Life Cycle of a Fluke Worm
The trematodes, or flukes, are a type of flatworm. Almost all of them are parasitic on vertebrates or mollusks, and some parasitize humans. Flukes have flat, unsegmented bodies with a leaf shape, and generally have a pair of suckers that allows them to cling to their hosts. Many flukes have fairly complicated life cycles, which may involve more than one host.
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Egg
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Flukes, like other types of worms, reproduce by laying eggs. Adult flukes lay their eggs into the intestines, tissue or bloodstream of their hosts. Some eggs have a spine or knob that cuts through the wall of the blood vessel or other tissue and allows them to migrate through the body. Eggs eventually make their way into the digestive system and are passed in the urine or feces.
Miracidium
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Eggs which are passed near or in fresh water hatch out larvae, called miracidia. This free-swimming form of the fluke lives in the water until it can find an intermediate host--usually a snail. This form of the fluke can actually multiply in snails independently. According to the University of South Carolina, blood flukes spend two generations in their snail hosts before emerging to infect other creatures. Flukes living in snails are called Sporocysts and Redia. Humans sometimes catch flukes by eating contaminated snails.
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Cercaria
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The form of fluke which emerges from the snail is called the cercaria. This form is also free-swimming, and can move through the water of its own accord. It may enter a human or animal host at this point, causing itching and discomfort, or it may find another intermediate host. This intermediate host may be a small animal, or even a plant. If the cercaria finds another intermediate host, it's referred to as a metacercaria. After living in this host for a while, it will eventually leave to find its definitive host and reach adulthood.
Infection
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The final host is usually infected through ingestion of the fluke. Raw and undercooked snails, shellfish and other water animals are a serious risk factor, especially in Africa and Southeast Asia. Once the fluke enters the final host, it settles in its preferred area, such as the lungs, liver, intestines or blood vessels, and absorbs nutrients from the host's body. Some people may develop immune problems from these parasites. Blockage of the bile ducts, respiratory passages or other body parts is also common. Fluke infection also increases the risk of some cancers.
Reproduction
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After growing to adulthood inside the final hosts, flukes find another member of their species to mate with. Most species of fluke are hermaphroditic--both male and female--but blood flukes have differentiated sexes. Flukes may also self-fertilize their own eggs if other members of the species are not available. Once the eggs have been fertilized, they pass through the host's tissue to begin the life cycle again.
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References
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