Third eyelid cartilage eversion in an adult mare

To describe the first report of third eyelid cartilage eversion in an adult American Quarter Horse mare. A 22-year-old American Quarter Horse mare presented to the University of Missouri Veterinary Health Center Equine Hospital for a 2-week history of a third eyelid cartilage abnormality of the left eye with no known recent trauma. Complete ophthalmic examination revealed third eyelid cartilage eversion of the left nictitans. The abnormal scrolled cartilage was surgically excised using a handheld cautery unit and submitted for histopathologic evaluation. Histopathologic findings displayed normal third eyelid cartilage, without evidence of neoplasia or inflammation. Mucosal hyperplasia and increased vascularity of the submucosa were observed. The horse healed well after electrocautery excision and normal third eyelid conformation was restored. To the authors' knowledge, this is the first report of an acquired, presumed spontaneous, third eyelid cartilage eversion in a horse.