If a horse has a corneal ulcer, or any noticable eye issues, it is important to contact a veterinarian immediately. If untreated, corneal ulcers can eat away at the cornea, and end up causing infection or permanent damage to the equine's eye.
Swelling, redness, eye discharge, squinting, and obvious eye pain can all by signs of a corneal ulcer. The surface of the eye may also look abnormal, dry, or dull. Your vet will be able to tell you if the symptoms are from a corneal ulcer, or from another eye problem entirely. In the case of a corneal ulcer, A vet may recommend protecting your horse's eye from the light through the use of an indoor stall, or a blinder. They may also teach you how to flush the horse's eye, and prescribe ointment to help prevent secondary infection.
If treated in a timely manner, under the care of a veterinarian, corneal ulcers should heal with no permanent damage, although as with any injury or illness, lasting effects are always possible.