TAPEWORMS: Cestoda

PHYSICAL CHARACTERISTICS

The body of most tapeworms is flat and much longer than it is wide, so that it looks like a tape or ribbon. The length varies from 0.02 inch (0.6 millimeter) to 98 feet (30 meters), the longest worms being found in sperm whales. Tapeworms are parasites that have no head, mouth, or digestive system. Parasites (PAIR-uh-sites) are animals or plants that live on or in other animals or plants, or hosts, without helping them and usually harming them. Tapeworms have a body covering through which they absorb nutrients from the host's intestine. This covering also protects the worms from the host's immune reactions and digestive acids. Tapeworms are whitish and as internal parasites they live in darkness.

The body of tapeworms has three regions: scolex (SKOH-leks), neck, and strobila (stroh-BYE-luh). The scolex is the head. It has spines, hooks, suckers, tentacles, glands releasing sticky secretions, or a combination of these structures that the worm uses to attach itself to the inner wall of the intestine of the final host, also called the primary host. Suckers are the most common attachment tool. Suckers are usually cup shaped and have powerful muscular walls. The neck is the region of the body just behind the scolex. It is usually short.

The strobila is behind the neck. It consists of a row of segments called proglottids (proh-GLAH-tuhds). The strobila is made up of anywhere from a few to more than one thousand proglottids but usually contains several dozen. Each proglottid starts development at the neck, and proglottids form one by one throughout the life of the tapeworm in the final host. Just behind the neck, the proglottids are short and narrow. When a new proglottid forms at the neck, already formed proglottids are pushed toward the rear, grow, and eventually contain the reproductive organs.

Behind the new proglottids, each strobila contains the following types of proglottids, from front to back: premature proglottids, with the beginnings of reproductive organs; mature proglottids, which contain functioning male and female reproductive organs; postmature proglottids, which contain developing eggs; and gravid (GRA-vuhd) proglottids, which contain ripe eggs. The gravid proglottids at the end of the worm break off and pass into the environment with the host's feces (FEE-seez). A few species of tapeworms have no proglottids.

GEOGRAPHIC RANGE

Tapeworms live all over the world.

HABITAT

Tapeworms live in almost all land, sea, and freshwater habitats where vertebrates live. Vertebrates (VER-teh-brehts) are animals with a backbone. Most adult tapeworms live in the intestines of the final hosts, but a few species live in the body cavity. Tapeworm larvae, which live in a host called the intermediate host before moving to the final host, live in various types of tissue, such as liver, lung, muscle, body cavity, brain, and sometimes even the eyes. Larvae (LAR-vee) are animals in an early stage that change form before becoming adults.

DIET

Tapeworms eat by absorbing nutrients from their hosts' intestines.

WATCH WHAT YOU EAT

People who eat raw fish often carry fish tapeworms. The fish tapeworm absorbs vitamin B₁₂, causing the host to be deficient in a vitamin that is essential for the development of red blood cells. Humans become infected with pork tapeworm and beef tapeworm when they eat undercooked or raw meat.

BEHAVIOR AND REPRODUCTION

Scientists know little about the behavior of tapeworms in the intestines of their hosts. It seems that most tapeworms attach themselves at a certain site of the intestinal wall and stay there for their entire lives.

Tapeworms follow this general scheme as their life cycle. The eggs, each holding an embryo (EHM-bri-yo), pass into the environment with the final host's feces and are eaten by the intermediate host. In the intestine of the intermediate host, the embryos hatch and, using their hooks, bore through the intestinal wall and into the body cavity or an internal organ. In the new location the embryos transform into larvae. In most species the larvae have a fully developed scolex identical to that of adult tapeworms. The larvae enter the final host when it eats the intermediate host. In the final host the scolices (SKOH-luh-seez, the plural of scolex) of the larvae attach to the intestinal wall. The necks of the larvae start production of proglottids, and the strobila forms. With further development of proglottids, the worm starts producing eggs, which are released with feces into the environment. Some tapeworms have more than one intermediate host.

Most tapeworms make both eggs and sperm. Each proglottid contains one set of male reproductive organs and one set of female reproductive organs. In most species the male organs mature first, and proglottids first act as male organs. In species in which the female organs develop first, sperm develop in the male organs when the eggs develop in the female organs. Sperm from one tapeworm enter the female reproductive organs of another tapeworm during mating and are stored for a while before joining with eggs for the start of development of embryos.

