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. The immune response to infection consists of neutrophil and macrophage migration into infected tissue.
Human blastomycosis presents as a pulmonary or extrapulmonary infection.
Three forms of pulmonary infection have been described. Asymptomatic infection is rarely identified because of the lack of an inexpensive, reliable screening test. Chest radiographs often demonstrate an alveolar process that may resemble a parenchymal mass. Acute pneumonia is the most common form of human blastomycosis. Alveolar or reticulonodular patterns without hilar lymphadenopathy are often seen on chest radiographs.