Moon blindness, or equine recurrent uveitis (ERU)

Moon blindness has been around a long time. In fact it may be one of the earliest veterinary diseases ever documented. In historical accounts of moon blindness, the disease flareups were thought to be tied to phases of the moon. Over the years, this disease has gone by several names, including periodic opthalmia. The name in vogue right now is equine recurrent uveitis (ERU). ERU is a chronic, painful, eye disease and is the most common cause of blindness in horses. ERU is incurable but early diagnosis and treatment can improve the long term prognosis. Symptoms of ERU vary, but usually inflammation is involved. After the initial occurrence of eye problems, problems recur both eyes repeatedly. With each occurrence, damage to the eye worsens. The prevalence of ERU is around 8% according to a 1992 article in Veterinary Clinics of North America: Equine Practice; 1992). Appaloosas are more likely than other breeds to develop ERU and 4 times more likely to become blind as a result of the ERU.

Symptoms
The first signs of ERU include inflammation in one or both eyes, corneal edema (causing a blue-white color to the cornea), red blood vessels in the corners of the eye, corneal ulcers, eye squinting, blinking, tripping, sensitivity to light/head shaking, runny nose, white spots on the eye, or bleeding in the eye. The eyes of the horse may constrict in darkness rather than dilate.

What causes ERU?
ERU has been a tough nut to crack. In the "days of yore" horsemen astutely noted that ERU was more prevalent in swampy areas and farms with poor drainage. Just a few few decades ago experts believed that ERU was caused by lack of riboflavin in the horse's diet. In relatively recent articles in The Horse magazine, I learned that ERU is thought to arise when an abnormal autoimmune response is triggered and the body attacks the eye and its internal structures. What precipitates this response? The most common indirect cause of ERU is infection with Leptospira, a spiral-shaped bacteria, or spirochetes, that can be found in areas with stagnant water. Other indirect causes can include other kinds of bacteria, parasites, viruses, or trauma. The working hypothesis is that horses contract an infection or sustain trauma, mount an immune-response, and fail to appropriately terminate that response. I read and re-read this article from The Horse magazine which provides detail on the immune process that triggers ERU. It made my eyes cross, but if you are interested in exactly how the immune system gets out of whack, that's the article to read.

Treatment
If the horse is experiencing painful eye spasms, atropine will be prescribed to relax/dilate the eye. However, atropine can affect gut motility and increase the risk of colic. If bacterial infection is present, antibiotics will may be used, while steroidsmay be used to control inflammation. Medication is usually applied topically, as much as 4-6 times a day. Bute, aspirin, or banamine can reduce inflammation. Banamine, a non-steroidal anti-inflammatory drug (NSAID), analgesic, and antipyretic may be the single most effective treatment of acute anterior uveitis in horses.

One new development in treatment is a delivery device that is surgically implanted in the eye. The implant releases cyclosporine A, an immunosuppressant, to "turn off" the cells that cause the inflammation in ERU. These implants can last five years.

Take home message
Now you've had an earful on ERU, rest assured that there is far more to the story than I can impart. But there is a take home message, embedded in a story. A few months after I bought Harv, way back in 1997, his eye got weepy and was partially closed. It didn't look that serious, just uncomfortable. Another boarder bullied me into calling a vet, saying "no eye injury is minor, and it could be serious." In this case, Harv recovered without incident, but I know now that eye problems are not to be fooled with. If you notice anything unusual about your horse's behavior (related to vision) or his/her eye, CALL THE VET. Any infection or inflammation can damage vision, and if god forbid there is a disease process, you can treat it early.


RESOURCES

From The Horse Magazine
Leptospira Not an Important Factor in Recurrent Uveitis, Researchers Say

Understanding Equine Recurrent Uveitis (ERU)

Other
Spotlight on Moon Blindness from Thoroughbred Times, Sept. 1998

Merck Veterinary Manual on ERU

Blind Appaloosas Web site

Equine Recurrent Uveitis: Information for Horse Owners