Bacterial Conjunctivitis in Cats: Symptoms, Treatment and When to See a Vet

 

Bacterial Conjunctivitis in Cats: Symptoms, Treatment and When to See a Vet

By Dr Duncan Houston

A red, swollen or sticky eye in a cat is often described as bacterial conjunctivitis. Sometimes that is correct. In other cases, bacteria are only one part of the problem.

Chlamydia felis and Mycoplasma felis can cause primary conjunctivitis, particularly in kittens and cats living in groups. Bacteria may also take advantage of tissue already damaged by feline herpesvirus, a corneal ulcer, trauma or an eyelid abnormality.

What matters most is not simply whether discharge is present. It is whether the eye is painful, whether the cornea remains clear and whether the underlying cause has been identified.

Quick Answer

Bacterial conjunctivitis causes redness, swelling and discharge from one or both eyes. Important feline causes include Chlamydia felis, Mycoplasma felis and secondary bacterial infection complicating herpesvirus or other eye disease.

A veterinary eye examination is needed to rule out a corneal ulcer, foreign body, uveitis, glaucoma and other painful conditions before treatment is selected. An eye that is held shut, appears cloudy or blue, contains a visible white or dark spot, or seems severely painful needs same-day veterinary care.

What Is Conjunctivitis?

The conjunctiva is the thin membrane lining:

• The inner surfaces of the upper and lower eyelids

• The third eyelid

• Part of the surface surrounding the eyeball

Conjunctivitis means this tissue has become inflamed.

Inflamed conjunctiva may become:

• Bright pink or red

• Swollen

• Thickened

• Moist

• Prone to discharge

• More visible over the surface of the eye

Conjunctivitis is a clinical sign rather than one diagnosis. Infectious, traumatic, immune-mediated and structural problems can all produce a similar red, sticky eye.

What Is Bacterial Conjunctivitis?

Bacterial conjunctivitis occurs when bacteria infect or significantly contribute to inflammation of the conjunctiva.

It can be divided into two broad groups.

Primary bacterial conjunctivitis

The bacterium itself is a major cause of the disease.

The two most important recognised organisms are:

• Chlamydia felis

• Mycoplasma felis

Secondary bacterial conjunctivitis

Bacteria colonise an ocular surface already damaged by another condition, such as:

• Feline herpesvirus

• A corneal ulcer

• A foreign body

• Eyelid trauma

• Poor tear-film function

• Entropion

• Another chronic inflammatory eye disease

Co-infections are also common. A 2024 study of 255 shelter cats with active ocular-surface disease found frequent detection of Mycoplasma, feline herpesvirus, feline calicivirus and Chlamydia, with many cats testing positive for more than one organism. This is why one positive PCR result does not always provide the complete diagnosis.

What Are the Signs of Bacterial Conjunctivitis?

Possible signs include:

• Red or pink conjunctiva

• Swollen eyelids

• Swelling of the conjunctiva, called chemosis

• A prominent or inflamed third eyelid

• Clear or watery discharge

• White or grey mucous discharge

• Yellow or green discharge

• Crusting around the eyelids

• Frequent blinking

• Squinting

• Holding the eye partly closed

• Pawing or rubbing at the face

• Sensitivity to bright light

• Sneezing or nasal discharge

• Reduced appetite or lethargy in more severely affected cats

The problem may begin in one eye and later involve both.

Does Yellow or Green Discharge Prove It Is Bacterial?

No.

Thick yellow or green discharge increases suspicion for bacterial involvement, but discharge colour does not identify the organism or prove that bacteria started the problem.

Viral inflammation can attract large numbers of neutrophils, producing thick discharge even when a virus is the primary trigger. A cat with feline herpesvirus may also develop a secondary bacterial infection after the ocular surface has been damaged.

This is why antibiotics are selected according to the complete clinical picture rather than the discharge colour alone. Current feline infectious disease guidance recommends antibiotics for severe disease or suspected secondary bacterial infection, not automatically for every watery or red eye.

Is the Problem Limited to the Conjunctiva?

This is the most important early distinction.

