EEVS Monthly Newsletter



Precipitates formed from
chronic ERU

Equine Recurrent Uveitis (ERU)

by Dr. Amanda Wilson


To conclude our eye series, we will discuss Equine Recurrent Uveitis. Also known as "moon blindness," periodic ophthalmia, and iridocyclitis, Equine Recurrent Uveitis is a serious disease of equine eyes and is the most common cause of blindness in horses.


The term "uveitis" refers to inflammation of the uvea (iris, ciliary body, choroid.) An over-active immune system causes congestion of the blood vessels within the eye and the blood vessels begin leaking proteins and other potentially damaging substances into sensitive parts of the eye, causing inflammation. This inflammation is painful and can have long-term consequences if left untreated.


Genetics do play a role, as we'll discuss later, but ERU can affect any horse in any age group and surprisingly horses can be as young as 4 to 8 years old at their first episode of the disease. Signs include:

  • Swelling around the eye/eyelids
  • Excessive "tears" or drainage from eye
  • Holding eye closed, unwilling to open eye
  • Hazy cornea (surface of eye)

One or both eyes may be affected initially. Eventually the unaffected eye can become diseased, however this is not always the case.


A corneal ulcer can also cause the signs above and is an emergency situation, call your veterinarian immediately if you observe these. Your veterinarian can differentiate ERU from other diseases that cause the signs above based on the horse's history and an ophthalmologic examination. Typically, a horse is diagnosed with ERU if two or more episodes of uveitis have been observed.


Appaloosas and Appaloosa-type horses have a higher risk of developing ERU. This is thought to be due to genetic factors of the immune system. There is also a link to Leptospirosis infection in horses and development of ERU. Horses with access to ponds or rivers may be at an increased risk of acquiring Lepto. Horses that have sustained trauma or injury to an eye sometimes develop ERU later on in life in that eye.


Prevention of ERU is difficult, especially in horses that are genetically predisposed. However, preventing injury to eyes by maintaining a safe environment is the best approach. Also, a quality fly mask should be worn daily by horses that are predisposed.


Treatment for ERU focuses on controlling the inflammation and the pain associated with it, as well as preventing permanent damage to the eye(s) that can lead to blindness. Your veterinarian may use a combination of topical and/or systemic non-steroidal anti-inflammatory drugs (NSAIDs) and corticosteroids to suppress inflammation. They also may use atropine, a drug that helps prevent permanent adherence of the iris to underlying structures. For long-term therapy, some horses may also be candidates for surgical placement of an implant in their eye that periodically secretes an immunosuppressive medication onto the eye.


Horses with chronic ERU can eventually become partially or completely blind in affected eye(s). The eyes very rarely have to be removed, but they can develop a "shrunken" appearance within the orbit. Horses that are able to gradually adjust to blindness are surprisingly adaptable as long as their environment remains consistent for the remainder of their life.


ERU can be a difficult disease to treat and manage, and can carry a poor prognosis for maintaining vision in affected eye(s).


Mark your calendars for Thursday, December 4th for our ever-popular Client Education meeting!

Dr. Kin will be discussing the second portion of Equine Metabolic Diseases and Dr. Wilson will be talking about Targeted Deworming Programs. We will be sending out more information soon, also check our Facebook and Twitter pages for details!
Exclusively Equine Vet Services PC
PO Box 721777
OKC, OK 73172
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"In riding a horse we borrow freedom."

- Helen Thomson