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A six-year-old female neutered domestic longhair cat from Crewe, Cheshire, UK, presented with a non-painful soft tissue swelling in the left hind limb. The cat had regularly spent time outdoors and was a known hunter. It was in good body condition without other clinical symptoms, and negative for Feline Immunodeficiency Virus (FIV) and Feline Leukaemia Virus (FeLV) by serology. The swelling was palpable within the muscles of the caudal aspect of the tibia. Radiography also revealed cortical and medullary sclerosis, and a 2 × 3 mm radiolucent lesion within the tibia (Fig 1a,b). Exploratory surgery showed a friable, fatty mass extending onto the tibia and infiltrating between the gastrocnemius and digital flexor muscle bellies. Histopathology identified the soft tissue swelling as a granulomatous steatitis, composed of epithelioid macrophages and multinucleated giant cells, with some neutrophils, lymphocytes and plasma cells (Fig 2a). The Ziehl-Neelsen stain demonstrated numerous intra- and extracellular acid-fast, up to 30 ∝ m long, slender bacteria (Fig 2b,c). The bone biopsies exhibited the same granulomatous inflammation, with collagen fibre deposition between bone fragments. The adjacent skeletal muscle was not involved.