Protozoa are single-celled eukaryotes that are major causes of disease and death in developing countries. Protozoa can replicate intracellularly within a variety of cells (e.g., Plasmodium in red cells, Leishmania in macrophages) or extracellularly in the urogenital system, intestine, or blood. Trichomonas vaginalis organisms are sexually transmitted flagellated protozoal parasites that often colonize the vagina and male urethra. The most prevalent pathogenic intestinal protozoans, Entamoeba histolytica and Giardia lamblia, are ingested as nonmotile cysts in contaminated food or water and become motile trophozoites that attach to intestinal epithelial cells. Bloodborne protozoa (e.g., Plasmodium, Trypanosoma, Leishmania) are transmitted by insect vectors, in which they replicate before being passed to new human hosts. Toxoplasma gondii is acquired either through contact with oocyst-shedding cats or by eating cyst-ridden, undercooked meat.
Fig. 9.4 MeningealbloodvesselswithangioinvasiveMucor species.Notetheirregularwidthandnearright-anglebranchingofthehyphae(arrow). (Courtesy of Dr. Dan Milner, Department of Pathology, Brigham and Women’s Hospital, Boston, Massachusetts.)
Parasitic worms are highly differentiated multicellular organisms. Their life cycles are complex; most alternate between sexual reproduction in the definitive host and asexual multiplication in an intermediate host or vector. Thus, depending on the species, humans may harbor adult worms (e.g., Ascaris lumbricoides), immature stages (e.g., Toxocara canis), or asexual larval forms (e.g., Echinococcus spp.). Once adult worms take up residence in humans, they usually do not multiply but they produce eggs or larvae that typically are passed in stool. Often, the severity of disease is proportional to the number of infecting organisms. For example, a burden of 10 hookworms is associated with mild or no clinical disease, whereas 1000 hookworms consume enough blood to cause severe anemia. In some helminthic infections, such as schistosomiasis, disease is caused by inflammatory responses to the eggs or larvae rather than the adult worms.
Helminths comprise three groups:
Roundworms (nematodes) are circular in cross-section and nonsegmented. Intestinal nematodes include A. lumbricoides, Strongyloides stercoralis, and hookworms. Nematodes that invade tissues include the filariae, such as Wuchereria bancrofti and Trichinella spiralis (
Fig. 9.5
Tapeworms (cestodes) have a head (scolex) and a ribbon of multiple flat segments (proglottids). They adsorb nutrition through their tegument and do not have a digestive tract. They include the fish, beef, and pork tapeworms that make their home in the human intestine. The larvae that develop after ingestion of eggs of certain tapeworms can cause cystic disease within tissues (Echinoccus granulosus larvae cause hydatid cysts; pork tapeworm larvae produce cysts called cysticerci in many organs).
Flukes (trematodes) are leaf-shaped flatworms with prominent suckers that are used to attach to the host. They include liver and lung flukes and schistosomes.
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Fig. 9.5 CoiledTrichinella spiralis larvawithinaskeletalmusclecell.
Ectoparasites are insects (e.g., lice, bedbugs, fleas) or arachnids (e.g., mites, ticks, spiders) that cause disease by biting or by attaching to and living on or in the skin.