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. 🐎👁️
This article is general veterinary education and does not diagnose or treat an individual animal. Always confirm medication names, doses and interactions with the veterinarian who knows your pet.
If your animal is struggling to breathe, collapsed, bleeding heavily, seizing, or has eaten something toxic, contact your vet or the nearest emergency clinic now.
Animal Poisons Helpline (AU) 1300 869 738 · Animal Poisons Helpline (NZ) 0800 869 738 · ASPCA Animal Poison Control (US) (888) 426-4435
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Ask A Vet articles are written by Dr Duncan Houston BVSc, a practising veterinarian, unless another author is named in the byline.
