Hearing in Dogs:
The Association with ColorThere are many reasons a dog or cat can be deaf, some inherited and some acquired. Probably the majority of elderly companion animals have some hearing loss, as do many elderly humans. This is not normally a cause for concern in a dog breeding program. There is, however, one type of hearing loss which is complete and can be distinguished by 5 weeks of age. This type probably has a strong hereditary componant. Where information is available on the hearing status of parents and offspring, animals deaf in one ear (not normally identifiable by behavior) are much more likely to produce totally deaf offspring.
Physically, this type of hearing loss appears to be due to a lack of melanocytes in the inner ear. Anything that affects the migration of the melanocytes from the neural crest is a suspect for association with deafness. This certainly includes some of the more extreme forms of white spotting (e.g., in Dalmations, English Setters, Australian Cattle dogs, Bull Terriers, and English Cockers) and the white associated with a double dose of the merle gene (Collies, Shetland Sheepdogs, Australian Shepherds, Catahoula Leopard dogs, and probably Great Danes, Foxhounds and double dapple Daschunds.) The association is detailed at Dr. Strain's site on deafness in dogs and cats.
It is very important to note that unilaterally deaf dogs (deaf in one ear) are much more likely than those with normal hearing to produce bilaterally (both ears) deaf puppies. Further, dogs are remarkably good at compensating for deafness. In order to be certain of the hearing status of a dog it is necessary to test the dog's hearing, using a prodecure called brainstem evoked auditory response (BAER). The test is no harder on the dog than the eye tests most responsible breeders now carry out on their breeding stock, and can be performed any time after about 5 weeks of age.
For the most part, BAER testing is available primarily at universities with strong veterinary programs. Some veterinarians are not even aware that it is possible on companion animals. Dr. Strain's site includes a link to BAER testing sites in the US and Canada. In addition, some breeds with a known high incidence of deafness sponsor clinics at which BAER testing can be carried out. I will try to list such clinics that accept dogs of other breeds.
Many breeders of dogs with large areas of white believe that pigment on the ears (or inside the ears) is associated with normal hearing. This is, however, an assumption that has never really been tested. It is the melanocytes within the inner ear, not those visible due to skin or hair pigmentation, that are associated with deafness. These inner ear melanocytes may be but are not necessarily associated with pigment in the vicinity of the ear.
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sbowling@mosquitonet.comupdated March 8. 2010
