A cat closing one eye is almost always a sign of pain or irritation in that eye. Cats instinctively squint or hold an eye shut to protect it, so even if your cat seems fine otherwise, something is bothering that eye and it needs attention. The cause can range from a minor irritation that clears up in hours to a serious condition that requires prompt veterinary care.
The Most Common Causes
Conjunctivitis, commonly called pinkeye, is the most frequent eye problem in cats. It happens when the thin membrane lining the eye becomes inflamed from an infection, allergy, or even dust. You’ll typically see watery discharge alongside the squinting. The color of that discharge tells you something: clear mucus usually points to a virus, while green or yellow mucus suggests a bacterial infection.
Corneal ulcers are another common culprit. These are essentially scratches or sores on the clear surface of the eye, often caused by a scuffle with another cat, a poke from a branch, or even a rough grooming session. Along with squinting, you may notice cloudiness in the eye, discharge, and your cat shying away from bright light or rubbing its face against furniture. Cornell University’s veterinary team notes that affected cats often behave as though they’re having trouble seeing, bumping into things or misjudging distances on the affected side.
Foreign material stuck under the eyelid, like a seed, a bit of litter, or a stray hair, can also cause one-sided squinting that comes on suddenly. If your cat was perfectly fine an hour ago and is now holding one eye shut, something may have gotten into it.
Feline Herpesvirus and Recurring Flare-Ups
Most cases of conjunctivitis in cats trace back to feline herpesvirus. This virus targets tissues that interface with the outside world, particularly the eyes and upper respiratory tract. The initial infection is usually the most severe, often occurring in kittenhood with sneezing, nasal congestion, and goopy eyes.
Here’s what many cat owners don’t realize: once a cat contracts herpesvirus, it carries the virus for life. The virus goes dormant but can reactivate during periods of stress, illness, or immune suppression. That means your cat might seem perfectly healthy for months, then suddenly start squinting one eye again. These flare-ups can affect one eye or both, but a single-eye episode is very common. If your cat has a history of upper respiratory infections as a kitten, herpesvirus is a likely suspect for recurring eye problems.
The Third Eyelid: A Built-In Warning Sign
Cats have a third eyelid, a pale membrane that sits in the inner corner of each eye. You normally don’t see much of it. When it slides up and becomes visible, covering part of the eye, that’s a sign something is wrong. Conditions that can cause it to protrude in one eye include conjunctivitis, trauma or swelling, glaucoma (elevated pressure inside the eye), and a condition called Haw’s syndrome, which is one of the more common eye issues in cats.
If you notice a whitish or pinkish film partially covering one eye alongside the squinting, that’s the third eyelid doing its job as a protective barrier. It’s not a diagnosis by itself, but it confirms the eye is in distress.
More Serious Possibilities
Two conditions that cause squinting and require faster intervention are glaucoma and uveitis (inflammation inside the eye). Glaucoma involves a buildup of fluid pressure that can damage the optic nerve and lead to blindness. In cats, glaucoma often affects one eye first, causing the pupil on that side to look noticeably larger than the other. This difference in pupil size, called anisocoria, is one of the clearest warning signs.
Uveitis, or inflammation of the inner structures of the eye, can make the eye look cloudy or reddened and may cause the pupil to constrict rather than dilate. Left untreated, chronic uveitis can lead to complications including lens displacement and secondary glaucoma. Both conditions cause significant pain, which is why the cat squints or holds the eye completely closed.
Signs That Need Urgent Attention
Some symptoms warrant a same-day vet visit rather than a wait-and-see approach:
- One pupil noticeably larger or smaller than the other. This can indicate glaucoma, uveitis, or nerve damage.
- Blood visible inside the eye. This suggests damage to internal eye structures.
- A fixed, dilated pupil that doesn’t respond to light. This can mean severe damage to the optic nerve.
- A deep divot or dark spot on the surface of the eye. Deep corneal ulcers that reach the innermost layers are considered pending perforations and need surgical treatment quickly.
- The eye appears to be bulging. This can signal dangerously high pressure inside the eye.
- Your cat won’t open the eye at all and seems very distressed. Significant, constant pain points to something beyond a minor irritation.
What Happens at the Vet
A veterinary eye exam is straightforward and usually painless for your cat. The vet will look at the eye’s surface, check for discharge, and assess pupil responses. One of the most common diagnostic tools is a fluorescein stain test: orange-tinted drops are placed on the eye, and any damaged tissue on the cornea absorbs the dye, glowing green under a special light. This makes even tiny ulcers clearly visible.
If dry eye is suspected, the vet may use a Schirmer tear test, which involves placing a small paper strip at the edge of the eye to measure tear production over one minute. Normal tear production in cats falls around 12 to 13 millimeters per minute. Values well below that range suggest the eye isn’t producing enough moisture to protect itself, a condition that can lead to chronic irritation, infections, and corneal damage if left unmanaged.
Recovery Timelines
Simple bacterial conjunctivitis typically clears up within about a week of starting prescribed treatment. Viral infections, especially herpesvirus flare-ups, can take longer and may require supportive care while the immune system does most of the work. Corneal ulcers vary widely depending on depth. A superficial scratch might heal in a few days with medication, while a deep ulcer could need weeks of treatment or even surgery.
Your vet will likely prescribe eye drops or ointment that you’ll need to apply at home. This is rarely a cat’s favorite activity, but wrapping your cat snugly in a towel and approaching from the side rather than head-on makes it more manageable.
Safe Home Care Before the Vet Visit
While you’re waiting for your appointment, you can gently clean discharge from around the eye using a cotton ball dampened with clean, warm water. Wipe from the inner corner outward, use a fresh cotton ball for each eye, and be careful not to touch the eyeball itself. This won’t treat the underlying problem, but it keeps the area clean and prevents dried discharge from sealing the eye shut.
Do not use human eye drops on your cat. Many over-the-counter products contain ingredients that are harmful to cats. Zinc sulfate, found in some redness-relief drops, is irritating to feline eyes. Certain glaucoma medications designed for humans are highly toxic to cats due to differences in how their bodies process the drugs. Even common antibiotic drops may contain formulations not tested or approved for feline use. Stick with warm water until your vet provides the right medication.