Mary Utter, DVM, PhD, Dipl. ACVO, assistant professor of ophthalmology at the University of Pennsylvania veterinary school, continued the discussion on eyes at the In-Depth session on Ophthalmology held during the 2007 American Association of Equine Practitioners Convention, held Dec. 1-5 in Orlando, Fla. She underscored the observation made by other presenters that equine recurrent uveitis (ERU) is the leading cause of blindness in horses. The prevalence of uveitis in the United States horse population is about 8% based on a 2005 study-this means there could be 736,000 horses with moon blindness in the United States even as you read this!

Utter pointed out that this is a syndrome of many subsets rather than a single disease. She compared it to laminitis, in that both are inflammatory processes involving multiple tissues in key functional areas, and both occur due to a variety of triggers. She noted, "Both of these diseases are poorly understood, both have a variable response to therapy, and both are bad for the horse."


Eyeball of horse with uveitis

An eye affected by uveitis. Note the yellow fluid in the bottom of the eye.

A horse can have a genetic predisposition to develop uveitis, but in most cases the disease process begins as some compromise to the blood-ocular barrier in which the blood vessels of the iris and ciliary body become leaky. White blood cells enter the eye along with their inflammatory cytokines and enzymes. She explained that molecular mimicry could play a role--an antigen might trigger the immune system, then similar antigens of the horse can subsequently trigger the immune system, which causes the clinical signs of ERU. The antigen (protein trigger) can either be proteins within the horse's own tissues (i.e., self-proteins) or might be due to a phenomenon known as epitope spreading, which describes how antibodies react to a single antigenic site (foreign trigger) on a protein, similar to the trigger for diseases such as asthma.

ERU is like an autoimmune response, tending to be a dynamic process with shifts in immune reactivity that cause a waxing and waning of uveitis episodes.

Suspected stimuli that might incite an attack of ERU are bacterial, viral, and parasitic diseases, including leptospirosis, onchocerciasis, strangles, brucellosis, toxoplasmosis, equine herpesvirus, and Lyme disease. Some breeds have a predilection for developing ERU, including Appaloosas and Warmbloods.

While leptospirosis is the most significant cause of uveitis in all species worldwide, Utter feels there also might be a "lepto link" in horses. Leptospirosis is caused by spirochete (spiral-shaped bacteria) that penetrate mucous membranes in the mouth and settle in the kidneys, causing an animal to shed the organism in urine for two to three months. Although the serum titer elevates within four to eight days following infection (antibodies are made by the horse), the virus persists for the animal's life. Initially, the illness is mild and clinical signs might be missed; months later the eye disease begins, with increasing damage with each flare-up. Horses at the highest risk are those with access to carrier species, such as cattle, deer, rats, or raccoons, and especially those in close proximity to water sources, since this organism can live for months in ground water. A leptospirosis vaccine is approved for cattle, but its use in horses is considered off-label. The vaccine should only be used on "at-risk" farms and only on horses with normal eye exams and negative serum titers.


80% of uveitis cases occur in Appaloosas

Utter reported on breed predilection for ERU, saying 80% of uveitis cases occur in Appaloosas, and generally both eyes are affected. The individuals most at risk are those with coat patterns with overall roan or light coat color, little pigment around the eyelids, and sparse manes and tails. In Germany some lines of Warmbloods are at risk.

In Appaloosas with a positive lepto titer, 100% lose sight in one eye and 50% go completely blind. If an Appaloosa has had no exposure to leptospirosis, there is a 70% chance of losing sight in one eye and a 29% chance of total blindness. In non-Appaloosa breeds, a horse with a positive lepto titer has a 50% chance of losing sight in one eye and a 17% chance of complete blindness, while if the non-Appaloosa horse has no previous exposure to lepto, it a 34% chance of losing vision in one eye and only a 6% chance of going blind.

Utter described how a horse with ERU will have an acute attack alternating with periods of quiescence (latency), yet damage is ongoing despite a lack of outward clinical signs. Each episode results in more permanent damage. During a flare-up, there is tearing, pupillary constriction (miosis), conjunctivitis and redness, decreased intraocular pressure, and other signs of inflammation. Chronic cases can have evidence of "footprints" of previous episodes, such as iris adhesions, increased pigmentation of the iris, and scarring on the optic disc. Cataracts are a common sequela, with the potential to lead to glaucoma (a group of diseases characterized by increased pressure within the eyeball, which can damage the optic nerve and cause blindness). Other indications of chronic inflammation and damage might be present, including band keratopathy (deposition of calcium just beneath the epithelium--or outermost layer--of the cornea), phthisis bulbi (degeneration and atrophy of the globe), or retinal detachment.

Utter stressed the importance of client education so subtle clinical signs can be recognized early to enable timely treatment implementation. Treatment goals focus on preserving vision, controlling active inflammation, and minimizing discomfort and permanent damage.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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