Deafness
By: George M. Strain, PhD
Deafness in dogs can result from many causes (aging, drug toxicity, noise,
infection, trauma), but the cause of significant concern to owners of English
Setters and other similarly affected breeds is congenital sensorineural
deafness, because it appears to be inherited. Although described as congenital
(present at or near birth), the deafness actually is not present until age
3-4 weeks. Cochlear development occurs normally to that point, but the vascular
supply to the cochlea (the stria vascularis) degenerates, so the hair cells
of the cochlea die. This is known as sensorineural deafness because the
sensory nerve cells, the hair cells, die. There is no treatment for the
hair cell loss, and the deafness is total for each affected ear. Thus, a
dog may be unilaterally deaf, bilaterally deaf, or bilaterally hearing.
Congenital deafness has been reported in sixty breeds, but it is uncommon
in most and usually not proven to be inherited. The cause of the degeneration
of the stria vascularis is not known for certain, but appears to result
from the absence of pigment cells (melanocytes) in the stria. My hypothesis
is that the melanocytes are somehow suppressed by the gene (piebald) responsible
for white in the hair coat; there is considerable evidence that congenital
deafness in many species (including humans) is associated with genes producing
white hair. Dogs that are bilaterally deaf become fairly obvious with time
(they don't wake up at meal time or in response to loud noises), but unilaterally
deaf dogs usually go undetected because they respond to sounds with their
good ear; their only deficit is difficulty in localizing sound, and they
quickly adapt to this. The only way to be certain of a dog's hearing status
is to have its hearing tested by the BAER test (see below). Behavioral testing
by the owner or a veterinarian will not detect unilaterally deaf animals,
and often does not detect bilaterally deaf dogs because they rely heavily
on visual and vibratory cues. Dogs that are bilaterally deaf do not make
good pets, and it is strongly recommended that they be put down by the breeder
before placement with pet owners who become devastated with the discovery
of the deafness and experience considerable difficulty in raising and caring
for these animals. This recommendation is certainly controversial, but there
is no shortage of available dogs without obvious genetic defects, and the
quality of life for a dog who is constantly being startled is questionable.
Because this defect appears to be hereditary, dogs with unilateral deafness
should not, in my opinion, be bred. I have shown with my research on the
Dalmatian breed that deafness incidence nearly doubles in breedings from
parents with three good ears between them compared to parents with four
good ears. It is very important to know the hearing status of your dog and
its potential mate before breeding them. Based on analysis of the results
of hearing tests I have performed on about 400 English Setters, the incidence
of deafness is 13.5% unilateral deafness and 2.8% bilateral deafness; in
Dalmatians, the most severely affected breed, incidence is 21.9% and 8.0%.
There is no difference in incidence between males and females, no difference
between left or right ears, and no apparent difference in incidence between
orange, blue, or tricolored English Setters. In other breeds blue-eyed dogs
have a greater than normal incidence of deafness, but blue eyes are too
rare in English to confirm the same effect. The likelihood of deafness being
present in dogs with at least one unilaterally deaf parent has been shown
to be significantly greater in the Dalmatian and English Cocker Spaniel
breeds, but insufficient data is available from English Setters to confirm
a similar effect. Nevertheless, it is extremely likely to hold true. The
mechanism of inheritance of deafness is not known. It does not appear to
be sex-linked or simple autosomal recessive or dominant: I have experimentally
bred deaf Dalmatian to deaf Dalmatian and gotten hearing puppies. The best
guess at this time is that at least two genes are involved or some form
of incomplete penetrance is present. I currently am isolating DNA from deaf
dogs for future studies of the genetic defect. Much research waits to be
performed before we completely understand this problem.
COPYRIGHT © 1996 by George M. Strain, PhD
By: George M. Strain, PhD
The hearing test known as the brainstem auditory evoked response (BAER)
or brainstem auditory evoked potential (BAEP) detects electrical activity
in the cochlea and auditory pathways in the brain in much the same way that
an antenna detects radio or TV signals or an EKG detects electrical activity
of the heart. The response waveform consists of a series of peaks numbered
with Roman numerals: peak I is produced by the cochlea and later peaks are
produced within the brain. The response from an ear that is deaf is an essentially
flat line. In the sample recordings shown below Puppy 1 heard in both ears,
Puppy 2 was deaf in the left ear, Puppy 3 was deaf in the right ear, and
Puppy 4 was deaf in both ears.
Because the response amplitude is so small (fractions of a microvolt) it is necessary to average the responses to multiple stimuli (clicks) to unmask them from the other unrelated electrical activity that is also present on the scalp (EEG, muscle activity, etc). The response is collected with a special computer through extremely small electrodes placed under the skin of the scalp: one in front of each ear, one at the top of the head, and one between and behind the eyes. It is rare for a dog to show any evidence of pain from the placement of the electrodes - if anything the dog objects to the gentle restraint and the irritation of wires hanging in front of its face. The stimulus click produced by the computer is directed into the ear with a foam insert earphone. Each ear is tested individually, and the test usually is complete in 10-15 minutes. Sedation or anesthesia are usually not necessary unless the dog becomes extremely agitated, which can usually be avoided with patient and gentle handling. A printout of the test results, showing the actual recorded waveform, is provided at the end of the procedure. Test results are confidential. The English Setter Association of America has initiated a closed hearing registry to collect results of BAER tests on English Setters in order to further research directed toward determining the cause of deafness and its mechanism of inheritance. (Interested individuals are invited to contact Jane Wooding [33 Hickory Lane, Ridgefield CT 06877, 203-438-5565], the coordinator of the registry.)
Strain, G. M. 1996. Aetiology, prevalence, and diagnosis of deafness
in dogs and cats.
(Commissioned review.) British Veterinary Journal 152:17-36.
COPYRIGHT © 1996 by George M. Strain, PhD
George M. Strain, PhD
Professor of Neuroscience
Louisiana State University
School of Veterinary Medicine
Baton Rouge, Louisiana 70803
Phone: 504-346-3268
Fax: 504-346-5736
Email: strain@vt8200.vetmed.lsu.edu
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