Clonorchis sinensis is a diminutive parasitic worm shaped like a flattened elongated flame. If you saw one, it might remind you of a small leech; however, you are unlikely to see one because of its small size – usually not more than 25 mm – and because it lives hidden away inside the bile ducts of the liver.
Life cycle of Clonorchis sinensis
Clonorchiasis – infection with Clonorchis sinensis – is common in East and Southeast Asia where fish is often eaten raw. Like many parasitic flukes, the life cycle involves a number of hosts:
- A larval parasitic stage is ingested in the raw flesh of a freshwater fish, often carp.
- As the fish tissue is broken down through digestion, the larva (metacercaria) is released into the small intestine.
- Larvae migrate to the bile ducts, probably by traveling up the common bile duct from the intestine. They arrive in the liver within hours.
- Larvae attach to the walls of bile ducts and mature in about a month to adult worms that can live at least eight years.
- Worms produce eggs, which are flushed down into the intestine with bile and passed to the outside world in the stool.
- Deposited in fresh water, the eggs are eaten by freshwater snails. They hatch in the snail.
- The parasites reproduce asexually in the snail’s internal organs.
- Upon exiting the snail, the parasite, now called a cercaria, seeks out a freshwater fish and invades its muscle tissue.
- Metacercariae remain dormant in the fish until it is eaten raw.
Geographical range of Clonorchis sinensis
The Chinese liver fluke is common in Korea, China, Japan, Taiwan, and Vietnam. People in other parts of the world may acquire the worms if they eat raw fish exported from those countries. Pickled, dried, and frozen fish may be involved.
In East and Southeast Asia, people raise carp in ponds where the fish feed on algae and water plants. These ponds have traditionally been fertilized with human sewage. This practice, combined with a cultural taste for raw fish has supported the life cycle of Clonorchis sinensis for generations, and resulted in 80% of people being infected with the parasite in some places.
Clonorchis sinensis is not an exclusively human parasite. Many mammals, and possibly all mammals, can be infected; thus, it would be extremely difficult to eradicate in the environment where appropriate species of snails are present. Birds may be susceptible as well, and may spread it from place to place.
Clonorchiasis – Chinese liver fluke infection
Infection with C. sinensis, or clonorchiasis, can cause inflammation and blockage of the bile ducts and, along with the similar liver fluke Opistorchis viverrini, C. sinensis is associated with cholangiocarcinoma – cancer of the bile ducts of the liver.
Sources
CDC. “Clonorchiasis.” DPDx Laboratory Identification of Parasites of Public Health Concern.
Roberts, Larry S., and John Janovy Jr. Foundations of Parasitology 6th ed. Boston: McGraw Hill, 2000
Rosemary Drisdelle - Rosemary Drisdelle has been published many times as a nonfiction writer and several times as a poet. Her first book, Parasites: Tales of ...
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