THORNY-HEADED WORMS: Acanthocephala

PHYSICAL CHARACTERISTICS

Thorny-headed worms are parasites that live in vertebrates as adults and in insects and crustaceans as larvae. 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. Vertebrates (VER-teh-brehts) are animals with a backbone. Crustaceans (krus-TAY-shuns) are water-dwelling animals that have jointed legs and a hard shell but no backbone. Larvae (LAR-vee) are animals in an early stage that change form before becoming adults.

Adult thorny-headed worms are tubular or slightly flat. Most are white or colorless, but some are yellow, brown, red, or orange. Adult thorny-headed worms are less than 1 inch (a few millimeters) to more than 2 feet (60 centimeters) long. Females usually are larger than males. The snout has hooks arranged in rows or lengthwise lines. The worm can retract the snout into its body. In some species the body is armed with spines. Inside their bodies, thorny-headed worms have a network of fluid-filled cavities. They have no digestive tract.

GEOGRAPHIC RANGE

Thorny-headed worms live all over the world.

HABITAT

Adult thorny-headed worms live in the intestines of mammals, birds, fishes, amphibians, and reptiles. Larvae live in crustaceans and insects.

DIET

Thorny-headed worms absorb nutrients from their hosts' intestines.

BEHAVIOR AND REPRODUCTION

When they attach themselves to the intestinal wall of their hosts, thorny-headed worms can cause serious, sometimes fatal, internal damage. In most cases, the more worms there are, the more serious is the damage.

Female and male thorny-headed worms mate in the intestines of their primary hosts. Fertilization (FUR-teh-lih-ZAY-shun), the joining of egg and sperm to start development, takes place inside the female's body. She releases the fertilized (FUR-teh-lyzed) eggs into the host's intestine, and they leave the host in its feces (FEE-seez). Outside the host, the eggs are eaten by intermediate hosts. The larvae hatch, bore into the intermediate host's intestinal wall, and develop there through two more stages. When a primary host, such as a bird, eats the intermediate host, such as an insect, the larvae enter the primary host and develop into adults.

Some species of thorny-headed worms have transport hosts. These hosts are vertebrates that eat intermediate hosts containing final-stage larvae but in which the larvae cannot develop into adult worms. The larvae do not die but stay in the transport host until it is eaten by a suitable primary host.

THORNY-HEADED WORMS AND PEOPLE

Very few species of thorny-headed worms cause disease in humans.

DRIVING THE HOST CRAZY

To make sure their life cycle is complete, thorny-headed worms can change the behavior of their hosts. For example, one species of crustacean normally swims to the bottom of the water when a duck is near. When infected with thorny-headed worms, however, this crustacean swims up and attaches itself to a rock, making itself easy prey for the duck, which becomes the primary host.

CONSERVATION STATUS

Thorny-headed worms are not considered threatened or endangered.

NO COMMON NAME (Moniliformis moniliformis): SPECIES ACCOUNTS

Physical characteristics: Moniliformis moniliformis worms are long, threadlike, and often coiled. Females are 4 to 11 inches (10 to 27 centimeters) long. Males are 1.6 to 2 inches (4 to 5 centimeters) long. The snouts of these worms have 12 lengthwise rows of seven to eight hooks.

Geographic range: Moniliformis moniliformis (abbreviated as M. moniliformis) worms live all over the world.

Habitat: The primary hosts of M. moniliformis worms are dogs, cats, and wild rodents, especially rats. The intermediate hosts are beetles and cockroaches. The transport hosts are toads and lizards.

Diet: M. moniliformis worms absorb nutrients from their hosts.

Behavior and reproduction: Adult M. moniliformis worms mature in five to six weeks in the intestines of primary hosts. Hatching of the first-stage larvae occurs 15 minutes to 48 hours after ingestion by the intermediate host. The larvae develop to their final stage in the intermediate host in about two months.

Moniliformis moniliformis and people: M. moniliformis worms cause disease in people. The symptoms include tiredness, ringing in the ears, and diarrhea.

Conservation status: M. moniliformis worms are not considered threatened or endangered. ∎

GIANT THORNY-HEADED WORM (Macracanthorhynchus hirudinaceus): SPECIES ACCOUNTS

Physical characteristics: Female giant thorny-headed worms are as long as 26 inches (65 centimeters) and are 0.3 to 0.4 inch (8 to 9 millimeters) wide. Males are as long as 4 inches (10 centimeters). The body is grayish brown with deep grooves on the surface. The snout has six spiral rows of six hooks each.

Geographic range: Giant thorny-headed worms live all over the world.

Habitat: Adult giant thorny-headed worms live in hogs, squirrels, moles, hyenas, and dogs. The larvae live in cockroaches and beetles.

Diet: Giant thorny-headed worms absorb nutrients from their hosts.

Behavior and reproduction: Female giant thorny-headed worms release a huge number of eggs that can survive more than three years in the primary host. The larvae develop for four to five months in the intermediate host. The worms reach adulthood two to three months after entering the primary host.

Giant thorny-headed worms and people: Giant thorny-headed worms cause disease in people and hogs. Hogs become infected when they ingest beetles while rooting for grubs. Humans become infected mainly in rural Asia, where people eat beetles and use them for medicine.

Conservation status: Giant thorny-headed worms are not considered threatened or endangered. ∎

FOR MORE INFORMATION

Books:

Moore, Janice. Parasites and the Behavior of Animals. New York: Oxford University Press, 2002.

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

Web sites:

Cole, Rebecca A. "Acanthocephaliasis." Field Manual of Wildlife Diseases. http://212.187.155.84/pass_06june/Subdirectories_for_Search/Glossary&References_Contents/BooksContents/BookRef36_FieldManualofWildlifeDiseases/33/chapter33.htm (accessed on February 18, 2005).