Clonorchiasis is caused by infection with a parasitic flatworm known as the liver fluke, specifically *Clonorchis sinensis*. This tiny worm lives in the bile ducts of the liver and causes disease by damaging the liver and biliary system.
The life cycle of *Clonorchis sinensis* is complex and involves multiple hosts. It begins when eggs of the parasite are passed in the feces of an infected human or animal. These eggs must reach freshwater to continue development. Once in freshwater, the eggs hatch into free-swimming larvae called miracidia, which then infect a specific type of freshwater snail. Inside the snail, the parasite undergoes several developmental stages, multiplying and transforming into another larval form called cercariae.
The cercariae leave the snail and swim freely in the water until they find and penetrate freshwater fish, particularly species like carp. Inside the fish, the cercariae encyst in the muscles or under the scales as metacercariae, which are the infective stage for humans. People become infected by eating raw, undercooked, or improperly prepared freshwater fish containing these metacercariae.
Once ingested, the metacercariae excyst in the small intestine and migrate through the bile duct system to the liver, where they mature into adult flukes. These adult flukes attach to the bile duct walls and start producing eggs, continuing the cycle. The presence of the flukes causes inflammation, irritation, and sometimes obstruction of the bile ducts, leading to symptoms and complications.
The main cause of clonorchiasis is therefore the consumption of raw or inadequately cooked freshwater fish harboring the infective larvae. This is common in areas where eating raw fish is part of the local diet and where sanitation conditions allow contamination of water bodies with human feces.
Infection can be asymptomatic, especially in light infections, but when symptoms occur, they often include upper abdominal pain, fatigue, and sometimes fever. Chronic infection may lead to more serious complications such as inflammation of the bile ducts (cholangitis), gallstones, bile duct obstruction, and even increased risk of bile duct cancer over many years.
Preventing clonorchiasis involves improving sanitation to prevent contamination of freshwater with parasite eggs, controlling snail populations, and educating people to avoid eating raw or undercooked freshwater fish. Cooking fish thoroughly kills the metacercariae and breaks the cycle of infection.
In summary, clonorchiasis is caused by a liver fluke parasite transmitted through eating raw or undercooked freshwater fish infected with the parasite’s larvae. The infection damages the bile ducts and liver, leading to a range of health problems if untreated.
