The causative agent for swimmer's itch is the larval, free-living and aquatic stage of a group of flatworms that are called schistosomes. Most of species that cause swimmer's itch use bird hosts for the adult parasite and aquatic snails as intermediate hosts for the larval stages. The life history of the dermatitis-producing worms is cyclic.

Eggs released from the adult worms that reside in the blood vessels (usually veins around the intestine) of the bird host, make their way into the digestive tract of the bird and then pass out of the host with the feces. If the eggs are deposited in water, they will hatch within an hour if conditions are right. The miracidium, an aquatic stage, is free-swimming, but nonfeeding. It has enough energy to keep it moving for about a day. Once the miracidium comes in contact with the proper snail it will either penetrate into the snail via the integument or it may enter through its mouth. Within the snail, the miracidium will elongate to form a reproductive sac called the sporocyst. This germinating structure will produce a second generation of sporocysts. In approximately a month, the sporocysts produce another stage, called the cercaria. This stage burrows out of the snail, becomes a second type of nonfeeding, swimming aquatic stage that must enter the bird host. It does this either by penetrating the skin of the bird or by being ingested and then entering the blood vessels in the walls of the pharynx or esophagus. In the bird host, the parasite migrates through various organs of the bird and finally matures in the blood vessels. The adult worms then begin producing large numbers of eggs which again are voided with the feces. Avian schistosomes usually complete their life cycle in two months, however, the specific time varies with each species.

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