Fasciola hepatica

• Fasciola hepatica are flatworm parasitic in liver and bile ducts of domestic animals and humans.
• Typically caused by Fasciola hepatica, 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.

Fasciola hepatica

Epidemiology & Risk Factors

• 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.

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Fasciola hepatica cycle

Transmission

• 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.

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Fasciola hepatica miracidium

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.

Fasciola hepatica prevalence

Life Cycle

• Immature eggs are discharged in the biliary ducts and passed in the stool. Eggs become embryonated in freshwater over ~2 weeks; embryonated eggs release miracidia, which invade a suitable snail intermediate host. In the snail, the parasites undergo several developmental stages (sporocysts, rediae, and cercariae). The cercariae are released from the snailand encyst as metacercariae on aquatic vegetation or other substrates. Humans and other mammals become infected by ingesting metacercariae-contaminated vegetation (e.g., watercress). After ingestion, the metacercariae excyst in the duodenumand 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. 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.

Disease

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

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Sheep liver with inflamed gall bladder and bile duct infected with adult liver flukes, Fasciola hepatica.

• 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.

Triclabendazole

• Triclabendazole, a benzimidazole compound active against immature and adult Fasciola parasites, is the drug of choice for treatment of fascioliasis. In February 2019, the U.S. Food and Drug Administration (FDA) approved triclabendazole for treatment of fascioliasis in patients at least 6 years of agepdf iconexternal icon.

• As with all medications, use of triclabendazole should be individualized.

• Triclabendazole is given orally, with food, to improve absorption. According to the FDA-approved product labelpdf iconexternal icon, the recommended dosage regimen (for patients at least 6 years of age) is two doses of 10 mg/kg given 12 hours apart.

• Triclabendazole resistance has been documented, particularly in infected animals but also in some infected humans.

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Fasciola hepatica metacercaria

• Additional Perspective About Therapy
• On the basis of limited data, nitazoxanide might be effective therapy in some patients. The drug is given orally, with food. The dosage regimen for adults is 500 mg po bid (twice a day) for 7 days.

• Praziquantel, which is active against most trematodes (flukes), typically is not active against Fasciola parasites. Therefore, praziquantel therapy is not recommended for fascioliasis.

• In some patients who have biliary tract obstruction, manual extraction of adult flukes (e.g., via endoscopic retrograde cholangiopancreatography [ERCP]) may be indicated.

• Triclabendazole
• Prevention & Control
• 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.

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