Giant forms of Blastomyces dermatitidis in the pulmonary lesions of blastomycosis. Potential confusion with Coccidioides immitis.

Typical yeast-phase cells of Blastomyces dermatitidis have a characteristic appearance in tissue sections. Fungal morphologic variation occurs infrequently in the lesions of blastomycosis, yet it can complicate the differential diagnosis, particularly if fresh tissue is not available for microbiologic culture. The authors report a case of pulmonary blastomycosis, confirmed by culture and direct immunofluorescence, in which some of the yeast-like cells were abnormally large. These giant yeast-like cells exceeded the size range accepted for the tissue forms of B. dermatitidis; therefore, coccidioidomycosis was considered initially in the differential diagnosis. Otherwise characteristic morphologic features of these cells, in particular multinucleation and the production of broad-based blastoconidia, helped resolve the differential diagnosis. The diagnosis can be confirmed by direct immunofluorescence or microbiologic culture.