How Horses See: Part Two

In my last blog post, I shared some information about normal equine vision from the University of Minnesota’s Trainers Series at the Leatherdale Equine Center.  In part two, I will recap the material on common eye diseases and disorders.

Part Two: Common Eye Problems

DIAGNOSIS

Just like humans, horses with eye problems may squint, have red eyes, have tears down their face, and their eyelids may swell up.  You might also notice your horse rubbing his eye on his leg or a fence post.  It’s easy to think, “Oh, he probably just caught a bit of hay or a speck of dust in his eye, he’ll be fine,” but unfortunately, in horses, severe eye problems look the same to an owner as minor ones.  Since early detection and treatment is crucial with major issues, it’s best to call the vet with any eye issues so they can test the eye to figure out the severity of the damage and treat it accordingly.

Corneal ulcers, when the cornea of the eye gets scratched, are one of the most common eye issues. However, that doesn’t mean they aren’t serious.  Equine eyes don’t have blood vessels in them so their eyes are more prone to developing infections and “melting ulcers” which are a very real threat to eye survival.  It’s important to seek veterinary attention promptly.

Equine Recurrent Uveitis (ERU) or moon blindness occurs when the inside of the eye becomes infected and is the most common cause of blindness in horses.  While the causes of this disease are not fully understood, vets have identified it as an auto-immune condition. The blindness is typically caused by cataracts, glaucoma, and deterioration of the retina.  Cataracts are most often seen in older horses, but can occur congenitally in foals as well.  Horses can also get conjunctivitis, aka pink eye, which is inflammation of the conjunctiva. 

You might also notice a small growth around your horse’s eye.  Squamous cell carcinoma, sarcoids, and melanomas are three types of cancer that can develop around a horse’s eyelid. These are more common in horses with white skin around the eyes and vets recommend such horses wear fly masks year-round to prevent excessive exposure to the sun. Early detection and surgical removal are recommended for cancerous growths around the eye.

TREATMENT

Since horses have extremely strong eyelid muscles, vets typically will sedate the horse in order to inspect it.  Then they often will apply a fluorescein or Bengal stain to the eye which coats irritated areas and highlights them.  This allows vets to determine the location, size, and depth of any ulcers.  Treatment typically involves an antibiotic topical ointment applied 2-4 times a day and an injection of banamine. 

Guardian Mask http://guardianmask.com/

Treatment of eye issues, especially in the case of recurring Uveitis, often involves special fly masks (like the Guardian mask above) with hard cups over the eyes to prevent the horse from rubbing the eye on everything as well as protecting the eye from debris and excessive sunlight.  

Use of normal fly masks can help prevent eye issues, especially irritation from flies and debris. Equine Sun Visor fly masks have a special solid fabric to block additional sun rays and act as sunglasses.

My horses wearing Equine Sun Visors - www.equinesunvisor.com

The best course of action is to prevent eye injuries when possible and always call your veterinarian about eye issues.

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How Horses See: Part One

Have you ever wondered how horses see their world?  I attended the first session of the Trainer Series at the University of Minnesota Leatherdale Equine Center class on equine vision*. Here's what I learned:

Part One: Normal Equine Vision

Kaleidoscope - photo by Molly Goossens Photography

COLOR

Research into equine vision tells us that horses see some colors but not all the range we do. They have what we call dichromatic vision. Essentially, horses are red-green color blind, so they cannot distinguish between red and green and do not see red, purple, or orange.  Mostly they see the world in muted shades of yellow, green, blue, and gray.  Because horses have difficulty differentiating solid colors, many jumps designed for horses are striped. 

Also, horses with blue eyes like Kaleidoscope pictured above, do not have any eye issues more than horses with brown eyes except they are prone to skin cancer with all of that pale skin.  Wearing fly masks year-round and sunscreen in the summer can help prevent painful sunburn and risk of cancer for horses with pale skin.

FIELD OF VISION

As prey animals, horses have excellent peripheral vision and are adept at detecting motion.  With an eye on each side of their head, horses have almost a full 360 degree field of vision.  They do have a blind spot directly in front of their noses up to 3-4 feet and directly behind them.  Most of their vision is monocular, i.e. input is from one eye only.  However, in front their eyes’ fields of vision overlap and they have binocular vision and depth perception. When a horse is ridden with a lower headset and in a round frame, they see the ground not far in front of them.  Imagine the trust they must have in their rider to not be looking out for possible threats while in this frame!  Ever wondered why horses have such long noses?  One reason is that their eyes are designed to be high enough up on their heads that they can see over tall grass when grazing.

CLARITY

Although horses only have approximately half the ability humans have to see details, horses have better detailed vision than cats or dogs. One reason for this might be that horses have unusually large eyes which allow more light to enter in. Their ability to focus on something very close to them in proximity is limited.  For this reason, horses often prefer to halt while far away from an object of interest, lift their head and use both eyes to observe.

LIGHT & DARK

Horses are gifted with extraordinary night vision for a non-nocturnal animal.  They can see quite well in the darkness once their eyes have adjusted to it.  However, their eyes take significantly longer to adjust to changes in lighting than human eyes do.  When leading a horse from bright daylight into a dim barn, allow time for their eyes to transition.

Horses are certainly well-adapted to being grazing prey animals but not necessarily so well suited to being domesticated animals used for riding and driving.  Keep this information in mind when you are handling and riding them, for your safety and for their comfort!

Elena - photo by Molly Goossens Photography

*Information shared with permission from "Equine Vision: What they See and What You Don't!" presentation by Kerry J. Kuhle, DVM

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