Q: Are horses with blue eyes more sensitive to light and prone to diseases than those with brown eyes? I have always heard that blue eyes are more susceptible to tumors and other complications.
A: Over the years a number of myths have circulated about blue-eyed horses. Eye color refers to the pigmentation of the iris, a membrane that lies under the clear cornea. In the center of the iris is the dark opening called the pupil. When the iris muscles contract or expand, the pupil changes size to regulate the amount of light entering the eye. Horses naturally have only two iris colors: blue or brown. Some horses will have both blue and brown coloration in their irises, a situation called "heterochromia iridis."
Horses with blue eyes are no more likely to develop any eye disease than are those with brown ones. However, horses with blue irises frequently have pink skin, rather than black, around the eyelids---and this is most likely the source of the myth that blue eyes are more intolerant of light. Pink skin lacks the protective pigment called melanin, and any areas of pink skin on a horse, including on the eyelids, are more susceptible to damaging sunburns than black skin and are also more likely to develop skin cancer (called squamous cell carcinoma).
If a horse has pink skin, usually found under white markings, it's a good idea to take steps to minimize his risk of cancer by reducing his exposure to ultraviolet (UV) light. Possibilities include using a UV-blocking fly mask during the day and restricting turnout time to early mornings, late evenings or overnight. And, of course, don't let your horse use a tanning bed!
Amber Labelle, DVM, MS, DACVO
University of Illinois at Urbana-Champaign
This article first appeared in EQUUS issue #429.