Equine recurrent uveitis (ERU), also known as moon blindness, is the most common cause of blindness in horses worldwide. It affects 2 to 25 percent of horses globally, with 56 percent of affected horses eventually becoming blind. More than 60 percent of affected horses are unable to return to previous levels of work. ERU is most often characterized by repeated episodes of inflammation of the uveal tract of the eye (the middle layer), involving one or both eyes. A subclinical manifestation, known as insidious uveitis, does not present as outwardly painful episodes, and instead is consistent low grade inflammation (not episodic) that causes cumulative damage to the eye. Cumulative damage caused by ERU can lead to cataracts, glaucoma, and eventually blindness. Although not all horses that experience a single episode of uveitis will develop ERU, they are at risk for disease.