Finding	More consistent with uncomplicated conjunctivitis	More concerning for corneal or internal eye disease
Eye position	Eye remains mostly open	Eye is held tightly shut
Corneal appearance	Clear, smooth and shiny	Blue, grey, white, rough, indented or dark
Pain	Mild irritation	Marked squinting, rubbing or light sensitivity
Discharge	Watery, mucous or mildly sticky	Discharge accompanied by severe pain or a visible corneal defect
Pupil	Similar to the other eye	Unusually large, small or irregular
Vision	Appears normal	Bumping into objects or sudden uncertainty
Urgency	Prompt examination if persistent	Same-day assessment

A corneal ulcer, uveitis or glaucoma may initially be mistaken for conjunctivitis. These conditions require a different treatment plan and can threaten vision.

What Causes Bacterial Conjunctivitis in Cats?

The main causes and contributing factors include:

1. Chlamydia felis

2. Mycoplasma felis

3. Secondary bacterial infection following feline herpesvirus

4. Corneal ulcers

5. Trauma or foreign material

6. Eyelid abnormalities

7. Tear-film disease

8. Overcrowding, stress and concurrent respiratory infections

Chlamydia felis

Chlamydia felis, previously called Chlamydophila felis, is an intracellular bacterium that primarily targets the conjunctiva.

It is particularly associated with:

• Kittens and young cats

• Cats younger than approximately nine months

• Shelters

• Breeding catteries

• Rescue environments

• Multi-cat households

• Recent introduction of a new kitten or cat

The infection often begins in one eye before progressing to both. Characteristic findings include marked conjunctival redness, swelling of the third eyelid, chemosis and discomfort. Discharge commonly begins watery and then becomes mucous or mucopurulent.

Does Chlamydia cause respiratory signs?

Respiratory signs are usually mild compared with the eye disease.

Some cats develop:

• Mild nasal discharge

• Sneezing

• Noisy nasal breathing

• Temporary fever

• Reduced appetite

A cat with respiratory signs but no conjunctivitis is less likely to have C felis as the primary cause.

Does Chlamydia cause corneal ulcers?

Corneal ulcers and keratitis are not typical features of uncomplicated C felis infection.

When a cat with suspected Chlamydia also has a corneal ulcer, feline herpesvirus, trauma or another cause should be considered.

How does Chlamydia spread?

Transmission normally requires close contact with infected ocular secretions.

The organism has poor survival outside the host, so direct contact is more important than long-term environmental contamination. Shared hands, grooming, bowls, bedding and equipment may still move fresh discharge between cats in crowded environments.

Mycoplasma felis

Mycoplasma felis can cause conjunctivitis and upper respiratory disease. It may act as a primary pathogen, complicate a viral infection or be detected in a cat without obvious disease.

Mycoplasmas have no cell wall. This feature affects which antibiotics can treat them because beta-lactam antibiotics, including penicillins and cephalosporins, do not act directly against organisms without a cell wall.

Possible signs include:

• Red conjunctiva

• Chemosis

• Clear or coloured eye discharge

• Nasal discharge

• Sneezing

• Lethargy

• Reduced appetite

• Coughing or breathing difficulty when lower respiratory disease is present

Is Mycoplasma always present in cats?

No.

Some Mycoplasma species may form part of the normal upper respiratory flora, and M felis can be found in healthy cats as well as diseased cats. It should not be described as universally present in every cat.

At the same time, recent evidence supports M felis as a genuine primary ocular pathogen in at least some patients. In the 2024 shelter study, Mycoplasma was frequently detected both alone and alongside other organisms in cats with active ocular disease.

The correct interpretation is therefore:

• Detection can be clinically important.

• Detection does not automatically prove causation.

• The amount and location of organism, clinical signs and presence of other pathogens all matter.

Secondary Bacterial Infection After Feline Herpesvirus

Feline herpesvirus type 1 is a major cause of conjunctivitis and corneal disease in cats.

Herpesvirus damages epithelial cells and may allow opportunistic bacteria to colonise the ocular surface. A cat may therefore have viral conjunctivitis with secondary bacterial involvement.

Clues supporting herpesvirus include:

• Recurrent episodes

• Recent stress or illness

• Sneezing or nasal discharge

• Corneal ulceration

• A branching or dendritic corneal lesion

• Previous herpes-related eye disease

• Disease that improves and later returns

Antibiotics may treat secondary bacteria, but they do not suppress herpesvirus replication. Antiviral treatment may also be required when active herpes disease is suspected, particularly when the cornea is involved.

