Periocular basal cell carcinoma in a donkey: case report.

In horses, basal cell neoplasms are uncommon benign tumors with slow growth mostly reported in the skin and eyelid. Malignant presentation is even more uncommon. This report describes a basal cell carcinoma (BCC) in the periocular region of a 10-year-old donkey. The animal presented good body condition and a neoformation in the right eye, noted 3 months prior to hospital admission. At the ophthalmological examination, the mass was identified above and adhered to the cornea, measuring 3.0 × 2.5 × 1.5 cm. Cytology revealed a suspicion of a malignant epithelial neoplasm, characterized by groups of cohesive cells that occasionally exhibited a palisade arrangement. Due to financial limitations, and advised of the higher risk of recurrence, the owner did not approve the tumor excision in association with chemotherapy and opted for exenteration of the globe. The procedure was performed under general anesthesia without complications, and with an appropriate surgical margin. On histopathology, the tumor was characterized by infiltrative, non-encapsulated neoplastic proliferation supported by thin fibrovascular stroma and organized into islands and nests, occasionally exhibiting a palisade pattern and delimiting spaces containing homogeneous eosinophilic material. The cells were polygonal with indistinct cell borders, eosinophilic cytoplasm, round to oval nucleus with a single, prominent nucleolus. On immunohistochemistry, the tumor was positive only for pan cytokeratin and negative for vimentin, Sox-10 and Melan-A. After 12 months of surgical excision, no recurrence or complications were related. This report emphasizes that in spite very uncommon, basal cell carcinoma may be included in current differential diagnoses of periocular neoplasms in horses and donkeys.