The cornea is the round, transparent surface that covers the frontmost components of the feline eye—the iris, pupil, and fluid-filled anterior chamber. Nourished by tears on its exterior surface and by aqueous humor (the fluid that fills the anterior chamber) on its interior surface, the cornea serves two important functions: (1) it protects the eye against assault by dust, germs, and other dangerous external substances; and (2) it focuses and controls the amount of light that enters the eye, thus facilitating accurate vision. In order to perform its sight-enabling function, the cornea must be free of any cloudy areas that could impede the entry of light into the interior of a cat’s eye.
Despite its normally strong and durable structure, the cornea’s surface is a sensitive tissue and is, therefore, subject to a variety of disorders that can interfere with its proper functioning. Most common among these disorders is the excessive loss of cells in the outermost layer of tissue (epithelium) covering the cornea. Called corneal ulceration, the condition will become progressively more severe as the cell loss outpaces the generation of new epithelial cells. In an advanced case, a perforation may develop on the corneal surface, allowing drainage of the intraocular contents. This can result in blindness and even the loss of the affected eye.
Feline corneal ulcers have a wide variety of potential causes, including scratches that a cat can sustain in a fight; ingrown eyelashes; a piece of dirt that becomes trapped beneath the eye lid; exposure to caustic chemicals; and viral or bacterial infection. According to Thomas Kern, DVM, associate professor of ophthalmology at Cornell University’s College of Veterinary Medicine, the most frequent cause is recurrent infection with the feline herpesvirus (FHV). He explains: “Most cats that are exposed to this virus are very young.
The virus may seem to disappear, but in fact it may go on living in some of the infected cat’s nerve ganglia for the rest of the animal’s life. And from time to time, the cat’s immune system will allow the virus to escape. If the virus attacks the surface of the eye, it will cause ulceration.” Fortunately, Dr. Kern notes, most cats that experience herpetic corneal ulcers “are not doomed to have them recurrently.”
The clinical signs of corneal ulceration include inflammation of the tissue surrounding the cornea; seepage of discharge from the eye; clouding of the cornea; and apparent hypersensitivity to bright light. An affected cat may squint, rub its eyes, and behave as if it is having vision problems. However, Dr. Kern points out: “The only corneal ulcers that we see are those that are symptomatic enough to be noticed. Many of them heal naturally within a few days to a week without treatment or, for that matter, without even being noticed by the owner.”
Diagnosis of cornneal ulceration is achieved by a review of the animal’s medical history and a thorough examination of the affected eye. A tentative diagnosis of an ulcer will typically be confirmed by means of a test using eye drops containing a dye called fluorescein. If an ulcer is present, the dye will bind to any damaged tissue, leaving a clearly visible greenish coloration on the ulcerated area.
Treatment will depend on the severity of the ulcer and will focus on controlling the inflammation, reducing the pain it is causing, and preventing the spread of any bacterial or fungal infection. (In cases of superficial ulcers, the use of an antibiotic ointment is typically effective in controlling such infection.) Since self-trauma may aggravate a corneal ulcer and interfere with the healing process, an affected cat may be required to wear an Elizabethan collar until the ulcer is completely healed.
“If we think that the corneal ulcer is probably caused by the herpesvirus,” says Dr. Kern, “we’ll prescribe a drug called cidofovir, which is given twice daily to prevent viral cells from replicating. That will usually take care of a simple ulcer within a week or two. Or we may prescribe lysine, which is an amino acid that also inhibits replication of the virus. Some cats experiencing recurrent corneal ulcers that we think are caused by the herpesvirus may have to be put on lysine therapy permanently.”