tapeworm

/tayp"werrm'/, n.

any of various flat or tapelike worms of the class Cestoidea, lacking an alimentary canal, and parasitic when adult in the alimentary canal of humans and other vertebrates: the larval and adult stages are usually in different hosts.

[1745-55; TAPE + WORM]

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Any of about 3,000 species (class Cestoda, phylum Platyhelminthes) of bilaterally symmetrical parasitic flatworms found worldwide.

Tapeworms range from 0.04 in. (1 mm) to more than 50 ft (15 m) long. The head bears suckers and often hooks for attaching to the liver or digestive tract of the host. Once attached, a tapeworm absorbs food through its body wall. The body is often divided into a head or scolex possessing the suckers and hooks, an unsegmented neck, and a series of proglottids (units containing both male and female reproductive organs) that continually form in a growth region at the base of the neck. Following fertilization, each mature proglottid containing thousands of embryos breaks off and is eliminated in the host's feces. The life cycle may require more than one host but otherwise somewhat resembles that of the roundworm trichina. Many species that infest humans belong to the genus Taenia; the intermediate host is implied by the name (e.g., beef tapeworm, T. saginata). Humans usually acquire tapeworms through fecal contamination of soil or water or inadequate cooking of meat or fish.

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▪ parasitic flatworm

also called  cestode 

      any member of the invertebrate class Cestoda (phylum Platyhelminthes), a group of parasitic flatworms containing about 5,000 species. Tapeworms, which occur worldwide and range in size from about 1 mm (0.04 inch) to more than 15 m (50 feet), are internal parasites (parasitism), affecting certain invertebrates and the liver or digestive tracts of all types of vertebrates—including humans, domestic animals, and other food animals, such as fish. Some attack a single host, others require one or two intermediate hosts as well as a final, or definitive, host during their life cycle. The disease caused by tapeworms is known as cestodiasis (q.v.).

      Tapeworms are bilaterally symmetrical (i.e., the right and left sides are similar). Some consist of one long segment; others have a definite head, followed by a series of identical segments called proglottids. The head, or scolex, bears suckers and often hooks, which are used for attachment to the host. The body covering is a tough cuticle, through which food is absorbed. There is neither a mouth nor a digestive tract. Tapeworms also lack a circulatory system and an organ specialized for gas exchange.

      Most tapeworms are hermaphroditic (hermaphroditism) (i.e., functional reproductive organs of both sexes occur in the same individual). They are usually self-fertilizing, and gonads of both sexes also occur within a single proglottid. The life cycle is complex. The pork tapeworm (Taenia solium, or Taeniarhynchus solium), found wherever raw pork is eaten, lives in the human intestine in its adult stage. Each proglottid, following fertilization, may contain as many as 40,000 embryos encased in separate capsules. If the embryos, which pass out with the host's feces, are eaten by a mammal such as a dog, camel, pig, monkey, or human being, the larva emerges in the digestive tract. It bores through the intestinal wall into a blood vessel and is carried to muscle tissue in which it forms a protective capsule (encysts) and is called a cysticercus, or bladder worm. If the cysticercus is eaten alive in raw meat, it attaches itself to the host's intestine and develops directly into a mature adult.

      The life cycle of the beef tapeworm (Taenia saginata, or Taeniarhynchus saginatis), which occurs worldwide where beef is eaten raw or improperly cooked, is much like that of the pork tapeworm. Man is the definitive host; cattle serve as the intermediate host.

      The fish tapeworm (Dibothriocephalus latus, or Diphyllobothrium latum), most common in waters of the Northern Hemisphere, infests humans and other mammals that eat fish, particularly bears and dogs. Fertilized eggs pass from the host's body in the feces. In a water medium they develop into a hairlike larva and are eaten by tiny crustaceans, which, in turn, are eaten by a fish. In the fish, the tapeworm larva encysts in muscle tissue. When the fish is eaten by a mammal, the larva attaches to the mammalian intestine and develops into a mature adult. Fish hosts of this tapeworm include trout, salmon, pike, and perch.

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