Equine recurrent uveitis (ERU), also known as moon blindness, is an autoimmune disease and the leading cause of blindness in horses. [1] The disease results from the immune system repeatedly attacking the structures of the eye causing tissue damage, inflammation, and eventually blindness.

The underlying cause of equine recurrent uveitis is usually unknown, but Appaloosa horses, Warmblood horses, and Icelandic horses appear to have a genetic predisposition. Horses that develop leptospirosis or other infectious diseases also have an increased risk of ERU.

Symptoms of ERU include cloudy eye (corneal edema), abnormal pupil responsiveness, and loss of vision. Some horses experience “flare-ups” of uveitis with more severe symptoms such as squinting and yellow-green fluid within the eye.

ERU is a lifelong disease often requiring ongoing management by owners. Treatment focuses on reducing inflammation and preventing flare-ups in affected horses. Treatment options include medical management, intraocular injections, and surgical intervention. Despite appropriate treatment, the prognosis for vision in an affected eye is poor.

Causes of Moon Blindness

Equine recurrent uveitis (moon blindness, periodic ophthalmia) is an autoimmune disease, meaning the horse’s own immune system attacks the tissues of the eye and causes damage. [2]

Under normal circumstances, the eye contains no white blood cells or other immune cells. [3] The blood-ocular barrier, a thin layer of cells lining the blood vessels of the eye, prevents white blood cells from entering the organ. [3]

Equine recurrent uveitis occurs when there is a disruption to the blood-ocular barrier, allowing white blood cells to enter the eye. [3] Since white blood cells are not normally present within the eye, the immune system identifies the eye tissues as foreign and targets them for destruction. [3] This results in widespread damage to the eye’s internal structures. [3]

The historical term “moon blindness” likely originated from the belief that horses developed the condition during the full moon, though there’s no scientific evidence supporting this notion. It’s more likely that the increased awareness of the condition during the night, when horses would display symptoms such as increased sensitivity to light, led to this association.


Autoimmune Disease Development

Like many other autoimmune diseases, the precise cause of ERU is usually unknown. [3] Studies of autoimmune disorders suggest there is an initial trigger that results in the immune system targeting antigens (recognizable proteins) on normal body tissues. [3] The immune system then identifies these tissues as abnormal and attempts to destroy them. [3]

Although it is not clearly understood, proposed causes for autoimmune disease development include: [3]

  • Molecular mimicry: a pathogen produces similar antigens to a normal body tissue
  • Bystander activation: the immune system activates against normal body tissues accidentally while trying to clear an infection
  • Epitope spreading: the immune system’s response to an infection starts targeting other proteins, including those produced by normal body tissues

Connection to Infectious Diseases

Studies show that leptospirosis, an infectious disease affecting the whole body, can trigger development of ERU. [2] Horses become infected by ingesting contaminated water or food containing urine from a Leptospira-infected animal. [3]

The development of ERU after Leptospira infection is likely due to molecular mimicry, where the Leptospira bacteria produce a protein that is similar to normal eye tissue proteins. [2] As the immune system attempts to clear the Leptospira infection, it also targets the eyes, resulting in ERU. [2]

For this reason, some researchers recommend vaccinating horses with a high risk for ERU against Leptospira. [3][4] Vaccination of horses currently affected with ERU is not recommended, as studies show it does not slow the progression of ERU. [3][5]

Other infectious diseases that could potentially trigger ERU include: [6]

Genetic Risk in Certain Breeds

The Appaloosa horse has an increased risk of developing ERU compared to other breeds, and often experience more severe symptoms. [3] Other breeds with appaloosa coat patterning, such as Ponies of the Americas and Knabstruppers, may also have an increased risk. [3][6]

Genetic analysis shows that the leopard complex spotting gene, which produces the leopard coat pattern, is associated with a higher risk of ERU. [3] The underlying cause of why leopard coat patterning increases the risk of ERU is unknown, however researchers speculate that the depigmentation (reduced pigment, producing a white coat) that occurs with leopard patterning may trigger uveitis. [1][6]

Both Warmblood horses and Icelandic horses also have a genetic predisposition to developing ERU. [1][6] Studies in Warmblood horses show an association between development of ERU and mutations on genes related to the immune system, suggesting a possible link. [6] Further research in Icelandic horses is necessary to determine the cause of ERU in this breed. [6][7]

Equine Eye Anatomy

Given the high rates of occurrence of eye injury and disease in horses, it can be helpful for horse owners to familiarize themselves with eye anatomy.



