Oriental liver fluke

Clonorchis sinensis

ORDER Opisthorchiida

FAMILY Opisthorchiidae

TAXONOMY

Clonorchis sinensis Cobbold, 1875. Some scientists now use the genus designation Opisthorchis for this fluke.

OTHER COMMON NAMES

English: Chinese liver fluke; French: Douve du foie chinoise, douve du foie orientale; German: Chinesischer Leberegel.

PHYSICAL CHARACTERISTICS

The adults are flattened cigar-shaped flatworms 0.4-1.0 in (10-25 mm) long and 0.1-0.2 in (3-5 mm) wide. The pointed front end has an oral sucker at its tip. Much of the anterior half of the animal is filled with a looping uterus. A small, slightly lobed ovary follows with two large branching testes located in the posterior half of the body. The genital pore opens about a fifth of the body length behind the oral sucker. The ventral sucker is circular and located slightly behind the oral sucker.

DISTRIBUTION

East Asia, including Japan, much of China, and Korea. HABITAT

This organism begins its life as an egg passed in the feces of a human or other mammal. It then infects snails, followed by one of more than 100 species of fish, and finally humans or other fish-eating mammals.

BEHAVIOR

After the eggs are passed into the water, they are eaten by the first intermediate host, which is a snail (Bulimus species or Parafossarulus manchouricus). The miracidia hatch in the intestine, form a sporocyte, generate rediae, and finally cercariae. Individual rediae can produce as many as 50 cercariae, which exit the snail into the water and begin the search for a second intermediate host. Clonorchis sinensis has an unusual method of searching; its cercariae sink until they hit either the bottom or some other structure, then swim to the surface to sink again. While bouncing back and forth between the surface and the bottom, cercariae that encounter a fish burrow through the skin and encyst in its muscle tissue. These second intermediate hosts are usually freshwater fishes of the Cyprinidae family, in addition to at least nine species of fishes from eight other families and possibly a few freshwater shrimp. The cercariae remain in the second intermediate host until the definitive host, which may be a human or other mammal, eats the fish. The flukes, now metacercariae, encyst in the host's small intestine and transform themselves into immature flukes, which then migrate to the bile ducts in the host's liver. There, the fluke matures in about three weeks and lays eggs that exit the host in the feces.

FEEDING ECOLOGY AND DIET

As a parasitic organism, Clonorchis sinensis relies on hosts to meet its dietary needs. Snails serve as hosts for its miracidia, fishes for its cercariae and metacercariae, and humans or other fish-eating predators for the adult flukes. Sporocysts form in snails. Metacercarial encyst in fishes. Adults live in the small bile ducts of the liver of definitive hosts and dine on their blood.

REPRODUCTIVE BIOLOGY

Adults are hermaphroditic. Adult liver flukes can lay as many as 4000 eggs per day. The operculate eggs are shaped like light bulbs, yellowish brown in color, and range from 0.0009-0.001 in (25-35 pm) long and 0.0004-0.0007 in (11-20 pm) wide. The miracidia are ciliated and ovate with a short anterior protrusion. The cercariae resemble tadpoles with long tails. The life span of Clonorchis sinensis is at least 10 years, reportedly reaching 30-40 years.

CONSERVATION STATUS Not listed by IUCN.

SIGNIFICANCE TO HUMANS

Clonorchiasis is a disease that commonly affects people in parts of Asia, where people may eat uncooked infected fishes. Infections in other parts of the world result from eating imported infected fishes. Transmission of Clonorchis sinensis via infected dried, salted, smoked, or pickled fish has also been reported. ♦

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