Moon blindness, otherwise known as recurrent uveitis, can be a devastating disease of the equine eye.
The term moon blindness comes from the ancient belief that the disease was associated with the changes of the lunar cycles. The "recurrent" or "periodic" part of the disease is the result of the propensity of this disease to recur in a rather unpredictable manner after the animal has once experienced the disease.
The uvea is an anatomical name for certain parts of the interior eye and, of course, "itis" means inflammation--so uveitis is an inflammation of the uvea. The uvea includes most of the interior parts of the eye that have a large blood supply. This is especially true for the iris--the colored part of the eye surrounding the pupil.
Who Suffers From Uveitis?
All species of animals, including humans, can suffer from some form of uveitis, but the horse has been plagued with the recurrent form of this disease for some time. In fact, this disease was recorded by veterinarians attending horses of Alexander the Great. This disease probably suffers from more folklore, ignorance, misconception, and anecdotal causes, cures, and treatments than any other human or veterinary disease. Unfortunately, the bottom line is that we really don't know very much about recurrent uveitis. Although there are many "predisposing" factors scattered about in the lay literature as well as specific horse predispositions, there is no age, sex, breed, moon cycle, or any other proven predisposing factor. It has been reported that up to 12% of a given population of horses in some areas of the United States can suffer from uveitis.
The Suspected Cause
The most commonly held explanation for recurrent uveitis with out trauma, is that inflammation of the
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uvea is caused by a delayed hypersensitivity reaction or is an autoimmune mediated phenomenon. Essentially there is an immune mediated reaction developing after exposure to a foreign substance. Some of the immune cells have a memory for the foreign substance and are suspected to persist on the uveal tissue. Then, when the horse (and subsequently the immune cells) are exposed to the same foreign substance at some time in the future, the reaction stimulates inflammation.
The autoimmune phenomenon occurs when the immune system, for whatever reason, stops recognizing a part of the body as part of the family and starts to reject it. It might be possible that after infection with one of the suspected causes of recurrent uveitis, immune proteins (antibodies) are produced that target the uveal tissue. If the protein structure of part of the infectious agent is similar to the protein structure of part of the uveal tissue, this situation might occur.
The primary signs of uveitis are squinting, tearing, and an increased sensitivity to light. Other observed signs are cloudiness to the cornea as well as within the eye, so it will be difficult to see the iris and pupil with a distinct, cloudy haze present. As the inflammation occurs within the eye, irritating chemicals are produced and released into the fluid in the anterior chamber (the space between the inside of the cornea and the iris which is normally filled with a clear fluid).
Before the treatment of uveitis is initiated, it is imperative that the diagnosis be made by a veterinarian and that the cornea be evaluated for the presence of a corneal ulcer. The two main drugs used in the treatment of uveitis are atropine and corticosteroids. The atropine serves several important purposes. Atropine works by paralyzing some of the muscles of the iris, thus stopping the painful spasm and allowing the pupil to dilate. The dilation of the pupil is very important because if it stays constricted and in contact with much of the lens in the presence of inflammation, the chances are greater for the iris to become scarred and attached to the lens.
The corticosteroids are potent anti-inflammatory drugs that are typically necessary to diminish the inflammation of a uveitic eye. As mentioned before, the main theoretical causes of uveitis involve an immune mediated inflammation--suppression of the ocular immune system is generally necessary to control uveitis.
Corticosteroids do not come as a risk-free treatment. Due to their potent immune suppressing abilities, the corticosteroids, if used on an eye in the presence of a corneal ulcer, can exacerbate the ulcer and predispose the eye to a fungal infection. Approximately 65% of horses which develop fungal eye disease have been treated with a topical corticosteroid. Despite this risk, the corticosteroids are necessary to control the inflammation associated with uveitis.
In addition, systemic pain medication such as bute or banamine, are often necessary.
Recurrent uveitis is the leading cause of blindness in horses. Treatment time is critical for a faster recovery and aids in the prevention of permanent vision obscuring adhesions and cataract formation.
Dr. Darin Peterson, DVM, was born and raised on a horse and cattle ranch in Rosholt, S.D., and received his B.S. in Animal Science from SDSU. He concentrates most of his work time with large animals. He can be reached at 701-347-5496 or email@example.com.