Recurrent painful inflammation and pathologic changes in the eyes of horses with equine recurrent uveitis (ERU, also known as moon blindness) could be caused by the persistence of invasive bacteria called Leptospira in affected horses' eyes and the resultant immune response directed to the area. Divers described recent research on leptospirosis, the disease caused by these bacteria.

Leptospira are spirochetes that cause devastating infections in humans, horses, and other animals. In horses, leptospiral organisms can cause abortion--sometimes sporadic cases and sometimes in storms--and they can cause urinary tract disease, including kidney infection and failure. The bacteria are shed in the urine of infected animals, and horses are exposed to it when they drink water in which infected animals have urinated. The risk of transmission is greatest for horses in pastures with standing water that are shared with other species of animals, such as cattle.

Although abortion and kidney infection are severe, the organism is getting increased research attention at present because of its association with ERU, which used to be called periodic ophthalmia or moon blindness because the condition waxes and wanes in severity and was thought to be influenced by lunar cycles.
Evidence suggests ERU might be the most common cause of horse blindness.

The Leptospira organisms reach the eye after dispersal in the blood. Targeting of the leptospires by cells and molecules of the immune system results in a condition known as anterior uveitis, the clinical signs of which are squinting, severe engorgement of the blood vessels of the sclera (the white part of the horse's eye), corneal edema (which makes the surface of the eye appear whitish or bluish-gray), tearing, pinpoint pupils, and a hazy appearance to the aqueous humor in the anterior chamber. Observation of such signs should prompt veterinary evaluation.

If the horse has recurrent flare-ups of uveitis, then cataracts, adhesions between the horse's lens and iris, degeneration of the vitreous body, and blindness can develop. Appaloosas are more likely to get ERU than other breeds.

Uveitis can have many causes, such as eye injuries, equine influenza virus, and Onchocerca and Strongylus parasites, but leptospirosis is increasingly being recognized as an important causative agent. Veterinarians have traditionally managed uveitis by focusing on control of the inflammation, but Divers suggested that an antimicrobial should be added to treatment protocols to help control the infection if leptospirosis is suspected or identified.

Even though antibodies can be found in the aqueous humor, the organism is able to persist, suggesting that other elements of an effective immune response might not be operant in the eye. Researchers are focusing on vaccine development and better treatments for intraocular infection.

About the Author

Kim A. Sprayberry, DVM, Dipl. ACVIM

Dr. Kim A. Sprayberry, DVM, Dipl. ACVIM, is an internal medicine specialist at Hagyard Equine Medical Institute in Lexington, Ky. When not working with horses, she enjoys pursuits in medical journalism and editing as well as kayaking and American southwest archaeology.

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