Merck Manual

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Overview of Fluke Infections


Richard D. Pearson

, MD, University of Virginia School of Medicine

Last full review/revision Mar 2020| Content last modified Mar 2020
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Flukes are parasitic flatworms. There are many species of fluke. Different species tend to infect different parts of the body.

Flukes may infect

  • Blood vessels of digestive or urinary system: Schistosoma species (schistosomiasis)

  • Intestines: Fasciolopsis buski, Heterophyes heterophyes, and related organisms (intestinal flukes)

  • Liver: Clonorchis sinensis, Fasciola hepatica, and Opisthorchis species (liver flukes)

  • Lungs: Paragonimus westermani and related species (lung flukes)

The life cycle of flukes is complex. It typically involves snails that live in fresh water. Infected snails release immature flukes that swim in the water (cercariae). In some species of flukes, the cercariae directly infect people who come in contact with them in the water. In other species, the cercariae first infect fish or crustaceans (such as crayfish or crabs) and form cysts in their flesh. Some flukes form cysts on aquatic plants. If people eat raw or undercooked fish or crustaceans or aquatic plants that contain the cysts, they can become infected. Flukes mature into adults in people. Depending on the species, adult flukes may live from 1 to more than 20 years.

Adult flukes release eggs. Eggs that are released into the digestive tract can be passed in stool. Eggs that are released into the urinary tract can be passed in the urine. If untreated stool or urine enters fresh water, the eggs hatch and infect snails, continuing the fluke life cycle.

Symptoms vary depending on which organ the adult flukes infect.

Diagnosis of fluke infections usually involves examining a sample of stool, urine, or sputum with a microscope to look for characteristic eggs. Sometimes blood tests are done.

Prevention of fluke infections is very important. People living in or visiting areas where flukes are common should avoid contact with contaminated freshwater and dispose of urine and feces in a sanitary way.

For treatment, praziquantel, a drug that eliminates flukes from the body, is effective for most, but not all fluke infections in people.

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