A corneal ulcer (ulcerative keratitis) is the most serious eye disease that veterinarians treat. Defined as a lesion in which the outer layer and some of the middle layer of the cornea have been lost, even simple ulcers can quickly progress to larger, more complicated ones if not diagnosed and treated promptly. A corneal ulcer can even lead to an infection of the eye’s inner structures.

Corneal ulcers can be classified by their cause. There can be mechanical causes such as abrasions, foreign objects, or ingrown eyelashes. Some corneal ulcers are caused by infectious organisms like bacteria, fungi, or viruses. If the horse has a paralyzed facial nerve, he might not be able to blink properly or close the eye to protect it. There is even a disease in which horses cannot produce tears called keratoconjuctivitis sicca, which causes the eyes to become dry and prone to damage.

You should contact your veterinarian immediately if your horse shows any of these signs:

• Squinting;
• Tearing;
• Unable to tolerate bright sunlight;
• A cloudy or blue eye;
• A red, swollen eye.

Your veterinarian will perform a complete ophthalmological examination of both of your horse’s eyes, including staining the corneas to check for an ulcer. Abrasions to the cornea are often difficult to see without specific dyes, even with the proper lighting and equipment. Defects in the outer layer of the cornea allow the dye to diffuse into the middle layer, and appear bright, fluorescent green