Corneal Ulcers and Trauma

A scratch from another cat, plant material, dust, a foreign body or rubbing against a rough surface may damage the cornea or conjunctiva.

Bacteria can then enter the damaged tissue.

Possible clues include:

• Sudden onset

• Only one eye affected

• Marked pain

• A visible scratch or eyelid wound

• A cloudy or rough cornea

• Rapid progression

• History of a fight or outdoor activity

A bacterial corneal infection is more serious than uncomplicated conjunctivitis because infection may extend deeper into the cornea and threaten the structural integrity of the eye.

Eyelid and Tear-Film Problems

Conjunctivitis may continue returning when an underlying mechanical problem remains untreated.

These include:

• Entropion, where the eyelid rolls inward

• Abnormal eyelashes

• Eyelid masses

• Hair rubbing against the eye

• Reduced tear production

• Poor tear quality

• Incomplete eyelid closure

• Nasolacrimal drainage problems

Antibiotic drops may temporarily reduce discharge while the physical cause continues irritating the eye.

Which Cats Are Most at Risk?

Risk is greater in:

• Kittens

• Recently adopted cats

• Shelter or rescue cats

• Cats from breeding establishments

• Cats living in crowded multi-cat environments

• Cats exposed to a new cat with eye or respiratory signs

• Cats under substantial stress

• Cats with feline herpesvirus

• Immunocompromised cats

• Cats with eyelid or tear-film abnormalities

C felis is particularly associated with young cats and group living. Mycoplasma transmission and disease are also promoted by overcrowding, stress and concurrent respiratory infections.

Is Bacterial Conjunctivitis Contagious?

It depends on the cause.

Chlamydia felis

C felis is contagious between cats through close contact with infected secretions.

Mycoplasma felis

M felis can spread directly through close contact, aerosols and grooming. It survives poorly away from the host, so direct exposure is more important than environmental persistence.

Secondary bacterial infection

A secondary infection caused by an opportunistic organism may not spread in the same way. However, the underlying herpesvirus or respiratory infection may be contagious.

Trauma or eyelid disease

Conjunctivitis caused by trauma, a foreign body or an eyelid abnormality is not contagious.

Until the cause is known, basic isolation and hygiene are sensible when multiple cats share the home.

How Worried Should You Be?

Severity	What it may look like	What to do
Lower immediate risk	Mild watery discharge, slight redness, eye remains fully open and cat is bright and eating	Monitor closely and arrange an examination if it persists beyond 24 hours
Moderate	Mucous or coloured discharge, conjunctival swelling, repeated blinking, third-eyelid redness or recurring episodes	Arrange a veterinary appointment within 24 hours
Severe	Eye held partly or fully shut, marked pain, heavy discharge, reduced appetite or rapid worsening	Seek same-day veterinary care
Critical	Cloudy or blue cornea, visible ulcer or crater, abnormal pupil, sudden vision loss, bulging eye, major trauma or chemical exposure	Go to an emergency veterinary hospital immediately

The colour and quantity of discharge matter less than pain, corneal clarity, pupil appearance and speed of progression.

What Else Can Look Like Bacterial Conjunctivitis?

Possible condition	Clues that may increase suspicion
Feline herpesvirus	Recurring episodes, stress-related flare-ups, respiratory signs or corneal ulcers
Chlamydia felis	Young cat, marked chemosis, one eye progressing to both and minimal respiratory disease
Mycoplasma felis	Conjunctivitis with respiratory disease, group exposure or co-infection
Foreign body	Sudden one-sided pain, outdoor exposure or material beneath an eyelid
Corneal ulcer	Severe squinting, light sensitivity, cloudiness or fluorescein uptake
Entropion	Eyelid rolls inward and fur rubs the cornea
Uveitis	Small or irregular pupil, cloudiness and internal eye inflammation
Glaucoma	Enlarged or firm eye, large pupil, cloudiness and severe pain
Eosinophilic keratitis	Raised pink, white or yellow corneal plaque with blood vessels
Corneal sequestrum	Tan, brown or black patch within the cornea
Tear-film disease	Sticky discharge, dull cornea and recurring surface irritation
Oral, nasal or orbital disease	One-sided eye changes with nasal discharge, facial swelling or pain opening the mouth

Co-infection and overlapping signs are common, so the appearance alone does not reliably identify one organism.

