A
tapeworm is a ribbonlike colony of parasitic flatworms, of the class Cestoda, that infest humans and other vertebrate animals. Species that most commonly infest humans are: the beef tapeworm, Taenia saginata, found worldwide; the pork tapeworm, T. solium, found mostly in Central and South America, Africa, and Asia; and the fish tapeworm, Dibothriocephalus latus, of parts of Europe, Africa, and the Americas.
A tapeworm consists of a head (scolex) that is equipped with hooks or suckers used to attach the parasite to the host's intestinal lining. The "body" is composed of segments known as proglottids, which are separate, sexually functional individuals that can synchronize their muscular activity to keep the colony mobile. The colony is looped back and forth so that the intestine remains unblocked; a 10-m (33-ft) pork tapeworm can inhabit the small intestine, which is 6 m (20 ft) long.
Younger
proglottids have testes; as they get older, the
testes shrivel up and are replaced by ovaries and a uterus. Proglottids having male sex
organs release sperm that travel to the
older proglottids and fertilize the eggs. Proglottids that contain developing embryos break away and are excreted with feces.
Larvae develop and, if the waste matter is eaten by an animal, become dormant and
encysted in the animal's muscle tissue. Humans eating poorly cooked animal meat can then ingest the encysted larvae. Adult tapeworm infestations can be eliminated with chemotherapy. Humans, however, can also serve as intermediary hosts, the larvae forming cysts up to 25 cm (10 in) in diameter in the brain, lungs, liver, or other organs. Such infections can only be treated surgically, if at all.
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