Etiology

The worms are acquired in fresh water by contact with their larval stages. The worm eggs, which are highly diagnostic for each species, are shed in the urine or feces of the human host and hatch to produce a minute short-lived larval stage called a miracidium. The miracidium of each species invades the tissues of a specific snail host where it undergoes asexual reproduction, eventually to produce the final larval stage. The final stage, the cercaria, is released daily in very large numbers from the snail, swims freely in the water, and then bores into the skin of the human host. In the human the parasite migrates to the liver via the heart and lungs, eventually to mature in the veins of the liver, gut, or bladder. Eggs appear in the urine or feces approximately 30 to 40 days after infection.

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