinical case, 698   Fibroma-myxoma virus, 371  Fidaxomicin (Dificid), 81, 142  Fifth disease, 308-309, 309/ 7081  Filaments, hepatitis B, 344, 344/   Filariasis, 4701, 481-482  Filariform larvae, 469, 474, 476  Filgrastim (granulocyte colony-  stimulating factor), 519  Filoviruses, 227/ 251, 329-330  Filtration, 104  Fimbriae, bacterial, 61, 11  First-set allograft reactions, 538  Fish, as reservoirs  for Anisakis simplex, 7031  for Clonorchis sinensis, 466  for Diphyllobothrium latum, 7031  for Erysipelothrix rhusiopathiae, 361  for Heterophyes heterophyes, 467  Fish tank granuloma, 190  Flaccid paralysis, in botulism, 138  in poliomyelitis, 335  Flagella, bacterial, 61, 11  Proteus species, 163  Flagellates, 424   clinical uses of, 82, 138, 142, 145, 625  mechanism of action of, 82  Flatworms, 423  Flaviviruses, 237, 251, 3551  proteases of, 2391    Loa loa transmission by, 484  Onchocerca volvulus transmission by,  483, 7031   Paragonimus westermani;  Schistosoma; Trematodes  Flumadine (rimantadine), 270, 2701, 316  Fluorescence-activated cell sorting test,  551, 552/   Fluorescent antibody assay, viral  identification by, 267  Fluorescent antibody tests, 66, 550, 550/  Fluorescent treponemal antibody-   Fluorocytosine (flucytosine), 791, 81, 4001  Fluoroquinolones, 80. See also specific  drugs   hemolytic-uremic syndrome and, 155  mechanism of action of, 80  pseudomembranous colitis due to, 141  structure of, 80, 80/   Follicular dendritic cells (FDCs), 515-516  Folliculitis, 6421, 645-646, 645/ 7071  hot tub, 166, 7071  Pseudomonas aeruginosa, 166, 7071  Staphylococcus aureus, 109, 110/ 7071  Fomites, 33, 331   Clostridium botulinum, 139  Clostridium perfringens, 140  Listeria monocytogenes, 144-145  Staphylococcus aureus, 351, 114  Yersinia, 222  Foot   Frameshift mutations, 18, 19  Francisella, classification of, 1061  Francisella tularensis, 178-179, 663s  disease caused by. See Tularemia  properties of, 178  reservoirs for, 7021  transmission of, 351, 361  Francisella tularensis vaccine, 961, 97  Friction rub, in pericarditis, 603  Fried rice, reheated, Bacillus cereus food  poisoning caused by, 137  FTA-ABS (fluorescent treponemal   allergic reactions to, 399, 401 b  antifungal therapy and, 400, 4001, 401 b  bacteria compared with, 3971  causing cutaneous and subcutaneous  mycoses, 679s   causing systemic mycoses, 679s-680s  cell walls of, drugs inhibiting synthesis  of, 75    pathogenesis by, 398-399, 3991, 401 b  reproduction by, 397-398  structure and growth of, 397-398, 3971,  398 f 400b-40lb  toxins of, 395, 401 b  Fungi imperfecti, 398  Fungizone (amphotericin B), 400  adverse effects of, 81, 400t  mechanism of action of, 81, 81/ 400t  structure of, 81, 81/   Pseudomonas aeruginosa, 7071  Staphylococcus aureus, 112-113, 707 1  Fusarium solani, 420  Fusion inhibitors, 3861, 388  Fusion proteins, of respiratory syncytial  virus, 321  Fusobacterium   Fusobacterium necrophorum, 631  Fusobacterium nucleatum, 662s  Fuzeon (enfuvirtide), 270, 270 1, 386 1, 388   of animal retroviruses, 370  of HIV, 378-379, 3801  Gag polyprotein, HIV, 381  Gal-Gal dimers, 154  GALT (gut-associated lymphoid tissue),  502, 514   Gametocytes, 434  banana-shaped, 436/ 437  clinical case, 697  crescent-shaped, of Plasmodium  falciparum, 436/ 437  Gamma globulin  for common variable   dihydroxypropoxymethylguanine),  2701, 2711, 272/ 273, 2771  Gardasil vaccine, 308  Gardnerella vaginalis, 145, 624, 6261, 658s  as normal flora, 261  Gas gangrene, 140, 140/ See also  Necrotizing fasciitis  Gastric carcinoma, Helicobacter pylori  infection associated with, 47  Gastric mucosal-associated lymphoid   Gastritis, Helicobacter pylori, 1591, 162  Gastroenteritis, 615-619, 6161  adenoviral, 304-305  febrile, Listeria monocytogenes, 144  foodborne. See Foodborne diseases  Listeria monocytogenes, 144  Norwalk virus, 338  Plesiomonas shigelloides, 221  rotaviral, 339   Staphylococcus aureus, 114  Gastroenteropathy, allergic, 5591  Gastrointestinal anthrax, 135-136  Gastrointestinal bleeding, in peptic ulcer  disease, 615   disease caused by exotoxins in, 421  infections of, 614-621  appendicitis, 619-620  diarrhea, 615-619  diverticulitis, 620, 620/  enteric fever, 620-621  enterocolitis, 615-619  esophagitis, 614-615  gastroenteritis, 615  typhoid fever, 620-621  mucosal epithelium of, adenovirus  infections and, 305  nonspecific host defense in, 53  normal flora of, 271, 28, 281, 30b, 119,  219, 414   in colon, 140, 142, 165, 167, 219, 221  as portal of entry  bacterial, 341  viral, 2541   malignant transformation and, 364-365  Gene therapy, 244-245  Generalized transduction, 20-21  Genetic disorders, in phagocytosis, 56  Genetic predisposition, to autoimmune  disorders, 569-570  Genetics, 18-22, 244-247  antibiotic resistance and, 88-89, 94b  chromosome-related resistance and,  88   Genital tract. See also Genitourinary tract  Chlamydia trachomatis infections of,  209, 2081, 209/   female. See also Cervical carcinoma;  Vagina; Vaginitis; Vaginosis  normal flora of, 220, 414, 415/  Genital tract cultures, 64-65  Genital ulcer disease, 622-623, 6241  Genital warts, 307, 307/  transmission of, 341 
ReviewOfMedicalMicrobiologyAndImmunologyByWarrenLevinson-14thEdition Review of Medical Microbiology and Immunology By Warren Levinson – 14th edition_djvu.txt