The Internet is a wonderful resource: It enables horse owners around the globe to exchange ideas on horsemanship and stable management quickly and freely. The downside is there’s a large amount of inaccurate information available. False information regarding equine eye health seems particularly common. Here I’ll address some frequently heard myths regarding horses’ eyes:
Myth 1: Horses with blue eyes have more eye problems. A horse with a “blue” eye actually has a blue iris. The iris is the colored part of the inside of the eye surrounding the pupil. While humans have an astonishing number of normal iris colors (from blue to green to brown to violet), horses typically have one of two: blue or brown. Some horses have irises that are both brown and blue; the medical term for a two-colored iris is “heterochromiairidis.” These are more common in horses with patterned coats such as Paints and Appaloosas. Blue irises are usually seen in horses with light-colored coats such as cremellos. Having a blue iris, however,does not make a horse any more likely to have intraocular problems, including equine recurrent uveitis.
Coat color, on the other hand, can be associated with eye problems. For instance, leopard Appaloosas are more likely to have difficulty seeing in the dark (if affected by a condition called stationary congenital night blindness), and chocolate-colored Rocky Mountain Horses are more likely to have multiple intraocular abnormalities, including blinding conditions such as retinal detachment. In these cases genetics are responsible for the relationship between breed, coat color, and eye diseases.
While having a blue iris might not make a horse more likely to have an eye disease, blue irises usually go hand in hand with pink skin. One of the most important known risk factors for developing squamous cell carcinoma (SCC, a form of skin cancer) is pink skin. Thus, horses with pink eyelids are more likely to get SCC. No relationship has been documented, however, between blue irises and SCC. Protect a horse with pink eyelids or excessive white on his face from ultraviolet (UV) light exposure (e.g., with a UV-blocking fly mask).
Myth 2: A horse’s “moon blindness” is worse when the moon is full. Equine recurrentuveitis (ERU), also called “moonblindness,” is the most common cause of blindness in horses. This autoimmunedisease is caused by the immune system attacking the inside of the horse’s eye. The underlying cause is not well-understood,and many researchers are working to determine why horses develop ERU.
Before the age of modern science, somehorse owners believed the phase of the moon influenced their horses’ ERU flare-ups. Now we understand that the immune system — not a celestial body — causes these episodes of active inflammation.
Myth 3: Feeding my horse a nutritional supplement will prevent eye disease. Horses do not usually develop age-related cataracts or macular degeneration like humans do (since horses do not have amacula, or central area of the retina). A number of vitamin supplements have been purported to delay these conditions’ progression in humans. However, there is no evidence to suggest that a vitamin can help keep equine eyes healthy. Follow your veterinarian’s nutritional recommendations to develop a balance diet for your horse.
Myth 4: My horse has lost 87.6% of the vision in his eye. Assessing what a horse can (and can’t) see is extremely challenging for horse owners, veterinarians, and veterinary ophthalmologists. Because horses are unable to tell us what they can and cannot see, we are left with indirect measures of vision. Furthermore, many visually impaired horses are well-adapted to their abnormal vision and can navigate familiar environments well. The “menacetest” is the most basic vision assessment, in which a threatening gesture (usually your open-palmed hand moving quickly towards the surface of the eye) should induce a horse with good eyesight to blink and/or avoid the gesture. But horses can have a positive menace test and still be significantly visually impaired. Other methods of vision assessment include the maze test, where a horse is asked to navigate an unfamiliar course of obstacles.
While there’s a lot of information out there on equine eye disease, ultimately,the best source of information about any eye disease is your veterinarian or veterinary ophthalmologist.
About the Author: Amber Labelle, DVM, MS, Dipl. ACVO, is an assistant professor of comparative ophthalmology at the University of Illinois Urbana-Champaign’s Veterinary Teaching Hospital. Her current research focuses on intraocular tumors in horses.