Unless your mare has had a bad breakup with the stallion next door, her watery eyes could indicate a serious problem.

Horses can exhibit eye pain (squinting, tearing, face rubbing) for a plethora of reasons and to varying degrees. The common cause is a corneal ulcer. In this article, we will discuss Equine Recurrent Uveitis (ERU). This particular condition is a result of inflammation inside the eye caused by an autoimmune disorder. Equine Recurrent Uveitis is aptly named because of its tendency to recur in one or both eyes. Some of its former names that you may be more familiar with include: "moon blindness" and "periodic ophthalmia" because of its erratically recurrent nature, compared by early observers to the phases of the moon. This particular disease is the result of an overly aggressive immune system in which the eye's own immune system reacts against normal ocular tissues early in the disease. Various problems are implicated to be the cause.

Recognizing ERU

The most obvious sign of eye pain is squinting. If you think your horse is squinting, compare the upper eyelids and eye lashes of each eye; identify the one with the eyelashes pointed down. Other signs of ocular pain include: repeated face rubbing and tears streaming down the face. It is important to remember that individual horses have different pain thresholds and will express pain differently. Over time, some horses may develop a tolerance to mild pain and show fewer signs.

In addition to pain, uveitis changes the eye color. The cornea may look to be a hazy blue or gray because it has more fluid than normal. The pupil may be smaller or difficult to see because of the cloudy cornea. The iris (colored part of the eye) may change color and the fluid space between the iris and cornea might be occupied by cells or proteins and may appear gray, tan or even red.

The real problems for the horse eye in response to uveitis begin when the horse experiences the second, third or subsequent attack.

When the immune system of the eye attempts to become more sophisticated and efficient, it reduces the checks and balances in the immune system, thus allowing the response to start more easily. Some inflammatory cells permanently stay inside the eye for surveillance against disease. This improves the reaction time when a new problem arises, but it also makes it easy to accidentally trigger an unnecessary attack.

Ultimately, if uncontrolled, permanent damage starts to add up and includes complete cataract, scars from the inflammation, glaucoma, death of nerve cells and shrinkage of tissues. All of the above contribute to vision loss and ongoing pain.

Appaloosas deserve special mention because they are affected by a particularly severe version of ERU. While many breeds respond to relatively minimal therapy, and only exhibit mild squinting and eye discharge, Appaloosas often require intensive treatment, have more severe clinical signs and relapse more frequently. The underlying genetic reason for this is not known, but the prognosis for controlling pain and continued vision is far poorer in this breed.


Treatment is aimed at reducing inflammation inside the eye. The initial cause is probably no longer present at this stage. In mild cases, treatment might be minimal. If clinical signs worsen, more aggressive therapy such as specific eye medications and oral anti-inflammatories are recommended. A fly mask is also highly recommended. It helps to keep the flies off the face as well as provide a barrier against the bright sun. For chronic or severe cases an implant that produces constant low levels of medication to "turn off" the cells that cause the inflammation in ERU may be necessary.

In summary, if your horse develops a painful eye (or eyes) but hasn't been diagnosed with ERU, the most important thing you can do is to have the horse examined by your veterinarian. It is important to distinguish ERU from other causes of painful eyes so that appropriate therapy may be instituted. If necessary your veterinarian can also advise you of a veterinary ophthalmologist.

There is probably nothing that can be done to prevent horses from becoming affected by ERU. Even though an infection is most likely the original cause, you shouldn't be concerned about it spreading to other horses in the barn. Outbreaks of uveitis are rare, usually obvious and most times don't result in recurrent uveitis.

Samantha Howard is a mixed animal veterinarian at Ellensburg Animal Hospital who writes a monthly column for The Daily Record. Send your questions to: askdoc howard@gmail.com



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