SYMPTOM

A change in eye colour

Owners notice one or both eyes looking a different shade than before, a cloudy blue-grey haze centred on the pupil, a darkening or lightening of the iris itself, a reddish tinge, or a patch of brown pigment spreading across the surface of the eye.

Normal Ageing Lens Change (Nuclear Sclerosis)

Nuclear sclerosis is a near-universal ageing change in the lens, appearing as a hazy blue-grey tint centred on the pupil in dogs typically from around six to seven years onward, and in cats usually from around eight to nine years. It results from the lens continuing to lay down new fibres throughout life, compressing and hardening the older central fibres, and light still passes through relatively normally so vision is largely preserved. This is the benign end of the eye-colour-change spectrum and is often mistaken by owners for a cataract, which it is not.

Cataract Formation

A cataract is a true opacity within the lens itself, appearing as a denser white or crystalline patch rather than the even blue-grey haze of nuclear sclerosis, and can be focal or progress to involve the whole lens. Inherited cataract is the most common cause in dogs, often at younger ages, and diabetes is the next most common: around three-quarters of diabetic dogs develop cataracts within a year of diagnosis, and these can form over weeks even when the diabetes is well controlled. A rapidly forming cataract can leak lens protein and set up inflammation inside the eye, which can in turn raise the pressure within it. Cataracts are less common as a primary finding in cats, where they more often follow uveitis or trauma.

Intraocular Inflammation (Uveitis)

Inflammation inside the eye alters iris colour by causing protein and inflammatory cells to accumulate in the fluid in front of the lens, giving the eye a duller, muddier, or darker appearance than its normal colour, sometimes with visible redness or a constricted pupil. Uveitis has a wide range of underlying triggers, infectious, immune-mediated, traumatic, or secondary to other eye disease, and in cats is a recognised feature of several systemic viral and infectious conditions. It is often painful, and continuing inflammation can lead to glaucoma, cataract, lens displacement or retinal detachment, any of which can cost sight.

Elevated Pressure or Corneal Change (Glaucoma)

Glaucoma raises pressure within the eye, and the resulting corneal swelling can produce a bluish haze across the front of the eye that owners often describe as a colour change rather than cloudiness. In dogs, primary glaucoma most often appears as a sudden attack in one eye: over hours the white of the eye turns red, the cornea becomes hazy blue, the pupil is wide and does not narrow in light, and the eye is painful, which can show as squinting, rubbing or a quiet, withdrawn dog. Pressure at these levels can damage the retina and optic nerve beyond recovery within hours to a few days, and the pressure itself cannot be judged by looking at the eye. Glaucoma in cats is usually secondary to uveitis or a tumour inside the eye and tends to develop more gradually with subtler signs, although sight can still be lost.

Iris Pigment Change

Brown pigment spreading across the iris is recognised particularly in older cats. Much of it is iris melanosis, a benign surface change that can remain stable for years. The same appearance can, however, be early feline diffuse iris melanoma, the most common tumour arising inside the cat's eye, which can spread to other organs; reported rates of spread range from around one in five to more than half of cases, rising with the depth of invasion. Early melanoma is also flat, and the two cannot be told apart by appearance, even on specialist examination, without a tissue sample, so a flat patch that spreads slowly is not reassurance on its own. Thickening of the iris, pigment shed into the fluid of the eye, a change in pupil shape or response to light, and raised eye pressure are features more often seen with melanoma.

Corneal Pigmentation or Surface Disease

Chronic irritation, reduced tear production, or an immune-mediated corneal condition can cause the surface of the eye itself, rather than the iris or lens behind it, to develop a brown or pigmented film, most recognised in breeds prone to pannus or dry eye. This changes the apparent colour of the eye from the outside in, rather than from a structure deeper within it.

