Case description: An 11-year-old Arabian mare was presented for investigation of a visible, pale-colored intraocular mass in the right eye.
Clinical findings: An intraocular mass was detected clinically and ultrasonographically as originating from the superior temporal quadrant of the ciliary body and iris and causing secondary glaucoma. The echodense mass was occupying the majority of the vitreous chamber and extended into the anterior chamber. The left eye appeared normal.
Treatment and Outcome: Enucleation was recommended for therapeutic and diagnostic purposes. No adjuvant treatment was given. Histopathological examination demonstrated a benign intraocular teratoid medulloepithelioma located at the ciliary body. Immunohistochemical studies showed that neoplastic cells were positive for vimentin, S-100 protein, neuron-specific enolase (NSE), and nestin and negative for glial fibrillary acidic protein (GFAP). Electron microscopy revealed abundant cellular matrix and blood vessels surrounding tumor cells, which had indented, round to oval nuclei. There were also apoptotic bodies and cells containing melanosomes of variable shape and size. Eight years later, the horse has had no recurrence and maintains normal vision in the left eye.
Clinical relevance: This is the first report of a benign teratoid intraocular medulloepithelioma in an adult horse and the ultrastructural and immunohistochemical characterization of a teratoid medulloepithelioma in this species.