Clonorchis sinensis

Clonorchis sinensis is a Trematode that causes the disease clonorchiasis.

Disease / Pathology

Clonorchis sinensis is transmitted by the ingestion of raw or improperly cooked fish infected with the metacercariae stage. Symptoms due to clonorchiasis are primarily due to biliary obstruction. The worms produce irritation in the biliary duct area as well as toxins resulting in inflammation and lesions. The inflammation is proportional to the amount of worms causing the infection.,

Obstruction of the bile ducts can result in bile retention. There is an infiltration of leukocytes and eosinophils. Fibrosis is common in chronic infections. Chronic infections can can lead to obstructive jaundice especially in the presence of biliary stones and/or liver abscesses. Recurrent pyogenic cholangitis can occur in infected patients. Long term chronic infections are linked to oncogenic changes resulting resulting in infected individuals developing neoplasms of the bile duct and to cholangiocarcinoma. Strong evidence linking these neoplasms to the parasite is the increased frequency of these neoplasms in areas where Clonorchis sinensis is endemic.

Location in the Host

Clonorchis sinensis is located in the bile ducts of the liver.

Geographic Distribution

Clonorchis sinensis is distributed throughout the Far East. The reservoir host are dogs, cats, and other fish eating mammals.

Life Cycle

Clonorchis sinensis is transmitted by the ingestion of raw or improperly cooked fish infected with the metacercariae stage. The metacercariae cyst will excyst in the host duodenum and localize in the biliary duct as adults. Eggs are produced and are passed in the stool. The embryonate eggs will develop in the snail after ingestion to produce the miracidia form which molts three times to the cercariae form which is free-swimming and encyst in the skin or flesh of fresh water fish. The uncooked or raw fish that is ingested transmits the parasite.

Morphology & Diagnosis

Clonorchis sinensis eggs are best detected in the feces during an ova & parasite examination. They are small eggs at 27-35u long and 27-35u wide.

They characteristically have a small knob located at the abopercular end; a seated operculum that gives the appearance of shoulders; and thick shell wall.

The adult worms are seldom seen but are flattened and hermaphroditic and 10-25mm long by 3-5mm wide. They are seen usually during a surgical procedure or biopsy.

Images

Clonorchis sinensis egg – note the characteristic small knob located at the abopercular end; a seated operculum that gives the appearance of shoulders; and thick shell wall – photo courtesy of CDC – Dr. Mae Melvin
Clonorchis sinensis egg Illustration pointing out characteristics