Equine Vision: Part 2

By Lauren Lamb

The cornea is a thin, transparent but extremely strong tissue. It supplies most the light refraction of the eye. Light refraction helps focus the light onto the optic nerve, which in turn helps generate an image in our brain.

The cornea of a horse is approximately 1.5 mm in thickness and is made up of five layers. The outer layer is the epithelium. The epithelium is the cornea’s best defense mechanism against infection from bacteria and fungus. Damage to the cornea will result in a significant increased risk of an infection in the cornea or deeper structures of the eye. The tear film, which is a layer of tears that covers the epithelium, along with the eyelids also help removed most of the bacteria that a horse’s eye is exposed to throughout the day.

A cornea ulcer may occur if a portion of the corneal epithelium along with some of the deep layers of the cornea are removed due to trauma or infection. A corneal ulcer can seem insignificant at first. However, if a corneal ulcer is not treated appropriately, it can quickly escalate to a severe, sight-threatening disease in a matter of hours. It is vital that you call your veterinarian immediately if you notice any clinical signs of a corneal ulcer. Your veterinarian will need to examine your horse’s eye to assess the size and depth of the corneal ulcer.

Clinical signs of a corneal ulcer include squinting, excessive tearing or watering, mucus on the eyelid, swelling of the conjunctival (pink tissue around the eye) and/or light sensitivity. For a horse that have subtle clinical signs and minimal pain, the best way to see if there is any pain or inflammation in the eye is to stand directly in front of the horse and look at their upper eyelashes. If a horse has a painful eye, then the eyelashes over that eye will be lower or tilted toward the ground compared to the opposite eye.

Learn more in the January issue of OKFR!