Diffuse leptomeningeal malignant histiocytosis in the brain and spinal cord of a Tibetan Terrier.

An 8-year-old male Tibetan Terrier showed prolonged astasia, complete paralysis, ticlike signs, and seizure and died 2 months after the onset of symptoms. Histopathologically, there was moderate to severe infiltration of pleomorphic histiocytic mononuclear cells bilaterally in the basiarachnoidal and ventricular areas of the brain. The spinal dura mater, arachnoidal space, and leptomeninges were also affected by infiltrative proliferation of these mononuclear cells. The infiltrating cells had the morphologic characteristics of histiocytes but exhibited moderate pleomorphism and atypia, with abundant mitotic figures. With immunohistochemistry and lectin histochemistry, most of the infiltrating cells were positive for lysozyme and lectin RCA-1 and negative for glial fibrillary acid protein, suggesting that they were of monocytic/histiocytic-origin. Positive proliferating cell nuclear antigen immunostaining demonstrated that most nuclei of the histiocytic cells were in the S phase of the cell cycle, consistent with a proliferating population of cells. Based on these findings, the case was diagnosed as diffuse leptomeningeal malignant histiocytosis.