Equine Recurrent Uveitis in Older Horses
Equine recurrent uveitis, or moon blindness, is the top cause of equine blindness. Learn the signs, why it is an emergency, treatment, and how to reduce flares.
Few conditions worry horse owners as much as a painful eye, and for good reason. Equine recurrent uveitis, widely known as moon blindness, is the leading cause of blindness in horses around the world. It is not a single event but a pattern of repeated inflammation inside the eye, with each flare adding to the damage. Older horses with a history of eye trouble, and breeds like the Appaloosa, are especially at risk.
The encouraging part is that early, aggressive treatment of each flare protects vision, and many horses with ERU live full, comfortable lives. This guide explains what uveitis is, how to recognize a flare, why it is a true emergency, and how treatment and daily management reduce the toll. It is educational information meant to support your equine veterinarian's advice, not to replace it. When in doubt about an eye, call your vet today.
Eye Protection and Flare Prevention Helpers
EquiVizor 95% UV Eye Protection Fly Mask
UV-blocking mask designed to help horses prone to uveitis
Cashel Crusader Fly Mask with Ears
Trusted fly mask that shields the eyes from flies and debris
Intrepid International Equine-Medi Lens Eye Hood
Shatterproof flexible lens hood for protecting a healing eye
A note before you shop: a fly mask and good fly control reduce triggers and protect the eye, but they do not treat a flare. An active uveitis episode is a veterinary emergency that needs prescription medication, not management products alone.
What Happens Inside the Eye
The uvea is the eye's middle layer, rich in blood vessels, that includes the iris and related structures. In uveitis, this layer becomes inflamed. The eye floods with inflammatory cells and fluid, the pupil clamps down painfully, and pressure and chemistry inside the eye are disturbed. A single bout is damaging enough, but ERU is defined by recurrence: the inflammation returns again and again, and each episode chips away at the delicate internal structures.
Over repeated flares, that accumulating damage can lead to cataracts, scarring, glaucoma, retinal injury, and ultimately blindness. This is exactly why stopping each flare quickly, and reducing how often they happen, is the heart of managing the disease.
Recognizing a Flare
A uveitis flare is painful, and the signs reflect that pain. Learn them so you can act fast.
- Squinting or holding the eye partly or fully closed
- Excessive tearing or watery discharge down the face
- Sensitivity to bright light, seeking shade or turning away
- A cloudy, hazy, or bluish look to the normally clear cornea
- A small, constricted pupil, called miosis
- Redness around the eye, swelling of the lids, or rubbing
Any one of these means a painful eye, and a painful eye in a horse is always urgent. Do not wait to see if it improves overnight, and do not reach for a leftover tube of ointment, because some eye medications are harmful if there is also a corneal ulcer. Call your veterinarian the same day so the eye can be examined and treated correctly.
Why It Is a True Emergency
Time matters with uveitis. Every flare that goes untreated allows inflammation to injure the eye further, and the difference between same-day treatment and a delay of a few days can be the difference between preserving and losing vision. Prompt anti-inflammatory therapy also relieves genuine pain. Treating a flare as an emergency is one of the most protective things an owner can do. For a wider view of urgent situations, see our guide to when to call the vet for a senior horse.
Causes and the Appaloosa Connection
ERU is fundamentally an immune-mediated disease. After an initial insult, the immune system continues to target tissue inside the eye, which drives the recurring pattern. The first trigger is often never identified, but it can follow eye injury, infection, or other illness, and in many areas exposure to the bacterium Leptospira is strongly linked to the disease.
Genetics play a clear role. Appaloosas are several times more likely to develop ERU than other breeds, and their disease tends to be more severe, more often involves both eyes, and is harder to control. Horses carrying Appaloosa ancestry or related coat-pattern genetics share some of that elevated risk. If you own an Appaloosa or a leopard-spotted horse, treat eye health as a priority.
Treatment and Long-Term Management
Treatment has two goals: stop the current inflammation fast and reduce future flares. Your veterinarian leads both.
| Approach | Purpose |
|---|---|
| Anti-inflammatory medication | Calms the active flare and protects the eye |
| Atropine | Relaxes the pupil and relieves painful spasm |
| Treating the underlying trigger | Addresses infection or injury when identified |
| Cyclosporine implant (surgical) | Reduces flare frequency in hard-to-control cases |
| UV fly mask and fly control | Lowers light and irritation that can provoke trouble |
Flares are treated aggressively, then medications are tapered slowly under veterinary direction, never stopped abruptly. For horses with frequent, poorly controlled recurrence, an equine ophthalmologist may place a sustained-release cyclosporine implant inside the eye, which can meaningfully cut the number of episodes. ERU is managed for life rather than cured, so a long-term partnership with your vet is essential.
