Moon Blindness in Horses

Causes, Treatment, and Prevention

View of horse's head, eyes and ears.
Equine Recurrent Uveitis, periodic ophthalmia or ERU can be mistaken for an eye infection. Image Credit:Jaclyn Sollars/ Moment /Getty Images

Moon blindness, an inflammatory eye condition, was historically named for its misperceived association with the moon's phases. We now know that moon blindness has nothing to do with the moon but can recur over the course of weeks or months. It causes pain and discoloration of the eye, and affected horses are very sensitive to bright sunlight. Some horse breeds seem to be more susceptible than others—appaloosas, in particular. Because this condition is progressive and can cause blindness, owners should seek veterinary help as soon as symptoms appear.

What Is Moon Blindness?

Technically known as equine recurrent uveitis (ERU), moon blindness is generally considered to be an autoimmune condition characterized by recurrent inflammatory episodes in one or both eyes. It is the most common cause of blindness in horses.

Symptoms of Moon Blindness

Symptoms of moon blindness involve one or both eyes. A horse will be reluctant to be in bright sunlight, exhibiting signs of pain or irritation that are not relieved by antibiotic treatment.


  • Inflammation
  • Redness of the eye area
  • Murkiness or white discoloration of the eye
  • Tearing (watering)
  • Squinting
  • Blindness

Some horses with moon blindness may suffer recurrent bouts of inflammation, followed by relatively painless periods. Others experience constant low-level inflammation. Owners may initially attribute redness, tearing, and squinting to an environmental irritant or eye infection, but measures to treat these conditions will have little effect on moon blindness.

Moon blindness is regarded as recurrent because it appears to clear up and then spontaneously reoccurs. For some horses, the recurrence may occur over years or flare-ups may occur far more frequently, episodes happening within weeks or even days.

Untreated, moon blindness can lead to cataract formation or prolonged inflammation, both of which are likely to lead to permanent blindness.

Causes of Moon Blindness

There are several possible causes for moon blindness, although a genetic predisposition to autoimmunity probably underlies external triggers, which may include:

Appaloosas and German warmbloods are two breeds that seem particularly vulnerable to moon blindness due to hereditary factors.

Diagnosing Moon Blindness in Horses

To diagnose moon blindness, your veterinarian will consider the horse's recent ocular heath history. Recurrent episodes of inflammation are indicative of moon blindness and help rule out other eye problems. In addition, your vet will perform extensive eye exams, including a fluorescein stain, to determine the health of the cornea and retina.

Treatment for Moon Blindness

Treatment options for moon blindness include steroids, non-steroidal anti-inflammatory drugs (NSAIDs), and mydriatics for inflammation and pain. During an acute flare of moon blindness, it's important to protect your horse from bright light with either a mask or by keeping it indoors.

Each time this condition reoccurs, you will need to resume treatment. Frequent flares may prompt some horse owners to resort to euthanasia. If only one eye is affected, the eye can be removed. Surgery to implant a drug-laden disc in the eyes seems to hold some promise, but it is not yet common practice.

Prognosis for Horses with Moon Blindness

Unfortunately, there is no cure for moon blindness, but early treatment can help slow the progression of the disease. More than half of the horses that develop moon blindness will eventually become blind.

How to Prevent Moon Blindness

Little can be done to prevent moon blindness, but providing good nutrition, a clean environment, fly control, and watchful care will help your horse be as healthy as possible. If you notice eye inflammation or discharge, call your veterinarian so that treatment, if needed, can be started promptly.

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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  1. Equine Recurrent Uveitis. UC Davis Veterinary Medicine Center for Equine Health.