Symptoms of moon blindness can develop in one or both eyes. [3] Studies show around 38-66% of horses develop uveitis in both eyes, with Appaloosas having a higher risk. [3] In Appaloosas, around 81% of horses develop uveitis in both eyes. [3]

There are three types of equine recurrent uveitis: classic, insidious, and posterior. Each type has slightly different symptoms. [2]

Classic Equine Recurrent Uveitis

Classic ERU is the most common type of recurrent uveitis in horses. In this syndrome, the symptoms are initially severe. [2] This is known as the acute phase, which typically has a sudden onset. [2]

Symptoms include: [2][5]

  • Squinting
  • Swelling of the eye
  • Tearing
  • Avoiding bright lights
  • Cloudy eye
  • Yellow-green fluid within the eye
  • Contraction of the pupil
  • Holding the eye closed

These initial symptoms typically resolve after 10-21 days. [2] The eye then enters the chronic phase, which is ongoing and less severe. [2]

Symptoms of chronic uveitis include: [2]

  • Cloudy eye
  • Scarring or color changes to the iris
  • Contraction of the pupil
  • Loss of the corpora nigra, small cysts along the top surface of the pupil

As the chronic phase of disease continues, horses also develop recurring episodes of acute uveitis. [2] These episodes typically have progressively more severe symptoms. [2]

Insidious Equine Recurrent Uveitis

Insidious ERU has similar symptoms to chronic, classic ERU, but horses do not develop episodes of acute uveitis. [2] In most cases, the first symptoms identified in this disease are blindness or clouding of the eye. [2] Insidious ERU is most common in Appaloosas and draft breeds. [2]

Posterior Equine Recurrent Uveitis

Posterior ERU only affects the back of the eye, producing few symptoms that are identifiable by the owner. [2] The main symptoms in this disease are reduced vision or blindness. [2] This type of ERU is most common in Warmbloods. [2]

Sudden changes in vision are a medical emergency requiring prompt veterinary attention. Rapid intervention provides the best chance of preserving vision.


A diagnosis of equine recurrent uveitis relies on an ocular examination and a thorough history of previous uveitis episodes. [3] ERU is not the only cause of uveitis, so the history of recurrence is an important factor in diagnosing ERU versus other causes of eye disease. [3]

Diagnostic tests that may help distinguish between ERU and other forms of uveitis include: [3][8]

  • Ophthalmic examination to identify ocular changes associated with long-term, repeated injury
  • Fluorescein dye application, which highlights any damage to the eye surface
  • Tonometry, measuring the pressure within the eyeball
  • Bloodwork
  • Measuring antibodies against Leptospira bacteria
  • Ultrasound of the eyeball

Referral to a specialist may be required.

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Equine recurrent uveitis is a lifelong disease that is manageable through treatment. [4] The initial treatment protocol is usually medical management, where medications are used to control episodes of uveitis and prevent future occurrences. [4]

If medical management is unsuccessful, then additional treatment options such as intraocular (into the eye) injections or surgical intervention can be considered. [4]

Medical Management

Medical management of ERU involves a combination of anti-inflammatory and immunosuppressive (reduces the effect of the immune system) medications. [4]

Common medications used for controlling ERU include: [4]

  • Corticosteroids, usually given every 4-6 hours
  • Non-steroidal anti-inflammatory drugs, usually given every 8-24 hours
  • Pupil-dilating medications, usually given every 4-24 hours

Medical management of ERU requires frequent medication administration for the remainder of the horse’s life. [4] In addition, immunosuppressive medications can leave horses vulnerable to other forms of disease, particularly infections. Some owners elect for enucleation (surgical removal of the eye) due to the ongoing time commitment required for medical management. [4]

Environmental Management

Another component of medical management is adjusting the environment to reduce the risk of uveitis flare-ups. [3]

Recommendations for environmental management include: [3]

  • Increasing insect control
  • Decreasing exposure to dust
  • Decreasing sun exposure
  • Splitting vaccinations into multiple appointments rather than administering them at the same time
  • Using a fly mask to protect the eyes from irritants

Intraocular Injections

Intraocular injections involve administering a medication directly into the eyeball for a stronger effect. [4] Veterinarians can perform this procedure under standing anesthesia, which is less risky than the general anesthesia required for surgical procedures. [4]

The medications most commonly injected into the eye during these procedures are: [4][9]

  • Triamcinolone, a steroid
  • Gentamicin, an antibiotic
  • Rapamycin, an antibiotic

Some studies show that fewer than 15% of horses have recurrence of uveitis after intraocular injection, making this method a promising treatment for ERU. [4]

Surgical Intervention

Surgical intervention aims to preserve vision while preventing future episodes of uveitis. The two main surgical techniques are cyclosporine implants and pars plana vitrectomy.

