The cornea is a thin and transparent, yet extremely strong tissue that supplies a majority of the eye's refractive, or light-bending, power. It is one of the most sensitive tissues in the body. The thickness of the equine cornea is about 1.5 mm, and it consists of four layers (see "Corneal Anatomy" on page 68):
- The outer epithelium is a barrier to bacteria and the tear film;
- The thick stroma is mostly collagen;
- Descemet's membrane is the very thin basement membrane secreted by the corneal endothelium; and
- The inner endothelium is only one cell layer thick and contains a pump that is essential for corneal transparency.
The environment of the horse constantly exposes him to bacteria. The species present vary depending on the season and geographic area, but Gram-positive bacteria (identified by a common laboratory test) are the most common species found in normal horse eyes. The mechanical action of the eyelids provides a continuous mechanism to sweep bacteria from the corneal surface. Tears are removed from the eye with each blink, decreasing the number of bacteria in the tear film. The eye's immune system and its epithelial barrier also help prevent bacterial adhesion and invasion into the cornea.
This is the most important disease of the horse cornea. Ulcerative keratitis is the medical term for corneal ulceration and means that the corneal epithelium and varying amounts of corneal stroma are missing. Equine corneal ulceration is a potentially sight-threatening disease requiring early clinical diagnosis, laboratory confirmation, and appropriate medical and surgical therapy. Corneal ulcers in horses might initially appear mild and unimportant, but serious complications are always a possibility.
Infection by bacteria and fungi should be considered in every corneal ulcer. Bacteria living on the surface of the cornea are able to attach to the corneal cells once the epithelium has been damaged. Bacteria recruit white blood cells that produce enzymes to the ulcer site; these enzymes digest collagen in the stroma and greatly speed the progression of an ulcer. Enzymatic activity in corneal ulcers is referred to as "melting" and gives a grayish, liquefied appearance around the ulcer. Melting is a major problem for the horse cornea!
In superficial corneal injuries in which only a small amount of epithelium is absent, healing is generally rapid, averaging about one millimeter of epithelial movement over the ulcer site per day. For example, a 10-mm diameter ulcer will, on average, heal in six days. If it becomes infected, healing can be delayed up to several weeks or longer.
Classification of Ulcers
By Depth--Superficial ulcerations or abrasions involve only epithelial cell loss and should heal rapidly if they don't get infected. The deeper the ulcer, the more the corneal stroma is involved, and the more likely healing will be slow with scarring of the cornea likely. Ulcers that penetrate to the Descemet's membrane are called descemetoceles (see "Corneal Anatomy" above). These deep ulcers are emergencies, as the eye is at extreme risk of corneal rupture. If a corneal ulcer ruptures, the iris will prolapse forward to plug the corneal hole. Iris prolapse is an emergency with surgery indicated for therapy.
By Etiology--There are multiple causes of corneal ulcers, including mechanical causes such as trauma, dirt, and plant foreign bodies, and eyelid disease.
Many bacterial species cause corneal ulcers in horses, but Gram-negative bacteria such as Pseudomonas and Gram-positive bacteria such as Staphylococcus and Streptococcus are particularly feared. Infection of corneal ulcers in horses is common.
Diagnosis of Ulcers
Corneal ulcers are painful; most horses with corneal ulcers squint. Stimulation of corneal sensory nerves causes painful inflammation of the iris (anterior uveitis). Other common clinical signs of corneal ulcers include tearing and corneal cloudiness.
All horses with painful eyes should be stained with fluorescein dye. Fluorescein stain will color exposed stroma, but not normal epithelium or Descemet's membrane. Cultures for bacteria and antibiotic sensitivity tests should be performed routinely when an ulcer is infected or slow to heal. If the corneal ulcer appears to be melting (see page 74) or if it hasn't responded to treatment, it should also be cultured.
Corneal ulcers in horses are serious problems no matter how small or superficial they might appear when first noticed. Bacterial infection and anterior uveitis are major concerns for horses with corneal ulcers, so they must be aggressively treated in order to preserve vision.
Subpalpebral (under the eyelid) tubing treatment systems might be necessary for medicating horses with painful eyes.
