Swimmer's Itch Underlying Cause
Swimmer's itch is caused by the larvae of a parasite, called a schistosome. A schistosome is a flat worm that lives in snails. Schistosomes live mainly in fresh water, but may be present in salt water. Larvae are immature offspring that are released into the water by these organisms. The parasites are more prominent in warm water.
These tiny organisms burrow into the skin and cause inflammation, which results in the rash. The rash usually occurs on areas of the skin that are covered by a swimsuit. The larvae die and the rash slowly improves with time, but may take several weeks.
- Verbrugge LM, Rainey JJ, Reimink RL, Blankespoor HD. Prospective study of swimmer's itch incidence and severity. J Parasitol. 2004 Aug;90(4):697-704. 
- Verbrugge LM, Rainey JJ, Reimink RL, Blankespoor HD. Swimmer's itch: incidence and risk factors. Am J Public Health. 2004 May;94(5):738-41.