KEEPING THE WINDOWS CLEAN
There’s no doubt about it, looking deeply into those big brown (and occasionally blue) equine eyes does reveal secrets of their soul… and reflects our own. We are naturally drawn to look at the eyeballs of animals, but horses have particularly large, prominently featured and expressive orbs. Like our own, horse eyes are responsive to the animal’s internal psyche as well as their external environment. Unlike us, the equine eye is structurally and anatomically distinct, resulting in significant differences between how and what the horse and human see. Horse eyes are placed widely apart towards the sides of their head. This placement gives them nearly a full 360 circle of view but also limits their binocular vision and therefore visual acuity and depth perception. Humans have eyes located toward the front which limits our field of view but increases our binocular vision, resulting in sharper focus and greater depth perception. Understanding that what we see compared to what the horse sees is inherently different, even in the very same surroundings, is an important concept to grasp for any up and coming “horse whisperer”.
The eye in general and horse eyes in particular are tough and incredibly delicate at the same time. Given the prominence of eye placement and the nature of the beast, the equine eye is also prone to injury. A cloudy or painful window to the soul is never a good thing, so it’s wise to consider eye injury from trauma or infection as a justifiable emergency, requiring prompt treatment to avoid catastrophe. When in doubt about any eye injury, call for a consult. Some common eye problems that you may encounter in your horse include, trauma and corneal damage (ulcer), lacrimal duct problems and recurrent uveitis.
Trauma or early disease in the eye may or may not be obvious. If there is no obvious wound, the most common indications of an eye problem are tearing, squinting or closed eye, light sensitivity, guarding or rubbing. Step back, look carefully at both eyes to appreciate subtle signs of pain. Depending on your own experience, it may be wise to consider any sign of eye pain as an emergency, worth a call to your veterinarian. If there is excessive tearing but no indication of discomfort in the eye itself (no squinting, redness etc.) the problem may be with the lacrimal system. While this is not itself an emergency, irritation from the tearing may cause the horse to rub its eye which may lead to more serious complications. For more information, see our
Sept 2018 newsletter
on this topic.
Equine Recurrent Uveitis (ERU), is inflammation of any part of the uveal tract (pigmented layers of the eye). It is the most common cause of impaired vision and blindness in the horse. Anterior uveitis refers to inflammation of structures like the iris and ciliary body. Posterior uveitis is inflammation of structures deeper in the globe, including the choroid and potentially the retina and optic nerve. The disease may be localized to a single segment of the uveal tract or it may affect multiple areas causing permanent damage to the eye resulting in blindness. ERU, also called “moon blindness” or “periodic ophthalmia” usually develops in stages, beginning with a sudden flare up that may subside and then reoccur. Episodes of inflammation often increase over time and the development of cataracts and glaucoma is not uncommon. The onset of uveitis can be triggered by bacteria, viral infection, trauma, intraocular parasites, or a variety of undetermined causes. Relatively recently, uveitis has been linked to an organism called leptospira. Leptospirosis is a zoonotic (transferable between animals and humans) spirochete bacterium with world wide distribution.The bacterium is transmitted to the horse from standing water, soil, bedding, feed or drinking water that is contaminated with urine from an animal that is a carrier or host for the organism. Leptospira live in various host mammals including skunks, raccoons, deer, and opossums, all of which can shed the bacteria via urine. All of these mammals live in the PNW and are often in shared environments with our horses. It has been reported that 40% to 70% of horses with ERU also had titers to leptospirosis. A vaccine against the organism was developed in 2015 to help prevent recurrent disease and/or minimize bacterial shedding of already infected animals.
As spooky as horses can be, they also have an uncanny ability to adapt to blindness. There are many stories of blind horses establishing a trusting relationship with “seeing eye” buddies (including humans) and being ridden or motoring around the pasture with no problems. There are even blind horses who have risen to remarkable levels of competition in several different sport disciplines. With these animals there’s no doubt that what lies inside their darkened windows is what counts after all.