Some cats have heterochromia iridis, which is an unusual condition that means the iris in each eye is a different color. The result is a cat with one blue eye and the other either green, yellow, or brown.
Heterochromia can either be genetic or acquired due to health problems. The three types of heterochromia are complete, sectoral, and central. All kittens are born with blue eyes because they have no melanin in the iris. Heterochromia occurs when melanin fails to develop in one of the eyes, resulting in one blue eye and one that reflects the cat’s actual eye color.
Even though odd eyes are a genetic defect, many owners and breeders see heterochromia as a trait that adds to the cat’s uniqueness. Cats with this condition will often fetch more money than cats with normal colored eyes.
The Anatomy of a Cat’s Eyes
A cat’s eyes are large compared to the size of its head. As described in the MSD Veterinary Manual, the bony cavity or socket that contains the eyeball is called the orbit. The orbit is a structure that is formed by several bones. The orbit also contains muscles, nerves, blood vessels, and the structures that produce and drain tears.
The eye’s white is called the sclera, which is a tough outer layer covered by the conjunctiva (a thin membrane). This covers the inside of the eyelids. In front of the eye is the cornea, a clear dome that protects the eye and lets in light.
The iris is the round, colored area of the eye, and behind it sits the lens. The retina then lines the back of the eye and is sensitive to light. This is what changes light rays into nerve impulses. The retina will send the beams to the brain, which converts them into an image.
Light enters through the pupils, which react quickly to changes in light. In the sun, the eyes will become small. In the darkness, they will expand so that the cat can see.
Why Do Cats Have Different Eyes?
An odd-eyed kitten is a term used to describe a cat with two different colored eyes. As discussed, this is caused by a condition called heterochromia iridium, a genetic anomaly most commonly found in white cats.
It’s not a medical condition, and it doesn’t cause any problems in affected cats. While it might appear alarming, cats with heterochromia – or odd eye syndrome – can see perfectly.
Cats get their eye color from pigment-producing cells in the iris called melanocytes. If no melanocytes are present, the eyes will be blue. A low number of melanocytes will cause green eyes, and a high number will cause orange coloration of the eye.
Active melanocytes will create a more intense color. Less active melanocytes will produce a lighter eye color. An individual cat’s genetics determines this. There are three types of heterochromia iridium:
- Complete heterochromia. The cat’s eyes have two independent colors. For example, one might be blue and the other green. Sometimes a cat’s genes will prevent melanin from reaching one of the eyes, so one will remain blue.
- Sectoral heterochromia. The iris of a single eye features two colors – one eye might contain a mixture of blue and green, for example. It happens when various levels of melanin spread throughout one iris.
- Central heterochromia. Different colors within the iris appear spiked or haloed.
Heterochromia iridium can either be hereditary from birth or acquired due to illnesses or medical problems.
Hereditary Heterochromia Iridis
A cat’s genes determine pigmentation. All kittens are born with blue eyes because there is no melanin in the iris. When a kitten reaches 7-12 weeks of age, melanin begins to move into the iris and changes the cat’s eye color.
Unless health issues or diseases hit later on in life, this eye color is permanent. The amount of melanin that moves into the iris will determine a cat’s actual eye color. This is most likely to be brown (including orange tones), yellow, or green.
All-white or white-spotted cats are more likely to carry the heterochromia gene. This is because the dominant white or white spotting gene that makes a cat completely white or bi-colored can interfere with the migration of melanocytes in one of the eyes. White-furred cat breeds with these genes, like the Japanese Bobtail, Turkish Angora, and the Turkish Van, are more likely to develop the condition.
Acquired Heterochromia Iridis
Acquired odd eyes can’t be passed on to kittens from the affected parent cats. Certain conditions can cause a change of color in a cat’s eye, which usually occurs later. Conditions include:
Uveitis is the inflammation of the eye’s uveal tract, consisting of the iris, choroid, and ciliary body. It’s a painful condition and, if left untreated, can lead to blindness. Uveitis can be an isolated problem or a symptom of other conditions. These include:
- Eye trauma
- Bacterial or fungal infection
- Feline herpes
- High blood pressure
Some cats with uveitis will paw at the sore eye, while others will try to avoid contact with the affected eye altogether. Other cats might keep the eye shut and may become aggressive if a human attempts to go near it.
Most cats with uveitis will avoid bright lights because this will cause pain or discomfort. Squinting, redness, and cloudiness in the eye are tell-tale signs that something is wrong.
Feline glaucoma is when the watery fluid (aqueous humor) in the front part of the eye behind the lens cannot drain properly. The fluid then accumulates, putting pressure on the optic nerve. Typical signs include a cloudy, milky coloration of the eye.
