Helminth-Trematode: Clonorchis sinensis

Clonorchis. sinensis is a fish-borne trematode (fluke) that inhabits the bile duct of mammals, including man. Clonorchiasis is prevalent in China, Korea, and Vietnam, and 15–20 million people are believed to be infected by this fluke. Humans are infected by eating raw or undercooked freshwater fish as preparations of filet, ‘sashimi,’ or congee, which harbor C. sinensis metacercariae. Freshwater snails act as a first intermediate host for the larval development of C. sinensis and after the parasite undergoes asexual multiplication, shed cercariae into water. The cercariae then penetrate freshwater fish and transform to metacercariae. Both small and large freshwater fish may serve as infection sources, depending on conditions in different locations. In humans, the C. sinensis larvae excysted in the duodenum creep up into the bile duct and mature into the ovigerous adult fluke. In the bile duct the infection induces hyperplasia of the bile duct epithelium, obstructive jaundice, liver enlargement, ascites, and cirrhosis, and infrequently cholangiocarcinoma. Human infections can be prevented by health education regarding the cooking of freshwater fish, keeping aquaculture systems free of contamination by snails and the feces of humans and reservoir animals, and by freezing freshwater fish.