Introduction
Cats have a variety of eye colors, from copper to green and every shade in between. The anatomy and physiology of feline eyes is similar to human eyes, with structures like the iris, pupil, lens and retina. However, there are some key differences. The tapetum lucidum, a reflective layer behind the retina, allows cats to see well in low light conditions. Cats also have a third eyelid called the nictitating membrane for extra eye protection.
A cat’s eye color is primarily determined by genetics. The gene for coat color is linked to eye color. For example, white cats often have blue eyes. However, kittens are born with blue eyes that can change color as they mature. Environment, nutrition and health can also influence eye color over time.
Common Feline Eye Colors
Feline eye colors encompass a wide range of shades. Some of the most common eye colors in cats include:
Amber Eyes: This is the most common eye color in cats, representing 50-60% of the feline population. Amber eyes appear yellow, yellow-green, or orange. The pigment responsible for this color is lipochrome. Amber eyes are very common in tabby cats.
Green Eyes: After amber, green is the second most common eye color in cats, representing about 20% of the population. Green eyes contain a moderate amount of lipochrome pigment along with a blue reflection from the tapetum lucidum behind the retina. This mix creates a vivid green color.
Blue Eyes: Blue eyes contain a very low amount of lipochrome, allowing the blue color from the tapetum lucidum to show through prominently. White cats or cats with white spotting are more likely to have blue eyes. Only about 5% of cats have blue eyes. https://www.catster.com/guides/types-of-cat-eye-colors/
Hazel Eyes: Hazel feline eyes appear as a mix of shades like green, gold, copper, and brown. The variety of colors comes from a moderate amount of lipochrome combined with the blue from the tapetum. https://www.litter-robot.com/blog/cat-eye-colors/
Changes in Eye Color Due to Illness
Certain illnesses and medical conditions can cause a cat’s eye color to change. Here are some of the most common causes:
Nuclear sclerosis, a condition in which the lens of the eye hardens, can cause a cat’s eyes to take on a cloudy, bluish-gray appearance. According to the American College of Veterinary Ophthalmologists, nuclear sclerosis usually develops in middle-aged to older cats as the proteins in the lens start to clump together and obscure the pupil.1
Cataracts, which cause clouding in the lens, may initially give a cat’s eyes a hazy, bluish tint. As cataracts worsen, the eyes can appear almost white. Cataracts typically start out minor but gradually progress over time.2
Glaucoma, increased pressure within the eye, can lead to a dilated pupil and eventually blindness if left untreated. The affected eye may seem enlarged or bulge outward compared to the normal eye.3
While changing eye color alone may not be cause for alarm, paired with other symptoms like squinting, redness, discharge or vision issues, it warrants a trip to the vet to identify the underlying condition.
Feline Upper Respiratory Infections
Feline upper respiratory infection (URI) is a common viral or bacterial infection in cats that affects the sinuses, throat, windpipe, and eyes. Some of the most common symptoms of feline URI include:
- Nasal discharge
- Sneezing
- Coughing
- Eye discharge
- Squinting or watery eyes
- Redness of the eyes
Eye symptoms are common with feline URI. The infection can cause conjunctivitis, which leads to eye discharge, redness, and swelling. Some cats may develop corneal ulcers on the eye as a result of the infection. One notable eye symptom is a change in eye color, especially in cats with light-colored eyes.
According to the Austin Animal Center, the conjunctiva – the thin tissue lining the inner eyelid and covering the front of the eyeball – can become inflamed during a URI infection. This inflammation causes increased blood flow to the eyes, which can temporarily change the eye color (source). For example, a cat with blue eyes may develop a reddish or orange tint during a bout of URI. Once the infection is treated and clears up, the eye color should return to normal.
Feline Leukemia Virus
The Feline Leukemia Virus (FeLV) is a retrovirus that infects cats. It suppresses the immune system and increases susceptibility to other diseases and cancers. FeLV can be transmitted between cats through bodily fluids such as saliva, blood, urine, and feces. It can lead to a variety of health problems including cancer, anemia, and ocular issues.
Some of the eye disorders associated with feline leukemia include:[1]
- Inflammation of the iris (uveitis)
- Abnormalities in pupil size (anisocoria)
- Impaired eye movement (ophthalmoplegia)
- Retinal hemorrhaging
- Retinal degeneration
While the exact cause of these ocular issues is unknown, it’s likely due to the FeLV virus infecting eye tissues or suppressing the immune system. Feline leukemia can lead to secondary infections, inflammation, and cancerous changes. Early detection is key, so routine veterinary care is important for FeLV positive cats.
Diabetes Mellitus
Diabetes is a fairly common endocrine disease in cats that can affect the eyes. It is caused by insufficient production of insulin by the pancreas. This results in elevated blood glucose levels, which can damage small blood vessels throughout the body, including in the eyes.
