Intracerebral Malignant Plasmacytoma in a Mule Deer (Odocoileus hemionus).

A wild, mature, gravid female mule deer (Odocoileus hemionus) was presented with marked neurological signs, including abnormal behaviour, circling and incoordination. The animal was humanely destroyed and submitted for diagnostic investigation. Grossly, a well-demarcated, 3 × 3 × 3 cm intracranial mass replaced the left olfactory bulb and frontal lobe. Histologically, there was a highly cellular, infiltrative and unencapsulated neoplastic mass of round cells with eccentrically located nuclei. Neoplastic cells were immunohistochemically labelled for lambda immunoglobulin light chain. Clusters of CD20- and CD79a-positive cells were scattered throughout the tumour and CD3- and Iba1-positive cells diffusely infiltrated the neoplasm. Ultrastructurally, the cytoplasm of the neoplastic cells had prominent rough endoplasmic reticulum with a variable degree of dilation. Histopathological, immunohistochemical and electron microscopical results were diagnostic for a solitary extramedullary plasmacytoma. Intracranial neoplasms are rarely diagnosed in wildlife species, but they should be included in the differential diagnosis for potential causes of central nervous system disease.