Dogs and Deafness: When Your Dog Loses its Hearing
For a dog to hear, the cells that transmit sounds must be intact, as must the brain cells that interpret the sounds. Hearing loss can be caused by congenital deafness, changes of old age, middle and inner ear infections, head injuries, wax and debris blocking the ear canals, tumors of the middle ear, and certain drugs and poisons. The antibiotics streptomycin, gentamicin, neomycin, and kanamycin can damage the auditory and vestibular nerves, causing both deafness and labyrinthitis. Hypothyroidism can be associated with a type of deafness that may respond to treatment with thyroid hormone.
Congenital deafness is caused by developmental defects in the hearing apparatus. Although present from birth, it is not evident until puppies are old enough to respond to sound. This happens from 11 days onward. Deafness may involve one or both ears. If only one ear is affected, you may not notice the deafness.
There is a connection between hereditary deafness and a gene for coat color. Dogs with predominantly white coats, and those with merle coats, are at increased risk for congenital deafness. The highest incidence occurs in Dalmatians, but at least 60 breeds and their crosses are affected. The deafness is due to a lack of pigment or melanin in the “hair cells” that detect sounds. If these dogs with merle or white coats have pigmented cells in their inner ears, even though the rest of their haircoat is lacking pigment, they can hear normally.
Senile deafness develops gradually, beginning at about 10 years of age. It is seldom total. Old dogs with deafness often retain some ability to hear high-pitched sounds, such as a dog whistle. The deafness may not be particularly noticeable unless there is also loss of vision.
A dog with significant hearing loss is less active, moves more slowly, is difficult to arouse from sleep, and fails to respond to commands. Shouting, clapping loudly when the dog is not looking, blowing a whistle, and other attention-getting sounds can be used to test the dog's hearing.
Stamping on the floor attracts a deaf dog's attention, because she can feel the vibrations. Always do this before waking or touching a deaf dog, so that you do not startle her.
Hearing tests can be done on puppies and adult dogs. Hearing is tested using an electroencephalogram (EEG)to record the brain waves produced in response to sounds of different frequencies. If the brain wave pattern remains unchanged, the sound was not heard. This procedure, called a brain stem auditory evoked response (BAER) test, is particularly useful for screening puppies who are at risk for congenital deafness. It is available at referral centers.
The Institute for Genetic Disease Control in Animals (GDC)maintains an open registry for inherited deafness. Results are used to better understand the mode of inheritance and develop data that can help in selecting normal breeding animals. Testing is done at a minimum age of 35 days, according to a BAER protocol. Most of GDC's operation has been transferred to OFA, so search there for deafness registry information, as well.
This article is excerpted from “Dog Owner’s Home Veterinary Handbook” with permission from Wiley Publishing, Inc.
Copyright © 2007 by Howell Book House. All rights reserved.