Equine recurrent uveitis (ERU), historically known as “moon blindness” or periodic ophthalmia, is the most common cause of blindness in horses worldwide and affects an estimated 2-25% of the equine population depending on region and breed. ERU is characterized by repeated episodes of painful intraocular inflammation (uveitis) separated by quiet periods, with each episode causing progressive, cumulative damage to internal eye structures. Without treatment, ERU leads to cataracts, glaucoma, retinal detachment, and ultimately irreversible blindness.

Overview

Uveitis refers to inflammation of the uveal tract — the iris, ciliary body, and choroid — which are the vascular, pigmented structures inside the eye. In ERU, an immune-mediated process drives recurrent bouts of inflammation. The initial trigger may involve infection (particularly Leptospira species), but subsequent episodes are driven by autoimmune responses against ocular antigens, with T-lymphocytes attacking the horse’s own eye tissues.

The disease may present in three patterns:

  • Classic ERU — Distinct episodes of acute uveitis with symptom-free intervals. Each episode causes incremental damage.
  • Insidious ERU — Low-grade, persistent inflammation without obvious acute flare-ups. Common in Appaloosas and draft breeds. Often diagnosed late because signs are subtle.
  • Posterior ERU — Primarily affects the back of the eye (vitreous, retina, choroid). More common in European warmbloods and draft breeds.

Causes & Risk Factors

  • Leptospirosis — Leptospira interrogans (various serovars) has been identified within the eyes of many ERU-affected horses. Leptospiral infection is considered the most significant triggering factor, though not all ERU cases involve Leptospira.
  • Autoimmune molecular mimicry — Leptospiral antigens share structural similarity with ocular proteins, leading the immune system to attack the eye
  • Breed predisposition — Appaloosas are 8 times more likely to develop ERU than other breeds. Other at-risk breeds include draft horses, warmbloods, and Paints.
  • Genetic factors — Specific equine leukocyte antigen (ELA) types are associated with increased susceptibility
  • Geographic factors — Higher prevalence in areas with warm, wet climates where Leptospira thrives (standing water, wildlife reservoirs)
  • Trauma or other ocular infection — May trigger the initial inflammatory episode

Symptoms

Acute Episode

  • Squinting (blepharospasm) and tearing (epiphora)
  • Swollen, puffy eyelids
  • Cloudy or bluish cornea (corneal edema)
  • Constricted pupil (miosis)
  • Red, congested conjunctiva
  • Sensitivity to light (photophobia)
  • Discharge (watery to mucopurulent)
  • Visible haziness in the front of the eye (aqueous flare from protein and cells)
  • Reduced appetite and depression due to pain

Chronic Changes (Between or After Episodes)

  • Cataracts (lens opacity — the most common long-term complication)
  • Iris color changes (darkening, irregularity)
  • Posterior synechia (iris adhered to the lens)
  • Vitreous degeneration (floaters, loss of clarity)
  • Retinal degeneration or detachment
  • Phthisis bulbi (shrunken, non-functional eye in end-stage disease)
  • Corneal scarring or pigmentation

Any horse showing sudden squinting, tearing, and a cloudy eye should be examined by a veterinarian within 24 hours. Corneal ulcers and uveitis can look similar but require different treatments, and misdiagnosis can lead to serious complications.

Diagnosis

  • Complete ophthalmic examination — Examination in a darkened stall using a direct ophthalmoscope or slit-lamp biomicroscope. Assessment of anterior chamber for flare, cells, and fibrin; pupil response; lens clarity; fundus examination.
  • Tonometry — Measurement of intraocular pressure (IOP). Low IOP suggests active uveitis; elevated IOP may indicate secondary glaucoma.
  • Ocular ultrasound — Evaluates internal structures when the eye is too cloudy for direct visualization. Can identify vitreal debris, retinal detachment, and lens subluxation.
  • Fluorescein staining — To rule out concurrent corneal ulceration (important because corticosteroid treatment for uveitis is contraindicated with active ulceration)
  • Leptospira serology and aqueous/vitreous PCR — Serum antibody titers and intraocular fluid testing can support Leptospira involvement
  • History of recurrence — A critical diagnostic criterion; multiple episodes strongly support ERU rather than a single traumatic uveitis

