CONDITION
Conjunctivitis
Conjunctivitis describes inflammation of the conjunctiva, the thin, moist tissue that lines the inner surface of the eyelids and covers the white of the eye. When this tissue becomes irritated or infected, it can swell, redden, and produce discharge. The change is often visible as redness around or within the eye, a watery or thicker discharge that may collect at the corner or along the lashes, and sometimes a narrowed appearance if the eye is uncomfortable. Owners often notice one or both eyes looking different: pinker than usual, weepy, or with a crust that builds up overnight. The pet may rub at the face, blink more frequently, or seem bothered by light. Sometimes the signs appear quickly after an obvious event, such as dust or pollen exposure; in other cases they develop gradually without a clear trigger. Conjunctivitis can occur alone or as part of a broader pattern involving other structures of the eye or body. This page explores what conjunctivitis can look like in daily life, the range of causes and mechanisms that can produce it, how it is distinguished from other conditions affecting the eye, and the forms of management that exist depending on what is driving the inflammation.
Why this matters now
Conjunctivitis can occur at any age and may develop suddenly or progress gradually. Seasonal patterns sometimes emerge, with pollen-related cases more common in spring and summer. Infectious causes may cluster in multi-animal households or after exposure to unfamiliar animals.
The condition may begin with mild watering or redness before progressing to more obvious discharge and discomfort. Without identification of the underlying cause, signs may persist, recur, or spread to the other eye. Redness, discharge and squinting are not specific to conjunctivitis. Corneal ulceration, uveitis and glaucoma produce the same outward signs and can threaten sight, and they are separated from conjunctivitis by examination, fluorescein staining and eye pressure measurement, not by appearance at home. Acute glaucoma in dogs and cats typically shows eye pressure above 40 to 60 mmHg, with a cloudy cornea, a dilated pupil and loss of sight. Where sight has already been lost, the outlook for its return is guarded, and treatment aims to lower the pressure rapidly to preserve as much sight as possible.
Signals & patterns
Early signals
Increased tear production or watery eyes
The eye may appear excessively wet or have clear discharge tracking down the face.
Mild redness around the eye margins
The conjunctival tissue may appear pink or slightly inflamed compared to normal.
Increased blinking frequency
Subtle discomfort may cause more frequent blinking or squinting, particularly in bright light.
Pawing at the eye or face
Animals may rub at the affected eye with a paw or against furniture.
Later signals
Thick, coloured discharge
Discharge may become yellow, green, or white and accumulate around the eye, suggesting possible bacterial involvement.
Marked swelling of the eyelids
Significant inflammation may cause visible puffiness around the eye.
Holding the eye partially or fully closed
Discomfort in conjunctivitis is typically mild. An eye held partly or fully closed is a sign of eye pain, which is also a feature of corneal ulceration, uveitis and glaucoma.
Click to read about the biological mechanisms
How this is usually investigated
Evaluation of conjunctivitis aims to identify the underlying cause and assess whether deeper eye structures are affected. Various examinations and tests may provide diagnostic clarity.
Ophthalmic examination
Fluorescein staining
Schirmer tear test
Bacterial or viral culture/PCR
Cytology or conjunctival swab
Tonometry (eye pressure measurement)
Options & trade-offs
Management depends on identifying and addressing the underlying cause. Various approaches may be appropriate depending on the specific diagnosis.
Topical anti-inflammatory preparations
Eye drops or ointments aimed at reducing inflammation and discomfort.
Trade-offs: Corticosteroid preparations are contraindicated where corneal ulceration is present, because they delay healing and can potentiate corneal infection, and corticosteroids are avoided in feline herpesvirus eye disease. Different formulations suit different causes of inflammation.
Antimicrobial therapy
Antibacterial or antiviral medications applied topically or given systemically.
Trade-offs: Appropriate when infection is confirmed or strongly suspected. Some require frequent application, which may be challenging for some pets and owners.
Artificial tears and lubrication
Lubricating preparations to support tear film and comfort.
Trade-offs: Helpful as supportive care and a routine part of dry eye management. May need frequent application throughout the day.
