One of the characteristics common to the dimorphic molds is the ability to convert the mold forms to the yeast forms by incubating subcultures in enriched media at 35°-37°C. The upper image illustrates a subculture of a mold colony suspected of being a dimorphic fungus. The colony was inoculated to the surface of blood agar and incubated for 3 days at 37°C. Note that the colonies have a prickly appearance, suggesting an intermediate stage of conversion. The lower image is a lactophenol blue mount of a portion of one of the prickly colonies. This fungus was identified as Blastomyces dermatitidis. The lactophenol blue mount illustrates a short hyphal segment that is converting into spherical yeast forms. A few individual yeast forms are also present. These yeast forms are relatively large and show a propensity toward budding with broad-based attachments. This is characteristic of the yeast conversion forms of Blastomyces dermatitidis.
B. dermatitidis survives in soil that contains organic debris (rotting wood, animal droppings, plant material). The incidence of blastomycosis in dogs in endemic areas can be quite high. Humans are susceptible to acquiring infections by close contact with dogs or through scratches and bites.
Blastomycosis usually presents as a cutaneous or a respiratory disease, although it may also affect bone, prostate, and other organs.
The geographic distribution of blastomycosis in the United States tends to be centered more in the upper Mississippi river drainage system and adjacent tributaries. At one time, blastomycosis was referred to as "Chicago disease." The reason why Chicago was singled out from other surrounding areas may have more to do with the availability of advanced medical centers where the diagnosis was more likely to be made rather than an overall increased prevalence.
To make the specific diagnosis, the physician must be aware of blastomycosis. Specimens sent to the laboratory may include sputum (specifically for fungal culture), biopsy material, and urine (occasionally, the organism can be isolated from urine as it often infects the prostate). B. dermatitidis may also be observed in a KOH preparation of pus from a skin lesion.