The Skin Test in an Epidemiologic Study of Tularemia in Montana Trappers

Fifty-three persons listed in the Rocky Mountain Laboratory and Montana State Board of Health records as having had tularemia at various times from 1925 to 1964 and 344 Montana fur trappers were examined for prior infection by Francisella tularensis. All but 2 persons in the case group showed delayed hypersensitivity to tularemia skin test antigen. All but 4 skin test reactors had agglutinins to F. tularensis. Reactions were as pronounced among persons infected many years ago as among those infected recently. Likewise the reactions of those who had had tick, rabbit or sheep-borne tularemia were no different from those who were infected from muskrat, beaver or water. Fifty-eight (17%) of Montana trappers were skin test reactors. Forty-two also had antibodies to F. tularensis, indicating a rather high frequency of past infection in this occupational group. However, among reactors only 13 (22%) had an illness associated with trapping activities and clinically characteristic of tularemia. The risk of infection was directly related to the degree of trapping activity. Among those who trapped an average of 500 or more animals per year nearly one-third showed skin sensitivity to F. tularensis. Although clinical tularemia was apparently infrequent, the high risk of infection merits consideration of vaccine prophylaxis in this occupational group.