What Is Conjunctivitis?
Conjunctivitis refers to inflammation of the conjunctiva — the thin, transparent membrane that lines the inner surface of the eyelids and covers the white part of the eye. In cats, it is one of the most frequently diagnosed ocular conditions and can arise from several distinct causes, each requiring a different management approach.
The hallmark signs are fairly consistent regardless of the underlying cause: redness of the conjunctival tissue, discharge from one or both eyes, and varying degrees of squinting or discomfort. What differs significantly between cases is the nature of the discharge, whether one or both eyes are involved, and whether other symptoms accompany the eye changes. These details often hold the key to identifying the underlying cause.
Viral Conjunctivitis: The Most Common Cause
In cats, the majority of conjunctivitis cases have a viral origin. Feline herpesvirus type 1 (FHV-1) is responsible for a substantial proportion of cases and deserves particular attention because of its lifelong implications.
FHV-1 is extraordinarily widespread in the cat population. Estimates suggest that the majority of domestic cats have been exposed, and once infected, the virus establishes a latent infection in the nervous system that persists for life. Stress — whether from environmental changes, illness, or multi-cat household dynamics — can trigger reactivation, causing recurrent bouts of conjunctivitis and upper respiratory signs.
Herpesvirus conjunctivitis typically presents with a clear to slightly mucoid discharge, marked redness, and squinting. Corneal involvement — specifically the development of dendritic ulcers, a pattern of corneal ulceration associated with herpesvirus — can occur in more severe or chronic cases. Affected cats may also show upper respiratory symptoms including sneezing and nasal discharge, as FHV-1 is one of the primary agents behind cat flu.
Feline calicivirus is another viral agent associated with upper respiratory disease in cats, though it is less commonly the primary cause of conjunctivitis than herpesvirus.
Bacterial Conjunctivitis
Bacterial infection is a common cause of conjunctivitis in its own right, and bacterial involvement also frequently complicates cases that begin as viral infections. When bacteria are the primary culprit, the discharge tends to be more purulent — thicker, yellower, or greener — and may cause the eyelids to become crusted or stuck together, particularly after sleep.
Chlamydia felis is among the most significant bacterial causes of feline conjunctivitis. Unlike many other bacterial pathogens, Chlamydia felis has a strong affinity for the conjunctival tissue and produces a particularly persistent and sometimes severe conjunctivitis. It typically begins in one eye before spreading to the other and is associated with a mucoid to mucopurulent discharge. Upper respiratory signs are usually mild or absent, which can help distinguish it from herpesvirus-related disease.
Mycoplasma species are also recognised as causes of bacterial conjunctivitis in cats, producing signs that can resemble those of chlamydial infection. Secondary bacterial infections with more common organisms such as Staphylococcus and Streptococcus species can occur alongside primary viral disease, complicating the clinical picture.
Allergic and Environmental Conjunctivitis
Though less common in cats than in humans or dogs, allergic conjunctivitis does occur. Environmental allergens — pollens, dust, moulds, and certain household cleaning products — can trigger an inflammatory response in the conjunctiva. Allergic conjunctivitis tends to produce a clear, watery discharge without the accompanying respiratory signs or systemic illness seen with infectious causes.
Both eyes are typically affected simultaneously, and the cat may show other signs of allergic disease such as skin irritation or sneezing. Identifying and removing the offending allergen, where possible, is central to management. Antihistamines may offer some relief in mild cases, though their efficacy in cats is variable.
Irritants such as cigarette smoke, aerosol sprays, or dusty litter can also cause conjunctival inflammation that mimics allergic disease. These cases usually resolve once the irritant is removed.
Getting the Diagnosis Right
Because the treatment for viral, bacterial, and allergic conjunctivitis differs substantially, accurate diagnosis is important. A veterinary examination should assess not just the eye itself but the broader clinical picture — whether other animals in the household are affected, the cat's vaccination history, recent stressors, and whether respiratory signs are present.
In straightforward cases with a classic presentation, your vet may begin empirical treatment while awaiting test results. Where the cause is unclear, conjunctival swabs can be sent for PCR testing to identify herpesvirus, Chlamydia felis, or Mycoplasma. This is particularly useful in chronic or recurrent cases.
A careful examination of the cornea is also important. Corneal ulceration changes the treatment approach — some medications that are safe on an intact cornea can be damaging or contraindicated when the corneal surface is compromised.
Treatment by Cause
For herpesvirus conjunctivitis, antiviral medications are the cornerstone of treatment. Topical cidofovir, where available, and oral famciclovir are the most evidence-supported options. L-lysine supplementation was previously widely recommended as a means of reducing herpesvirus replication, but more recent evidence has cast doubt on its clinical efficacy, and it is no longer endorsed by many veterinary ophthalmologists.
Managing stress in cats prone to herpesvirus flare-ups is an equally important part of long-term care. Environmental enrichment, Feliway diffusers, and addressing any sources of social conflict in multi-cat households can meaningfully reduce the frequency of recurrences.
Bacterial conjunctivitis is treated with appropriate topical or systemic antibiotics depending on the causative organism. Chlamydia felis responds well to doxycycline, which is typically given systemically rather than topically for reliable efficacy. All cats in a household where Chlamydia felis is confirmed should be treated, as it spreads readily through direct contact.
Allergic conjunctivitis management focuses on allergen avoidance and, where necessary, topical anti-inflammatory treatment. Your vet will advise on the most appropriate option based on the severity of signs and your cat's individual history.
When to Seek Help Promptly
Any cat showing eye discharge, redness, or squinting should be seen by a vet rather than having home remedies applied. The eye is a delicate structure, and conditions that appear mild can deteriorate quickly — particularly if corneal ulceration is involved. Early, targeted treatment gives the best chance of a full recovery and protects your cat's long-term vision.