Inherited deafness is a problem common to over forty breeds of dogs in the UK. The problem arises in the development of the embryo through the failure of certain cells to be in the right place at the right time. As a result, the failure of the blood supply to the cochlea causes degeneration of the hair cells, which are the crucial link in the transmission of sound. This loss of hair cells is permanent and irreversible. Deafness can be detected as early as four weeks of age, and the ideal age at which to test is between five and six weeks. The tradition method of testing has been to make a loud noise and see if the puppy responds. Clearly such a method cannot distinguish between puppies which are deaf in one ear and those which can hear normally. It is important to detect if a puppy can hear in both ears, because if dogs deaf in one ear are bred from the incidence of deafness in the breed is increased. The only method of testing each ear separately is by the BAER (brainstem auditory evoked response) method. In this, the dog has three fine electrodes inserted just under the skin on appropriate parts of the head, and an earphone is used to make clicks of known volume in one ear while the other is covered. The response, if any, is picked up by the electrodes and indicated on a computer monitor. The cells which are responsible for deafness during embryonic development are also involved with the inheritance of coat colour. It is primarily white dogs (in many breeds) which suffer from deafness, but coloured Bull Terriers can be deaf, even solid coloured ones have been known to be deaf. In Bull Terriers the incidence of deafness in the UK is unknown as too few breeders test their stock. However, USA data show that about one in six white Bull Terriers are unilaterally deaf (deaf in one ear) whilst about one in twenty are bilaterally deaf (totally deaf); in the coloureds about one in forty are unilaterally deaf, and total deafness is rare. UK experience is consistent with these figures. Veterinary advice is that unilaterally deaf puppies are suitable for pets, but should not be bred from. Most responsible breeders euthanase totally deaf puppies because dogs are controlled by tone of voice - which is impossible with a deaf dog. There is no clear mode of inheritance of deafness, but the incidence is reduced when BAER testing is used to remove totally deaf and unilaterally deaf dogs from the breeding stock.
Ordinary vets do not have the equipment or expertise to conduct BAER tests, indeed there are only six centres in the UK at which dogs can be tested. Your vet will be able to refer you to the nearest testing centre. Dogs which have been tested are issued a certificate which states the hearing status of both ears.