Printable Fact Sheet
Swimmer's itch (Schistosome dermatitis) is a temporary skin infection acquired by bathers in some lakes when they accidentally become involved in the life cycle of the trematode worm. Swimmer's Itch appears during warm weather in early June and reaches a peak in July.
The adult worms are parasitic in the blood vessels of aquatic birds, particularly waterfowl, shorebirds and some mammals. Following mating, the females lay eggs in the intestinal veins of the host. These eggs migrate into the intestines and are passed into the water along with droppings. The eggs hatch into tiny free-swimming animals called miracidia. The miracidia find and penetrate the tissues of certain fresh water snails where additional development occurs and cercariae, minute free-swimming, fork-tailed, colourless animals are released. It is the penetration of those cercariae through the skin of bathers which results in swimmer's itch. Man is an accidental host, and had the cercariae found their natural host, they would complete their life cycle to adult form. Because man is an unsuitable host, after skin penetration the cercariae dies.
The cercariae penetrates the skin of bathers as water droplets evaporate. As the cercariae penetrate the skin, a temporary, prickly, itching sensation occurs, sometimes followed by a general inflammation of the affected area. Areas mostly affected are the chest, stomach, and the back of legs. These symptoms usually subside quickly leaving the victim with tiny red spots at the site of penetration. Several hours later, the bather will experience a distinct itching sensation and the red spot will enlarge to form separated bumps up to ¼ inch in diameter. These lesions do not spread to other areas of the body, nor do they spread from person to person. The degree of reaction varies with the sensitivity of the individual and with the severity of the infestation. With repeated exposure, the body usually becomes more sensitive, rather than immune. Generally the rash heals within a week or more, but may persist because of scratching and secondary infections arising.
Prevention and Treatment
Rubbing with a water repellant substance (petroleum jelly) on the skin prior to swimming may assist in repelling the cercariae from penetrating. Bathers should always rub down briskly with a coarse towel when emerging from the water since the cercariae do not penetrate the skin until the water evaporates. This towel should not be used again until it has been laundered. Treatment of Swimmer's Itch is symptomatic and self-limiting. Calamine lotion may be used to relieve the itching.