Equine Recurrent Uveitis: Moon Blindness in Horses

Moon blindness in horses — the colloquial name for equine recurrent uveitis (ERU) — is the leading cause of blindness in horses worldwide, and one of the most painful eye conditions a horse will experience. Despite the romantic name, there is nothing mystical about the disease: it is a chronic, immune-mediated form of uveitis in horses that flares unpredictably and, untreated, will rob a horse of its sight. UK veterinary surveys suggest 2–5% of British horses are affected at some point in their lives, with leptospirosis playing a smaller role here than in continental Europe but still a recognised trigger. This guide explains how ERU develops, how to spot it early, how vets diagnose and treat it in 2026, and how to manage a horse living with the condition.

What Is Moon Blindness?

Moon blindness is the historical name for equine recurrent uveitis, a chronic inflammatory disease of the uveal tract — the pigmented middle layer of the eye that contains the iris, ciliary body and choroid. Uveitis simply means inflammation of the uvea; the disease becomes recurrent uveitis when two or more episodes occur in the same eye, separated by quieter periods.

Each flare causes intense pain, swelling and clouding of the eye. The damage from each episode accumulates: the lens becomes cataractous (see our guide to equine cataracts), the retina scars, the iris adheres to the lens (synechiae), and pressure inside the eye climbs (glaucoma). Over months and years, sight is progressively lost, often ending in complete blindness in the affected eye.

Why Is It Called Moon Blindness?

The name dates back at least to the 17th century, when horse owners noticed the disease flared and faded in cycles and assumed the moon was responsible. The Roman writer Vegetius used a luna-related term, and the description “moon-eye” appears in 18th-century British veterinary texts. We now know the moon has nothing to do with it: the cyclic flares reflect the immune-mediated nature of the disease, not lunar phases. The name has nonetheless stuck, alongside “periodic ophthalmia” and the modern clinical term equine recurrent uveitis.

How Common Is ERU in the UK?

British prevalence figures sit between 2% and 5% of the horse population, with higher rates in certain breeds (see below). Worldwide, the MSD Veterinary Manual reports a prevalence of 2–25%, depending on region and breed. UK ophthalmology referral centres — the Animal Health Trust before its closure, and now the equine ophthalmology services at Liphook, Rossdales and the Royal Veterinary College — report ERU as one of their commonest reasons for equine eye referral. Climate plays a role: damp, low-lying yards with rodent populations carry higher leptospirosis risk, which in turn raises ERU risk.

Breeds Most at Risk

Genetic predisposition is well-established for several breeds:

• Appaloosa: eight times more likely to develop ERU than other breeds, with severe disease at younger ages. Linked to the leopard-complex (LP) gene that produces their spotted coat patterns.
• German and continental Warmbloods: a particular MHC haplotype (ELA-A9) carries elevated risk.
• Knabstrupper and other spotted breeds: share the leopard-complex genetic risk profile of Appaloosas to a lesser degree.
• The Icelandic Horse: reported internationally to develop a particularly aggressive, vision-threatening form of ERU.
• Draft horses: Belgian and Percheron lines are reported to have higher rates.

That said, any horse of any breed can develop ERU. UK reported cases include Thoroughbreds, native ponies, cobs and Arabs.

What Causes Equine Recurrent Uveitis?

ERU is fundamentally an autoimmune disease: the horse’s own immune system attacks ocular proteins, and once the cycle starts the eye becomes a target organ regardless of the original trigger. The triggers themselves include:

• Leptospirosis infection — particularly serovars Pomona and Bratislava. The bacteria invade the eye and persist, driving chronic inflammation. Common in mainland Europe and the United States; less prevalent but recognised in the UK, especially on yards with cattle, rodents or stagnant water.
• Eye trauma — a corneal ulcer, foreign body or blunt blow that starts an inflammatory cascade. An untreated ulcer can progress to equine keratitis and chronic uveitis.
• Parasitic infection — Onchocerca cervicalis and other parasites, more common historically and pre-routine ivermectin use.
• Bacterial or viral infection — including equine herpesvirus and Brucella.
• Genetic susceptibility — in Appaloosas and certain warmblood lines.
• Idiopathic (unknown cause) — in many UK cases the original trigger is never identified.

