We’ve all heard the saying, “No hoof, no horse.” But a horse’s eyesight is just as important as those four feet. In the wild, poor vision could mean the difference between life and death if a horse couldn’t spot predators in time to avoid danger. Even a modern, domesticated horse can have his performance life drastically altered or cut short if eye trouble leads to blindness or distorted vision. And some eye troubles, such as cancer, can lead to issues affecting the horse’s overall health.
No wonder we get excited when science pushes forward our ability to diagnose and treat disorders of the equine eye. Here, researchers from universities across the country share some of their latest advancements to help our horses maintain healthy eyes and good vision.
Imaging and Diagnostics: Corneal In Vivo Confocal Microscopy
For any type of health problem, a correct diagnosis is essential to treatment, and various forms of imaging play an important role.
Equine eye issues are no different, and one of the newer techniques giving veterinarians better insights is corneal in vivo confocal microscopy, according to Eric C. Ledbetter, DVM, DACVO, an associate professor of ophthalmology and ophthalmology section chief at Cornell University’s Hospital for Animals.
This imaging technique centers around a confocal microscope, which uses a narrow beam of light to take multiple two-dimensional images of the eye at different depths. The images are automatically reconstructed into a three-dimensional view. In use, the confocal microscope is attached to an adjustable table mounted on a mobile desk, says Dr. Ledbetter. It’s run by a computer with a standard monitor for viewing.
The exam is often performed at a veterinary hospital utilizing standing sedation and topical anesthetic.
Using this procedure, veterinarians can evaluate all layers of the horse’s cornea, including deep corneal lesions that are otherwise not accessible for diagnostic evaluation, says Dr. Ledbetter. It’s also useful for diagnosing other corneal issues, including infections, various autoimmune conditions, tumors, foreign bodies and more.
Dr. Ledbetter and his Cornell colleagues were the first to report the use of this imaging technique in horses and are the only group to have published descriptions of its use in horses. (The technique has shown promise in other species, including dogs.)
“This is a noninvasive imaging technique that allows the cornea to be evaluated in real time with magnification and resolution similar to histology of a standard biopsy but without any cutting or other damage to the horse’s tissues,” says Dr. Ledbetter. “I often explain it as a virtual biopsy.”
Also, a small camera can project the images immediately for the veterinarian to interpret. Fortunately, this means the horse owner doesn’t have to wait days to hear back on results from a biopsy.
The equipment required for the technique is expensive—typically costing more than $50,000 for a unit, according to Dr. Ledbetter. The cost to perform an examination varies by facility, but Dr. Ledbetter notes that it is relatively low and can prevent the need for other forms of diagnostic testing. In addition, the procedure can potentially reduce treatment costs while improving outcomes by providing a specific diagnosis early in the course of the disease.
Corneal in vivo confocal microscopy is available only in a limited number of locations worldwide. However, its availability is spreading, says Dr. Ledbetter, who notes the technology has been used at Cornell for several years to evaluate equine eyes.
Treatment Delivery Method: Direct Medication Injection
Giving eye drops is a common method for medicating the equine eye. However, it can be time-consuming for the owner to administer, and over time the horse can become difficult to treat as he tries to avoid the drops. In addition, drops can be an inefficient way to administer medication, particularly to the cornea, says Brian Gilger, DVM, MS, DACVO, DABT, professor of ophthalmology at the North Carolina State University College of Veterinary Medicine.
“The cornea is normally about 1 mm thick,” he explains. “When it becomes diseased, it becomes much thicker. Using standard eye drops, it’s very difficult to get medication down deep enough.” That can mean that the horse faces surgery and the possibility of a corneal transplant.
Now, says Dr. Gilger, a new type of needle makes it very simple to inject medication exactly where it’s needed. “It’s a completely new injection device,” he says. “It precisely injects where you want the drug to go and does not rely on treating the entire ocular surface.”
With the horse standing but tranquilized—and typically at a veterinary clinic—the veterinarian administers a local topical anesthetic, then uses imagery (via high-frequency ultrasound) to guide the injection and deliver medication directly into the affected area of the cornea.
Dr. Gilger adds, “The goal is to provide treatment about every three days instead of every two to four hours with eye drops.”
Dr. Gilger says the procedure can effectively heal fungal keratitis (inflammation of the cornea caused by fungal infection) and other diseases where the entire eye doesn’t need to be treated. As a bonus, the treatment can cost just $200 to $300 versus, for example, standard fungal keratitis treatments, which can run $1,000 to $2,000.
“This type of treatment will probably completely replace eye drops in the next few years,” Dr. Gilger predicts.
Treatment Procedure: Corneal Cross-Linking
The horse’s cornea is a common place for injuries or ulcers, which may stem from various causes, including trauma, fungus or bacteria. Corneal issues can cause serious problems, including scarring and loss of sight. In some cases, corneal ulcers open the door to bacterial or fungal infection. This can accelerate the ulceration and lead to a jelly-like or liquefied appearance of the eye, often referred to as “corneal melting.” If medical treatment isn’t effective for corneal issues, the horse may require surgery, which could include inserting a catheter to provide continuous medication to the eye.
A new technique called corneal cross-linking has the potential to fix the problem through a nonsurgical, in-patient procedure, according to Dr. Gilger.
With corneal cross-linking, the veterinarian applies riboflavin (vitamin B) eye drops to the surface of the cornea. (Dr. Gilger notes that improved techniques for application are being developed, such as local injection.) The veterinarian then shines a UV light onto the eye, which activates the riboflavin, creating bonds between the collagen fibers in the eye. This stabilizes and strengthens the cornea to prevent melting and also kills the infection, says Dr. Gilger.
