Conjunctivitis, the most common of all feline eye disorders, is an inflammation of the thin mucous membrane (conjunctiva) that lines the inner surface of a cat’s eyelids and coats the outer surface of the eyeball. Many cats will experience at least a mild episode of the condition at some point in their lives.

According to Thomas Kern, DVM, associate professor of ophthalmology at Cornell University’s College of Veterinary Medicine, the conjunctiva serves several purposes. Most important, he notes, this slippery membrane provides the eyeball with lubrication by functioning as a conduit for tears that fall onto its surface and are distributed by what he refers to as “the blinking phenomenon.” Furthermore, the conjunctiva harbors certain antibodies that may help an animal ward off some eye infections. Nevertheless, he points out, there are several microorganisms that cats commonly carry, and the feline system’s inflammatory immune response to these bacteria and viruses is responsible for the great majority of feline conjunctivitis cases.

The clinical signs of the condition can be evident in either one or both eyes and will typically be observed as well in the third eyelid—the membrane positioned in the inner corner of a cat’s eye, between the lower eyelid and the eyeball. The signs include squinting, frequent blinking, and the presence of a discharge that, depending on the cause of the conjunctivitis, can be either colorless and watery or thick and dark-colored. The condition also tends to cause the conjunctiva and third eyelid to become swollen and red.

Environmental irritants, such as dust or airborne chemical substances can cause conjunctivitis, as can exposure to certain outdoor plants—a condition called allergic conjunctivitis. The most frequent cause by far, however, is infection with the herpesvirus, the calicivirus, or one of two bacteria—chlamydophila or mycoplasma. Conjunctivitis, Dr. Kern adds, is also relatively common in cats whose immune systems have been compromised by infection with the feline immunodeficiency virus (FIV) or the feline leukemia virus (FeLV).

All cats, regardless of breed or gender, are susceptible to conjunctivitis, and the condition is not heritable. However, age is clearly a determining factor. “Although conjunctivitis can occur in older cats, this is an infection that occurs primarily in young animals,” says Dr. Kern, “and it is most prevalent in catteries and other multiple-cat environments. In the course of their development, young cats may pick up a virus or a bacterium from an older cat. And, like kids, they play together and pass it around. Sooner or later, they all get it.” Although most affected cats will eventually develop an immunity to the condition and will not experience recurrences, Dr. Kern notes that episodes of conjunctivitis may recur periodically in those animals who carry the herpesvirus—“just as cold sores appear from time to time in humans who carry it.”

In most cases, he points out, conjunctivitis will self-resolve with no medication at all. However, he advises, owners should seek veterinary care if a cat has apparent eye discomfort and discharge to rule out more serious eye disorders. Says Dr. Kern: “Most veterinarians will prescribe antibiotic eyedrops or an ointment to be used three or four times a day for two to three weeks, by which time the discharge and other signs should disappear. If we suspect that we’re dealing with herpesvirus, we’ll use topical antiviral drugs along with antibiotics. We won’t be able to cure it, but we will try to eliminate the infection from the surface of the eye and let it heal.”