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Infections of humans caused by the transmission of disease agents that naturally live in animals. People become infected when they unwittingly intrude into the life cycle of the disease agent and become unnatural hosts. Zoonotic helminthic diseases, caused by parasitic worms, involve many species of helminths, including nematodes (roundworms), trematodes (flukes), cestodes (tapeworms), and acanthocephalans (thorny-headed worms). Helminthic zoonoses may be contracted from domestic animals such as pets, from edible animals such as seafood, or from wild animals. Fortunately, most kinds of zoonotic helminthic infections are caused by rare human parasites.
The best-recognized example of a food-borne zoonotic helminthic disease is trichinosis, caused by the trinchina worm, Trichinella spiralis, a tiny nematode. People commonly become infected by eating inadequately prepared pork, but a sizable proportion of victims now contract the worms by eating the meat of wild carnivores, such as bear. Trichinosis is usually a mild disease, manifested by symptoms and signs of intestinal and muscular inflammation, but in heavy infections damage done by the larvae to the heart and central nervous system can be life threatening. Because of public awareness about properly cooking pork and federal regulations about feeding pigs, trichinosis has become uncommon in the United States. People who eat inadequately prepared marine fish may become infected with larval nematodes. Of the many potential (and rare) helminthic zoonoses from wild animals in the United States, Baylisascaris procyonis is particularly dangerous. The nematode is highly prevalent in raccoons, the definitive host. See Medical parasitology, Nemata
infectious diseases whose causative agents adapted in the course of evolution to parasitism in certain animal species. Zoonoses include cattle plague, hog cholera, and pasteurellosis. Zoonoses may arise among human beings under certain sanitary and economic conditions that favor a given mechanism of transmission of the causative agent. However, the causative agents of zoonoses, in contrast to zooanthroponoses, cannot circulate among human beings. Man represents a biological dead end for them because he is not part of the epizootic process and does not participate in the causative agent’s evolution as a parasitic species.