Problems with Cataracts?

Q: I am thinking of buying a 6-year-old mare that has cataracts in both of her eyes. What kind of problems would I be in for if I decide to buy this sweetheart?

via e-mail

A: If you are seriously interested in this horse, you should invest in an examination by a veterinary ophthalmologist. The specifics of the cataracts will affect the prognosis. The term "cataract" only means that there is an opacity to the lens of the eye; that opacity might be a very small spot on the lens or encompass the entire lens. The lens of the eye is normally clear and acts like a camera lens focusing the beam of light entering the eye on the retina in the back for transmission to the brain via the optic nerve. The lens is like a Zip-Lock bag filled with clear Jello; there is a firm capsule on the front and back surface and soft, clear tissue in the middle. Cataracts develop when some process occurs that causes any part of the clear capsule and/or center to become opaque and therefore affect the beam of light entering the back of the eye.

Diffuse cataracts affecting the entire lens occur in foals and are considered a form of congenital eye problems. (Has this horse had them since birth?) The most common cause of cataract formation in the adult horse is inflammation caused by the advanced stages of anterior uveitis or moonblindness (see The Horse, "Periodic Ophthalmia," July 1998, and "Common Problems," April 2000). There are other causes in the adult horse, but they usually only affect one eye.

The first step in evaluation of the cataract is to determine if it involves the anterior (front) capsule, the posterior (back) capsule, the central part of the lens, or any combination of these structures. It then is noted how much of the lens is affected. Where is the cataract within the lens? How dense is the cataract (how much light is making it through)? After that exam, your veterinarian will attempt to determine how much the cataracts affect vision, a task easier said than done.

How does the horse react to her environment? In most horses, you can have a surprisingly large degree of vision loss and not really notice any abnormalities in the way that horse interacts with the environment. Horses have an amazing ability to compensate for vision loss, especially if one eye is primarily affected and the loss was gradual in development. It might be dangerous to ride a horse with compromised vision, but if the horse had been ridden, did she shy right or left frequently? Did she hold her head in an odd position? Did she have problems in dim light environments?

If the cataracts are small and not severely affecting vision, the next question typically is: What will happen over the next few years? It is completely unpredictable.

Several horses under my care have small, central cataracts that do not appear to affect vision greatly, and they have stayed exactly the same for five years. I have others that have progressed from small, relatively insignificant cataracts to causing complete blindness within a few years. Only frequent examination will tell you how small cataracts will change over time.

Can something be done? In foals, cataracts can be removed surgically with relative success. The adult horse situation generally is more complicated.

On a final note, should you end up taking on this horse, you must consider the danger in owning a horse with potentially limited sight. In addition, you could be held libel for any injury occurring to people, other animals, and even property damage should it be related to a suspected visual deficit.

About the Author

Michael Ball, DVM

Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.

Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at or by calling 800/582-5604.

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