A liver fluke is a parasitic flatworm that infects the liver and bile ducts of humans and animals. Several species cause disease in people, and infections occur across more than 70 countries on every continent except Antarctica. Most infections happen by eating contaminated raw or undercooked freshwater fish, or by consuming aquatic plants like watercress that carry the parasite’s larvae.
Types of Liver Flukes
Two families of liver flukes infect humans. The first, Fasciolidae, includes the common liver fluke (also called the sheep liver fluke) and a larger relative that primarily affects animals but occasionally infects people. These parasites are found worldwide, especially in regions where sheep, cattle, or goats are raised. You get infected by eating raw aquatic vegetation, particularly watercress, that has the parasite’s larvae attached to it.
The second family, Opisthorchiidae, includes the Chinese liver fluke, the Southeast Asian liver fluke, and the cat liver fluke. These species are concentrated in East and Southeast Asia, and infection comes from eating raw or undercooked freshwater fish that harbor the larvae. In parts of Thailand, where the Southeast Asian liver fluke is most common, bile duct cancer rates are among the highest in the world, with age-standardized rates reaching 33.4 per 100,000 in men.
How Liver Flukes Enter and Move Through the Body
When you swallow contaminated food, the larvae break free from their protective cysts in your small intestine. What happens next depends on the species, but the common liver fluke’s journey is particularly dramatic. The newly released juvenile parasites burrow through the intestinal wall within hours, since their survival drops quickly if they stay in the gut. Once through, they enter the abdominal cavity and migrate toward the liver.
Within about a week of ingestion, the parasites reach the liver by penetrating its outer capsule. They then spend weeks tunneling through liver tissue, using specialized enzymes to break down the surrounding structure as they go. This migration phase is the most physically destructive part of the infection. After roughly 12 weeks, the flukes reach the bile ducts, where they mature into adults and begin producing eggs. Those eggs pass into the intestines and out in stool, completing the cycle if they reach freshwater.
In that freshwater environment, the eggs hatch and infect specific species of freshwater snails. Inside the snail, the parasite multiplies and eventually releases a new larval form that either attaches to aquatic plants (in the case of the common liver fluke) or burrows into freshwater fish (for the Asian species). At least 20 snail species across multiple continents serve as intermediate hosts.
Symptoms During the Acute Phase
The acute phase corresponds to the weeks when juvenile flukes are actively migrating through your liver tissue. Symptoms during this period can include fever, malaise, and abdominal pain, particularly in the right upper area beneath the ribs. An enlarged liver is common. Some people also develop hives, cough, and shortness of breath as the immune system reacts to the parasites moving through tissue. Blood tests during this phase typically show a sharp rise in a type of white blood cell called eosinophils, which the body deploys against parasitic infections.
Severe illness is uncommon in adults, but young children sometimes experience intense abdominal pain. Nausea, vomiting, diarrhea, loss of appetite, and weight loss can all occur.
Symptoms During the Chronic Phase
Once the flukes settle into the bile ducts, the infection enters a chronic phase that can last months, years, or indefinitely. Many people have no symptoms at all during this stage. The only detectable sign may be a mildly elevated eosinophil count on routine blood work. Some people never develop noticeable problems.
For those who do develop symptoms, the issues stem from the flukes physically blocking or irritating the bile ducts. This can cause inflammation of the bile ducts (cholangitis), gallbladder inflammation, gallstones, jaundice, and in some cases pancreatitis. These symptoms may come and go as the flukes shift position. With the Asian liver fluke species, chronic infection also produces indigestion, constipation or diarrhea, and intermittent abdominal pain, with severity depending on the number of flukes present and how long the infection has persisted.
Bile Duct Cancer Risk
Chronic infection with certain liver fluke species is an established risk factor for cholangiocarcinoma, a cancer of the bile duct lining. This link is strongest for the Southeast Asian liver fluke and the Chinese liver fluke. In Asian countries, liver fluke infection is the leading risk factor for this cancer, and the presence of the parasite alongside other carcinogens in the diet magnifies the risk further.
Cholangiocarcinoma is an aggressive cancer. Most patients have tumors that are too advanced for surgical removal at the time of diagnosis. In Thailand’s northeastern provinces, where Southeast Asian liver fluke prevalence is highest, bile duct cancer rates are dramatically elevated compared to other regions. This geographic overlap between fluke prevalence and cancer incidence is one of the clearest pieces of evidence linking the two.
Diagnosis
Detecting a liver fluke infection depends on the stage. During the acute phase, when the parasites are still migrating, eggs won’t appear in stool samples because the flukes haven’t yet matured and started reproducing. Blood tests looking for antibodies and elevated eosinophil levels are more useful at this stage, along with imaging that may show tracks of damage through the liver.
During the chronic phase, the standard approach is examining stool samples under a microscope to identify fluke eggs. However, egg shedding can be intermittent, so multiple samples may be needed. After treatment, a stool sample taken three months later that shows no eggs is generally considered evidence of a cure.
Treatment
The treatment depends on the species. For the common liver fluke, the standard medication is taken as two oral doses about 12 hours apart. With correct dosing, this approach cures 75% to 95% of patients. For the Asian liver fluke species, a different antiparasitic is used, also taken orally over a short course.
Treatment is straightforward when the infection is caught, but the challenge is that many people with chronic infections don’t know they’re infected. In endemic areas, reinfection is common if eating habits don’t change.
How To Avoid Infection
Prevention comes down to what you eat and how it’s prepared. For the common liver fluke, the risk food is raw aquatic vegetation, especially watercress. Thoroughly washing these plants reduces risk but doesn’t eliminate it, since the larvae can be firmly attached. Cooking aquatic greens before eating them is the most reliable safeguard.
For the Asian liver fluke species, the risk food is raw or undercooked freshwater fish. Dishes like sashimi, ceviche, or fermented fish preparations made from freshwater species caught in endemic areas carry the highest risk. Cooking fish thoroughly kills the larvae. Freezing, pickling, smoking, and drying are not consistently reliable methods for destroying them. If you’re traveling in East or Southeast Asia, sticking to fully cooked freshwater fish is the simplest way to protect yourself.