By Dr. George M. Strain,
Louisiana State University Comparative Biomedical Sciences,
School of Veterinary Medicine, Baton Rouge, Louisiana 70803
[Every effort has been made to ensure accuracy of information. However,
this is not a substitute for prompt veterinary care. Any similarity
to other publications is unintentional. Published
online at Sealyhealthguard.org, 7/18/11]
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NOTE: For an explanation of "congenital" see www.medterms.com/script/main/art.asp?articlekey=10766
Congenital deafness in dogs (or other animals) can be acquired [caused
by intrauterine infections, ototoxic drugs like gentamicin, liver
disorders, or other toxic exposures before or soon after birth] or
inherited. Inherited deafness can be caused by a gene defect that
is autosomal dominant, recessive, sex-linked, or may involve multiple
genes (more on this later). It is usually impossible to determine
the cause of congenital deafness unless a clear problem has been observed
in the breed, or carefully planned breedings are performed. In this
article I will discuss what is currently known about the genetics
of deafness in dogs so that breeders can make the best informed decisions
possible when attempting to reduce or eliminate deafness.
Congenital deafness has been reported for approximately 80 breeds,
with the list growing at a regular rate; it can potentially appear
in any breed but especially those with white pigmentation. Deafness may have been long-established in a breed but kept hidden
from outsiders to protect reputations.
The disorder is usually associated with pigmentation patterns, where
the presence of white in the hair coat increases the likelihood of
deafness. Two pigmentation genes in particular are often associated
with deafness in dogs: the merle gene (seen in the Collie, Shetland
Sheepdog, Dappled Dachshund, Harlequin Great Dane, American Foxhound,
Old English Sheepdog, and Norwegian Dunkerhound among others) and
the piebald gene (Bull Terrier, Samoyed, Greyhound,
Great Pyrenees, Sealyham Terrier, Beagle, Bulldog,
Dalmatian, English Setter).
However, not all breeds with these genes have been reported to be
affected. The deafness, which usually develops in the first few weeks
after birth while the ear canal is still closed, usually results from
the degeneration of part of the blood supply to the cochlea (the stria
vascularis). The nerve cells of the cochlea subsequently die and permanent
deafness results. The cause of the vascular degeneration is not known,
but appears to be associated with the absence of pigment producing
cells (melanocytes) in the blood vessels. All of the function of these
cells are not known, but one role is to maintain high potassium concentrations
in the fluid surrounding the hair cells of the cochlea; these pigment
cells are critical for survival of the stria.
Deafness in the Doberman, which is also accompanied by vestibular
(balance) disturbance, results from a different mechanism, where hair
cell death is not the result of degeneration of the stria. Deafness
may also occur later in life in dogs from other causes such as toxicities,
infections, or injuries, or due to aging (presbycusis); these forms
of deafness almost never have a genetic cause in animals and thus
do not present a concern in breeding decisions.
The prevalence of congenital deafness in different breeds is seldom
known because of the limited number of studies In the Dalmatian, where
the incidence is highest, 8 percent of all dogs in the US are bilaterally
deaf and 22 percent are unilaterally deaf. In the English Setter,
English Cocker Spaniel, Australian Cattle Dog, and Bull Terrier, where
fewer numbers of dogs have been hearing tested, the incidence appears
to be about one third to one half that of Dalmatians.
Unilateral or bilateral deafness is found in 75percent of all white
Norwegian Dunkerhounds, but the incidence in normal-color dogs is
unknown. Other breeds with a high incidence are the Catahoula and
Australian Shepherd. The incidence of all types of deafness in the
general dog population is low, reported to be 2.56 to 6.5 cases per
10,000 dogs seen at veterinary school teaching hospitals, but these
data predate the availability of hearing testing devices and so are
much lower that actual values. Recognition of affected cases is often
difficult, because unilaterally deaf dogs appear to hear normally
unless a special test (the brainstem auditory evoked response, BAER)
is performed; facilities to perform the BAER are usually only available
at veterinary schools (see list of BAER Testers). It should be noted
that a unilaterally deaf dog can be as great a genetic risk for transmission
of deafness to its offspring as is a bilaterally deaf dog.
The method of genetic transmission of deafness in dogs is usually
not known. There are no recognized forms of sex-linked deafness in
dogs, although this does occur in humans. The disorder has been reported
to have an autosomal recessive mechanism in the Rottweiler, Bull Terrier,
and Pointer, but this suggestion is not reliable because the reports
were before the availability of BAER testing and the ability to detect
unilaterally deaf dogs. References usually state that deafness transmission
in most other breeds is autosomal dominant, but this is false, as
will be discussed below. Pigment-associated inherited deafness is
not restricted to dogs. Similar defects have been reported for mice,
mink, pigs, horses, cattle, cats, and humans. Deafness in blue-eyed
white cats is common and is known to be passed on as an autosomal
dominant defect. Blue eyes, resulting from an absence of pigment in
the iris, is common with pigment-associated deafness but is not, in
and of itself, an indication of deafness or the presence of a deafness
gene; however, in several breeds dogs (Dalmatian, English Setter,
English Cocker Spaniel, Bull Terrier) with blue eyes are statistically
more likely to be deaf. Waardenburg's syndrome, a human condition,
presents with deafness, a stripe of white in the hair and beard, blue
or different colored eyes (even in blacks and asians), no pigment
behind the retina, and minor structural deformities around the nose
and eyes. This is an autosomal dominant disorder with incomplete penetrance,
which means that individuals that inherit the disorder may not show
all components of the syndrome - i.e., they may not be deaf. Incomplete
penetrance of a defect greatly complicates the determination of mode
of inheritance. At present there is no documentation that incomplete
penetrance is a factor in any canine deafness.
