Equine Recurrent Uveitis (ERU) is a troubling disease for horses which can affect and eventually destroy vision in one or both eyes. It was previously referred to as “moonblindness”, because it was thought for many years that during an actute phase of the disease had to do with the cycling of the moon. A common is myth is that only appaloosa horses are affected by this disease; in reality while they are more predisposed to it, it can affect horses of any breed.
The root cause of uveitis is not known, however scientists and vets believe it can be developed/triggered by any one or combination of the following:
- Bacterial organisms (Leptospira, Brucella, Streptococcus)
- Viral agents (equine influenza, equine viral arteritis, parainfluenza type 3)
- Parasites (Onchocerca, Strongylus, Toxoplasma)
- Miscellaneous (endotoxemia, tooth root abscesses, neoplasia)
(List excerpted from NC State University, leaders in ERU research)
While horses can be exposed to the viral agents or bacterial organisms, they may never be subject to ERU. A “trigger” (such as traumatic injury to the eye) is what causes the immune system to respond, causing antibodies to travel to the site of the injury, in this case the eye, and attempts to repair the damage. It is this autoimmune response that is considered a “flare up” where horses exhibit the signs of ERU. Painful constriction of the pupil, watery eyes, squinting or keeping the eye shut completely are all signs. After the initial flare up, the horse can go days, months or even years without another flare up. During the flare up, horses are generally put on a course of corticosteroids and given eye drops multiple times a day.
As per its name, ERU is recurrent, which generally means most horses will experience flare ups throughout the remainder of their lifetime. ERU tends to cause more damage to the eye during each flare up, making it degenerative to the point of vision loss in one or eventually both eyes. A small percent of horses experience only one or two flare ups in their lifetime, whereas a horse with a very aggressive case of ERU could experience flare ups every 6-8 weeks.
A key factor to controlling the disease is management of the horse both during flare ups and during latent phases. Keeping a journal of the horse’s schedule (more specifically, any changes to it), the weather, medications, etc. are all ways to uncover what your horse is sensitive to, therefore helping avoid conditions which may trigger a flare up. Many horse owners choose not to turn their horses out during the daytime, especially during a flare up when sunlight can cause both pain and exacerbate the condition. Some medications specifically note to avoid daylight turnout while the horse is receiving them, as the sun can damage the eye even further.
A popular item among ERU horse owners is the Guardian Mask which has been specifically developed for horses with uveitis and other eye conditions. Many horse owners have annedotally reported a decrease in flare ups after using this mask. While it has an unusual appearance, it protects the horse’s eyes from 95% of the sun’s rays. It is available for both pasture and riding use.
Another big step in the right direction is research done by NC State University, specifically Dr. Brian Gilger and his colleagues. Dr. Gilger has developed an implant for horses suffering from uveitis, which slowly releases medication into the eye over a five year period. This is called the cyclosporine implant, and has been done by major equine hospitals all over the United States. This surgery will also be performed on a small sample of horses in Canada this year, which many hope will mean more widespread availability for horses in the country. While the implant is not considered a “cure”, horse owners are reporting a dramatic decrease in flare ups after having the surgery done.
For horses that do not respond to conventional treatment, blindness can result. Many owners opt to have enucleation performed, which is the removal of the eye. Once the eye is removed, the horse will cease to experience pain and the resulting “headaches” from flare ups. Many horses have continued to be ridable, and some have successful competitive careers once adjusting to seeing from one eye.
ERU is a very damaging and painful disease for horses, and upsetting for their owners. Fortunately, many horses can live a long and happy life with proper management of this ocular disease.