When to see a vet urgently
A cat with eye symptoms needs prompt assessment. Particularly urgent signs include:
- Your cat keeps an eye closed or rubs it vigorously, or the eye's surface looks cloudy.
- Your cat breathes with its mouth open or breathing is clearly laboured.
- A kitten or adult cat becomes weak and does not eat or drink normally.
Do not wait for eye discharge to become pus-like before contacting a vet. You can find a vet near you.
Overview
Chlamydia felis is a bacterium that multiplies inside cells of the eye and respiratory tract linings. In cats, infection most often appears as chronic conjunctivitis and less often as systemic disease. It spreads easily through contact between animals, but survives poorly in the environment and is rapidly inactivated.
The conjunctiva is a thin mucous membrane on the inner eyelids and around the edges of the front of the eyeball. When inflamed, it becomes red and swollen. The third eyelid, visible at the inner corner, may also become more prominent. The conjunctiva helps spread tears over the eye during blinking and has local defence functions too. Conjunctivitis does not itself mean that the clear cornea is ulcerated.
Red eyes alone do not identify the cause. Herpesvirus, a foreign body or a structural eyelid problem, among other things, can produce similar symptoms. Chlamydiosis cannot be recognised from discharge colour alone.
Causes
The main sources are affected cats and carriers of Chlamydia felis. Transmission routes include:
- direct contact with an infected cat;
- droplets from sneezing or coughing;
- care equipment, food bowls and litter trays;
- from mother to kittens.
Transmission usually requires close contact, and transfer of eye discharge between cats is a key route. Spread through equipment is possible, particularly in heavily contaminated group housing. This is not usually an infection persisting in the environment for long periods in the way some resilient viral diseases do. When assessing mother-to-kitten transmission, close nursing contact must be considered; finding bacteria in the reproductive tract alone does not prove a particular route.
Risk factors
- living in a shelter or breeding establishment;
- kittens under one year old;
- crowded conditions with many animals together;
- stress, including moving home, and weakened immune defences;
- lack of vaccination in a high-risk environment.
Young age and close group living are the clearest practical risks. Stress or reduced immunity in the list cannot confirm an individual cat's infection, and information on their independent effects is limited. Older cats can also become ill. Lack of vaccination becomes relevant when there is a real exposure risk in the environment.
Typical symptoms
Chlamydiosis most often causes eye symptoms in cats:
- eye inflammation, including conjunctivitis, redness and swollen eyelids;
- mucus-like or pus-like eye discharge;
- sneezing and a runny nose;
- coughing, less commonly;
- fever;
- tiredness and reduced appetite.
Symptoms may begin in one eye and later spread to both. Initially clear watering can become thick discharge. A swollen conjunctiva can look very dramatic even while the cat remains lively and eats. Normal appetite therefore does not exclude infection.
Nasal symptoms alone without eye signs are less consistent with chlamydiosis. A corneal ulcer is not typical. If the eye's surface is damaged, other causes or a concurrent infection are investigated too.
Severe cases may involve:
- lung damage, including pneumonia in kittens;
- lameness from reactive arthritis;
- reproductive disorders, rarely.
Pneumonia is rare in chlamydiosis and should not be regarded as a usual consequence. Evidence linking lameness and reproductive disorders specifically to this bacterium is weak. These symptoms need their own assessment and should not automatically be attributed to chlamydiosis.
Complications and consequences
Without treatment, chlamydiosis can lead to:
- chronic conjunctivitis;
- reduced immune defences and secondary diseases;
- pneumonia;
- reduced quality of life because of ongoing eye inflammation.
The most common persistent problem is continuing or recurrent eye inflammation and discomfort. Not every cat develops the serious consequences listed. However, an untreated cat can shed the bacterium for a long time even when symptom severity varies. A new flare-up does not necessarily mean an entirely new eye disease.
It is useful to know that Chlamydia felis may also infect people, making it a potential zoonosis, but human infections are rare and usually cause mild conjunctivitis.
Diagnosis at the vet
Diagnosis is confirmed using:
- eye discharge smears and cytology;
- PCR testing to detect Chlamydia felis;
- culture, rarely because it is difficult;
- exclusion of other causes of conjunctivitis, including FHV-1, FCV and mycoplasmas.
Sample quality and examination of the whole eye
PCR looks for the bacterium's genetic material, or DNA. Because it lives inside cells, the sample needs cells from the conjunctival surface, not just discharge from the eye corner. Details of sampling and previous medication help interpretation. Not seeing the bacterium microscopically does not rule out infection, as intracellular changes are not always detected.
