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Clonorchiasis

[Clonorchis sinensis]

Causal Agents

The trematode Clonorchis sinensis (Chinese or oriental liver fluke).

Life Cycle

Life cycle of Clonorchis sinensis

Embryonated eggs are discharged in the biliary ducts and in the stool The number 1. Eggs are ingested by a suitable snail intermediate host The number 2. Each egg releases a miracidia The number 2a, which go through several developmental stages (sporocysts The number 2b, rediae The number 2c, and cercariae The number 2d. 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 The number 3 Infection of humans occurs by ingestion of undercooked, salted, pickled, or smoked freshwater fish The number 4. After ingestion, the metacercariae excyst in the duodenum The number 5 and ascend the biliary tract through the ampulla of Vater The number 6 Maturation takes approximately 1 month. The adult flukes (measuring 10 to 25 mm by 3 to 5 mm) reside in small and medium sized biliary ducts. In addition to humans, carnivorous animals can serve as reservoir hosts.

Geographic Distribution

Endemic areas are in Asia including Korea, China, Taiwan, and northern Vietnam. Clonorchiasis has been reported in non-endemic areas (including the United States). In such cases, the infection is found in Asian immigrants, or following ingestion of imported, undercooked or pickled freshwater fish containing metacercariae.

Clinical Presentation

Most infections are asymptomatic. Most pathologic manifestations result from inflammation and intermittent obstruction of the biliary ducts. In mild cases, manifestations include dyspepsia, abdominal pain, diarrhea or constipation. With infections of longer duration, the symptoms can be more severe, and hepatomegaly and malnutrition may be present. In rare cases, cholangitis, cholecystitis, and chlolangiocarcinoma 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 (larger, abopercular) end, a small knob or hooklike protrusion is often visible. The miracidium is visible inside the egg.

Figure A: C. sinensis egg: the small knob at the abopercular end is visible in this image.

Figure B: C. sinensis egg. Note the operculum resting on "shoulders;" image taken at 400× magnification.

Figure C: C. sinensis egg; images taken at 400× magnification.

Figure D: C. sinensis egg; images taken at 400× magnification.

C. sinensis adults.

 

Clonorchi sinensis adults are flattened and measure approximately 10-25 mm long by 3-5 mm wide. Like other flukes, they are hermaphroditic, with a single ovary situated anterior to two branches testes. Adults reside in the biliary passages of the liver of the definitive host.

Figure A: Adult of C. sinensis.

Figure B: Adult of C. sinensis stained with carmine. Clearly visible in this image are the oral sucker (OS), pharynx (PH), ceca (CE), acetabulum, or ventral sucker (AC), uterus (UT), vitellaria (VT) and testes (TE).

Snail intermediate hosts of C. sinensis.

 

Clonorchis sinensis requires two intermediate hosts for completion of its life cycle. The first is a snail and the second is cyprinoid fish (although at least eight families of fish may serve as intermediate hosts for C. sinensis). Among the snails, over 100 species in several general may serve as the primary intermediate host. In China, Japan, Korea, Taiwan and other endemic areas, Parafossarulus manchouricus is the most common snail host. Other genera include Bithynia, Tarebia, Alocinma and Bulimus.

Figure A: Shells of Parafossarulus manchouricus, the most common snail host of C. sinensis in endemic areas in southeast Asia. Image courtesy of the Web Atlas of Medical Parasitology and the Korean Society for Parasitology.

Figure B: Bithynia sp., another common intermediate host of C. sinensis. Image courtesy of Michal Maňas.

Laboratory Diagnosis

Diagnosis is based on microscopic identification of eggs in stool specimens. However, the eggs of Clonorchis are practically indistinguishable from those of Opisthorchis. The adult fluke can also be recovered at surgery.

Serologic testing is currently not available for Clonorchis infection in the United States.

More on: Morphologic comparison with other intestinal parasites.

Treatment Information

Treatment information for clonorchiasis can be found at: https://www.cdc.gov/parasites/clonorchis/health_professionals/index.html

DPDx is an education resource designed for health professionals and laboratory scientists. For an overview including prevention and control visit www.cdc.gov/parasites/.

  • Page last reviewed: February 20, 2018
  • Page last updated: February 23, 2018
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