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Medical Index
First Posted: Feb 8, 2007
Nov 13, 2018

Equine Recurrent Uveitis

by Brian C. Gilger, DVM, MS, Dipl. ACVO
Professor, Ophthalmology

Equine recurrent uveitis (ERU) (also known as moon blindness, iridocyclitis, and periodic ophthalmia) is a major ophthalmic disease of the horse and is the most common cause of blindness in this species. Fortunately, recent advances in the treatment of horses with ERU have led to the successful management of this disease in some horses. The prevalence of uveitis in the United States horse population is about 8% based on a 2005 study. There could be aa many as 736,000 horses with moon blindness in the United States at this time. (April 2008)

Updates: "Genetic Ties - Research has shown that certain breeds, such as German Warmbloods and Appaloosas, are more at risk of developing ERU than other horses, such as Thoroughbreds. Appaloosas, in particular, are 8.3 to 12 times more at risk, as well as more likely to become blind as a result. ...researchers have recently identified a group of genes associated with the MHC class I haplotype ELA-A9 that has been linked to ERU in German Warmbloods. In one study, 41% of cases carried these genes. Another genetic marker we now know to be associated with ERU is leopard complex spotting in Appaloosas. Laboratories such as that at the University of California, Davis, now offer testing for it. ...

Leptospirosis - Many environmental influences and microbial triggers might be to blame for ERU's development, but the most common is Leptospira, ... This pathogen (disease-causing organism) exists in soil everywhere, particularly in tropical, flood-prone, and mild climates. While leptospirosis exposure in horses is common, ...disease development is rare, and ocular disease typically occurs 12 to 24 months later. ...two theories as to how Leptospira might trigger disease: Either the bacteria penetrate the eye, causing persistent low-grade inflammation, or they mimic an existing antigen (molecule that induces an immune response), causing the horse's immune system to become confused and attack the eye itself, not the organism. ..." Equine Recurrent Uveitis: The Latest

This page discusses some important facts about ERU, its causes, and potential treatment options for the affected horse.

What Is Equine Recurrent Uveitis?

Uveitis refers to inflammation of the uveal tract of the eye. The uveal tract consists of three components: the iris (colored portion of the eye responsible for opening and closing the pupil), the ciliary body (produces a watery fluid that maintains the shape of the eye) and the choroid (back portion of the eye responsible for supplying blood and nutrients). Equine recurrent uveitis is the term used for eye inflammation that recurs periodically. Equine recurrent uveitis occurs to varying degrees among horses and may affect one, or both eyes. After a horse is affected, ERU recurs at varying intervals, each time causing additional ocular damage that leads to permanent changes in the internal structures of the eye.

What Causes Recurrent Uveitis in Horses?

Any injury to a horse's eye may result in the development of uveitis and subsequent ERU. The two most common causes of ocular injury are trauma and infectious diseases. Trauma to the eye may result from a blunt blow or by penetration of the globe (eyeball) by a sharp object. Other common causes include:

  • Bacterial organisms (Leptospira, Brucella, Streptococcus)
  • Viral agents (equine influenza, equine viral arteritis, parainfluenza type 3)
  • Parasites (Onchocerca, Parasites Strongylus (Strongylus vulgaris, large strongyles, commonly known as the blood worm, is a common horse parasite in the phylum Nematoda), Toxoplasma (A single-celled parasite called Toxoplasma gondii causes a disease known as toxoplasmosis), Leptospirosis, Brucellosis (infection by bacteria of the genus Brucella), Equine Herpesvirus, Strangles, and Lyme Disease)
  • Miscellaneous (endotoxemia, tooth root abscesses, neoplasia)
  • Regardless of the inciting cause, it is believed that the ERU is immune-mediated. This means that once the primary infection resolves, subsequent episodes of eye inflammation
  • are the result of over action of the horse's own immune system in response to active immune cells such as T-lymphocytes.
  • Some breeds have a predilection for developing ERU, including Appaloosas and Warmbloods. In fact, 80%,of uveitis occurs in Appaloosas.
What Treatment Is Available for Recurrent Uveitis in Horses?

The main goals of therapy for ERU are to preserve vision and reduce and control ocular inflammation in an attempt to limit permanent damage to the eye. In horses where a definite inciting cause has been identified, treatment is directed at eliminating the primary problem, and initial tests to isolate an inciting agent are performed. These tests may consist of a complete blood count, biochemistry profile, conjunctival biopsy, and serology for bacterial and viral agents. More often, however, one particular cause cannot be isolated. In these instances, therapy is directed at the allaying of symptoms and reducing ocular inflammation.

Because vision loss is a common long-term manifestation of ERU, initial therapy must be aggressive. In acute cases, treatment in the form of systemic and local therapy consisting of antibiotics, corticosteroids and anti-inflammatory drugs is used, many times simultaneously. Initial therapy is instituted for at least two weeks, and may be continued for an additional two weeks after the resolution of clinical signs. In severe cases, local subconjunctival injections of corticosteroids may be indicated as an adjunct to therapy. In most instances, a subpalpebral lavage catheter is placed to facilitate delivery of topical medications.

Unfortunately, certain horses are never able to completely eliminate their uveitis and for these horses alternative forms of therapy must be considered.

Although traditional therapy may be used to control recurrent flare-ups, no therapy has proven to be 100% effective in eliminating ERU. Some horses respond well to long-term topical and systemic therapy and are able to be managed in this fashion. Other horses, however, do not respond to traditional therapy and may experience recurrences of uveitis more frequently. Fortunately, studies investigating the use of an immunosuppressive drug known as cyclosporin are currently underway in an attempt to manage these horses. This treatment involves the ocular implantation of a cyclosporin delivery device, which requires general anesthesia.

How Will I Know If My Horse Has Recurrent Uveitis?

The equine eye is a highly specialized structure and therefore responds to inflammation in a variety of ways. One sign is a cloudy, red, painful eye. Horses with this condition are typically blepharospastic (holding the eyelids closed or squinting) and may be sensitive to light due to the painful nature of this disease. Other signs include ocular discharge (which may be evident as tearing), corneal edema (bluish tinge to the cornea) and conjunctival hyperemia or scleral injection (prominent vessels in the whites of the eye). Any of these signs may indicate a potential problem and should alert the owner and veterinarian to the possibility of ERU.

What Is the Long-Term Effect of Equine Recurrent Uveitis?

The recurrent nature of ERU allows multiple opportunities for damage to occur in the internal structures of the eye, and therefore the prognosis is guarded. Some manifestations of damage include cataract formation, degeneration of the globe with pthisis bulbi (shrinking of the eye) and blindness.

Your Veterinarian's Role

Your horse's veterinarian plays a key role in the successful management of ERU. Although vision loss is a common result of ERU, early recognition by your veterinarian, with possible referral to a veterinary ophthalmologist for persistent or nonresponsive cases, can have a positive effect on the eventual outcome.

Update: "...Ophthalmology in horses was declared such an important topic at the 2009 American Association of Equine Practitioners (AAEP) Convention in early December that two Table Topics were held. In one session approximately 175 veterinarians attended, and in the second session about 120 practitioners were in attendance.

The discussion in the first Ophthalmology Table Topic started with a review from Ann Dwyer, DVM, on the AAEP-sponsored Focus on Ophthalmology Meeting that was held in Raleigh, N.C., in October 2009. Dwyer reported the attendance for the Focus meeting was excellent, and the veterinarians attending the meeting found the conference to be very helpful in improving their abilities to diagnosis and treat equine ophthalmic disease.

Amber Labelle, DVM, then gave a brief overview of the suspected causes and mechanisms of equine recurrent uveitis (ERU or moon blindness), including a novel surgical therapy using Suprachoroidal Cyclosporine A Implants.

Dwyer and Labelle agreed ERU is a very frustrating disease to treat. While progress has been made in treatment options and understanding of the pathophysiology and various risk factors of the syndrome, ERU is still a major cause of blindness in horses. Both veterinarians repeatedly emphasized the need for routine eye exams, especially as part of annual or bi-annual wellness exams, careful documentation of all findings, and aggressive therapy for intraocular inflammation..." Equine Eye Problems Discussed by Veterinarians

For More Information:

A Look at Equine Recurrent Uveitis and Leptospirosis
Equine Recurrent Uveitis Biomarker Research a Real Eye Opener
Genetic risk factors for insidious equine recurrent uveitis in Appaloosa horses
Advances in Equine Ophthalmology
Equine Uveitis
"Watery Eyes Can Indicate a Serious Problem: Equine Recurrent Uveitis"
The Top 7 Things to Know about Equine Recurrent Uveitis

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