TAPEWORMS AND PEOPLE

Fifty-seven species of tapeworms live in humans. Six of these species are considered a public health problem because they cause serious diseases. Tapeworms also are dangerous to animals kept by people, such as horses, sheep, cattle, dogs, cats, rabbits, and domestic birds. Tapeworms infect the fish that people eat and reduce production in fish farms.

Tapeworms and More Tapeworms

Scientists are constantly finding new species of tapeworms. Thirty to forty species were discovered each year between 1992 and 2002.

CONSERVATION STATUS

Tapeworms are not considered threatened or endangered.

BROAD FISH TAPEWORM (Diphyllobothrium latum): SPECIES ACCOUNTS

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 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. ∎

DOG TAPEWORM (Echinococcus granulosus): SPECIES ACCOUNTS

Physical characteristics: Adult dog tapeworms are 0.1 to 0.2 inch (3 to 6 millimeters) long. The body consists of the scolex, a short neck, and 3 to 5 proglottids. The scolex has 30 to 36 hooks. The gravid proglottids are very long.

Geographic range: Dog tapeworms live all over the world.

Habitat: Adult dog tapeworms live in the intestines of meat-eating mammals, mainly dogs, wolves, and jackals. The larvae live in the liver, lungs, and muscles of plant-eating mammals such as sheep, cattle, camels, pigs, goats, and horses. The habitat includes areas where humans live and work, such as pastures, farms, and villages.

Diet: Dog tapeworms absorb nutrients from their hosts.

Behavior and reproduction: The released gravid proglottids of dog tapeworms can crawl, and the larvae probably climb up grasses. Some gravid proglottids may stay around the anus of the dog, contaminating its hair with eggs. Embryos hatching from eggs in the intestine of the intermediate host travel to the liver or the lungs, and sometimes to the muscles or even the eyes. The embryos grow very slowly and transform into a covered ball. Inside this ball numerous scolices and additional larvae balls develop in a process that can continue for 20 to 30 years. Meat-eating mammals, the final hosts, become infected when they eat a liver or another organ containing a larvae ball. In the final host's intestine, each scolex produces an adult tapeworm.

Dog tapeworms and people: The disease caused by dog tapeworms is one of the most serious parasitic diseases for humans in Asia, Africa, South America, and Europe. Humans are infected as intermediate hosts, meaning larvae balls develop in their internal organs. Scientists are researching drug treatments, but until these drugs are developed, surgery remains the only method of treatment. The disease caused by dog tapeworms also is dangerous for many plant-eating domestic animals, such as sheep, pigs, goats, cattle, camels, and horses.

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

NO COMMON NAME (Proteocephalus longicollis): SPECIES ACCOUNTS

Physical characteristics: The body of Proteocephalus longicollis worms is about 9 inches (22 centimeters) long and 0.08 inch (2 millimeters) wide. The scolex has small suckers and a small suckerlike organ at the tip. There are reproductive openings on the sides of the proglottids.

Geographic range: Proteocephalus longicollis (abbreviated to P. longicollis) worms live in Europe, Asia, and North America.

Habitat: P. longicollis worms live in freshwater fishes, mainly trouts, salmons, whitefishes, and smelts, that live in lakes and rivers in the northern hemisphere.

Diet: P. longicollis worms absorb nutrients from their hosts.

Behavior and reproduction: Scientists do not know how P. longicollis worms behave. The worms make both eggs and sperm. Eggs released in water are eaten by tiny crustaceans, which are the intermediate hosts. The larvae develop in the body cavity of the crustacean. Fishes become infected by eating crustaceans containing the P. longicollis larvae.

Proteocephalus longicollis and people: P. longicollis worms infect fishes raised in fish farms.

Conservation status: P. longicollis worms are not considered threatened or endangered. ∎

FOR MORE INFORMATION

Books:

Knutson, Roger M. Fearsome Fauna. New York: W. H. Freeman, 1999.

Ruppert, Edward E., Richard S. Fox, and Robert D. Barnes. Invertebrate Zoology. 7th ed. Belmont, CA: Thomson-Brooks/Cole, 2004.

Zimmer, Carl. Parasite Rex. New York: Free Press, 2000.

Web sites:

"Do You Know These Parasites?" Wonderwise.http://net.unl.edu/wonderwise/12parasi/kidactivity/activity.htm (accessed on February 7, 2005).

"Echinococcosis." American Family Physician.http://www.aafp.org/afp/20020901/821ph.html (accessed on February 7, 2005).

"Tapeworms: The Cestodes." WormLearn.http://home.austarnet.com.au/wormman/wltape.htm (accessed on February 7, 2005).