Blastomycosis as a cause of lameness

A 2-year-old spayed female boxer was presented with a 3-week history of right front lameness. The owner had noticed a firm swelling on the lateral aspect of the distal part of the ante-brachium, just proximal to the radiocarpal joint. Radiographs of the limb showed an oval area of bone lysis in the distal part of the ulna approximately 5 cm long by 2 cm wide (Figure 1).

Figure 1.

A lytic lesion in the distal region of the ulna of a 2-year-old boxer caused by Blastomycosis osteomyelitis.

The lesion was surgically curetted and samples from it submitted for histopathologic analysis. The remaining defect was filled with a combination of autogenous cancellous bone from the proximal part of the ipsilateral humerus and a silica-based synthetic particulate bone substitute (Consil; Nutramax Laboratories, Edgewood, Maryland, USA) (Figure 2).

Figure 2.

Postoperative view of the lesion after surgical curettage and packing with autogenous cancellous and synthetic graft material.

The histopathologic diagnosis on the curetted material was pyogranulomatous osteomyelitis due to Blastomyces dermatitidis. The owner declined treatment options and the dog was euthanized.

Blastomyces dermatitidis is a soil-inhabiting fungal organism that causes systemic infection by inhalation. One of the obstacles to the diagnosis of this condition is the recognition that B. dermatitidis is endemic to most parts of southern Canada. Most veterinary textbooks commonly found in North American small animal hospital libraries are authored and published in the USA and, without exception, list the disease as endemic to the valleys of the Mississippi, Missouri, and Ohio rivers, as well as the mid-Atlantic states (1–4). In fact, human and animal cases have been documented in every Canadian province, except Prince Edward Island and Newfoundland (5), and the fungus was demonstrated as being endemic in Saskatchewan more than 2 decades ago (6).

Areas of organic soil, especially near water, may contain the fungus, which can be inhaled through the normal sniffing activity of most dogs. Young, large breed males are most commonly affected. From an initial fungal pneumonia, the organism produces pyogramulomatous inflammation wherever it spreads, including lymph nodes, skin, eyes, central nervous system, prostate, and bone (1–4). Osteomyelitis is found in 10% to 15% of cases, most often as a solitary lesion in the epiphyseal region of long bones distal to the stifle or elbow (4). These lesions involve the forelimbs more often than the hindlimbs. Radiographically, osteolysis is prominent in most cases with periosteal proliferation and soft tissue swelling being present in about 50% of the cases, although not noted in this case (2,4).

Itraconazole, 5 mg/kg bodyweight, PO, q24h for 6 to 8 wk, or until signs of active disease are no longer present, is the therapy of choice. Seventy-five percent of cases will be cured, while the remainder will develop respiratory signs or will relapse weeks to months later and die (1–4). The prognosis is good if the dog does not develop serious respiratory signs in the first 10 d and if fewer than 3 body systems are affected by the fungus (1–4).