How Do Vets Diagnose Bacterial Conjunctivitis?

Diagnosis begins with a complete eye examination rather than immediately selecting an antibiotic.

The aims are to:

1. Confirm that the conjunctiva is inflamed.

2. Determine whether the cornea is damaged.

3. Assess whether the eye is painful.

4. Look for a structural or internal eye problem.

5. Decide whether laboratory testing will change treatment.

6. Identify whether other cats may need investigation or treatment.

History and Physical Examination

Your veterinarian may ask:

• When did the signs begin?

• Did one eye become affected before the other?

• Are other cats showing signs?

• Has a new cat recently entered the home?

• Does your cat live in a shelter, rescue or breeding environment?

• Is there sneezing or nasal discharge?

• Has the problem occurred before?

• Has your cat recently been stressed, boarded or hospitalised?

• What eye medication has already been used?

• Is your cat eating and behaving normally?

• Has there been trauma or a cat fight?

The entire cat may be examined for fever, dehydration, respiratory disease and enlarged lymph nodes.

Fluorescein Staining

Fluorescein dye adheres to areas where the protective corneal epithelium is missing.

It helps identify:

• Corneal ulcers

• The location and size of a defect

• A leaking or perforated cornea

• Whether steroid-containing medication is unsafe

• Whether referral or intensive treatment may be needed

An eye should not be treated as uncomplicated conjunctivitis until significant corneal disease has been considered.

Tear Testing

A Schirmer tear test may be used to assess tear production.

Low tear production is less common in cats than dogs, but tear-film abnormalities can contribute to chronic conjunctivitis and corneal disease.

Intraocular Pressure

Tonometry measures pressure inside the eye.

It can help detect:

• Glaucoma

• Uveitis

• Other internal eye disease

Both conditions can cause redness and pain while requiring very different treatment from bacterial conjunctivitis.

Cytology

Conjunctival cytology involves collecting cells from the inflamed tissue and examining them under a microscope.

It may reveal:

• Neutrophilic inflammation

• Eosinophilic inflammation

• Bacteria

• Chlamydial inclusion bodies

• Abnormal epithelial cells

• Evidence supporting another inflammatory process

Cytology can strongly support C felis when numerous characteristic inclusions are present. However, inclusions are most likely to be found early in infection and may be confused with other intracellular material.

Cytology is much less reliable for diagnosing M felis. In an 88-cat study, Mycoplasma-like organisms were visible in only half of the PCR-positive cats and were also seen in one PCR-negative cat.

PCR Testing

PCR is the preferred laboratory method for detecting C felis.

A proper conjunctival swab should collect epithelial cells because Chlamydia lives inside those cells. Poor sampling can produce a false-negative result.

PCR panels may test for:

• Chlamydia felis

• Mycoplasma felis or Mycoplasma species

• Feline herpesvirus

• Feline calicivirus

• Bordetella bronchiseptica

• Other pathogens

Does a positive PCR confirm the cause?

Not always.

A positive C felis result in a young cat with marked chemosis and compatible signs is likely to be meaningful because PCR positivity is relatively uncommon in healthy cats.

Mycoplasma and herpesvirus require more cautious interpretation because they may be detected in cats without active disease. Cats may also test positive for several organisms at the same time.

Can treatment affect PCR results?

Yes.

Whenever practical, diagnostic samples should be collected before antibiotics, antivirals or topical medication are started. The laboratory should be contacted regarding sample handling because some eye dyes and medications can affect particular test methods.

Bacterial Culture

Bacterial culture is not required for every uncomplicated case of conjunctivitis.

It becomes more valuable when:

• A corneal ulcer appears infected

• Rod-shaped bacteria are seen on cytology

• The disease is severe

• Previous antibiotics have failed

• Infection repeatedly returns

• An unusual organism is suspected

• Antimicrobial resistance is a concern

Culture results should be interpreted with the examination because normal ocular organisms may occasionally grow from a sample without being the primary cause.

