Dealing With Deteriorating Vision in Horses

Would you know if your horse was losing his eyesight? Moreover, what could you do? 

It might appear that your horses are grazing in the field without a care in the world when, in reality, all of their senses, particularly their vision, are in “red alert” mode, actively monitoring the environment for potential danger. Their large eyes with horizontally fashioned, elliptically shaped pupils help maximize their ability to scan the horizon.

“Not only are horses reliant on their vision for safety as a prey species, they also require excellent vision as athletes,” says Ann E. Dwyer, DVM, a private practitioner at Genesee Valley Equine Clinic, in Scottsville, New York, and past president of the American Association of Equine Practitioners (AAEP). “Thus, declining eyesight in these animals can have devastating consequences for their handler or rider, other horses in the herd, and themselves.”

As many owners know from personal experience, ocular tissues are extremely sensitive. Infection, trauma, dry eye, and increased intraocular pressure (glaucoma) can range from extremely irritating to downright agonizing for people. An acutely red, painful, and irritated eye in your horse that he continues to rub clearly indicates a problem mandating a veterinary visit. But do you think you would recognize deteriorating eyesight or other nonacute or nonemergency ocular issues in your horse? 

Researchers on a soon-to-be published study found that recognizing ocular abnormalities is no easy feat. In fact, one might say it’s much like not seeing the forest for the trees.

“We surveyed hundreds of horse owners in Queensland, Australia, and only 3.3% of those owners felt that their horse had a medical concern involving their eyes. We subsequently conducted complete ocular examinations in 339 of the 974 horses we obtained completed surveys for and found that almost 88% actually had abnormal ocular findings,” says lead author Fernando Malalana-Martinez, DVM, GPCert(EqP), Dipl. ECEIM, FHEA, MRCVS, senior lecturer in equine internal medicine at the University of Liverpool’s Institute of Veterinary Science, in the U.K.

“An estimated 1 to 2% of the American equine population currently suffers unilateral (in one eye) or bilateral (in both) blindness, equivalent to approximately 95,000-190,000 horses. This is a substantial number of horses, making vision loss an important issue in equine operations,” Dwyer adds.

Let’s take a closer look at potential causes of ocular abnormalities and vision loss in horses. We’ll also describe behaviors and signs you can watch for that suggest visual impairment. Finally, we’ll review management strategies for helping horses deal with deteriorating eyesight and blindness. However, we haven’t included details regarding treatment of specific ocular conditions because they lie beyond this article’s scope.

Dwyer encourages owners to maintain an open mind as they read this article. “It’s very important for owners to recognize that many horses without any vision at all can be successfully managed and potentially even continue to compete athletically,” she says. “Deteriorating vision is not synonymous with a death sentence.”

Leading Ocular Abnormalities

Trauma

Considering the large size of the horse’s eyes relative to his head and the proximity of those eyes to the ground, where dust and debris, vegetation, and other horses’ tails and feet tend to aggregate, it’s no surprise that trauma remains a leading cause of equine ocular issues.

“Trauma causing injury to the surface of the eye, called the cornea, is usually readily observable,” says Dwyer. “A red, painful, swollen eye that the patient holds closed with an obvious defect or even embedded foreign body makes diagnosis relatively straightforward.”

Indeed, data collected in the above-cited survey by Malalana-Martinez and colleagues showed that “owners more readily identify corneal lesions (than other types of ocular abnormalities), which often occur in cases of trauma.”

Malalana-Martinez did additionally note, however, that “very few owners reported an ocular traumatic injury as a specific entity—0.3% of 974 horses.”

Trauma cases must be addressed immediately because secondary infections—both bacterial and fungal—can develop rapidly, potentially leading to more advanced and serious disease, including melting corneal ulcers.

“Some horses lose vision if ulcerative keratitis (fungal infection of the cornea) advances to infection within the globe (eyeball) or the corneal disease becomes so severe that enucleation (eye removal) is required,” says Dwyer. “Fungal keratitis that is severe frequently results in vision loss, even if the keratitis is eventually controlled.

“A simple injury can quickly manifest into an even larger, more complex, expensive, and potentially eye-threatening condition,” she adds. “Never wait to have any horse with any eye problem examined by a veterinarian.”

Cataracts and retinal atrophy

“In addition to trauma, leading causes of ocular abnormalities noted in our study included cataracts and age-related retinal atrophy,” says Malalana-Martinez.

