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tapeworm

tapeworm, name for the parasitic flatworms forming the class Cestoda. All tapeworms spend the adult phase of their lives as parasites in the gut of a vertebrate animal (called the primary host). Most tapeworms spend part of their life cycle in the tissues of one or more other animals (called intermediate hosts), which may be vertebrates or arthropods.

Anatomy and Function

An adult tapeworm consists of a knoblike head, or scolex, equipped with hooks for attaching to the intestinal wall of the host (which may be a human), a neck region, and a series of flat, rectangular body segments, or proglottids, generated by the neck. The chain of proglottids may reach a length of 15 or 20 ft (4.6–6.1 m). Terminal proglottids break off and are excreted in the feces of the host, but new ones are constantly formed at the anterior end of the worm. As long as the scolex and neck are intact the worm is alive and capable of growth. A rudimentary nervous system and excretory system run the length of the worm, through the proglottids. However, there is no digestive tract; the worm absorbs the host's digested food through its cuticle, or outer covering.

Reproduction

Each proglottid contains a complete set of male and female reproductive organs that produce the sex cells. Fertilization is internal; in most species cross fertilization between two adjacent worms is necessary, but in a few species self-fertilization may occur between two proglottids of the same worm, or within the same proglottid. In some species the fertilized eggs are shed continuously and leave the host's body in the feces; in others the fertilized eggs are stored until the proglottid is filled with them and the entire proglottid is then shed. The eggs develop into embryos with a hard outer shell; these do not hatch until they are eaten by a suitable intermediate host.

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.

Consequences of Infestation

Intestinal tapeworm infestation frequently occurs without symptoms; occasionally there is abdominal discomfort, diarrhea, constipation, or weight loss. The presence of tapeworm proglottids in clothing, bedding, or feces is the usual sign of infestation. Treatment is typically with albendazole or praziquantel, which kill the worm.

The most serious tapeworm infestation in humans is caused by the ingestion of T. solium eggs through fecal contamination, which results in the person serving as the intermediate, rather than the primary, host. The embryos migrate throughout the body, producing serious illness if they lodge in the central nervous system. In many poorer regions of the world, the larvae of T. solium are a major cause of human epilepsy. The embryos of the dog tapeworm encyst in various internal organs of humans, most commonly in the liver. The cysts produced by these embryos are called hydatid cysts, and the infestation of the liver is called hydatid disease.

Classification

Tapeworms are classified in the phylum Platyhelminthes, class Cestoda.

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Cestoda

Cestoda

The class Cestoda consists of long, flat, ribbonlike worms that are commonly called tapeworms. Tapeworms are obligatory parasites , ones that cannot survive independent of a host, that live in the intestines of vertebrate hosts. They form an extremely varied group, and nearly every vertebrate species is associated with a different parasitic cestode. Most cestodes make use of one or more intermediate hosts to bring them into the body of the ultimate host. Some cestodes can achieve impressive lengthsworms of up to 15 meters (50 feet) have been observed.

Characteristics of Cestodes

All tapeworms share a body plan. At the front end is a head region called the scolex. The scolex maintains a hold on the host's digestive tract and has many suckers and hooks for this purpose. The scolex also contains the tape-worm's sense organs, which consist primarily of cells sensitive to touch and chemical stimuli, as well as the modest concentration of nervous tissue that makes up the tapeworm brain.

The scolex is followed by a short neck region and a trunk, which is divided into a series of segments known as proglottids. New proglottids are produced in the neck region. As these form, older proglottids are pushed back toward the rear of the animal. The proglottids house the reproductive organs, which mature gradually as proglottids move to the back. Tapeworms are hermaphroditic , so that each proglottid includes both male and female gonads and generates both sperm and eggs. A tapeworm can reproduce sexually, either through self-fertilization or cross-fertilization with another tapeworm, or asexually, by breaking off proglottid segments at the end of the trunk. These reproductive traits are admirably adapted to reproduction in an environment (in the body of a host) in which worms are not guaranteed to encounter individuals of the same species.

Proglottids and fertilized eggs exit the host's digestive tract along with the host's excrement. In most tapeworm species, eggs or proglottids are first ingested, or taken in, by an intermediate host, often an arthropod or a different vertebrate species. The cestode may develop into a larval form or may become temporarily dormant within the intermediate host. The ultimate host becomes infested with the cestode when it consumes an infested intermediate host.

