The basis of deafness
from external factors, many different genes are known to cause deafness in both
laboratory mice (Steel 1995) and dogs (Strain 1996), this attributable to
specific types of abnormalities within the inner ear.
The type associated with
white coat colour is described as sensorineuronal. It has been shown in mice
that the presence of pigment cells is essential for normal inner ear
development. They normally colonize the stria vascularis. However, in their
absence, as is also well documented in the dog, the stria vascularis
degenerates. As this provides the blood supply to the cochlea, damage to this
structure occurs and the sensory hair cells necessary for hearing die.
Clearly the effect is variable as BAER testing has demonstrated that one, both, or
neither ear may be affected.
Pigment cells are invariably absent from the stria
of deaf mice which have a white coat colour attributable to pigment cell
The relationship of
deafness with white coat colour and blue eyes is therefore clear. In all cases
the lack of pigment cells is responsible. The fewer the number and the more
limited pigment cell spread, the greater the proportion of the coat lacking
these cells and appearing white. Similarly, there is also the greater the risk
of one or both eyes being unpigmented to give the blue appearance. And, most
importantly for this report, there is also the greater risk of pigment cells
being absent from the stria of one or both ears to result in unilateral or
On the basis of these findings there is no need to
postulate specific single or multiple genes for deafness or blue eyes in pigment
cell deficient white dogs. All the effects are attributable to the s
The incidence of deafness
Table 2 presents some of
the estimates of deafness in Dalmatians. While there was some indication of a
region by region variation within America and also national variations, the data
overall are remarkably consistent.
At all locations, the incidences were high,
and the problem has not been not restricted to particular lines or sections of
the breed. This in itself speaks against a gene for deafness being involved.
Moreover, the BAER testing results showed that the estimates of bilaterally deaf
dogs minimise the problem.
The frequency of unilaterally affected animals in
all regions was generally two to three times higher than that of totally deaf
animals. A very high proportion of the breed (20 - 30%) therefore suffers some
level of the defect.
The association of blue
eyes with deafness
The association of blue
eyes with deafness in white dogs has been recognised since the first reported
case in 1896( Rawitz, in Hayes 1981).
Some of the recent key evidence is
summarised in Table 3.
It may be seen that the risks of both bilateral and
unilateral deafness in blue eyed dogs are about 2 - 3 times higher than in brown
eyed dogs, and even the presence of one blue eye signals almost the same high
level of risk.
Greibrokk (1994) has attributed to the relatively low incidence
of deafness in Norwegian Dalmatians (Table 2.) to breeder selection against blue
eyes. This may also be true for British Dalmatians which also show a lower
incidence of deafness (Wood and Lakhani 1997) than their counterparts in America
where blue eyes are reportedly (Greibrokk 1994) tolerated for show purposes.
The link of blue eyes with
deafness is also suggested in Strain et al's (1992) report of a single
American Dalmatian dog which was thought to be "free of a gene for deafness" on
the basis of that he produced only 13 (6.2%) unilaterally deaf and 2 (1%)
bilaterally deaf puppies among 210 recorded in 25 litters, as compared with
21.8% and 8.0%, respectively, in the study overall (Table 2.).
only 2 of the puppies (1%) had blue eyes compared with 10.6% recorded elsewhere
in the Strain study. Greibrokk (1994) has also pointed out that a lower
incidence of deafness was achieved in Norway by selecting against blue eyes than
achieved in America (Strain et al 1992) by selecting against unilateral
and bilateral deafness.
Selection against blue
eyes and hence for eye pigmentation, implies selection for more pigment cells or
greater pigment cell spread. It is therefore to be expected that the
probability of pigment cells reaching the inner ear will be higher to cause a
reduction in the incidence of deafness.
The association of
pigmented patches with a reduced risk of deafness
Many investigators have
pointed to the reduced incidence of deafness in Dalmatians with pigmented
patches (Strain et al 1992; Holliday 1992, Greibrokk 1994, Famula et
al 1996; Strain and Tedford 1996).
The most compelling data are presented
in Table 4. Bilateral deafness in patched animals was consistently lower (about
2%) than that found in dogs without patches (about 8.4%). Likewise, the
frequency of unilateral deafness was also substantially reduced (8.5% to 23.5%).
The relationship between
patching and lower incidence of deafness was also seen among the progeny of the
single male described by Strain et al (1992) that produced a low
incidence of deafness. Among his 210 puppies a high proportion (21.9%) were
patched compared with the lower overall frequency (9.8%) in the main study.
Consistent with the
association between patching and a reduced incidence of deafness in Dalmatians
is the observation that in Bull Terriers, where there appears to be breeder
tolerance of head patching, the incidence of deafness is lower than found in
Dalmatians (Table 1).
In laboratory mice it has also been noted that the more
extreme the amount of white areas in the coat, the greater the likelihood of an
absence of pigment cells in the inner ear and the greater the risk of deafness
Just as selection against
blue eyes has been found to reduce the incidence of deafness, it may be expected
that selection for patches would have the same effect.
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