Lancet fluke

Dicrocoelium dendriticum

ORDER Plagiorchiida

FAMILY Dicrocoeliidae

TAXONOMY

Dicrocoelium dendriticum Rudolphi, 1819, intestine of Xiphias gladius, a swordfish (probably in error).

OTHER COMMON NAMES

English: Lancet liver fluke; French: Petite douve du foie; German: Kleiner Leberegel, Lanzettegel.

PHYSICAL CHARACTERISTICS

Adult lancet flukes have translucent bodies shaped like long, thin leaves. Both oral and ventral suckers are located toward the front of the body, with the foremost oral sucker a bit smaller than the others. This species averages about 0.02-0.06 in (5-15 mm) long and 0.04-0.08 in (1-2.5 mm) wide.

DISTRIBUTION

Northeastern United States; Australia; northern and central Europe; Asia; and Africa.

HABITAT

Dicrocoelium dendriticum prefers dry habitats. It begins its life as an egg in the feces of its definitive hosts. Its first intermediate hosts are terrestrial snails, including Helicella species and Cionella lubrica; its second intermediate hosts are ants (Formica fusca); and its definitive hosts include various mammals, including sheep, cattle, and pigs, as well as cottontail rabbits, deer, and wood-chucks. In snails the miracidia hatch in the intestine, then migrate through the intestinal wall. In ants, metacercariae encyst in the gaster. In the definitive hosts, immature flukes leave the metacercaria cysts and migrate to the common bile duct.

BEHAVIOR

The eggs of lancet flukes are found on dry land, where they are eaten by snails, including Cionella lubrica in the United States. Hatched miracidia develop into sporocysts that in turn produce a second generation of sporocysts. The sporocysts transform into cercariae and migrate to the snail's lung cavity. The snail encases as many as 400 cercariae in mucus, then ejects the slime balls thus formed through its respiratory pore. Ants (Formica fusca) become the second intermediate host by ingesting the slime balls. While most of the infection in ants occurs in the hemocoel (spaces between the cells and tissues), some still-immature flukes make their way into the sube-sophageal ganglion (cluster of nerve cells underneath the eosphagus), which causes the ants' behavior to change. The ants are impelled to climb to the tips of grass blades. When the temperature drops at night, the ants' jaws clamp onto the grass, and keep them attached to the grass until temperatures rise the next morning. Herbivores grazing early in the morning inadvertently eat the ants with the foliage and become infected with the flukes. The metacercariae travel to the new host's bile duct, gall bladder, and pancreatic ducts, where they mature and lay their eggs. The mammals eliminate the eggs in their feces, and the life cycle repeats.

FEEDING ECOLOGY AND DIET

Lancet flukes meet their nutritional needs through various hosts, including snails, ants, and plant-eating mammals. In snails, mother sporocysts form, giving rise to daughter sporo-cysts and finally cercariae.

REPRODUCTIVE BIOLOGY

Adults are hermaphroditic. The dark brown, ovate eggs are op-erculate, about 0.0014-0.0019 in (36-48 pm) long and 0.0008-0.0012 in (22-30 pm) wide. The cercariae are long with nipped-in "waists." The fluke's anterior portion is ovate and the posterior portion tapered.

CONSERVATION STATUS Not listed by the IUCN.

SIGNIFICANCE TO HUMANS

When left untreated, infected livestock may develop progressive hepatic cirrhosis. In sheep, the condition can reduce reproductive capacity and wool production. ♦

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