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Dealing With Deteriorating Vision in Horses

Dealing With Deteriorating Vision in Horses

By Stacey Oke

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.”
 

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