Specifically, 34.3% of the horses had cataracts and 31.8% had senile or age-related retinal atrophy, which is degeneration of the membrane lining the back of the eye that essentially transmits information from the eye to the brain. An additional 10.1% of examined horses also had so-called “bullet-hole” lesions in their retinal tissues, which are pin-sized defects that might or might not impair vision. 

“Cataracts are areas of focal or diffuse cloudiness within the lens of the eye,” says Dwyer. “As with other species, cataracts can affect vision, and both mature and hypermature (beyond full development) cataracts can be blinding.”

“In terms of the retina, I think probably senile atrophy is more significant to these horses than bullet-hole lesions, as these have to be quite extensive to significantly affect a horse’s vision,” says Malalana-Martinez.

Uveitis

The most common cause of blindness in horses is equine recurrent uveitis (ERU), also referred to as moon blindness, a condition most owners have at least heard of, particularly in the Appaloosa world. Uveitis itself is defined simply as inflammation of the uvea, which comprises several tissues inside the eye, including the iris. Classic signs suggestive of uveitis include a red, painful, cloudy eye accompanied by miosis, a profound constriction of the pupil. Unfortunately, even with aggressive treatment, about half of all horses with uveitis eventually suffer severe vision loss.

“Reasons for loss of vision associated with ERU vary,” says Dwyer. “Some horses suffer detached retinas, others go blind from maturing cataracts, others lose vision when the eye is so damaged that it just scars in on itself and becomes what we call phthisical.”

Glaucoma

Defined as a “multifactorial neurodegenerative ocular disease,” glaucoma typically appears as eyes that are “big and blue,” either over the whole corneal surface or a portion of it. This condition is relatively uncommon in horses and was not identified in any horses in Malalana-Martinez’s study.

“When it does occur, glaucoma frequently occurs secondary to uveitis and requires aggressive multimodal therapy to address the underlying changes in the eye,” Dwyer says.

Other

Less common causes of vision impairment and loss stem from a wide variety of injuries/diseases, including:

• Cranial trauma, especially training accidents in which a horse flips over, causing skull trauma that affects the visual pathways.
• Neoplasia (abnormal growths) involving the globe, retrobulbar region of the orbit behind the eye, or brain region near the optic nerves. Neoplastic conditions can either arise primarily in the orbit or can start in the paranasal sinuses and spread or expand into the orbit.
• Congenital defects in which foals are born with rudimentary, nonfunctional eyes.

“There is actually a genetic condition called ‘multiple congenital ocular anomaly’ that has been linked to the silver dapple coat color,” says Dwyer. “Like many genetic linked diseases in horses, it presents with a spectrum of severity, ranging from minor variants to blinding sequelae. I have seen this syndrome in horses of this coat color, as well as Miniature Horses and some other breeds that are both silver dapple and other coat colors.”

Dwyer also describes having seen West Nile virus cause acute blindness in a few horses within 48 hours of onset of clinical signs.

What Impaired Vision Looks Like

Based on the available data, it seems horses are masters of disguise, frequently hiding deteriorating eyesight behind their lovely lashes. Owners shouldn’t feel bad about not recognizing their horses’ diminished vision; veterinarians attest that even the most experienced horseperson can be fooled about what a horse can or cannot see.

“Ridden horses with major ophthalmic abnormalities, including extensive and bilateral lens opacification (a “fogginess” to the lenses) commonly show no behavioral evidence of visual compromise,” said Andrew G. Matthews, BVM&S, PhD., Dipl. ECEIM, honorable Member ACVO, FRCVS, an equine ophthalmologist from Scotland, during a presentation at a recent AAEP Focus on Ophthalmology session. “On the contrary, however, horses with minor abnormalities such as central focal lens opacities or vitreal ‘floaters’ can exhibit behavior suggestive of visual dysfunction.”

This can include headshaking, shying, and/or exaggerated startle responses.

“Other horses with gradually declining vision, which occurs more commonly than a sudden loss of vision, can begin to show signs of hesitation or uncertainty in certain situations, herd behavior may change, and they can actually be observed bumping into objects or obstacles,” adds Dwyer.

