Osteomyelitis Associated With Disseminated Blastomycosis in Nine Dogs

Blastomycosis, a common disease entity of dogs in the southeastern United States, may be encountered elsewhere. Blastomyces dermatitidis, the cause of this disease, is a soil‐borne fungal organism. B. dermatitidis is introduced into the host by inhalation of infective spores. This results in a primary lung infection. Dissemination of the organism occurs by lymphohematogenous means and may involve any body tissue. Other tissues frequently involved are the skin, male genitals, lymph nodes, eyes, and bone. Osteomyelitis was diagnosed in nine dogs with disseminated blastomycosis. Six of nine dogs had solitary bone lesions. All nine dogs had a clinical lameness. Only two dogs had clinical signs of respiratory disease. A total of 25 bone lesions were observed in these dogs, with 19 lesions located in the tubular bones of the extremities. Typically, osteolytic lesions appeared at the ends of long bones; only two lesions were observed proximal to the stifle. No extremity lesions were seen proximal to the elbow. Approximately half of the bone lesions had an associated periosteal response or soft tissue enlargement, while no sinus or fistulous tracts were observed. Blastomycosis was confirmed in all dogs by a positive reaction to the gel immunodiffusion test and/or identification of the B. dermatitidis organism. 0rganisms were retrieved and identified from a variety of tissues, including three bone biopsy specimens and one joint aspirate. In seven of nine dogs, initial diagnosis was by identification of the organism. The radiographic differential diagnoses included primary bone neoplasia, soft tissue neoplasia with secondary bone involvement, fungal osteomyelitis, and bacterial osteomyelitis. These differential diagnoses were based on the distribution, localization, and aggressiveness of the lesions observed. The 32 percent incidence of blastomycosis‐associated osteomyelitis reported here is significantly higher than reported previously.