Swimmer’s Itch

Children are more likely targets because of their tendency to play in shallow water.

Beverly Louis, BSc(Pharm), Drug Information Pharmacist, British Columbia Drug & Poison Information Centre

Swimmer’s itch (or cercarial dermatitis) is a rash resulting from an allergic reaction to a parasite called a Schistosome (a type of flatworm), which has a complex life cycle. Although some schistosomes can cause serious infections in humans, the ones found in the lakes and ponds of North America tend to infect birds (e.g., ducks) and snails.

This parasite lives as an adult worm in ducks. The worm releases eggs, which are then excreted in the duck feces into the water. The eggs hatch and release larvae, which have only 24 hours to find a specific snail host before they die. If it finds the right host, the larva enters the snail, where after three to four weeks of development, it is released into the water as a cercaria (a free-swimming larval form of the parasite). The microscopic cercaria then swims to the surface searching for an avian host to complete its life cycle. It is at this stage that the parasite may encounter a human swimmer and penetrate the skin.

Although some cercariae can penetrate skin while it is still wet, others only penetrate dry skin. Once they enter the skin, the cercariae quickly die. Swimmer’s itch results in individuals who have been previously exposed to the cercariae and who have become sensitized to the dead parasite.

As the cercariae penetrate the skin, the person may feel a tingling sensation. Approximately a third of people will develop an allergic reaction to the parasite and experience possible itching, swelling and a rash. Scratching should be avoided as it will increase the risk of infection. Symptoms usually begin within a couple of hours and may last for up to two weeks.

Although swimmer’s itch can affect anyone, children are more likely targets because of their tendency to play in shallow water, where the cercariae are often found. As well, children tend to run in and out of the water without drying off. The parasite may then enter their skin once the infested water on their skin evaporates. Swimmer’s itch is not contagious.

Many people may not require treatment. However, if the itching is bothersome, cool compresses, baking soda baths or pastes, calamine lotion, topical antihistamines and colloidal oatmeal baths have been recommended. If the rash is severe, oral antihistamines and topical corticosteroids can be used.

To help prevent swimmer’s itch, individuals should shower off immediately after coming out of the water and then briskly dry themselves with a towel. This measure, however, will not affect the cercariae that have already penetrated the skin.