Humans as Tapeworm Hosts
Human tapeworm infestations are most common in regions where there is fecal contamination of soil and water and where meat and fish are eaten raw or lightly cooked. In the case of the human tapeworm most common in the United States (the beef tapeworm, Taenia saginata) the usual intermediate host is a cow, which ingests the proglottid while drinking or grazing. The round-bodied embryos, equipped with sharp hooks, hatch and bore through the cow's intestinal wall into the bloodstream, where they are carried to the muscles. Here each embryo encloses itself in a cyst, or bladder; at this stage it is called a bladder worm. During the bladder worm stage the embryo develops into a miniature scolex; it remains encysted until the muscle is eaten by a primary host, in this case a human. If the scolex has not been killed by sufficient cooking of the meat, it sheds its covering and attaches to the intestinal wall, where it begins producing proglottids.
A human tapeworm common in Mexico, the pork tapeworm (T. solium), has a similar life cycle, with a pig as the usual intermediate host. The fish tapeworm, Diphyllobothrium latum, transmitted to humans from fish, especially pike, is common in Asia and in Canada and the northern lake regions of the United States. This tapeworm has a more elaborate life cycle, involving both a fish and a crustacean as intermediate hosts. The dwarf tapeworm, Hymenolepsis nana, is transmitted through fecal contamination and is common in children in the southeastern United States. There are also several tapeworms for whom humans are the usual intermediate host; among these, the dog tapeworm, Echinococcus granulosis, spends its adult phase in the intestines of dogs.
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