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Causes of Moon Blindness

January 23, 2011

Learn more about this auto-immune disease commonly referred to as moon blindness that affects horses’ eyes from the American Association of Equine Practitioners.


What exactly is “moon blindness,” and what causes it?


“Moon blindness” is another term for Equine Recurrent Uveitis, called ERU for short. ERU is a disease seen only in horses. It is an auto-immune disease, meaning the immune system, rather than defending the horse from foreign invaders like bacteria or fungi, attacks the inside of the eye. Although there is a tremendous amount of research going on about ERU, no one is sure of the exact initiating cause. There is some evidence that previous infection with the leptospira bacteria can lead to ERU, but other causes have been implicated, including trauma, flu virus, parasites and strangles.

Regardless of the initial cause of ERU, the end result is the same: self-perpetuating immune system attack of the inside of the eye that can lead to blindness. The most common ways that ERU causes blindness are cataract formation and retinal detachment. Early detection of the disease is key for successful management, although some horses continue to deteriorate despite the best treatments available.

The clinical signs of ERU can be very subtle in the early stages (cloudiness or mild tearing) or very dramatic (severe ocular pain, squinting and blindness). Any horse with an abnormal-appearing eye (cloudy, glassy, red) or signs of eye pain (squinting, tearing) or changes in vision, should be examined by a veterinarian.

-Dr. Amber Labelle a member of the American Association of Equine Practitioners
Ohio State University, Columbus, Ohio