Bit & Bridle

Equine Uveitis (ERU)

Other Names

  • Moon Blindness
  • Periodic Ophthalmia
  • ERU
  • Recurrent Uveitis
  • Equine Recurrent Uveitis

What is Equine Uveitis (ERU)?

Equine uveitis is an inflammation of the uveal tract — the vascular middle layer of the eye, comprising the iris, ciliary body, and choroid. Equine recurrent uveitis (ERU) is the most important ophthalmic condition in horses worldwide, and is the leading cause of blindness in the species. It is characterised by repeated episodes of painful intraocular inflammation separated by periods of apparent quiescence, though immune-mediated damage continues even between visible flare-ups.

The condition has been recognised for centuries — the historical name “moon blindness” referred to the apparent association with lunar cycles, which was attributed to the episodic, waxing-and-waning nature of the condition. Each inflammatory episode causes cumulative damage to the delicate structures of the eye, progressively impairing vision and ultimately leading to blindness if not managed appropriately.

ERU is most common in certain breeds, particularly Warmbloods, Appaloosas, and Draught breeds, and affects horses of all ages, though it typically first presents in horses between 4 and 8 years of age.

Causes

Equine uveitis can be triggered by various initiating factors, with immune-mediated mechanisms perpetuating the condition:

  • Leptospirosis: Leptospira infection is the most important infectious cause of ERU, responsible for a significant proportion of cases in many parts of Europe. The bacteria can persist within the eye, triggering chronic immune-mediated inflammation. Leptospira pomona is most strongly implicated.
  • Immune-mediated (idiopathic): In many horses, the immune system mounts an inappropriate attack against intraocular proteins — a form of autoimmune disease. The trigger may have been an infection that has since resolved, but the immune response has become self-perpetuating.
  • Trauma: A single significant eye injury can trigger uveitis, which may then become recurrent.
  • Onchocerca cervicalis: A parasitic worm whose microfilariae (larvae) can trigger inflammatory reactions in the eye, particularly in the anterior uvea.
  • Systemic disease: Various systemic infections (including strangles, EHV, and others) may occasionally trigger uveitis, though leptospirosis is the most clinically important.
  • Breed predisposition: Appaloosas are disproportionately affected — studies suggest the gene encoding the leopard complex coat pattern is linked to increased ERU susceptibility through immune mechanisms.

Symptoms and Signs

Acute episodes:

  • Photophobia (squinting, reluctance to open the eye in light)
  • Epiphora (excessive tearing)
  • Blepharospasm (spasm of the eyelids — the horse holds the eye tightly closed)
  • Cloudiness or haziness of the cornea (due to corneal oedema)
  • Hypopyon — white/yellow fluid visible in the lower anterior chamber of the eye (pus cells)
  • Miosis — abnormal constriction of the pupil
  • Redness and swelling of the conjunctiva
  • Sensitivity around the eye on palpation

Chronic and recurrent changes:

  • Posterior synechia — adhesions between the iris and the lens (iris is “stuck” and may show an irregular pupil margin)
  • Lens opacity — cataract formation
  • Vitreous floaters — strands of debris visible in the vitreous humour
  • Retinal degeneration and detachment in advanced cases
  • Phthisis bulbi — a shrunken, degenerate, blind eye in end-stage disease
  • Gradual or complete vision loss

Diagnosis

Diagnosis requires thorough ophthalmic examination, ideally by a veterinarian with specific equine ophthalmology experience:

  • Complete ophthalmic examination: Assessment of all ocular structures under mydriasis (pharmacological pupil dilation using tropicamide drops). Includes slit lamp biomicroscopy of the anterior segment and indirect ophthalmoscopy of the posterior segment.
  • Intraocular pressure (IOP) measurement: Tonometry measures the pressure within the eye. In acute uveitis, IOP is typically low (hypotony). Elevated IOP indicates secondary glaucoma — a serious complication.
  • Leptospira testing: Serology (MAT test) and/or PCR on blood or aqueous humour (fluid aspirated from the anterior chamber) can detect leptospiral involvement. Aqueous humour PCR is particularly useful.
  • Differential diagnosis: The acute eye examination must differentiate uveitis from other painful eye conditions including corneal ulceration, glaucoma, and conjunctivitis — each requiring different management. A horse with a painful closed eye should always be examined by a vet promptly; self-treatment or assumption can cause significant harm.

