The Epidemiology and Ecology of Blastomycosis, Coccidioidomycosis and Histoplasmosis

Blastomycosis, coccidioidomycosis and histoplasmosis are the major pulmonary mycoses of humans. Each is considered to be a primary pulmonary disease acquired by inhalation of infectious spores from the environment. Blastomycosis occurs predominantly in the Mississippi River Valley and Southeastern portions of the U.S.A. Clinical cases have also been noted from a few other areas of the world. The etiologic agent, Blastomyces dermatitidis, is not readily recoverable from nature, but the disease is not contagious and must be acquired from some exogenous source poorly defined at present. Blastomycosis is common in dogs but unusual in other species although a wide variety of animals can be infected experimentally. The disease is more common in middle age, in blacks, and in males.

Coccidioidomycosis is restricted to the Western Hemisphere where most cases occur in the semiarid regions of Southwestern U.S. and contiguous areas of Mexico. Limited endemic foci are found in South and Central American locations. The arthroconidia of Coccidioides immitis occur in the soil of endemic areas from whence they are inhaled by humans and other animals. The disease is not customarily contagious. Handling cultures in the laboratory is a serious biohazard. The basis for an apparent racial predisposition to develop disseminated disease is unknown.

Two clinical entities distinguishable by their etiologic agents are recognized to comprise histoplasmosis. Histoplasmosis caused by Histoplasma capsulatum var. capsulatum is a cosmopolitan disease with the greatest known concentration of cases located in the eastern half of the U.S. and most of Latin America. The fungus thrives especially well in habitats enriched by the feces of birds, e.g., chickens, grackles, and starlings, and in a large number of species of bats in whom it causes an infection which may involve the intestinal tract. The disease is not contagious. A large variety of animals, aside from humans, are susceptible to infection.

Save for one exceptional report all cases of the other form of histoplasmosis caused by Histoplasma capsulatum var. duboisii have occurred in Africa. Prevalence of the disease is unknown. It is presumed that susceptible individuals inhale infectious spores from the soil but the ecology of the fungus is currently unknown. Two species of lower animals are known to sustain infections. Demographic facts are more sparse than with the other pulmonary mycoses discussed.