Successful treatment of chronic canine blastomycosis in a Labrador retriever with sterile histopathology

A four-year-old, neutered male Labrador retriever from Minnesota, USA was presented to a dermatology referral clinic for evaluation of waxing and waning paronychia and multifocal cutaneous nodules of five months’ duration. The condition was managed before referral with antibiotics and glucocorticoids resulting in a partial response, but new lesions continued to develop. Skin biopsies made both before and after referral revealed pyogranulomatous dermatitis and fibrosis with negative fungal stains. However, acetate tape impression cytology performed at time of referral detected a single blastomyces organism; thus, a definitive diagnosis of blastomycosis was made. Treatment with fluconazole followed by generic itraconazole resulted in rapid, full recovery in three months’ time. Thoracic radiographs at the time of blastomycosis diagnosis were within normal limits, despite history of mild cough. This case highlights the potential pitfalls in diagnosis and treatment of canine blastomycosis.