Treatment

Management of ERU has two goals: controlling acute episodes and preventing recurrence:

  • Acute episode management:
    • Topical atropine (1%) — dilates the pupil, reduces pain from ciliary spasm, and prevents iris-lens adhesions (posterior synechia). Applied 2–4 times daily during acute flares; use carefully in horses with colic risk.
    • Topical corticosteroids (prednisolone or dexamethasone drops) — reduce intraocular inflammation. Contraindicated if corneal ulceration is present.
    • Systemic NSAIDs (flunixin meglumine or phenylbutazone) — systemic anti-inflammatory support.
    • Dark stabling and fly mask — reduce photophobia and protect the eye.
  • Chronic management and prevention of recurrence:
    • Cyclosporin A intravitreal device (CsA implant) — a surgical implant placed within the vitreous humour that continuously releases cyclosporin A, an immunosuppressive drug, directly into the eye. A major advance in ERU management; shown to significantly reduce the frequency and severity of recurrent episodes. Performed under general anaesthesia at specialist equine hospitals.
    • Systemic antibiotics (doxycycline) — where leptospirosis is implicated, antibiotic treatment may reduce bacterial load and slow disease progression. Some evidence supports long-term doxycycline use in leptospiral ERU.
    • Vitrectomy — surgical removal of the vitreous humour (the gel within the eye) can remove antigens, antibodies, and inflammatory mediators, reducing recurrence rates. Less commonly performed than the CsA implant but offers good results in specialist hands in Europe.
    • Enucleation — surgical removal of the eye. Considered when the eye is permanently blind, causing chronic pain, or when medical management is failing. Horses adapt well to life with one eye.

Prevention

  • Leptospirosis control: Address leptospiral exposure risk — rodent control, clean water sources, isolation of aborting mares, and vaccination where available. See the leptospirosis entry.
  • Parasite control: Appropriate worming programmes targeting Onchocerca cervicalis (ivermectin-based products are effective against microfilariae).
  • Prompt treatment of eye injuries: Any eye trauma should be assessed and treated promptly by a vet to minimise the risk of initiating chronic uveitis.
  • Reduce UV exposure: UV-protective fly masks are beneficial for horses with chronic uveitis, as light exposure can exacerbate inflammation.
  • Breeding considerations: Appaloosa owners should be aware of the breed’s elevated ERU risk and ensure regular ophthalmic monitoring.

Prognosis

Without treatment, ERU follows a progressive course — each episode of inflammation causes more cumulative damage, and blindness is a common outcome over years. With appropriate management:

  • Cyclosporin A implants significantly reduce recurrence rates in many horses and can preserve vision for years beyond what would otherwise be possible.
  • Horses with leptospiral ERU treated with antibiotics and cyclosporin show the best outcomes.
  • Appaloosas generally have a more aggressive disease course and a higher risk of progression to blindness despite treatment.
  • Blind horses can live comfortably and safely in stable management — the welfare of a well-managed blind horse should not be automatically considered poor.

When to Call the Vet

A horse with a closed, squinting, or weeping eye should be examined by a vet the same day — ideally within hours. Eye conditions can deteriorate rapidly and some, such as corneal ulceration, require urgent specific treatment. Do not apply any eye medication without veterinary advice, as corticosteroids (which are appropriate for uveitis) are contraindicated and potentially damaging if an ulcer is present. If your horse has a known history of ERU, contact your vet at the start of any acute episode rather than waiting to see whether it resolves.

Frequently Asked Questions

Can a horse with uveitis still be ridden?

Many horses with well-managed ERU — particularly those that are unilaterally affected and retain good vision in the healthy eye — can continue to be ridden safely. Horses adapt surprisingly well to unilateral vision loss. However, the horse’s vision, temperament, and the demands of the work must all be considered. A horse that is acutely inflamed, in pain, or significantly vision-impaired should not be ridden. Your vet will advise on fitness to ride based on the individual horse’s status.

Is ERU the same as “moon blindness”?

Yes — moon blindness is the historical common name for equine recurrent uveitis. The name arose because episodes appeared to coincide with phases of the moon (due to the regular but unpredictable recurrence pattern). It is the same condition, and the two terms are used interchangeably. The modern, preferred name is equine recurrent uveitis (ERU).

What is the cyclosporin implant and does it work?

The cyclosporin A (CsA) intravitreal implant is a small device surgically placed inside the eye that slowly releases cyclosporin, a potent immunosuppressant, directly into the vitreous humour over a period of years. Multiple studies have shown that the implant significantly reduces the frequency and severity of uveitis episodes in the majority of horses. It is considered the most effective currently available treatment for managing recurrent ERU. It is performed under general anaesthesia at specialist equine hospitals and referral centres.

Will my horse go blind from uveitis?

Not necessarily, but ERU is the leading cause of equine blindness, and the risk increases with each inflammatory episode. Horses that receive prompt, appropriate treatment during flare-ups, and who are managed with preventive measures (including cyclosporin implantation where appropriate), have a significantly better chance of retaining useful vision. The outcome varies with the individual horse, the underlying cause, and the frequency and severity of episodes. Regular monitoring by a veterinarian experienced in equine ophthalmology is essential.