Flukes: Trematoda

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FLUKES: Trematoda

LANCET FLUKE (Dicrocoelium dendriticum): SPECIES ACCOUNTS


Flukes (FLOOKS) are flatworms that are parasites (PAIR-uh-sites), which are animals or plants that live on or in other animals or plants, or hosts, without helping them and usually harming them. Flukes usually are leaf shaped and have suckers that they use for attaching to and feeding on their hosts. Flukes can be as small as one–thirty-second of an inch (1 millimeter) or as long as 23 feet (7 meters), but most are one-eighth to 2 inches (5 millimeters to 5 centimeters) long. Flukes have a hard covering that keeps them from being dissolved by the stomach juices of their hosts.

There are two types of flukes. One type has a direct life cycle, meaning there is only one host, often freshwater snails, in which development from egg to adult occurs. The other type of flukes has an indirect life cycle, meaning they infect different hosts during the various stages of life. Most of these flukes have two stages of development and at least two hosts.


Flukes live all over the world.


The habitat of a fluke is the same as that of its host and can change as the fluke goes from host to host.


Flukes cannot live without nourishment from a host organism. In some species the larvae (LAR-vee), or animals in an early stage that change form before becoming adults, do not eat. Adult flukes eat blood cells, mucus, and body cells.


Flukes with an indirect life cycle begin life as eggs in a primary host and then pass with the host's feces (FEE-seez) or waste into water or onto land. After the eggs hatch, the larvae move to another host, called the intermediate host, which is often a mollusk. Mollusks (MAH-lusks) are animals with a soft, unsegmented body that may or may not have a shell. The flukes change form, exit the host, and move to another intermediate host, which is frequently another mollusk, a fish, or an amphibian, and change form again. The life cycle continues when a new primary host eats the second intermediate host, at which point the fluke infects the primary host. Primary hosts often are mammals and birds.

In some species the first-stage larvae do not feed. For this reason, they must find a first intermediate host very quickly, usually within one or two days of hatching. In moving from the first to the second host, most flukes use environmental cues, such as light or water turbulence, to seek the new host. Some species also follow a chemical trail. In some species, however, the larvae seem to stumble upon rather than track their hosts. Some flukes with an indirect life cycle skip the second intermediate host and invade the primary host directly. Others live on plants rather than in a secondary host. The primary host then becomes infected by eating the fluke-infested vegetation.

Flukes with an indirect life cycle use asexual and sexual reproduction. Asexual (ay-SEK-shuh-wuhl) means without, and sexual means with, the uniting of egg and sperm and the transfer of DNA from two parents. When the first-stage larvae reach their destination within the first intermediate host, the asexual phase begins when the larvae lose their hairlike fibers and change into another form of larvae. The new larvae produce more of the same type of larvae or a transformed type. With asexual reproduction, the number of invading flukes can multiply very quickly inside the first intermediate host.

Did You Know?

Scientists estimate that as many as one-half of all animal species are parasites.

In the first intermediate host, the asexually produced larvae transform into free-living young flukes. The young flukes swim to the second intermediate host, which is typically prey for the primary host. Once on or in the second intermediate host, the young flukes transform again. It is only after the flukes finally enter the primary host and become adults that they use sexual reproduction either by mating with other flukes or by fertilizing (FUR-teh-LYE-zing) themselves. Fertilization (FUR-teh-lih-ZAY-shun) is the joining of egg and sperm to start development. Almost all flukes make both eggs and sperm. Blood flukes have separate sexes, and the adult females and males mate with each other.

Flukes with a direct life cycle use only sexual reproduction. The entire life cycle occurs in one host, usually a mollusk. Although predators may eat the host species and temporarily harbor the worms, the worms can survive for only a short time in the predator's digestive tract and cannot reproduce or develop there.


Flukes pose a health threat to humans. Fluke diseases cause weakness, diarrhea, bleeding, fever, abdominal pain, and other severe symptoms.


After malaria, the disease caused by human blood flukes is the most common parasitic disease in the world, affecting more than two hundred million people in more than seventy-five countries.


Flukes are not threatened or endangered.

LANCET FLUKE (Dicrocoelium dendriticum): SPECIES ACCOUNTS

Physical characteristics: Adult lancet flukes have pearly bodies shaped like long, thin leaves. The suckers are on the mouth and on the bottom of the worm toward the front of the body. The mouth sucker closest to the front is a bit smaller than the other suckers. Lancet flukes are about three-sixteenths to five-eighths of an inch (5 to 15 millimeters) long and one–thirty-second to a little more than one-sixteenth of an inch (2 millimeters) wide.

Geographic range: Lancet flukes live in the northeastern United States, Australia, northern and central Europe, Asia, and Africa.

Habitat: Lancet flukes live in dry habitats. The primary hosts are mammals such as sheep, cattle, pigs, rabbits, deer, and woodchucks. The first intermediate hosts are land-dwelling snails, and the second intermediate hosts are ants.

Diet: Lancet flukes feed on the cells of their hosts.

