BAER testing for white cats

In cats, inherited congenital (present from birth) deafness is seen almost exclusively in white coated individuals. The deafness is caused by degeneration of the auditory apparatus of the inner ear and may affect one ear (unilateral) or both ears (bilateral).

Breeding studies have defined the relationship between deafness in white cats and blue eye colour. The gene responsible is an autosomal dominant gene termed W (for White). This gene appears to be pleiotropic – ie, it has more than one effect, being responsible for the white coat colour and also blue eyes and deafness. However, while the gene has complete penetrance for white coat colour (all cats that carry the gene will have a white coat), it has incomplete penetrance for blue eye colour and for deafness (but these two are strongly linked). Thus deafness is strongly linked to the white coat colour and blue eye colour, but not all white cats or white cats with blue eyes are necessarily deaf. The variable penetrance of deafness and eye colour may be caused by interplay with other genes and/or environmental factors.

The risk of deafness in relation to coat colour and eye colour is shown in the figure below. If deafness occurs, it may be either unilateral or bilateral.

Overall, deaf cats with white coat colour and one or both blue eyes, make up around about 1-1.5% of the total cat population. However, the prevalence of white cats does vary in different geographies.

If a white cat has 2 blue eyes, it is 3-5 times more likely to be deaf than a cat with 2 non-blue eyes, and a cat with 1 blue eye is about twice as likely to be deaf as a cat with 2 non-blue eyes. In addition, longhaired white cats are 3 times more likely to be bilaterally deaf. In a feral situation deaf white cats experience strong negative natural selection pressure as:

• They are deaf
• They are photophobic (intolerant of bright light because of the blue eyes)
• They have reduced vision in low light conditions

However, among pet cats it is much more common to find white cats, probably simply due to selective breeding (human preference and intervention). Many cat breeds are known to have the white coat gene and can, therefore, produce deaf white individuals. The GCCF now insists on white cats registered on the active register being checked for deafness (e.g. using BAER testing – brainstem auditory evoked response … this is a simple non-invasive test that can be performed at specialist centres to determine accurately whether deafness is present).