A "down under" lesion on the muzzle of a dog

A 10‐year‐old, castrated, male Labrador Retriever was presented to a local veterinary practice for investigation of a firm, deeply pigmented, alopecic, subcutaneous mass (8 mm in diameter) on the left side of the muzzle. A fine‐needle aspirate of the mass was submitted for cytologic evaluation to the University of Florida. Microscopically, the preparation contained a predominant population of histiocytes that contained variable numbers of intracytoplasmic, negative‐staining, filamentous structures consistent with Mycobacterium sp. A presumptive diagnosis of canine leproid granuloma syndrome was based on the cytologic findings and location of the lesion. Acid‐fast staining revealed bright pink, acid‐fast organisms within the histiocytic cells, supporting the diagnosis. The bacteria were not detected in histopathologic sections or by a polymerase chain reaction (PCR) test 1 week later, however, possibly because of spontaneous remission. Canine leproid granuloma syndrome is a common disease in Australia, but is uncommon in dogs in North America. It is caused by a novel, unnamed Mycobacterium species and usually affects the skin and subcutaneous tissues of the head and ears. A diagnosis usually can be made in Wright's‐Giemsa and acid‐fast–stained cytologic specimens, however, definitive diagnosis requires PCR testing at a specialized laboratory.