The procedure itself can cost around $500 and is widely used in Europe and beginning to see more use in the U.S. One drawback, says Dr. Gilger, is that it is currently a lengthy procedure. It takes 30 minutes or more to administer the eye drops and another 30 minutes for the UV light application, he explains. Additional research is now focused on testing different techniques to speed up the process.
Treatment Procedure: Stem-Cell Therapy
Immune-mediated keratitis is a common equine eye condition, says Dr. Gilger. With IMMK, the horse’s own immune system essentially attacks cells in the cornea. It can lead to inflammation, cloudiness of the eye and blood vessels growing into the cornea. It is considered a chronic disease that can wax and wane for years and ultimately lead to blindness, says Dr. Gilger.
Treatment typically involves daily application of steroids. However, the medication can have long-term side effects. “Topical use of steroids puts the horse at risk for developing corneal infections, such as fungal keratitis, and can result in corneal degeneration when used for a long period of time,” says Dr. Gilger. “Furthermore, if the eye is injured when topical steroids are being used, then the eye heals much slower than normal.” Yet if treatment is stopped, the problem can come back.
Dr. Gilger has been working on a stem-cell-based solution with his NCSU colleague Lauren Schnabel, DVM, PhD, DACVS, DACVSMR, an assistant professor of equine orthopedic surgery and stem-cell researcher, along with ophthalmology resident Amanda Davis.
“We can now safely harvest stem cells from [the IMMK patient’s] bone marrow, grow [more cells] in the lab, inject them back into the horse’s eye and stop the immune response plus aid with healing,” says Dr. Schnabel.
Harvesting the cells costs about $1,800 and involves using a needle inserted into a bone (often the horse’s sternum) to remove stem cells. During the procedure, the horse is sedated, and a local anesthetic is used to numb the site. Stem cells not used for the initial treatment are frozen for future use, and any repeat injections run about $500, says Dr. Gilger.
While the treatment takes a couple of weeks to develop—time for the stem cells to grow in the lab—since IMMK is a chronic condition, waiting a little longer for a treatment won’t make much difference to the horse, says Dr. Schnabel.
During research on the procedure, the stem-cell therapy cleared the disease in a few patients—some of whom needed no additional treatment. Other horses showed a lesser response but still improved. This difference is an inherent challenge when stem cells are taken from the patient, says Dr. Schnabel.
“There is variation in the quality of stem cells from patient to patient,” she explains. “What you get from a 3-year-old horse is different [than] what you get from a 20-year-old horse or a horse that might have other issues.”
That’s why future research may include using cells from a horse other than the patient—a horse with more optimal stem cells—to remove some of that variability in results. Researchers will also be working to determine the best delivery method for the treatment—one that will ensure the best distribution and work most effectively over the long term, says Dr. Schnabel.
More to Come
These are just a few of the promising advancements in equine eye care now available to veterinarians. More research is constantly in the works. There’s little doubt that owners can expect more innovation soon to help keep their horses’ eyes healthy and vision crystal clear.
7 Insights into Equine Vision
1. From light to images. As light enters the horse’s eye, it passes through the cornea, aqueous humor, lens and vitreous humor. These structures refract, or bend, the light and focus it onto the retina. At this point, the light is converted to an electrical signal that travels out of the eye, along the optic nerve, to the brain. The brain interprets it as images, or vision.
2. Binocular and monocular vision. The horse’s eyes are placed on the side of his skull, giving him a wide arc of monocular vision on each side and a roughly 65-degree arc of binocular vision in front. With monocular vision, the horse sees different things through each eye, although the brain shares the information from each side. (This lays to rest the myth that what a horse sees with one eye isn’t recognized through the other eye.) Binocular vision allows the horse to see the same thing with both eyes at once. With this wide range of vision from monocular and binocular viewing, horses can see more of the world around them at any given moment than a human. When your horse spooks or startles, it may be caused by something you can’t see without turning your head.
3. Blind spots. This eye placement also gives the horse blind spots—notably behind himself and for a space of about 4 feet in front of him. To adjust for this front vision limitation, a horse will naturally raise, lower or tilt his head to see better—which could explain why so many horses reflexively move their head upward as you raise a hand to their forehead. It’s also worth noting that a horse ridden with his nose on or behind the vertical cannot see directly in front of himself.
4. Night vision. Horses do have fairly good vision in low-light conditions. That’s because their large eyes, combined with their horizontal pupils, help them capture more light.
5. Color vision. Horses can see some colors, although they probably don’t see them as vibrantly as people do. In general, they can see the green–yellow spectrum and the blue–gray spectrum. They can’t see red.
6. Eye care. To help keep your horse’s eyes in good health, reduce the chance of irritation from dust, insects and UV light. A fly mask may help as well as providing shade during peak sunlight hours. Also work to decrease the risk of trauma by paying attention to stabling and pasture. Low tree branches, thorny plants, hooks or other sharp objects can be hazardous.
7. Signs of trouble. If you notice that your horse is squinting, rubbing an eye or suddenly acting head shy, these could be signs of eye trouble. Also watch for tearing, squinting, redness, cloudiness or lumps and bumps around the eye. If you suspect a problem, don’t hesitate to call your veterinarian since treatment delays could increase the risk of a serious issue.
This article was originally published in the September 2018 issue of Practical Horseman.