We want to continue to see their flocks arrive in the spring, their goslings hatch in the summer, and hear them honking southward in the fall. The schistosome requires two hosts, one consisting of one of several types of snail, and the other one or more species of waterfowl. Adult worms found in blood vessels of waterfowl produce eggs that are passes in feces. On exposure to water, eggs hatch and the next phase of the parasite, called a miracidium, infects a molluscan host, such as a snail. The parasite develops in the snail, and produces another form, called cercariae, which can penetrate the skin of birds to complete the cycle.
Humans are accidental hosts of the cercariae.
The parasite can penetrate the skin but can go no further. Symptoms include reddening and itching of exposed skin in the water or immediately after emerging. A raised rash may develop later. The more often a person has been exposed to the parasite, the more intense the reaction. The Centers for Disease Control reports that swimmer's itch seems to be worst in conditions of calm water and unusually warm weather.