What is the primary intermediate host for Clonorchis sinensis and what disease results from chronic infection?

• A. Freshwater snails; clonorchiasis with risk of cholangiocarcinoma
• B. Freshwater fish; clonorchiasis
• C. Mosquitoes; lymphatic filariasis
• D. Pigs; taeniasis

Answer: (A)

Clonorchis sinensis uses a freshwater snail as the first intermediate host. The parasite develops inside the snail, then releases stages that infect freshwater fish (the second intermediate host), where metacercariae form in the fish tissue. Humans acquire the infection by eating raw or undercooked infected fish, and the adult flukes reside in the bile ducts, causing clonorchiasis. Chronic infection keeps the biliary tract inflamed and damaged, and it is associated with an increased risk of cholangiocarcinoma, cancer of the bile ducts. So the primary intermediate host is the freshwater snail, and the disease from chronic infection is clonorchiasis with risk of cholangiocarcinoma.