A fluke is any of a large group of flat leaf-shaped worms that parasitize many kinds of animals, including snails, fish, birds, and mammals. Some flukes live on external structures, such as the gills or skin of fish, but many are internal parasites that feed on the host's tissues and body fluids, causing a variety of disorders. Fluke diseases in man are serious health problems.
Some flukes complete a life cycle in only one host, but many have more complicated life cycles. For example, blood flukes (Schistosoma) have three larval forms and invade two hosts, certain freshwater snails and man. Adult blood flukes live in the blood vessels of human beings, and their eggs pass out of the body with the feces or urine, depending on the kind of blood fluke. If deposited in water, the eggs hatch into larvae that swim around until they find a suitable snail to invade. If they do not find a host within 24 hours, the larvae die. Once inside the snail the larvae transform to the second larval form, which reproduces asexually and gives rise to the third form, called the cercaria. The cercariae escape from the snail and swim around. If they encounter a human host, the cercariae penetrate the skin, enter a blood vessel, and later reach the vessels in the walls of the intestinal or urinary tract. There they mature into adult flukes, which mate and produce eggs.
Blood flukes cause schistosomiasis. The disease varies, depending on the species of blood fluke. The most common species affect the liver and the bladder. Schistosomiasis is especially widespread in places where human feces are used as fertilizer and where the snail hosts are common around irrigation ditches.
Many flukes have even more complex life cycles than the blood flukes. For example, the Chinese liver fluke (Opisthorchis sinensis) has four larval forms and requires three hosts, snail, fish, and man.
Flukes make up the class Trematoda of the phylum Platyhelminthes.
