Equine Recurrent Uveitis at a Glance
ERU Cause Distribution
Comparison
| Feature | Classic ERU | Insidious ERU | Posterior ERU |
|---|---|---|---|
| Onset | Sudden, painful, photophobic | Subtle, gradual | Usually acute |
| Signs | Squinting, tearing, cloudiness | Mild cloudiness only | Vision loss, no pain |
| Primary Site | Anterior chamber, iris | Anterior chamber | Retina, choroid |
| Breed Predilection | Appaloosa, Warmblood | Draft breeds | All breeds |
ERU Treatment Flow
ERU Prevention Checklist
| Vaccinate against Leptospira if recommended regionally | |
| Minimize fly exposure around eyes (fly mask) | |
| Use UV-blocking fly mask in bright conditions | |
| Check eyes daily for squinting, tearing, cloudiness | |
| Maintain topical NSAID during high-risk seasons | |
| Schedule ophthalmic exams twice yearly | |
| Consider suprachoroidal cyclosporine implant for recurring cases | |
| Keep atropine and flurbiprofen on hand for acute flares |
Equine Recurrent Uveitis (ERU), also called moon blindness, is the most common cause of blindness in horses worldwide. It involves repeated episodes of intraocular inflammation that progressively damage the uveal tract -- iris, ciliary body, and choroid -- leading to cataracts, glaucoma, retinal detachment, and permanent vision loss. Appaloosas are 8 times more likely to develop ERU than other breeds. There is no cure; treatment focuses on controlling inflammation during episodes and preventing recurrences.
| ERU Phase | Key Signs | Management |
|---|---|---|
| Acute episode | Squinting, tearing, cloudy cornea, miosis, photophobia | Atropine q4–6h + topical steroids + systemic NSAID; continue 1–2 wks after signs clear |
| Inter-episode | Near-normal appearance; silent structural damage accumulating | Annual ophthalmic exam; baseline fundic photography |
| Advanced disease | Cataracts, glaucoma, retinal detachment | CsA implant or vitrectomy; enucleation for blind painful eye |
What Triggers ERU Episodes
ERU has multiple causes. The most common in the United States is Leptospira species infection -- serovars Pomona and grippotyphosa are most frequently implicated. Leptospires invade ocular tissue, triggering an immune-mediated inflammatory response that recurs long after the initial infection clears. Other causes: Borrelia burgdorferi (Lyme disease), equine herpesvirus-1, Toxoplasma gondii, trauma, and idiopathic (no identifiable cause). The eye's immune privilege breaks down, and the immune system attacks intraocular proteins, perpetuating a cycle of inflammation with each episode.
Signs of an Active Uveitis Episode
Acute signs: squinting (blepharospasm), excessive tearing (epiphora), cloudy or blue-tinged cornea (corneal edema), constricted pupil (miosis), sensitivity to light (photophobia), and a dull, painful appearance to the eye. The eye often feels warmer than normal. Episodes last 1-4 weeks if untreated. Between episodes, the eye may appear nearly normal, but cumulative damage accumulates with each recurrence. By the time an owner notices cloudiness, significant structural damage has often already occurred.
Diagnosis and Veterinary Evaluation
Diagnosis requires ophthalmic examination by a veterinarian, ideally an equine ophthalmologist. Findings include: flare (protein in the anterior chamber), hypopyon (white blood cells settling in the anterior chamber), posterior synechia (adhesions between iris and lens), and vitreous haze. Leptospiral antibody titers (micro-agglutination test) can identify exposure but do not confirm ERU causation. Genetic testing for the Appaloosa LP gene helps assess breed-specific risk. Baseline examination and fundic photography establish a reference for tracking progression.
Treatment During Acute Episodes
Topical atropine (1% ophthalmic solution) dilates the pupil and reduces pain from ciliary spasm; apply every 4-6 hours during acute episodes. Topical corticosteroids (prednisolone acetate 1%) reduce inflammation; alternate with atropine. Systemic NSAIDs (flunixin meglumine 1.1 mg/kg IV or oral twice daily) reduce systemic inflammation. Systemic corticosteroids (dexamethasone) are used in severe cases. Treatment continues 1-2 weeks after signs resolve. Never skip doses -- incomplete treatment accelerates structural damage.
Surgical Options for Recurring Cases
Intravitreal cyclosporine implant (CsA implant) is a surgical option for horses with frequent recurrences. A slow-release implant is sutured to the sclera, delivering cyclosporine to the vitreous for up to 4 years. Studies show 66-80% of horses have reduced recurrence frequency, and some go into long-term remission. Cost: $1,500-3,000. Vitrectomy (removal of the vitreous) is another option, particularly for Leptospira-positive horses -- removing the vitreous removes the antigenic reservoir that drives recurrences. Enucleation (eye removal) is indicated for a blind, painful eye that no longer responds to medical management.
Sources: Gilger BC. Equine Uveitis. Veterinary Clinics of North America: Equine Practice 2010; Dwyer AE. Practical field ophthalmology. AAEP Proceedings 2011; Leutenegger CM et al. Equine recurrent uveitis and Leptospira. Veterinary Microbiology 1999; Brinkmann et al. Appaloosa LP gene and ERU prevalence study.