In his presentation Matthews described the following abnormalities and how they can potentially cause visual dysfunction:

• Retinal issues presumably impact visual acuity (this is the 20/20 type of vision reported in humans, ­indicative of clearness of vision) and color perception;
• Bullet-hole lesions on the fundus, at the back of the eye, have been associated with significant visual disability;
• Older horses with senile retinal degeneration show decreased vision in dim lighting conditions; and
• Dense opacities on the lens can disrupt the passage of light, creating blind spots.

Looking Beyond What We See

The reason identifying impaired vision can be tricky is because horses might be able to adapt their behavior when faced with ocular dysfunction, especially when vision loss occurs gradually.

Owners, together with their veterinarians, can use these three tests to help assess equine vision:

1. The dazzle reflex This reflex involves the horse’s involuntary aversion response (e.g., blinking, globe retraction, third eyelid protrusion, and/or head movement) to intense illumination of the eye. 
2. The menace response This test is conducted by making a small, threatening hand gesture toward the horse’s eye. A horse that can see should blink.
3. Obstacle course testing Also referred to as maze testing, this is a pretty reliable method of assessing vision—probably more so than the menace response, which is known to be somewhat subjective. “If a horse that is unilaterally blind has the other eye blindfolded and is presented with a maze of buckets or other solid objects, it will either freeze or stumble into the obstacles,” says Dwyer. “When the visual eye is uncovered the horse will be able to navigate the maze.”

Supporting Horses With Decreased Vision/Blindness

So, the question now remains: How safe is a blind 1,000-pound domesticated prey species with a flight instinct? Our sources agree that attitudes have changed in terms of managing aging equids, and that owners of blind or visually impaired horses should avoid jumping to conclusions regarding the functional consequences of ocular disease or damage.

Many organizations exist to support owners of blind/visually impaired horses, and several owner-friendly publications are readily available. In a nutshell, Dwyer suggests owners focus on the following key areas when dealing with a horse affected by vision loss:

• Social interactions Visually impaired horses typically don’t do well in herd situations. They are often relegated to the hollows of the pecking order, frequently being excluded from food and water. The majority thrive with a pasture buddy, be it a calm companion horse, goat, or other friend. Some prefer to live in solitude.
• Environmental organization Once set up appropriately for safety, try not to move anything in a blind horse’s enclosures. Check all fencing, windows, footing, hooks for hanging, and feed and water buckets, etc. for sharp edges to avoid injury.

“Blind horses tend to use their muzzles to ‘read’ their environment, much like humans read braille,” says Dwyer. “We want to be certain their sensitive muzzles will not be injured when navigating their environment.”

Additionally, horses with visual dysfunction benefit from “cues” that indicate where certain objects are in a pasture. Such cues include stone footing near gates, rubber mats near feed/water sources, and rubber tires around trees, to name a few.

In addition to simply keeping blind or partially blind horses as pasture pets, some owners continue to actively train these animals. Again, horses tend to rely more heavily on other sensory cues, such as hearing, smell, and touch. Along with auditory cues, owners need to reassure their horses frequently with touch, consistently approaching the horse from the same side each time (e.g., at the front of the left shoulder).

“Religiously following training and handling routines will help visually impaired horses learn what is expected of them,” says Dwyer. “This type of training also strengthens the bond between horse and owner.”

Take-Home Message

“While ocular issues and visual impairment can occur at any life stage,” says Malalana-Martinez, “our study clearly shows that ocular pathology is common in horses 15 years or older, and that increased aging is associated with increased occurrence of eye abnormalities and cataracts. With the aging equine population continuing to grow as horses are increasingly treated as pets rather than beasts of burden, this means ocular issues will also increasingly need to be addressed.

“Knowledge of diseases affecting older horses is essential to provide adequate preventive or therapeutic measures to maximize their athletic careers and quality of life,” he adds.

It’s important that veterinarians perform a systematic ocular exam annually, particularly in aging horses. Speak with your veterinarian about any concerns you have at your horse’s next routine physical.

Dwyer stresses the importance of recognizing that vision loss affects each horse’s welfare differently and, so, your veterinarian’s recommended treatment will vary with it. And, again, have your veterinarian out at the first sign of a potential ocular issue impacting vision.

“Some blind horses seem to retain their globes with no apparent discomfort, but others experience signs of chronic discomfort,” she says. “It can be hard to judge the discomfort, but veterinarians who have enucleated blind eyes that appear to be bothering the horse have often been told by the owners that the horse shows improvement in temperament and attitude once they’ve recovered from surgery.”