Moon Blindness in Horses 101: Causes, Symptoms, and Treatment

Moon blindness is the common term for a condition in horses that most health professionals now call “equine recurrent uveitis (ERU)”. This chronic, painful disorder is the most frequent cause of blindness in horses.

Experts believe that records about moon blindness in horses pre-date records of human diseases. People call it “moon blindness” because previously horse owners erroneously thought that the moon caused the condition.

While not contagious, it happens more often in Appaloosas than other breeds. It occurs more often in some areas or countries where it may affect between 10 to 20 percent of the horses.

It usually strikes one eye but may be in both, especially in Appaloosas. Once a horse contracts the disease, it frequently reoccurs. This recurrence may be within weeks, months or years.

History of ERU

Archeologists found documentation of moon blindness in horses in the Egyptian pyramids at Giza over 4500 years ago. Over the millennia, the disease has had several names including iridocyclitis, recurrent iridocyclitis, periodic ophthalmia and chronic intraocular inflammation. The term “moon blindness in horses” came about in the 1600s. People believed that the cycles of the moon caused it.

Causes of ERU

Moon blindness in horses is a complicated condition that can have several causes. This is one reason that it is hard to diagnose quickly. Many times a horse will have uveitis, a common condition of the eye. This can be from injury to the eye, a simple infection or other eye condition. Recurrent uveitis is the more serious one.

Researchers are racing to find a cure or prevention. They do not agree on the exact cause. Some veterinarians speculate that there are several diseases grouped under the common name of ERU.

Researchers have found that horses infected with Leptospira, a bacteria found in stagnant water, are more prone to ERU. Here are other possible causes:

  • Parasites;
  • Bacterial infections;
  • Viral infections;
  • Systemic infections;
  • Fungi;
  • Pollen;
  • Vitamin deficiencies;
  • Autoimmune deficiencies;
  • Trauma to the eye.

In addition to this list, some experts blame Equine flu, tooth or hoof abscesses and even some worming medications. Not all of these causes have been substantiated. Consult your veterinarian for further advice.

Symptoms of ERU

The symptoms appear to the layman as uveitis, or simple eye infection. The eyes can appear as tearful, reddish and puffy. This appearance is similar to the results of fly bites, an injury from brushes or other irritation.

Since ERU can lead to blindness and is extremely painful for the horse, a person should consult a veterinarian as soon as he or she notices eye irritation. Here are some of the symptoms that horse handlers should watch for:

  • Redness in the eye tissues;
  • Puffy eyelids;
  • Weepy eyes or whitish discharge from the eyes;
  • Squinting;
  • Sensitivity to light;
  • Pupil constriction;
  • Cloudiness in one eye, or in both eyes in severe cases.

It is common for the animal to not exhibit signs of pain even though it is hurting. Sunlight, fluorescent lights, and other things can cause pain in an affected horse.

Complications That Can Occur with Moon Blindness in Horses

Since the symptoms can appear to be simple eye irritation, many horse owners do not immediately seek veterinarian attention. Untreated, the condition quickly can lead to further complications, including blindness. Despite the modern developments to counter the effects, it is a very serious problem for horses.

With each occurrence, more damage occurs. Usually, this is from scarring from the disorder. In some cases, the veterinarian or specialist may recommend euthanasia to prevent the animal’s prolonged suffering.

Some of the complications can include:

  • Cataracts;
  • Glaucoma;
  • Detachment of the retina;
  • Calcification of the cornea;
  • Atrophy of the eye;
  • Blindness and more.

Diagnosing ERU

When a veterinarian encounters possible moon blindness in horses, he or she performs several tests including a physical examination, a comprehensive eye exam, blood tests and more. Other eye diseases and conditions have the same symptoms as ERU. Since there are several suspected causes for the condition, a thorough exam is necessary.

Once the health practitioner rules out the diagnosis of a simpler uveitis, he or she narrows down the possible cause of the condition. This information helps to determine the treatment he or she will prescribe.

One of the most telling symptoms is a darkening in the eye’s iris and irregularities in the area around the iris.

A veterinarian usually will consult an equine ophthalmologist or an eye specialist. The ophthalmologist may want to see the horse at his or her facility to confirm the diagnosis.

Treatment for ERU

No one has found a cure for moon blindness in horses. The best treatments lessen the horse’s discomfort when episodes happen and minimize the damage that each episode does. The disease is progressive, meaning that each episode adds to the damage in the eye.

Veterinarians have several methods of treating ERU, depending on the severity of the episode and the condition of the eye. In milder cases, the treatment may be steroids and non-steroidal anti-inflammatory drugs to reduce inflammation.

For more severe cases, a veterinarian may use an implant that very slowly delivers cyclosporine directly to the eye. Often, board-certified ophthalmologists and clinical specialists perform this procedure at specialty facilities. A specialist performs this complex procedure while the horse is under anesthesia. This method is not common in all parts of the world.

Another option is to remove the eye that has recurring episodes. It is typical for ERU to affect only one eye. Appaloosas are more likely to have it in both eyes, but other breeds may experience this severity also.

Researchers are experimenting with antibiotics, such as penicillin and streptomycin.

Since experts do not know the exact cause of ERU, it is impossible to prevent it. However, veterinarians recommend horse owners provide good nutrition, a sanitary environment that is free of stagnant water and fly control. Horse owners and handlers should exercise vigilant watchfulness for eye problems that indicate moon blindness in horses.

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