A Technique for Creating Localized Subcutaneous Blastomyces Granulomas in Rats

Our purpose was to develop a simple, reliable method for creating subcutaneous Blastomyces dermatitidis nodules in rats and to describe the histologic appearance of these lesions. We used B. dermatitidis isolated from a dog with blastomycosis to prepare a Blastomyces yeast suspension. Four rats were used to test initial dose concentrations of 10⁵, 10⁶, 10⁹, and 10¹⁰ yeast organisms. The dose was administered subcutaneously over the distal tibia in a volume of 0.1 ml. We then inoculated 35 additional rats with 10⁹ or 10¹⁰ yeast organisms. Rats were euthanized 7, 10, 14, 21, or 28 days after inoculation, and the histologic appearance of the nodules was described. A full post-mortem examination sought evidence of systemic spread of Blastomyces organisms. We successfully induced subcutaneous Blastomyces abscesses in 34 of 37 rats injected with 10⁹ or 10¹⁰ organisms. Nodules first appeared 3 to 7 days after injection and reached 2 to 15 mm in diameter by 7 to 28 days after inoculation. Histologically the lesions were characterized by a necrotic center surrounded by a layer of viable yeast and granulomatous inflammation. Live yeast organisms were recovered from all lesions. No adverse effects or systemic spread of Blastomyces organisms were observed. We conclude that subcutaneous Blastomyces abscesses can be induced safely and reliably in rats after injection of 10⁹ and 10¹⁰ organisms. Histologically, the experimentally induced lesions share both similarities to and differences with lesions of naturally occurring blastomycosis.