Swimmer's itch: Encyclopedia - Swimmer's itch

Swimmer's itch

Swimmer’s itch, or cercarial dermatitis, is a short-term, immune reaction occurring in the skin of humans that have been infected by water-borne trematode parasites. Symptoms, which include itchy, raised papules, commonly occur within hours of infection and do not generally last more than a week.

The trematodes that cause swimmer’s itch are schistosomes that mostly parasitize snails and birds, particularly waterfowl (one exception is Schistosomatium douthitti, which infects snails and rodents). These groups are not to be confused with schistosomes of the genus Schistosoma, which infect humans and cause the serious human disease schistosomiasis. Two examples of schistosome genera that infect birds but can accidentally infect humans, giving rise to swimmer’s itch, are Trichobilharzia and Gigantobilharzia. Avian schistosomes such as these cannot complete their life cycles in humans. The itchy papules are caused by localized inflammatory immune reactions, each bump corresponding to the penetration site of a single parasite, which dies within hours.

Swimmer's itch - Life cycles of non-human schistosomes

Adult stages of avian schistosomes are found in the gastrointestinal tract of the bird host. The worm releases eggs that are shed in the bird’s feces . Upon immersion in water, a short-lived, non-feeding, free-living stage known as the miracidium emerges. The miracidium swims using cilia, following chemical and physical cues that increase its chances of finding the first host in its life cycle, a snail. After infecting a snail it develops into a sporocyst, which in turn undergoes asexual reproduction, yielding large numbers of another short-lived, free-living stage, the cercaria. Cercariae use a tail-like appendage (often forked in genera causing swimmer’s itch) to swim to the surface of the water, as well as other physical and chemical cues, in order to locate the next and final host in the life cycle, a bird. After infecting a bird, the parasite develops into a schistosomulum and eventually migrates to the gastrointestinal tract where it matures and, if it encounters a mate, sheds eggs to begin the cycle anew.

Swimmer's itch - Risk factors

Humans usually become infected with avian schistosomes after swimming in lakes or other bodies of slow-moving freshwater. 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 has been postively correlated with increased risk of infection in epidemiological studies in Europe and North America, and shallow inshore waters -- snail habitat -- undoubtedly 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 swimmer's itch "hotspots". Children may become infected more frequently than adults but this probably reflects their tendency to swim for longer periods inshore, where cercariae also concentrate.

Swimmer's itch - Control measures

Various strategies, targeting either the mollusc or avian hosts of schistomes, have been used by lakeside residents in recreational areas of North America to deal with outbreaks of swimmer's itch. In Michigan, for decades authorities used copper sulphate as a molluscicide to reduce snail host populations and thereby the incidence of swimmer's itch. The results with this agent have been inconclusive, possibly because

• snails become tolerant
• local water chemistry reduces the molluscicide's efficacy
• local currents diffuse it
• adjacent snail populations repopulate a treated area.

More importantly, perhaps, copper sulphate is toxic to more than just molluscs, and the effects of its use on aquatic ecosystems are not well understood. Another method targetting the snail host, mechanical disturbance of snail habitat, has been also tried in some areas of North America, with promising results. Finally, some work in Michigan suggests that administering praziquantel to hatchling waterfowl can reduce local swimmer's itch rates in humans.

Public education of risk factors, a good alternative to the above mentioned interventionist strategies, can also reduce human exposure to cercariae.

See also

Adapted from the Wikipedia article "Swimmer's itch", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki