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Equine recurrent uveitis (ERU)

Quick facts

  • ERU is an auto-immune disease affecting horses' eyes.
  • There is no known cause or cure for ERU.
  • Appaloosas are 8.3 times more likely to get uveitis than all other breeds combined.
  • ERU can reduce your horse’s quality of life and cause cataracts, eye cancer and blindness.
  • Treatment includes a combination of medications.
  • Research may reveal why appaloosas are more likely to get ERU and could lead to a genetic test.

What is equine recurrent uveitis (ERU)?

Equine recurrent uveitis (moon blindness or periodic ophthalmia) is one of the most common eye problems in horses and the leading cause of blindness. It’s an immune-mediated disease, which means the body's immune system attacks its own eye tissues.

The cause of ERU is unknown and there’s no cure, which makes this disease a challenge for owners and veterinarians.

Is your horse at risk?

About 10 to 25 percent of horses in the United States suffer from ERU. The exact number of affected horses is unknown. Appaloosas, paints, drafts and warmbloods are at higher risk for ERU. Standardbreds and thoroughbreds appear to be less affected. But no horse, regardless of age and breed, is safe from ERU.


  • Appaloosas are 8.3 times more likely to develop uveitis (of any type) than all other breeds combined.
  • Appaloosas are four times more likely to go blind as a result of ERU.
  • Twenty-five percent of horses diagnosed with ERU are appaloosas.
  • Leopard appaloosas are more at risk than those with blankets or dark, solid-type patterns.

Impacts of ERU

ERU may

  • Decrease your horse’s quality of life.
  • Affect your relationship together.
  • Make training hard.
  • Decrease performance.
  • Prevent your horse from competing because of drug withdrawal times.

Diagnosing ERU

Detecting and treating ERU right away is crucial to preserve your horse's eyesight and quality of life.

Diagnosing ERU can be hard in the beginning because it’s hard to tell the difference between other cases of uveitis. A clear diagnosis of ERU typically occurs after the horse has several recurrent uveitis episodes in one or both eyes.

Symptoms of ERU

Some symptoms are easy to recognize if you take the time to look at your horse's eyes regularly. Others require special training to detect, which is where your veterinarian comes in. Your veterinarian will be able to detect more specific clinical signs during a complete eye exam.

Symptoms you can look for 

  • Squinting.
  • Holding the eye shut in response to pain.
  • Sensitive to light.
  • A lot of tearing.
  • Bluish or cloudy look to the cornea.
  • White portion (sclera) of the eye looks bloodshot.
  • Pupil is smaller than normal.


Possible outcomes of ERU

Eye damage continues even with early and aggressive treatment. Horses can develop

  • Scarring within the eye.
  • Cataracts.
  • Glaucoma, eye cancer.
  • Chronic pain.
  • Phthisis bulbi (shrunken eye).
  • Ultimately blindness.

Reducing pain, swelling, and further damage

ERU has no cure. Therefore, the treatment goals for active episodes of uveitis include

  • Decreasing the swelling within the eye.
  • Controlling the horse's pain.
  • Minimizing the damage to the structures of the eye.
  • Delaying the onset of blindness.


Each horse and case are 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.

Hospital treatment

Horses can quickly become bitter towards drugs. Your veterinarian may recommend hospitalizing your horse and placing 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. Veterinarians can deliver medication through the catheter directly onto the eye without handling the horse’s face.

New surgical treatments are showing promise for providing long-term control of ERU. But they require a skilled eye doctor and proper equipment. Early studies show these procedures increase the time between recurrences, shorten the length of active episodes, and delay the onset of blindness. Hopefully these new treatments will be more widely available and provide horse owners with more rewarding treatment options for ERU.

ERU research

The Equine Genetics and Genomics Laboratory at the University of Minnesota is studying the horse genome to find an answer for why ERU affects many appaloosas. This discovery could lead to a genetic test that individuals can use and incorporate into breeding decisions. Under proper use this test could decrease the prevalence of ERU in the appaloosas. Researchers are also searching for possible bacterial and viral causes of ERU.

Author: Sarah Wahlert, DVM

Reviewed in 2021

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