The cornea is the transparent outer portion of the eyeball. It is lubricated by tears and protected by the eyelids. It is extremely sensitive and reacts to environmental factors, as well as problems occurring locally in the eye and from within the body. Traumatic injury is probably the most common cause of corneal problems.

The cornea consists of several layers. The outermost portion is called the epithelium. This layer is an effective barrier against invading bacteria. Under the epithelium is the stroma. The stroma makes up about 90 percent of the corneal substance. Two additional layers are located under the stroma. These layers are thin, however they are important in protection and corneal cell regeneration.

In a simple injury, where only a small amount of epithelium is damaged, corneal healing is very rapid. Acorneal ulcer occurs when the protective epithelium and stroma are disrupted. The deeper the disruption, the more severe the ulcer. Deep ulcers heal much more slowly than superficial ulcers or abrasions.

Some causes of corneal ulcers include: traumatic injury, inherited dystrophies, feline herpesvirus (in cats), and nutritional deficiencies. They can also occur secondary to tear deficiencies (keratoconjunctivitis sicca) and endocrine disorders (diabetes mellitus and hypothyroidism).

Pain is an important symptom in animals with corneal ulcers. Blinking, squinting, and intolerance to light are symptoms reflecting a painful eye. The normally transparent cornea develops a cloudiness and small blood vessels invade the stroma. The eye becomes watery as tear production is exaggerated.

Prompt treatment is necessary, as untreated corneal ulcers can rapidly lead to blindness. A corneal ulcer is an emergency condition. Immediate veterinary attention is needed.

Treatment often requires long-term use of ophthalmic medication. Temporary artificial lenses or surgery (third eyelid flap) is often necessary for the treatment of deep corneal ulcers.