Case summary A 3-year-old neutered male indoor British Shorthair cat was referred for a 2-week history of intermittent right forelimb lameness. Radiographic examination showed a diaphyseal monostotic, expansile, fusiform, lytic lesion in the right ulna. CT further defined the lesion and also demonstrated ipsilateral pulmonary consolidation. Histology was conclusive of osteomyelitis, and microbiology and fluorescence in situ hybridisation analysis (FISH) were negative on aerobic and anaerobic bacterial culture, as well as fungal culture. Clinical and radiographic improvement was seen after anti-inflammatory treatment and a short initial period of antibiosis.

Relevance and novel information This is an unusual monostotic diaphyseal cortical location for osteomyelitis in cats and, moreover, may represent a rare case of sterile osteomyelitis. To our knowledge, non-traumatic osteomyelitis in this location in cats has not been reported in the veterinary literature.