Liver flukes are small parasites that live in the bile ducts of mammals, including humans, and live off the nutrients they can acquire there. Liver flukes can cause the liver to swell and the bile ducts to become blocked. In some animals, such as sheep or goats, they can cause extensive damage to the liver that kills the animal. Liver flukes are flatworms with suckers similar to leeches. Most are between 15 and 30mm wide, 30 to 100mm long and 2 to 5mm thick.
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The life cycle of the liver fluke starts when an adult lays eggs in the bile duct. The eggs continue through the digestive tract until they leave the body through the faeces. When the eggs come into contact with water, they hatch.
Tiny life forms, called miracidiums, emerge from the eggs. They are able to swim using cilia, which are tiny hairlike appendages. These miracidiums swim around in the water until they find a snail, which serves as its intermediate host. They will die if they don't find a snail within a few hours of hatching.
The miracidiums multiply and develop within the snails for five to seven weeks and then exit the snails as cercariae, which look like tadpoles. They exit the snails and secrete a protective coating that covers them. They become attached to plants and continue to develop into metacercariae.
The metacercariae can now infect any mammal that ingests them, usually when they eat the vegetation the metacercariae are attached to. Humans are most often infected by eating contaminated watercress. After ingestion, the metacercariae migrate through the intestinal wall and toward the liver, where they begin to develop into adults.
Once in the liver, it takes six to eight weeks for the metacercariae to migrate through the liver to the bile ducts. It takes about three or four months for the metacercariae to develop into adults and begin producing eggs, completing the life cycle. The adult liver flukes are usually found in capsules in the bile ducts, with two or more flukes in each capsule.
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