Moon Blindness (ERU) Vet Guide 2025 – Dr Duncan Houston 🐴👁️

By Dr Duncan Houston, BVSc – in-depth expertise on equine recurrent uveitis (ERU), aka moon blindness: understanding disease, managing flare-ups and emerging treatments.

Introduction

Equine Recurrent Uveitis (ERU), commonly called “moon blindness,” is the leading cause of blindness in horses. Characterized by recurring inflammation of the uveal tract, ERU has no true cure. This comprehensive 2025 guide explores its causes, clinical signs, diagnostic protocols, proactive management, treatment options—including new therapies—and owner-focused prevention tools. 👁️

📌 What Is Moon Blindness?

ERU is episodic inflammation in the uveal tract—the iris, ciliary body, and choroid. Episodes range from sudden painful flare-ups to insidious chronic inflammation. Repeated episodes over time can destroy ocular structures, leading to scarring, cataracts, glaucoma, and vision loss :contentReference[oaicite:3]{index=3}.

🔬 Causes & Risk Factors

Immune-mediated disease: often follows infections (Leptospira spp, Streptococcus, viruses) or trauma—triggering autoimmune responses :contentReference[oaicite:4]{index=4}.
Infectious influence: Leptospira infections in the eye correlate with severe ERU, especially in Appaloosas :contentReference[oaicite:5]{index=5}.
Genetic predisposition: Appaloosa horses up to 8× more likely to develop ERU; Leopard complex gene implicated :contentReference[oaicite:6]{index=6}.
Breed susceptibility: Appaloosas, Warmbloods, Drafts, Paints, Quarter Horses commonly affected :contentReference[oaicite:7]{index=7}.
Environmental factors: wetter climates may have higher leptospiral exposure :contentReference[oaicite:8]{index=8}.

📋 Types of ERU

Classic: Acute painful episodes separated by clear intervals. Leads to structural damage :contentReference[oaicite:9]{index=9}.
Insidious: Subtle, chronic low-grade inflammation with few external signs; often bilateral and damaging :contentReference[oaicite:10]{index=10}.
Posterior: Involves choroid and retina, causing visual impairment with less anterior eye signs :contentReference[oaicite:11]{index=11}.

⚠️ Clinical Signs & Complications

During active flare-ups, horses may show:

Redness, tearing, squinting, light sensitivity
Corneal edema, aqueous flare, fibrin, miosis
Secondary issues: cataracts, synechia, iris atrophy, glaucoma, retinal degeneration, optic atrophy :contentReference[oaicite:12]{index=12}.

Insidious or posterior ERU may only manifest as gradual vision loss, without obvious signs :contentReference[oaicite:13]{index=13}.

🔍 Diagnosis

History of relapse/remitting signs
Comprehensive ophthalmic exam: fluorescein stain, slit-lamp, intraocular pressure, fundoscopy :contentReference[oaicite:14]{index=14}.
Bloodwork and ocular fluid tests for leptospiral antibodies
Ultrasound may identify posterior inflammation or structural damage

🩺 Treatment Strategies

ERU therapy is twofold: control acute flares and minimize recurrence.

Acute Management

Topical corticosteroids (prednisolone acetate, dexamethasone) reduce inflammation :contentReference[oaicite:15]{index=15}.
Mydriatics (atropine) relieve pain and prevent synechia :contentReference[oaicite:16]{index=16}.
Topical NSAIDs (flurbiprofen, diclofenac) offer alternative anti-inflammatory effects :contentReference[oaicite:17]{index=17}.
Systemic NSAIDs (flunixin meglumine) help manage inflammation and pain :contentReference[oaicite:18]{index=18}.
Antibiotic injections: Intravitreal gentamicin shows promise in leptospira-related ERU cases :contentReference[oaicite:19]{index=19}.
Intracapsular steroids: For persistent posterior inflammation

Long-Term Management & Preventing Relapse

Suprachoroidal cyclosporine implants release anti-inflammatories over time :contentReference[oaicite:20]{index=20}.
Maintain topical steroids or NSAIDs between episodes in recurrent cases
Vitrectomy surgically removes infected vitreous and reduces recurrence—effective especially in Europe :contentReference[oaicite:21]{index=21}.
Tailored antibiotic therapy when leptospiral involvement suspected or proven :contentReference[oaicite:22]{index=22}.
Regular eye exams to catch inflammation early before damage occurs

🔬 Emerging Therapies

UF and collaborators are testing immunomodulatory eye drops (SOCS1-KIR peptides) showing promise in restoring vision and reducing inflammation :contentReference[oaicite:23]{index=23}. Additionally, AI-based diagnostic imaging may help early detection of subclinical ERU :contentReference[oaicite:24]{index=24}.

🧩 Prognosis

ERU is incurable, but early treatment can preserve vision. Many horses manage well for years, though repeated flares increase risk of blindness :contentReference[oaicite:25]{index=25}.

Prognosis varies: classic ERU often has poorer outcomes; insidious/posterior forms carry high risk of gradual vision loss :contentReference[oaicite:26]{index=26}.

🛡️ Prevention & Owner Strategies

Immediate care: Don’t ignore eye redness or discomfort
Remove flies/sunlight: Use UV-protective fly masks and ensure clean, shaded turnout
Hygiene: Clean gear, prevent eye trauma or contamination
Control leptospirosis: Secure water sources and rodent control
Breed awareness: Monitor at-risk Appaloosas and others closely
Annual eye exams: Especially for predisposed breeds

📊 Summary Comparison

Stage	Signs	Treatment	Goal
Acute	Red, painful, cloudy eye	Steroids, atropine, NSAIDs	Pain relief, prevent damage
Chronic	Docking vision, cataracts, synechia	Cyclosporine implants, vitrectomy	Long-term control, preserve vision
Preventive	No signs	Fly mask, sanitation	Reduce flare frequency

📲 Ask A Vet Support & Closing

Moon blindness is a lifelong challenge—but vigilance and proactive care can maintain vision and comfort. Early identification, rapid treatment, and strategic prevention are essential.

Want customized eye-health reminders, consultation support, or treatment planning? Ask A Vet offers expert protocols, outcome tracking, and tele-vet access through our app. Download the Ask A Vet App today to protect your horse’s sight and wellbeing. 🐎👁️