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Equine recurrent uveitis (ERU), also known as moon blindness, is a chronic condition characterized by eye inflammation, drainage, pain, and sensitivity to light. Affected horses can have flare-ups as often as once a month, leading to the outdated belief that the condition had some connection to the moon’s phases.

As the disease progresses, the retina and other structures of the eye begin to degenerate. Glaucoma and cataracts also occur frequently in affected horses, limiting sight. ERU is the leading cause of blindness in horses.

ERU has not been definitively linked to an identified virus or bacteria, and the disease may have an autoimmune rather than an infectious cause. No protective therapy exists, and once a horse is affected, flare-ups can be treated but not prevented. Horses with leptospirosis have a greatly increased risk of getting ERU, as do horses of Appaloosa, Hanoverian, and other warmblood breeding. In the general equine population, ERU affects about 10 to 15% of horses.

In past years, the only treatment option was to medicate the horse’s eyes several times a day, a procedure that was time-consuming and challenging for some owners and caregivers. Recently, this chore has been replaced for some horse owners by the development of an implanted device that delivers cyclosporine directly to the affected tissues.

The implant has been tested for several years and results continue to be positive, with horses achieving therapeutic levels of the drug by 30 days after implantation. About 75% of horses with implants have retained sight for three to five years in affected eyes. This is a higher percentage of sight than in horses that do not have implants. The procedure also significantly decreases the frequency of flare-ups. Implants are effective in treating ERU for at least three years and possibly longer before they must be replaced.

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