Moon Blindness In Horses: Understanding Equine Recurrent Uveitis

Lonely horse pasturing on the field in the nighttime

For a well-adjusted horse, few senses are as crucial to health and well-being as his sight. Sharp vision ensures that he can carefully choose his path through a paddock, recognize his handler or handlers, find his way to food and water, and stay aware of boundaries and dangers. Equine recurrent uveitis – also called ERU or moon blindness in horses – is one of the most prevalent health threats to a horse’s vision. This persistent immune system reaction causes blindness in slightly more than half of the horses it affects and can also potentially cause discomfort, pain, and partial vision loss in those that do retain their vision.

Why Is It Called Moon Blindness?

It would be logical to assume that the colloquial name for this equine eye issue comes from bright light exposure – e.g., looking up at the sun or moon too long while riding or grazing. Another good guess would be that the name is derived from the milky-white appearance of a horse’s ERU-affected eye. The truth, however, is a little more unusual: when the disease was first cataloged in the 1600s, the farmers and horse-handlers at the time believed that equine recurrent uveitis was related to the phases of the moon. This belief and observation, although incorrect, was so widely held that the disorder was named after the moon. 

With a similar ailment being described as far back in history as the pyramids of Giza, where it was said to have affected cavalry horses at the time, ERU may be one of the first recorded horse-related diseases. Today, the colloquial “moon blindness” remains as well-known as the “correct” scientific term, Equine Recurrent Uveitis. 

What Causes Moon Blindness In Horses?

In the many years since it was first recorded, science has safely ruled out moon phases as the culprit behind ERU. So what does cause this persistent eye inflammation? Some other early rumors cautioned against damp environs for horses – such as marshy pastures, stalls with damp bedding or feed, or fields irrigated with town sewage. As it turns out, it’s not the water itself that is linked to moon blindness, but rather spiral-shaped bacterial organisms that use it to travel and breed. The bacterial strains Leptospira and Spirochetes, the latter includes the tick-borne Lyme disease, are typically linked to episodes of ERU. It is not the bacteria itself responsible for the disorder, but it does play a part. The bacterial presence acts as a catalyst, triggering the telltale ongoing inflammation episodes that damage structures within the horse’s eye or eyes. 

Ensuring that equine bedding, fields, and feed are as dry and free of unwanted moisture as possible will help lower ERU risk in horses. However, another red flag that even the most careful equine professional can’t overcome is genetics.

Does Moon Blindness Affect All Horses?

Nice young appaloosa horse running

Any horse can become infected with equine recurrent uveitis, but some breeds are far more susceptible than others. For example, according to extensive research, Appaloosa horses are approximately eight times more likely to be affected by moon blindness. They are also more likely to experience it bilaterally (in both eyes) when it does occur. 

Scientists have connected ERU and the genetic strain that causes white spotting or “leopard spotting” in Appaloosas. Studies indicated that horses that carry two copies of the gene are exponentially more likely to experience moon blindness over counterparts with one copy, or no copies, of the gene. Researchers are quick to caution that this may be a matter of correlation rather than causation, but where one appears, the other is likely to be found. Much like undesirable genetic disorders in canines, selective breeding can and does play a role in reducing the possibilities of ERU potential. Still, for breeders looking to achieve a certain equine look or lineage, the rewards may outweigh the risks.

Is Moon Blindness Contagious Or Zoonotic?

While it can be a difficult prognosis for an affected horse, the good news is that he won’t typically pose a danger to his paddock-mates or human handlers. ERU is not typically transmitted from horse to horse through touch. However, the triggering bacteria can be passed on through an affected horse’s urine, shared food, or shared water, as well as through human handler touch, should they fail to wash their hands from animal to animal. Thankfully, ERU is not zoonotic – which means it isn’t capable of “jumping” from horse to human. You should still take care to avoid touching an affected eye and a non-affected eye on the same horse. If the condition doesn’t present as bilateral, to begin with, a human handler can unwittingly spread it this way. 

Can Moon Blindness In Horses Be Cured?