Addressing underlying causes
Management of allergies, eyelid abnormalities, or other contributing factors.
Trade-offs: Long-term management may be needed for allergic or conformational causes. Some conditions may warrant surgical correction.
Common misconceptions
"Conjunctivitis is always contagious"
In dogs, many cases result from allergies, environmental irritants, dry eye, or conformational issues and are not transmissible. In cats, infectious agents such as feline herpesvirus and Chlamydia felis are recognised causes, and Chlamydia felis spreads through close contact between cats.
"Red eyes will resolve on their own"
Redness is shared by conjunctivitis, corneal ulceration, uveitis and glaucoma, and which of these is present cannot be judged from the appearance of the eye at home. Conjunctivitis itself can also last: in cats, untreated chlamydial conjunctivitis can persist for weeks or months.
"Human eye drops are safe for pets"
Some human eye preparations are unsuitable for a particular eye: corticosteroid drops, for example, delay healing where a corneal ulcer is present. Vets do prescribe some human eye products for dogs and cats under the prescribing cascade, where no authorised veterinary product is suitable, so whether a product is a human or a veterinary one does not show whether it is appropriate for a given eye.
Related conditions
Keratoconjunctivitis Sicca (Dry Eye)
Keratoconjunctivitis sicca can cause or worsen conjunctivitis, because insufficient tear production leaves the conjunctiva vulnerable to drying, irritation, and secondary inflammation. In some cases, what appears as persistent conjunctivitis may reflect an underlying problem with tear production rather than a primary conjunctival disorder.
Corneal Ulcers
Corneal ulcers and conjunctivitis often occur together, as both involve disruption to the eye's outer surface and can result from the same initiating events: trauma, foreign material, or anatomical irritation. Conjunctivitis may develop secondary to a corneal injury, and the discharge or inflammation seen in conjunctivitis can sometimes obscure an underlying ulcer until the eye is examined more closely.
The page on this condition is being checked again and is offline for now.
Uveitis
Uveitis involves inflammation of structures deeper inside the eye, and may initially be mistaken for conjunctivitis by owners noticing redness, squinting, or discharge. The two conditions can occasionally occur together, particularly when an infectious or immune-mediated process affects multiple parts of the eye, and distinguishing between them often requires closer examination of the eye's internal structures.
Cherry Eye
Cherry eye (prolapse of the third eyelid gland) can lead to secondary conjunctivitis, as the displaced gland may irritate the conjunctiva or disrupt the normal tear film. The visible swelling and redness seen with cherry eye can resemble conjunctivitis, though the underlying cause and appearance on closer inspection differ.
Entropion
Entropion, in which the eyelid rolls inward, often causes chronic or recurrent conjunctivitis because the fur and skin along the lid margin rub continuously against the conjunctiva and cornea. Until the anatomical problem is addressed, conjunctivitis may persist or return despite treatment of the inflammation itself.
Glaucoma
Glaucoma, a rise in pressure inside the eye, shares redness of the eye with conjunctivitis. Sudden glaucoma typically also shows a dilated, slow-moving pupil and a cloudy cornea, and it is identified by measuring eye pressure.
Feline Herpesvirus (FHV-1)
Feline herpesvirus is a recognised infectious cause of conjunctivitis in cats, with conjunctival redness and a watery discharge from the eyes and nose. Cats are thought to remain latent carriers for life after a first infection.
Chlamydophila Conjunctivitis in Cats
Chlamydia felis causes conjunctivitis in cats, mostly those under one year of age. It may begin in one eye and generally spreads to the other, with marked swelling of the conjunctiva and a discharge that starts watery and later becomes thicker.
Noting when signs first appeared and whether one or both eyes are affected can help guide discussions with a veterinary professional. Observing the character of any discharge, response to bright light, and whether the pet can open the eye fully provides useful information. Identifying any potential triggers such as recent exposure to other animals, new environments, or seasonal patterns may also be relevant.
Last reviewed: 10 October 2026 · Dr Alastair Greenway MRCVS