Equine Sarcoid Arising From the Medial Caruncle With Invasion of the Third Eyelid in a Quarter Horse Gelding.

Objective: To describe the clinical and histologic features of a sarcoid arising from the medial caruncle with spread to the third eyelid of a horse.

Animal studied: A 20-year-old Quarter Horse gelding.

Procedures: An incisional biopsy of the medial caruncle mass was initially performed, with histopathology suggesting a sarcoid or other spindle cell neoplasm. The mass grew rapidly following the biopsy, expanding to affect the third eyelid and adjacent superior palpebral conjunctiva. A complete third eyelid and medial caruncle excision with resection of affected palpebral conjunctiva and adjunctive cryotherapy was subsequently performed.

Results: Histopathology revealed the mass to be consistent with an equine sarcoid as evidenced by a densely cellular population of neoplastic spindle cells arranged into interlacing bundles with collagenous stroma and with hyperplastic overlying epithelium extending into the neoplasm as thin rete ridges. Over 90% of neoplastic cells showed strong hybridization signaling for bovine papillomavirus types 1 and 2, supporting an underlying viral cause consistent with the diagnosis of a sarcoid.

Conclusions: This paper describes the first reported case of an equine sarcoid arising from the medial caruncle with spread to the third eyelid, treated with third eyelid and medial caruncle excision and adjunctive cryotherapy. Veterinarians evaluating horses with masses affecting the medial caruncle and third eyelid should consider sarcoids as a differential diagnosis. Rapid growth of cutaneous sarcoids following incisional biopsy is common, and the same may be true for sarcoids that arise from the medial caruncle or third eyelid.