Moon Blindness in Horses

Which Breeds Are Affected?

ERU reportedly affects up to an estimated 25% of the worldwide horse population of 60 million animals (including 10.3 million in the U.S.). While any horse can be affected, Appaloosas are particularly susceptible — eight times higher than other breeds. Genetic research has identified that the LP allele — responsible for causing the white spotting pattern in Appaloosas — carries a risk factor for ERU in the breed. Horses carrying two copies of the LP gene have Congenital Stationary Night Blindness, or the inability to see in low-light. Quarter horses, Paint horses, Warmbloods and draft breeds also have a high occurrence of ERU in the U.S. horse population, while Standardbreds and Thoroughbreds are less affected. Gender and age are not predisposing factors for the disease.

What Are the Causes of ERU?

The cause of ERU has been long debated with several possible viral, protozoal, bacterial, parasitic as well as noninfectious links investigated. Vitamin deficiencies (especially vitamins A, C and the B complex), physical trauma to the eye, the equine flu, abscesses (tooth, hoof) and deworming medication have also been suggested. What actually causes ERU is complex and likely multi-factorial but points to a genetic predisposition to autoimmune issues that correlate to external triggers. Likely a combination of genetics, environmental factors and underlying cause of the original episode are involved.

An infectious organism, the bacterium Leptospira spp, has been most studied as a causative factor for ERU. It is a common cause of ERU in the US and Europe, and exposure level has been found to be as high as 77.5% in some studies. There is a definitive increase in the incidence of vision loss and complications with ERU-affected horses and leptospirosis. Leptospirosis is a bacterial disease spread via a wildlife host through urine of infected animals, most commonly reported in humid, tropical climates. After leptospiral bacteria enter the bloodstream (leptospiremia phase) by way of the intestines they tend to settle in the kidneys, uterus, or the uveal tract of the eye. Ridding the body of infections in any of these areas is very difficult. This organism is a common cause of abortion, and of uveitis, and rarely kidney disease. However, it can persist in the kidneys without displaying symptoms, shedding the organism in urine (leptospiruria) for up to three months, which can cause perpetuation of leptospiral spread in herd situations. Fortunately, there is an equine-approved vaccine for leptospirosis, which has been shown to be effective at preventing leptospiremia and leptospiruria. For horses in high-risk areas, ask your veterinarian about the vaccine. In horses already dealing with leptospiral uveitis, it is best to focus on treatment and management of the flares. While this suggests increased exposure to Leptospira spp, you may consider vaccinating other horses on the property. Current evidence shows that the Leptospira organisms form an impenetrable biofilm layer that protects the bacteria from the antibiotics and from the immune system, making the organism both difficult to detect and to fight, leading to the recurrent inflammatory episodes.

Because horses positive for the LP gene tend to develop the insidious non-recurrent form of uveitis and are typically negative for Leptospira spp, some researchers feel it should be considered a separate disease. For this reason, the term “leopard coat pattern uveitis” has been suggested.

Treatments Available

Each ERU case is unique, and treatment varies depending on the individual horse. Early recognition is always important, but long-term prognosis is guarded for affected horses. The goal for every case is to preserve vision and maintain comfort. While some horses respond well to treatment, others can develop additional related problems, such as cataracts, glaucoma, eye shrinkage and retinal damage, which are each difficult to treat and all can potentially lead to blindness.

While there is no known cure for ERU, multiple medical treatments are available with the goal of eliminating or slowing the progression of inflammation in the eye, minimizing damage and controlling pain. Treatment may include a combination of topical corticosteroids (prednisolone, dexamethasone), oral (systemic) non-steroidal anti-inflammatory (NSAIDs, flunixin meglumine, phenylbutazone) medications, mydriatics (atropine) and immunosuppressive drugs. In severe cases, specific corticosteroids may need to be injected into the eye. Antibiotics may be prescribed if a bacterial infection is suspected. Active episodes typically require at least four weeks of treatment during which the horse may need to remain indoors, out of sunlight or wear a mask to protect the eyes from bright light. Unfortunately, none of these treatments necessarily prevent recurring episodes. During an active flare, treatment will need to resume.

Surgical Interventions

• In horses positive for Leptospira spp, a vitrectomy can be performed under general anesthesia, removing the majority of the core vitreous — the gel-like fluid that fills the eye — which is replaced with saline. The source of inflammation is removed by replacing the vitreous fluids containing Leptospira spp and its accompanying protective biofilm. Although this is a challenging surgery to perform, in Leptospira spp-positive cases, this procedure has a high rate of resolution. However, this procedure is less likely to be helpful in leopard coat pattern uveitis.
• The cyclosporine implant is a small, disc-shaped device that is surgically implanted into the sclera of the eye and slowly releases the medication, cyclosporine A, an immune-modulating T-cell inhibitor and anti-inflammatory drug. The implant can provide therapeutic dosages for up to three years. It often reduces the need for topical and oral medications and has been shown to be very beneficial in improving vision, controlling inflammation and minimizing episodes and recurrence.
• Finally, enucleation — the surgical removal of the eye(s) — is often necessary for ongoing painful eyes.