Behavior and reproduction: Lancet flukes begin life as eggs in the feces of their primary hosts. The eggs are picked up and carried by snails. The larvae form slime balls in the snail, which ejects them through its breathing pore. The larvae are picked up by ants and cause cramping in the ants' jaws. The cramping causes the ants to clamp down on blades of grass and become stuck. The larvae then are eaten by grazing animals, such as sheep and cattle, and develop to the adult stage in the animal's liver. The eggs leave the host in its feces, and the cycle starts over. Adult lancet flukes make both eggs and sperm and either mate with other flukes or fertilize themselves.

Lancet flukes and people: Lancet flukes cause liver disease in farm animals.

Conservation status: Lancet flukes are not threatened or endangered. ∎


Physical characteristics: Female human blood flukes are thin, cylindrical, and one-half to 1 inch (1.3 to 2.5 centimeters) long. Males are a little bit shorter and thicker. Males have small spiny suckers on the mouth and belly and a wrinkled back dotted with small bumps. Females usually spend their lives attached to males. Both males and females are white.

Geographic range: Human blood flukes live in the warm regions of the world, especially South America, Africa, the Caribbean, and the Middle East.

Habitat: The larvae of human blood flukes live in freshwater snails. The adults live in veins in the abdomens of mammals such as rodents, dogs, cattle, baboons, and humans.

Diet: Human blood flukes feed on blood in the abdominal veins of their hosts.

Behavior and reproduction: The eggs of human blood flukes hatch in freshwater areas and develop into larvae, which follow chemical, light, and gravitational cues to find and then penetrate the soft tissues of snails. The larvae transform into another type of larvae and swim out of the snails. The larvae actively seek out the next host by targeting fatty acids in the skin. They then penetrate the skin of a secondary host, which may be a person or other mammal. Once in the host, the larvae become immature flukes, travel to the circulatory system, and travel to veins near the large intestine. Once in the veins, the flukes mature, mate, and lay eggs, many of which leave the host's body with feces. The cycle begins again when the eggs make their way into the freshwater habitat of the snail. Adult male human blood flukes usually live joined with the females, the female remaining in the male's spine-covered reproductive canal, a groove that runs along the lower surface of the body.

Human blood flukes and people: Infection with human blood flukes causes disease in humans. The condition causes abdominal pain, diarrhea, intestinal bleeding, tiredness, and a decrease in red blood cells, leaving the victim weak and vulnerable to other diseases.

Conservation status: Human blood flukes are not threatened or endangered. ∎


Physical characteristics: Adult Fasciola hepatica (abbreviated to F. hepatica) are a little more than 1 inch (3 centimeters) long and three-eighths of an inch (1 centimeter) wide. They have a spiny outer covering. The front end has a mouth sucker and a cone-shaped tip, and the rear end is tapered. The sucker on the fluke's lower surface is larger than the mouth sucker.

Geographic range: F. hepatica live all over the world but mainly in Europe, Mexico, and Central America.

Habitat: F. hepatica live in swampy freshwater areas inhabited by snails. Snails are their sole intermediate hosts. The primary hosts include grazing mammals such as sheep, cattle, and horses; farm animals such as hogs; pets such as dogs, cats, and rabbits; and humans.

Diet: F. hepatica feed on the lining of the ducts, or tubes, in the liver, causing hardening of the ducts.

Behavior and reproduction: The eggs of Fasciola hepatica, which are deposited in the environment in the primary host's feces, hatch in freshwater areas, usually within about ten days, longer if temperatures are cool. These flukes have been known to survive in particularly cold water for several years. The embryos develop into larvae, which quickly swim to and penetrate the soft tissue of snails. The larvae produce more larvae, which transform. Larvae in their final stage live in the snails for four to eight weeks, then exit and swim to plants lying just below the water line. Passing plant-eating animals become infected when they eat the plants, often grass. Humans typically become infected by drinking water containing flukes or by eating greens such as watercress. The flukes travel to the abdominal cavity in the first twenty-four hours, then to the liver over the next few days. Within six to eight weeks, the flukes reach the liver ducts, where they mature and lay eggs. The eggs are then carried to the intestine and pass into the feces. The flukes sometimes spread to the lungs as well as the liver.

Fasciola hepatica and people: Humans infected with F. hepatica may have symptoms ranging from skin inflammation to pneumonia. Fluke infection can result in massive bleeding in horses, a reduction of milk production in dairy cattle, and death in sheep.

Conservation status: Fasciola hepatica are not threatened or endangered. ∎



Aaseng, Nathan. Invertebrates. New York: Venture, 1993.

Silverstein, Alvin, Virginia Silverstein, and Robert Silverstein. Invertebrates. New York: Twenty-First Century, 1996.

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

Web sites:

Frey, Rebecca J. "Fluke Infections." AhealthyMe. http://www.ahealthyme.com/article/gale/100084581 (accessed on December 20, 2004).

Frisby, Holly. "Dicrocoelium dendriticum (Lancet Fluke)." PetEducation.com http://www.peteducation.com/article.cfm?cls=2&cat=1621&articleid=731 (accessed on December 20, 2004).