Walsh & Hoyt: Blastomyces Dermatitidis (Blastomycosis)

Blastomyces dermatitidis is a dimorphic fungus. It causes a systemic disease called blastomycosis (or North American blastomycosis) that is characterized by the formation of inflammatory masses that are part abscess and part granuloma, i.e., pyogranulomas. The mycelial form of the fungus produces branching hyphae that usually are 23 min diameter. Arising at right angles to the hyphae are conidiophores that produce single terminal conidia that vary from 2 to 10 micrometers in diameter and are round or oval. The yeast form of the fungus consists of multinucleate cells that usually are 815 micrometers in diameter. The cells have thick walls that are highly refractile. The yeast cells of B. dermatitidis reproduce by single buds with a broad base between parent and bud. The daughter cell often is nearly as large as the mother cell before detachment. These characteristics can be used to distinguish this organism from other fungi both in vitro and in vivo.