How Is Bacterial Conjunctivitis Treated?

Treatment depends on:

• The suspected organism

• Whether the cornea is ulcerated

• Whether the infection is localised or systemic

• Whether respiratory signs are present

• Whether one cat or several cats are affected

• The cat’s age and health

• Whether medication can be given reliably

The correct treatment for C felis is not necessarily the correct treatment for herpesvirus with a secondary bacterial infection.

Treatment for Chlamydia felis

Systemic antibiotic treatment is more effective than topical treatment alone because C felis is an intracellular organism and may persist at sites beyond the visible surface of the eye.

Doxycycline is commonly the first-choice treatment. Current feline infectious disease guidance recommends a course of at least four weeks to improve the likelihood of eliminating infection, commonly continuing for at least two weeks beyond clinical resolution. Shorter courses can make the eyes look better while leaving the organism present.

When doxycycline is unsuitable, a veterinarian may select another systemic antibiotic according to the cat’s age, tolerance and diagnosis. Amoxicillin-clavulanate may be considered as an alternative in selected young kittens.

Topical antibiotic medication may be prescribed as an adjunct, particularly when discharge is heavy, but it should not replace adequate systemic treatment for confirmed chlamydial disease.

Why Does Chlamydia Need Such a Long Treatment Course?

Clinical signs often improve rapidly because inflammation decreases once bacterial replication is suppressed.

The organism may still remain within conjunctival cells after the eye appears normal.

Research using quantitative PCR found that seven, 14 and sometimes 21 days of doxycycline failed to eliminate infection in some cats. At least 28 days was required for reliable elimination within that experimental population.

Stopping early can result in:

• Recurrence

• Continued shedding

• Reinfection of other cats

• Persistent infection within a group

A cleaner-looking eye is not the same as microbiological clearance.

Treatment for Mycoplasma felis

Doxycycline is also commonly selected for suspected M felis infection.

Other antibiotics may be considered when doxycycline is unsuitable or ineffective. The choice should account for the fact that mycoplasmas lack cell walls, making penicillin-type and cephalosporin antibiotics ineffective against the organism itself.

A topical tetracycline antibiotic may be used alongside systemic medication in selected ocular cases.

The ideal treatment duration is less clearly defined than it is for C felis. Clinical signs may improve within approximately one week, but longer treatment may be needed in persistent or PCR-confirmed infection. The veterinarian should determine the duration from the cat’s response, diagnostic findings and wider respiratory involvement.

Treatment for Secondary Bacterial Infection

A topical ophthalmic antibiotic may be appropriate when bacteria have complicated:

• Feline herpesvirus

• A superficial corneal ulcer

• Minor trauma

• Eyelid inflammation

• Another ocular-surface disease

The medication is selected according to:

• Whether an ulcer is present

• Ulcer depth

• Cytology

• Likely bacteria

• Previous medication

• Culture results where indicated

• Dosing practicality

Systemic antibiotics are not automatically required for every localised ocular infection.

The underlying cause must also be treated. An antibiotic may reduce discharge without correcting herpesvirus replication, entropion, a foreign body or an unstable tear film.

Does Your Cat Need Antiviral Treatment?

Antiviral treatment may be required when feline herpesvirus is considered an important part of the disease.

Clues include:

• Corneal ulceration

• Dendritic or branching corneal lesions

• Recurrent stress-associated episodes

• Concurrent upper respiratory signs

• Previous herpesvirus disease

• Poor response to antibiotics alone

Treatment may involve topical antiviral medication, oral famciclovir or both, depending on the severity and distribution of disease.

Antibiotics do not treat herpesvirus. Antiviral medication does not treat C felis or Mycoplasma. Some cats genuinely need treatment addressing more than one organism.

Doxycycline Must Be Given Safely

Doxycycline tablets or capsules should not be given dry to a cat.

Medication can lodge within the feline oesophagus, causing severe inflammation and potentially an oesophageal stricture. Doxycycline should be followed by an appropriate amount of food or water, or supplied in a suitable liquid formulation according to the prescribing veterinarian’s instructions.

Contact your veterinarian if a cat receiving doxycycline develops:

• Difficulty swallowing

• Repeated swallowing movements

• Gagging

• Regurgitation

• Drooling after medication

• Pain while eating

• Sudden refusal of solid food

Do not crush or reformulate medication without checking because taste, stability and dosing accuracy may change.

