Because the horse is a prey animal, his vision is much different from our own. With its head facing forward, the horse has a 350-degree visual field. With his head up, he sees with both eyes (binocular vision) for 65 to 80 degrees, with blind spots near his face and tail. He uses peripheral vision when his head is down, but he can’t do both at the same time. In addition, it is believed that horses see red and blue colors, but not yellow or green.
Eye problems are among the most common reasons vets are called to look at horses, according to the U.S. Department of Agriculture (USDA), along with trauma, colic and laminitis. Unfortunately, eye problems in the horse can be difficult to diagnose. It is important to know how your horses’ eyes looks normally so you can quickly spot when they appear abnormal. For example, tears drain nasally. So, if the horse has tears running down its face or has pus in the corner of an eye, he could have a blocked or infected tear duct. And, the eyelashes usually stick straight out. When they are drooping down, it normally indicates pain.
If you suspect a problem, it is best to call the veterinarian right away, because the longer the eye goes without treatment, the worse it can get. Furthermore, if you decide to treat the eye yourself, using the wrong medication could cause permanent damage. For example, therapies recommended for some eye problems can make a corneal ulcer worse.
What problems should the horse professional watch out for? Dennis Brooks, DVM, PhD, Dipl. ACVO, professor of ophthalmology at the University of Florida, puts corneal ulcers at the top of the list. At the Horseman’s Day during the American Association of Equine Practitioner’s (AAEP) 2003 annual convention, he said, “There are really only two ophthalmic diseases: Corneal ulcers and everything else.”
A corneal ulcer is also called ulcerative keratitis. It occurs when there is a lesion in the outer layer (and at times the middle layer) of the cornea. Small lesions can get worse quickly and lead to infection if they are not promptly diagnosed and treated. They can be caused by abrasions, foreign objects or ingrown eyelashes as well as bacteria, fungi or viruses. Brooks lists squinting, tearing, being unable to tolerate bright sunlight, a cloudy or blue eye and/or a red, swollen eye as symptoms of an ulcer.
The fluorescein dye test is the basic test. The orange stain detects the corneal ulcer because the defect in the outer layer allows the dye to go through to the middle layer where it shows up as a bright, fluorescent green. Small lesions usually heal quickly with treatment, but deep or infected ones need more aggressive therapy. Your vet may prescribe one ointment for the pain and another for the infection. An antibiotic containing any corticosteroid should not be used, because it can cause enlargement of the lesion and subsequent loss of the eye. A contact lens may be used to cover and protect non-infected defects while they heal.
In an article written for the AAEP’s website, Dr. Brooks advises everyone to remember the following about corneal ulcers.
- “Corneal ulcers are frequently not clearly visible.
- Even with proper examination lighting, all red or painful eyes must be stained with fluorescein and rose Bengal dyes.
- A slowly progressive, indolent course often belies the seriousness of the ulcer.
- Corneal ulcers in horses may rapidly progress to eye rupture.
- Topical corticosteroids are bad when the cornea retains fluorescein stain.
- Uveitis caused by a corneal ulcer or stromal abscess may be very difficult to control.”
Other Common Problems
Recurrent or persistent uveitis is commonly known as moon blindness. It is an autoimmune system disease that is one of the most common causes of equine blindness in the United States. This is a very painful condition that is more prevalent in Appaloosas than in other breeds. Affected horses may frequently run into objects in the dark even though they are in familiar surroundings. Treatment helps manage the condition, but there is no cure.
Glaucoma in horses is caused by a change in the clear eye fluid that causes an increase in the intraocular (within-the-eye) pressure. While the incidence is fairly low, glaucoma is being seen with greater frequency. The condition seems only to occur in one eye. Symptoms include an enlarged eye, pain, dilated pupils, corneal edema, decreased vision, optic nerve atrophy and corneal striae (stretch marks). Treatment consists of a combination of drugs and surgery to decrease the intraocular pressure. Anti-inflammatories may also help. The condition is particularly difficult to control in Appaloosas, but the reason is unknown. Blindness is the final outcome due to damage to the optic nerve.
Squamous cell carcinoma (cancer) is often seen in horses without pigment around the eye and/or in older horses because of their long-term exposure to the sun’s ultraviolet rays. A tumor may occur on the edge of the lid, in the corner of the eye, or on the third eyelid. Because treatment is expensive and the outcome often uncertain, it is best to take the tumor location and size into consideration as well as the age, health and value of the animal. Prompt surgical removal is usually followed by radiation or chemotherapy. If left untreated, the horse could lose the eye, or tumors may invade the whole side of the horse’s head.
Fly masks help keep the sun off white skin and prevent cancer as well as help allergic horses, but it is important to keep them clean. Dr. Brooks feels that fly masks are safe and do not cause additional problems.
The most common disease of the lens is the cataract, or clouding of the lens. The formation of cataracts may be hereditary or occur from trauma. A small cataract may not drastically affect a horse’s vision. However, if cataracts are large or progressively become worse, the horse will lose vision or possibly become completely blind. Cataracts in horses are rarely treated, but when they are, a veterinarian surgically removes them.
Eyelid lacerations can happen any time a horse catches himself on a post, fence, brush, etc. Inflammation and corneal ulcers may accompany the cut. Prompt treatment, which typically consists of a few stitches along with gentle cleansing and antibiotics, usually leads to healing without scarring. If neglected, cuts can lead to infection and production of proud flesh.
The bottom line is to take all eye conditions seriously. Brooks feels the eye has tremendous healing ability, but you need a quick, accurate diagnosis and appropriate treatment to give the horses in your care the best chance at a complete recovery.