Pneumococcal infections occur with increased frequency in two clinical settings: (1) chronic diseases such as CHF, COPD, or diabetes; and (2) congenital or acquired defects in immunoglobulin production (e.g., acquired immune deficiency syndrome [AIDS]). In addition, decreased or absent splenic function (e.g., sickle cell disease or after splenectomy) greatly increases the risk for overwhelming pneumococcal sepsis. You will recall that the spleen contains the largest collection of phagocytes in the body and is the major organ responsible for removing pneumococci from the blood. The spleen also is an important site for
Table 13.5 The Pneumonia Syndromes and Implicated Pathogens
Streptococcus pneumoniae Haemophilus influenzae Moraxella catarrhalis Staphylococcus aureus Legionella pneumophila Enterobacteriaceae(Klebsiella pneumoniae) andPseudomonas spp. Mycoplasma pneumoniae Chlamydia pneumoniae Coxiella burnetii (Qfever)
Respiratorysyncytialvirus,humanmetapneumovirus,parainfluenzavirus(children);influenzaAandB(adults);adenovirus(militaryrecruits)
Gram-negativerodsbelongingtoEnterobacteriaceae (Klebsiella spp.,Serratia marcescens, Escherichia coli)andPseudomonas spp.
S. aureus (usuallymethicillin-resistant)
Anaerobicoralflora(Bacteroides, Prevotella, Fusobacterium, Peptostreptococcus),admixedwithaerobicbacteria(S. pneumoniae,
S. aureus, H. influenzae, andPseudomonas aeruginosa)
NocardiaActinomycesGranulomatous:Mycobacteriumtuberculosisandatypicalmycobacteria,
Histoplasma capsulatum, Coccidioides immitis, Blastomyces dermatitidis
Anaerobicbacteria(extremelycommon),withorwithoutmixedaerobicinfection
S. aureus, K. pneumoniae, Streptococcus pyogenes, andtype3pneumococcus(uncommon)
Mycobacteriumaviumcomplex(MAC)InvasiveaspergillosisInvasivecandidiasis“Usual”bacterial,viral,andfungalorganisms(listedabove) production of antibodies against polysaccharides, which are the dominant protective antibodies against encapsulated bacteria.
The presence of numerous neutrophils in sputum containing the typical gram-positive, lancet-shaped diplococci supports the diagnosis of pneumococcal pneumonia, but it must be remembered that S. pneumoniae is a part of the endogenous flora in 20% of adults, and therefore false-positive results may be obtained. Isolation of pneumococci from blood cultures is more specific but less sensitive (in the early phase of illness, only 20% to 30% of patients have positive blood cultures). Pneumococcal vaccines containing capsular polysaccharides from the common serotypes are used in individuals at high risk for pneumococcal sepsis.
Both encapsulated and unencapsulated forms of H. influenzae are important causes of community-acquired pneumonias. The former can cause a particularly life-threatening form of pneumonia in children, often after a respiratory viral infection. Adults at risk for developing infections include those with chronic pulmonary diseases such as chronic bronchitis, cystic fibrosis, and bronchiectasis. H. influenzae is the most common bacterial cause of acute exacerbations of COPD. Encapsulated H. influenzae type b was formerly an important cause of epiglottitis and suppurative meningitis in children, but vaccination against this organism in infancy has significantly reduced the risk.