White Rex & Deafness

A study undertaken by the Nordic Rex Cat Association (Nordiska Rexringen) 2005

Information was submitted by Rex owners/breeders
via the Nordiska Rexringen website, e-mail, telephone and mail.
Compilation of Cornish Rex data: Eine Palm
Compilation of Devon Rex data: Camilla Söderström
Authors: Pernilla Ohlsson, Camilla Söderström, Eine Palm
Translation: Dominic Phillips, Camilla Söderström
Edited: Elizabeth Brown

There have been several studies on white cats and deafness, but as far as we know this is the first specific breed study. Earlier studies were on domestic white cats. The purpose of this study is to investigate how common deafness is in white Rex.

Previous documented studies have found, respectively, a prevalence of deafness of 64.9% and 85% in cats with two blue eyes, 39.1% and 40% in cats with one blue eye (odd-eyed), and 16.7% and 22% in cats with yellow eyes. These figures are taken from three well-known studies (Bosher & Hallpike, 1965; Mari, 1973; Bergsma & Brown, 1971; reviewed by Delack, 1984). Their studies involved 256 white domestic cats in total.

Taking the above figures into consideration, a similar result was forecast for our study of 247 white Rex cats (123 Devon Rex and 124 Cornish Rex) and indeed our studies conclude that there is a higher risk of deafness in blue-eyed Rex than in yellow-eyed Rex. However, the percentage of deaf cats in our study is significantly lower. One possible explanation for the apparently lower number of deaf cats in our study might be that there are cats with undetected partial deafness. Relatively few cats were tested with the objective Brainstem Auditory Evoked Response (BAER) test to see whether the cat really has full hearing in both ears. If a large percentage of the Rex in our study who were reported as hearing are in fact partially deaf, we can at least state that these cats compensate for it so well that neither their owner, breeder or veterinarian could detect it by their behaviour. This concludes that we can expect a partially deaf cat to live as good a life as a fully hearing one despite its disability.

White cats carrying the underlying cs Siamese dilution pigment gene can have blue eyes without deafness, and it has been suggested that the presence of this gene explains why purebred white cats are less often deaf than mixed-breed white cats (Pedersen, 1991) Data supporting this is not available.

Just as in earlier studies, we found that the so-called kitten cap has no statistically proven significance as to whether the cat is hearing or not. (Kitten cap: a proportion of kittens may possess a spot or smudge of coloured fur on the head, due to a few surviving pigment-producing cells, this rarely persists into adulthood.)
The results of this study have been useful in ascertaining the percentage of deaf white Rex cats; the studies will however continue in the hope that other breed clubs where white cats exist will follow suit and perform similar studies, ultimately giving more material for comparison.
Below you can read the statistics for each breed followed by a brief summary of our overall findings.

DEVON REX
This study focuses on Devon Rex born after the requirement for a positive hearing test before being allowed to breed. The study includes cats born in the mid-1990s with the majority of the cats being born from 2000 onwards. Approximately 95% of all white Devon Rex born during this period were entered into this study.

Data was received for 123 Devon Rex. All reports are unfortunately incomplete and in some statistics some cats could not be included since the required information was not reported (for example: whether the cat was born with a kitten cap).

Eye colour
Blue: 16%
Yellow/orange: 63%
Odd-eyed: 16%
Green: 5%
(20% of the 123 reports gave no information on eye colour)

Kitten cap
Cats born with cap: 76.5%
Cats born without cap: 23.5%
(There was no information on presence/absence of a kitten cap in 30% of the 123 cats.)

Hearing
Cats hearing: 97.5%
Deaf cats: 2.5% (3 cats). Two being yellow-eyed and one being blue-eyed.

Hearing test
‘Noise’ test done by a veterinarian: 94
BAER test: 4

BAER tests were only done in 4 cases. In one case to confirm that a suspected cat actually was deaf, in one case to confirm that the white mother of a deaf kitten was hearing (which she was). In the two other cases, BAER tests were done prior to exporting. BAER testing is the only safe way to prove whether a cat is deaf in one or both ears.

Controlled tests not performed by a veterinarian but the breeder/owner who considered the cat to be hearing: 9

There was no information on the methods of testing the remaining cats.

CORNISH REX
Data was received for 124 Cornish Rex. All reports are unfortunately incomplete and in some statistics some cats could not be included since the required information was not reported. All cats included were born after the requirement for a positive hearing test before breeding.

Eye colour
Blue: 25.94%
Yellow/orange: 54.3%
Odd-eyed: 19%
Green: 0.8%
(6.5% of the 124 reports gave no information on eye colour)

Kitten cap
Cats born with cap: 47% (of which 4 cats are deaf)
Cats born without cap: 53% (of which 4 cats are deaf)
(There was no information on the presence/absence of a kitten cap in 60% of the 124 cats)

Hearing
Cats hearing: 92%
Deaf cats: 8% (10 cats) of which 6 are blue-eyed, 2 yellow-eyed and 2 odd eyed being partially deaf.

Hearing test
‘Noise test’ performed by a veterinarian: 72
BAER test: 10

Controlled test not performed by a veterinarian but the breeder/owner who considered the cat to be hearing: 10

There was no information on the methods of testing the remaining cats.

Conclusion:
If we add the results of both the Devon and Cornish Rex the results read as follows:
247 reports of white cats
5.3% deaf, or partially deaf, cats
15% deaf among the blue-eyed
3.2% deaf among the yellow-eyed
5.3% deaf among the odd-eyed (partly deaf on blue side)
0% deaf among green-eyed cats

We would like to thank all participants that have given reports on their white Rex cats. Without which the study could not have been carried out.

Reference:
Delack, J.B. (1984). Hereditary deafness in the white cat. Compendium on Continuing Education for the Practicing Veterinarian 6, 609-619.