Vet Connect: The Equine Eye and Its Concerns

Many people do not think to worry about their horse’s eyes, since the majority of equine are considered working animals. Horses were made to be in the outdoors, so one would think they have all the necessary means to “handle the outdoors.” Unfortunately, for this reason, horses are known to have more severe eye injuries and state of disease once they finally make it to the veterinarian. Owners do not usually notice if their horse’s eye is cloudy or irritated. They usually only realize the issue as their horse is scratching on a post, not turning the poles or barrels the way they used to, or not staying with the cows as they did in the past.

The horse’s eyes are very complex in the way they are constructed. Details on the makeup of the eye help to show how issues can arise and where they are most commonly seen. The eye is connected to the brain which communicates through nerve signals. It is a sphere filled with fluid and is fragile. For this reason, it sits inside the eye socket, a circular structure made of bone. The cornea is also an important part of the eye; it is the outermost layer of the eye and is like a window, controlling the entry of light into the eye. Just inside the cornea is the anterior chamber. This chamber holds a thick, clear fluid called aqueous humor. The aqueous humor is very important for supplying nutrition to the cornea and lens, nutrients such as amino acids and glucose. The colored part of the eye is known as the iris, which functions to control the size and diameter of the pupil. Finally, the pupil functions to determine the amount of light allowed in the image-forming part of the eye.

Horses can injure their eye on branches, halters, dust—and anything else you can think of. Signs to watch for are redness, swelling, eye rubbing, blinking, and excessive drainage. If any of these start to occur, it is ideal to get the horse to a veterinarian as soon as possible. If delayed too long before being seen, the owner risks severe and long-term damage to the horse’s eye.

Certain conditions are known to affect horses more than other animals. To list a few: traumatic injuries, equine recurrent uveitis, squamous cell carcinoma, cataracts, and corneal disorders. Out of those listed, 90 percent of horses that come into the clinic have corneal disease, squamous cell carcinoma, and equine recurrent uveitis. Another big one that affects horses’ eyes is physical trauma to the cornea. Equine recurrent uveitis is one of the most common and one of the most challenging to treat and, in severe cases, will lead to blindness. The cowboy term of ERU is moon blindness. ERU is an autoimmune disease that is clinically diagnosed with excessive squinting, redness, cloudiness, and watery discharge that will increase in frequency and severity. There are three types of equine recurrent uveitis: affecting the front of the eye (classic), subclinical (insidious), and the back of the eye (posterior). Unfortunately, there is no real cure for ERU. Topical and oral medications have been used to try to manage these conditions. Steroids can be used to help with inflammation and any existing ocular damage, but in severe cases these do not help, and the horse may need to undergo eye surgery. Research being done using stem cells to help fight off ERU is in the very early stages. Corneal ulcers or any injury to the cornea are also big issues in the eye due to the cornea being constructed by collagen fibers, which lack blood supply. It is very difficult for the cornea to fight off bacterial or fungal infections. In this case, they need to be treated aggressively with topical antibiotics or antifungals. The most common eye cancer is squamous cell carcinoma. These tumors tend to develop on the third eyelid or on the inside of the eyelid. These cancer spots appear most like warts. Common treatment involves freezing (cryotherapy) to remove. The removal of the mass or sometimes even the eye may be necessary.

Remember to check your horse’s eyes periodically and monitor for any signs listed above. Watch for subtle behavior changes to help catch problems early on. Don’t delay in having an eye checked. Eye concerns are commonly considered an emergency and need to be dealt with promptly.