When to see a vet urgently
A red or discharging eye needs prompt assessment, particularly if:
- Your cat keeps it closed or has marked pain and light sensitivity.
- The surface becomes cloudy, pupils differ in size or vision suddenly worsens.
- A sharp object has struck the eye or damage is visible.
- Eye symptoms occur with obvious weakness or not eating.
Do not wait for pus-like discharge before making contact. You can find a vet near you.
Overview
The conjunctiva protects and moistens the eye and helps prevent microbes entering. Inflammation disrupts its function, causing discharge, swelling and light sensitivity. Conjunctivitis can affect one or both eyes, be acute or chronic, and be infectious, allergic, traumatic or secondary to another eye disease.
The conjunctiva lines the inner eyelids and part of the eyeball surface. It helps spread tears during blinking and contributes to local defences. However, it does not provide complete protection against infections. The third eyelid visible at the inner corner can also swell with inflammation.
The conjunctiva and cornea are different structures. The cornea is the clear front surface of the eye, and damage can cause similar squinting and discharge. Visible redness is therefore not automatically treated as a problem confined to the conjunctiva.
Causes
The most common causes of conjunctivitis in cats include:
- Infections: viruses including FHV-1 and calicivirus, and bacteria including Chlamydia felis and mycoplasmas
- Bacterial infections involving staphylococci, streptococci and mycoplasmas
- Allergies to dust, pollen, household chemicals or cat litter
- Irritation from smoke, dust or chemicals
- Eye injuries and foreign bodies
- Parasites, such as Thelazia
- Systemic diseases, including immune system or autoimmune diseases
Not every bacterium listed is an equally common primary cause. Staphylococci and other bacteria may be involved in secondary inflammation after an injury, foreign body or viral infection, for example. Finding bacteria alone therefore does not always explain how the eye problem began.
An allergic reaction and chemical irritation are different too. Smoke or dust can irritate without a confirmed allergy. With prolonged or recurrent symptoms, eyelid position, the cornea and other local causes are investigated too. Several factors may sustain the problem in the same cat.
Risk factors
Conjunctivitis is more common in:
- Kittens, with weaker immune defences and high infection risk
- Cats living in shelters or boarding facilities
- Unvaccinated cats
- Cats with chronic viral infections, including FHV-1, FIV and FeLV
- Cats living in dusty or smoky spaces
Young age and environments with several cats particularly increase the relevance of infectious eye disease. However, conjunctivitis in a kitten alone does not establish true immunodeficiency. Risk factors differ between causes, and vaccination does not protect against every listed pathogen or structural eye problem.
Main symptoms
Typical signs of conjunctivitis include:
- Red eyes and swollen conjunctiva
- Increased tears or pus-like discharge
- Eyelids sticking together after sleep
- Light sensitivity and squinting
- Itching, with paw rubbing of the eyes
- Reduced appetite and tiredness in infectious cases
Both watery and pus-like discharge can accompany eye disease needing investigation. Colour alone does not distinguish herpesvirus, Chlamydia and other causes. Herpesvirus may also involve nasal symptoms and corneal damage; Chlamydia infection may produce particularly marked conjunctival swelling and begin in one eye.
Appetite and activity may remain normal despite eye symptoms. Conversely, impaired vision, cloudiness or a pupil change is unsuitable for home monitoring as simply mild superficial inflammation. These may involve internal eye inflammation or altered pressure, for example.
Complications and consequences
Without treatment, conjunctivitis can lead to:
- Chronic inflammation
- Corneal ulcers
- Adhesion of the conjunctiva to the cornea, called symblepharon, particularly in herpesvirus infection
- Impaired vision
A corneal ulcer may be the original cause of redness or occur with concurrent infection. It is not an inevitable consequence of every conjunctivitis case. Likewise, vision risk depends on which structures are damaged and disease severity.
After infection, herpesvirus can remain dormant and reactivate later. Returning symptoms therefore do not always mean a new infection acquired from elsewhere. Persistent or changed symptoms require reassessment of other causes too, rather than attributing everything to a previous herpesvirus diagnosis.
Diagnosis at the vet
Diagnosis is based on:
- An eye examination
- Discharge sampling and cytology
- PCR testing and bacterial culture to identify infection
- A Schirmer tear test to assess tear production
- Fluorescein testing to rule out corneal damage
The surface and internal eye are assessed separately
Examination also checks beneath the third eyelid and other eyelids for foreign bodies. Fluorescein staining helps identify corneal surface damage. The Schirmer test measures tear production, and eye pressure measurement looks for conditions including glaucoma or changes consistent with internal inflammation.
Not every cat needs the same samples. Cytology shows inflammatory cells and may reveal microbes. Culture and susceptibility testing are considered particularly for suspected bacterial infection, with sampling preferably before topical antibiotics begin. Tell the person collecting samples about previous eye medication.