Can Steroid Eye Drops Be Used?

Not before the cornea has been examined.

Steroid-containing drops can worsen active herpesvirus replication, delay corneal healing and allow an ulcer to deepen. This includes combination products containing both an antibiotic and a corticosteroid.

Steroids may be appropriate for selected immune-mediated eye conditions, but only after:

• A corneal ulcer has been ruled out

• The diagnosis has been established

• Intraocular pressure has been considered

• An appropriate monitoring plan is in place

Never reuse a previous steroid eye medication for a new red or painful eye.

Cleaning the Eyes Safely

External discharge can be softened and removed using:

• Sterile saline

• Clean gauze dampened with lukewarm water

• A veterinarian-recommended eye cleanser

Wipe gently from the inner corner outward.

Use separate gauze for each eye and each cat.

Do not apply:

• Hydrogen peroxide

• Chlorhexidine directly to the eye

• Human mouthwash

• Tea

• Essential oils

• Contact-lens disinfectant

• Human redness-relief drops

• Homemade salt solutions of uncertain concentration

Cleaning improves comfort and prevents dried discharge sealing the eyelids. It does not replace antimicrobial or antiviral treatment.

How to Give Eye Drops or Ointment

1. Wash your hands.

2. Prepare the medication before handling your cat.

3. Position your cat facing away from you.

4. Support the chin gently.

5. Lift the upper eyelid or lower the bottom eyelid slightly.

6. Apply the prescribed amount without touching the bottle tip to the eye, eyelids or fur.

7. Allow the cat to blink.

8. Reward calm behaviour.

9. Wash your hands again.

When several eye medications are prescribed, ask your veterinarian about the order and interval between products. Drops are generally applied before ointments because ointment can form a barrier across the ocular surface.

Contact the clinic if medication administration is becoming impossible. An impractical treatment schedule is not a moral test of your commitment. It is a reason to discuss a more workable plan.

Should Every Cat in the Household Be Treated?

Not automatically, but exposed cats require consideration when C felis is confirmed or strongly suspected.

In a multi-cat household, cattery or shelter, apparently healthy cats may carry or incubate infection and later reinfect the treated cat.

Veterinarians may recommend:

• Examination of every cat

• PCR testing of selected cats

• Simultaneous treatment of exposed cats

• Temporary separation

• Extended treatment in an endemic group

• Risk-based vaccination after the outbreak is controlled

In breeding catteries with endemic infection, current guidance generally recommends treating all cats for at least four weeks, with some groups requiring six to eight weeks.

Do not medicate every household cat with one animal’s leftover prescription. Each cat’s weight, age, health and concurrent medication must be considered.

How Quickly Should the Eye Improve?

With effective treatment for C felis, visible improvement often begins within approximately 48 hours.

That early response does not justify stopping treatment. The complete prescribed course remains necessary to reduce persistent infection and recurrence.

Mycoplasma-associated signs may improve within approximately one week, although the time required for organism elimination is less certain.

Contact the clinic if:

• The eye becomes more painful at any point

• The cornea becomes cloudy

• A white, blue or dark spot appears

• The pupil changes

• Discharge increases rapidly

• Appetite falls

• A kitten becomes lethargic

• There is no meaningful improvement within 48 to 72 hours

• Signs return soon after medication stops

• Medication cannot be administered

A corneal ulcer normally requires a planned recheck even when the eye appears more comfortable.

When Is Bacterial Conjunctivitis an Emergency?

Go to an emergency veterinary hospital immediately if your cat has:

• A bulging, ruptured or penetrated eye

• Major eye or facial trauma

• Chemical exposure

• Sudden apparent blindness

• A severely cloudy or blue eye

• An abnormal fixed pupil

• Uncontrolled bleeding

• Open-mouth breathing

• Blue, grey or very pale gums

• Collapse or profound weakness

Seek same-day veterinary care if:

• The eye is held tightly shut

• Your cat is repeatedly pawing at the eye

• The cornea appears white, blue, grey or dark

• A crater, line or spot is visible on the cornea

• The pupil looks different from the other eye

• The eye is markedly swollen

• Thick discharge is rapidly accumulating

• A kitten’s eyelids are swollen or sealed together

• Your cat has stopped eating

• The condition is worsening over several hours

A mildly watery eye in a bright cat may not require a midnight emergency visit. It should still be examined if it remains abnormal the following day.

