Each papule corresponds to the penetration site of a single parasite. After locating a bird, the parasite penetrates through the skin (usually the feet), dropping the forked tail in the process. Inside the circulatory system, the immature worms (schistosomula) develop into mature male and female worms, mate and migrate through the hosts circulatory system (or nervous system in case of T. regenti) to the final location (veins feeding the gastrointestinal tract) within the host body. There they lay eggs in the small veins in the intestinal mucosa from which they make their way into the lumen of the gut, and are dumped into the water when the bird defecates. One European species, Trichobilharzia regenti, instead infects the bird hosts nasal tissues and larvae hatch from the eggs directly in the tissue during drinking/feeding of the infected birds. Risk factors
Humans usually become infected after swimming in slow-moving rivers, lakes or ponds. Some laboratory evidence indicates snails shed cercariae most intensely in the morning and on sunny days, and exposure to water in these conditions may therefore increase risk. Duration of swimming is correlated with increased risk of infection in Europe and North America, and shallow inshore waters may harbour higher densities of cercariae than open waters offshore. Onshore winds are thought to cause cercariae to accumulate along shorelines.
Studies of infested lakes and outbreaks in Europe and North America have found cases where infection risk appears to be evenly distributed around the margins of water bodies as well as instances where risk increases in endemic swimmers itch "hotspots". Children may become infected more frequently and more intensely than adults but this probably reflects their tendency to swim for longer periods inshore, where cercariae also concentrate. Stimuli for cercarial penetration into host skin include unsaturated fatty acids, such as linoleic and linolenic acids. These essential fatty acids are found in many edible crops, derived plant oils, sun lotions and creams based on plant oils. Control
Various strategies targeting the mollusc and avian hosts of schistosomes have been used by lakeside residents in recreational areas of North America to deal with outbreaks of swimmers itch. In Michigan, for decades, authorities used copper sulfate as a molluscicide to reduce snail host populations and thereby the incidence of swimmers itch. The results with this agent have been inconclusive, possibly because:

Snails become tolerant
Local water chemistry reduces the molluscicides efficacy
Local currents diffuse it
Adjacent snail populations repopulate a treated areaMore importantly, perhaps, copper sulfate is toxic to more than just molluscs, and the effects of its use on aquatic ecosystems are not well understood.Another method targeting the snail host, mechanical disturbance of snail habitat, has been also tried in some areas of North America and Lake Annecy in France, with promising results. Some work in Michigan suggests that administering praziquantel to hatchling waterfowl can reduce local swimmers itch rates in humans.