Malignant histiocytosis of the brain in three dogs.

Three dogs (two Rottweilers and a Flat-coated retriever) showed various neurological signs, including apathy, depression, circling, a partial decrease in functions associated with cranial nerves, seizures, hyperaesthesia, proprioceptive deficits, and increased spinal reflexes. In all three cases, necropsy revealed a solid, distinct, white mass in the brain and multiple, poorly demarcated, firm nodular proliferations in the lung; in one case the liver was also affected. Histopathological examination showed loosely aggregated, pleomorphic cells, with abundant eosinophilic cytoplasm. The neoplastic cells sometimes contained vacuoles or phagocytized cells. Binucleated and multinucleated giant cells, and mitotic figures, were common. Immunohistochemically, the tumour cells reacted strongly for lysozyme and vimentin, but there was no reaction for S-100 protein, cytokeratin, CD3 or CD79a. The histological and immunohistochemical examinations indicated a histiocytic origin of the tumour cells and malignant histiocytosis was therefore diagnosed.