The eyes may have it

Dr. Jennifer Thompson

Something amiss in a horse’s eye will creep out even veteran owners. But given the list of possible injuries, infections and diseases that affect horse eyes, you’re likely to encounter a problem at some point.

Dr. Jennifer Thompson, Lodi/Madison Equine Clinic explained that horses have the largest eyes of any land-bound mammal. A horse eye is larger than the eye of an elephant, she said. Horse eyesight sweeps 250 degrees around the animal leaving blind spots in the coverage directly ahead and directly behind.

“Horses have 20/33 visual accuracy compared to 20/50 for a dog and 20/330 for a rat,” Thompson said. “The horse has a visual streak area and a point of best focus. They’ll position their heads to get that point of focus.”

A horse will tilt its head to get a point of focus on a specific object or point. They’ll raise their heads to focus on distant subjects and lower and tilt their heads to focus on things nearby or on the ground. Outside of the point of focus area, horses appear to have great sensitivity to movement explaining a tendency to sometimes startle over things unnoticed by to humans.

“Horses also see better in dark than light. The tapetum lucidum is a layer of tissue in the eye that reflects extra light at night and allows the horse to see better in the dark,” Thompson said.

Horses see two colors: blues and reds. For example, a red apple appears green to a horse creating a situation similar to color blindness in people. High contrast areas also are more noticeable to horses and can create problems when trying to move the animals from bright sun into a dark barn or trailer.

“But the more contrast there is between objects the less trouble you may have with other things. Contrasting colors can help a horse distinguish fencing. The hunt fence with contrasting colors leads to less rails down in a competition,” Thompson said.

When it comes to injuries and diseases of the eye, Thompson suggested that you begin by knowing what a healthy eye looks like. The eye should be bright, blinking, clear, constant colors, reactive pupils and free of any discharge.

Once you see the eye closed, swollen, a color change, drainage, or a non-reactive pupil you have a good indication something is haywire. Thompson referred to such conditions as “An unhappy eye.”

Among common eye maladies is a corneal ulcer. A grain of sand, dust, weed seed or hay chaff, for example, in the eye under the eyelid can scratch the cornea. If the abrasion is deep enough and left undetected you can end up with the ulcer. Treatment involves cleaning and perhaps additional care as advised by a veterinarian, Thompson said.

Pink eye or conjunctivitis is also on the list of common potential problems. Pink eye is treatable with medicine and largely preventable with the use of a fly mask. Flies are notorious for spreading pink eye and fly masks help close the door on that method of transmission.

Sensitivity to light along with some pain, swelling and small flecks of color in the eye can fall into a category known as uveitis or ERU, equine recurrent uveitis. It isn’t always clear what causes ERU, Thompson said, but if you can’t get it under control the disease can cause blindness.

Horses have a tendency to injure eyes, too, Thompson added. “The majority of eye injuries that I see in my practice come from water or grain buckets. People let them wear out and get bent and out of shape and when the horse puts its head in to eat or drink it catches and eye or eyelid as it pulls its head back out,” Thompson said.

While there is a long list of thing that can affect a horse’s eye, Thompson said that most of them are manageable. “Even a blind horse or a horse with one eye can lead an otherwise healthy and productive life. I had a client that kept a blind horse and it lived many years. You have to make some adjustments but it can be done,” she said.