Conjunctivitis is the most common eye disorder in cats. It’s an inflammation of the conjunctiva, the thin mucous membrane that lines the inside of a cat’s eyelids and coats the outer surface of the eyeball. The condition causes redness, swelling, and discharge from one or both eyes, and it almost always involves the third eyelid, the small membrane tucked into the inner corner of each eye. Most cases are caused by viral or bacterial infections, and while conjunctivitis is usually treatable, ignoring it can lead to serious complications including permanent vision damage.
What Causes Conjunctivitis in Cats
The vast majority of feline conjunctivitis cases are infectious, driven by the cat’s immune system reacting to viruses or bacteria. The most common culprit is feline herpesvirus (FHV-1), which is so widespread that serologic studies show exposure rates as high as 97% in cat populations worldwide. After initial exposure, more than 80% of cats become persistently infected. The virus hides dormant in nerve tissue near the brain and can reactivate during periods of stress, illness, or anything that weakens the immune system. When it reactivates, the virus travels along nerve fibers back to the eye’s surface, triggering a new bout of conjunctivitis.
Bacterial infections are the other major cause. Chlamydia felis is particularly common in young cats under nine months of age. Because the bacterium can’t survive long outside a host, transmission requires close contact between cats, usually through eye discharge. This makes it especially common in shelters, catteries, and multi-cat households. Contaminated objects like bedding or food bowls can also spread it in crowded environments.
Less frequently, conjunctivitis can result from allergies, irritants like dust or cigarette smoke, or physical trauma to the eye. But infections account for the large majority of cases veterinarians see.
How to Recognize It
The hallmark signs are redness, swelling, and discharge. You’ll notice your cat’s eyes look puffy, and the tissue around and inside the eyelids appears obviously pink or red. The third eyelid often becomes swollen and more visible than usual, sometimes partially covering the eye.
The type of discharge can shift over the course of the illness. Early on, it tends to be watery and clear. As the condition progresses or if bacteria become involved, the discharge often turns thicker, becoming cloudy, yellowish, or greenish. Cats with conjunctivitis frequently squint or hold the affected eye partially closed, and you may see them pawing at their face. Some cats also sneeze or develop nasal discharge, particularly when a herpesvirus or chlamydial infection is involved, since these pathogens affect the entire upper respiratory system.
Chlamydial conjunctivitis has a distinctive pattern: it typically starts in one eye and then spreads to both within a few days. Herpesvirus conjunctivitis can affect one or both eyes from the start and may include small ulcerated areas on the conjunctival surface where the virus has destroyed tissue.
What Happens at the Vet
A veterinarian will start by examining the eye’s surface and surrounding tissues, looking for redness, swelling, and the character of any discharge. Several specific tests help determine the cause and rule out more serious problems.
A tear production test is often done first, before any drops are applied to the eye. A small paper strip is placed inside the lower eyelid to measure how many tears the eye produces in one minute. While dry eye is rare in cats, it can occur with herpesvirus infections, and this test helps catch it.
A fluorescein stain is one of the most important parts of the exam. The vet applies an orange dye to the eye’s surface, then shines a blue light on it. The dye sticks to any areas where the outer layer of the cornea is damaged, making ulcers glow bright green. This is critical because corneal ulcers require more aggressive treatment and can worsen quickly if missed.
In some cases, the vet may gently collect cells from the conjunctival surface using a small brush or spatula. Examining these cells under a microscope can help distinguish between viral, bacterial, and allergic causes. This is done before the fluorescein stain, since the dye can interfere with bacterial testing.
Treatment Depends on the Cause
For bacterial conjunctivitis, including chlamydial infections, antibiotic eye drops or ointments are the standard treatment. Your vet will prescribe a specific product and schedule, which typically involves applying medication to the affected eye several times a day for one to several weeks.
Herpesvirus conjunctivitis is trickier because the virus can never be fully cleared from the body. Antiviral eye medications are the primary treatment for active flare-ups. Some are applied as frequently as every few hours during the first day, then tapered to several times daily. Newer formulations require only twice-daily application, which is much easier for both cat and owner. In more severe cases, oral antiviral medication may be prescribed alongside the eye drops. The goal is to control the active infection and reduce damage to the eye’s surface, not to eliminate the virus entirely.
If a corneal ulcer is present, antibiotic eye drops are added to prevent secondary bacterial infection of the damaged tissue, regardless of whether the original cause was viral. Ulcers require close follow-up to make sure they’re healing properly.
Complications of Untreated Conjunctivitis
Left untreated, conjunctivitis can progress from an uncomfortable nuisance to a sight-threatening problem. Corneal ulcers are one of the most common complications. The constant inflammation and viral damage to the eye’s surface create open wounds on the cornea that can deepen, become infected, and in severe cases perforate the eye.
Symblepharon is a particularly serious complication, most often seen in kittens with herpesvirus infections. When the conjunctival surface is badly damaged, scar tissue forms and creates adhesions, essentially fusing the inner eyelid to the eyeball or gluing layers of conjunctiva together. This can restrict eye movement, block tear drainage, and permanently impair vision. In very young kittens whose eyelids haven’t yet opened, a severe infection trapped behind closed lids can destroy the eye’s surface entirely.
These complications are largely preventable with prompt treatment, which is why any cat showing signs of eye redness, swelling, or discharge should be examined sooner rather than later.
Keeping Your Cat’s Eyes Clean at Home
While your cat is being treated, you’ll likely need to keep the eye area clean between medication doses. A sterile saline eye wash, applied directly to the eye or the surrounding tissue, can help soften and remove crusted discharge. Wash your hands before and after handling the eye area or applying any medication. When using eye wash or drops, avoid touching the tip of the bottle to the eye or any surface to prevent contamination.
Use a separate clean cloth or cotton ball for each eye to avoid spreading infection from one side to the other. Never use human eye drops or medications on your cat unless specifically directed by your veterinarian, as some ingredients safe for people are harmful to cats.
Can You Catch It From Your Cat
The main bacterial cause, Chlamydia felis, does have a low zoonotic potential, meaning transmission to humans is theoretically possible but rare. The risk comes from direct contact with an infected cat’s eye discharge or respiratory secretions. Simple hygiene measures, washing your hands after handling a cat with conjunctivitis and avoiding touching your own eyes, are enough to minimize any risk. Feline herpesvirus is species-specific and does not infect humans.
Why Some Cats Get Repeated Flare-Ups
If your cat seems to develop conjunctivitis over and over, herpesvirus is the most likely explanation. Because the virus persists in nerve tissue for life, any stressor can trigger reactivation: a move to a new home, the introduction of a new pet, boarding, illness, or even changes in routine. Each reactivation sends the virus back to the eye surface, restarting the cycle of inflammation and discharge.
Managing stress is a genuine part of managing recurrent herpesvirus conjunctivitis. Keeping your cat’s environment stable, minimizing disruptions, and addressing any underlying health issues can reduce the frequency of flare-ups. Some veterinarians also recommend long-term supplements that may help support the immune system’s ability to keep the virus suppressed, though the evidence for these varies. The key thing to understand is that recurrent conjunctivitis in a cat isn’t a failure of treatment. It’s the nature of the virus, and the goal shifts from curing the infection to controlling it and catching flare-ups early.