Reducing Flares Day to Day
Between flares, your job is to lower the triggers. A UV-blocking fly mask is one of the most practical tools, shading the eyes and reducing the bright light that bothers an affected horse. Keep flies under control with repellent, sheets, and clean surroundings, since irritation and rubbing do the eyes no favors. Maintain a calm routine, address any eye injury promptly, and keep up regular veterinary eye checks so subtle changes are caught early. If your horse develops cataracts from repeated flares, our guide to cataracts and vision loss in horses covers what comes next.
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Living With a Horse That Has ERU
An ERU diagnosis is serious, but it is far from hopeless. Many horses live comfortably for years when every flare is caught and treated quickly and triggers are kept low. Even horses that lose sight in one or both eyes adapt remarkably well with consistent handling and a stable, predictable environment, a subject we cover in our vision-loss guide.
The horses that fare best have owners who never gamble with a painful eye. Learn the signs, keep a UV fly mask on hand, control the flies, and call your vet the moment something looks off. With that vigilance, recurrent uveitis becomes a manageable lifelong condition rather than the end of your horse's bright, active years.
Related Senior Horse Health Guides
- Cataracts and Vision Loss in Horses - What happens when sight fades and how horses adapt.
- Melanoma in Grey Horses - Another eye-area concern that needs prompt vet attention.
- Signs a Senior Horse Is in Pain - Reading the quiet discomfort of an aging horse.
- When to Call the Vet for a Senior Horse - The signs that mean do not wait.
Frequently Asked Questions
What is equine recurrent uveitis?
Equine recurrent uveitis, or ERU, often called moon blindness, is repeated episodes of painful inflammation inside the eye. Each flare causes swelling and damage within the eye's interior structures, and because the bouts return again and again, the cumulative harm builds over time. ERU is the most common cause of blindness in horses worldwide. It can affect one eye or both. The recurring nature is the defining feature: even after a flare settles, another can follow weeks or months later.
What are the signs of uveitis in a horse?
A flare is painful and usually obvious once you know what to look for: squinting or holding the eye shut, excessive tearing, sensitivity to bright light, and a cloudy, hazy, or bluish appearance to the normally clear cornea. The pupil often becomes small, called miosis, and the eye may look red around the edges. Some horses rub the eye or become head shy. Any of these signs means a painful eye, and a painful eye in a horse is an emergency that needs same-day veterinary care.
Is equine uveitis an emergency?
Yes. A suddenly painful, weepy, cloudy, or squinting eye should be treated as an emergency, and you should call your veterinarian the same day. Uveitis flares cause real damage with every episode, and prompt anti-inflammatory treatment limits that damage and eases the horse's pain. Delays let inflammation injure the internal eye structures, raising the risk of permanent vision loss. Do not apply leftover ointments on your own, since some eye medications are harmful if the cornea is also ulcerated. Let the vet examine first.
What causes equine recurrent uveitis?
ERU is an immune-mediated disease: the horse's own immune system keeps attacking tissue inside the eye, which is why episodes recur. The original trigger can be hard to pin down and may follow infection, eye injury, or other illness, and in many regions exposure to the bacterium Leptospira is strongly associated with it. Appaloosas are notably predisposed and tend to have more severe, harder-to-control disease, and other breeds with Appaloosa heritage share some of that risk. Often a single root cause is never identified.
How is equine uveitis treated?
Treatment aims to stop the inflammation fast and control the pain. Your veterinarian typically prescribes anti-inflammatory medication, often combined with atropine to relax the eye and ease painful spasm of the pupil, and addresses any underlying trigger. Flares are managed aggressively and tapered carefully under veterinary guidance. For horses with frequent, hard-to-control recurrence, a surgical option such as a sustained-release cyclosporine implant placed by a specialist can reduce the number of flares. Treatment is lifelong management rather than a one-time cure.
Are Appaloosas more likely to get uveitis?
Yes. Appaloosas are several times more likely to develop ERU than other breeds, and their disease tends to be more severe, more often affects both eyes, and is harder to control, which makes them more likely to go blind from it. Horses with Appaloosa ancestry or certain coat-pattern genetics can carry elevated risk too. If you own an Appaloosa, watch the eyes especially closely, keep a UV-blocking fly mask on in bright conditions, and have any squinting or cloudiness checked without delay.
Can a horse with uveitis still have a good life?
Absolutely. Many horses with ERU live comfortably for years when flares are caught early and managed well, and even horses that lose vision in one or both eyes adapt remarkably with consistent handling and a stable environment. The keys are fast treatment of every flare, reducing triggers with a UV fly mask and good fly control, and close partnership with your vet. With diligent management, ERU is a condition you live alongside rather than one that ends your horse's working or retirement years.
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