What Should You Do Right Now?

1. Look at the cornea

Without touching the eye, check whether the clear front surface remains:

• Transparent

• Smooth

• Shiny

• Similar to the other eye

Cloudiness, discolouration or an irregular surface increases the urgency.

2. Assess comfort

Watch for:

• Squinting

• Repeated blinking

• Hiding

• Light avoidance

• Pawing

• Face rubbing

• Reduced appetite

• Irritability when the head is approached

Pain is more concerning than discharge alone.

3. Check whether one or both eyes are affected

A problem beginning in one eye and progressing to both can fit C felis.

A persistently one-sided, severely painful eye makes trauma, foreign material, an ulcer or a structural abnormality more important.

4. Clean only the external discharge

Use sterile saline or clean gauze dampened with lukewarm water.

Do not force the eyelids apart or rub the cornea.

5. Prevent rubbing

A correctly fitted Elizabethan collar may be needed when your cat is scratching the eye.

6. Separate affected cats where practical

Until the cause is known:

• Use separate bowls

• Use separate bedding

• Wash your hands between cats

• Avoid sharing eye-medication bottles

• Clean fresh discharge promptly

• Keep vulnerable kittens away from affected cats

7. Take photographs

Photograph both eyes in similar lighting.

A short video can help show:

• Blinking

• Squinting

• Discharge production

• Behaviour around light

• Whether one eye is worse

8. Record previous medication

Bring the names or photographs of any drops, ointments or tablets already used.

Do not apply another dose of an old medication until a veterinarian confirms that it is appropriate for the current problem.

9. Arrange a veterinary examination

As a practical guide:

• Mild watery discharge lasting only a few hours: monitor closely

• Persistent redness or discharge: examination within 24 hours

• Thick discharge, swelling or recurrence: prompt examination

• Squinting, cloudiness or visible corneal change: same-day care

• Trauma, rupture, chemical exposure or sudden blindness: emergency care

Common Mistakes Owners Make

Assuming coloured discharge identifies the cause

Green or yellow discharge increases suspicion for bacterial involvement but does not distinguish Chlamydia, Mycoplasma, secondary infection or viral inflammation.

Using leftover eye drops

A medication prescribed for conjunctivitis may be dangerous when the new problem is a corneal ulcer, glaucoma or uveitis.

Stopping doxycycline when the eye looks normal

Clinical improvement occurs before reliable elimination of C felis. Ending the course early increases the risk of persistence and recurrence.

Giving doxycycline tablets dry

Dry tablets can lodge in the oesophagus and cause serious injury.

Treating only one cat during a household outbreak

Untreated exposed cats can maintain infection and reinfect the patient.

Relying on topical medication alone for Chlamydia

Systemic treatment is more effective and is needed to improve the likelihood of eliminating the organism.

Assuming a positive Mycoplasma PCR explains everything

Mycoplasma may be detected in healthy cats and commonly occurs alongside other pathogens. The result must fit the clinical findings.

Using steroid drops before staining the cornea

A hidden ulcer can worsen rapidly under inappropriate corticosteroid treatment.

Waiting until the eye is completely closed

Repeated squinting already indicates discomfort. Treatment is usually simpler before corneal damage progresses.

Can Bacterial Conjunctivitis Be Prevented?

Not every case can be prevented, but transmission and recurrence can often be reduced.

Helpful measures include:

• Isolate cats with active eye or respiratory discharge where practical.

• Wash hands between cats.

• Use separate eye-medication bottles.

• Clean bowls, carriers and bedding.

• Avoid overcrowding.

• Reduce conflict and stress within multi-cat homes.

• Quarantine and monitor newly adopted cats.

• Complete every prescribed antibiotic course.

• Investigate recurring disease rather than repeatedly using empirical drops.

• Maintain appropriate core vaccination against feline herpesvirus and calicivirus.

• Discuss C felis vaccination when the household has a genuine ongoing exposure risk.

