Important: primary bacterial conjunctivitis is rare in cats. Conjunctivitis is most often caused by herpesvirus (FHV-1), Chlamydia felis and mycoplasmas, with staphylococci and streptococci becoming involved secondarily. Antibiotic eye drops should therefore not be used just in case without an examination: treatment is selected according to the cause.
The reference to rarity means a primary infection with ordinary bacteria, not that bacteria cannot start the disease. Chlamydia felis is a bacterium, and mycoplasmas can also cause primary inflammation. They should therefore not be interpreted as non-bacterial alternatives to viral disease.
When to see a vet urgently
Contact a vet promptly if an eye problem involves any of the following:
- your cat keeps the eye tightly closed or rubs it constantly because of pain;
- the surface of the eye becomes cloudy or has visible damage;
- the eye has been injured;
- the conjunctiva or third eyelid swells enough to cover the eye.
Overview
The conjunctiva is a thin mucous membrane covering the inner surfaces of the eyelids and part of the eyeball. Normally, it protects the eye from microbes and dust. A bacterial infection causes the conjunctiva to become inflamed, swollen and produce pus-like discharge. The condition may affect one or both eyes and can be acute or chronic.
Conjunctiva also covers the third eyelid, the membrane visible at the inner corner of the eye. Inflammation may therefore be particularly obvious there. The conjunctiva and cornea are different structures: the cornea is the clear front surface of the eye. As well as examining the red conjunctiva, the vet needs to establish whether this surface is damaged too.
Pus-like discharge is consistent with bacterial involvement, but its colour or thickness alone cannot distinguish bacterial, viral and mixed infections. Discharge can also become thick during a viral infection. The eye's appearance therefore cannot safely tell you which medicine would help or whether antibiotics are needed.
Causes
Common pathogens in cats include:
- staphylococci,
- streptococci,
- Chlamydia,
- Pasteurella multocida,
- Mycoplasma species.
Predisposing factors include:
- eye injuries, scratches and foreign bodies,
- weakened immunity,
- chronic respiratory infections,
- contact with sick animals,
- poor hygiene in the living environment.
The microbes listed are not equally important as causes. Some bacteria are also part of the normal microbial community on a healthy eye's surface and only cause problems alongside a viral infection, foreign body or eyelid abnormality, for example. Finding a bacterium alone is therefore not enough to show that it explains the symptoms.
Chlamydia felis lives inside host cells and usually spreads through eye discharge during close contact. It does not survive well outside the cat. In an environment with several cats, direct contact and transfer of secretions help infection spread. However, an eye can also become red without bacterial infection, through dust or chemical irritation, for example.
Risk factors
- kittens and elderly cats,
- cats living in shelters or breeding establishments,
- animals with chronic respiratory infections,
- cats recovering from illness or otherwise weakened.
Any cat can develop conjunctivitis, but infectious conjunctivitis is seen particularly in young cats and environments with several cats. Ageing is not a shared specific risk factor for all the bacterial infections listed. Chlamydia infection, for example, predominantly affects young animals. Risk factors are therefore assessed according to the suspected cause, contacts and other aspects of health.
Symptoms
Typical symptoms include:
- redness and swelling of the conjunctiva,
- thick, pus-like discharge, yellow or green in colour,
- eyelids and eyelashes sticking together after sleep,
- itching and rubbing the eyes with a paw,
- increased tear production,
- sensitivity to light,
- reduced appetite and activity because of feeling generally unwell.
Chlamydial conjunctivitis may begin in one eye and later spread to both. The discharge may initially be clear and watery. Marked swelling of the conjunctiva and redness of the third eyelid are consistent with the disease, but do not confirm it. Your cat may otherwise eat and behave fairly normally, so good general health does not rule out an eye infection.
At the appointment, explain which eye developed symptoms first and whether other cats at home have similar signs. Mention sneezing, nasal discharge and previous eye problems too. Squinting can be a sign of pain. It does not show whether the problem is confined to the conjunctiva, as a corneal ulcer often causes similar closing of the eye.
Possible complications
Without treatment, bacterial conjunctivitis can lead to:
- chronic inflammation,
- corneal damage, including keratitis and ulcers,
- reduced vision or loss of sight,
- infection spreading to other structures of the eye.
These serious consequences are not the inevitable progression of ordinary superficial inflammation. Corneal ulcers are not usually part of chlamydial conjunctivitis. If the cornea is damaged, herpesvirus, injury or another concurrent disease should be investigated, for example. A bacterial infection developing on top of viral damage can deepen a corneal lesion and threaten the eye.
Diagnosis at the vet
Diagnosis is based on:
- a clinical examination,
- sampling and bacterial culture,
- cytological examinations to identify the pathogen,
- eye examinations to assess the cornea and the back of the eye.
What does the eye examination investigate?
