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Mycobacterium malmoense as an extrapulmonary pathogen of cats
  1. U. Hetzel, Dr rer nat Dr med vet FTA Pathologie1,
  2. C. E. Dixon, BVSc1,
  3. A. E. Just, BVSc2,
  4. R. J. Birtles, PhD3 and
  5. A. Kipar, Dr med vet habil DiplECVP FTA Pathologie1
  1. 1Division of Veterinary Pathology, School of Veterinary Science, University of Liverpool, Crown Street, Liverpool, L69 7ZL, UK
  2. 2Nantwich Veterinary Hospital, Nantwich, Crewe Road End, Nantwich CW5 5TD, Cheshire, UK
  3. 3School of Environment and Life Sciences, University of Salford, Greater Manchester M5 4WT, UK
  1. 1E-mail for correspondence: akipar@liverpool.ac.uk

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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.

FIG 1:

Cat with soft tissue swelling at hind limb. (a) Radiograph of left tibia, showing the soft tissue swelling (*), the focal cortical and medullary sclerosis with loss of trabecular bone structure (arrow) and a 2 mm × 3 mm radiolucent lytic lesion (arrowhead). (b) Radiograph of unaltered right tibia for comparison

FIG 2:

Histopathology of soft tissue swelling between muscles of left hind limb. (a) Granulomatous steatitis with multinucleated giant cells and focal lymphocyte aggregates (arrow). Ad – adipocyte. HE stain. Bar = 40  ∝ m. (b) Closer view of multinucleated giant cell and epithelioid macrophages containing bundles of long, slender unstained bacteria (arrows). HE stain. Bar = 10  ∝ m. (c) The bacteria are acid-fast in the …

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