Choroid Plexus Papilloma of the Fourth Ventricle of the Plexus in a Horse

An 11-year-old Andalusian male horse was presented with a primary complaint of progressing hind limb ataxia. Postmortem examination of the brain revealed a bilateral, grayish, verrucose neoplasm arising from the cerebellopontine angle, which was diagnosed as a choroid plexus papilloma. Histopathologic examination showed a delimited and expansive growth with a branching arboriform pattern and papillae lined with a single layer of cells covering a congestive fibrovascular stroma. The cytoplasm of epithelial cells lining the choroid plexus was strongly positive for pancytokeratins and weakly for glial fibrillary acidic protein (GFAP). To our knowledge, this is the first macroscopic, histopathologic, and immunohistochemical description of a choroid plexus papilloma affecting the fourth ventricle of a middle-aged horse.