In simple Mendelian genetics, each dog carries two copies of each
gene, one from each parent. The possible outcomes of breedings can
be demonstrated with tables showing the genotype of both parents and
the possible combinations in their offspring. If deafness is carried
as a theoretical simple autosomal recessive gene (d), the breeding
of two hearing carriers (Dd) will result, on average, in 25 percent
affected dogs (dd), 50 percent hearing carriers (Dd), and 25 percent
free of the defect (DD). The breeding of a carrier to a dog free of
the defect (Table 2) will result in no affected dogs but 50 percent
carriers and 50 percent free. The breeding of an affected dog to a
carrier will result in 50 percent affected, 50 percent carriers, and
no free. Finally, the breeding of an affected dog to a dog free of
the defect will result in 100 percent carriers and no affected or
free.
If instead the deafness is carried as a simple autosomal dominant
gene (D), the breeding of an affected dog (Dd) to a free dog (dd)
would result on average in 50 percent affected and 50 percent free.
Dogs with the genotype DD would be unlikely to occur unless two deaf
dogs had been bred. All of the above assumes that incomplete penetrance
is not acting. If more than one gene (recessive and/or dominant) is
involved in producing the deafness, the possible combinations become
much more complicated. In humans more than 50 different autosomal
recessive or dominant deafness genes or loci have been identified.
The children of two deaf parents with two different recessive deafness
genes can be unaffected but carry both genes. If deafness in dogs
results from more than one recessive gene, the possible outcomes of
breedings are more numerous and determination of the mechanisms of
transmission will be difficult.
As stated above, deafness is often associated with the merle (dapple)
gene, which produces a mingled or patchwork combination of dark and
light areas. This gene (M) is dominant so that affected dogs (Mm)
show the pattern, which is desirable in many breeds. However, when
two dogs with merle are bred, 25 percent will end up with the MM genotype.
These dogs usually have a solid white coat and blue irises, are often
deaf and/or blind, and are sterile. Breeders of these dogs breeds
know not to breed merle to merle. In this case the deafness is neither
dominant nor recessive, but is linked to a dominant gene that disrupts
pigmentation and secondarily produces deaf dogs.
Genetic transmission of deafness in dogs with the piebald (sp) and
extreme white piebald (sw) pigment genes, such as the Dalmatian, is
less clear. These genes affect the amount and distribution of white
areas on the body. Deafness in Dalmatians does not appear to be autosomal
dominant, since deaf puppies result from hearing parents. It does
not appear to be a simple recessive disorder: we have bred pairs of
deaf Dalmatians and obtained bilaterally hearing and unilaterally
hearing puppies, when all should have been deaf if the disorder was
recessive.
These findings might be explained by a multi-gene cause, the presence
of two different autosomal recessive deafness genes, or a syndrome
with incomplete penetrance. Further studies (in progress) will be
required to determine the mechanisms. Several candidate genes known
to cause pigment-related deafness in humans or mice have been eliminated
as the possible cause of pigment-associated deafness in Dalmatians.
Whole-genome screens will hopefully identify the cause in this and
other breeds.
Recent studies have shown that deafness in Dobermans, which do not
carry the merle or piebald genes, results from direct loss of cochlear
hair cells without any effects on the stria vascularis. Vestibular
(balance) system signs, including head tilt and circling, are seen,
and the deafness is transmitted by a simple autosomal recessive mechanism.
A similar pathology has been described for the Shropshire Terrier.
So what should breeders do when deafness crops up? The most conservative
approach would be to not breed the affected animal and not repeat
the breeding that produced deafness. It is frequently recommended
(Le. Dalmatian Club of America) that bilaterally deaf puppies should
be euthanized, since they make poor pets, are difficult to train,
are prone to startle biting, frequently die from misadventure (cars),
and require excessive care.
There is considerable controversy on this point, and there is no
question that many people have successfully raised deaf dogs. For
every story of a problem deaf dog there seems to be a story of one
that was successfully raised. Unfortunately, there is no way to predict
how a deaf puppy will turn out. Unilaterally deaf dogs can make good
pets but should never be bred.
When deafness is uncommon in a breed, affected dogs should not be
bred, but this does not mean that all related dogs are a risk and
must be retired from breeding. An understanding of simple autosomal
recessive and dominant patterns, as explained above, can allow the
breeder to make better informed decisions and likely avoid future
deaf animals without sacrificing a breeding line that has been shaped
over many years.
However, extreme caution must be used when line breeding of dogs
related to deaf dogs, whether the deafness is unilateral or bilateral.
To make these decisions in an informed manner for breeds with known
deafness, it is important that advantage be taken of hearing testing
facilities at veterinary schools. Unilaterally deaf dogs cannot be
detected by other means, and these dogs will pass on their deafness
genes.
END