Swimmer's itch usually happens within hours (1 to 48 hours) of being in the water. At first, it may cause a tingling, itching or burning feeling of the skin. Small reddish spots then appear, sometimes turning into larger rashes and blisters, with intense itching.
It usually only affects skin which directly touches the water. Skin that is covered by tight swimwear is usually fine.
Swimmer’s itch usually gets better without treatment and goes away by itself in 1-3 weeks.
As swimmer’s itch is an allergic reaction, people in the same water can have different reactions. But the more often someone has contact with the flatworms, the more serious (immediate and intense) their reaction may be.