LATE EYE COLOUR CHANGE TO BLUE
Kittens’ eyes naturally change from infant blue eyes to the adult colour around 6-8 weeks old, often not reaching their final adult hue for many months. More rarely an adult cat’s eyes change back to blue. In the case of white or white-marked cats, one eye may remain blue – this is congenital heterochromia and is related to the genes that produce white fur.
I can’t find any information on both eyes changing colour at the same time, but there is a similar condition in humans called acquired heterochromia iridis where one eye (rarely both) changes colour later in life. In humans, loss of eye pigment is often associated with illness or trauma. Because the cat mentioned here has been examined and is healthy, it is more likely that there is a random mutation and the iris has simply stopped producing pigment.
Batu is a tawny spotted castrated male Ocicat, born in June 2014. His breeder, Ulla Haapanen, received photos of Batu from his owners, showing that his eye colour has slowly changed within the last year from yellow-green to blue. The gradual change began at the age of 2 years and progressed within a year. By March 2017 he had vivid blue eyes. All five of Batu’s full siblings have normal yellow or green eye colour, as do his parents.
Neither of the cat's parents carry a colourpoint gene and there is no white spotting. Vitiligo (depigmented patches) most usually shows as random white patches on the coat and is not usually symmetrical. Batu's eyes changed from baby-blue to yellow-green in the normal way, but later changed to vivid blue. According to the vet, there’s nothing wrong with Batu. He went to see a vet who specialised in eye diseases. Batu’s intra-ocular pressures were normal (this is not glaucoma), his eyesight is normal, his eyes are completely fine – they are just clear blue instead of yellow-green. Because he is a neuter, it isn’t possible to see if the condition can be passed on genetically.
In 2020, Ms. L. Bercand of Gabchaly cattery in France wrote to me about a black female Sphynx whose eyes changed colour at the age of 6 years. Layka was born in Belgium in 2014 and until March 20o she had gold to green eyes. Since March 2020 her eyes began to change colour, first becoming solid green. In late April Layka started to develop bluish spots in her eyes. In May Layka had totally blue or aquamarine eyes. An ophthalmology specialist confirmed that Layka had no underlying pathology and was in very good shape. In fact nothing in Layka's behaviour has changed. Layka underwent further examinations to look for any pathological abnormalities, but nothing was found.
STUDIES INTO LATE EYE COLOUR CHANGE
There is very little information on late eye colour change that isn’t linked to illness. According to “The Color of the Human Eye: A Review of Morphologic Correlates and of Some Conditions that Affect Iridial Pigmentation” (Pascal D. Imesch, MD, Ingolf H.L. Wallow, MD, And Daniel M. Albert, MD in Survey of Ophthalmology Volume 41, Supplement, 2nd February 1997), iris may not remain constant throughout life, but this needs further study. I have excerpted some relevant findings and annotated them with reference to the cats featured on this page.
Differences in eye colour are due to variable amounts of melanin pigment granules within a constant number of melanocytes (melanin producing cells) in the superficial stroma of the iris. These melanocytes reach their genetically determined amount of melanin in early childhood and their melanin content usually remains constant in adulthood. In cats this is when the kitten eye colour changes to the adult eye colour. Except for blue, eye colour depends on the density of melanin granules in the cells. Iris colour is due to either the number and distribution of melanocytes, or to their melanin content, or to a combination of these factors. The number and size of melanin granules, not the number of melanocytes, seems to be the main difference between darker or lighter eye colours. In fact, all eye colours seem to have the same number of melanocytes. Little is known about the regulation of the melanin content. Melanin is derived from tyrosine. The rate of melanin production is controlled by an enzyme called tyrosinase. Metabolism of melanin in the iris usually occurs slowly and is difficult to detect.
Diseases such as Horner's syndrome and Fuchs' heterochromic iridocyclitis cause a decrease in eye pigmentation in one eye, but not in both. These diseases have other symptoms which weren’t found in these cats. That means something else is going on. It seems that melanin is no longer being produced in the stromal melanocytes, or that melanin is being broken down as fast as it is being produced.
A study by Carino et al, found that changes in iris colour could occur spontaneously in 10-20% of a normal (human) population until past adolescence. (Carino OB, Matheny AP, DeRousseau CJ, Bito LZ: Changes in iridial pigmentation past childhood through adolescence (abstract). Invest Ophthalmol Vis Sci 35 (Suppl): 1530, 1994)
Although eye colour was thought to remain the same (barring trauma or disease), there are some congenital conditions that affect it. Most studies relate to heterochromia, where the eyes are different colours. Laties studied heterochromia in rabbit eyes following sympathectomy (surgical cutting of a sympathetic nerve or removal of a ganglion to block nerve signals) and measured the tyrosinase activity. He also examined the adrenergic innervation of iris melanocytes in different animals, including cats. An adrenergic nerve fibre is a neuron that uses the neurotransmitter adrenaline (epinephrine), noradrenaline or dopamine. Innervate means stimulated by nerve fibres.
Laties confirmed that melanocytes are closely related to adrenergic innervation. All the sympathectomized rabbits showed a decrease in eye colour and tyrosinase activity in the eye served by the blocked sympathetic nerve. Regulation of the pigment content of stromal iris melanocytes appeared to be influenced by the sympathetic nervous system, even after childhood. (Laties AM: Ocular melanin and the adrenergic innervation to the eye. Trans Am Ophthalmol Soc 72:560-605, 1974). Unlike Laties’ rabbits, the cats on this webpage had colour change in both eyes.