View source for Swimmer's itch

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{{Infobox_Disease |
  Name           = {{PAGENAME}} |
  Image          = cercarial_dermatitis_lower_legs.jpg |
  Caption        = [[Cercarial]] [[dermatitis]] on lower legs, four days after spending a day in the shallows of a lake. |
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{{SK}}  Rice paddy itch, kubure; kobanyo; sawah; hoi con; duckworms itch; clam digger's itch; duck itch; cercarial dermatitis
==Overview==
'''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 parasitic [[Schistosomatidae | schistosomes]] that use both snails and vertebrates as hosts in their life cycles. Most cases are caused by parasites that use waterfowl as the vertebrate host. These avian schistosomes cannot complete their [[parasitic life cycles | life cycles]] in mammals, but can [[parasitic life cycles | accidentally infect]] humans, giving rise to mildly itchy spots on the skin. Within hours, these spots become raised papules that are more intensely itchy. The papules are caused by localized inflammatory [[immune]] reactions, each corresponding to the penetration site of a single parasite, which dies in the skin within hours. 

The schistosomes that give rise to swimmer’s itch should not to be confused with those of the genus ''[[Schistosoma]]'', which infect humans and cause the serious human [[disease]] [[schistosomiasis]], or with larval stages of thimble jellyfish (''Linuche unguiculata''), which give rise to [[seabather's eruption]].

Since it was first described in Michigan in 1928{{ref|Cort}}, swimmer's itch has been reported from around the world. Some suggest [[Incidence (epidemiology)| incidence]] may be on the rise{{ref|Hjorngaard}}, although this may also be attributed to better monitoring.

== Etiology ==
The [[genus| genera]] most commonly associated with swimmer’s itch in humans are ''Trichobilharzia'' and ''Gigantobilharzia''. Trematodes in these groups normally complete their life cycles in birds. However, swimmer’s itch can also be caused by schistosome parasites of non-avian vertebrates, such as ''Schistosomatium douthitti'', which infects snails and  rodents. Other [[taxon | taxa]] reported to cause the reaction include ''Bilharziella polonica'' and ''Schistosoma bovis''.

==Life cycles of non-human schistosomes==
The non-[[human]] schistosomes use two hosts in their life cycles. One is a snail, the other is a bird or mammal. Schistosomes are gonochoristic and sexual reproduction takes place in the vertebrate host. In genera that infect birds, adult worms occur in tissues and veins of the host’s [[gastrointestinal tract]], where they produce eggs that are shed into water with host feces. One European species, ''[[Trichobilharzia regenti]]'', instead infects the bird host’s [[nasal]] [[Biological tissue|tissues]], where its eggs are shed with [[tears|lachrymal secretions]]. 

Once a schistosome egg is immersed in [[water]], a short-lived, non-feeding, free-living stage known as the [[miracidium]] emerges. The miracidium uses [[cilium | cilia]] to follow chemical and physical cues thought to increase its chances of finding the first intermediate host in its life cycle, a 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 [[genus | 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 (definitive) host in the life cycle, a bird. After infecting a bird, the parasite develops into a schistosomulum and migrates through the host’s circulatory system (or nervous system in case of ''T. regenti'') to the final location within the host body where it matures and, if it encounters a mate, sheds eggs to begin the cycle anew.

==Risk factors==
Humans usually become infected with avian schistosomes after swimming in lakes or other bodies of slow-moving fresh water. 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 positively correlated with increased risk of infection in Europe{{ref|chamot}} and North America{{ref|lindblade}}, 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.{{ref|leighton}} 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{{ref|chamot}} as well as instances where risk increases in endemic swimmer's itch "hotspots"{{ref|leighton}}. [[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.{{ref|verbrugge}} Stimuli for cercarial penetration into host [[skin]] include [[unsaturated]] [[fatty acids]], such as [[linoleic]] and [[Linolenic acid|linolenic]] acids. These substances occur naturally in human skin and are found in sun lotions and creams based on plant oils.

