Corneal ulcers are the most common eye injury that affects horses. Equine corneal ulcers can easily become serious, difficult to treat, vision threatening, and therefore career threatening problems. Corneal ulcers can affect any horse, of any age, and in any place.
The cornea is the transparent surface of the front of the eyeball. It consists of four layers. The outer epithelial layer, which is only eight to ten cells thick and is replaced with new cells every five to seven days. The second layer is the stroma which makes up 90% of the thickness of the cornea, Descemet’s membrane which lies beneath the stroma is the third, and the fourth is the innermost endothelial layer.
A corneal ulcer is defined as a break in the epithelial layer and into the stroma. Typically an ulcer results from trauma to the cornea due to such things as fox tail, grass awns, or dust particles. Typically, a secondary infection begins shortly after the damage occurs and is caused by a bacteria or a fungus that is already present around the eye of the horse. The equine cornea is particularly predisposed to the development of corneal ulceration because of its large size and the horse’s inclination to trauma.
Signs associated with corneal ulcers include squinting, excessive tear production, reddening of the whites of the eye, avoidance of light, and discharge. Some ulcers will look like there is water on the cornea or will have a blue haze, due to the swelling and have a small pupil. Most horses will be reluctant to have you touch the eye or try to open the eye lids. They will prefer to stand in the shade and squint constantly. Corneal ulcers may be obvious to the naked eye but often need to be stained with fluorescein to be visualised. Fluorescein is an orange dye, which becomes fluorescent green and binds temporarily to the surface where the epithelial layer has been lost.
Uncomplicated, superficial abrasions may heal quickly with minimal treatment. Deeper ulcers, which become infected, can rapidly become serious problems. A wide variety of bacteria are present on the surface of the normal eye. Many of these can cause disease once the cornea is injured. Some of these bacteria produce enzymes called collagenases, which destroy the substance of the stroma, leading to large, rapidly deteriorating ulcers, which have a gelatinous, “melting” appearance. If unchecked, these continue to worsen, exposing Descemet’s membrane and finally leading to rupture of the eyeball. A common complication of corneal injuries is inflammation of the uveal tract, known as uveitis. Horses are far more prone to uveitis than other species. If unchecked uveitis can lead to chronic pain, cataract formation, scarring within the eyeball, increased pressure within the eyeball, and damage to the retina leading to blindness.
Treatment of corneal ulcer involves controlling pain and inflammation, eliminating or preventing infection, and preventing secondary complications. Ensuring no foreign material is present is also very important. The horse may be treated with topical drops and ointments or it may require antibiotics and anit-inflammatories be given orally or in the muscle. Horses should be kept in the stable out of sunlight. It may be necessary to apply a patch over the affected eye to reduce discomfort. In more severe and deeper ulcers it may be necessary to debride the area or surgically repair the ulcer with another tissue.
Equine corneal ulcers can quickly become severe, painful and vision threatening problems. The majority of corneal ulcers and abrasions which are treated quickly and appropriately will respond well and heal in a few days. However; those which are neglected, not noticed or inappropriately treated can quickly become major problems with a guarded prognosis for future eye sight.
If you have any questions regarding the above information or any questions/concerns in general, please contact Twin Valley VHS at 745-6642.
Dr. Justin Noble DVM
Twin Valley Veterinary Health Services