A cat holding one eye shut is almost always reacting to pain or irritation in that eye. Cats instinctively squint or close an eye completely when something is wrong with it, whether that’s a scratch on the surface, an infection, a foreign object, or inflammation deeper inside the eye. This reflex, called blepharospasm, is an involuntary muscle contraction that clamps the eyelid shut to protect the eye from further damage.
Why Cats Squint or Close One Eye
The squinting reflex is driven by nerve pathways that connect pain-sensing areas of the eye to the muscles controlling the eyelid. When the surface of the eye (the cornea) or the tissues around it detect irritation, signals travel to the facial nerve, which triggers the eyelid to contract. This happens on the same side as the problem, which is why only one eye closes. The contraction can range from a partial squint to a fully shut eye depending on how much pain is involved.
Because cats are prey animals as well as predators, they tend to hide discomfort. A closed eye is one of the more obvious pain signals a cat will give you, and it usually means the problem has progressed enough to cause real discomfort.
Common Causes
Conjunctivitis
Conjunctivitis, or inflammation of the pink tissue lining the eyelid, is one of the most frequent reasons for a cat to hold one eye shut. You may notice excessive tearing, reddened tissue around the eye, or discharge that ranges from clear and watery to thick, yellow, or greenish. Infectious agents like bacteria, viruses, and fungi are the most common culprits. In many cases a virus starts the inflammation, and then bacteria move in and create a secondary infection that worsens the discharge.
Feline Herpesvirus
Feline herpesvirus (FHV-1) is by far the most common viral cause of eye problems in cats. Exposure rates in the general cat population run as high as 97%, meaning nearly every cat encounters this virus at some point. After the initial infection clears, the virus hides in nerve tissue and can reactivate later in life. Stress is the biggest trigger for flare-ups: a move to a new home, the arrival of a new pet, boarding, illness, or even giving birth can bring symptoms roaring back. About 45% of cats carrying the latent virus will shed it again spontaneously or during stressful periods.
Herpesvirus flare-ups typically cause squinting, watery or mucus-like discharge, redness, and sometimes sneezing or nasal congestion. These episodes can recur throughout a cat’s life.
Corneal Ulcer
A corneal ulcer is an open sore on the clear surface of the eye. It can result from a scratch (often from another cat), a foreign body like a grass seed, or a viral infection that damages the corneal tissue. Signs include squinting, discharge, cloudiness of the eye, and sensitivity to bright light. An affected cat may paw at the eye or seem disoriented.
Corneal ulcers range from superficial scratches that heal in a few days with treatment to deep erosions that threaten the entire eye. In advanced cases, the ulcer can perforate the corneal surface, allowing the contents of the eye to drain out. This can lead to blindness or loss of the eye entirely, which is why any suspected corneal ulcer needs veterinary attention quickly.
Foreign Object
A blade of grass, a bit of litter, dust, or even a loose hair can get trapped under the eyelid or stick to the corneal surface. This causes immediate, intense squinting. Sometimes the cat’s natural tear production will flush the object out on its own within a few hours, but if the squinting persists, the object may be lodged and needs to be removed professionally.
Entropion
Entropion is a condition where the eyelid rolls inward, causing the fur and eyelashes to rub directly against the corneal surface. This creates constant irritation, discharge, and squinting that won’t resolve on its own. Persian cats and other flat-faced breeds are most prone to entropion because of their facial structure. Intact male Maine Coons are also predisposed due to their pronounced facial jowls. The condition typically requires minor surgery to correct the eyelid position permanently.
Glaucoma and Uveitis
Problems with pressure inside the eye can also cause squinting. Glaucoma means the pressure is too high, which creates a deep, aching pain. Uveitis is inflammation inside the eye that often lowers the pressure. Both conditions cause the cat to hold the affected eye shut, and both can lead to vision loss without treatment. You might notice the eye looks cloudy, the pupil is an unusual size, or the eye itself appears swollen or sunken compared to the other one.
What the Discharge Tells You
The type of fluid coming from the eye offers clues about the cause. Clear, watery discharge often points to irritation, allergies, or the early stages of a viral infection. As the problem progresses or bacteria get involved, the discharge typically thickens and turns yellow or green. Thick, crusty discharge that seals the eye shut overnight is a strong sign of bacterial involvement. A complete absence of discharge combined with a tightly closed eye may suggest a deeper problem like glaucoma, uveitis, or a foreign body lodged behind the eyelid.
What a Vet Visit Looks Like
A veterinarian will start by examining the eye’s surface and surrounding tissues with a light and magnification. Two common tests help narrow down the problem. The first is a fluorescein stain, where an orange dye is applied to the eye’s surface. The dye sticks to any damaged areas of the cornea and glows green under blue light, revealing ulcers or scratches that are invisible to the naked eye.
The second is tonometry, a painless test that measures the pressure inside the eye. This is essential for detecting glaucoma (high pressure) or confirming uveitis (low pressure). If dry eye is suspected, a tear test can measure how much moisture the eye produces by placing a small paper strip inside the lower eyelid for 60 seconds and measuring how far the tears travel along it.
Most of these tests take just a few minutes and don’t require sedation.
Why You Should Not Use Human Eye Drops
It’s tempting to reach for eye drops you have at home, but human eye medications are often dangerous for cats. Antibiotic ointments containing neomycin and polymyxin B (the active ingredients in products like Neosporin) have caused life-threatening allergic reactions in cats. Steroid-containing drops are equally risky: if your cat has a corneal ulcer you can’t see, steroids will prevent healing and can cause the ulcer to deepen rapidly, potentially leading to a ruptured eye. Artificial tears designed for humans may also contain preservatives that irritate feline eyes.
The safest thing you can do at home is gently wipe away any discharge with a warm, damp cloth, using a fresh area of the cloth for each wipe. Don’t press on the eye or try to force it open.
Signs That Need Urgent Attention
Some eye problems are true emergencies where hours matter. Get your cat to a vet immediately if you notice any of the following: the eye is bulging forward or looks like it’s popping out of the socket, there’s visible damage to the eye’s surface (a cut or puncture), the clear part of the eye looks gelatinous or is melting rather than staying smooth, or the eye has suddenly changed size or shape. A rapidly worsening corneal ulcer can rupture within a single day, and an eye that has been pushed out of the socket (proptosis) needs treatment as soon as possible to have any chance of saving vision.
Even in less dramatic situations, a cat that has kept one eye closed for more than 24 hours, has colored discharge, or seems to be in increasing pain should be seen promptly. Eye conditions in cats tend to escalate quickly, and what starts as a minor irritation can become a serious threat to the eye within days.