Cataracts are opacities of the eyes’ lenses. In short, the lens turns white and doesn’t let light through. And since light is required for your horse—or any animal—to process visual information, the problem is clear.

Although cataracts are frequently considered a process of aging in humans, in horses they’re found at any age, and are the most frequent congenital vision defect in foals. Appaloosas are among the equine breeds that are particularly prone to this inheritable condition.

“Horses manifest varying degrees of blindness as cataracts mature,” says Dennis Brooks, DVM, PhD, a professor of ophthalmology at the University of Florida in Gainesville and diplomate with the American College of Veterinary Ophthalmologists.

Symptoms: Dr. Brooks says that very small, developing lens opacities are common and not associated with blindness. As cataracts mature and become more opaque, the degree of blindness increases. “If the cataract matures slowly and the horse has time to adapt to this reduction in vision, the owner may not notice any changes in behavior,” Dr. Brooks explains. “But sudden loss of sight due to rapid cataract formation can result in the horse displaying nervousness, shyness, and refusal to perform functions it was trained to do.”

Diagnosis: Since the lens is located deep within your horse’s eye, your veterinarian will need to make the diagnosis with an ophthalmoscope or slitlamp. Dilating the pupils may be necessary.

Treatment: Most veterinary ophthalmologists recommend surgically removing cataracts for foals under 6 months old if the horse is healthy and free of other eye conditions such as uveitis, and if the foal’s personality allows you to apply the drops that must be administered for several weeks before and several months after surgery. This is a significant time commitment for you, the owner.

The most common surgical procedure breaks up and emulsifies the lens nucleus and cortex with an ultrasonic titanium needle in a silicone sleeve that’s inserted into the horse’s cornea through a small incision. The affected lens tissue is then sucked out of the eye.

In foals, results are usually very good. However, in adult horses with cataracts caused by equine recurrent uveitis (also referred to as periodic ophthalmia or moon blindness), results are often poor due to scar tissue forming inside the eye during surgery.