Clinical disease most often involves lungs, skin, bones, and genitourinary tract. In nature or on media at room temperature, Blastomyces dermatididis grows as a mycelial form. In humans or on enriched media at 37oC, the organism grows as a yeast form. Yeast forms have a characteristic thick cell wall that is highly refractile. These cells reproduce by single buds with a broad base between parent and bud.
Definitive diagnosis requires the growth of the organism from clinical specimens.
Presumptive diagnosis and initiation of treatment can be made by visualization of the characteristic yeast in clinical specimens or histopathologic sections. The presence of pyogranulomas on histopathologic examination should alert one to the possibility of blastomycosis. Yeast forms may be difficult to see on routine hematoxylin and eosin stains. Because the skin lesions result from dissemination, surgical excision is not curative and hence not useful. Amphotericin B is the treatment of choice for all clinical forms of blastomycosis.