The genera most commonly associated with swimmers itch in humans are Trichobilharzia and Gigantobilharzia. It can also be caused by schistosome parasites of non-avian vertebrates, such as Schistosomatium douthitti, which infects snails and rodents. Other taxa reported to cause the reaction include Bilharziella polonica and Schistosoma bovis. In marine environments, especially along the coasts, swimmers itch can occur as well.These parasites use both freshwater snails and vertebrates as hosts in their parasitic life cycles as follows:

Once a schistosome egg is immersed in water, a short-lived, non-feeding, free-living stage known as the miracidium emerges. The miracidium uses cilia to follow chemical and physical cues thought to increase its chances of finding the first intermediate host in its life cycle, a freshwater snail. After infecting a snail, it develops into a mother sporocyst, which in turn undergoes asexual reproduction, yielding large numbers of daughter sporocysts, which asexually produce another short-lived, free-living stage, the cercaria. Cercariae use a tail-like appendage (often forked in genera causing swimmers itch) to swim to the surface of the water; and use various physical and chemical cues in order to locate the next and final (definitive) host in the life cycle, a bird. These larvae can accidentally come into contact with the skin of a swimmer. The cercaria penetrates the skin and dies in the skin immediately. The cercariae cannot infect humans, but they cause an inflammatory immune reaction. This reaction causes initially mildly itchy spots on the skin. Within hours, these spots become raised papules which are intensely itchy.
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.