Conjunctivitis in Cats: An Extremely Common Condition
Conjunctivitis — inflammation of the conjunctiva, the pink tissue lining the inner eyelids — is one of the most frequently diagnosed conditions in feline practice. Unlike in dogs, where conjunctivitis may be allergic or secondary to a structural problem, feline conjunctivitis is predominantly infectious in origin. Understanding the specific causes in cats is important because treatment depends on the underlying agent, and using the wrong approach can lead to prolonged illness, complications, and in some cases, unnecessary recurrence.
The Main Causes of Conjunctivitis in Cats
Feline Herpesvirus Type 1 (FHV-1)
Feline herpesvirus type 1 is the most common single cause of conjunctivitis in cats, particularly in kittens and cats in rescue environments or multi-cat households. FHV-1 is one of the pathogens responsible for feline upper respiratory tract infection (cat flu), and most cats are exposed early in life. After the initial infection, the virus does not leave the body. Instead, it establishes lifelong latency in the trigeminal nerve ganglion — a cluster of nerve cells near the base of the skull. The virus can reactivate at any point and resume active replication, typically triggered by stress. Common triggers include rehoming, the arrival of a new pet or person in the household, illness, surgical procedures, or boarding.
During a reactivation episode, the cat sheds the virus and typically develops conjunctivitis, which may be unilateral or bilateral. Concurrent respiratory signs — sneezing and nasal discharge — are common but not always present. FHV-1 can also directly infect corneal cells, causing painful dendritic (branching) corneal ulcers that are characteristic of herpesvirus infection.
L-lysine was once widely recommended as a supplement to suppress FHV-1 replication, based on the theory that the virus requires arginine and that lysine might compete with it. However, subsequent studies have not demonstrated clinical benefit in cats, and the World Small Animal Veterinary Association (WSAVA) no longer recommends its routine use. For moderate to severe FHV-1 flares, systemic treatment with famciclovir — an oral antiviral — is the evidence-based approach and has been shown to reduce viral shedding and improve clinical signs.
Chlamydophila felis
Chlamydophila felis is an obligate intracellular bacterial pathogen that is a significant cause of conjunctivitis in cats, particularly in multi-cat households and breeding colonies. It typically causes a unilateral conjunctivitis initially — affecting one eye first — before spreading to the second eye within days to weeks. The discharge is often mucopurulent (thick and yellowish), and the affected eye may appear swollen and uncomfortable. Respiratory signs, if present, are usually mild compared to FHV-1.
Chlamydophila felis has some zoonotic potential: human infection is possible through close contact with an infected cat, though it is uncommon, and immunocompetent individuals rarely develop serious illness. Immunocompromised individuals should take appropriate precautions when handling affected animals.
Treatment requires systemic antibiotics — doxycycline given orally for at least four weeks is the standard of care. Topical tetracycline ointment is used as an adjunct but is not sufficient on its own, as Chlamydophila can persist at sites beyond the conjunctiva. A vaccine against Chlamydophila felis is available in the UK as part of some polyvalent feline vaccination products and is recommended for cats at higher risk due to their living situation.
Mycoplasma felis
Mycoplasma felis is another bacterial cause of feline conjunctivitis. It is often found alongside other pathogens, particularly FHV-1, and its precise role as a primary pathogen versus an opportunistic agent is still debated. It responds to doxycycline and topical tetracycline.
Signs of Conjunctivitis in Cats
- Discharge from one or both eyes, ranging from clear and watery in early stages to thick, yellow-green, and sticky as the condition progresses or secondary infection develops
- Squinting or blepharospasm — partial or complete closure of the affected eye, indicating pain or discomfort
- Swollen or reddened eyelids
- Protrusion of the third eyelid (nictitating membrane) in the inner corner of the eye
- Rubbing at the face or eye
- Sneezing or nasal discharge if the conjunctivitis is associated with cat flu (FHV-1)
Unilateral conjunctivitis — affecting only one eye — is a useful clue that the cause may be Chlamydophila felis or a local problem such as a foreign body. Bilateral conjunctivitis from the outset is more suggestive of FHV-1 or systemic disease.
Diagnosis: Why Testing Matters
Because the treatment for FHV-1 differs from the treatment for Chlamydophila felis, and because the consequences of mismanagement can be significant, PCR (polymerase chain reaction) testing of conjunctival swabs is often recommended to identify the causative agent. This is particularly important in kittens, cats with recurrent conjunctivitis, and cats in multi-cat environments where an accurate diagnosis enables appropriate quarantine and management decisions. A vet will also perform a careful eye examination to check for corneal ulcers, using fluorescein stain, before prescribing any topical medication — steroid-containing drops are contraindicated in the presence of a corneal ulcer.
A Special Case: Ophthalmia Neonatorum in Kittens
Ophthalmia neonatorum is a condition seen in very young kittens whose eyelids have not yet opened (they normally open at around 10 to 14 days of age). If infection is present in the sealed eye, discharge accumulates beneath the fused eyelids, causing swelling and pressure. If this is not treated promptly, the infection can cause severe damage to the developing eye and permanent vision loss. If you notice a swollen, bulging area beneath a young kitten's sealed eyelids, seek veterinary attention immediately. Do not attempt to force the eyelids open yourself without veterinary guidance.
Treatment Summary
- FHV-1 mild cases: supportive care, minimise stress, warm saline cleaning of discharge
- FHV-1 moderate to severe cases: oral famciclovir (systemic antiviral), topical antiviral drops where available (idoxuridine — availability varies in the UK), management of corneal ulcers if present
- Chlamydophila felis: oral doxycycline for a minimum of four weeks, topical tetracycline ointment as adjunct
- All cases: an Elizabethan collar if the cat is rubbing at the eye; good hygiene to prevent spread in multi-cat households
Managing Conjunctivitis in Multi-Cat Households
Both FHV-1 and Chlamydophila felis spread readily between cats through direct contact and shared surfaces. If one cat in a household is diagnosed, other cats should be monitored closely. Affected cats should be isolated from unaffected individuals where practicable. Food bowls, bedding, and litter trays should not be shared during active infection. Wash hands thoroughly between handling affected and unaffected animals. Discuss whether vaccination against Chlamydophila felis is appropriate for the other cats in the household with your vet.
When to See Your Vet
Any conjunctivitis lasting more than 24 to 48 hours should be assessed by a vet. Do not attempt to treat feline eye conditions with dog eye drops or human preparations — some ingredients that are safe in other species are toxic to cats. Early, appropriate treatment leads to faster resolution, reduces the risk of corneal damage, and limits spread to other cats in the household.