A journal on the US National Library of Medicine warns that while glaucoma is relatively uncommon in cats, it’s likely that many feline cases go unrecognized. It usually begins in one eye and then spreads to the other, leading to blindness.
Portosystemic Liver Shunt
Copper-colored eyes can be a sign of portosystemic liver shunt, which is an abnormal vessel that allows blood from the animal’s intestine to bypass the liver. The result is that toxins, proteins, hormones, and nutrients absorbed by the intestines bypass the liver. Instead, they will circulate throughout the body, causing liver function deterioration.
Not all cats with portosystemic liver shunt will develop copper-colored eyes, but it can be a direct symptom.
Cat Breeds Most Likely to Develop Odd Eye
Cats with white or bi-colored coats are more likely to develop heterochromia. According to Cornell Feline Health Centre, white cats with blue eyes are at the highest risk of developing congenital deafness. 80% of white cats with two blue eyes will start to show signs of deafness when they are about 4 days old due to cochlear degeneration.
Furthermore, a white cat with two blue eyes is 3-5 times more likely to be deaf than a cat with two non-blue eyes. A cat with one blue eye is about twice as likely to be deaf as a cat with two non-blue eyes.
This is because the white gene can occasionally cause the degradation of the cochlea and disrupt the melanocyte migration into one or both eyes. The cochlea is an integral part of the inner ear for a cat’s hearing, resulting in irreversible deafness in one or both ears. The most likely cat breeds to develop odd eye are as follows:
Japanese Bobtail cats are characterized by their short, stubby tails, which look more like rabbit tails than a classic long cat’s tail. Legend has it that Buddhist monks once owned the breed.
You’re likely to see many establishments house figurines of a short-tailed cat with one arm raised throughout Asia. They depict the Japanese Bobtail and are believed to bring good luck to the store owner.
However, the breed’s true genetics aren’t similar to other Japanese cats, causing many experts to question their origin. Many Japanese Bobtails will later develop gold eyes, so cats with heterochromia are most likely to have one blue and one gold eye. This is due to their genes, as many have pure white coats or white coats with brown and grey spots.
Turkish Angora cats are adored for their cheerful personalities and playful curiosity. They can create strong bonds with humans and often become protective of their owners. Thanks to their luxurious, silky coat, they’re known as the ballerina of the cat world.
Because the Turkish Angora has either the white or white-spotted gene, both heterochromia and deafness are common conditions of the breed.
The Turkish Van is graceful and muscular, known for being able to swim and catch fish. Their coat is silky and smooth with no undercoat, making it low maintenance and easy to brush. The body is usually a chalky white, while the head will display a series of colored markings separated by a vertical white blaze.
Interestingly, even though most Turkish Vans are white, heterochromia is not the breed’s most dominating feature. There are no Turkish Van cats with green eyes, but they can be amber, blue, or a mixture of both.
The Khao Manee is a rare breed of cat originating from Thailand, with an ancient ancestry that can be traced back thousands of years. Khao Manee means “white gem,” which reflects their brilliantly white coat and diamond-shaped eyes. They’re considered to be the rarest cat breed in the world and, as a result, sell for thousands of dollars. A Khao Manee with heterochromia is even more popular than one with two normal eyes.
Many Khao Manees will retain at least one blue eye from kittenhood, where the melanin has failed to move to the iris. This is what gives them their unique and highly sought-after look.
Though hairless, the Sphynx is another cat breed that’s prone to developing oddly-colored eyes. Their suede-like skin is covered in a fine down and comes in almost every color or pattern, including tortoiseshell, pointed, or tabby. They’re known for their smart, curious, and engaging personalities, and they will do anything to get attention.
Their large, round eyes will usually be yellow or light green, while the blue eye is usually vivid and beautifully deep in color. This is why odd eye Sphynxes make such sought-after cats.
How Rare Are Odd Eyed Cats?
Heterochromia is rare in most cats but more common in other breeds. It rarely occurs in dark-haired or black cats because they don’t have the white or white spotting gene. It’s important to note that a cat with white fur will not automatically develop odd eyes as they grow older – it’s down to the individual cat’s genes.
To put into context the rarity of odd eye syndrome, white cats make up around 5% of the cat population. Of this, between 1-2% of the population will have either two blue eyes or one blue eye.
When it comes to feline heterochromia, there is no need to panic. If you notice that one eye remains blue in your cat, it’s rarely a cause for concern. It’s merely due to the eye’s melanin levels and is most commonly associated with the white or white-spotting gene.
If your cat is older and you notice its eye color begins to change, there might be an underlying health problem. It might not be serious, and it is likely to be due to the cat’s age.