PCR detects a pathogen's genetic material. Testing for Chlamydia requires enough conjunctival cells because the bacterium lives inside cells. A positive herpesvirus result alone does not prove it causes the current symptoms, while a negative result does not reliably exclude infection. Results are compared with symptoms and other findings.
If systemic disease is suspected, a general examination and blood tests may also be needed. Abnormal tissue may be biopsied. The aim is to ensure an eyelid abnormality, tumour or another problem requiring separate treatment is not left underlying recurrent redness.
Treatment and follow-up
Treatment depends on the cause
A topical antimicrobial may be prescribed for confirmed or reasonably suspected bacterial infection. However, antibiotics do not eliminate herpesvirus or remove a foreign body. Viral eye disease may need an antiviral selected by the vet, alongside treatment of any corneal damage and pain.
In Chlamydia infection, eye treatment alone is generally insufficient to eliminate infection. The vet also assesses oral treatment and the needs of contact cats in the household. Do not share or swap medicines between cats on your own.
If symptoms result from eyelid malposition or another structural cause, correction may be needed. Internal eye inflammation and glaucoma need their own treatment. A product that helped a previous eye infection may therefore be unsuitable for a new symptom.
Safe use of eye medicines
Use only a product prescribed for the current situation. Steroid-containing eye medication can worsen a corneal ulcer and delay healing. Local anaesthetic used during examination is also not used for home pain relief. Do not treat the eye with old drops or those prescribed for another animal.
Ask for a practical demonstration of drops or ointment if your cat resists. The bottle tip must not touch the eye or hairs. If you cannot carry out the prescribed administration, tell the practice so treatment can be made practical. Remove discharge gently according to the instructions provided.
Monitor eye opening, discharge quantity and general condition. Do not change instructions simply because redness decreases. Increasing pain, cloudiness or worsening vision needs reassessment. Follow-up can also confirm corneal healing, which is not always apparent from outward condition.
Prognosis varies with the cause. Many superficial inflammations can be controlled, but herpesvirus, for example, may cause recurrent signs. Treatment and monitoring may therefore continue differently even in two cats whose eyes initially looked similar.
Prevention
To reduce conjunctivitis risk:
- Keep vaccination against respiratory infections up to date
- Keep the home clean and avoid dust and smoke
- Choose safe litter and toys
- Monitor chronic eye diseases and viral infections
- Avoid injuries and contact with affected animals
Vaccinations reduce disease severity from certain respiratory viruses, but do not guarantee complete prevention of infection or eye symptoms. Contact instructions for an affected cat depend on the suspected cause. Wash your hands after handling discharge and avoid contaminating bottle tips when giving medicines.
Reducing tobacco smoke and dust supports eye health even without an allergy diagnosis. A new one-sided eye problem in an otherwise healthy-looking cat needs examination and should not automatically be attributed to familiar household dust. Prevention does not replace examination of a painful eye.
Important for cat owners
Conjunctivitis is common and usually treatable in cats, but can cause serious complications without care. Remember: redness, discharge and light sensitivity warrant a veterinary visit. Earlier treatment improves the chances of preserving eye health.
This material is for information and does not replace treatment instructions. Contact a vet as soon as the first signs of illness appear.
Frequently asked questions
Why does one of my cat's eyes have discharge but the other does not?
Several causes may begin on one side, such as a foreign body, corneal damage or Chlamydia infection. The side affected alone does not determine the diagnosis. Pain, cloudiness or keeping the eye closed increases urgency.
Can I use previously prescribed eye drops?
Not without reassessment. Similar redness may result from a different disease, and some drops can harm a corneal ulcer. Tell the vet about previous medication and follow instructions for the current situation.
Do antibiotics cure every eye infection in cats?
No. They do not treat viruses, eyelid abnormalities, foreign bodies or eye pressure disorders. In bacterial infection, the need and medicine choice depend on findings. Recurrence or incomplete response requires review of the cause.
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
- Conjunctivitis, Cornell Feline Health Centre, accessed 2026-11-03.
- Conjunctivitis in Cats, VCA Animal Hospitals, accessed 2026-11-03.
- GUIDELINE for Feline Herpesvirus infection, European Advisory Board on Cat Diseases, accessed 2026-11-03.
- GUIDELINE for Chlamydia felis, European Advisory Board on Cat Diseases, accessed 2026-11-03.
- Corneal Ulcers in Cats, VCA Animal Hospitals, accessed 2026-11-03.
- AntibioticScout: Katze – Konjunktivitis, Universität Zürich, accessed 2026-11-03.
- Uveitis in Cats, VCA Animal Hospitals, accessed 2026-11-03.