==Control measures and treatment==
Various strategies, targeting either the [[mollusc]] or avian hosts of schistosomes, 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. {{ref|blankespoor1991}}  

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 targeting the snail host, mechanical disturbance of snail habitat, has been also tried in some areas of North America{{ref|leighton}} and Lake Annecy in France, with promising results. Some work in Michigan suggests that administering [[praziquantel]] to hatchling waterfowl can reduce local swimmer's itch rates in humans.{{ref|blankespoor2001}} Work on schistosomiasis showed that water-resistant topical applications of the common insect repellent [[DEET]] prevented schistosomes from penetrating the skin of mice.{{ref|salafsky}}

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

Orally adiministered [[hydroxyzine]], an [[antihistamine]], is sometimes prescribed to treat swimmer's itch and similar dermal allergic reactions.

==See also==
*[[Dermatitis]]
*[[Schistosomiasis]]

==References==
#{{note|Cort}} Cort, WW (1928) '''Schistosome dermatitis in the United States (Michigan).''' Journal of the American Medical Association 90: 1027 – 1029
#{{note|Hjorngaard}} Hjorngaard Larsen, A, J Bresciani and K Buchmann (2004) '''Increasing frequency of cercarial dermatitis at higher latitudes.''' Acta Parasitologica 49: 217 – 221
#{{note|chamot}} Chamot, E, L Toscani and A Rougement (1998) '''Public health importance and risk factors for cercarial dermatitis associated with swimming in Lake Leman at Geneva, Switzerland.''' Epidemiology and Infection 120: 305 – 314
#{{note|lindblade}} Lindblade, KA (1998) '''The epidemiology of cercarial dermatitis and its association with limnological characteristics of a northern Michigan lake.''' Journal of Parasitology 84: 19 – 23
#{{note|leighton}} Leighton, BJ, S Zervos and JM Webster (2000) '''Ecological factors in schistosome transmission, and an environmentally benign method for controlling snails in a recreational lake with a record of schistosome dermatitis. ''' Parasitology International 49: 9 – 17
#{{note|verbrugge}} Verbrugge, LM, JJ Rainey, RL Reimink and HD Blankespoor (2004) '''Prospective study of swimmer’s itch incidence and severity'''. Journal of Parasitology 90: 697 – 704
#{{note|blankespoor1991}} Blankespoor, HD and RL Reimink (1991) '''The control of swimmer’s itch in Michigan: Past, present, and future.''' Michigan Academician 24: 7 – 23
#{{note|blankespoor2001}} Blankespoor, CL, RL Reimink and HD Blankespoor (2001) '''Efficacy of Praziquantel in treating natural schistosome infections in common mergansers.''' Journal of Parasitology 87: 424 – 426
#{{note|salafsky}} Salafsky, B, K Ramaswamy, Y He, J Li and T Shibuya (1999) '''Development and evaluation of lipodeet, a new long-acting formulation of N,N-diethyl-m-toluamide (DEET) for the prevention of schistosomiasis.''' American Journal of Tropical Medicine and Hygiene 61: 743 – 750

==External links==
* [http://www.cdc.gov/ncidod/dpd/parasites/cercarialdermatitis/factsht_cercarialdermatitis.htm CDC]
* {{DermNet|arthropods/swimmers-itch}}
* [http://www.lib.uiowa.edu/hardin/md/swimmersitch.html Links to pictures of swimmer's itch (Hardin MD/Univ of Iowa)]
* [http://www.schistosomes.cz Schistosome Group Prague, a member of the European Cercarial Dermatitis Network (ECDEN)]
* [http://www.aocd.org/skin/dermatologic_diseases/swimmers_itch.html]


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[[Category:Dermatology]]
[[Category:Inflammations]]
[[Category:Parasitic diseases]]
[[Category:Diseases]]
[[Category:Primary care]]
[[Category:Emergency medicine]]