Equine Recurrent Uveitis (Moon Blindness): What Every Practitioner Should Know
Understanding the most common cause of blindness in horses
Equine recurrent uveitis (ERU), also known as moon blindness, represents the most common cause of blindness in horses worldwide. This immune-mediated inflammatory disease affects the uveal tract through distinct stages, each presenting unique diagnostic and therapeutic challenges.
Recognizing ERU’s Two Forms
ERU manifests in two primary forms that every equine practitioner must differentiate. The acute form presents with classic inflammatory signs: blepharospasm, epiphora, photophobia, conjunctival injection, and a visibly painful eye.
Far more dangerous is the chronic form, which Dr. Rachel Allbaugh, DACVO, emphasizes as particularly insidious because it lacks outward signs of pain. “Unfortunately, horses with insidious equine recurrent uveitis, they don’t have any of the outward signs of pain and it’s only this bonfire burning inside of the eye as a smoldering situation,” Dr. Allbaugh explains. This silent inflammation progresses relentlessly, causing irreversible damage while the horse appears comfortable.
Clinical Recognition
The hallmark of ERU lies in recognizing intraocular inflammation through systematic examination:
Aqueous flare serves as the gold standard clinical sign. Using a direct ophthalmoscope on the tiny circle beam setting, hold the device one centimeter from the eye while observing from a 90-degree angle. In affected eyes, you’ll observe a “headlight in the fog” effect as the focused beam reveals suspended protein.
Pupillary changes manifest as miosis due to ciliary muscle spasm. Posterior synechiae may develop, creating permanent adhesions between iris and lens, leaving characteristic “footprints” on the lens capsule.
Iris color changes become apparent in horses with blue irises, which turn yellowish or reddish during inflammation. Brown irises show hyperemia and may develop hyperpigmentation.
Vitreal changes include the characteristic “serum-stained” or “pond water” appearance in chronic cases, where proteins create a yellowish-green hue.
Etiology and Risk Factors
Dr. Allbaugh identifies Leptospira as a significant trigger for ERU. “We do have lepto as an inciting cause treating with systemic antibiotics to clear that and hopefully minimize the risk of recurring flares,” she notes.
Certain breeds show dramatically increased susceptibility. Appaloosas demonstrate the highest risk, with the condition linked to the leopard complex (LP) gene mutation. Dr. Allbaugh emphasizes that “Appaloosas with the leopard complex and German warm bloods I strongly deter breeding these affected horses” due to the genetic component. Draft breeds also show increased predilection. Practitioners should maintain heightened vigilance when examining these high-risk breeds.
Treatment: Aggressive Early Intervention
Successful ERU management requires aggressive anti-inflammatory therapy initiated promptly:
Topical steroids form the cornerstone of therapy, with prednisolone acetate preferred for optimal corneal penetration. “Neopolydex has the dexamethasone alcohol form which also does penetrate well,” making it an acceptable alternative.
Systemic anti-inflammatories provide crucial support, with flunixin meglumine as Dr. Allbaugh’s preferred choice. Treatment duration extends well beyond clinical resolution: “we’re treating for at least two to four weeks past the resolution of clinical signs because again it’s a bonfire in the eye and I want to make sure I have all those embers quieted down.”
Atropine provides comfort by blocking ciliary muscle spasm, dilating the pupil to prevent synechiae formation, and stabilizing the blood-aqueous barrier.
Advanced Therapeutic Options
For refractory cases, surgical interventions offer additional hope:
Intravitreal gentamycin provides rapid inflammatory control but carries risks. “The intravitreal injection has been shown to have pretty rapid response at combating the inflammation but it does have the risk of rapidly forming complete cataracts and retinal degeneration,” Dr. Allbaugh cautions.
Supraciliary cyclosporine implants represent a promising long-term option, particularly for Appaloosas. These implants, lasting 14-16 months, are placed in the deep scleral tissue and have shown remarkable success in maintaining comfort and vision.
Complications and Prognosis
ERU progression leads to devastating complications: cataracts develop as a direct consequence of chronic inflammation, though cataract surgery success remains limited due to concurrent retinal damage. Secondary glaucoma may require additional management with dorzolamide and timolol. Calcific band keratopathy can result from steroid therapy, while retinal detachment represents end-stage disease.
Prevention and Genetic Counseling
The genetic component of ERU, particularly in Appaloosas, demands responsible breeding practices. Dr. Allbaugh advocates against breeding affected horses or creating homozygous leopard complex offspring. “Let’s not breed him to horses who are known to have the leopard complex to create more homozygotes who’s not going to have the automatic ERU risk factor,” she recommends.
ERU represents one of the most challenging ophthalmic conditions in equine practice, demanding early recognition and aggressive treatment. The insidious nature of chronic ERU makes it particularly dangerous, as irreversible damage progresses silently.
Success depends on practitioner education, owner awareness, and prompt intervention. For high-risk breeds, regular ophthalmic examinations become critical. As Dr. Allbaugh emphasizes, “early recognition is your key and close monitoring by clients” remains fundamental to preserving vision and comfort.
Prevention through responsible breeding practices and rapid intervention upon clinical recognition remain our most powerful tools against this sight-threatening disease.
This article is based on content from Dr. Rachel Allbaugh, DACVO, presented through VetOnIt CE continuing education programs. Learn more about advanced equine ophthalmology at VetOnIt CE On-Demand.
Tags: Ophthalmology, Equine, Large Animal