Because of the cestodes' parasitic lifestyle, certain organ systems are unnecessary. The most obvious of these is the digestive tract, which is absent from the group. Because cestodes live in an environment that is not only rich in nutrients, but one in which the nutrients are already well processed, further digestion is unnecessary. Instead, food absorption occurs over the entire surface of the cestode body, in an ectodermal, or skin, layer known as the integument. The integument is covered with tiny projections called mitotrichia, which increase the surface area available for absorption.

Subclasses of Cestodes

Cestodes are divided into two subclasses, Cestodaria and Eucestoda. Cestodaria is a small subclass of relatively small tapeworms that are parasites to elasmobranch fishes (sharks, rays, and chimeras). The trunks of cestodarians are not segmented into proglottids. The rear of the body includes a small sucker. Eucestoda is a much more diverse group, and includes all other cestodes. Eucestodes are characterized by the presence of proglottids.

see also Phylogenetic Relationships of Major Groups.

Jennifer Yeh

Bibliography

Brusca, Richard C., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, 1990.

Gould, James L., and William T. Keeton. Biological Science, 6th ed. New York: W. W. Norton and Co., 1996.

Hickman, Cleveland P., Larry S. Roberts, and Allan Larson. Animal Diversity. Dubuque, IA: Wm. C. Brown, 1994.

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Cestoda

Cestoda A class of flatworms (see Platyhelminthes) comprising the tapeworms – ribbon-like parasites within the gut of vertebrates. Tapeworms are surrounded by partially digested food in the host gut so they are able to absorb nutrients through their whole body surface. The body consists of a scolex (head), bearing suckers and hooks for attachment, and a series of proglottids (or proglottides), which contain male and female reproductive systems. The life cycle of a tapeworm requires two hosts, the primary host usually being a predator of the secondary host. Taenia solium has humans for its primary hosts and the pig as its secondary host. Mature proglottids, containing thousands of fertilized eggs, leave the primary host with its faeces and develop into embryos and then larvae that continue the life cycle in the gut and other tissues of a secondary host (see bladderworm).

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Tapeworm

Tapeworm

What Is the Life Cycle of the Beef Tapeworm?

What Is the Life Cycle of the Pork Tapeworm?

What Are the Signs and Symptoms of Tapeworm Infection?

How Do Doctors Diagnose and Treat Tapeworm Infection?

Resource

Tapeworms are long, flat, intestinal worms found in humans and many other animals.

KEYWORDS

for searching the Internet and other reference sources

Cestodes

Infestation

Neurocysticercosis

Tapeworms, also called cestodes (SES-todes), infect humans worldwide, although they are rare in the United States. The most common species in humans are Taenia saginata, the beef tapeworm, and Taenia solium, the pork tapeworm. As adults, these worms stay in the intestines and usually do little harm. But if people become infected with the cysts (immature stage) of the pork tapeworm, they can develop a condition called cysticercosis (sis-ti-ser-KO-sis), which can damage the brain. This is a major health problem in many tropical countries.

What Is the Life Cycle of the Beef Tapeworm?

The adult beef tapeworm is usually a whopping 15 to 30 feet long (4.5 to 9 meters) and lives in the small intestine. An infected person usually has only one or two worms. The tapeworms use their head, called the scolex, to attach themselves to the intestinal wall. They have 1,000 to 2,000 body segments, called proglottids, each containing 80,000 to 100,000 eggs.

The eggs can survive for months or years in the environment. When cattle or other herbivores (plant-eaters) eat egg-contaminated vegetation, the eggs hatch and burrow through their intestinal wall. The larvae* burrow into muscles and form fluid-filled cysts, which are protective capsules. If humans eat raw or undercooked beef containing cysts, the cysts develop over a 2-month period into adult tapeworms. Adult beef tapeworms can live for more than 30 years.

* larvae
are worms at an intermediate stage of the life cycle between eggs and adulthood.

What Is the Life Cycle of the Pork Tapeworm?

The adult pork tapeworm is about half as long as the beef tapeworm, usually 8 to 11 feet (2.5 to 3.5 meters) long. It also has a scolex for attaching to the intestinal wall and a body of about 1,000 proglottids. Each proglottid contains about 50,000 eggs.

