Red eyes in cats almost always signal some form of inflammation or irritation, and the causes range from a mild infection that clears up quickly to serious conditions that threaten your cat’s vision. The most common culprit is conjunctivitis, an inflammation of the thin tissue lining the eyelids. But redness can also point to corneal damage, increased pressure inside the eye, or even a systemic illness. Understanding what else is happening alongside the redness helps narrow down what’s going on.
Conjunctivitis: The Most Common Cause
Conjunctivitis is responsible for the majority of red eyes in cats. The conjunctiva, a thin pink membrane that lines the inner surface of the eyelids, becomes inflamed and visibly swollen. You’ll typically notice squinting, frequent blinking, and discharge that can be either watery and clear or thick and dark-colored depending on the cause. The third eyelid, that extra membrane in the inner corner of the eye, often becomes puffy and red as well.
The most frequent triggers are infections. Feline herpesvirus is the leading cause by a wide margin, followed by calicivirus and two types of bacteria: chlamydophila and mycoplasma. Herpesvirus is especially persistent because once a cat is infected, the virus stays dormant in the body and can flare up during periods of stress, illness, or immune suppression. Bacterial conjunctivitis often produces thicker, darker discharge compared to the watery eyes more typical of viral infections.
Allergies, dust, and environmental irritants can also inflame the conjunctiva, though these are less common in cats than in dogs or humans.
Corneal Ulcers and Scratches
The cornea is the clear outer surface of the eye, and it’s surprisingly easy to damage. A scratch from another cat, a poke from a branch, or even rubbing against rough fabric can create an erosion or ulcer. When this happens, the conjunctiva turns an angry red and can swell or puff up around the eye, a reaction called chemosis. Your cat may squint, paw at their face, or keep the affected eye partially closed.
Deeper ulcers cause a foggy or cloudy patch on the cornea itself. If an ulcer becomes infected, the area develops a yellow or tan appearance as bacteria produce enzymes that dissolve the corneal tissue. The cornea can actually soften and appear to be melting, which is a veterinary emergency since the eye can perforate. Vets diagnose corneal damage using a fluorescent dye that sticks to any damaged area and glows bright green under a special lamp, making even tiny erosions visible.
Uveitis: Inflammation Inside the Eye
Uveitis involves inflammation of the structures inside the eye rather than on the surface. It’s the most common way systemic diseases show up in a cat’s eyes, and it’s more serious than conjunctivitis because it can permanently damage vision if untreated. The eye appears red, but the redness looks deeper. You might also notice cloudiness, a change in iris color, or pupils that seem different sizes.
What makes uveitis concerning is what it can signal about your cat’s overall health. Several major feline diseases cause uveitis as one of their symptoms:
- Feline immunodeficiency virus (FIV) commonly triggers inflammation in the front of the eye, with the immune system sending waves of white blood cells into the eye tissue.
- Feline leukemia virus (FeLV) can cause tumor cells to invade the eye’s inner structures, sometimes restricting how the iris moves.
- Feline infectious peritonitis (FIP) typically causes inflammation in both eyes at once, with damaged blood vessels leaking cells and protein into the front chamber of the eye.
- Toxoplasmosis, a parasitic infection, can produce new blood vessel growth and small inflammatory nodules on the iris.
Because uveitis so often reflects a bigger problem, a vet finding it will usually recommend blood tests to check for these underlying infections.
Glaucoma: Pressure Buildup
Glaucoma occurs when fluid inside the eye doesn’t drain properly, causing pressure to build. Normal eye pressure in cats averages around 12 mmHg, though it naturally fluctuates throughout the day, running a few points higher at night and lower in the afternoon. Readings of 25 mmHg or more, or a difference of 12 mmHg or more between the two eyes, raise a red flag.
Glaucoma isn’t especially common in cats, but when it occurs, it’s a medical emergency. High pressure can destroy vision rapidly. The eye looks red, may appear swollen or enlarged, and your cat will likely seem uncomfortable or withdrawn. One tricky aspect of feline glaucoma is that pressure can fluctuate significantly from one measurement to the next, so a single normal reading doesn’t rule it out if other signs are present.
Flat-Faced Breeds Are at Higher Risk
If you have a Persian, Himalayan, or another flat-faced breed, chronic eye redness may be partly structural. These cats have been bred to have shallow eye sockets, which means their eyes sit more prominently and are more exposed to the environment. The more extreme the flat face, the more the eyeball protrudes beyond the orbit.
This anatomy creates a few overlapping problems. Protruding eyes are prone to a condition called exposure keratitis, where the cornea dries out and becomes inflamed simply because the eyelids can’t fully cover it. These breeds also have severely distorted tear drainage ducts, so tears can’t flow away from the eye normally. The result is chronic overflow tearing, staining on the fur below the eyes, and a higher baseline risk of corneal damage and infection. If you own a flat-faced cat, some degree of eye irritation may be ongoing, but persistent redness still warrants a vet visit to rule out ulcers or other complications.
Signs That Need Immediate Attention
Not every case of red eyes is an emergency, but several warning signs should prompt a same-day or immediate vet visit. Cloudiness in the eye, a visible change in iris color, or swelling of the iris all warrant urgent evaluation. Dilated pupils that don’t respond to light can indicate retinal detachment, and if treatment is delayed even a few hours, the resulting blindness is often permanent.
Even straightforward conjunctivitis should be seen by a vet within 24 hours, since the underlying cause determines the treatment. Bacterial infections respond to antibiotics, while herpesvirus requires antiviral medications. Using the wrong type won’t help and may allow the condition to worsen. Suspected corneal ulcers need immediate attention because infection can escalate quickly from a minor scratch to a sight-threatening emergency.
What Treatment Looks Like
Treatment depends entirely on the cause. For bacterial infections like chlamydophila, oral antibiotics tend to work more reliably than eye drops alone. Antibiotic eye drops or ointments are still commonly used alongside oral medication, especially to control surface-level infection and prevent secondary bacterial complications during viral flare-ups.
Viral conjunctivitis from herpesvirus is managed with antiviral medications. Since the virus lives permanently in the body, treatment focuses on controlling flare-ups rather than curing the infection. Cats with recurring herpesvirus episodes may need antiviral treatment periodically throughout their lives, particularly during stressful events like boarding, moving, or the introduction of a new pet.
Corneal ulcers are treated with topical antibiotics to prevent or fight infection while the surface heals. Minor erosions often resolve within a week, but deeper ulcers may need more aggressive intervention. Your cat will likely need to wear a cone to prevent them from rubbing the eye during healing.
For uveitis and glaucoma, treatment targets both the eye symptoms and any underlying disease. If blood tests reveal FIV, FeLV, or another systemic condition, managing that illness becomes the priority alongside controlling the eye inflammation. Glaucoma specifically requires medications to reduce eye pressure, and in severe or unresponsive cases, surgical options may be discussed.