Clonorchis sinensis has a complex life cycle. The definitive host (a fish-eating mammal; this includes humans, cats, dogs) harbors the adult worm in its biliary system. The adult worms lay eggs, which are released in the host's feces. If the feces reach freshwater, the eggs can be ingested by the first intermediate host, freshwater snails. Within the snail, the parasite can transform to a free-swimming stage, cercariae. The cercariae can then penetrate the skin of a freshwater fish or shrimp, the second intermediate host. The organism encysts within the skin or musculature of the fish. If the fish or shrimp is then eaten raw or undercooked, the fluke can hatch in the duodenum and move into the definitive host's biliary system. Within 4 weeks, the fluke matures to the point where it begins producing eggs. Adult flukes are thought to be able to survive in humans for 20 years.
Most infections are asymptomatic, but clinical manifestations of clonorchiasis can be roughly correlated to the individual's worm burden (number of eggs per gram of stool):
- Low worm burden (<100) may be asymptomatic or have vague abdominal symptoms. Signs may include occasional jaundice and eosinophilia.
- Moderate worm burden (<1000) may have more obvious abdominal signs including fevers, weight loss, colicky pain, and distention. Occasionally an acute syndrome of fever, abdominal pain, urticaria, and eosinophilia – reminiscent of acute schistosomiasis (Katayama syndrome) – may occur.
- High worm burden (up to 25 000) may present with an acute right upper quadrant pain syndrome.