Blastomycosis is a systemic disease caused by Blastomyces dermatitidis, a broad-based dimorphic fungus found in the soil around the Ohio and Mississippi River valleys, the Great Lakes, and southeastern states; the primary route of infection is the respiratory tract. Common sites of disseminated infection include the skin, subcutaneous tissue, eyes, bones, lymph nodes, brain, and testes.1,2
Dermatologic signs can be seen with blastomycosis; 30% to 50% of dogs have cutaneous signs at diagnosis.1,4 Incidence of dogs presented solely for localized cutaneous signs is not well described, although one study noted only 13% of dogs with blastomycosis had no radiographic pulmonary changes.4 Thoracic radiographs were unremarkable in this case.
Diagnosis is made by identifying the organism via cytology, histopathology, culture, or antigen testing.5 Histoplasmosis should be ruled out in patients that live in states where both Histoplasma spp and Blastomyces spp are endemic, as Histoplasma spp can cross-react with Blastomyces spp quantitative antigen testing on both urine and serum.5 Cytology of the affected digit in this case could have resulted in diagnosis, but more extensive sampling under anesthesia was preferred.
Definitive diagnosis involves respiratory sampling, which is not often pursued without other evidence of involvement. In humans, primary cutaneous blastomycosis is rare, and differentiation from secondary cutaneous blastomycosis with negative thoracic radiography is difficult.6,7