As the larvae mature, they pass again into the water to search out a bird to inhabit, but sometimes select a human by mistake. Swimmers' itch is the rash which develops when human antibodies destroy the parasites. The larval stages of the parasite concentrate near the water surface where they can contact birds and mammals. Wind will tend to bring the organisms into the shallows, often some distance from the snail hosts in which they had lived. The highest incidence of infection seems to be late in the day after a period of incoming wind.
Humans are unsuitable hosts for the swimmers' itch larvae, which die soon after entering the skin.
Hours later, the tingling sensation will disappear, and the red spots will enlarge and become itchy like an insect bite. The degree of discomfort varies with the sensitivity of the individual, the severity of the infestation and prior exposure. The condition may persist for a few days. Avoid dense aquatic vegetation where snails are likely to occur. Enter the water from a boat or wharf, rather than wading in from the beach, to limit exposure.