Moon Blindness

by Katharine Mertens, DVM

     “Mom, why did the moon make that horse go blind?”  my five year old son asked me as we finished up a call to a teenage Appaloosa gelding with a tearing eye.

     Little kids have a way of pointing out the ridiculous.  “That would be a terrible thing for the moon to do, wouldn’t it,” I replied.  “But Mr. Moon didn’t make the horse go blind.  In fact, the horse isn’t blind.  His eye is hurting from a disease called Moon Blindness. We’re going to help the horse feel better, but unfortunately . . . he might go blind eventually.”

     “Will he be blind when the moon comes out again?” my son persisted.

     “No, but I think you’re onto something about where this name comes from.  This disease has been around for ages and ages.  It comes and goes, just like the phases of the moon.  Sometimes the horse is fine, but every now and then he’ll look like that horse did.  One or both eyes will start to have tears, maybe even squint closed a little bit, and the eye can look cloudy.”

     “Cloudy like the moon?”

     “No, I mean the horse will get better and worse, better and worse—just like the moon gets bigger and smaller, bigger and smaller.”

     “Oh.”  My little buddy thought this over for a minute.  “Are we almost home?”

     Indeed.

     As we headed that way, I reviewed the difficult news I had to give the owner we just left.

     Moon Blindness is the common name for the disease we veterinarians discuss as Equine Recurrent Uveitis (ERU).  It is an immune-mediated disease unique to horses, and the leading cause of blindness for these animals.  The Appaloosa breed is most commonly affected, although any breed, of either gender, can present with the disease.  Usual age at time of diagnosis is early to mid-teens.

     Calling the disease immune mediated means that it is perpetuated by the immune system itself, rather than by an outside cause.  Ideally, the immune system releases inflammatory devices to combat invading germs such as bacteria, viruses, parasites, or fungus.  But occasionally, the immune system gets aberrantly stimulated and starts to release these weapons against its own host system. 

     For horses with moon blindness, this aberrant stimulation results in a slippery slope of progressive eye pain, swelling (often noticed as cloudiness), and vision loss.  Some prior infections seem to be linked to ERU, but there has been no demonstrated correlation that certain infections always cause ERU. 

     Without a proven cause, ERU is difficult to treat.  We’re left trying to modulate the immune system itself, with anti-inflammatory drugs used both topically (on or in the eye), and systemically (throughout the body).  A major frustration for horse owners and veterinarians, however, is the lack of success for any treatment to significantly alter the course of the disease towards eventual blindness.

     We do what we can to keep the patients comfortable.  In the case of the Appaloosa I’d just visited, I left him with topical atropine ointment to dilate his pupil, topical steroid ointment to reduce inflammation directly at the eye, and oral banamine paste to reduce inflammation systemically.  I asked to recheck the horse within 2-3 days, and again after two weeks.

     This would constitute the acute phase of treatment.  Dilating the pupil would relieve the painful muscle spasm of a constriction, as well as keep the pupil open as far as possible in case it started to stick to the nearby lens or cornea.  That’s a common complication of eye inflammation: proteins released within the eye create a very “sticky” environment, where the pupil might adhere to adjacent structures and no longer be able to dilate or constrict in response to illumination changes.  This is one cause of blindness.  Other causes include cataract formation in the lens itself, and detachment of the retina at the back of the eye.

     As for chronic treatment of ERU, the jury is mostly out.  What we all want to find is something to prevent or slow down the recurring nature of the disease.  Daily aspirin is often suggested as therapy, but the scientific evidence for effectiveness is lacking.  Horses who have suffered only a few bouts of inflammation may be helped by surgical implants to the eye of immune-modulating drugs, a hospital procedure recently billed at a few thousand dollars.

     No matter what, horses with any signs of eye pain—like the tearing and squinting that prompted the Appaloosa’s call—need to be examined immediately to reduce pain and prevent as much damage as possible.  Also, a thorough eye exam is warranted during any pre-purchase examination to check for signs of prior inflammation and damage within either eye.

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Katharine Mertens, DVM is the owner of Mertens Mammals, LLC, a mobile, equine veterinary practice based in Boring.  You can reach the practice at (503) 663-6400.