Geographic Variation in the Prevalence of Histoplasmin Sensitivity

¹ Statisticians of the Tuberculosis Program, Division of Special Health Services, Public Health Service, Department of Health, Education, and Welfare, Washington, D. C.

This paper reports the results of a detailed mapping of geographic variation in the prevalence of histoplasmin sensitivity throughout the United States. The material is based on histoplasmin skin tests of 68,452 young adults who were lifetime residents of single counties. The data on this group were drawn from observations on more than 150,000 persons x-rayed and skin tested with various antigens in three cooperative research programs directed by tuberculosis research workers in the Public Health Service.

Skin tests and residence histories were obtained, recorded and interpreted as uniformly as possible to provide comparable material on a large homogeneous population.

A summary map of the United States shows the area of highest prevalence, from 60 to 90 per cent positive reactors, in the central states stretching in a triangle from Ohio to Iowa, extending southeast through Missouri and Arkansas to Louisiana, with a third side cutting from Ohio southwest through Kentucky, Tennessee and Mississippi.

The rate of positive reactors decreases, on the whole, with increasing distance from this triangular area. An area of moderately high prevalence in which 30 to 60 per cent of the young adults tested were positive reactors, forms an uneven ring around the central triangle varying in width up to about 300 miles. Next comes a ring of intermediate prevalence ranging from 10 to 30 per cent and including a considerable area westward. Histoplasmin sensitivity was practically absent among lifetime residents of the Atlantic and Pacific coastal areas, except for scattered islands of fairly high frequency.

The present project was undertaken with the intention of supplying a nationwide map of histoplasmin sensitivity. Such a map is a requisite for studies of the determinative factors in the growth and spread of H. capsulatum, and in the conditions of its transmission to man. With some degree of precision, the localities of varying risk have been defined over the country's entire area of differing geographic and climatic conditions. This analysis explains neither why nor how human beings become infected with Histoplasma capsulatum but it does measure the variation in the risk of infection over the 3,000,000 square miles of the United States.