The Three Types of ERU

UK equine ophthalmologists recognise three clinical patterns:

• Classic ERU: distinct, painful flares of inflammation lasting days to weeks, separated by quiet periods of weeks to months. The most common pattern.
• Insidious ERU: low-grade, near-constant inflammation without obvious flares. Easily missed; the horse never appears acutely sore but progressively loses vision. Particularly common in Appaloosas.
• Posterior ERU: inflammation predominantly in the back of the eye (retina, choroid, vitreous). Less ocular pain, more rapid retinal damage. Common in European warmbloods.

Symptoms of Moon Blindness in Horses

An acute flare presents dramatically. Look for:

• Squinting (blepharospasm) — the horse holds the eye partly or fully closed.
• Tearing — copious, often clear; the horse may rub the eye on a leg or fence.
• Photophobia — aversion to bright light; horse seeks the back of the stable.
• Cloudy or hazy cornea — a blue-grey or white film over the eye.
• Red, bloodshot conjunctiva — not to be confused with simple equine keratoconjunctivitis, which a vet will rule out.
• Constricted (miotic) pupil — small, often pin-point.
• Yellow or greenish material in the front chamber — pus or fibrin.
• Visible discomfort — reduced appetite, head shaking, dullness.

Insidious ERU shows none of this drama. Watch instead for a slowly developing cataract, persistent mild squinting in bright light, dark spots on the iris (corpora nigra deformities) or a pupil that no longer dilates fully — and book a thorough ocular examination if they appear. Reduced vision in dim light has separate causes too, such as equine night blindness (CSNB), so an accurate diagnosis matters.

How Vets Diagnose ERU

Diagnosis combines history, full ophthalmic examination and ancillary tests. A first-opinion vet will typically:

• Take a full history of previous eye episodes, breed, vaccinations and turnout.
• Examine the eye with a focal light and direct ophthalmoscope, after dilating the pupil with tropicamide (unless contraindicated by elevated pressure).
• Stain the cornea with fluorescein to rule out an active ulcer (a critical decision — topical steroids will worsen an ulcer).
• Measure intraocular pressure with a tonometer (TonoVet or Tono-Pen) to detect glaucoma or hypotony.
• Refer complex or recurrent cases to an equine ophthalmologist for slit-lamp biomicroscopy, gonioscopy and ocular ultrasound where the cornea is too cloudy for direct examination — the same specialists who handle other equine orbital conditions.
• Consider serology and aqueous humour PCR for leptospirosis where indicated.

Treatment Options

The aim of treatment is to reduce inflammation, control pain, preserve vision, and lengthen the gaps between flares. Standard British protocol, broadly in line with the approach used by UK equine practices, involves:

• Topical corticosteroids (prednisolone acetate 1% or dexamethasone) four to six times daily during a flare, tapering as the eye settles. Only used once an ulcer has been ruled out.
• Topical atropine to dilate the pupil and prevent the iris adhering to the lens (synechiae), and to relieve ciliary muscle spasm pain.
• Topical NSAIDs (diclofenac, flurbiprofen) where steroids are contraindicated.
• Systemic NSAIDs — flunixin meglumine (Finadyne) or phenylbutazone (bute) for pain and to reduce systemic inflammation.
• Antibiotics if infection is suspected or confirmed.
• A UV-protective fly mask — an absolute must for any horse with ERU, year-round in the UK during daylight hours.

All medication must be prescribed and dosed by your vet; the regimes above are described for understanding, not as a recipe to self-medicate. Topical eye medication in horses is famously challenging. A subpalpebral lavage system — a soft tube placed under the upper eyelid, exiting at the side of the neck — allows medication to be delivered without needing to handle the painful eye repeatedly. Most UK referral centres can place one as an outpatient procedure.

The Cyclosporine Implant

For horses with frequent flares despite medical management, a suprachoroidal cyclosporine implant offers years of disease control from a single surgery. The implant, placed between the sclera and choroid, slowly releases cyclosporine A directly into the eye for around three to four years, dramatically reducing flare frequency in many cases.

UK availability is limited to a small number of referral centres — typically the RVC, Rossdales, Liphook and a handful of regional specialist hospitals. The procedure is best suited to early-to-mid stage ERU, before significant cataract or glaucoma develops. Discuss with an equine ophthalmologist as soon as a horse has had two or more flares.

When Enucleation Becomes Necessary

Enucleation — surgical removal of the eye — sounds drastic, but for a horse with end-stage ERU, intractable pain, secondary glaucoma or a phthisical (shrunken) eye, it is often the kindest option and dramatically improves quality of life. Horses adapt to monocular vision within days. Many British eventers, dressage horses and happy hackers continue to perform well after enucleation, with appropriate adjustments.