Blood Within the Eye (Hyphaema)

A red tinge within the eye itself, either a layer of blood settling at the bottom of the clear front chamber or an eye that looks red throughout, usually reflects bleeding inside the eye. Trauma is one cause. In older cats, high blood pressure is one of the more frequent underlying conditions, often alongside kidney disease or an overactive thyroid, and the same pressure can detach the retina and cause sudden blindness. In any pet, bleeding into the eye can also reflect uveitis, a tumour inside the eye, or a clotting disorder such as anticoagulant rodenticide poisoning, in which bleeding elsewhere in the body may not be visible and can be fatal. Redness of the white of the eye without blood inside it is a separate finding, seen with conjunctivitis, uveitis and glaucoma.

Lens Displacement (Lens Luxation)

In some dogs, particularly terriers such as the Jack Russell and Miniature Bull Terrier and other breeds such as the Lancashire Heeler, an inherited weakness of the fibres holding the lens lets it slip out of place, most often between three and eight years of age. When the lens moves forward into the front of the eye it can block fluid drainage and raise the pressure within hours, giving a painful, red eye with a blue-white haze, and the risk to sight follows the same timescale as acute glaucoma. The inherited form usually affects both eyes, though not always at the same time. In cats, lens displacement more often follows long-standing uveitis or glaucoma.

Corneal Ulceration

An ulcer on the surface of the eye can make part or all of the cornea look cloudy, blue-white or grey, usually with squinting, tearing or pawing at the eye. Superficial ulcers often heal with treatment, while deep ulcers and melting ulcers, in which infection and enzymes soften the cornea into a grey, gelatinous area, can progress to perforation of the eye, sometimes within 24 hours. The depth of an ulcer cannot be judged by looking at the eye at home, and flat-faced breeds are prone to ulcers that deepen.

Why timing matters

Early observation

Early on, many colour changes are subtle enough to be noticed only in certain lighting, or when comparing a recent photo to an older one, and two common gradual patterns, nuclear sclerosis and iris pigment spread, both begin this way. Nuclear sclerosis is reliably benign, but in a cat a spreading patch of iris pigment can look identical to early diffuse iris melanoma, so its gradual onset is not on its own reassuring.

Later presentation

In the slower processes, such as spreading iris pigment or a forming cataract, the colour change tends to become more obvious over time and may be joined by other signs such as squinting, redness, or a visible size difference between the pupils, with any asymmetry between the two eyes becoming more pronounced. In the faster-moving conditions described above, these accompanying signs are typically present from the outset rather than appearing later.

Nuclear sclerosis and iris pigment spread that proves benign progress over months to years, and a change on that timescale can be documented with dated photographs. Acute glaucoma, forward lens displacement, hyphaema and deep or melting ulcers sit at the other end of the range and change over hours, with permanent loss of sight, or of the eye, possible within hours to a few days. Photographs cannot measure eye pressure or ulcer depth, and they cannot separate benign iris pigment from early melanoma in a cat, so a record kept over weeks describes only the slow patterns.

Conditions commonly associated

Nuclear Sclerosis

A very common cause of an owner-noticed eye colour change in older dogs, reflecting a normal ageing process of the lens rather than disease.

Cataracts in Dogs

Lens opacity can alter the eye's appearance and, unlike nuclear sclerosis, can progress to affect vision.

Uveitis

Intraocular inflammation commonly changes iris colour and clarity and can arise from a wide range of underlying triggers; continuing inflammation can lead to glaucoma, cataract or lens displacement.

Glaucoma

Raised intraocular pressure can produce a bluish corneal haze that presents to owners as a colour change. In dogs this often develops over hours with a red, painful eye and a wide, fixed pupil, and sight can be lost permanently within hours to a few days.

Corneal Ulcers

Surface injury or infection of the cornea can alter the eye's clarity and apparent colour, often with discomfort. Deep and melting ulcers can progress to perforation of the eye, sometimes within 24 hours.

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Pannus (Chronic Superficial Keratitis)

Progressive pigmentation across the cornea changes the eye's surface colour, recognised particularly in predisposed breeds.

