Congenital sensorineural deafness in cats
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Congenital sensorineural deafness occurs in domestic cats with a white coat. It is a congenital deafness caused by a degeneration of the inner ear. Deafness can occur in white cats with yellow, green or blue irises, although it is mostly likely in white cats with blue irises. In white cats with mixed-coloured eyes (odd-eyed cats), it has been found that deafness is more likely to affect the ear on the blue-eyed side. White cats can have blue, gold, green, or copper coloured odd eyes.
In one 1997 study of white cats with varying degrees of hearing deficiency, 72% of the animals[clarification needed] were found to be totally deaf. The entire organ of Corti was found to have degenerated within the first few weeks after birth; however, even during these weeks no brain stem responses could be evoked by auditory stimuli, suggesting that these animals had never experienced any auditory sensations. It was found that some months after the organ of Corti had degenerated, the spiral ganglion also began to degenerate.
Domesticated white cats with blue eyes and white coats are often completely deaf. Whether or not this is a result of Waardenburg syndrome remains unclear. Deafness is far more common in white cats than in those with other coat colors. According to the ASPCA Complete Guide to Cats, "17 to 20 percent of white cats with nonblue eyes are deaf; 40 percent of "odd-eyed" white cats with one blue eye are deaf; and 65 to 85 percent of blue-eyed white cats are deaf."
The gene that causes a cat to have a white coat is a dominant masking gene. As a result, the cat will have an underlying coat colour and pattern. When the dominant white gene is present, however, that pattern will not be expressed. A cat that is homozygous (WW) or heterozygous (Ww) for this gene will have a white coat despite the underlying pattern/colour. A cat that lacks this dominant masking gene (ww) will exhibit a coat colour/pattern. There are several sources for a white cat to have blue eyes. If the underlying coat pattern is one of a pointed cat (also referred to as a Siamese pattern), the blue eyes may come from the genetics of the pointed gene. A common misconception is that all white cats with blue eyes are deaf. It is possible to have a cat with a naturally white coat without this gene, as an extreme form of white spotting, although this is rare – some small non-white patch usually remains.
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