Nematodes

The roundworms form an extremely large yet fairly homogeneous assemblage, most of which are free-living (nonparasitic). Some parasitic nematodes, however, may cause disease in humans (zoonosis), and others cause disease limited to human hosts (anthroponosis). Among the latter, several are enormously abundant and widespread. See Nemata

The giant roundworm (Ascaris lumbricoides) parasitizes the small intestine, probably affecting over a billion people; and the whipworm (Trichuris trichiura) infects the human colon, probably affecting a half billion people throughout the tropics. Similarly, the hookworms of humans, Necator americanus in the Americas and the tropical regions of Africa and Asia, and Ancylostoma duodenale in temperate Asia, the Mediterranean, and Middle East, suck blood from the small intestine and cause major debilitation, especially among the undernourished. The human pinworm (Enterobius vermicularis) infects the large intestine of millions of urban dwellers. Most intestinal nematodes, which require a period of egg maturation outside the human host before they are infective, are associated with fecal contamination of soil or food crops and are primarily rural in distribution.

The nonintestinal nematodes are spread by complex life cycles that usually involve bloodsucking insects. One exception is the guinea worm (Dracunculus medinensis), a skin-infecting 2–3-ft (0.6–1-m) worm transmitted by aquatic microcrustaceans that are ingested in drinking water that has been contaminated by larvae that escape from the skin sores of infected humans. Such bizarre life cycles are typical of many helminths. Other nematodes of humans include (1) the filarial worms, which are transmitted by mosquitoes and may induce enormously enlarged fibrous masses in legs, arms, or genitalia (elephantiasis), and (2) Onchocerca volvulus, which is transmitted by blackflies (genus Simulium) and forms microscopic embryos (microfilariae) in the eyes causing high incidence of blindness in Africa and parts of central and northern South America.

A more familiar tissue-infecting nematode of temperate regions is Trichinella spiralis, the pork or trichina worm, which is the agent of trichinosis. The tiny spiraled larvae encyst in muscle and can carry the infection to humans and other carnivorous mammals who eat raw or undercooked infected meat.

Trematodes

Parasites of the class Trematoda vary greatly in size, form, location in the human host, and disease produced, but all go through an initial developmental period in specific kinds of fresh-water snails, where they multiply as highly modified larvae of different types. Ultimately, an infective larval stage (cercaria) escapes in large numbers from the snail and continues the life cycle. Each trematode species follows a highly specific pathway from snail to human host, usually by means of another host or transport mechanism. These include the intestinal, liver, blood, and lung flukes. See Schistosomiasis, Trematoda

Cestodes

Tapeworms, the other great assemblage of parasitic flatworms, parasitize most vertebrates, with eight or more species found in humans. Their flat ribbonlike body form consists of a chain of hermaphroditic segments. Like the trematodes, their life cycles are complex, although not dependent on a snail host. The enormous beef tapeworm of humans, Taenia saginata, is transmitted by infected beef (“measly beef”) from cattle that grazed where human feces containing egg-filled tapeworm segments contaminated the soil. Other tapeworms include the pork, dog, and broad (or fish) tapeworms. See Cestoda

Protozoa

Of the many protozoa that can reside in the human gut, only the invasive strain of Entamoeba histolytica causes serious disease. This parasite, ingested in water contaminated with human feces containing viable cysts of E. histolytica, can cause the disease amebiasis, which in its most severe form is known as amebic dysentery. Another common waterborne intestinal protozoon is the flagellate Giardia lamblia, which causes giardiasis, a mild to occasionally serious or long-lasting diarrhea. See Protozoa

Other flagellate parasites infect the human skin, bloodstream, brain, and viscera. The tsetse fly of Africa carries to humans the blood-infecting agents of trypanosomiasis, or African sleeping sickness, Trypanosoma brucei gambiense and T. brucei rhodesiense. The infection can be fatal if the parasites cross the blood-brain barrier. In Latin America, the flagellate T. cruzi is the agent of Chagas' disease, a major cause of debilitation and premature heart disease among those who are poorly housed. The infection is transmitted in the liquid feces of a conenose bug (genus Triatoma) and related insects. The infective material is thought to be scratched into the skin or rubbed in the eye, especially by sleeping children. See Trypanosomatidae

Another group of parasitic flagellates includes the macrophage-infecting members of the genus Leishmania, which are transmitted by blood-sucking midges or sand flies. Cutaneous leishmaniasis is characterized by masses of infected macrophages in the skin, which induce long-lasting dermal lesions of varying form and severity. The broad spectrum of host-parasite interactions is well exemplified by leishmaniases. The various manifestations of the disease are the result of the particular species of agent and vector, the immunological status of the host, the presence or absence of reservoir hosts, and the pattern of exposure.

Two remaining major groups of protozoa are the ciliates and the sporozoans. The former group is largely free-living, with only a single species, Balantidium coli, parasitic in humans (and pigs). This large protozoon is found in the large intestine, where it can cause balantidiasis, an ulcerative disease. The sporozoans, on the other hand, are all parasitic and include many parasites of humans. The most important are the agents of malaria. Other disease agents are included in the genera Isospora, Sarcocystis, Cryptosporidium, and Toxoplasma. Pneumocystis, a major cause of death among persons with acquired immune deficiency syndrome (AIDS), was formerly considered a protozoon of uncertain relationship, but now it is thought to be a member of the Fungi. See Acquired immune deficiency syndrome (AIDS), Malaria, Sporozoa

Toxoplasma gondii, the agent of toxoplasmosis, infects as many as 20% of the world's population. It can penetrate the placenta and infect the fetus if the mother has not been previously infected and has no antibodies. As with most medically important parasites, the great majority of Toxoplasma infections remain undetected and nonpathogenic. The parasite primarily affects individuals lacking immune competence—the very young, the very old, and the immunosuppressed. See Medical bacteriology, Medical mycology, Parasitology, Zoonoses