Cercarie die upon penetration into human skin, thus halting the schistosome life cycle. Miracidia mature within snails and produce free-swimming cercariae, which either reinfect birds or infect humans. The eggs hatch and liberate ciliated miracidia into the water, which infect snails (intermediate host). Adult worms then produce eggs that are passed in avian feces. The life cycle begins as cercariae infect birds (definitive host), maturing into adult worms within blood vessels. Swimmer’s itch occurs when free-swimming cercariae penetrate the skin of humans (incidental host) and cause an allergic skin reaction (types I and IV hypersensitivity). Human schistosomes may additionally cause systemic schistosomiasis. Cases are less often due to human schistosomes (e.g., Schistosoma haematobium, Schistosoma japonicum, Schistosoma mansoni, Schistosoma mekongi, Schistosoma intercalatum). The Trichobilharzia genus is commonly implicated. Swimmer’s itch is most often caused by non-human schistosomes (over 20 species), particularly avian schistosomes. What is the Cause of the Disease? Etiology Children may be at a higher risk due to more exposure to shallow water and they may be less likely to towel dry. Resident or migratory birds (definitive hosts), snails (intermediate host), and humans (incidental host) must all be present in order for a person to contract swimmer’s itch. Presence of hosts is also an important factor. Warm months may increase intermediate host snail populations leading to increased transmission.
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