Managing a Horse with ERU at Home

Practical day-to-day management:

• UV-blocking fly mask: wear during all daylight turnout, year-round.
• Reduce dust and ammonia: deep, well-ventilated bedding, clean stables, soaked or steamed hay if dust is an issue.
• Manage flies: repellent, fly fringes, and ideally turnout at dawn/dusk with stabling during peak fly hours in summer.
• Reduce leptospirosis risk: control rodents on the yard, fence off standing water, and avoid grazing alongside cattle where possible.
• Vaccination: a Leptospira vaccine for horses is licensed in the US and has been used off-label in some UK referral cases; discuss with your vet.
• Routine ocular checks every 3–6 months — insidious flares are often picked up only by a careful eye exam.
• Keep a flare diary: dates, triggers, severity, treatment used and response. Invaluable for ophthalmology referrals.

Can ERU Be Prevented?

The original cause cannot always be prevented, particularly where the trigger is genetic or unknown. What you can do is reduce known risk factors:

• Treat any eye injury or ulcer aggressively and early — do not let a corneal ulcer simmer.
• Maintain a robust deworming programme guided by faecal egg counts.
• Use UV protection on horses with light-pigmented eyes.
• Investigate any unexplained eye episode promptly — the second flare is more easily controlled if the first was managed early and thoroughly.

Life Expectancy and Prognosis

ERU does not directly shorten life expectancy. With aggressive early management, many horses retain useful vision in the affected eye for years; with the cyclosporine implant, flares can stop almost entirely. Without treatment, however, more than half of affected horses go blind in the affected eye, and bilateral blindness occurs in around 20% of severe cases. Ridden careers can continue with one eye removed; horses with bilateral blindness can live good lives in a stable, well-known environment with experienced handling.

When to Call the Vet Urgently

Any horse holding an eye closed, tearing heavily or showing a cloudy cornea is a same-day veterinary call. Eye conditions in horses move fast: a corneal ulcer can perforate in 24–48 hours, and an untreated ERU flare causes permanent damage with every passing day. Do not wait. Do not apply leftover ointment from a previous case — a steroid drop on an undiagnosed ulcer can blind the eye. Call the vet.

Frequently Asked Questions

Can a horse recover from moon blindness?

Individual flares can settle completely with prompt treatment, and many horses regain comfortable vision between episodes. The underlying disease, however, is incurable: ERU is a chronic, lifelong condition. The aim is to control flares, slow vision loss, and where possible eliminate flares entirely with a cyclosporine implant.

Is moon blindness in horses painful?

Yes — an active ERU flare is genuinely painful. The inflammation causes the ciliary muscle to spasm and the eye becomes acutely sensitive, which is why affected horses squint, tear, shy from light and may go off their feed. Pain relief is a central part of treatment, and any horse showing these signs needs same-day veterinary attention rather than “watching and waiting”.

Is moon blindness in horses caused by leptospirosis?

Leptospirosis is one of the most clearly identified triggers globally, particularly serovars Pomona and Bratislava. In the UK it is less common than in the US or mainland Europe but still recognised, especially on yards with cattle or rodents. Many UK cases have no identifiable infectious cause and are presumed primarily autoimmune.

Is moon blindness hereditary?

Genetic susceptibility is well-documented in Appaloosas (linked to the leopard-complex LP gene) and in certain warmblood lines (specific MHC haplotypes). The disease itself is not inherited as a single Mendelian trait, but the predisposition can be passed on, which is one reason responsible Appaloosa breeders screen breeding stock.

What does an early flare of moon blindness look like?

The earliest signs are usually subtle: mild squinting in bright light, slightly increased tearing, a pupil that is smaller than the other side, and a faintly cloudy or hazy cornea. A horse showing any of these signs warrants a same-day veterinary examination — early treatment is far more effective than catching a fully developed flare 48 hours in.

Can my horse still be ridden with ERU?

Between flares, yes — many horses with ERU continue to compete and hack. During an active flare the horse is in pain and should not be ridden. Horses that have undergone enucleation (eye removal) adapt remarkably well to monocular vision and can often return to their previous level of work, sometimes within weeks of surgery. For a clear clinical overview, Horse & Hound has a useful vet-reviewed summary.