Tapeworm, a parasitic flat worm. The tapeworm has a long, flat body made up of a head, a neck, and a chain of segments, or sections. A chain can consist of 3 to more than 4,000 segments and range in length from less than one inch (2.5 cm) to more than 75 feet (23 m).

Adult tapeworms live in the intestinal tract of vertebrates (animals with backbones), including humans. Tapeworms have no digestive system. Through their body walls, tapeworms directly absorb digested food from the intestine of the host (the animal they infect).

There are more than 1,500 species of tapeworms. Of these about 25 are known to infect humans. Tapeworm infections are prevalent in many areas of the world, but are not common in the United States. In the United States, infections are due primarily to three species: the beef tapeworm, the fish tapeworm, and the dwarf tapeworm (the smallest species that infects humans).

Tapeworm Infections

Beef- and fish-tapeworm infections develop after a person eats tapeworm-infected beef or fish that has not been adequately cooked. Dwarf-tapeworm infection is transmitted by eating food contaminated with human feces.

The head of the tapeworm is attached to the host's intestine with suckers or hooks. New tapeworm segments are produced continually, while older segments usually pass out of the intestine with the host's feces. The older segments consist of sacs containing thousands of eggs.

The eggs usually will not develop into new tapeworms in the host. In most cases, the eggs must be eaten by a second animal (cattle or fish, in the case of beef- and fish-tapeworm infections). The second animal is called an intermediate host. In the intermediate host, the eggs hatch into larvae (immature forms). The larvae eventually lodge in the intermediate host's muscles, where they develop protective capsules. When an individual eats the larvae-infected meat, the capsules are digested and new tapeworms emerge.

Dwarf tapeworms do not require an intermediate host; the eggs can develop into adults within a single host. The infection is thus spread by eating food contaminated with the eggs, rather than meat contaminated with the larvae.

Symptoms, Treatment, and Prevention

Symptoms of tapeworm infection are not distinct, and infections often go undetected. Frequently there are no symptoms. Most infections cause no serious harm to the host. In some cases, however, abdominal pain, nausea, diarrhea, and anemia can result. Contrary to popular belief, weight loss and increased appetite do not usually accompany tapeworm infection.

When infection is known to exist, the tapeworms can be killed with drugs that do not harm the host. Prevention of tapeworm infection depends on good personal hygiene and thorough cooking or washing of foods.

Tapeworms belong to the class Cestoda. The beef tapeworm is Taenia saginata; the fish tapeworm, Diphyllobothrium latum; the dwarf tapeworm, Hymenolepis nana.