A Geographical Study of "Swimmers' Itch" in the United States and Canada

Abstract

A skin eruption developing after swimming or wading in certain lakes in midwestern U.S. and Canada, known technically as "cercarial or schistosome dermatitis" is a disease of place; that is, it can be acquired only at certain places and is not communicable from person to person. Its occurrence is more severe in children who wade in shallow onshore waters where the larvae are most abundant than in adults. Mechanism of transmission involves the trematode worm; the definitive host, either bird or mammal; intermediate host, a snail; and man, an accidental intruder into the cycle. This is a 4-factor complex and may involve a 5th, bacteria, when secondary infection occurs. The major endemic area extends from parts of the U.S. near the Great Lakes north-west into Canada. The general distr. of reported cases suggests that infection of snails occurs during migration of aquatic birds, and that outlying foci arise by crossing over of birds from one flyway to another. Reported cases in Tenn. Valley Authority, where migratory waterfowl were formerly uncommon, coincides with increased use of this area by birds from the Mississippi flyway. Cases of proved cercarial dermatitis are acquired in lakes; rivers are less favorable and there are no proven marine cases. Physical environmental factors influencing dermatitis distr. are the multitude of lakes and sloughs within the area of Pleistocene glaciation. Here lies the breeding grounds of migratory water-fowl, the center of the present distr. of the fresh-water pulmonate mollusca, and environments more or less favorable for the schistosomes, including the most important Stagnicola emarginata. Larger glacial lakes with clear water and sandy beaches relatively free of vegetation are suitable habitats. Chemical composition of water is important in limiting distr. of snail and cercarial dermatitis; neutral or moderately acid pH is less favorable than where pH is on alkaline side. Wave-swept beaches are unsuitable; snails and dermatitis are more widely distr. on leeward shores. Max. infection of dermatitis occurs in mid-July to August during the warmer season. Incidence of schisto-some infection of snail hosts varies. This may be due to variation in number and distr. of snails, to pollution of water, the migratory behavior of bird hosts, and the habits of man. Human factors influencing the distr. of the dermatitis include the increased use of automobiles and development of roads which make lakes more accessible to a greater number of people; this provides a rise in the number of cases of cercarial dermatitis in the "silent zone" or areas where the definitive and intermediate hosts exist. Swimmers prefer lakes with sandy rather than muddy bottoms and it is in the sandy environment that the pathogenic complex is completed. Voluntary controls include: restriction of swimming in endemic areas which is undesirable for recreational and financial reasons; control of the static host, the snail, by use of Cu which may be impractical in large lakes. Involuntary influences on distribution by bird hosts has been brought about by reduction of breeding grounds through agriculture, lowering of water table, and by introduction of carp. In spite of decreasing waterfowl population, cases of swimmers'' itch are increasing. This is partially due to increased use of larger lakes by birds, and the impounding of water for various purposes. Thus the silent zone of cercarial dermatitis is spreading farther south, and more summer visitors are gaining access to lakes in the northern endemic areas.