Sporadic infection in endemic regions accounts for the majority of cases of human and canine blastomycosis. The fungus is found throughout the midwestern and southeastern portions of the United States, particularly along the Ohio and Mississippi River valleys, north central Wisconsin, North Carolina, Minnesota, and Illinois. Blastomycosis is rarely reported outside of North America, primarily from Africa. Although difficult to isolate from soil, B. dermatitidis has been recovered from earth enriched with rotted wood, bird droppings, or animal droppings.
Soil surrounding bodies of water within the endemic region is particularly rich in fungi.
Epidemics are unusual but have been described after excavation of contaminated soil in endemic regions. Individuals who spend large amounts of time in wooded areas within endemic regions, such as hunters or forestry workers, are at highest risk for infection. The pathogenesis of blastomycosis is similar to that of histoplasmosis. Inhalation of spores results in alveolar inoculation and germination to yeast forms. Although pulmonary macrophages eliminate the majority of spores before infection occurs, those that survive produce pneumonitis and may disseminate hematogenously.