Are blue eyes more vulnerable?
Q:Are horses with blue eyes more susceptible to cataracts or other ocular problems than are horses with brown eyes? I have very light eyes and have been told I should protect them just as I do my skin. Should I put the equivalent of “sunglasses” on my blue-eyed horse?
A:The color of a horse’s eye is associated with the presence (brown eyes) or absence (blue eyes) of the pigment melanin within the iris. This is determined by coat color, specifically around the eyes, so horses with blue eyes are most likely to have large areas of pink skin (white hair) on their head.
Blue eyes in horses and all species are more sensitive to sunlight. That’s because the top layer of a blue iris has less pigment than the top layer of a brown iris. The lack of pigment in the iris means more light reaches the sensitive structures at the back of the eye, such as the retina.
Clinically this translates to difficulty focusing or even pain experienced in bright light. For this reason, we recommend equipping a blue-eyed horse with a mask that blocks more than 90 percent of ultraviolet (UV) light (Equine Sun Visor, Equivizor, Kensington Uviator) when turning out during daylight.
Blue irises are also more likely to have thin areas (iris hypoplasia) or even full thickness defects (colobomas). These abnormalities, which are present at birth, can make a horse’s eyes even more sensitive to the sun and may impair the pupil’s ability to constrict (narrow) in bright light. In severe cases of iris hypoplasia the thin tissue can bulge, giving it a mass-like appearance. Although this can look concerning, these areas are generally not a problem for the horse.
Finally, horses with blue irises generally have light pink conjunctiva surrounding the eye, which—like other non-pigmented tissue around the eye—is at a higher risk for UV light-associated tumors such as squamous cell carcinoma and hemangiosarcoma. Because they can invade the local bone and lymph nodes, these cancers can be life threatening.
The key to preventing them is to protect a horse with a UV-blocking mask as mentioned earlier.