Even a positive result is assessed alongside symptoms: the bacterium can sometimes be detected in a cat without symptoms. Several causes of eye inflammation may be present together. Antibody testing is generally not the first choice for investigating a current eye problem.
The vet also checks the cornea and eyelids. Fluorescein staining helps find a corneal ulcer, and tear production or eye pressure is assessed according to the situation. This avoids treating a painful eye solely as presumed chlamydiosis.
Treatment
- the first-choice antibiotic is oral doxycycline, prescribed by the vet;
- all household cats are treated simultaneously, including those without symptoms, otherwise infection can return;
- water or a little food is given after a doxycycline tablet, as a tablet lodged in the oesophagus can cause an oesophageal stricture;
- eye ointments help as additional treatment, but without oral treatment the bacterium remains in the body.
Carrying out treatment and monitoring recovery
The antibiotic treatment described above is not an instruction to start medication yourself. The vet selects the product and treatment duration according to the cat's situation. Eyes may look better before infection is controlled, so do not stop prescribed treatment on your own. Considering contact cats without symptoms reduces the chance of reinfection.
Oral treatment reaches bacteria beyond the eye's surface. A topical eye product alone is therefore generally insufficient to eliminate infection. If giving medication is not possible, ask for practical help or a suitable alternative. Do not leave a cat to swallow a dry tablet; individual instructions are provided to support safe swallowing.
Monitor the amount of discharge, how well the eyes stay open and appetite. Report increasing pain, cloudiness or stalled improvement. Do not use drops left over from a previous eye infection without reassessment. A steroid-containing product, for example, can be harmful with a corneal ulcer.
The prognosis with appropriately selected treatment is generally good. Recurrent symptoms require reassessment of medication administration, contact cats and other eye diseases. Treatment response is not judged solely because one eye already looks healthy.
Prevention
- risk-based vaccination: Chlamydia vaccination is not a core vaccination, but is recommended in breeding establishments and households with several cats where disease has already been identified;
- isolation of sick animals;
- regular disinfection of premises and care equipment;
- reducing crowding;
- monitoring cats' immune health.
Vaccination may reduce symptom severity, but does not guarantee that a cat will not become infected or spread the bacterium. Vaccinated cats with symptoms therefore still need examination. Vaccination does not replace treatment of a confirmed infection either.
Wash your hands after handling discharge and before caring for another cat. Wash contaminated bedding and equipment with cat-safe cleaning products too. In a household with several cats, coordinate isolation and cleaning with the shared treatment plan so contacts without symptoms are considered. A visibly tidy room does not show that no infectious cat is present in the group.
Important for cat owners
Chlamydiosis is a common infection in cats, particularly in crowded conditions. Remember that it reduces quality of life and can occasionally infect people. The most important preventive measures are isolation of sick cats and less crowded conditions, with vaccination in breeding establishments where disease has occurred. At the first signs of conjunctivitis and sneezing, take your cat to a vet for an accurate diagnosis and treatment.
This article provides general information and does not replace veterinary care. If you notice signs of illness in your cat, contact a vet.
Frequently asked questions
Can chlamydiosis affect only one eye?
Yes. Symptoms often begin on one side even if both eyes later become inflamed. However, one affected eye does not confirm chlamydiosis, as a foreign body or corneal damage, for example, can cause similar squinting and discharge.
Do cats without symptoms need to be considered in treatment?
Yes. A cat living in the same home can sustain infection without visible symptoms. Tell the vet about every contact cat so investigation and treatment of the whole group can be planned together. Do not share one cat's prescription medicine with others on your own.
Is feline Chlamydia the same as the human sexually transmitted infection?
No. Ordinary human chlamydial infection and feline Chlamydia felis infection are caused by different bacteria. Cat-associated eye infection in people is extremely rare. If someone in the household develops eye symptoms, they should also mention the cat's confirmed infection to their healthcare professional.
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.
- Chlamydial Conjunctivitis in Cats, VCA Animal Hospitals, accessed 2026-11-03.
- Chlamydial Conjunctivitis in Animals, Merck Veterinary Manual, accessed 2026-11-03.
- Chlamydia Felis in Cats, Cats Protection, accessed 2026-11-03.
- Conjunctivitis, Cornell Feline Health Centre, accessed 2026-11-03.
- Conjunctivitis in Cats, VCA Animal Hospitals, accessed 2026-11-03.
- Corneal Ulcers in Cats, VCA Animal Hospitals, accessed 2026-11-03.