Epidemiology & Risk Factors

Unstained adult Fasciola hepatica fluke fixed in formalin (30 mm by 13 mm)

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 2 million.

Fasciola hepatica is found in more than 50 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 less widespread. 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.

Special conditions are needed for fascioliasis to be present in an area, and its geographic distribution is very patchy (focal). The eggs passed in the stool of infected mammals have to develop (mature) in a suitable aquatic snail host to be able to infect another mammalian host. Requirements include sufficient moisture and favorable temperatures (above 50°F) that allow the development of miracidia, reproduction of snails, and larval development within the snails. These factors also contribute to both the prevalence and level (intensity) of infection. Prevalence is highest in areas where climatic conditions promote development of cercariae.

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Infective Fasciola larvae (metacercariae) are found in contaminated water, either stuck to (encysted on) water plants or floating in the water, often in marshy areas, ponds, or flooded pastures. People (and animals) typically become infected by eating raw watercress or other contaminated water plants. The plants may be eaten as a snack or in salads or sandwiches. People also can get infected by ingesting contaminated water, such as by drinking it or by eating vegetables that were washed or irrigated with contaminated water. Infection also can result from eating undercooked sheep or goat livers that contain 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.