Equine recurrent uveitis

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Equine Recurrent Uveitis

Moon Blindness, Recurrent Iridocyclitis, Periodic Ophthalmia

Equine recurrent uveitis (ERU), also referred to commonly as moon blindness, is the leading cause of blindness in horses, characterized by repeated and unpredictable episodes of inflammation of the uveal tract of the eye. Any horse breed is susceptible to ERU, however Appaloosas and draft breeds are predisposed. Appaloosa horses are eight times more likely to develop the disease in their lifetime, and four times more likely to go blind as a result of the disease.

ERU is an immune-mediated disease (meaning it lacks a definite etiology and involves tissue damage caused by the body's immune system), however it is suspected that other factors are involved, including:
  • Corneal injury
  • Trauma
  • Neoplasia (tumor)
  • Intraocular or systemic infections
The severity of ERU clinical signs observed in horses, as well as the frequency of episodes in horses, varies with each individual horse.

A horse can have an individual episode of primary uveitis without necessarily having ERU. A horse is classified as having ERU or breed associated insidious disease once they have three or more episodes of primary uveitis. Blindness does not usually occur on the first episode, it is the cumulative effects of repeated episodes that lead to the onset of glaucoma, cataracts (develops in 25% of affected horses), degeneration, or retinal detachment.

Latest Research
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.


Frequent squinting
Tearing or watering eyes
Corneal cloudiness
Eyelid swelling
Sensitivity to light
Contraction of the pupil


  • History
  • Clinical signs
  • Eye exam
  • Response to treatment
  • Laboratory tests


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.
Topical corticosteroids and systemic NSAIDs
Acupuncture therapy
Suprachoroidal cyclosporine implant surgery
Pars plana vitrectomy


  • 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.

Scientific Research References

Good Overviews

Clinical Trials

Also Consider

Commonly Affected Breeds

Appaloosa iconHanoverian icon