Clonorchiasis is a parasitic infection caused by the liver fluke Clonorchis sinensis, also known as the Chinese or Oriental liver fluke. This flatworm can measure 10 to 25 mm in length and resides in the bile ducts of the liver in infected individuals. Clonorchiasis is widely prevalent across East Asia, particularly in countries such as China, South Korea, Vietnam, and parts of Russia. It is estimated that over 15 million people are currently infected, with approximately 200 million individuals living in areas where they are at risk of contracting the infection.
How the Infection Occurs
The life cycle of Clonorchis sinensis involves multiple hosts, beginning with eggs passed in the feces of infected mammals entering freshwater. These embryonated eggs are then ingested by specific freshwater snails, which serve as the first intermediate host. Inside the snail, eggs hatch into miracidia, developing into cercariae.
Mature, free-swimming cercariae are released from the snail into the water. They penetrate freshwater fish, particularly carp and minnows, and encyst in the flesh as metacercariae. Humans become infected by consuming raw or undercooked freshwater fish containing these metacercariae. After ingestion, the metacercariae excyst in the duodenum and migrate into the bile ducts, where they mature into adult flukes within about one month. Adult flukes can live for many years, even up to 25 to 30 years, continuously producing eggs passed in the host’s stool, perpetuating the life cycle.
Signs and Diagnosis
Many individuals with clonorchiasis, particularly those with light infections, may not experience symptoms. When symptoms occur, they are generally gastrointestinal and can range from mild to severe. Common complaints include abdominal pain, indigestion, fatigue, and diarrhea. In heavier or chronic cases, more pronounced symptoms include right upper quadrant abdominal pain, an enlarged and tender liver, and jaundice (yellowing of the skin and eyes).
Diagnosis of clonorchiasis relies on microscopic examination of stool samples to identify Clonorchis sinensis eggs. Eggs usually become detectable in stool approximately three to four weeks after infection. However, Clonorchis sinensis eggs are very similar to Opisthorchis species, another type of liver fluke, which can complicate identification. If stool examinations are inconclusive or to assess liver and bile duct involvement, imaging studies are used. Ultrasound, CT, or MRI can reveal diffuse dilatation of the peripheral intrahepatic bile ducts, a characteristic finding in clonorchiasis.
Treatment and Prevention
Praziquantel, an anthelmintic drug, is the primary medication for clonorchiasis, highly effective against the liver fluke. It typically has a cure rate ranging from 80% to 90%, though cure rates can vary in areas with heavy endemicity or high worm burdens. Dosage and duration are determined by infection severity and individual patient factors.
Prevention largely centers on avoiding raw or undercooked freshwater fish, especially where the parasite is common. Thoroughly cooking freshwater fish until it reaches a safe internal temperature kills metacercariae, eliminating infection risk. Avoiding raw, lightly salted, smoked, or pickled fish from endemic areas is also important. Beyond individual dietary practices, public health measures play a significant role in controlling clonorchiasis. These measures include health education programs, promoting safe food handling, and ensuring proper sanitation to prevent water contamination.
Potential Health Consequences
Chronic or untreated clonorchiasis can lead to serious long-term health complications due to persistent adult flukes in the bile ducts. The worms cause ongoing inflammation and obstruction within the biliary system. This can result in recurrent pyogenic cholangitis, a severe bacterial infection of the bile ducts. Flukes and their eggs can also contribute to cholelithiasis (gallstones), which can further obstruct bile flow and lead to pain and inflammation.
A concerning health consequence of chronic Clonorchis sinensis infection is an increased risk of cholangiocarcinoma, a rare but aggressive bile duct cancer. The International Agency for Research on Cancer (IARC) classifies Clonorchis sinensis as a Group 1 carcinogen, meaning it is definitively carcinogenic to humans. This association is strong, with studies indicating that chronic clonorchiasis can significantly increase the odds of developing cholangiocarcinoma, with some estimates suggesting a relative risk 5 times higher. The chronic inflammation and cellular changes induced by the parasite contribute to the development of this cancer.