Moon Blindness (Recurrent Uveitis) in Horses – Vet Guide 2025 👁️🐴

By Dr Duncan Houston, BVSc

Introduction

Also known as Recurrent Uveitis (ERU), moon blindness is the **leading cause of equine blindness**, characterized by recurring internal eye inflammation. Though once thought tied to lunar cycles, current research confirms it’s **immune-mediated**, with infections like Leptospira often implicated. This 2025 guide dives deep into recognizing, diagnosing, managing, and preventing ERU—empowering horse owners to act early and protect their horse’s vision.

Why Early Diagnosis Is Crucial

• Each flare causes cumulative damage: cataracts, synechiae, glaucoma, and eventual blindness.
• Symptoms mimic other eye diseases—accurate diagnosis guides safe treatment.
• Misuse of over-the-counter drops, like steroids in ulcers, can worsen disease.

What Is Recurrent Uveitis?

ERU affects the uvea: the iris, ciliary body, and choroid. It recurs episodically—one flare subsides, another emerges. Over time, flare-ups lead to permanent changes, vision loss, and chronic pain.

Causes & Predisposition

• Immune-mediated response: Horse’s immune system attacks eye tissue after exposure to certain antigens.
• Infectious triggers: Bacteria like Leptospira, Streptococcus, or viruses may initiate inflammation.
• Breed predisposition: Appaloosas are at highest risk in the U.S.; warmbloods and others also affected.
• Environmental trigger: Stress, trauma, or systemic illness often precede flares.

Recognizing Symptoms

Symptoms can mirror other eye problems—only a vet can differentiate. Key signs include:

• 🌟 **Pain**: squinting, light avoidance, tearing
• 🌟 **Corneal changes**: blue/white haze (corneal edema)
• 🌟 **Iris changes**: constricted pupil, irregular margins
• 🌟 **Discharge**: usually watery
• 🌟 **Systemic signs**: low-grade fever, dullness

Diagnostic Steps

1. **Comprehensive ophthalmic exam** to assess ensemble of signs
2. **Tonometry** for intraocular pressure
3. **Ultrasound** to evaluate internal structures, especially when corneal opacities block view
4. **Labs**: test for leptospiral antibodies or other infections
5. **Ulcer screening**: fluorescein dye to rule out co-existing ulcer

Important Differentials

• Corneal ulceration: Requires topical antibiotics, NOT steroids
• Glaucoma: Raised pressure contraindicates atropine
• Infectious keratitis or tumors: Persisting signs need tailored care

Treatment Strategy

1. During Acute Flares

• **Topical corticosteroids** to suppress inflammation
• **Atropine** 1% to dilate pupil and relieve pain—but never use if glaucoma present
• **Systemic NSAIDs** (flunixin, phenylbutazone) for comfort
• **Antimicrobials** if infection suspected
• Regular rechecks—evaluating response and monitoring glaucomatous changes

2. Between Flares (Maintenance)

• **Periodic exams** every 3–6 months to catch early recurrences
• **Disked or topical anti-inflammatory** therapy as needed
• **Leptospira vaccination** or treatment if seropositive
• **Owner vigilance** for early signs like watery eye or squinting

3. Chronic & Refractory Cases

• **Immunomodulatory therapy** (e.g., cyclosporine implants in the eye)
• **Surgical lens extraction** if cataracts limit vision—refer to veterinary ophthalmologist
• **Enucleation** (eye removal) for blind, pain-filled eyes—to improve quality of life

Why You Can’t DIY Treat 🚫

• Over-counter eye drops often contain medication harmful if misused (e.g., steroids in ulcers).
• Improper treatment can exacerbate disease or delay care.
• Complexity demands accurate diagnosis and imaging—only vets can determine presence of glaucoma, infection, or deeper damage.

Prevention & Long-Term Management

• **Early detection** saves vision—training staff to report subtle ocular signs helps.
• **Manage leptospiral exposure**: control rodents, standing water; test and treat horses.
• **Stress reduction**: minimize competition, ensure rest, protect from trauma and bright light.
• **Fly control**: eyes are vulnerable—use masks, stalls, screens.
• **Health maintenance**: dental, deworming, colic prevention for holistic well-being.

Breed-Specific Considerations: Appaloosas

• Higher ERU rates—screening recommended
• Breeding stock: assess for uveal pigment anomalies and avoid propagation of predisposition
• Baseline ophthalmic exams starting at Yearling age, repeated regularly

Owner-Initiated Care

• Do NOT medicate eyes before vet exam—delay in proper treatment can be devastating.
• Use **clean warm water** to gently clear discharge between vet visits.
• Keep a **flare diary**: record date, symptoms, treatment, and outcomes.

Case Study: ERU Flare in a Warmblood Mare

1. Day 0: Tearing right eye, squinting; cloudy cornea noted.
2. Day 1: Vet exam reveals low pressure, signs of uveitis; fluorescein negative.
3. Days 1–5: Topical dexamethasone q6h, atropine q8h, systemic flunixin; mare bright and eating.
4. Day 7: Cornea clear, iris responds; taper medications.
5. 3-Month Follow-up: Normal eye exam—owner monitoring signs weekly.

This mare remains sighted and is kept on twice-yearly vet eye checks.

Conclusion

Recurrent Uveitis (Moon Blindness) is a serious, chronic condition—but early recognition, meticulous vet care, and strategic management can save vision and quality of life. With recurrent episodes, every vet visit and treatment counts.

See anything off in your horse’s eye—flare, tearing, discomfort? Contact Ask A Vet at AskAVet.com or via our app for instant, expert guidance, treatment prioritization, and follow-ups. 🩺📱