Inflammatory pseudotumor in a cat with cutaneous mycobacteriosis

A 5-year-old, castrated male, domestic Shorthair Cat had an ulcerated mass with fistulous tracts on the left hind paw. Homogeneous tan tissue diffusel y infiltrated the dermis and subcutis of the paw and extended proximally so that, short of amputation, complete excision was not feasible. Biopsy spec imens consisted of granulation tissue with marked proliferation of spindle cells. Neutrophils and histiocytic cells were scattered among the spindle c ells. The histiocytic cells had abundant foamy or vacuolated cytoplasm, but features of granulomatous inflammation, such as epithelioid macrophages or granuloma formation, were not observed. The initial impression was inflamm atory granulation tissue, but the degree of fibroplasia prompted inclusion of fibrosarcoma in the differential diagnosis. Cutaneous mycobacteriosis wa s diagnosed when numerous acid-fast bacteria were identified with Kinyoun's stain; Mycobacterium avium was subsequently cultured. The cat was euthanat ized because of lack of response to enrofloxacin therapy. At necropsy, lesi ons were localized to the hind limb. Not only is mycobacteriosis an uncommo n cause of cutaneous masses in cats, but this case was unusual because of t he lack of granuloma formation and the similarity of the mass to a spindle cell tumor.