The primary treatment for bacterial corneal ulcers is intensive administration of antibiotics. Broad-spectrum antibiotics are usually administered three to six times per day to kill bacteria in the eyes of horses with ulcers. Culture and sensitivity tests can be used to guide the choice of antibiotics. Bacitracin/neomycin/polymyxin B, tobramycin, or chloramphenicol are good antibiotics to use initially in the treatment of many ulcers in horses. Gentamicin or ciprofloxacin should be used for melting ulcers.
Prevention of collagen breakdown and ulcer progression is also important in ulcer therapy. Enzymes derived from white blood cells in the tears can be powerful forces in the destruction of the corneal stroma, and can cause rapid deepening of ulcers. Topical corticosteroids increase this damaging enzyme activity and should not be used in the treatment of corneal ulcers.
Serum from the blood of the horse contains proteins with anti-melting activity. Blood is drawn from the horse or foal, spun down, and the serum drawn off and stored in an eye dropper for use as medication. Serum can be placed in the eye hourly for melting ulcers in horses. Ethylenediaminetetraacetic acic (EDTA, a chelating agent and anticoagulant at 0.2%) can also be administered several times a day as eye medication to reduce corneal melting. Acetylcysteine (5-10%) is used topically for its melting enzyme-inhibiting properties. It must be kept refrigerated and the horse treated every one to two hours.
Treatment of the anterior uveitis found with corneal ulcers in horses is critical. Topical 1% atropine is usually given to relieve pain and dilate the pupil. Atropine must be administered with great care only by a veterinarian as this medication might cause colic in horses and has contributed to fatalities. Topical atropine once or twice a day by itself does not likely cause significant colic in adult horses; however, when combined with pain, other medications, and hospitalization, it can contribute to serious colic signs.
Phenylbutazone (Bute) or flunixin meglumine (Banamine) is used orally, intramuscularly, or intravenously to relieve eye pain.
Corticosteroid therapy interferes with the immune system of the eye and is contraindicated in the management of equine corneal ulcers.
Adjunctive Surgical Therapy
To increase lost corneal thickness and strength, deep corneal ulcers threatening rupture of the eye and corneas that have ruptured (iris prolapse) require conjunctival flap placement or corneal transplantation surgery. (see photos on page 74). Surgery is required to save the horse's vision if iris prolapse occurs.
Infection inside the eye following rupture of a corneal ulcer is severely painful and can make the horse very ill. There is a grave prognosis for saving the eye. To spare the unfortunate horse this severe discomfort, enucleation or removal of the eye is the humane alternative.
Brooks, D.E. Equine Ophthalmology. In: Gelatt, KN (ed), Veterinary Ophthalmology, 3rd ed. Philadelphia: Lippincott Williams and Wilkins, 1999.
Editor's note: This is the fifth article in a series of eye articles by Dr. Brooks. See the first article, "Eye Anatomy and Physiology," article Quick Find #2797 at www.TheHorse.com for more information on eye anatomy.
CORNEAL ULCER CAUTIONS
- Corneal ulcers are frequently not clearly visible. For that reason, all horses with red or painful eyes should be stained with fluorescein dye for diagnosis.
- A slowly progressive mild course often belies the seriousness of the ulcer in its early stages.
- Regardless of the initial cause, all ulcers in horses have the potential to progress to blindness.
- Corneal ulcers in horses can rapidly progress to rupture of the eye due to enzymes in the tears causing "melting."
- Topical corticosteroids should not be used when the cornea retains fluorescein stain (indicative of an ulcer).
- Uveitis always exists with corneal ulcers in horses.
- The deeper the ulcer, the more aggressive is the medical therapy, and the greater the likelihood that surgical therapy will be required.--Dennis Brooks, DVM, PhD, Dipl. ACVO
About the Author
Dennis E. Brooks, DVM, PhD, Dipl. ACVO, is a professor of ophthalmology at the University of Florida. He has lectured extensively, nationally and internationally, in comparative ophthalmology and glaucoma, and has more than 140 refereed publications. He is a recognized authority on canine glaucoma, and infectious keratitis, corneal transplantation, and glaucoma of horses.
POLL: Colic Surgery