Treatment & Medications

Acute Episode Management

  • Topical corticosteroids — Dexamethasone 0.1% or prednisolone acetate 1% ophthalmic drops, applied every 4-6 hours during acute inflammation. Only after confirming no corneal ulcer is present.
  • Topical atropine (1%) — Dilates the pupil to relieve ciliary spasm pain and prevent posterior synechia. Applied every 6-12 hours until the pupil dilates, then tapered.
  • Systemic NSAIDs — Flunixin meglumine (1.1 mg/kg IV or oral) to reduce intraocular inflammation and provide analgesia. Phenylbutazone is an alternative.
  • Topical NSAIDs — Diclofenac or flurbiprofen ophthalmic drops as adjunctive anti-inflammatory therapy
  • Subconjunctival corticosteroid injection — Triamcinolone or methylprednisolone injected under the conjunctiva in severe cases, performed by the veterinarian
  • Dark environment — Stall rest in a darkened stall to reduce photophobia and pain

Long-Term Management and Prevention of Recurrence

  • Cyclosporine suprachoroidal implant — A sustained-release cyclosporine device surgically placed beneath the sclera that delivers immunosuppressive medication directly to the eye for 3-5 years. Studies report significant reduction in flare-ups (up to 80-90% of treated horses have no recurrence). This is the most effective long-term intervention for ERU.
  • Intravitreal gentamicin injection — An alternative procedure where gentamicin is injected into the vitreous humor to reduce Leptospira organisms and modulate the immune response. Effective in many cases, particularly in European studies.
  • Core vitrectomy — Surgical removal of vitreous humor (which contains inflammatory mediators and Leptospira organisms). Performed under general anesthesia at referral hospitals. Most effective in Leptospira-positive cases.
  • Enucleation — Removal of a blind, painful eye. The most humane option when the eye has no visual potential and is a source of chronic pain.

Prognosis

  • ERU is a progressive disease; each inflammatory episode causes cumulative, irreversible damage
  • Without treatment, most affected horses eventually become blind in the affected eye (and 30-50% develop disease in the contralateral eye)
  • Cyclosporine implants and vitrectomy have significantly improved long-term outcomes, with many horses maintaining useful vision for years
  • Appaloosas tend to have bilateral, insidious disease with a more guarded prognosis
  • Early, aggressive treatment of acute episodes and pursuit of preventive surgical options offer the best chance of preserving vision
  • Blind horses can often be managed safely with environmental modifications, a consistent routine, and a sighted companion

Frequently Asked Questions

Why is it called “moon blindness”? The name dates to antiquity, when the recurring nature of the disease — with episodes appearing to wax and wane with lunar cycles — was attributed to the influence of the moon. We now understand the recurrent episodes are immune-mediated, not lunar.

Can ERU be prevented? There is no vaccine for ERU. Risk reduction includes minimizing exposure to Leptospira (managing standing water, controlling wildlife access to water sources, and maintaining clean environments). Genetic testing and breeding decisions in high-risk breeds may help long-term.

Should I pursue surgery (cyclosporine implant or vitrectomy)? If your horse has experienced two or more episodes of uveitis, consultation with a veterinary ophthalmologist about surgical options is strongly recommended. The earlier these interventions are performed (before significant structural damage), the better the visual outcome.

Can a horse with ERU still be ridden? Many horses with ERU in one eye or with controlled disease continue to be ridden and perform. Horses with limited or no vision in one eye adapt remarkably well, though they may be startled by objects approaching from the blind side. Discuss safety considerations with your veterinarian and trainer.

This information is for educational purposes only and does not replace professional veterinary advice. Consult a veterinary ophthalmologist for diagnosis and management of equine recurrent uveitis.