Cyclosporine Implants

Cyclosporine implants are devices that a surgeon embeds into the eye’s surface. [4] These devices contain the immunosuppressant cyclosporine. Once embedded, the device releases the drug slowly over a period of up to 3 years. [4]

Constant release of immunosuppressant medication means that owners do not need to constantly apply medications to the eye surface. [3] Additionally, the device delivers medication directly into the eye, improving the efficacy of the medication. [3]

Studies show that cyclosporine implants are very effective in preserving vision and preventing future occurrences of acute uveitis. One study showed that 85% of eyes receiving a cyclosporine implant maintained vision after surgery, with 31% of them maintaining vision for up to 2 years. [10]

Another study on 186 eyes showed that eyes with implants had significantly fewer flare-ups per month than untreated eyes. [11]

Pars Plana Vitrectomy

Pars plana vitrectomy is a surgical procedure where the surgeon removes affected vitreous, the gel-like substance that maintains the shape of the eye. [3] Removing the affected vitreous removes the inflammatory cells causing damage, returning the eye to an unaffected state. [3]

Horses undergoing pars plana vitrectomy have a good prognosis, with 73% of horses showing no further episodes of uveitis. [3] Around 70% of eyes maintain some vision after the procedure. [3]

However, there is a high risk of complications associated with this surgery, including damage to the lens, hemorrhage within the eye, and damage to the retina. [3]


The long-term prognosis for vision is guarded to poor in horses that have multiple episodes of uveitis. [2] One analysis showed that 56% of horses lose some vision in one or both eyes by 10 years after diagnosis. [2]

Appaloosas and horses positive for Leptospira have the poorest prognosis and more frequently develop complete blindness. [3]

Economic Impact

Equine recurrent uveitis is a lifelong disease that can have a significant economic impact for owners. [3]

Economic impacts include: [3][12]

  • Inability to perform or reduced performance
  • Expense of diagnostics and ongoing treatment
  • Loss of value of the horse

A study examining the economic impacts of ERU showed that nearly half of horses who returned to a performance career performed at a reduced level. [12] 29% of horses did not return to performance directly due to reduced vision from ERU. [12]

The same study also showed that ERU horses commonly develop musculoskeletal or gastrointestinal disorders due to ERU and its treatment. [12] These conditions may further increase costs associated with managing horses with ERU. [12]

For these reasons, some owners elect to move forward with euthanasia or enucleation of the affected eye. [3][5][13] Many owners also rehome horses with ERU due to the ongoing economic concerns associated with their management. [3][12]


Equine recurrent uveitis, commonly known as moon blindness, is an autoimmune disease causing repeated immune system attacks on ocular structures.

  • Appaloosa horses, Warmblood horses, and Icelandic horses have a genetic predisposition to ERU
  • Certain infections, such as leptospirosis, may increase the risk of ERU development
  • Symptoms include cloudiness of the eye, contraction of the pupil, and reduced vision
  • ERU is a lifelong disease that requires ongoing management through medications, intraocular injections, or surgical intervention
  • The long-term prognosis for vision in affected eyes is poor

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  1. Bellone. R. R., Genetics of Equine Ocular Disease. Veterinary Clinics of North America: Equine Practice. 2020.View Summary
  2. Gilger. B. C., Recurrent Uveitis. Equine Clinical Immunology. 1st ed. Wiley. 2016. doi: 10.1002/9781119086512.ch15.
  3. Gilger. B. C. and Hollingsworth. S. R., Diseases of the Uvea, Uveitis, and Recurrent Uveitis. Equine Ophthalmology. 1st ed. Wiley. 2016.
  4. McMullen. R. J. and Fischer. B. M., Medical and Surgical Management of Equine Recurrent Uveitis. Veterinary Clinics of North America: Equine Practice. 2017.View Summary
  5. Malalana. F., What’s New in Equine Recurrent Uveitis?. In Practice. 2020.
  6. Kingsley. N. B. et al., A Review of Investigated Risk Factors for Developing Equine Recurrent Uveitis. Veterinary Ophthalmology. 2023.View Summary
  7. Hack. Y. et al., A Genetic Investigation of Equine Recurrent Uveitis in the Icelandic Horse Breed. Animal Genetics. 2022.View Summary
  8. Allbaugh. R. A., Equine Recurrent Uveitis: A Review of Clinical Assessment and Management. Equine Veterinary Education. 2017.
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  10. Gilger. B. C. et al., A Novel Bioerodible Deep Scleral Lamellar Cyclosporine Implant for Uveitis. Investigative Ophthalmology & Visual Science. 2006.View Summary
  11. Gilger. B. C. et al., Long-Term Outcome after Implantation of a Suprachoroidal Cyclosporine Drug Delivery Device in Horses with Recurrent Uveitis. Veterinary Ophthalmology. 2010.View Summary
  12. Gerding. J. C. and Gilger. B. C., Prognosis and Impact of Equine Recurrent Uveitis. Equine Veterinary Journal.View Summary
  13. Sandmeyer. L. S. et al., Equine Recurrent Uveitis in Western Canadian Prairie Provinces: A Retrospective Study (2002–2015). The Canadian Veterinary Journal. 2017.