Editor’s Note: This article was revised by the author to reflect new and updated information in November 2017.

Equine recurrent uveitis is one of the most frequent causes of equine blindness worldwide

Horses exhibit eye pain for a variety of reasons and to varying degrees. As we’ve discussed previously, the most common cause is corneal ulceration. In this article, we’ll discuss uveitis, which requires a very different kind of treatment than that used for ulcers. Uveitis means inflammation inside the eye; it can be combined with other words to describe exactly where in the eye the inflammation occurs. While acute uveitis occurs suddenly and might be caused by a systemic disease or corneal ulcer, this type of uvetitis gets better with treatment of the causative condition and does not occur again. Equine recurrent uveitis (ERU) is an autoimmune disease that can be controlled, but occurs again and again. Equine recurrent uveitis (ERU) can’t be cured; it is named for its tendency to recur in one or both eyes. The disease has also been called “moon blindness” and “periodic ophthalmia” because of its erratically recurrent nature, compared by early observers to the phases of the moon. It is a result of an overly aggressive immune system, an autoimmune disease in which the eye’s own immune system reacts against normal tissue early in the disease. Various causes are implicated, with Leptospira and other bacteria suspected to be the cause in many horses in the United States and Europe. Equine recurrent uveitis is one of the oldest diseases known in veterinary medicine. Even ancient Egyptian horses might have been affected, as described in hieroglyphics in the tombs of the pharaohs. In modern times, ERU is one of the most frequent causes of equine blindness (5-25% of U.S. horses might be affected) with costs being high in the United States. Luckily, the great majority are mild or even unrecognized forms of the disease, requiring occasional or no therapy.

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