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Infection by Clonorchis sinensis, the Chinese liver fluke, is endemic in areas of Japan, Korea, China, Taiwan, Southeast Asia, and the far eastern part of Russia. An estimated 15 million people are infected (13 million in China); in some communities, prevalence can reach 80%. Opisthorchiasis is principally caused by Opisthorchis felineus (regions of the former Soviet Union) or Opisthorchis viverrini (Thailand, Laos, Vietnam). Clonorchiasis and opisthorchiasis are clinically indistinguishable. Parasite eggs are shed into water in human or animal feces, where they infect snails, which release cercariae, which infect fish. Human infection follows ingestion of raw, undercooked, or pickled freshwater fish containing metacercariae. These parasites excyst in the duodenum and ascend into the biliary tract, where they mature and remain for many years, shedding eggs in the bile (eFigure 35–24).

eFigure 35–24.

Life cycle of Clonorchis sinensis (Chinese liver fluke). Embryonated eggs are discharged in the biliary ducts and in the stool. Eggs are ingested by a suitable snail intermediate host

image. Each egg releases a miracidia
image, which go through several developmental stages (sporocysts
image, rediae
image, and cercariae
image). The cercariae are released from the snail and after a short period of free-swimming time in water, they come in contact and penetrate the flesh of freshwater fish, where they encyst as metacercariae
image. Infection of humans occurs by ingestion of undercooked, salted, pickled, or smoked freshwater fish
image. After ingestion, the metacercariae excyst in the duodenum
image and ascend the biliary tract through the ampulla of Vater
image. Maturation takes approximately 1 month. The adult flukes (measuring 10–25 mm by 3–5 mm) reside in small- and medium-sized biliary ducts. In addition to humans, carnivorous animals can serve as reservoir hosts. (From Global Health, Division of Parasitic Diseases and Malaria, CDC.)

Most patients harbor few parasites and are asymptomatic. An acute illness can occur 2–3 weeks after initial infection, with fever, malaise, abdominal pain, anorexia, tender hepatomegaly, urticaria, and eosinophilia. The acute syndrome is difficult to diagnose, since ova may not appear in the feces until 3–4 weeks after onset of symptoms. In chronic heavy infections, findings include abdominal pain, anorexia, weight loss, and tender hepatomegaly. More serious findings can include recurrent bacterial cholangitis and sepsis, cholecystitis, liver abscess, and pancreatitis. An increased risk of cholangiocarcinoma has been documented.

Early diagnosis is presumptive, based on clinical findings and epidemiology. Subsequent diagnosis is made by finding characteristic eggs in stool or duodenal or biliary contents (eFigure 35–25). The stool Kato-Katz test is widely used; performing repeated tests improves sensitivity. Imaging studies show characteristic biliary tract dilatations with filling defects due to flukes. Serologic assays for clonorchiasis with excellent sensitivity are available but cannot distinguish between past and current infection. Molecular tests have been developed but are not widely used.

eFigure 35–25.

Adult trematodes. Clonorchi sinensis adults are flattened and measure approximately 10–25 ...

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