Does Chlamydia Vaccination Prevent Infection?

The C felis vaccine is considered non-core.

It is not routinely required for every pet cat. It may be recommended for:

• Breeding catteries

• Shelters with a previous history of chlamydial disease

• Long-term multi-cat environments

• Cats with continuing high exposure risk

Vaccination reduces the severity of clinical disease but does not reliably prevent infection or shedding. It should therefore complement hygiene and outbreak management rather than replace them.

There is currently no vaccine for M felis. Prevention relies on reducing overcrowding, controlling concurrent infections and practising good hygiene within cat communities.

Can Humans Catch Chlamydia felis?

Human infection appears to be rare, but it is possible.

Several human cases of chronic follicular conjunctivitis linked to infected cats have now been documented, including three cases reported in 2024. There is still no evidence that C felis represents a major population-level zoonotic risk.

Sensible precautions include:

• Wash hands after cleaning eye discharge.

• Avoid touching your own eyes.

• Do not share face cloths or towels.

• Keep medication bottle tips clean.

• Use gloves when handling heavy discharge where appropriate.

• Seek medical advice if a person exposed to an infected cat develops persistent conjunctivitis.

Extra care is sensible for immunocompromised household members.

This is feline Chlamydia felis, not the common human sexually transmitted infection caused by Chlamydia trachomatis.

Will Your Cat Be Okay?

The prognosis for uncomplicated bacterial conjunctivitis is generally good when the correct cause is treated.

Cats with C felis often improve rapidly after appropriate antibiotics, although the full course must be completed. Mycoplasma-associated disease is also usually treatable, but concurrent herpesvirus or respiratory infection may prolong recovery.

The prognosis becomes more guarded when there is:

• A deep corneal ulcer

• Corneal melting

• Perforation

• Severe neonatal infection

• Delayed treatment

• A retained foreign body

• Entropion or another untreated mechanical cause

• Chronic herpesvirus disease

• Inability to administer treatment

• Repeated reinfection within a group

The goal is not simply to make the discharge disappear. It is to protect the cornea, remove the underlying cause and prevent another episode.

Frequently Asked Questions

Can bacterial conjunctivitis in cats go away on its own?

A very mild episode may improve, but persistent redness, discharge or swelling should be examined. Chlamydial conjunctivitis can continue for weeks or months without effective treatment.

Is bacterial conjunctivitis contagious between cats?

C felis and M felis can spread between cats through close contact. A secondary bacterial infection caused by trauma or another local eye problem may not be contagious.

Can I use human antibiotic eye drops on my cat?

No. Human products may contain an unsuitable antibiotic, preservative or corticosteroid. The eye should first be examined for an ulcer and other painful conditions.

How long does Chlamydia treatment take?

Treatment commonly lasts at least four weeks and may continue for two weeks beyond resolution of clinical signs. Group outbreaks may require longer treatment and simultaneous management of exposed cats.

Why did the eye improve and then become red again?

Possible reasons include an antibiotic course that was too short, reinfection from another cat, untreated herpesvirus, a structural eyelid problem, resistant bacteria or an incorrect original diagnosis.

Final Thoughts

Bacterial conjunctivitis in cats is usually treatable, but the red eye should not be diagnosed by discharge colour alone.

Chlamydia felis commonly causes marked conjunctival swelling in young cats and often begins in one eye before spreading to both. Mycoplasma felis can be a genuine ocular pathogen, but positive results need context because the organism may also be detected in healthy cats. Feline herpesvirus, corneal ulcers, trauma and eyelid abnormalities remain important alternatives and contributing causes.

The most important decision is whether the cornea is still clear and whether the eye is painful.

A comfortable cat with mild watery discharge may be monitored briefly. Persistent redness or discharge deserves an examination within 24 hours. An eye that is closed, cloudy, discoloured or severely painful should be assessed the same day.

Do not use leftover steroid drops, stop a long antibiotic course early or give doxycycline tablets dry. Accurate diagnosis and a practical treatment plan protect both the eye in front of you and every other cat sharing the home.

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If you are unsure whether your cat’s red or discharging eye can wait, or you need help monitoring treatment between veterinary examinations, ASK A VET™ can help you assess the warning signs and decide what to do next.