ThedaCare Health Matters By: P. Michael Shattuck, MD, Emergency Department phyisician

Do you know about swimmer’s itch?

Soon the waters in the local lakes will be warming enough that we can go into the water. Swimmers in some of the lakes in this area are prone to contract a condition called “swimmer’s itch.” The more technical term is cercarial dermatitis. It seems that every year in the late spring and early summer people get an itchy rash that develops within a day of being in a lake. The rash occurs only in areas of the body that have been under the water. It most commonly presents on the lower legs of someone who was out wading and raking the debris away from the beach. The rash is self-limited, lasting about a week. It is caused by a parasite in the water that penetrates the skin causing a reaction in the skin. To understand how this happens, you need to know about the life cycle of a type of flatworm parasite. The life cycle depends on other creatures that are in the lake. Specifically the lake needs to have snails and certain birds or mammals that inhabit it. The worm is the adult stage in the life cycle and it lives in a bird or mammal. Ducks, geese, and muskrats are common hosts of the adult worm. The worms lay eggs near the intestinal tract of the host and those eggs drop into the lake when the host defecates in the water. Once in the water, the eggs hatch into a microscopic larva that will seek out a mollusk, most commonly a snail. Here it develops into another stage that reproduces in the snail and then leaves to swim in the water in search of another host to which it can attach and penetrate the skin. In this stage the organism is known as a cercaria which swims freely in the water and can survive about 24 hours. When it finds a suitable host, it penetrates the skin and migrates to the intestinal tract to start the cycle over again. 

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