SYMPTOM
Eye redness
Redness may show in the white of the eye, the pink lining of the eyelids or the third eyelid, in one or both eyes. It often comes with discharge, squinting or excess blinking, rubbing at the eye, or a cloudy look to the surface of the eye.
Conjunctivitis
Inflammation of the membrane lining the eyelids and covering the white of the eye is a common cause of a red eye in both species. In cats, feline herpesvirus, Chlamydia felis and Mycoplasma are recognised infectious causes. In dogs, recognised causes include allergy, dry eye (keratoconjunctivitis sicca), eyelid and eyelash problems, and bacterial infection. It usually produces diffuse redness with watery to mucoid discharge and only mild discomfort.
Corneal Disease
Disease of the clear surface of the eye (the cornea) includes ulcers, the dark corneal sequestrum seen in cats, eosinophilic keratitis in cats and chronic superficial keratitis (pannus) in dogs. Ulcers are painful and usually cause squinting, watering and a red, bloodshot eye, while pannus is not painful. Flat-faced dog breeds are at much higher risk of corneal ulcers and German Shepherds are predisposed to pannus. In cats, sequestrum is seen more often in Persians and Siamese, and both sequestrum and eosinophilic keratitis have been linked to feline herpesvirus.
Uveitis and Intraocular Bleeding
Inflammation inside the eye, or bleeding into the front chamber (hyphaema) following trauma or a bleeding disorder, produces deep, diffuse redness with cloudiness and discomfort rather than the surface redness seen in conjunctivitis. In cats, systemic infections such as toxoplasmosis are a recognised trigger for uveitis. Uveitis can lead to glaucoma.
Eyelid Conformation Abnormalities
Eyelids that roll inward (entropion) or extra eyelashes that grow from the edge of the eyelid (distichiasis) let hairs rub on the eye, causing ongoing redness and in some cases corneal ulcers. Eyelids that roll outward (ectropion) leave the lining of the eye exposed to dust and irritants, which keeps it red. Entropion and ectropion are more common in breeds with loose facial skin or heavy lower lids, such as Basset Hounds, Bloodhounds and Cocker Spaniels.
Environmental Irritation
Dust, pollen and other mild airborne irritants can cause brief redness and watering as the eye's normal protective response, which settles once the irritant has cleared. A grass seed or grit lodged in the eye, often behind the third eyelid, is different: it can scratch the cornea and cause an ulcer, and usually shows as a closed or squinting eye with rubbing and discharge.
Glaucoma
Raised pressure inside the eye gives a red, engorged white of the eye, a cloudy or bluish cornea, a dilated pupil that may react slowly, pain (which in dogs usually shows as a change in behaviour rather than as squinting), and in some cases an enlarged or bulging eye. These signs can appear very suddenly. Eye pressure cannot be judged by looking at the eye at home; it is measured with a tonometer in the clinic. In dogs, glaucoma is often inherited (primary), while in cats it mostly follows uveitis or a tumour inside the eye.
Why timing matters
Early observation
Some causes of a red eye change within hours to days. In dogs with primary glaucoma, vision is usually lost within hours to days of the pressure rising, and deep corneal ulcers can worsen quickly and cause the eye to rupture.
Later presentation
Redness that persists or keeps returning often reflects an ongoing cause, such as dry eye, an eyelid or eyelash problem or chronic corneal disease, which keeps irritating the eye until it is found.
Surface redness with discharge and mild discomfort fits conjunctivitis, while deep redness with a cloudy eye, squinting, a change in pupil size or reduced vision points to the cornea or the inside of the eye. These cannot be told apart reliably by appearance alone; measuring eye pressure and staining the cornea are clinic tests.
Conditions commonly associated
Corneal Ulcers
Inflammation around the ulcer causes visible redness of the conjunctiva and sclera.
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Uveitis
Redness of the eye is a hallmark sign of inflammation within the uveal tract.
Corneal Sequestrum in Cats
Irritation and inflammation around the corneal lesion commonly redden the surrounding eye tissue.
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Distichiasis
Ongoing surface irritation from the misplaced lashes leaves the conjunctiva looking pink or inflamed.
Ectropion
Chronic exposure of the unprotected conjunctival surface leaves it persistently pink and inflamed.
Eosinophilic Keratitis in Cats
Eosinophilic infiltration of the cornea inflames the surrounding tissue, giving the eye a reddened appearance.
Hyphaema
Blood collecting in the front chamber gives the eye a reddish or pinkish tinge that is visible from the outside, sometimes as a distinct pooled layer.
Pannus (Chronic Superficial Keratitis)
The blood vessels that grow into the cornea as part of pannus can give the surrounding eye a reddened appearance.
Toxoplasmosis
Inflammation inside the eye caused by this parasite makes the affected eye appear red and uncomfortable.
Glaucoma
Raised pressure inside the eye makes the white of the eye look red and engorged, often with a cloudy or bluish cornea and a painful eye.
Conjunctivitis
Inflammation of the lining of the eyelids and the white of the eye gives diffuse redness, usually with discharge.
Entropion
Inward-rolling eyelids let hairs rub on the eye surface, keeping the conjunctiva red and irritated.
Keratoconjunctivitis Sicca (Dry Eye)
Too little tear film leaves the eye surface irritated, and dry eye is a recognised cause of conjunctivitis and redness in dogs.
Feline Herpesvirus (FHV-1)
Herpesvirus infection in cats commonly causes reddened conjunctiva with eye discharge, and can also affect the cornea.
When to explore further
When the symptom is persistent or worsening
When other signs are present alongside
When daily activities or quality of life are affected
When there is uncertainty about the cause
When the red eye is held shut, squinted or rubbed, which points to pain
When the eye surface looks cloudy or bluish, the eye looks enlarged or bulging, the pupils differ in size, a crater is visible on the eye, or vision seems reduced
Noting when this symptom occurs, any triggers, and how it changes over time can provide useful information for veterinary discussions.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS