The Top 7 Things to Know about Equine Recurrent Uveitis

Editor's note: This article is part of TheHorse.com's ongoing coverage of topics presented at the 2012 Western Veterinary Conference held in February.

When equine recurrent uveitis (ERU) develops, it often looks inconspicuous at best. Maybe your horse's eye is a little watery, or he's squinting in the bright sunlight. It's nothing to worry about, right? Wrong. At a recent veterinary convention, one veterinarian discussed just how serious ERU is if left untreated.

During a presentation at the 2012 Western Veterinary Conference, held Feb. 19-23 in Las Vegas, Nev., Jacquelin Boggs, DVM, MS, Dipl. ACVIM, discussed equine recurrent uveitis with a veterinary audience. Following are the top seven things horse owners should know about this potentially blinding disease:

1. Uveitis is the leading cause of blindness in horses. "While data on the incidence of uveitic horses is lacking, uveitis is the leading cause of blindness in horses worldwide," said Boggs, an equine technical services veterinarian for Pfizer Animal Health.

2. ERU has a variety of presentations and subsets of disease. The disease can be caused by a variety of factors, and often has an insidious onset, Boggs relayed. "Under the 'umbrella' of ERU are several clinically recognizable syndromes based upon either observed inflammation or the stage of ocular disease in the horse at the time of examination," Boggs said. She noted that ophthalmologists have developed two terms used to describe ERU subsets: classic uveitis and insidious uveitis. "Classic uveitis cases demonstrate bouts of severe inflammation and pain in one or both eyes, with periods in between the bouts where the eye is ' quiet, ' " she explained. "In contrast, horses with insidious uveitis have historically never been observed to show outward signs of pain, but the eye develops typical signs of chronic ERU."

Clinical signs of ERU are multiple and various, she said, and include:

  • Blepharospasm (squinting or closing of the eyelid in response to eye pain);
  • Epiphora (watery eyes);
  • Lacrimation (tearing);
  • Photosensitivity (sensitivity to light); 
  • Corneal haze;
  • Mild aqueous flare (small "floaters" in the front chamber of the eye);
  • Iris color change;
  • Corpora nigrans atrophy (degeneration of the soft brown, irregular body at the edge of the iris);
  • Vitreous haze (clouding of the gel that fills the eyeball between the lens and the retina);
  • Miosis (pupillary constriction, which is the "hallmark" of ERU); and
  • Retinal scarring.

In some cases, she noted, if a veterinarian examines a horse between bouts, the eye might look quiet at first look. Closer ocular evaluation, however, might reveal structural changes not readily visible to the naked eye, she said.

3. ERU is an immune-mediated disease with multiple triggers. Researchers still aren't sure exactly what causes ERU. What they do know is that when the eye's immune system "checks and balances" fail, the body mistakes healthy eye tissue for diseased tissue. This leads to ocular cloudiness, inflammation, and the development of ERU.

Boggs relayed that researchers have speculated on some "triggers" for ERU development including bacteria, viruses, parasites, vaccinations, host conditions, and individual genetics; why each of these triggers prompts ERU, however, remains unclear. "The exact mechanisms are topics of ongoing research," she said.

4. Researchers note an association between leptospirosis and ERU. "Of all the possible infectious triggers, leptospirosis (a bacterial disease) is the most significant worldwide," Boggs said, citing a report that estimated leptospirosis involvement in at least 60% of ERU cases in the northeastern United States. She explained that scientists and veterinarians have developed several theories as to why leptospirosis triggers ERU; however, researchers are still working to fully understand the disease-causing leptospires' involvement.

Boggs also noted there appear to be several risk factors for leptospirosis, including sharing close quarters with cows, pigs, and/or deer; living in close proximity to bodies of water (i.e., streams, ponds, etc.); drinking piped pond water; the presences of persistent ground water; and rat infestations. However, the disease is not always associated with these factors, and some farms seem endemic (disease is widespread on the farm). She stressed that in her own experience, she is aware of leptospirosis-associated cases of ERU on farms with few or none of the aforementioned risk factors.

5. Certain breeds are more likely to develop ERU. Appaloosas, European Warmbloods, and draft horses appear to be predisposed to developing ERU, Boggs said. "In western New York, Appaloosas are 8.3 times more likely to suffer from uveitis than other breeds (as reported by Ann Dwyer)," she relayed. Specifically, she noted, Appaloosas with light coat patterns (i.e., roan or white) and sparse manes and tails that lack pigment around the eyes appear at higher risk for developing the disorder. While the reason behind this remains unknown, researchers hypothesize that affected horses have a genetic predisposition for the disease resulting from abnormalities in some of the proteins within a gene set that's also involved in the immune system.

6. Immune system modulation is the main goal of treatment, and affected horses' response to treatment is variable. Boggs stressed that, prior to beginning treatment, veterinarians should ensure the owner fully understands the potential burdens associated with treating an ERU horse. "Not only will there be exhaustive time and financial resources put towards treating recurrent bouts, but many uveitic horses will ultimately go blind, despite therapy," she said, adding that some horses will eventually necessitate enucleation (surgical removal of the eye and associated structures) as a result of ERU. But the upside, she said, is that many horses respond well to treatment and can continue leading very productive lives.

"Treatment is aimed at decreasing inflammation, minimizing ocular damage from repeated bouts of inflammations, and managing pain," Boggs said. She explained corticosteroids (either topical, or intravenous or intramuscular injections) are the mainstay of ERU therapy to control immune responses and the inflammatory response. Veterinarians also administer systemic non-steroidal anti-inflammatory drugs (NSAIDs) to treat severe pain and topical atropine ointment to prompt pupil dilation. Boggs noted that in cases that don't respond well to this treatment, alternative drugs and ointments are currently showing promise.

Boggs also discussed a relatively new implant surgery currently being used to treat ERU. Suprachoroidal Cyclosporine A (CsA) implant surgery is becoming the treatment modality of choice in cases of clearly documented recurrent episodes when finances dictate. Cyclosporine is a drug that suppresses the overresponsive immune system. "This involves surgically instilling a small sustained-release device into the space surrounding the eye to allow direct administration of cyclosporine continually, at prescribed doses, for up to three years," Boggs relayed. Early results are promising in that 81% (of patients) have less inflammation and ERU attacks. Post-surgery, it take 30 to 45 days for the cyclosporine levels to become therapeutic.During that time Boggs recommended treating patients with the aforementioned traditional therapeutic regimen.

7. The long-term visual prognosis is guarded. Boggs cited one study that followed 160 cases of ERU over 11 years. She relayed that 56% of affected horses lost vision in one or both eyes. Further, ERU horses are at increased risk for developing secondary complications, some of which can cause blindness. She noted that in the same study 27% of the 160 horses developed corneal ulcers along with ERU (for this reason, Boggs stressed it is very important that owners do not instill steroids into the eye without consulting their veterinarian to ensure an ulcer is not present).

Other complications that could hinder an ERU horse's recovery include corneal scars, cataracts, glaucoma, iris atrophy, calcific band keratopathy (corneal calcium deposits), posterior synechia (adhesion of the iris to the cornea or the lens), lens luxation (movement away from normal position), vitritis (inflammation of the jelly in the posterior portion of the eye), and phthisis bulbi (eyeball degeneration and atrophy).

Take-Home Message While some horses with ERU will develop further complications, many horses live comfortable and functional lives. Knowing what to look for can help owners alert veterinarians to a potential case of ERU early, thus allowing treatment to begin sooner and giving the animal a better chance of recovery.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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