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Extension > Agriculture > Livestock > Horse > Horse health > Equine recurrent uveitis (ERU)

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Equine recurrent uveitis (ERU)

Sarah Wahlert, veterinary student, University of Minnesota

The exact cause of the disease has yet to be discovered and no cure exists, making this disease a challenge for owners and veterinarians alike.

Equine recurrent uveitis (ERU), also known as moon blindness or periodic ophthalmia, is one of the most common eye conditions in horses and the leading cause of blindness. It is an immune-mediated disease, meaning the body's immune system attacks its own tissues in the eye. The exact cause of the disease has yet to be discovered and no cure exists, making this disease a challenge for owners and veterinarians alike. While the exact number of affected horses is unknown, it is estimated that as many as 10-25% of horses in the United States suffer from ERU. Appaloosas, paint horses, drafts, and warmbloods are at higher risk, while standardbreds and thoroughbreds appear to be less affected. Appaloosas are 8.3 times more likely to develop uveitis (of any type) than all other breeds combined and 4 times more likely to go blind as a result of equine recurrent uveitis (ERU). Of the horses diagnosed with equine recurrent uveitis, 25% are appaloosas. It has also been shown that leopard appaloosas are more at risk than those with blankets or dark, solid-type patterns.

But no horse, regardless of age and breed, is safe from this disease. ERU can impact your horse's quality of life and your relationship together, as well as interfere with training, decrease performance, and prevent participation in competitions because of drug withdrawal times. Prompt detection and treatment is crucial to preserving your horse's eyesight and quality of life.

Diagnosing equine recurrent uveitis can be difficult in the beginning because it cannot easily be differentiated from other causes of uveitis. A definitive diagnosis of ERU is typically only made after the horse has had several recurrent episodes of uveitis in one or both eyes. Some symptoms are fairly easy to recognize if you take the time to look at your horse's eyes on a regular basis. Others require special training to detect, which is where your veterinarian comes in. You may notice your horse squinting or holding the affected eye shut in response to pain and sensitivity to light. There may also be excessive tearing. A closer look at the eye may reveal a bluish or cloudy appearance to the cornea, the white portion of the eye (sclera) may look bloodshot, and the pupil may be smaller than normal. Your veterinarian, who is trained to examine eyes, will be able to detect more specific clinical signs during a complete ophthalmologic exam.

The damage to the eye is progressive, even with prompt and aggressive treatment. Horses can develop scarring within the eye, cataracts, glaucoma, chronic pain, phthisis bulbi (shrunken eye), and some will ultimately become blind.

There is no cure for equine recurrent uveitis. Therefore, the recommended treatment for active episodes of uveitis is aimed at decreasing the inflammation within the eye, controlling the horse's pain, minimizing the damage to the structures of the eye, and delaying the onset of blindness. Each horse and situation is unique, but treatment includes a combination of topical and systemic anti-inflammatories, steroids, and immunosuppressive drugs. Treatment of active episodes typically requires at least four weeks of medications. Horses can quickly become resentful of this and your veterinarian may recommend hospitalization and placement of a subpalpebral lavage catheter. This small tube, only slightly larger than a spaghetti noodle, runs over the top of the horse's head and through the upper eyelid. This allows for the delivery of the medications directly onto the eye without asking the horse to tolerate handling of his face. New surgical treatments are showing promise for providing longer-term control of ERU but they require a skilled ophthalmologists and the proper equipment. Initial studies have shown that these procedures increase the time between reoccurrences, shorten the duration of active episodes of uveitis, and delay the onset of blindness. Hopefully in the near future these new treatments will be more widely available and will provide horse owners with more rewarding options for managing their horse's ERU.

The Equine Genetics and Genomics Laboratory at the University of Minnesota is currently studying the horse genome in an attempt to find an answer for why so many appaloosas are affected. This discovery could lead to the development of a genetic test that when used properly and incorporated into breeding decisions, could decrease the prevalence of this condition in the appaloosa population. Researchers are also searching for possible bacterial and viral causes of ERU.
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