Tapeworms: Cestoda

Physical characteristics: The strobila of broad fish tapeworms is about 30 feet (9 meters) long and has three thousand to four thousand proglottids. The scolex is finger shaped.

Geographic range: Broad fish tapeworms live in Scandinavia, the Baltic states, Russia, the United States, Canada, Ireland, Japan, around some lakes and large rivers in Africa, and in South America.

Habitat: The final hosts of broad fish tapeworms are fish-eating mammals such as dogs, cats, bears, seals, and humans. The first intermediate hosts are crustaceans (krus-TAY-shuns), which are water-dwelling animals that have jointed legs and a hard shell but no backbone. The second intermediate hosts are fishes. All the hosts live in or near rivers and freshwater lakes.

Diet: Broad fish tapeworms absorb nutrients from their hosts.

Behavior and reproduction: Scientists do not know how broad fish tapeworms behave. These worms make both eggs and sperm. Eggs The final hosts of broad fish tapeworms are fish-eating mammals such as dogs, cats, bears, seals, and humans. (Illustration by Brian Cressman. Reproduced by permission.) are released and pass into the environment with the final host's feces. The embryos hatch from the eggs in water. The embryos are covered by hairlike fibers and can swim several hours until being eaten by a small crustacean, the first intermediate host. In the intestine of the crustacean, the embryos lose the hairy covering, bore into the host's body cavity, and feed on nutrients in the host's body fluids. It takes 20 to 25 days for the embryos to develop into long larvae. The larvae make the host crustacean sick and slow, turning it into easy prey for fish. When the infected crustacean is eaten by a fish, which is the second intermediate host, the larvae travel from the fish's intestine into the muscles and turn into the next stage of larvae. People and animals, which are the final hosts, are infected with the larvae when they eat infected fish. After about two weeks in the host's intestine, the larvae transform into mature worms and start producing eggs.

Broad fish tapeworms and people: The disease caused by broad fish tapeworms is one of the most widespread diseases caused by tapeworms. The symptoms are diarrhea, abdominal discomfort, weakness, and in some cases, anemia (uh-NEE-mee-uh), which is a deficiency of red blood cells. The drug treatment is very efficient.

Conservation status: Broad fish tapeworms are not considered threatened or endangered. ∎