Conjunctivitis in Dogs and Cats: Causes, Symptoms, and Treatment

Conjunctivitis — inflammation of the conjunctiva, the thin membrane lining the inner eyelids and covering the white of the eye — is the single most common eye condition in dogs and cats. The conjunctiva acts as a first-line immune barrier for the eye, which means it reacts to a wide range of irritants, allergens, and infections.

"Conjunctivitis is a symptom, not a diagnosis. When I see a red, weepy eye, my job is to determine why the conjunctiva is inflamed — because treatment for allergic conjunctivitis is completely different from treatment for a bacterial or viral cause. Getting the underlying diagnosis right is essential." — Dr. Sarah Chen, DVM

The causes differ significantly between dogs and cats:

In Dogs

• Allergic conjunctivitis — the most common cause; linked to environmental or food allergies. Often bilateral (both eyes)
• Bacterial infection — usually secondary to another problem (dry eye, allergies, foreign body)
• Keratoconjunctivitis sicca (KCS / dry eye) — inadequate tear production leads to chronic inflammation
• Foreign body — grass seed, dust, or debris lodged under the eyelid
• Breed-related anatomy — brachycephalic breeds (Pugs, Bulldogs, Shih Tzus) have prominent eyes prone to exposure and irritation

In Cats

• Feline herpesvirus (FHV-1) — the most common cause, often acquired as a kitten; can flare throughout life
• Chlamydophila felis — bacterial infection causing one-sided then bilateral conjunctivitis
• Mycoplasma — another bacterial cause
• Allergic — less common than in dogs but does occur

The symptoms of conjunctivitis vary depending on the cause, but there are universal signs to watch for:

Common Signs (All Causes)

• Redness — the white of the eye and inner eyelids appear pink or red, with visible blood vessels
• Discharge — ranges from clear and watery to thick, yellow-green, or crusty depending on the cause
• Squinting or blinking — the pet holds the affected eye partially or fully closed
• Pawing at the eye — indicating discomfort or itching
• Swollen eyelids — puffy, sometimes to the point of partially closing the eye

Clues by Discharge Type

• Clear, watery discharge → allergic conjunctivitis, early viral infection, or irritant exposure
• White or grey mucus → dry eye (KCS), chronic allergic
• Yellow or green, thick discharge → bacterial infection (primary or secondary)
• Bloody discharge → trauma, foreign body, or more serious condition — seek urgent care

One Eye vs Both Eyes

• Both eyes simultaneously → allergic cause most likely (dogs), viral cause (cats)
• One eye first, then the other → infectious cause, especially Chlamydophila in cats
• One eye only → foreign body, duct blockage, injury, or unilateral condition

Any eye symptoms that worsen over 24 hours or are accompanied by a cloudy cornea, visible injury, or severe pain (eye held completely shut) should be seen by a vet the same day. Eye conditions can deteriorate rapidly, and delayed treatment can result in permanent vision loss.

Your vet will perform several tests to determine the underlying cause before prescribing treatment:

Diagnostic Tests

• Fluorescein stain — an orange dye applied to the eye that highlights corneal ulcers or scratches under blue light. Essential to rule out corneal damage before prescribing certain medications
• Schirmer tear test — a small paper strip placed inside the lower eyelid for 60 seconds measures tear production. Low results indicate dry eye (KCS)
• Tonometry — measures intraocular pressure to rule out glaucoma (which can mimic severe conjunctivitis)
• Cytology/swab — for infectious causes, especially in cats where herpesvirus or Chlamydophila is suspected

Treatment by Cause

Allergic conjunctivitis:

• Topical antihistamine or mast cell stabiliser eye drops
• Short courses of topical steroids (only if the cornea is confirmed intact via fluorescein stain)
• Long-term allergy management — addressing the underlying atopic dermatitis reduces eye flare-ups

Bacterial conjunctivitis:

• Topical antibiotic eye drops or ointment (chloramphenicol, fusidic acid, or fluoroquinolone)
• Treatment of the underlying cause (foreign body removal, dry eye management)

Viral conjunctivitis (cats — herpesvirus):

• L-lysine supplementation (evidence is mixed but widely used)
• Topical antiviral (cidofovir eye drops) for severe or recurrent cases
• Systemic famciclovir for severe herpesvirus flares
• Supportive care — keeping eyes clean, maintaining humidity

Dry eye (KCS):

• Ciclosporin (Optimmune) or tacrolimus eye drops — immunomodulators that stimulate tear production
• Artificial tears for supplementation
• Lifelong treatment in most cases

While conjunctivitis requires veterinary diagnosis, there's plenty you can do at home to support treatment and reduce recurrence:

Cleaning Discharge

• Use a clean, damp cotton pad (one per eye — never share between eyes) to gently wipe discharge from the inner corner outward
• Warm (not hot) water or sterile saline solution works well — avoid anything with additives
• Clean 2–3 times daily or as needed to keep the eye area comfortable
• Never use human eye drops (Visine, Clear Eyes) on pets — many contain vasoconstrictors that are harmful

Applying Eye Medication

• Approach from behind or the side to avoid startling your pet
• Gently tilt the head slightly upward
• Pull the lower eyelid down slightly to create a pocket
• Apply drops or a thin line of ointment into the pocket — avoid touching the tip to the eye
• Hold the eye closed gently for a few seconds to distribute medication
• Reward immediately with a treat

Prevention

• For allergy-prone dogs: wipe eyes and face after walks (especially during high-pollen seasons), manage underlying allergies, consider seasonal allergy strategies
• For brachycephalic breeds: daily eye checks, keep facial folds clean and dry, use artificial tears if recommended by your vet
• For cats prone to herpesvirus flares: minimise stress (a key trigger), maintain good nutrition, keep the environment calm during changes (new pets, moves, visitors)
• For all pets: avoid direct exposure to irritants (smoke, strong perfumes, aerosol sprays, dusty environments)

Most conjunctivitis cases are straightforward, but eye conditions can escalate rapidly. Know these emergency signs:

Seek Same-Day Veterinary Care If:

• The eye is held completely shut — indicates severe pain, possible corneal ulcer or foreign body
• Cloudiness or blue-white haze on the cornea — suggests corneal oedema or ulceration
• A visible scratch, dent, or spot on the eye surface
• Sudden onset of intense redness with a dilated or misshapen pupil — possible glaucoma or uveitis
• The eye appears to bulge forward — possible abscess behind the eye (retrobulbar) or glaucoma
• Bleeding from or around the eye
• Any trauma to the eye — cat scratch, thorn, impact

Why Speed Matters

The eye is one of the most time-sensitive organs in veterinary medicine:

• Corneal ulcers can perforate (create a hole through the cornea) within 24–48 hours in severe cases, especially if infected with Pseudomonas bacteria — this can lead to loss of the eye
• Glaucoma causes irreversible retinal damage within hours of pressure rising — early treatment can save vision, delay cannot
• Foreign bodies cause progressive damage the longer they remain — a grass seed behind the third eyelid can ulcerate the cornea within a day

The general rule: if you're unsure whether an eye problem is serious, err on the side of seeing a vet sooner rather than later. A quick eye exam is one of the most straightforward vet visits and can prevent catastrophic outcomes. For a broader overview of eye conditions, see our guide to common eye problems in pets.