The eggs hatch in the water producing miracidia which invade snails. Here the miracidia develop into infective cercaria which are released into the water where they again penetrate waterfowl, and on goes the life cycle. However, if a swimmer happens to be in water containing cercaria, they may decide to penetrate the swimmer's skin rather that the duck's. The cercaria tend to swim near the surface of the water and are attracted to the warm temperature of the skin. Swimmer's at greatest risk are those in warm, shallow, calm water near aquatic vegetation which may harbor snails.
Children often get swimmer's itch when wading and playing in shallow and weedy water.
The reaction to cercarial penetration depends upon whether the exposure was a first or subsequent exposure. My understanding is that the itching and subsequent rash is caused by the immune system's reaction to the foreign organisms in the skin, in a similar way that scabies mites cause itching. The difference between swimmer's itch and scabies is that the schistosome cercaria soon die in human skin and are therefore not infectious to others, whereas scabies mites live and reproduce in human skin, thereby causing an ongoing infectious condition. This lasts only 2 or 3 days. However, if your patient has been recently swimming in a lake in this area, swimmer's itch is certainly first on the list.