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Health Issues Parasites Type of Issue: Environmental health, epidemics, public health Definition: Diseases borne by parasites, or organisms that live within other, “host” organisms; parasites travel to their hosts via vectors which may include fleas, mosquitoes, rats, and other animals. Parasites are organisms that take up residence, temporarily or permanently, on or within other living organisms for the purpose of procuring food. They include plants such as bacteria and fungi; animals such as protozoa, helminths (worms), and arthropods; and forms such as spirochetes and microscopic viruses. The study of parasitism is a study of symbiosis. Symbiosis occurs when two organisms, known as symbionts, live in close association with each other, usually with one organism living in or on the body of the other. Such a living arrangement is called a symbiotic relationship. In a symbiotic relationship, the symbionts are usually, but not always, of different species, and the relationship need not be beneficial or damaging to either organism. When one member of a symbiotic relationship actually harms its host or in some way lives at the expense of the host, it is then a parasite. The word “parasite” is derived from the Greek parasitos, which means “one who eats at another’s table” or “one who lives at another’s expense.” A parasite may harm its host by causing a mechanical injury, such as boring a hole into it; by eating, digesting, or absorbing portions of the host’s tissue; by poisoning the host with toxic metabolic products; or by robbing the host of nutrition. It has been found that most parasites inflict a combination of these conditions. Parasites and Disease Medical parasitology is the study of human diseases caused by parasitic infection. It is commonly limited to the study of parasitic worms (helminths) and protozoa. The science places nonprotozoan parasites in separate disciplines, such as virology, rickettsiology, and bacteriology. The branches of parasitology known as medical entomology and medical arthropodology deal with insects and noninsect arthropods that serve as hosts and transport agents for parasites, as well as with the noxious effects of these pests. The medical study of parasitic fungi (molds and yeasts) that cause human disease is called mycology. Throughout history, human welfare has suffered greatly because of parasites. Fleas and bacteria killed one-third of the human population of Europe during the seventeenth century, and malaria, schistosomiasis, and African sleeping sickness have killed additional countless millions. Despite successful medical campaigns against yellow fever, malaria, and hookworm infections worldwide, parasitic diseases in combination with nutritional deficiencies are the primary killers of humans. Medical research suggests that parasitic infections are so widespread that if all the known varieties were evenly distributed among the human population, each living person would have at least one. Most serious parasitic infections occur in tropical, less modernized regions of the world, and because most of the planet’s industrially developed and affluent populations live in temperate regions, many people are unaware of the magnitude of the problem. On an annual basis, 60 million deaths occur worldwide from all causes; of these deaths, half are children under five years of age. Fifty percent of these, 15 million child deaths per year, are directly attributable to a combination of malnutrition and intestinal parasitic infection. It must be noted that less than 15 percent of the world’s present population is served by adequate clean water supplies and sewage disposal programs, and that almost all intestinal parasitic infections are the result of ingesting food or water contaminated with human feces. The transmission of parasitic diseases involves three factors: the source of the infection, the mode of transmission, and the presence of a susceptible host. The combined effect of these factors determines the dispersibility and prevalence of a parasite at a given time and place, thus regulating the incidence of a parasitic disease in a population. Because of host specificity, other humans are the chief source of most human parasitic diseases. The various manifestations of any human parasitic disease are a result of the particular species of parasite involved, its mode of transport, the immunological status of the host, the presence or absence of hosts, and the pattern of exposure. Humans transmit parasitic diseases to one another through the intestinal tract, nose and mouth, skin and tissue, genitourinary tract, and blood. It is fecal discharge, however, that offers the most convenient and common means for a parasite or its ova and larvae to leave its host, since the majority of parasites inhabit the gastrointestinal tract. For this reason, the proper disposal of fecal material becomes the most important method of preventing the spread of parasitic disease. Since most parasites inhabit the intestinal tract, food and water are also important means of transmitting parasitic infections. The infective organism may be present in contaminated drinking water, in animal and fish flesh used as food, in human feces used as fertilizer, or on the hands of food handlers. Arthropods are one of the main sources of parasitic diseases in humans. Arthropods act as both mechanical carriers of and intermediate hosts to many diseases—bacterial, viral, rickettsial, and parasitic—which they transmit to humans. In most tropical countries, basic preventive medicine for many devastating parasitic diseases depends on the control or eradication of insects and arachnids.  