Managing the blue eye blues: Equine corneal disease

Changes in the equine eye certainly get attention. For example, it is hard to miss when a normally brown-eyed horse suddenly develops a uniformly whitish-blue eye — or when a horse gets on the track with a bright open eye and finishes its workout with a tearing, squinting eye.

Eye trauma in the horse is a common event. The equine eye is the largest of any land mammal, and its side-facing position and relatively shallow socket (allowing horses to have a nearly 360-degree visual field) combine to make bumps, scrapes and other damage routine

Cynthia Cook, DVM, PhD, of Veterinary Vision Inc. in California explains, "The equine cornea, which is the transparent covering of the globe of the eye, is a common site for injury and disease and, as such, is a cause for many calls to equine practitioners. The most common cause of corneal disease is trauma, and racehorses are especially prone because of debris thrown up by the hooves of other horses during racing and training."

Causes

There are many causes of equine ocular damage and disease, and many of the common conditions can lead to a dramatic discoloration. The cornea is normally transparent to allow light passage for normal vision. When the cornea is damaged, regardless of the cause, it responds by swelling. It is this intense swelling that creates a blue or cloudy eye.

Corneal edema can be caused by direct trauma. Horses often bump their eyes during play or fighting, experience damage from cross ties or lead lines or from becoming cast in a stall. Damage to the surface of the eye and cornea can come from scrapes or punctures involving trees, shrubs and plant material (Photos 1 & 2). Additionally, trauma resulting from rubbing because of insect irritation, extremes of dust and airborne debris or while a horse is wearing a fly or face mask can also result in corneal abrasions and edema.

Photo 1: A differential diagnosis for corneal edema is a foreign body, either in the cornea itself (such as a small plant awn or sliver from a stick) or under the lids or third eyelid. This horse has a cocklebur under its lower lid and, amazingly, even though the lids are red and irritated and the horse was in considerable pain, the bur did not cause any damage to the cornea itself. (PHOTOS: COURTESY OF DR. KENNETH L. MARCELLA)

The blue eye will likely be associated with swelling of the lids and/or related parts of the face if a traumatic blow has been involved. Skin rubs and lacerations may also be noticed. Insect bites and rubs more commonly involve either the upper or lower lids only, and this distinction can help in determining a possible cause for a cloudy eye.

Photo 2: A close-up of the bur foreign body removed from the horse in Photo 1.

Corneal scratches are painful, and squinting (blepharospasm) and tearing are almost always noted. Initially, these corneal scratches are not visible on the surface of the eye, and there is no clouding of the cornea. Blunt trauma to the eye with a closed eyelid may result in a blue eye without any corneal scrape or visible damage.

A number of other serious ocular diseases can present as a blue eye as well. Keratitis is inflammation of the cornea (this part of the eye is largely made up of keratin cell types). The cause of keratitis is often infectious, viral, bacterial or fungal, but in some cases the cause is unknown. All types of keratitis are associated with a blue discoloration of the cornea.

Glaucoma is another disease with multiple causes and is characterized by increased pressure within the eye. Typically, the horse's eye is enlarged, blue and painful. Measurements of ocular pressure can be taken, which will help you formulate a diagnosis.

Uveitis is an important complication of virtually all trauma to the equine eye and involves irritation and inflammation of the sensitive tissues that line the inside of the eye (Photo 3). J. Rowan Blogg, BVSc, Dipl. ACVO, warns owners and veterinarians not to overlook uveitis and says, "When an injury is sustained on or near the eye or sometimes even to the head, the horse should be treated vigorously for uveitis."

Photo 3: Chronic uveitis can lead to a smaller, collapsing eye, corneal edema with or without neovascularization (new blood vessels invading the perimeter of the cornea) and thickening of the anterior chamber of the eye.

Treatment

Treatment for a blue or cloudy eye should be aggressive and started immediately. Staining the eye surface with a fluorescent dye and rose bengal dye is generally the first step in diagnosing corneal damage. The normal cornea is impermeable to dust, debris and stains. If some damage to this outer layer has occurred, then dye will adhere to the defect, making it easily visible. When corneal scratches or abrasions are detected, topical antibiotics, along with antifungal agents and anti-inflammatory medications, are applied frequently (four to six times daily) to the eye surface.

Photo 4: This horse showed a case of severe diffuse corneal edema but without any signs of trauma or pain.

If uveitis is suspected, then nonsteroidal anti-inflammatory medications will be used both systemically and topically. Atropine or other drugs designed to dilate the pupil, promote drainage of ocular fluid and reduce ocular pain are also commonly used. Additional systemic drugs used to lower intraocular pressure (e.g., mannitol) may be indicated in suspected glaucoma cases. Monitoring the degree and extent of bluing to the cornea, as well as the level of pain and irritation, may give an indication of healing and recovery, though corneal edema in some cases can take a long time (months) to resolve (Photos 4 & 5).

Photo 5: The same horse as in Photo 4 after months of treatment. The corneal edema is beginning to clear at the corner of the eye.

Immune-mediated keratitis

The one common thread in almost all cases of a blue eye in horses is that these animals are in pain and uncomfortable. This is exactly why these cases are difficult to miss. An agitated horse with a squinty, closed, tearing, cloudy eye that stands in its stall rubbing its face on everything is hard to overlook. The pain from glaucoma, uveitis or corneal abrasions is very evident.

Much more confusing, however, are those rare cases in which a horse will acutely develop a blue eye without any trauma or history of problems and with virtually no clinical signs. These horses are often fine when turned out but return with a blue eye. They are normal in all other aspects and have no swelling, tearing or any evidence of ocular pain. They do not typically take up dye when their eye is stained, and the results of an ocular ultrasonographic examination is normal. They are functionally blind because of the extreme amount of corneal edema and tend to show multiple punctuate nonulcerative lesions to the cornea (small indentations or dimples covered by normal corneal tissue).

It is now thought that a large proportion of these horses may be affected with immune-mediated keratitis (IMMK). IMMK is problematic worldwide, and this disease was discussed at the recent Equine Ophthalmology Consortium in Vienna, Austria. Susan Keil, DVM, MS, Dipl. ACVO, attended this conference and says, "IMMK is likely more than one disease lumped together under one name, and its cause and pathophysiology are not yet understood."

Horses with IMMK show corneal edema and a very blue eye, but the damage to the cornea does not resemble fungal or bacterial eye disease. Brian Gilger, DVM, MS, Dipl. ACVO, Dipl. ABT; Tammy Michau, DVM, Dipl. ACVO; and Jacklyn Salmon, BS, of the Department of Clinical Sciences, College of Veterinary Medicine at North Carolina State University recently reviewed a number of cases of IMMK and concluded that, "in all cases of IMMK, signs of uveitis or severe discomfort were not observed."

While true IMMK cases are still uncommon, this disease should be considered when encountering a horse with a blue eye, especially if that horse is not in pain. If pain is an important part of the presentation, then swift and aggressive treatment should be sought. Assessing for corneal abrasions or damage, glaucoma and uveitis will cover most of these types of eye cases. Appropriate treatment should be initiated quickly, and, in most instances, positive outcomes for horses with blue eyes can be achieved.

Dr. Marcella is an equine practitioner in Canton, Ga.