As well as the surface of the eye, the vet checks the eyelids and their inner surfaces, looking for a foreign body or a structure rubbing against the eye. Tear production and eye pressure are measured when needed. These can help investigate irritation related to dryness or another eye disease resembling conjunctivitis, for example. Fluorescein staining is used to look for damage to the corneal surface, as a small ulcer is not always visible to the naked eye.
Routine bacterial culture and PCR testing answer different questions. Culture and susceptibility testing can help select a medicine for an ordinary bacterial infection. Chlamydia is usually investigated with a PCR test that looks for the microbe's genetic material. The sample must also contain conjunctival cells. Cytology, the microscopic examination of cells, can show inflammation and bacteria, but cannot reliably identify every cause.
Before sampling, mention every eye medicine already used, as a topical antibiotic may reduce the chance of a successful culture. Where possible, samples are planned before medication starts. A negative herpesvirus PCR result alone does not rule out its involvement, because the virus is not always being shed at the time of sampling. A positive result alone does not establish the cause of symptoms either. Examination findings, the course of symptoms and sample results are assessed together.
Treatment and follow-up
Treatment targets the confirmed or reasonably suspected cause. An ordinary bacterial infection may be treated with a topical antibiotic chosen by the vet. Chlamydial infection usually requires systemic antibiotic treatment, so eye drops alone may not be enough to eliminate the infection. Significant herpesvirus-related eye disease may also require antiviral medication. An antibiotic does not eliminate a virus.
During treatment, pain control and gentle cleaning of discharge can improve comfort. Ask the practice to show you how to give the prescribed medicine. If you cannot handle the eye or administer medication safely, say so: medication that cannot be given in practice will not help. Do not switch to a previously used product on your own, as a similar-looking symptom may have a different cause this time.
Steroid-containing eye medicines can slow the healing of a corneal ulcer and cause serious complications. They should not be started on an unexamined red eye without veterinary advice. Rubbing the eye can also worsen damage, and the vet may recommend a protective collar. If pain increases or the eye becomes cloudy, contact the practice again without waiting for the scheduled review.
The prognosis depends on the cause and whether the cornea is healthy. The outlook for treated chlamydial conjunctivitis is generally good. Herpesvirus can remain dormant in the body and reactivate later, so recurrence does not always mean a new bacterial infection. Continue treatment as prescribed even if the discharge decreases sooner. Where needed, repeat staining confirms that any corneal damage has healed.
Prevention
- maintain good hygiene in your cat's living environment,
- provide appropriate eye care after injury or irritation,
- arrange regular veterinary checks,
- vaccinate against infections that may predispose to conjunctivitis, including herpesvirus and calicivirus,
- avoid contact with sick animals.
Vaccinations can reduce the severity of symptoms of certain infectious diseases, but do not protect against all conjunctivitis or guarantee prevention of infection. The need for a Chlamydia vaccine is assessed separately according to exposure; it is not automatically needed for every cat. Limiting a sick cat's contacts and preventing the spread of discharge complement vaccine protection.
If infection is suspected, keep the sick cat's care supplies separate from those of other animals and clean your hands between handling them. In a home with several cats, tell the vet about all contacts: in some Chlamydia situations, treatment of contact cats is also considered to prevent reinfection. However, the same medicines are not started for every cat simply because one has a red eye.
This text provides general information and does not replace veterinary care. If your cat develops symptoms, contact a vet.
Frequently asked questions
Can you tell from the discharge whether bacteria caused the inflammation?
Not with certainty. Thick yellowish discharge can be associated with bacterial infection, but also occurs with other infections. An examination checks for problems such as corneal damage and investigates the cause of inflammation before a medicine is selected.
Can a cat's eye infection spread to another cat?
Yes, if the cause is infectious. Chlamydia, for example, usually spreads through eye discharge during close contact. Inflammation caused solely by irritation or eyelid structure does not spread in the same way. The vet advises on the necessary contact restrictions according to the cause.
Why does the eye become red again after antibiotic treatment?
Possible reasons include an untreated underlying problem or reactivation of dormant herpesvirus. Recurrence alone therefore does not show that the antibiotic was ineffective. Re-examination reassesses the diagnosis and corneal condition; old drops should not automatically be restarted.
This article is for general information only and is not a guide to treatment. If you notice the first signs of illness, contact your vet.
Paw4care editorial team
Sources
- GUIDELINE for Chlamydia felis, European Advisory Board on Cat Diseases, accessed 2026-11-03.
- Conjunctivitis, Cornell University, accessed 2026-11-03.
- AntibioticScout: Katze – Konjunktivitis, Universität Zürich, accessed 2026-11-03.
- Conjunctivitis in Cats, VCA, accessed 2026-11-03.
- Corneal Ulcers in Cats, VCA, accessed 2026-11-03.
- GUIDELINE for Feline Herpesvirus infection, European Advisory Board on Cat Diseases, accessed 2026-11-03.
- Feline Herpesvirus: Therapeutic Review, MSPCA-Angell, accessed 2026-11-03.