A mare comes in from the field one afternoon with an eye that is red, irritated and tearing. Her owners chalk it up to flies and buy a fly mask. While the mask helps keep insects away from her eyes, the mare continues to suffer with painfully puffy, weepy eyes. By the end of the week, they notice that the eye looks cloudy and the mare is reluctant to come out of her stall into the sunlight. Alarmed, they call the veterinarian who concludes the examination with a worrisome diagnosis: uveitis, which is more formally known as periodic ophthalmia, or ERU, for equine recurrent uveitis.
The mare’s red and weepy eye is treatable with atropine to help dilate the pupil and reduce the discomfort, followed by antibiotic eye drops and anti-inflammatory drugs. However, uveitis, commonly known as “moon blindness,” often recurs.
Moon blindness is the most common cause of blindness in horses and mules. It affects up to 12 percent of horses worldwide and about 25 percent of Appaloosas (Quarter Horses being the next most vulnerable breed). The symptoms include:
- cloudiness on the surface of the eye
- a type of scarring called butterfly lesions
- redness and puffiness around the eye
- muscle spasms which keep the eye tightly closed
- rubbing the eye
- head shaking
- loss of balance or stumbling
- pain and irritability
- excessive squinting and blinking
- a sensitivity to bright light (photophobia)
- constricted pupils
In the worst cases, symptoms can escalate to include:
- detachment of the retina
- calcification of the cornea
- atrophy of the eye
- eventual blindness in one or both eyes
The clinical signs may reappear every few weeks or months once a horse has suffered an initial episode. Each flare-up can last up to a couple of weeks, and with each succeeding episode, the damage accumulates.
Aggressive treatment as soon as symptoms are noticed can minimize the detrimental effects, but many horses still gradually lose their vision.
The disease usually affects one eye at a time, but it’s not uncommon for the second eye to also develop symptoms, sometimes turning a horse completely blind.
Ancient Root Of The Problem
According to equine ophthalmologist Dennis Brooks, DVM, Ph.D., of the University of Florida in Gainesville, moon blindness has been plaguing horses for millennia.
“There are medical historians who think that moon blindness is the oldest recorded disease, older even than recorded human disease,” said Brooks. “It’s an old, old disease, but only in the last 10 years or so have we begun to understand what its causes are.”
Bacterial, fungal, viral and parasitic infections have all been implicated in ERU, with a bacterial disease called leptospirosis being one of the most common triggers. Horses can contract leptospirosis through exposure to contaminated feed or water, and the bacteria can also linger for years in soil where cattle have lived or where springtime flooding has occurred. ERU that develops in the wake of a leptospirosis infection may take a year or two to surface, long after the infection has gone.
Another possible cause of ERU is a parasitic infection called onchocerciasis. Horses contract the parasite, Onchocerca cervicalis, when they’re bitten by Culicoides flies, which are tiny midges. The parasitic larvae, called microfilariae, create weeping sores on the withers, the underside of the mane and the midline of the abdomen, and when they get in the tissues of the eye, they can cause intense irritation.
Ironically, broad-spectrum dewormers such as ivermectin, normally considered a benefit to equine health, can sometimes set off uveitis when they kill off large quantities of microfilariae in the eyes of an infected horse. This is because the immune system reacts more violently to dead microfilariae than live ones.
ERU can also take hold thanks to any of a number of common viral infections, such as influenza and equine herpes virus. Trauma to the eye can begin the destructive cascade as well. Corneal ulcers, intraocular surgery or any blunt or invasive force sustained by the eye can make the tissues vulnerable to ERU, as can allergies to certain pollens.
Said Brooks, “[ERU is] probably an immune-mediated disease. Infection can start it or trauma can start it. Something starts it, and whatever that initiating factor is, it heals, and you’re left with the autoimmune disease.”
In an autoimmune disease, the horse’s own immune system mounts an inappropriate inflammatory response. In the case of ERU, it’s against proteins in the eye. The T-lymphocyte response launched by the immune system triggers intense inflam-mation in the eye and sets the stage for long-term damage.
“What happens in these horses is that they get little collections of T-lymphocytes within their eyes. This is why the disease gets worse, and it keeps coming back, and it goes through these cycles. These lymphocytes are in little follicles, little structures which aren’t supposed to be [in the eye]. It took us a long time to figure that out,” Brooks said.
“It doesn’t just attack the eye,” he added. “It’s in the brain of the horse, and it may be in other organs too. It often starts somewhere else and ends up in the eye. The eye’s the only thing we notice, but horses that have an ERU-affected eye also have inflammation in their brains. We didn’t know that until maybe 20 years ago.”
Brooks confirmed that Appaloosas are at particular risk for moon blindness. “In cases where both of the horse’s eyes are affected, 80 percent of the time it’s an Appaloosa,” he said. “In non-Appaloosas, the rate at which both eyes are affected is only 20 percent.”