Conjunctivitis in cats: causes, treatment and when it is not conjunctivitis
Conjunctivitis is the most common eye problem in cats, and it nearly always starts with a virus many cats carry for life. It responds well to treatment, but there are two common mistakes: using eye drops from home, which can make an ulcer worse, and mistaking for conjunctivitis an eye that actually has something more serious.
What it is and what it looks like
Conjunctivitis is inflammation of the conjunctiva, the pink membrane lining the inside of the eyelids and the white of the eye. A cat with conjunctivitis has a red eye, swollen membranes (sometimes so swollen that it looks as if flesh is coming out of the eye), watering or discharge that may be clear, yellowish or greenish, and it squints and rubs the eye. It can affect one or both eyes. It often comes with sneezing and nasal discharge, because the most common infectious causes attack the eyes and the airways at the same time. If what you mainly see is chronic watering without redness in a flat-faced cat, it is probably something else: we cover it in the article on watery eyes.
The main cause: feline herpesvirus
Feline herpesvirus type 1 is responsible for a large share of conjunctivitis in cats, and forms part of what is known as the feline respiratory disease complex, or cat flu, together with calicivirus. Almost every cat is infected at some point, often as a kitten, and the virus stays latent in the nerves for life. At times of stress, such as a house move, an illness or a change at home, it can reactivate and cause a new episode. In kittens and young cats the first infection is usually severe, with fever, nasal discharge and eyes stuck shut; in adults, flare-ups tend to be milder. Herpesvirus can also cause corneal ulcers, which is why any feline conjunctivitis deserves a stain test to check for one.
Chlamydia, mycoplasma and other causes
Chlamydia felis is a bacterium that causes persistent conjunctivitis, often starting in one eye and spreading to the other, with very swollen conjunctiva and few respiratory signs. It is more common in young cats living in groups and responds well to oral veterinary antibiotic treatment, which is usually extended to every cat in the household. Mycoplasmas can also cause it. Non-infectious causes include foreign bodies, such as a seed or grass awn under the eyelid, which usually affect one eye suddenly; misplaced eyelashes or eyelids, such as entropion, more common in Persians; allergies; and irritation from smoke, cleaning products or litter dust.
In newborn kittens
Kittens open their eyes at around seven to ten days old. If before that you see swollen, bulging eyelids, or pus coming from the slit, there is an infection behind the closed lids, ophthalmia neonatorum, and you need to see the vet without delay: it can damage the cornea permanently. In slightly older kittens whose eyes are already open, it is very common to find their eyes stuck together with dried discharge. You can clean them gently with gauze soaked in lukewarm sterile saline, wiping from the inner corner outwards and using a fresh piece for each eye, but that does not replace a vet visit: a kitten with conjunctivitis usually has a respiratory infection too and may stop eating.
When it is not conjunctivitis (and is urgent)
Some eye problems look like conjunctivitis and are far more serious. See the vet the same day if the cat keeps the eye completely shut and will not open it, if the surface of the eye looks cloudy, bluish or has a spot on it, if one pupil differs from the other, if the eye looks bigger or sunken, if there is blood inside the eye, if the eye has changed colour, or if there has been a knock or a scratch from another cat. It could be a deep ulcer, uveitis (inflammation inside the eye that is sometimes a sign of diseases such as FIP, FeLV or FIV), glaucoma or a penetrating wound. An eye can be lost within days.
What the vet will do
The vet will examine the eyes with light and magnification, look under the eyelids and behind the third eyelid for foreign bodies, and apply a drop of fluorescein, a dye that stains corneal ulcers green when they are invisible to the naked eye. Tear production and eye pressure may be measured. In persistent conjunctivitis or in groups of cats, a PCR test on a conjunctival sample helps establish whether it is herpesvirus, chlamydia or mycoplasma. Depending on the result, treatment may be a veterinary antibiotic ointment or eye drops, an oral antibiotic if it is chlamydia, antiviral eye treatment if there is a herpes ulcer, or simply cleaning and monitoring if it is a mild flare-up.
What it costs in Spain in 2026
As a guide: consultation with fluorescein test, €40 to €70; consultation with a veterinary ophthalmologist, €70 to €130; conjunctival PCR, €50 to €120; tonometry, €20 to €40; veterinary eye drops or ointments, €10 to €30; antiviral eye treatment, €20 to €50. Oral treatment for chlamydia for several cats can cost around €30 to €80. If there is a deep ulcer or a corneal sequestrum that needs surgery, €500 to €1,500. A simple, well-treated conjunctivitis usually comes to under €100.
What not to do
Do not use eye drops from your medicine cabinet or human pharmacy products, and above all never drops containing corticosteroids unless the vet has ruled out an ulcer: in a cat with herpesvirus, a corticosteroid can turn a superficial ulcer into a perforation. Do not use camomile or herbal infusions in the eye, because the residue irritates and can become contaminated. No home remedies with salt or vinegar either. Do not use drops left over from the dog or another cat. And do not let a red eye that is not improving go for more than a couple of days. Lysine, very popular for feline herpes some years ago, has not proved effective in studies, and current guidelines no longer recommend it.
Cats with repeated flare-ups
A cat carrying herpesvirus can have bouts of conjunctivitis all its life, usually linked to stress. The best tool is reducing that stress: stable routines, hiding places, high spots, slow introductions to new animals, gradual changes. For flare-ups, ideally have a plan agreed with your vet so you know what to do and when to go back. For severe flare-ups or repeated ulcers, there are oral and topical antiviral treatments the vet can prescribe. And vaccination, although it does not prevent infection, greatly reduces the severity of episodes.
Prevention
Up-to-date core vaccinations: the trivalent vaccine protects against herpesvirus, calicivirus and panleukopenia, and although it does not completely prevent infection, it makes episodes much milder. Newly adopted kittens should spend a few days apart from the other cats at home. In homes with many cats, good ventilation, enough food bowls and litter trays and frequent cleaning reduce spread. Avoid smoke and aerosols. And in flat-faced breeds such as the Persian or the Exotic, clean the folds around the eyes daily with gauze and saline, because they are more prone to irritation.
Most feline conjunctivitis is caused by herpesvirus and flares up with stress. It needs a stain test to rule out ulcers before treatment. Never use eye drops with corticosteroids without a vet, and go to the emergency vet if the eye is cloudy, fully closed or the pupils differ.
Educational content from Michipedia, translated from the Spanish original. It does not replace the advice of a vet or a feline behaviour specialist. Every cat is an individual.