The Chinese liver fluke is a small worm that parasitizes the bile ducts and livers of humans, dogs, cats, pigs, and several wild animals in China, Japan, Korea, and Indochina. It was discovered in 1875, and recently, it was estimated that 20 million individuals in China alone are infected. Eggs are laid in the bile ducts, pass in the feces, and if they reach the proper freshwater snail, undergo a series of stages in this intermediate host. Eventually, free-swimming larvae are formed, which penetrate and encyst the skin or muscles of fish, expecially those of the carp family. Human beings and other definitive hosts become infected by eating the cysts (metacercaria) in raw or poorly cooked fish. Raw fish are a delicacy in many Asian countries, and fish are sometimes raised in ponds fertilized with human feces. Encysted metacercaria larvae are resistant to smoking, pickling, salting, and drying. Imported fish have caused human cases in Hawaii, and the popularity of Asian cuisine poses a potential danger to gourmets far beyond Asia.

Light infections are often asymptomatic. Heavy infections may produce diarrhea, fever, jaundice, and abdominal pain. Bile duct blockage and liver abscesses occur in chronic cases, and Clonorchis sinensis has been tentatively linked to liver cancer. Diagnosis is made by microscopic examination of the feces to discover the characteristic eggs. Drug therapy is sometimes successful. Preventive measures include rural sanitation, regulation of fish-farming methods, and cooking fish thoroughly. It is unlikely, however, that long-established culinary practices can be changed.