The life cycle is similar to the beef tapeworms except that the worms infect pigs instead of cows. When humans eat raw or undercooked pork containing cysts, the cysts develop into adult tapeworms in humans. Adult pork tapeworms can live up to 25 years.

Cysticercosis

Pork tapeworms also can cause a more serious infection, called cysticercosis. This happens if people eat or drink something contaminated with human waste containing pork tapeworm eggs. The eggs hatch into cysts in the intestines, and the cysts travel through the blood to the rest of the body, especially the muscles and brain.

What Are the Signs and Symptoms of Tapeworm Infection?

Beef tapeworm infections produce only mild symptoms that may include diarrhea, abdominal pain, and weight loss. Pork tapeworm infections generally produce no symptoms. Cysticercosis, however, can cause muscle pain, weakness, and fever. If the central nervous system is involved, it can also cause epilepsy* or inflammation of the brain and the membranes around it (meningoencephalitis).

* epilepsy
is α disorder in which α person repeatedly has seizures, sudden attacks in which the person may jerk, grow rigid, or lose consciousness briefly.

How Do Doctors Diagnose and Treat Tapeworm Infection?

Eggs and proglottids can be seen in stool samples by microscopic examination. But to tell which tapewormbeef or porkis involved, a scolex would have to be removed and examined. This is seldom done, as doctors usually can prescribe the same medication for both types of infection. Stools are checked at 3 and 6 months after treatment to ensure that the infection is gone.

Cysticercosis is diagnosed by examining the muscles or brain with a CT scan* that can show the cysts. Blood tests for antibodies, which are substances the body makes to fight the infection, can confirm the diagnosis. Cysticercosis is also treated with medication but, in rare instances, cysts may be removed surgically.

* CT scans
or CAT scans are the short form for computerized axial tomography, which uses x-rays and computers to view structures inside the body.

Prevention Tapeworm infection may be prevented by thoroughly cooking meat until juices run clear and the centers are no longer pink. This ensures that any tapeworm cysts in the meat are destroyed.

See also

Encephalitis

Parasitic Diseases

Worms

Resource

The U.S. National Center for Infectious Diseases has a fact sheet about Cysticercosis at its website. http://www.cdc.gov/nciod/focus/vol6no4/dpd.htm

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tapeworm

tapeworm (cestode) (tayp-werm) n. any of a group of flatworms that have a long thin ribbon-like body and live as parasites in the intestines of humans and other vertebrates. The body of a tapeworm consists of a head (see scolex), a short neck, and a chain of separate segments (see proglottis). Humans are the primary hosts for some tapeworms (see Taenia). However, other genera are also medically important (see Echinococcus).

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tapeworm

tapeworm Parasitic intestinal worm; infection is acquired by eating raw or undercooked infected pork (Taenia solium), beef (T. saginata), or fish (Diphyllobothrium latum). Eggs are shed in the faeces and infect the animal host. Cysticercosis is infection of human beings with the larval stage by ingestion of eggs from faecal contamination of food and water.

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Cestoda

Cestoda (tapeworms; phylum Platyhelminthes) A class of parasitic worms all of which lack a gut. The genitalia are normally repeated. The majority of forms possess a scolex which may have suckers. Mature individuals lack cilia. All are endoparasitic (see PARASITISM), most mature individuals living within vertebrates.

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tapeworm

tape·worm / ˈtāpˌwərm/ • n. a parasitic flatworm (class Cestoda), the adult of which lives in the intestine of humans and other vertebrates. It has a long ribbonlike body with many segments that can become independent, and a small head bearing hooks and suckers.

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tapeworm

tapeworm Parasite of the genus Taenia, which colonizes the intestines of vertebrates, including human beings. Caught from eating raw or under-cooked meat, it may cause serious disease.

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tapeworms

tapeworms See Cestoda.

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tapeworms

tapeworms See CESTODA.

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tapeworm

tapewormaffirm, berm, confirm, firm, germ, herm, midterm, perm, sperm, squirm, term, therm, worm •pachyderm • echinoderm •wheatgerm • endosperm •gymnosperm • isogeotherm •ragworm • flatworm • threadworm •tapeworm •eelworm, mealworm •silkworm • ringworm • inchworm •blindworm • lobworm • roundworm •slow-worm • screw worm •woodworm •bookworm, hookworm •bloodworm • lugworm • lungworm •earthworm

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