One complication of feline diabetes is the development of cataracts. According to research, the incidence of cataracts is much lower in diabetic cats compared to diabetic dogs [1]. Cats have lower levels of aldose reductase, an enzyme involved in sugar cataract formation [2]. So while diabetic cataracts can occasionally occur in cats, they are relatively uncommon.
Diabetic cataracts in cats tend to develop slowly over months to years. They often start out as small opacities in the lens that eventually enlarge and spread. This results in cloudiness and blurred vision. On eye exam, the veterinary ophthalmologist will see the characteristic changes in the lens associated with diabetic cataracts.
Cataract surgery may be recommended for cats with mature cataracts and significant vision impairment. After cataract removal, medications will still be needed to control the diabetes. Tight regulation of blood glucose levels is important to prevent recurrence of the cataracts.
Hypertension
Hypertension, or high blood pressure, is another common condition that can cause changes in a cat’s eye color and appearance. According to a study published in the Journal of Feline Medicine and Surgery, hypertension is prevalent in geriatric cats, with over 20% of cats over 10 years old affected (https://journals.sagepub.com/doi/10.1177/1098612X18818668).
High blood pressure puts strain on the tiny blood vessels in the retina at the back of the eye. This can cause damage and abnormalities that lead to changes in eye appearance. According to Animal Eye Care, common manifestations of feline hypertensive retinopathy include retinal hemorrhage, retinal edema, retinal detachment, and retinal degeneration (https://animaleyecare.com/common-eye-diseases/feline-hypertensive-retinopathy/).
A study in PubMed Central found that hypertension can cause retinal lesions, flame-shaped hemorrhages, retinal edema, and retinal detachment in cats (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC340053/). These changes often begin with a subtle lightening in eye color as the retina thins and transparency increases.
Treatment for feline hypertension involves medication to lower blood pressure and protect the eyes from further damage. With prompt treatment, some cats will regain partial vision, though significant damage is often irreversible.
Anisocoria
Anisocoria is a condition in which a cat’s pupils are unequal in size. The pupil is the opening in the center of the eye that allows light to enter. With anisocoria, one pupil will be dilated (enlarged) while the other is constricted (small). This affects the appearance of the cat’s eyes, making them look asymmetric.
Anisocoria occurs when the iris sphincter muscle, which controls constriction of the pupil, is damaged or not working properly. This causes one pupil to remain dilated while the other responds normally to light. Sometimes the condition affects both eyes, but one pupil is more dilated than the other.
According to VCA Animal Hospitals, in addition to the pupils being different sizes, other signs of anisocoria may include a change in eye color, discharge from the eye, inflammation of the eyelid, or a difference in vision between the two eyes (source).
While anisocoria may seem alarming, it does not necessarily indicate a vision or health problem. However, it can be a symptom of several feline illnesses, so veterinary examination is recommended if this condition develops.
When to See the Vet
Cat owners should contact their veterinarian immediately if they notice sudden changes in their cat’s eye color or observe other symptoms involving the eyes. Some signs that warrant a veterinary examination include:
- Excessive discharge from one or both eyes
- Redness or inflammation of the eye area
- Sudden change in iris color in one or both eyes
- Clouding or change in the cornea
- Swelling around the eyes
- Spots or lesions on the eyeball
- Rubbing or scratching at the eyes
- Squinting or blinking more than normal
- Visible third eyelid
While some minor color variations can occur as cats age, dramatic shifts in eye color or heterochromia (two different colored eyes) should always be evaluated by a veterinarian. Certain illnesses like glaucoma, cataracts, corneal ulcers and cancer can change a cat’s eye color and cause vision problems. Early veterinary care is crucial to diagnose and treat the underlying disease and prevent permanent damage or blindness (Trumann Animal Clinic, 2022).
Conclusion
In summary, there are several illnesses that can cause noticeable changes in a cat’s eye color. Feline upper respiratory infections, feline leukemia, diabetes, hypertension, and anisocoria are some of the most common diseases linked to alterations in eye color. These changes are often indicative of an underlying health issue that requires veterinary attention. While some changes may be subtle, it is important for cat owners to monitor their pet’s eyes for any differences and seek veterinary care if anything seems abnormal.
Proper diagnosis and treatment of illnesses that affect eye color are essential for a cat’s wellbeing. Left untreated, these diseases can lead to further complications or even threaten the life of a beloved feline. By scheduling regular veterinary checkups and staying alert to changes in eye appearance, cat owners have the best chance of catching problems early when they are most treatable. With prompt veterinary care for any eye color changes, cats have a good prognosis for managing illness and enjoying a long, fulfilling life.
