Medical parasitology

The study of diseases of humans caused by parasitic agents. It is commonly limited to parasitic worms (helminths) and theprotozoa. Current usage places the various nonprotozoan microbes in distinct disciplines, such as virology, rickettsiology, andbacteriology.

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 humanhosts (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 thewhipworm (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, andAncylostoma duodenale in temperate Asia, the Mediterranean, and Middle East, suck blood from the small intestine andcause major debilitation, especially among the undernourished. The human pinworm (Enterobius vermicularis) infects thelarge intestine of millions of urban dwellers. Most intestinal nematodes, which require a period of egg maturation outside thehuman host before they are infective, are associated with fecal contamination of soil or food crops and are primarily rural indistribution.

The nonintestinal nematodes are spread by complex life cycles that usually involve bloodsucking insects. One exception isthe guinea worm (Dracunculus medinensis), a skin-infecting 2–3-ft (0.6–1-m) worm transmitted by aquatic microcrustaceansthat 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 aretransmitted 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 theagent of trichinosis. The tiny spiraled larvae encyst in muscle and can carry the infection to humans and other carnivorousmammals 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 gothrough an initial developmental period in specific kinds of fresh-water snails, where they multiply as highly modified larvae ofdifferent types. Ultimately, an infective larval stage (cercaria) escapes in large numbers from the snail and continues the lifecycle. Each trematode species follows a highly specific pathway from snail to human host, usually by means of another hostor 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 foundin humans. Their flat ribbonlike body form consists of a chain of hermaphroditic segments. Like the trematodes, their lifecycles are complex, although not dependent on a snail host. The enormous beef tapeworm of humans, Taenia saginata, istransmitted by infected beef (“measly beef”) from cattle that grazed where human feces containing egg-filled tapewormsegments 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 seriousdisease. This parasite, ingested in water contaminated with human feces containing viable cysts of E. histolytica, can causethe disease amebiasis, which in its most severe form is known as amebic dysentery. Another common waterborne intestinalprotozoon 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 humansthe blood-infecting agents of trypanosomiasis, or African sleeping sickness, Trypanosoma brucei gambiense and T. bruceirhodesiense. The infection can be fatal if the parasites cross the blood-brain barrier. In Latin America, the flagellate T. cruzi isthe agent of Chagas' disease, a major cause of debilitation and premature heart disease among those who are poorlyhoused. The infection is transmitted in the liquid feces of a conenose bug (genus Triatoma) and related insects. The infectivematerial is thought to be scratched into the skin or rubbed in the eye, especially by sleeping children. SeeTrypanosomatidae

Another group of parasitic flagellates includes the macrophage-infecting members of the genus Leishmania, which aretransmitted by blood-sucking midges or sand flies. Cutaneous leishmaniasis is characterized by masses of infectedmacrophages 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 theparticular species of agent and vector, the immunological status of the host, the presence or absence of reservoir hosts, andthe pattern of exposure.

Two remaining major groups of protozoa are the ciliates and the sporozoans. The former group is largely free-living, with onlya single species, Balantidium coli, parasitic in humans (and pigs). This large protozoon is found in the large intestine, where itcan cause balantidiasis, an ulcerative disease. The sporozoans, on the other hand, are all parasitic and include manyparasites of humans. The most important are the agents of malaria. Other disease agents are included in the generaIsospora, Sarcocystis, Cryptosporidium, and Toxoplasma. Pneumocystis, a major cause of death among persons withacquired immune deficiency syndrome (AIDS), was formerly considered a protozoon of uncertain relationship, but now it isthought 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 theplacenta and infect the fetus if the mother has not been previously infected and has no antibodies. As with most medicallyimportant parasites, the great majority of Toxoplasma infections remain undetected and nonpathogenic. The parasiteprimarily affects individuals lacking immune competence—the very young, the very old, and the immunosuppressed. SeeMedical bacteriology, Medical mycology, Parasitology, Zoonoses