Advances in Equine Ophthalmology

Advances in Equine Ophthalmology

Treatment and research breakthroughs help to wage the war against equine vision problems.

Photo: Anne M. Eberhardt/The Horse

The more researchers study equine physiology, the more evidence suggests that so-called bad behavior could be as much physical as training and behavioral. Back pain, for instance, might be the root cause of many under-saddle issues. Now, equine vision problems are emerging as the culprit behind some cases of spookiness and other undesirable behaviors.

In a lecture at the Dressage at Devon (Pa.) show in September, Chelsey Miller, DVM, a resident in veterinary ophthalmology at the University of Pennsylvania's New Bolton Center in Kennett Square, described some of the exciting advances in her field.


A cataract is a clouding of the lens of the eye, giving the eye a clouded, milky-white appearance. Most people are familiar with the term because almost all humans develop cataracts as they age.

Equines can also develop cataracts, which in horses aren't age-related, Miller said. Cataracts can cause anything from light sensitivity to nearly complete blindness. Some horses manage just fine with cataracts, even when one eye is virtually blind; a cataract is a greater liability in equestrian disciplines requiring keen depth perception, such as jumping and polo. (Oddly, though a one-eyed racehorse is "legal," the Hackney-pony and Paso Fino breeds mandate two "visual eyes" for competition eligibility, she said.)

Surgical removal of cataracts in humans is a relatively simple office procedure. The surgery is more complicated in equines because general anesthesia is required, Miller explained. In fact, cataract removal is the most complex procedure in veterinary ophthalmology, she said.

The process is called phacoemulsification. A probe vibrating at ultrasonic frequencies liquefies and breaks up the cataract, which is then vacuumed out of the eye, Miller said.

In the past, the required minimum two to three months of post-operative care and regular medicating of the eye was practically as challenging as the cataract surgery itself, Miller said. During this time, horses often get resentful of having the eye area handled, and the continual manipulating of the eyelid can damage the surgical site. Thankfully, today veterinarians can install a temporary catheter that delivers the eye meds via a tiny tube, no direct handling necessary. If the horse is quiet and not inclined to rub his head, he can even be hand-walked or turned out in a small paddock with a fly mask on during the rehab period, she said.

Miller cautioned that cataract surgery doesn't give the horse perfect vision. However, veterinary ophthalmology researchers at North Carolina State University are looking into the use of an artificial lens to improve near- or farsightedness, she said. Experts use a technique called retinoscopy, in which different lens refractions are used to estimate a horse's vision, to help determine the degree of near- or farsightedness. (Most horses are farsighted, she said.)

Equine cataract surgery isn't undertaken lightly. First, there's the cost, which can run into the thousands (Miller was reluctant to state price ranges for the record). Second, there are risks of complications. For the latter reason, Miller said, most veterinarians are reluctant to do surgery if the horse has some vision.

"It's possible you could start with an eye with some vision and end up with no vision," she said.

Equine Recurrent Uveitis

Equine recurrent uveitis (ERU) is a chronic, immune-mediated eye disease that's not yet fully understood, according to Miller. ERU is essentially an immune response gone haywire. An antigen (a substance that provokes the production of antibodies) gets into the horse's eye and triggers a continuous, inappropriate autoimmune response. The result can be pain, glaucoma, and eventually blindness.

There might be no obvious signs of eye trauma prior to an ERU episode, Miller said. However, any bout of eye trouble--tearing, a scratch, swelling--should be followed up with twice-yearly checkups because such incidents can set the stage for ERU.

The Appaloosa breed is the most at risk for ERU, representing eight out of 10 cases, said Miller. Researchers speculate that Appys might produce a type of protein that makes them more susceptible to ERU, but the connection isn't fully understood and more research is needed, she said.

A bright spot in this otherwise puzzling and frustrating disease, said Miller, is the development of an optical implant impregnated with the immunosuppressant drug cyclosporine. The implant can reduce ERU flare-ups to fewer than once a year. An implant lasts for four to five years, and many horses never need a second implant, she said.

The implant requires general anesthesia. Costs can vary greatly, depending on the specifics of the case--but the cost is "not much more for two eyes because the horse is already anesthetized," Miller said.

Of the implant, Miller said: "ERU is still a chronic disease: You still can have flare-ups; you still can wind up with a blind eye. But it's the best we have so far."

Not every horse is a good candidate for every procedure, of course. A thorough evaluation by a veterinary ophthalmologist is a necessary first step in treating any eye condition, Miller said.

Vision Problems and Spookiness

Veterinary ophthalmologists know that some spooky behavior can have physical causes. For instance, horses (and alpacas, cows, and goats) have in each eye a structure called a corpora nigra. It's a pendulous cyst on a stalk that hangs down over the top of the eye. Its function is unknown; researchers postulate that it may function as a sunshade of sorts. A corpora nigra can grow abnormally large and can even move around in the eye. It's reasonable to suspect that a growth that shades part of a horse's field of vision could well lead to spooking as objects appear and disappear suddenly.

Although diseases of the equine eye are well studied and documented, less is known about the effects of vision problems on horse behavior, Miller said. Richard J. McMullen, Jr., DrMedVet, CertEO, assistant professor of ophthalmology at North Carolina State University's College of Veterinary Medicine in Raleigh, is conducting vision studies on this topic, Miller said. This groundbreaking research could help us learn more about why our equine friends behave as they do.

About the Author

Jennifer O. Bryant

Jennifer O. Bryant is editor-at-large of the U.S. Dressage Federation's magazine, USDF Connection. An independent writer and editor, Bryant contributes to many equestrian publications, has edited numerous books, and authored Olympic Equestrian, published by Eclipse Press and available at or by calling 800/582-5604. More information about Jennifer can be found on her site,

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