Clonorchiasis

Man is an incidental host for the oriental liver fluke Clonorchis sinensis, a parasite of fish-eating mammals of the Far East. Millions of people in China and Southeast Asia are infected with this organism. The adult worms live in the distal biliary passages , where they produce eggs which pass down the biliary system and out of the body in the faeces. The eggs are ingested by snails, in which they hatch into miracidia. These multiply within the snail, producing large numbers of cercariae that emerge into the water. The cercariae penetrate under the scales of certain freshwater fish, in which they encyst as metacercariae, the forms which are infective when inges-ted by mammals.

Man acquires the disease by eating inadequately cooked fish. The metacercariae encyst in the duodenum and pass through the ampulla of Vater, where they mature into adult worms inside the bile ducts. Most infected individuals are asymptomatic, but heavy infection may produce cholangitis and hepatitis. Infection with this fluke is associated with an increased incidence of adenocarcinoma arising from the epithelium of the bile ducts. Diagnosis is made by finding the characteristic eggs in the faeces. No satisfactory treatment for this infection is available.