A liver fluke is a parasitic flatworm that infects the liver and bile ducts of humans, typically after eating contaminated raw fish or aquatic plants. At least 2.4 million people across more than 70 countries are currently infected, according to the World Health Organization. Several species cause disease in humans, and while many infections produce mild or no symptoms, chronic infection can lead to serious liver damage and even bile duct cancer.
Species That Infect Humans
Six liver fluke species are responsible for the vast majority of human infections. They fall into two broad groups based on how they reach the human body.
The fish-borne flukes, which you get from eating raw or undercooked freshwater fish, include the Chinese liver fluke (found in China, Korea, Vietnam, and eastern Russia), the Southeast Asian liver fluke (concentrated in Thailand, Laos, Cambodia, and Vietnam), and the cat liver fluke (present in Italy, Germany, Russia, Belarus, and Ukraine). These species settle inside the bile ducts, where they can live for years.
The plant-borne flukes travel a different route. The sheep liver fluke and its larger relative, the giant liver fluke, are found across Asia, Africa, Europe, Latin America, and Oceania. People become infected by eating raw aquatic plants, particularly watercress, that carry the larvae. A sixth species, the lancet liver fluke, is found in North Africa, Asia, Europe, and parts of Canada, though human infections are uncommon.
How Infection Happens
Every liver fluke species passes through freshwater snails before it can infect a human. The journey differs slightly depending on the species, but the Chinese liver fluke offers a clear example of the full cycle.
Infected humans or animals pass fluke eggs in their stool. When those eggs reach freshwater, snails consume them. Inside the snail, the parasite develops and eventually releases microscopic larvae into the water. For fish-borne species, these larvae burrow into the flesh of freshwater fish. When a person eats that fish raw, undercooked, or lightly pickled, the larvae survive digestion and travel to the bile ducts of the liver. There, they mature into adult worms. The entire process from ingestion to egg-producing adult takes about three months.
For the sheep and giant liver flukes, larvae attach to aquatic vegetation instead of fish. Eating unwashed watercress, mint, or other freshwater plants is the primary route. Once swallowed, these larvae burrow through the intestinal wall, cross the abdominal cavity, and physically tunnel through liver tissue before settling into the bile ducts.
Symptoms in the Acute Phase
The acute phase begins when larvae are actively migrating through tissue, and it’s most noticeable with the plant-borne flukes like the sheep liver fluke. As immature worms bore through the liver, the immune system reacts strongly. Common symptoms during this stage include fever, diffuse abdominal pain (especially in the upper right side), headache, nausea, loss of appetite, weight loss, weakness, and night sweats. Many people also develop hives or itchy skin as part of an allergic response. Blood tests during this phase typically show a spike in a type of white blood cell called eosinophils, a hallmark of parasitic infection.
Fish-borne flukes like the Chinese and Southeast Asian species don’t tunnel through the liver the same way, so the acute phase is often milder or goes completely unnoticed.
Symptoms in the Chronic Phase
Once the flukes reach the bile ducts and mature, the infection enters its chronic phase. This stage is often silent. Many people carry the parasites for years without obvious symptoms, which is part of what makes liver flukes so insidious.
When symptoms do appear, they stem from the physical presence of worms and their eggs inside the bile ducts. Flukes attach to the duct lining with suckers, causing irritation and low-grade inflammation. Over time, this can lead to thickening and scarring of the bile duct walls. Some people experience intermittent upper abdominal pain, indigestion, or diarrhea. If worms or scar tissue block the bile duct, obstructive jaundice can develop, turning the skin and eyes yellow.
The Link to Bile Duct Cancer
The most serious long-term consequence of liver fluke infection is cholangiocarcinoma, a cancer of the bile duct lining. This connection is strongest with the Chinese liver fluke and the Southeast Asian liver fluke, both classified as carcinogens. The highest incidence of bile duct cancer in the world is recorded in the Khon Kaen province of northeast Thailand, which also has the highest prevalence of Southeast Asian liver fluke infection, a correlation that has been studied extensively.
The path from infection to cancer involves multiple overlapping mechanisms. The flukes physically damage the bile duct lining as they feed and move, creating ulcers where eggs can become trapped and trigger chronic inflammation. The parasites also release proteins that are absorbed by bile duct cells, altering gene expression in ways that promote abnormal cell growth. Meanwhile, the immune system’s sustained inflammatory response floods the area with signaling molecules that, over years or decades, can push damaged cells toward cancerous transformation. It’s the accumulation of this genetic and cellular damage over a long period that eventually leads to tumor formation in some chronically infected individuals.
This cancer risk is largely confined to regions where reinfection is common, meaning people who eat raw fish regularly over many years. A single, treated infection carries far less risk.
How Liver Flukes Are Diagnosed
The standard diagnostic method is a stool examination under a microscope, looking for fluke eggs. Because the parasites don’t shed eggs continuously, doctors typically collect three separate stool samples on different days to improve the chances of detection. This approach works well for chronic infections, where adult flukes are actively producing eggs.
During the acute phase, stool tests are often negative because the larvae haven’t yet matured into egg-laying adults. In these cases, blood tests that detect antibodies against the parasite can help. The limitation of antibody testing is that it can’t distinguish between a current infection and one that was treated in the past, so it’s less useful for follow-up.
Imaging with ultrasound, CT, or MRI can sometimes reveal dilated bile ducts or even the flukes themselves, particularly in heavy infections. These scans are more commonly used when doctors suspect complications like bile duct obstruction.
Treatment
Treatment depends on which species is involved. For the sheep and giant liver flukes, the World Health Organization recommends triclabendazole, which is the only effective drug for these species. It’s taken as one or two oral doses with a fatty meal, which helps the body absorb the medication. Most people clear the infection after a single course.
For the fish-borne flukes (Chinese, Southeast Asian, and cat liver flukes), praziquantel is the standard treatment. It’s effective at killing the adult worms in the bile ducts. After treatment, follow-up stool exams confirm the infection has cleared.
Bile duct damage from long-standing infections doesn’t always reverse after the parasites are eliminated, which is why early detection and treatment matter.
Prevention
Avoiding liver flukes comes down to how you prepare food. For fish-borne species, thoroughly cooking freshwater fish kills the larvae. Eating raw, smoked, pickled, or lightly salted freshwater fish in endemic areas carries real risk. Freezing fish at sufficiently low temperatures for extended periods can also destroy larvae, though home freezers may not reliably reach the temperatures needed.
For plant-borne species, the key is avoiding raw aquatic plants in areas where livestock graze near water. Watercress is the most common culprit. Cooking these plants eliminates the risk. Washing alone is not considered reliable because the larvae attach firmly to plant surfaces.
In endemic regions, preventing human and animal waste from contaminating freshwater sources breaks the parasite’s life cycle at its starting point. Mass treatment programs in parts of Southeast Asia have reduced infection rates, though reinfection remains common where dietary habits persist.