Changes in the equine eye certainly get attention. For example, it is hard to miss when a normally brown-eyed horse suddenly
develops a uniformly whitish-blue eye — or when a horse gets on the track with a bright open eye and finishes its workout
with a tearing, squinting eye.
Eye trauma in the horse is a common event. The equine eye is the largest of any land mammal, and its side-facing position
and relatively shallow socket (allowing horses to have a nearly 360-degree visual field) combine to make bumps, scrapes and
other damage routine
Cynthia Cook, DVM, PhD, of Veterinary Vision Inc. in California explains, "The equine cornea, which is the transparent covering
of the globe of the eye, is a common site for injury and disease and, as such, is a cause for many calls to equine practitioners.
The most common cause of corneal disease is trauma, and racehorses are especially prone because of debris thrown up by the
hooves of other horses during racing and training."
There are many causes of equine ocular damage and disease, and many of the common conditions can lead to a dramatic discoloration.
The cornea is normally transparent to allow light passage for normal vision. When the cornea is damaged, regardless of the
cause, it responds by swelling. It is this intense swelling that creates a blue or cloudy eye.
Corneal edema can be caused by direct trauma. Horses often bump their eyes during play or fighting, experience damage from
cross ties or lead lines or from becoming cast in a stall. Damage to the surface of the eye and cornea can come from scrapes
or punctures involving trees, shrubs and plant material (Photos 1 & 2). Additionally, trauma resulting from rubbing because
of insect irritation, extremes of dust and airborne debris or while a horse is wearing a fly or face mask can also result
in corneal abrasions and edema.
Photo 1: A differential diagnosis for corneal edema is a foreign body, either in the cornea itself (such as a small plant
awn or sliver from a stick) or under the lids or third eyelid. This horse has a cocklebur under its lower lid and, amazingly,
even though the lids are red and irritated and the horse was in considerable pain, the bur did not cause any damage to the
cornea itself. (PHOTOS: COURTESY OF DR. KENNETH L. MARCELLA)
The blue eye will likely be associated with swelling of the lids and/or related parts of the face if a traumatic blow has
been involved. Skin rubs and lacerations may also be noticed. Insect bites and rubs more commonly involve either the upper
or lower lids only, and this distinction can help in determining a possible cause for a cloudy eye.
Photo 2: A close-up of the bur foreign body removed from the horse in Photo 1.
Corneal scratches are painful, and squinting (blepharospasm) and tearing are almost always noted. Initially, these corneal
scratches are not visible on the surface of the eye, and there is no clouding of the cornea. Blunt trauma to the eye with
a closed eyelid may result in a blue eye without any corneal scrape or visible damage.
A number of other serious ocular diseases can present as a blue eye as well. Keratitis is inflammation of the cornea (this
part of the eye is largely made up of keratin cell types). The cause of keratitis is often infectious, viral, bacterial or
fungal, but in some cases the cause is unknown. All types of keratitis are associated with a blue discoloration of the cornea.
Glaucoma is another disease with multiple causes and is characterized by increased pressure within the eye. Typically, the
horse's eye is enlarged, blue and painful. Measurements of ocular pressure can be taken, which will help you formulate a diagnosis.
Uveitis is an important complication of virtually all trauma to the equine eye and involves irritation and inflammation of
the sensitive tissues that line the inside of the eye (Photo 3). J. Rowan Blogg, BVSc, Dipl. ACVO, warns owners and veterinarians
not to overlook uveitis and says, "When an injury is sustained on or near the eye or sometimes even to the head, the horse
should be treated vigorously for uveitis."
Photo 3: Chronic uveitis can lead to a smaller, collapsing eye, corneal edema with or without neovascularization (new blood
vessels invading the perimeter of the cornea) and thickening of the anterior chamber of the eye.