Fascioliasis is a parasitic infection typically caused by Fasciola hepatica, which is also known as “the common liver fluke” or “the sheep liver fluke.” A related parasite, Fasciola gigantica, also can infect people. Fascioliasis is found in all continents except Antarctica, in over 70 countries, especially where there are sheep or cattle. People usually become infected by eating raw watercress or other water plants contaminated with immature parasite larvae. The young worms move through the intestinal wall, the abdominal cavity, and the liver tissue, into the bile ducts, where they develop into mature adult flukes that produce eggs. The pathology typically is most pronounced in the bile ducts and liver. Fasciola infection is both treatable and preventable.

What is fascioliasis?

Fascioliasis is an infectious disease caused by Fasciola parasites, which are flat worms referred to as liver flukes. The adult (mature) flukes are found in the bile ducts (the duct system of the liver) of infected people and animals, such as sheep and cattle. In general, fascioliasis is more common in livestock and other animals than in people.

Two Fasciola species (types) infect people. The main species is Fasciola hepatica, which is also known as “the common liver fluke” and “the sheep liver fluke.” A related species, Fasciola gigantica, also can infect people.

In what parts of the world is fascioliasis found?

Fascioliasis is found in more than 70 countries, especially where sheep or cattle are reared. Fasciola hepatica is found in all continents except Antarctica. Fasciola gigantica has been found in some tropical areas. Except for parts of Western Europe, human fascioliasis has mainly been documented in developing countries.

How do people get infected with Fasciola?

People get infected by accidentally ingesting (swallowing) the parasite. The main way this happens is by eating raw watercress or other contaminated freshwater plants. Another way people might get infected is by ingesting contaminated water, such as by drinking it or by eating vegetables that were washed or irrigated with contaminated water.

Can Fasciola be spread directly from one person (or animal) to another?

No. Fasciola cannot be passed directly from one person to another. The eggs passed in the stool of infected people (and animals) need to develop (mature) in certain types of freshwater snails, under favorable environmental conditions, to be able to infect someone else.

Under unusual circumstances, people might get infected by eating raw or undercooked sheep or goat liver that contains immature forms of the parasite.

Can people get infected with Fasciola in the United States?

Yes. It is possible, but few cases have been reported in published articles. Some cases have been documented in Hawaii, California, and Florida.

However, most reported cases in the United States have been in people, such as immigrants, who were infected in countries where fascioliasis is well known to occur.

What are the signs and symptoms of Fasciola infection, and when do they begin?

Some infected people don’t ever feel sick.

Some people feel sick early on in the infection, while immature flukes are passing (migrating) from the intestines through the abdominal cavity and liver. Symptoms from the acute (migratory) phase can start as soon as a few days after the exposure (typically, 1–2 weeks) and can last several weeks or months.

Some people feel sick during the chronic phase of the infection, when adult flukes are in the bile ducts (the duct system of the liver). The symptoms, if any, associated with this phase can start months to years after the exposure. For example, symptoms can result from inflammation and blockage of bile ducts.

During both phases of the infection, clinical features can include fever, malaise, abdominal pain, eosinophilia, hepatomegaly (an enlarged liver), and abnormal liver tests.

What should people do if they think they might be infected with Fasciola?

See their health care provider.

How is Fasciola infection diagnosed?

The infection typically is diagnosed by examining stool (fecal) specimens under a microscope. The diagnosis is confirmed if Fasciola eggs are seen. More than one specimen may need to be examined to find the parasite. Certain types of blood tests also may be helpful for diagnosing Fasciola infection.

Can fascioliasis be treated?

Yes. Fascioliasis is a treatable disease. Triclabendazole is the drug of choice. It is given by mouth, usually in one or two doses. Most people respond well to the treatment.

How can Fasciola infection be prevented?

People can protect themselves by not eating raw watercress and other water plants, especially from Fasciola–endemic grazing areas. As always, travelers to areas with poor sanitation should avoid food and water that might be contaminated. No vaccine is available to protect people against Fasciola.

Fascioliasis occurs in many areas of the world and usually is caused by F. hepatica, which is a common liver fluke of sheep and cattle. In general, fascioliasis is more common and widespread in animals than in people. Even so, the number of infected people in the world is thought to exceed two million.

Fasciola hepatica is found in focal areas of more than 70 countries, in all continents except Antarctica. It is found in parts of Latin America, the Caribbean, Europe, the Middle East, Africa, Asia, and Oceania. Fasciola gigantica is found in fewer geographic regions. Human cases have been reported in the tropics, in parts of Africa and Asia, and also in Hawaii.

In some areas where fascioliasis is found, human cases are uncommon (sporadic). In other areas, human fascioliasis is very common (hyperendemic). For example, the areas with the highest known rates of human infection are in the Andean highlands of Bolivia and Peru.

