Cytology and histopathology of canine leproid granuloma syndrome

To describe the histopathology of canine leproid granuloma syndrome.

Histological examination of biopsy specimens taken from dogs with leproid granuloma syndrome. Biopsies were acquired from four veterinary pathology practices after initial examination showed acid-fast bacilli or lesions suggestive of a mycobacterial aetiology. Tissue from 30 cases formed part of a retrospective survey while a further 7 cases were obtained prospectively.

Tissue samples were fixed in formalin and paraffin embedded. Sections were stained with haematoxylin and eosin and Ziehl-Neelsen stains. Slides were evaluated for the type of inflammatory response and the number of bacteria. In a few cases smears made from crush preparations and needle aspirates were stained with DiffQuik and acid-fast stains.

The common cytological feature seen in DiffQuik stained smears was numerous, often spindle-shaped, macrophages with variable numbers of lymphocytes and plasma cells and lower numbers of neutrophils. Usually few to moderate numbers of medium length bacilli were detected within macrophages or extracellularly. Histologically, lesions were chiefly pyogranulomatous within the subcutis and dermis. Pyogranulomas were composed chiefly of epithelioid macrophages and neutrophils but plasma cells and small lymphocytes were scattered throughout the lesions in which giant cells were seen. Lesions were pyogranulomatous in 36 cases and granulomatous with small suppurative foci in one. The numbers of acid-fast bacilli present were very low to low in 22 cases, moderate in 6 and regionally numerous to numerous in 9. Bacteria were pleomorphic, ranging from long, slender filaments in parallel sheaves to short and variably-beaded bacilli or highly beaded to coccoid organisms. The morphology was more uniform in DiffQuik stained smears than in fixed tissue sections.

The pathology of canine leproid granuloma syndrome is highly uniform and is suggestive of saprophytic mycobacterial involvement. The morphological diversity of acid-fast bacilli probably results from differences in staining characteristics rather than indicating different species of Mycobacterium. While approximately half of the cases had only few organisms present, the veterinary practitioner using a Romanovsky stain such as DiffQuik on aspirated material might expect to obtain a rapid diagnosis in many cases. This would allow differentiation of the syndrome from neoplastic and other diseases of the skin and direct appropriate treatment at the initial presentation.