Moon Blindness in Horses

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Moon blindness is a common and painful condition of one or both eyes in horses and is the most common cause of equine blindness worldwide. Named so because people in the 1600s thought that the timeline of the disease followed lunar phases, moon blindness is also known by its medical term equine recurrent uveitis or ERU. Owners may notice their horses squinting or tearing, and the cornea, or clear layer that covers the eye, may turn whitish. Horses with moon blindness may go blind if the disease goes untreated. Certain breeds, such as Appaloosas, are at higher risk. Appaloosas are 8 times more likely to develop ERU than other breeds due to an inherited genetic risk factor. Learn here how to look for symptoms of moon blindness and how your veterinarian may diagnose and treat the disease. ERU can be treated to alleviate symptoms and delay progression of the disease, but be aware there is no cure. 1

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What is Moon Blindness?

Moon blindness (ERU) is a group of immune-mediated diseases that cause repeated bouts of inflammation within the inner portion of the eye. This inflammation leads to extensive pain and often blindness. While there are many processes that cause one bout of uveitis, or inflammation within the eye, two or more bouts lead to a diagnosis of moon blindness. This means if your horse has an episode one time, it is critical to watch carefully for another over the following few months to years. 

The eye has a very complicated anatomy, but a few key features are the cornea, which is the outer clear layer that allows light to penetrate, and the anterior portion directly underneath the cornea. The deeper layer of this anterior portion is the iris, which expands and contracts depending on how much light is in the environment. Behind the iris is the lens, which allows an animal to focus on an object, followed by the posterior portion called the vitreous body, and finally the retina, which captures the image to send to the brain. 

The area affected by moon blindness is known as the uveal tract, which consists of the iris, the muscles that control the iris (ciliary body), and the largest part of the uvea (the choroid), which is the blood vessel layer that surrounds the vitreous body and supplies blood to the outer part of the retina. 

Eye anatomy
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Inflammation in the eye causes pain and loss of the barrier between blood vessels and the interior of the eye. This causes the haziness of the interior portion of the eye and the loss of clearness in the cornea. Initially, pressure in the eye is decreased relative to normal. Recurrent bouts lead to blindness through a few different mechanisms. The lens may dislocate and/or develop cataracts, there may be increased pressure in the eye (glaucoma), and scarring may occur in the cornea and retina. Moon blindness usually affects one eye but affects both in about 20 percent of horses with ERU.


Symptoms of moon blindness differ between the relatively early stages of the disease (acute signs) or if the eye has had multiple bouts of ERU and the disease is at its end stages (chronic signs). Most acute symptoms can be noticed by an observant owner. In contrast, most chronic symptoms other than a small eye (phthisis bulbi) or blindness require an ophthalmic (eye) exam by your veterinarian.

Some horses have a low-level amount of inflammation (subclinical) that can cause the eye to progress to show chronic signs without acute signs. A veterinarian should examine your horse’s eyes at their annual exam to look for any chronic signs.

Symptoms of Moon Blindness

Acute symptoms

  • Tearing
  • Squinting
  • Pain
  • Excessive ocular discharge
  • Redness in the tissues around the eye (conjunctiva)
  • Whitish appearance to the outer clear layer of the eye (corneal edema)
  • Haziness in the eye anterior chamber (aqueous flare)

Chronic symptoms

  • The iris may be visualized adhered to the lens (posterior synechia)
  • Cataract development in the lens
  • Lens dislocation
  • Glaucoma
  • Detached retina
  • Shrinking of the eye (phthisis bulbi)
  • Corneal and retinal scarring
  • Blindness


Moon blindness affects 2-25 percent of horses worldwide. The causes of moon blindness are multi-factorial and poorly understood. It is thought that an inflammatory insult, such as an infection, parasite, or trauma may cause the initial bout of inflammation or uveitis. The horse’s immune system then sets off an abnormal and damaging cycle of inflammation which may wax (flare) and wane (periods of normalcy). When genetics is not implicated in breeds that are pre-disposed, the bacterium Leptospira is often implicated as the inciting factor

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Any horse may get ERU, but certain breeds are at higher risk. These include Appaloosas, Warmblood, American Quarter Horse and Draft horses.

