What exactly is happening inside a cat's eye when it turns pink and swollen?

Conjunctivitis is inflammation of the conjunctiva, the thin, moist membrane that lines the inside of the eyelids and covers the front of the eyeball up to the edge of the cornea, including the surface of the third eyelid cats have in the inner corner of each eye. When this tissue becomes irritated or infected, blood vessels dilate and the membrane swells, which is why the whites and lid margins turn red or pink and sometimes look almost fleshy. Cornell Feline Health Center identifies conjunctivitis as one of the most common ocular conditions seen in domestic cats, affecting animals of any age but appearing with particular frequency in kittens and cats in group housing such as shelters (Cornell Feline Health Center, 2021).

The condition can affect one eye or both, and it can be acute, developing over a day or two, or chronic, persisting or recurring over months. Because the conjunctiva sits directly against the cornea and drains toward the nasal passages, inflammation there frequently overlaps with sneezing, nasal congestion, and other upper respiratory signs, which is a key clue vets use when narrowing down a cause (Cornell Feline Health Center, 2021).

What are the first signs an owner usually notices at home?

The earliest changes are often subtle: a cat blinking more than usual, holding one eye slightly narrowed, or rubbing a paw across its face. As inflammation progresses, owners typically see redness of the pink tissue around the eye, watery to thick discharge that can mat the fur below the eye, and eyelids that look puffy or partially closed. The American Association of Feline Practitioners notes that ocular discharge and conjunctival redness are among the most frequently reported reasons cats are brought in for eye evaluation, and that discharge character (clear versus mucoid versus pus-like) helps clinicians distinguish viral from bacterial involvement (American Association of Feline Practitioners, 2021).

Cats with viral causes often show discharge that starts clear and watery and becomes thicker and yellow-green over several days as secondary bacterial infection sets in, sometimes alongside sneezing or a stuffy nose. A cat that squints hard, keeps the eye shut, or paws at its face persistently is showing signs of pain, not just irritation, and that behavior alone is worth a same-day call to the vet rather than a wait-and-see approach.

Why do so many cats — especially shelter or newly adopted kittens — get pink eye?

Feline herpesvirus-1 (FHV-1) is the single most common infectious cause of conjunctivitis in cats, and Cornell Feline Health Center estimates that the vast majority of cats have been exposed to the virus at some point in their lives, with the virus capable of lying dormant and reactivating during stress, such as a move to a new home or a shelter stay (Cornell Feline Health Center, 2021). Chlamydia felis and Mycoplasma species are the next most frequent bacterial culprits, and both spread efficiently in crowded, high-turnover environments like shelters and multi-cat households, which is why recently adopted kittens are disproportionately affected.

Non-infectious triggers matter too: allergens, dust, cigarette smoke, foreign material like grass awns or eyelashes rubbing the eye, and anatomical issues in flat-faced breeds such as Persians can all cause conjunctival irritation without any contagious agent involved. The USDA-supported ASPCA notes that any cat with chronic or recurrent redness that doesn't fit a typical infectious pattern should be evaluated for underlying allergic or structural causes rather than treated repeatedly with the same drops (ASPCA, n.d.).

How does a veterinarian figure out what's actually causing it?

Diagnosis starts with a full ophthalmic exam, including fluorescein staining, a dye applied to the eye's surface that glows green under blue light to reveal corneal ulcers or scratches that would change treatment and rule out herpesvirus-associated keratitis. Cornell Feline Health Center recommends this stain test essentially any time a cat has ocular discharge with squinting, because starting a steroid-containing drop on an ulcerated cornea can make damage dramatically worse (Cornell Feline Health Center, 2021).

For cases that are severe, recurrent, or not responding to first-line treatment, vets may swab the conjunctiva for PCR testing to identify FHV-1, Chlamydia felis, or Mycoplasma DNA specifically, since treatment differs meaningfully between a viral and a bacterial cause. Merck Veterinary Manual notes that a tentative diagnosis is often made on clinical signs and response to treatment alone in first-time, uncomplicated cases, reserving lab testing for chronic or refractory conjunctivitis (Merck Veterinary Manual, 2023).

What treatments actually work, and does a cat need antibiotic drops every time?

Treatment depends on the confirmed or suspected cause. Bacterial and chlamydial conjunctivitis typically responds to topical antibiotic ointments or drops such as those containing terramycin or erythromycin, usually given for 2 to 3 weeks, while herpesvirus-driven cases may need an antiviral eye medication or an oral supplement like L-lysine, though Cornell Feline Health Center notes evidence for lysine's benefit is mixed and it should not replace veterinary-directed treatment (Cornell Feline Health Center, 2021).

Systemic doxycycline is frequently the treatment of choice for Chlamydia felis infections because topical therapy alone often fails to fully clear it, and the American Association of Feline Practitioners recommends treating all cats in a household when chlamydial conjunctivitis is confirmed, since it spreads easily between housemates (American Association of Feline Practitioners, 2021). Steroid-containing eye medications are avoided or used only under close veterinary supervision because they can worsen an undiagnosed herpesvirus infection or a corneal ulcer.

What can owners do at home while the eye heals?

Gently wiping away discharge with a warm, damp, clean cloth several times a day keeps crust from gluing the eyelids shut and makes the cat more comfortable between medication doses. Medication should always be given for the full course prescribed, even after the eye looks normal, because stopping early is a common reason bacterial conjunctivitis relapses within days.

An Elizabethan collar or soft recovery cone may be needed if the cat paws at the eye enough to risk self-trauma, and isolating a newly diagnosed cat from other cats in the household for the first several days reduces transmission risk, particularly with suspected herpesvirus or chlamydial infection. Reducing stress — keeping routines predictable, providing hiding spots, and minimizing environmental changes — also matters, since stress is a well-documented trigger for herpesvirus flare-ups in cats with latent infection (Cornell Feline Health Center, 2021).

Will this keep coming back, and can it be prevented long-term?

Most straightforward bacterial or chlamydial conjunctivitis resolves fully within 2 to 3 weeks of appropriate treatment and doesn't recur. Herpesvirus-associated conjunctivitis is different: because the virus becomes latent in nerve tissue after the first infection, cats can experience repeat flare-ups throughout life, especially during stressful periods, and Cornell Feline Health Center notes that recurrence is common enough that owners of FHV-1-positive cats should watch for early squinting or discharge and start treatment quickly at the first sign (Cornell Feline Health Center, 2021).

Core vaccination against feline herpesvirus and calicivirus, recommended by the American Association of Feline Practitioners as part of routine kitten and adult vaccine series, reduces the severity of herpesvirus-related conjunctivitis even though it doesn't prevent latent infection outright (American Association of Feline Practitioners, 2021). Keeping litter boxes, food bowls, and bedding clean, and quarantining new cats for at least 2 weeks before introducing them to an existing household, lowers the chance of introducing chlamydial or mycoplasmal infections that spread quickly in multi-cat homes.