Case description: This clinical report describes a 10-year-old female spayed German Shepherd dog cross that was presented with cellulitis of the left proximal forelimb and osteomyelitis of the left proximal humerus, and was ultimately diagnosed with metastatic osteosarcoma.
Clinical findings: The diagnosis of cellulitis and osteomyelitis was made using ultrasound, radiography, cytology and histopathology, all of which were consistent with cellulitis and osteomyelitis. Cultures were negative.
Treatment: The patient was treated using two surgical debridements and long-term broad-spectrum antibiotic drugs. Despite surgical and medical treatment, the dog's condition progressed. A lytic lesion of the left proximal humerus was identified radiographically.
Outcome: One hundred forty-one days after initial presentation, the dog was presented with a non-weight bearing lameness of the left forelimb. An amputation was scheduled. Preoperative computed tomography scan of the thorax revealed gross metastatic disease to the lungs. The patient was euthanatized and a post-mortem examination revealed osteosarcoma of the left proximal humerus with widespread metastasis.
Clinical relevance: To our knowledge, this case is the first reported case of osteomyelitis masking osteosarcoma in a dog. It serves as a reminder to maintain a high index of suspicion when managing cases with a signalment, history and radiographic lesion that are consistent with a primary bone tumour.