The trematode Clonorchis sinensis (Chinese or oriental liver fluke) is an important foodborne pathogen and cause of liver disease in Asia. This appears to be the only species in the genus involved in human infection.
Clonorchis sinensis eggs are discharged in the biliary ducts and in the stool in an embryonated state . Eggs are ingested by a suitable snail intermediate host . Eggs release miracidia , which go through several developmental stages (sporocysts , rediae , and cercariae ). 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 . Infection of humans occurs by ingestion of undercooked, salted, pickled, or smoked freshwater fish . After ingestion, the metacercariae excyst in the duodenum and ascend the biliary tract through the ampulla of Vater . Maturation takes approximately one month. The adult flukes (measuring 10 to 25 mm by 3 to 5 mm) reside in small and medium sized biliary ducts.
The host range is relatively broad and C. sinensis is able to infect multiple species of snails and more than 100 species of freshwater fish as intermediate hosts. Many of the fish species belong to the Cyprinidae family, which includes carps and minnows. Additionally, C. sinensis metacercariae have been recovered from some species of shrimp in China and may contribute to disease transmission in these locations, although prevalence and metacercarial burdens are much lower than in cyprinid fish. Other than humans, domestic canids and felids, swine, mustelids, and other piscivorous mammals can serve as definitive hosts.
Endemic areas are in East Asia (including Korea, China, Taiwan, and northern Vietnam) and into far eastern Russia. Cases of clonorchiasis have been reported in non-endemic areas, including the United States, typically in Asian immigrants, or following ingestion of infected undercooked/pickled freshwater fish imported from endemic areas.
Symptoms are related to worm burden; most infections are of light burden and asymptomatic. Clinical manifestations result from inflammation, intermittent obstruction of the biliary ducts, mechanical injury resulting from worms feeding on mucosal tissue, toxic effects of worms’ metabolic products, and secondary bacterial infections. In mild cases, manifestations include mild abdominal symptoms. Patients with high worm burdens often have other nonspecific symptoms such as abdominal pain (particularly in the right upper abdominal quadrant) and various somatic symptoms (e.g. headache, dizziness). With infections of longer duration, the symptoms can be more severe, and hepatomegaly and malnutrition may be present. More rarely, biliary complications (e.g., cholangitis, cholelithiasis, cholecystitis, and cholangiocarcinoma), pancreatitis, and liver abscesses may develop.
Clonorchis sinensis eggs.
Clonorchis sinensis eggs are small, ranging in size from 27 to 35 µm by 11 to 20 µm. The eggs are oval shaped with a convex operculum that rests on visible “shoulders” at the smaller end of the egg. At the opposite (abopercular) end, a small knob or hook-like protrusion is often visible. The miracidium is visible inside the egg. Eggs of C. sinensis are highly morphologically similar to Opisthorchis spp.
C. sinensis adults.
Clonorchis sinensis adults are flattened, lance shaped, and measure approximately 10 to 25 mm long by 3 to 5 mm wide. The oral and ventral suckers (acetabulum) are relatively small. Like other flukes, they are hermaphroditic. The two testes are located posterior to the ovary, and are highly branched–a feature which separates it from the related Opisthorchis spp. (rounded testes). Adults reside in the biliary passages of the liver of the definitive host.
Snail intermediate hosts of C. sinensis.
Diagnosis is usually based on microscopic identification of eggs in stool specimens. However, the eggs of Clonorchis are practically indistinguishable from those of Opisthorchis . Adult flukes have been recovered at surgery or may be spontaneously passed after anthelmintic treatment. Morphologic examination of the intact adult fluke allows definitive species identification.
Serologic testing is currently not available for Clonorchis infection in the United States.
Standard precautions for the processing of stool samples apply. C. sinensis eggs in feces are not infectious to humans.
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