page_title: Swimmer's itch
section_title: Cause
hierarchical_section_title: Swimmer's itch / Cause
caption_attribution_description: Typically, hosts of avian schistosomes are migratory water birds, including shorebirds, ducks, and geese. Adult worms are found in the blood vessels and produce eggs that are swallowed and passed in the feces . On exposure to water, the eggs hatch and liberate a ciliated miracidium that infects a suitable molluscan intermediate host . The parasite develops in the intermediate host, usually a certain species of snail , to produce free-swimming cercariae that are released under appropriate conditions and penetrate the skin of the birds to complete the cycle . Humans are inadvertent and inappropriate hosts; cercariae may penetrate the skin but do not develop further . A number of species of dermatitis-producing cercariae have been described from both freshwater and saltwater environments, and exposure to either type of cercariae will sensitize persons to both.
context_page_description: Swimmer's itch or cercarial dermatitis, is a short-term allergic immune reaction occurring in the skin of humans that have been infected by water-borne schistosomes. Symptoms, which include itchy, raised papules, commonly occur within 1–2 days of infection and do not generally last more than 2–3 weeks. However, people repeatedly exposed to cercariae develop heavier symptoms with faster onset. Cercarial dermatitis is common in freshwater, brackish and marine habitats worldwide. Incidence may be on the rise, although this may also be attributed to better monitoring. Nevertheless, the condition has been regarded as emerging infectious disease.
  There are no permanent effects to people from this condition. Orally administered hydroxyzine, an antihistamine, is sometimes prescribed to treat swimmer's itch and similar dermal allergic reactions. In addition, bathing in oatmeal, baking soda, or Epsom salts can also provide relief of symptoms.
context_section_description: Swimmer's itch probably has been around as long as humans. The condition was known to exist as early as the 1800s, but it was not until 1928 that a biologist found that the dermatitis was caused by the larval stage of a group of flatworm parasites in the family Schistosomatidae. The genera most commonly associated with swimmer's 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 habitats, especially along the coasts, swimmer's 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 swimmer's 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. 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 host's 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 host's nasal tissues and larvae hatch from the eggs directly in the tissue during drinking/feeding of the infected birds.
caption: Life-cycle of swimmers itch
exif: {}