Infective Fasciola larvae (metacercariae) are found in contaminated water—typically, stuck to (encysted on) water plants or, potentially, floating in the water—such as in marshy areas, ponds, or flooded pastures. The main way people (and animals) become infected is by eating raw watercress or other contaminated water plants (for example, if the plants are eaten as a snack or in salads or sandwiches). Some data suggest people also might get infected by ingesting contaminated water, such as by drinking it or by eating vegetables that were washed or irrigated with contaminated water. Under unusual circumstances, infection might result from eating raw or undercooked sheep or goat liver that contains immature forms of the parasite.

The possibility of becoming infected in the United States should be considered, despite the fact that few locally acquired cases have been documented. The prerequisites for the Fasciola life cycle exist in some parts of the United States. In addition, transmission because of imported contaminated produce could occur, as has been documented in Europe.

Causal Agent

The trematodes Fasciola hepatica (also known as the common liver fluke or the sheep liver fluke) and Fasciola gigantica are large liver flukes (F. hepatica: up to 30 mm by 15 mm; F. gigantica: up to 75 mm by 15 mm), which are primarily found in domestic and wild ruminants (their main definitive hosts) but also are causal agents of fascioliasis in humans.

Although F. hepatica and F. gigantica are distinct species, “intermediate forms” that are thought to represent hybrids of the two species have been found in parts of Asia and Africa where both species are endemic. These forms usually have intermediate morphologic characteristics (e.g. overall size, proportions), possess genetic elements from both species, exhibit unusual ploidy levels (often triploid), and do not produce sperm. Further research into the nature and origin of these forms is ongoing.

Life Cycle

Immature eggs are discharged in the biliary ducts and passed in the stool -1 . Eggs become embryonated in freshwater over ~2 weeks -2 ; embryonated eggs release miracidia -3 , which invade a suitable snail intermediate host -4 . In the snail, the parasites undergo several developmental stages (sporocysts -4a , rediae -4b , and cercariae -4c ). The cercariae are released from the snail -5 and encyst as metacercariae on aquatic vegetation or other substrates. Humans and other mammals become infected by ingesting metacercariae-contaminated vegetation (e.g., watercress) -6 . After ingestion, the metacercariae excyst in the duodenum -7 and penetrate through the intestinal wall into the peritoneal cavity. The immature flukes then migrate through the liver parenchyma into biliary ducts, where they mature into adult flukes and produce eggs -8 . In humans, maturation from metacercariae into adult flukes usually takes about 3–4 months; development of F. gigantica may take somewhat longer than F. hepatica.

Life cycle image and information courtesy of DPDx.

Human fascioliasis is usually recognized as an infection of the bile ducts and liver, but infection in other parts of the body can occur.

In the early (acute) phase, symptoms can occur as a result of the parasite’s migration from the intestines to and through the liver. Symptoms can include gastrointestinal problems such as nausea, vomiting, and abdominal pain/tenderness. Fever, rash, and difficulty breathing may occur.

During the chronic phase (after the parasite settles in the bile ducts), the clinical manifestations may be similar or more discrete, reflecting inflammation and blockage of bile ducts, which can be intermittent. Inflammation of the gallbladder and pancreas also can occur.

The standard way to be sure a person is infected with Fasciola is by seeing the parasite. This is usually done by finding Fasciola eggs in stool (fecal) specimens examined under a microscope. More than one specimen may need to be examined to find the parasite. Sometimes eggs are found by examining duodenal contents or bile.

Infected people don’t start passing eggs until they have been infected for several months; people don’t pass eggs during the acute phase of the infection. Therefore, early on, the infection has to be diagnosed in other ways than by examining stool. Even during the chronic phase of infection, it can be difficult to find eggs in stool specimens from people who have light infections.

Certain types of blood tests can be helpful for diagnosing Fasciola infection, including routine blood work and tests that detect antibodies (an immune response) to the parasite.

The first step is to make sure the diagnosis is correct. For more information, patients should consult their health care provider. Health care providers may consult with CDC staff about the diagnosis and treatment of fascioliasis.

The drug of choice is triclabendazole. The drug is given by mouth, usually in two doses. Most people respond well to the treatment.

No vaccine is available to protect people against Fasciola infection.

In some areas of the world where fascioliasis is found (endemic), special control programs are in place or are planned. The types of control measures depend on the setting (such as epidemiologic, ecologic, and cultural factors). Strict control of the growth and sale of watercress and other edible water plants is important.

Individual people can protect themselves by not eating raw watercress and other water plants, especially from Fasciola-endemic grazing areas. As always, travelers to areas with poor sanitation should avoid food and water that might be contaminated (tainted). Vegetables grown in fields that might have been irrigated with polluted water should be thoroughly cooked, as should viscera from potentially infected animals.