Causes of Moon Blindness

  • Trauma
  • Cancer or tumor in the eye
  • Viral infections
  • Bacterial infections, especially Leptospira
  • Genetic factors in Appaloosa, Warmblood, or Draft breeds


The most important first step in diagnosing a horse with the acute symptoms described above is differentiating between moon blindness and a corneal ulcer. Moon blindness is an autoimmune disease that results in abnormal and exaggerated inflammation, while a corneal ulcer is when a horse scrapes its eye against foreign material, causing a cut on the sensitive cornea. In the case of an ulcer, squinting, tearing, corneal edema, and aqueous flare are all symptoms that can lead to an appropriate diagnosis and facilitate treatment.

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It is critical that you call your veterinarian if you notice any acute signs of moon blindness. Your veterinarian will closely examine the eye with a specialized light. They will then apply a stain (fluorescein) to the eye which will bind to a corneal ulcer. If there is no stain uptake on the cornea, a horse with the acute signs listed above probably has uveitis rather than an ulcer. Diagnostics for glaucoma associated with moon blindness involve taking the interior pressure of the eye with a gentle and highly specialized instrument called a tonometer. Your vet may also take blood samples to test for levels of Leptospirosis.


The major goals in the treatment of moon blindness are to preserve vision, decrease pain during an attack, and reduce the frequency of recurrence. After the initial diagnosis of uveitis, your veterinarian will prescribe an ointment to apply to your horse’s eye that contains corticosteroids. Corticosteroids are drugs that dampen the abnormal immune response in moon blindness. Therefore, it is critical to differentiate ERU and corneal ulcers as topical corticosteroids may delay healing of corneal ulcers and can lead to superinfection with fungi or bacteria.

Another ointment that your veterinarian may have you apply on the horse’s eye, called atropine, reduces the painful spasm that occurs when the iris and ciliary body are inflamed. Your veterinarian will probably also have you give your horse a non-steroidal anti-inflammatory called flunixin meglumine (Banamine) orally to provide pain control and decrease inflammation. Antibiotics may be given if your horse is Leptospira positive. Treatments usually are continued for 2-4 weeks and require regular rechecks with your veterinarian during that time.

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If your horse keeps having breakthrough flares, your veterinarian may have you see another veterinarian specializing in eye problems (ophthalmologist). They will do a complete eye exam with special instruments to make recommendations. These recommendations may include injections of antibiotics directly into the eye or surgery to place a low-level immune system suppressor (cyclosporine) implant

If your horse’s pain cannot be controlled or if the eye is blind, it may be more comfortable to have the eye removed (enucleation). 


Prognosis for long-term vision is improved with prompt diagnosis in a flare, but just under 50 percent of horses with moon blindness eventually lose vision. This may take years, so a horse with ERU often can have a normal quality of life with good management for some time. However, once chronic symptoms occur, the pain associated with ERU and eye changes may make them more spooky, irritable, or unable to return to their previous performance levels even if a horse has some vision. Horses that are candidates for cyclosporine implants usually do better long-term.

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Some horses require that one of their eyes be removed because of uncontrollable moon blindness. A horse with one eye may go on to have a good athletic career using other keen senses and the broad range of vision of its remaining eye. Horses that need both eyes removed or are blind in both eyes are rarely rideable but may have a good quality of life with some environmental adjustments.


Uveitis is hard to prevent, especially since it has so many causes. However, it is critical for a horse caretaker to identify the acute signs to try and stop uveitis from becoming recurrent or moon blindness. Good fly control, regular deworming, and keeping horses away from stagnant ponds will reduce exposure to environmental causes of uveitis.

If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.
Article Sources
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