Moon blindness is a chronic inflammatory eye condition in horses, formally called equine recurrent uveitis (ERU). It is the leading cause of blindness in horses worldwide. The name comes from an old belief that flare-ups were linked to lunar cycles, but the condition is actually driven by immune system dysfunction, often triggered by bacterial infection. Each episode of inflammation damages structures inside the eye, and without management, repeated flare-ups progressively destroy vision.
Why It Happens
The most common infectious trigger is a group of bacteria called Leptospira. These organisms are widespread in the environment, particularly in standing water, wet pastures, and areas with rodent activity. After a horse is exposed, the initial infection may cause little obvious illness. But in susceptible horses, the immune system develops a problematic response: proteins on the surface of Leptospira closely resemble proteins in the horse’s own eye tissues. The immune system begins attacking the eye itself, setting off recurring waves of inflammation long after the original infection has cleared.
Not every case of moon blindness traces back to Leptospira. Trauma, other infections, and genetic factors can also play a role. Appaloosas are at significantly higher risk than other breeds and tend to develop a more aggressive form of the disease. Draft breeds, warmbloods, and certain other breeds also appear more susceptible. Horses kept in warm, humid climates or in environments with poor drainage and high rodent populations face greater exposure to the bacteria that most commonly trigger the condition.
Three Forms of the Disease
Moon blindness doesn’t look the same in every horse. It takes three distinct forms, and recognizing which one your horse has matters for both treatment and long-term outlook.
Classic ERU is the most common. It follows a pattern of painful, obvious flare-ups separated by quiet periods where the eye looks relatively normal. Each flare damages the eye a little more, so the quiet periods are deceptive. The horse may seem fine between episodes, but internal structures are gradually deteriorating.
Insidious ERU is harder to catch. The inflammation is persistent but low-grade, and the horse often shows no outward signs of pain. Because there’s no dramatic flare-up to prompt an exam, the disease can silently destroy eye tissues over months or years before anyone notices vision loss. This form is particularly common in Appaloosas.
Posterior ERU targets structures at the back of the eye, including the retina and the gel-like substance that fills the eyeball. Retinal degeneration is common with this form, and a horse can lose significant vision even when the front of the eye looks normal on casual inspection.
Signs to Watch For
During an active flare-up, the signs are hard to miss. The affected eye will be red, watery, and partially or fully closed. The horse squints and avoids light. The cornea (the clear front surface of the eye) may turn hazy or blue-gray from fluid buildup. You might notice the pupil is constricted and doesn’t respond normally to light, or that the eye looks “cloudy” deeper inside.
Between flare-ups, or in horses with the insidious form, the signs are subtler. You might notice the eye looks slightly smaller than the other, or that the pupil is an irregular shape. The dark, frilly structures along the top of the pupil (called corpora nigra) may appear shrunken or ragged. Cataracts can develop, giving the lens a whitish or yellowish appearance. Some horses become head-shy on one side, resist bridling, or startle more easily, all behavioral hints of compromised vision that owners sometimes attribute to temperament.
A veterinarian diagnoses ERU after observing two or more episodes of uveitis, or by identifying characteristic damage during an eye exam. The exam typically involves checking pupil responses, measuring the pressure inside the eye (which drops during active inflammation), staining the cornea to rule out ulcers, and examining the interior of the eye in a darkened setting. Blood tests for Leptospira antibodies can help confirm whether the bacteria played a role, though a single positive test isn’t definitive on its own.
How Flare-Ups Are Treated
When a flare-up hits, the priority is stopping inflammation fast and keeping the pupil dilated to prevent internal scarring. Treatment typically involves anti-inflammatory eye drops applied multiple times a day, sometimes as often as every four to six hours in severe cases. A separate drop is used to keep the pupil open, which reduces pain and prevents the iris from sticking to the lens (a common complication called posterior synechia that can permanently distort the pupil).
Oral anti-inflammatory medication is usually given alongside the eye drops, especially in the first few days. If an active Leptospira infection is suspected, a course of antibiotics lasting several weeks may be added. Treatment continues for two to four weeks past the point where all visible signs of inflammation have resolved, then is slowly tapered. Stopping too early risks a rebound flare.
The challenge with moon blindness is that treating individual episodes, no matter how aggressively, doesn’t prevent the next one. Each flare causes cumulative damage: scar tissue inside the eye, cataracts, retinal degeneration, and changes to the gel filling the eyeball. Over time, even well-treated horses can lose functional vision.
Surgical Options
For horses with confirmed Leptospira-associated ERU, a surgery called vitrectomy offers the best chance of stopping the cycle. The procedure removes the gel inside the eye (the vitreous), along with the inflammatory debris and bacteria trapped within it. A large study tracking 654 horse eyes over periods up to 18 years found that vitrectomy prevented recurrence in 96.3% of cases. Long-term vision was preserved in roughly 75 to 81% of eyes, and 94.3% of operated eyes were saved from removal entirely. The key finding: horses that had surgery earlier in the disease course, before extensive damage accumulated, had the best outcomes.
Another option is a sustained-release implant placed inside the eye that slowly delivers an immune-suppressing medication over several years. Research at NC State has shown these implants can release medication for more than four years, and over 80% of horses with implants had their ERU controlled long-term. This approach is still considered somewhat experimental but has shown strong results, particularly for horses that aren’t ideal candidates for vitrectomy.
Daily Management Between Episodes
Preventing flare-ups involves reducing both immune triggers and environmental irritants. UV-blocking fly masks with 90 to 95% UV protection are one of the most practical tools. Ultraviolet light can worsen eye pain, swelling, and cloudiness in horses with ERU, so consistent use of a high-quality mask during turnout helps reduce the frequency and severity of episodes.
Environmental management also matters. Minimizing standing water, improving pasture drainage, and controlling rodent populations around barns all reduce Leptospira exposure. Keeping stalls clean and dust-free helps limit irritation to already-compromised eyes. Some owners find that turnout during lower-light hours (early morning, evening) is better tolerated than midday sun.
Routine eye exams, ideally every six to twelve months, allow a veterinarian to catch subtle changes before they become irreversible. This is especially important for Appaloosas and other high-risk breeds, where the insidious form can progress quietly. Catching early signs like mild cloudiness, slight changes in pupil shape, or decreased eye pressure can prompt treatment before significant vision is lost.
Long-Term Outlook
Moon blindness is a lifelong condition. Without intervention, most affected horses will eventually lose vision in the affected eye, and many develop the disease in both eyes. The insidious form is particularly damaging because it progresses without the painful flare-ups that prompt owners to seek help. Horses with chronic ERU can have appreciable retinal degeneration and significant vision compromise even when the front of the eye looks relatively normal.
With consistent management, early surgical intervention when appropriate, and regular veterinary monitoring, many horses retain useful vision for years. Horses that do lose sight in one or both eyes can still adapt remarkably well in a familiar, stable environment, though their use under saddle or in performance settings becomes more limited and requires careful safety considerations. The single most important factor in preserving vision is early detection. By the time obvious damage is visible to the naked eye, much of it is irreversible.