Distribution and Incidence

H. capsulatum has been isolated from soil of more than 50 countries. It is most common in temperate climates along river valleys and has been found in North, Central, and South America; India; Southeast Asia; and rarely Europe.

By far the most heavily endemic area in the world is the east central United States, particularly the Mississippi and Ohio River valleys. It is most prevalent in the states of Ohio, Kentucky, Indiana, Illinois, Kentucky, Tennessee, and Arkansas. Surrounding states also have many infections.

Infection is almost universal in the most heavily endemic areas. Skin test surveys reveal that over 90 percent of persons living in some counties in the central United States have had histoplasmosis before age 20 (Edwards et al. 1969). Based on skin test surveys, there are probably 40 to 50 million people in the central United States who have had histoplasmosis, and there are several hundred thousand new cases each year. The number of serious infections requiring diagnosis and treatment is very small, perhaps 1 or 2 percent of the total.

In contrast to the highly endemic central United States, only a handful of individual cases have been documented in Europe. Skin test surveys show about 0.5 percent of the population positive to histo-plasmin. In Italy, positive skin tests are slightly more common (about 2 percent) (Mantovani 1972).

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