The Common Parasites There are four major groups of parasites that most often invade human hosts: nematodes, trematodes, cestodes, and protozoa. Most nematodes, or roundworms, are free-living, and nematodes are found in almost every terrestrial and aquatic environment. Most are harmless to humans, but some parasitic nematodes invade the human intestinal tract and cause widespread debilitating diseases. The most prevalent intestinal nematodes are Ascaris lumbricoides, which infects the small intestine and affects more than a billion people; the whipworm Trichuris trichiura, which infects the colon and is carried by an estimated 500 million individuals; the human hookworms Necator americanus and Ancylostoma duodenale, which suck blood from the human small intestine and cause major debilitation among undernourished people; and Enterobius vermicularis, the human pinworm, which infects the large intestine and is common among millions of urban dwellers because it is easily transmitted from perianal tissue to hand to mouth. Nonintestinal, tissue-infecting nematodes are spread most often by hyperparasitic bloodsucking insects such as mosquitoes, biting flies, and midges. The most common tissue-infecting nematode is Trichinella spiralis, the pork or trichina worm, which is the agent of trichinosis. Other important parasitic nematodes include Onchocerca volvulus, which is transmitted by blackflies in tropical regions and causes blindness, and the mosquito-transmitted filarial worms that are responsible for elephantiasis. Trematodes, or flatworms, are commonly called flukes. Flukes vary greatly in size, form, and host living location, but all of them initially develop in freshwater snails. The human intestinal fluke, the oriental liver fluke, and the human lung fluke are all transmitted to humans by the ingestion of raw or undercooked aquatic vegetables, fish, or crustaceans. An important group of trematodes consists of the blood flukes of the genus Schistosoma, which enter the body through skin/water contact and are responsible for schistosomiasis. Cestodes, commonly called tapeworms, are parasitic flatworms that parasitize almost all vertebrates, and as many as eight species are found in humans. The two most common cestodes—Taenia saginata, the beef tapeworm, and Taenia solium, the pork tapeworm—are transmitted to humans by infected beef or pork products obtained from livestock that grazed in fields contaminated by human feces, or by contaminated water. The resulting disease, cysticercosis, which is potentially lethal, develops mostly in the brain, eye, and muscle tissue. Another animal-transmitted cestode is the dog tapeworm, Echinococcus granulosus, which dogs ingest by eating contaminated sheep viscera and then pass on to humans, who ingest the parasite’s eggs after petting or handling an infected dog. The human infestation of E. granulosus results in hydatid disease. Probably the most dramatic of the cestode parasites is the gigantic tapeworm Diphyllobothrium latum, which may reach lengths of 10 meters and a width of 2 centimeters. This tapeworm is transmitted to humans by the ingestion of raw or undercooked fish. This tapeworm, like most cestodes, can be effectively treated and killed by drugs, but if the worm merely breaks, leaving the head and anterior segments attached, it can regenerate its original body length in less than four months. Protozoa that can infect human hosts are found in the intestinal tract, various tissues and organs, and the bloodstream. Of the many varieties of protozoa that can live in the human intestinal tract, only Entamoeba histolytica causes serious disease. This parasite, which is ingested in water contaminated by human feces, is responsible for the disease amebiasis, also known as amebic dysentery. A less serious, though common, waterborne intestinal protozoan is Giardia lamblia, which causes giardiasis, a common diarrheal infection among children in day care and campers who ingest water fouled by animal waste. Another group of protozoa parasites specializes in infecting the human skin, bloodstream, brain, and viscera. Trypanosoma brucei, carried by the African tsetse fly, causes the blood disease trypanosomiasis (African sleeping sickness). In Latin America, infection by the protozoa Trypanosoma cruzi results when the liquid feces of the reduviid (assassin) bug is rubbed or scratched into the skin; it causes Chagas’ disease, which produces often fatal lesions of the heart and brain. Members of the protozoan genus Leishmania are transmitted by midges and sandflies, and their parasitic infestation manifests in long-lasting dermal lesions and ulcers; the destruction of nasal mucous, cartilage, and pharyngeal tissues; or in the disease kala-azar, resulting in the destruction of bone marrow, lymph nodes, and liver and spleen tissue. Two other types of protozoa are parasitic to humans. The first is the ciliate protozoa, which are mostly free-living, and of which only a single species, Balantidium coli, is parasitic in humans. This species is responsible for balantidiasis, an ulcerative disease. The second is the sporozoans, all of which are parasitic. Many species of sporozoans are harmful to humans, the most important being Plasmodium, the agent of malaria. The sporozoan parasite Toxoplasma gondii, the agent of toxoplasmosis, is responsible for encephalomyelitis and chorioretinitis in infants and children and is thought to infect as much as 20 percent of the world’s population. Pneumocystis, a major cause of death among persons with acquired immunodeficiency syndrome (AIDS), was formerly considered a result of sporozoan infection but is now thought to be fungal. Randall L. Milstein See AlsoEnvironmental diseases; Epidemics; Food poisoning; Lice, mites, and ticks; Lyme disease; Plague; Worms; Zoonoses. For Further Information Despommier, Dickson D., Robert W. Gwadz, and Peter J. Hotex. Parasitic Diseases. 4th ed. New York: Springer-Verlag, 2000. This source provides a list of parasitic diseases of current concern to public health professionals. It also describes the assessment of and treatment options for a variety of these diseases. Klein, Aaron E. The Parasites We Humans Harbor. New York: Elsevier/Nelson Books, 1981. An overview of common human-infecting parasites written for the general reader. The text is nontechnical, easy to follow, and presents numerous examples. Noble, Elmer R., and Glenn A. Noble. Parasitology: The Biology of Animal Parasites. 5th ed. Philadelphia: Lea & Febiger, 1982. A graduate-level textbook on parasites that infect animals. The comprehensive and highly technical text deals with the biological, environmental, and pathological aspects of parasites. Well illustrated with anatomical drawings and photographs. Roberts, Larry S., and John Janovy, Jr. Gerald D. Schmidt and Larry S. Roberts’ Foundations of Parasitology. Rev. 6th ed. Dubuque, Iowa: Wm. C. Brown, 2000. A graduate-level textbook covering all aspects of parasitology. The text is highly technical and intended for the informed reader. The book is well illustrated, but sensitive readers may be disturbed by many of the case study photographs. |
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