Fungal Infections

Species/common name:

Blastomyces dermatitidis

Telemorph Ajellomyces dermatitidis

Natural habitat

B. dermatitidis is very rarely isolated from the environment. It inhabits decaying wood material and so moist soil rich in organic material is probably its favoured niche, such as woodpiles, riverbanks and beaver dams.  

Geography

B. dermatitidis is endemic in North America especially in Wisconsin, Mississippi, Ohio and Missouri including the Mississippi and Namekagon river valleys. It has also been isolated from cases in subsahran Africa, India and central and south America with some strain variations.

Frequency

Very rarely isolated from the environment, but PCR detection rate slightly higher. Also a pathogen of dogs.

Diseases

Pneumonia in non-immunocompromised and immunocompromised individuals.

Skin disease in non-immunocompromised people.

Rarely dissemination to other organs, especially bones and genitourinary tract.

Culture peculiarities

Like other dimorphic moulds, B. dermatitidis is a mould at 25°C and a yeast at 37°C. In the laboratory the conversion from yeast to mould is slow and may not be seen. At 25°C  growth is slow to moderately rapid, but cultures should be retained for 4 weeks. Colony texture is membranous and downy to woolly. With a surface color of white to light brown. The reverse is pale to brownish. It does not produce arthroconidia. Conversion to a yeast form at 37°C is best seen an enriched medium such as brain heart infusion, with blood. Growth rate is variable with a colony texture typically creamy and yeast-like and granular to verrucose on the surface, white to beige.

A positive identification can be made with molecular tools.

Antifungal resistance (intrinsic and acquired)

Isolates are susceptible to miconazole, ketoconazole, itraconazole, voriconazole, posaconazole and amphotericin B.  Fluconazole and echinocandins are relatively inactive.  

Biosafety level 2

This organism must be handled in a biosafety level 3 laboratory.

Industrial use:

None

Images

Example of cultures of Blastomyces dermatitidis at 2 different temperatures showing dimorphism.

PAS stained biopsy showing multiple broad-based budding yeasts most consistent with B. dermatitidis.

High magnification of B. dermatitidis showing broad-based budding and thick walls.

Endemic regions of blastomycosis in N. America.

 

 

 

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