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Equine Health Resource

Equine Recurrent Uveitis, ERU

Synonyms: Moon Blindness, Periodic Opthalmia, Recurrent Opthalmia

Although poorly understood, ERU is the most common cause of blindness in horses. It is thought to be an auto-immune disease. In its classic form, the body’s defenses attack the internal structures of the front chamber of the eye (the uvea, which is the colored iris) causing the ciliary muscles to spasm and constrict the pupil.

Why the immune system gets confused and attacks the eye is not completely understood. The bacterial infection Leptospirosis may be involved. In some cases, it results from an underlying eye injury and it can appear after other illnesses. In most cases the cause is never determined, however there may be a genetic component.

Regardless of the cause, a cycle of inflammation and damage begins, including adhesions of the iris to the lens (posterior synechia). “Active” periods of pain and inflammation (that usually last two to three weeks) are interspersed with periods of relative “calm.”

ERU can end in the destruction of the eye, including detachment of the retina and blindness. It is notoriously difficult to treat. It tends to recur after treatment ends, and often makes surprise re-appearances later. This condition is much more common in the Appaloosa breed. It is important to consider this condition in any case of stubborn, repeated or puzzling eye inflammation.


  • Will my horse need to be treated for life?
  • What are the options for treatment?

    Treat eye conditions promptly, aggressively and completely. Be on the lookout for this condition in Appaloosa horses. Treat systemic illness conditions completely. Monitor the eyes for subtle signs of inflammation and contact your vet promptly, before the condition progresses.

    Some horses require life long monitoring and treatment. Use a good quality fly-mask with UV protection and decrease exposure to direct sunlight. Maintain good barn management to reduce dust and other debris.

    Helpful Terms & Topics in HSVGWritten, Reviewed or Shared by Experts in Equine Health

    Helpful Outside ResourcesCredible Equine Health Information on the Internet

    Author: Doug Thal DVM Dipl. ABVP


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