Equine Recurrent Uveitis in Horses

Equine recurrent uveitis, or ERU, is repeated inflammation of the uveal tract inside the eye. You may also hear it called moon blindness or periodic ophthalmia. It can affect one eye or both, and each flare can leave behind a little more damage than the last.

ERU matters because it is one of the most common causes of blindness in horses. During a flare, the eye is often painful, watery, cloudy, and sensitive to light. Over time, repeated inflammation can lead to scarring, cataracts, glaucoma, retinal damage, or a shrunken blind eye.

This condition is often described as immune-mediated, meaning the horse's immune system keeps driving inflammation even after the original trigger is gone. In some horses, especially in North America, prior Leptospira infection appears to play an important role. Appaloosas are especially prone to ERU and are more likely to develop disease in both eyes.

The good news is that many horses can be made more comfortable and keep useful vision for a long time with early treatment, careful monitoring, and a plan that matches the horse, the eye findings, and the pet parent's goals.

ERU is a complex disease rather than one single infection or injury. In many horses, it behaves like an immune-mediated condition. That means the eye keeps developing inflammation because the immune system is reacting abnormally inside the eye.

A major trigger in some horses is prior exposure to Leptospira bacteria. In North America, Leptospira interrogans serovar Pomona has been strongly associated with ERU. Not every horse with leptospiral exposure develops ERU, and not every ERU case is caused by leptospirosis, but the connection is important enough that your vet may discuss it during the workup.

Other factors can include previous eye trauma, inflammation secondary to corneal disease, and less commonly other infectious causes. Genetics also matter. Appaloosas have a markedly increased risk, including a higher chance of bilateral and more insidious disease, which suggests inherited susceptibility plays a role.

In real life, many cases likely result from a mix of genetics, environment, immune response, and prior infectious exposure. That is one reason treatment plans vary so much from horse to horse.

Your vet diagnoses ERU by combining the horse's history with a careful eye exam. The pattern of repeated painful eye episodes is a big clue. During the exam, your vet may look for corneal haze, aqueous flare, a constricted pupil, low eye pressure, synechiae, cataract, retinal scars, or other changes that suggest current or past uveitis.

Because several eye problems can look alike, ruling out other causes is essential. Your vet may use fluorescein stain to check for a corneal ulcer before choosing certain eye medications. A tonometer may be used to measure eye pressure, especially if glaucoma is a concern. In some cases, sedation, nerve blocks, or referral to an equine ophthalmologist helps complete a safer and more detailed exam.

If the case is severe, recurrent, or being considered for surgery, additional testing may be recommended. This can include ocular ultrasound when the inside of the eye is hard to see, bloodwork, and sometimes testing related to Leptospira exposure. Your vet may also discuss whether referral procedures such as a suprachoroidal cyclosporine implant or vitrectomy are appropriate.

Early diagnosis matters. The sooner a flare is recognized and treated, the better the chance of reducing pain and preserving vision.

There is no guaranteed way to prevent ERU, especially in horses with genetic risk. Still, thoughtful management may reduce triggers, help you catch flares earlier, and support long-term eye comfort. A UV-blocking fly mask is often a practical first step, particularly for horses that seem light-sensitive.

Good husbandry also matters. Work with your vet on routine wellness care, parasite control, dental care, and vaccination planning. Reduce eye irritation where you can with effective fly control, cleaner bedding, and prompt attention to any eye trauma or corneal disease.

Because leptospiral exposure is linked to ERU in some horses, it is reasonable to discuss environmental risk reduction with your vet. That may include limiting access to stagnant water, improving drainage in wet areas, and reducing contact with wildlife-contaminated water sources when possible. These steps cannot eliminate risk, but they may be part of a broader prevention plan.

For horses already diagnosed with ERU, the most important prevention strategy is early recognition of the next flare. Keep medications, your vet's instructions, and a plan for urgent recheck ready. Fast treatment can make a meaningful difference in pain control and vision preservation.