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Moon Blindness: Learn to recognize this potentially devastating eye disease.

By Tracy Williams
Moon blindness, technically known as equine recurrent uveitis or ERU, is the largest known cause of equine blindness. Documented cases occur throughout history, and they were originally termed “moon blindness” because past horse owners thought these recurring attacks were related to the phases of the moon. Today, we are more equipped to defining, treating and possibly preventing this painful eye condition.

Acute or Constant

Uveitis is a general term for inflammation inside the eye. Acute uveitis can be caused by a number of factors, such as corneal ulcers or trauma and may or may not occur again once the source of the inflammation is resolved. Thus, the first step in diagnosing whether or not your horse has recurrent uveitis is determining if this is a first-time case or your horse’s constant companion. Typically, a horse is not diagnosed with ERU until it has experienced two or three attacks. Unlike acute uveitis, the recurrent strain cannot be eradicated; it is an autoimmune disease and can only be controlled. It is caused by an over-enthusiastic immune system reacting to a possible problem and taking out healthy eye tissue in addition to the perceived threat. ERU can appear and recur in one or both eyes.

An Overactive Immune SystemAccording to Dennis Brooks, DVM and professor of ophthalmology at the University of Florida, ERU’s initial cause can be a number of factors: trauma, viral or bacterial infections, some parasites or systemic infection. Anything that breaches the barrier between the bloodstream and the eye (called the blood-ocular barrier) can trigger an immune response that can go haywire. It is the immune response that causes more problems than the actual infection. A common cause of ERU is Leptospira, a bacterium often found in stagnant water. Leptospira is especially

Blind Horse by Paula da Silva
Equine recurrent uveitis is the largest known cause of blindness in the horse.

difficult because it has a similar molecular makeup to certain eye structures, which encourages the eye’s immune system to attack healthy eye tissues in addition to the invading bacteria.

 

The goal of the eye’s immune system is to keep the cornea and lens transparent to optimize vision. According to Dr. Brooks, the blood-ocular barrier accomplishes this by preventing large molecules and cells from entering the eye. When trauma or inflammation allows this material to enter, eye cells spot these agents and release chemical signals to alert the immune system. Antibodies are imported into the eye to nab the invaders, but unfortunately in an ERU horse, these antibodies can attack eye structures, being unable to distinguish between healthy and diseased tissues. This increases inflammation, and to cope, the eye continues to shuttle in more antibodies, further increasing inflammation and resulting in more damage.

With the immune system attacking healthy tissue, numerous problems erupt. If left unchecked, ERU can cause the iris to stick to the lens or the cornea, distorting the pupil; the lens may become cloudy; the retina can detach; and nerve cells can die, destroying vision. As these attacks recur, the immune system becomes more and more efficient at its methods, and permanent damage builds causing cataracts, scars, glaucoma, nerve cell death and blindness.

Diagnosing ERU

 ERU by Linda Sherrill
Squinting, face rubbing and watery eyes are common symptoms of ERU. As the disease progresses cloudiness might be seen in the eye.
The most obvious signs of ERU result from the intense pain: squinting, face rubbing and watery eyes. Owners can also notice a change in the eye’s appearance – a cloudiness builds as fluid increases, the pupil typically shrinks, and the eye reddens. It is important to note that these signs are also common with corneal ulceration (causes acute uveitis), and you must differentiate between the two as treatment options are vastly different. If you treat your horse for ERU when an ulcer lurks in the eye, the treatment plan can cause the ulcer to rapidly worsen. Your veterinarian can perform a fluorescein dye test to rule out ulcers before proceeding with treatment.

Treatment
Because ERU cannot be cured, the goal in treatment is to decrease inflammation. In mild cases, NSAIDS like phenylbutazone (Bute) or flunixin meglumine (Banamine) are routinely given, but if signs worsen and ulcers are ruled out, a veterinarian will likely recommend antibiotics to prevent infection, corticosteroids to control inflammation and atropine to decrease pain and keep the pupil from shrinking. Treatment becomes more difficult if these episodes are often recurring.

A relatively new treatment option is becoming increasingly viable, however, according to Brian C. Gilger, DVM and professor of ophthalmology at North Carolina State University.		 Moon Blindness By Linda Sherrill
Research shows that Appaloosas are eight times more likely to develop ERU than other breeds and are also more likely to go blind as a result.

He explains that researchers have developed a surgical implant, placed into the eye that can leak cyclosporine into the eye tissues. Cyclosporine suppresses the immune system, thus shutting off cells that cause inflammation. These implants last for several years and are proving to decrease duration and severity of inflammation as well as tissue damage. The best candidates for this option are those that have almost no active inflammation but who have frequent bouts of ERU.

If you suspect your horse of having equine recurrent uveitis, monitor its eyes closely, noting any change in appearance or symptoms of pain and inflammation. Armed with this information, your veterinarian can recommend a treatment plan that best suits your horse’s individual needs. The consequences may be dire, but with prompt action, your horse’s odds instantly improve for a future of clear-sightedness.
ERU and Appaloosas
Historically, through mostly anecdotal evidence, Appaloosas seem to be especially susceptible to ERU, and in addition they suffer from a particularly difficult branch of the disease, one that is difficult to diagnose. In classic ERU cases, the horse experiences a flare-up that increases inflammation and triggers a pain response that alerts owners to a problem. Appaloosas, however, will often experience a chronic, low-grade inflammation that often doesn’t necessarily trigger obvious eye pain. Thus, the disease can go unnoticed until cataracts or blindness develops. Recent research is discovering more concrete links between Appaloosas and the disease in hopes of finding a preventative measure. The first study recognizing a connection was published in Animal Genetics in 1988. This study evaluated 16,242 horses over 12 years. The researchers concluded: “In the uveitis diagnosis category, Appaloosas had a significantly higher risk of developing uveitis…relative to Thoroughbreds.” The results were confirmed in an additional study published in 1995 in the Journal of the American Veterinary Association. This study observed the link between Leptospira and uveitis, but also discovered more evidence in favor of the Appaloosa’s predisposition. The Appaloosa by Jean-Marie Pluchon
study concluded that Appaloosas were possibly eight times more likely to develop uveitis than other breeds and also more likely to go blind as a result. The question remains as to why. Currently scientists believe the research supports a genetic link, a possibility being explored by Mark Rutherford, a researcher at the University of Minnesota’s College of Veterinary Medicine. His project suggests that there is a “susceptibility allele” in the Appaloosa’s genetic code. With further study, it might be possible to use breeding management to decrease prevalence of the disease. For further information, visit www.blindappaloosas.org.


Tracy Williams began actively researching eye diseases after helping treat editor Jill Haight’s Arabian Warlord who suffered from an eye ulcer. Warlord’s vision is completely restored

 


Equus Caballus, the magazine of the domestic horse, has been dedicated to the proper care and feeding of horses, ponies, donkeys and mules for over seven years. This site is a compilation of over 400 archived articles and new features about nutrition, health and equine management.

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