Moon Blindness in Horses - What is Equine Recurrent Uveitis?

What is Equine Recurrent Uveitis (eru), UC Davis Center for Equine Health, moon blindness HORSES, IS MY HORSE BLIND? types of horses that go blind, insidious uveitis

UC Davis Center for Equine Health

Equine recurrent uveitis (ERU), also known as moon blindness, is the most common cause of blindness in horses worldwide. It affects 2 to 25 percent of horses globally, with 56 percent of affected horses eventually becoming blind. More than 60 percent of affected horses are unable to return to previous levels of work. ERU is most often characterized by repeated episodes of inflammation of the uveal tract of the eye (the middle layer), involving one or both eyes. A subclinical manifestation, known as insidious uveitis, does not present as outwardly painful episodes, and instead is consistent low grade inflammation (not episodic) that causes cumulative damage to the eye. Cumulative damage caused by ERU can lead to cataracts, glaucoma, and eventually blindness. Although not all horses that experience a single episode of uveitis will develop ERU, they are at risk for disease.

Equine recurrent uveitis is hypothesized to be a complex autoimmune disease influenced by both genetic and environmental factors. Appaloosa horses are particularly susceptible to ERU, and in particular to bilateral disease, which suggests that genetics plays a significant role in ERU risk in this breed. Appaloosas are eight times more likely to develop ERU than other breeds and significantly more likely to become blind in one or both eyes. Genetic studies have identified LP, the allele causing the white spotting pattern, as an ERU risk factor in the breed, with homozygotes being at highest risk (LP/LP, or an organism with identical pairs of genes/alleles). However, not all homozygotes are affected, and work is ongoing to unravel other risk factors (both genetic and non-genetic). Other breeds with a high occurrence of ERU in the United States include American Quarter Horse, Thoroughbred, Warmblood, Hanoverian, and American Paint Horse. Genetic studies also identified a genomic location associated with disease in Warmbloods, but no genetic test is available for Warmblood breeds.

Related: Understanding Equine Vision and Eye Disease

Infectious organisms, particularly Leptospira spp, have also been associated with ERU. Increased incidences of complications and vision loss have been reported in leptospirosis-associated ERU affected horses, especially Appaloosas. The exact mechanisms by which Leptospira spp cause ERU are unknown. Higher prevalence rates of ERU are reported in tropical, temperate climates than in dry, arid climates. This may be due to differences in Leptospira spp in the environment.

There is currently no cure for ERU.

Clinical signs

Episodes of redness, tearing, and squinting may be early indicators of eye issues. Equine recurrent uveitis can affect one eye or both eyes, and may cause more severe signs in one eye than the other. The disease tends to increase in severity with repeated episodes. However, the insidious form often does not present with any outwardly painful episodes.

What is Equine Recurrent Uveitis (eru), UC Davis Center for Equine Health, moon blindness HORSES, IS MY HORSE BLIND? types of horses that go blind, insidious uveitis

Close up of the eye of an Icelandic horse affected with moon blindness. Photo: Wiki/Christian Muellner

The disease is classified into three syndromes:

  • Classic - This form is the most common and is characterized by periods of apparently painful, active inflammation of the eye(s) separated by periods of no or low levels of inflammation. The repeated attacks often lead to vision loss. Breeds predisposed to this form include Warmbloods and Icelandic horses, among others.
  • Insidious - Horses with insidious ERU typically have persistent, low-grade inflammation in the eyes. The condition is often not outwardly painful, but leads to gradual destruction of ocular tissues and degeneration of structures in the eye, resulting in vision loss. This form is most common in Appaloosas and has been observed in Draft breeds.
  • Posterior - Posterior ERU is characterized by inflammation of structures in the back of the eye (vitreous, retina, choroid). Retinal degeneration is common, and this form is most often seen in Warmbloods and Draft breeds.


A diagnosis of ERU is made by clinical and ophthalmic examinations and history of documented recurrent or persistent inflammation in one or both eyes. It is important to examine the front (anterior) and back (posterior) parts of the eyes to identify signs consistent with ERU and exclude other ocular diseases. A fluorescein stain may be used to assess the health of the cornea and distinguish from other ocular diseases. Complete blood count (CBC) and serum biochemistry may be performed, along with serologic testing for Leptospira spp.

Common eye examination findings associated with ERU include corneal edema, aqueous flare, posterior synechia, corpora nigra atrophy, cataract formation, vitreous degeneration, and retinal edema or degeneration.


Treatment for ERU is focused on eliminating or reducing inflammation in the eye(s), preserving vision, alleviating pain, and minimizing the recurrence of episodes of inflammation. Topical corticosteroids (prednisolone, dexamethasone), non-steroidal anti-inflammatory drugs (NSAIDs) (flurbiprofen, diclofenac, suprofen, bromfenac) and mydriatics (atropine) are used to reduce inflammation and minimize damage during an active episode. However, these treatments are not necessarily effective in preventing recurrence of disease.

Related: Diseases with Horse to Human Transmission

Injections of corticosteroids (methylprednisolone, gentamicin or triamcinolone) into the eye(s) are used to treat severe cases. Systemic medications include NSAIDs (flunixin meglumine, phenylbutazone) and corticosteroids (dexamethasone, prednisolone).

Surgical options to treat ERU are suprachoroidal cyclosporine implant and core vitrectomy. The cyclosporine implant is a sustained-release device that provides therapeutic dosages of cyclosporine A for up to three years after it is implanted. This approach has been shown to successfully control inflammation and minimize recurrences. The core vitrectomy, performed under general anesthesia, removes the core of the vitreous to remove debris (including organisms such as Leptospira spp) trapped in vitreous fluids. The fluid is then replaced with saline. This approach can improve vision, minimize episodes, and delay the progression of clinical signs.

Enucleation (removal of the eye) is recommended in ERU-affected eyes that are painful or have become blind.


Early diagnosis and intervention are associated with the best prognosis for ERU-affected horses. Long-term prognosis is guarded. Current treatments can slow the progression of inflammation in the eye, but are not curative. More than 60 percent of affected horses are unable to return to previous levels of work and approximately 56 percent of ERU-affected horses eventually become blind. ERU-affected horses with glaucoma or cataracts are more likely to become blind and are also more likely to require removal of the affected eye (enucleation).


Genetic risks for ERU have been reported in Appaloosas (insidious ERU) and German Warmbloods (posterior ERU). Risk for ERU in Appaloosas can be evaluated using the LP genetic test. Horses with two copies of the LP mutation (homozygous LP/LP) are at a higher risk of developing ERU than horses with no leopard spotting pattern (N/N). It is important to note that research is needed to determine if the LP mutation is responsible for ERU, or whether LP is simply being inherited along with the causal mutation (i.e., located nearby on the DNA). However, horses who test LP/LP should have their eyes examined frequently by a veterinary ophthalmologist for early detection and treatment if and when inflammation is detected.

Printed with the kind permission of the UC Davis Center for Equine Health. The UC Davis Center for Equine Health is dedicated to advancing the health, welfare, performance and veterinary care of horses through research, education and public service.

Main Photo: Shutterstock/Oraziopuccio