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CLONORCHIS SINENSIS (Chinese liver fluke)

Infection with the trematode Clonorchis sinensis (Chinese or oriental liver fluke) results mostly in inflammation and intermittent obstruction of the biliary ducts.

Life Cycle

The adult flukes deposit fully developed eggs that are passed in the faeces. The parasite then has a complicated life cycle involving two intermediate hosts - firstly snails and then freshwater fish. In the latter, infective larvae migrate to the tissues including muscle (often near the caudal fin). The body of the larvae then round up to form the highly resistant stage called metacercariae. These metacercariae are infective after about 23 days. To continue the lifecycle, the infected fish must then be eaten (undercooked or raw, dried, or salted).

Symptoms

The sites in the body affected are the gall bladder, bile duct and liver.
For this parasite, as for many other flatworm diseases, the severity of disease is dependent on the number flukes the individual host harbours, as follows:

Most infections consist of less than 100 parasites and the majority of these are asymptomatic.
For infections involving up to 1000 flukes, non-specific symptoms such as diarrhoea, abdominal pain and spleen enlargement may be seen.
In heavier infections, which may consist of many thousands of parasites (up to 21,000 flukes have been observed in a single infection), symptoms may include:

fever
acute pain in the upper right quadrant of the liver
enlargement and cirrhosis of the liver
jaundice
altered heart beat
loss of weight

The presence of the parasites in the bile duct leads to inflammation and fibrosis of the bile duct; this may be followed by changes in cell types often leading to cancer. The disease itself is rarely fatal.

Control

The most effective means of control of this parasite is to cook the fish, although for cultural reasons this may not be feasible.

Additional Information

The infective cysts are destroyed at temperatures above 50^oC but are resistant to salting and drying.
The global number of human cases is estimated at between 20 and 30 million.
As well as humans, most fish-eating mammals may also be infected, acting as reservoir hosts for the parasite.

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Parasites of the intestinal tract - US site

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