Equine recurrent uveitis (ERU)

Veterinary advice should be sought before applying any treatment or vaccine.

Equine Recurrent Uveitis (ERU)

Moon Blindness, Recurrent Iridocyclitis, Periodic Ophthalmia

Equine recurrent uveitis (ERU), also known as moon blindness, is the leading cause of blindness in horses. 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. It is a painful, immune-mediated disease in which the horse's own immune system attacks the tissues within the eye. It is characterized by repeated uveitis episodes, triggered by exposure to a parasite, virus, bacteria, or other irritant which gets in the eye. Studies show that approximately 20% of affected horses develop ERU in both eyes. Appaloosas, paint horses, drafts, and warmbloods are at higher risk of ERU, 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 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.

Clinical Signs of Horses with ERU

Horses with ERU usually present with symptoms related to having intense eye pain, such as excessive tearing (lacrimation) or ocular discharge, swelling around the eye, frequent squinting or blinking, partially closed or fully closed eyelid, constricted pupil, vision loss, and redness in the whites around the eyes.

Secondary complications of chronic episodes of uveitis are glaucoma, phthisis bulbi (shrunken eye), cataracts (develops in 25% of affected horses), retinal detachment, and/or irreversible blindness.

Diagnosis of ERU

A horse can have an individual episode of primary uveitis without necessarily having ERU. A definitive diagnosis of ERU is typically only made after the horse has had several recurrent episodes of uveitis in one or both eyes.

It is really important that your vet is able to initially rule out whether the source of your horse's painful eye isn't caused by ulcerative keratitis or stromal abscessation. This is done by conducting a fluorescein dye test during an eye examination. This is because corticosteroids are often the preferred treatment for horses with ERU, it is not good for horses to be given this medication if they have a corneal ulcer or abscess.

Treatment of ERU

The major goals of treatment of ERU are to preserve vision, decrease pain, and prevent or minimize the recurrence uveitis episodes. Treatment for uveitis depends on the underlying cause and the severity of the symptoms. Most cases are treated with topical anti-inflammatory medications and atropine for pupil dilation to reduce the pain and inflammation.

The University of California-Davis is currently conducting two different clinical trials related to ERU in horses. The title and requirements to participate in the trials are provided below:
  • Efficacy of Autologous Mesenchymal Stem Cells (MSC) for Treatment of Equine Recurrent Uveitis Following Intravenous Injection: Eligible horses must be those who have had multiple episodes of anterior uveitis within the past 12-18 months. The trial involves injecting MSCs into the horses intravenously either during a period of inflammation or during a quiescent period.
  • Genomic investigation of Equine Recurrent Uveitis in Appaloosa horses: Eligible horses include Appaloosas with known pedigrees. The purpose of the study is to determine what the genetic factors are contributing to the development of ERU in horses of the Appaloosa breed.
Click here to find additional information regarding either of these clinical trials at University of California-Davis.


Tearing or watering eyes
Corneal cloudiness
Eyelid swelling
Sensitivity to light
Contraction of the pupil
Vision loss


  • History
  • Clinical signs
  • Eye exam
  • fluorescein dye test
  • Response to treatment



Stem cell therapyClinical trials are currently being conducted at UCDavis to study the efficacy of Autologous Mesenchymal Stem Cells (MSC) for treatment of ERU in horses.
Sustained release intravitreal cyclosporine A implantsMay last up to 5 years.
Suprachoroidal Triamcinolone Injections
Pars plana vitrectomyA surgical procedure which has been used successfully to remove fibrin, inflammatory cells, and debris trapped in the vitreous to improve vision and delay the progression of the clinical signs.
Topical atropineGiven to help reduce synechiae formation by inducing mydriasis and relieving spasm of ciliary body muscles.
AntibioticsWhen Leptospira organisms are involved.
Homeopathic remediespoultices of chamomile and oral methylsulfonylmethane


  • Practice effective fly control at the barn.
  • Drain stagnant ponds or restrict horse's access into swampy pastures with stagnant water present.
  • Minimize horses contact with wildlife.


poor for a cure to preserve vision, but the disease can be controlled.

Scientific Research

General Overviews

Clinical Trials

Risk Factors

  • Genetics - 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 ERU. Of the horses diagnosed with equine recurrent uveitis, 25% are appaloosas.

Horse Case Stories

Also Consider

Commonly Affected Breeds

Appaloosa icon