Keratoconjunctivitis Sicca (Dry Eye)

Chronic reduced tear production can lead to corneal pigmentation and a darkened eye surface over time.

Sudden Acquired Retinal Degeneration Syndrome (SARDS)

SARDS causes sudden blindness in dogs without a true change in eye colour. The pupils are often wide and slow to respond to light, and over the following months the eye shine can look brighter, which some owners read as a change in the eye's appearance.

Hyphaema

Blood in the front chamber of the eye gives it a red tinge. In older cats it can reflect high blood pressure, and in any pet it can reflect trauma, uveitis, a tumour inside the eye or a clotting disorder such as anticoagulant rodenticide poisoning, in which hidden bleeding elsewhere in the body can be fatal.

Lens Luxation

Forward displacement of the lens, inherited in several terrier and other breeds, can raise eye pressure within hours and give a painful eye with a blue-white haze, carrying the same risk to sight as acute glaucoma.

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Systemic Hypertension in Cats

High blood pressure in older cats can cause bleeding inside the eye, seen as a red tinge, and can detach the retina, leading to sudden blindness with widely dilated pupils; the light-sensing cells of a detached retina are often damaged beyond recovery, and sight is less likely to return the longer the retina stays detached.

When to explore further

A colour change confined to one eye, or a difference in appearance between the two eyes where previously they matched, is a distinguishing feature worth describing precisely, since several of the categories above more often start unilaterally while normal ageing lens change is typically symmetrical.

A change that appears over hours or a day or two, particularly with a red white of the eye, a hazy blue cornea, a pupil wider than in the other eye, blood in the eye, squinting, tearing or pawing at the eye, matches the pattern of acute glaucoma, lens displacement, uveitis, hyphaema or a deep ulcer. In these conditions eye pressure or corneal damage can advance within hours, sight can be lost permanently within hours to a few days, and neither the pressure nor the depth of an ulcer can be judged at home.

In a cat, a pigmented patch on the iris cannot be classed as benign by its appearance, because early melanoma is also flat and can spread in a similar way. A patch that is raised or thickened, pigment flecks floating in the front of the eye, a pupil that changes shape or responds less to light, or a red or cloudy eye adds weight to the malignant possibility.

A colour change occurring in a diabetic dog, or developing rapidly alongside increased thirst and urination, adds context linking the eye change to a metabolic process rather than a primary eye condition.

Signs of failing sight, such as bumping into furniture, hesitance on stairs or reluctance to move in dim light, add detail whichever colour pattern is present. Sight lost suddenly, over hours or a few days, is a different pattern from a gradual decline. It fits acute glaucoma or lens displacement, and in an older cat a retina detached by high blood pressure, where the chance of sight returning falls the longer the retina stays detached.

A red tinge inside the eye of an older cat can be the first visible sign of high blood pressure, which can also detach the retina and cause sudden blindness. In any pet, blood in the eye alongside bruising, bleeding gums, blood in urine or faeces, weakness or breathing difficulty fits a clotting disorder such as anticoagulant rodenticide poisoning. Bleeding into the chest or abdomen cannot be seen from outside and can be fatal, and weakness or breathing difficulty can mean it has already started.

For a slow change in an eye that is comfortable, open and not red, a series of close-up photographs taken over several weeks in consistent, even lighting gives a clearer picture of whether a change is progressing than memory alone. Flash can distort apparent eye colour. Comparing both eyes side by side in the same photograph, and noting whether the change is centred on the pupil, spread across the iris, or affecting the clear surface of the eye, helps characterise which structure is involved. A photographic record is not suited to an eye that has changed over hours or days, or one that is red, squinting, watering, bloodstained, has a hazy blue or blue-white film on the clear front of the eye rather than a tint centred on the pupil, or has a wider pupil than the other, since pressure and corneal damage in those patterns can progress within hours and cannot be seen in a photograph. In a cat, photographs can show iris pigment spreading but cannot show whether it is benign.

Last reviewed: 10 October 2026 · Dr Alastair Greenway MRCVS