A fluke worm is a type of parasitic flatworm that belongs to the class Trematoda in the phylum Platyhelminthes. There are more than 10,000 species of flukes, and they occur worldwide, ranging in size from about 5 millimeters to several centimeters. Flukes parasitize members of all vertebrate classes, but most commonly parasitize fish, frogs, and turtles. They also parasitize humans, domestic animals, and invertebrates such as mollusks and crustaceans. Some flukes are external parasites, while others attach themselves to internal organs or the lining of the mouth, gills, or cloaca.

There are two families of liver flukes that cause disease in humans: Opisthorchiidae (which includes species of Clonorchis and Opisthorchis) and Fasciolidae (which includes species of Fasciola) . Clonorchis is a liver fluke parasite that humans can get by eating raw or undercooked fish, crabs, or crayfish from areas where the parasite is found. Liver flukes infect the liver, gallbladder, and bile duct in humans. While most infected persons do not show any symptoms, infections that last a long time can result in severe symptoms and serious illness. 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 life cycle of flukes is complex and 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. Adult flukes release eggs, which can be passed in stool or urine. If untreated stool or urine enters fresh water, the eggs hatch and infect snails, continuing the fluke life cycle.

Symptoms of fluke infections vary depending on which organ the adult flukes infect. Infections that last a long time can result in severe symptoms and serious illness. It is important to know that liver fluke infection can be easily prevented by ensuring that freshwater fish and watercress are thoroughly cooked before consuming them. Liver fluke infection can be treated with medication or surgery, and it is possible to eradicate liver flukes completely.