Unfortunately, because ERU is closer to an autoimmune disease (rather than an infection that can be targeted directly), there is currently no cure. This is not to say that there aren’t various treatments that can help affected horses achieve better vision, longer use of their eyes, and a reduction of ocular discomfort, but the underlying condition will always remain. One of the best ways a horse handler can help their horses is through observation, keeping track of telltale signs when they appear. 

If a horse experiences redness, weeping, seems visually disoriented, or presents any other eye issue without a known cause, a handler should record it on a calendar. If eye issues are observed regularly that “come and go,” you should consult an equine vet for a more in-depth examination to rule out (or determine) if moon blindness is the cause. 

Treatments for horses diagnosed with ERU can vary, depending on the stage and severity of the inflammation. For the vast majority of affected horses, approximately 9 in 10, medications such as topical eye drops and oral medications are used to treat resulting inflammation, leading to discomfort, pain, and damage within the eye. These medications may be steroidal or non-steroidal, depending on the individual horse’s tolerance and potential interaction with other ongoing medications. 

Sometimes, a sustained-release implant may also be used; this is inserted into an anesthetized horse’s eye under a cut flap. Once inserted, the implant remains in place to deliver ongoing medication. While the initial procedure is unavoidably invasive, it ensures that the horse receives a steady dose of necessary inflammation-reduction medication without the need for constant drops or pills.

Uveitis vs. Recurrent Uveitis In Horses

Herd of Horses Back to the Pasture in the Countryside

Inflammation in the eye is only considered ERU or moon blindness when ongoing, hence the “recurrent” part of the term. Upon seeing the trademark milky color in their horse’s eye or eyes, some handlers jump to conclusions and assume their equine is affected by ERU. However, there are several reasons why inflammation can occur in the eye – everything from errant flying insects out in the field to a stubborn branch whipping across his face during a brisk ride. Pinning down the cyclical or recurrent nature of equine eye inflammation is a must for proper diagnosis. Otherwise, there can be a lot of worry and anxiety over a temporary issue.

Equine recurrent uveitis will:

  • Appear more than once: Some equine experts liken it to rheumatoid arthritis – having joint pain once or twice isn’t necessarily a cause for concern. Still, weekly episodes point to an ongoing issue.
  • Have no known physical cause: If a horse has had a hard run through a dusty field or an overgrown trail, give it some time and examine the area around the eye carefully for physical scratches or debris before fearing the worst.
  • Flare-up as an immune system response: This means it can be triggered by other illnesses, such as equine blood-borne viruses, or even stress from a demanding show schedule, challenging vet visit, or a move. Rule out external factors of the physical, health, and emotional variety before considering the potential of moon blindness.
  • Be more of a risk factor in damp or wet environments: Ensure that feed and water is changed regularly and promptly and that stalls are kept clean and dry. Again, as opportunistic bacteria trigger it, it is impossible to keep a horse 100% safe, but the risk factors can and should be lowered whenever possible. 

If a horse is suspected of suffering from moon blindness, the best course of action is to consult with an equine veterinarian. Always be as comprehensive as possible when reporting observed eye inflammation incidents, and adhere to any medication regimens as prescribed. If more advanced treatments are needed, up to and including eye surgery or eye removal, an equine vet will be able to offer the best guidance and options for the affected horse. 

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Sources Cited:

1)  Young, Amy. “Equine Recurrent Uveitis.” UCDavis |Veterinary Medicine Center for Equine Health (ceh.vetmed.ucdavis.edu), March 3, 2020, https://ceh.vetmed.ucdavis.edu/news/equine-recurrent-uveitis. Accessed April 15, 2021.

2)  Thomasy, Sara M., DVM, PhD, DACVO, Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California-Davis. “Equine Recurrent Uveitis.” Merck Manual Veterinary Manual (merckvetmanual.com), June 2019, https://www.merckvetmanual.com/eye-diseases-and-disorders/equine-recurrent-uveitis/overview-of-equine-recurrent-uveitis. Accessed April 15, 2021.

3) Oakford, Glenye Cain. “Your Horse Has Uveitis? Here’s What You Need T