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White Coat Colour and Deafness.

Continued .....

The basis of deafness

Apart 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 deficiency.

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 bilateral deafness.

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 gene.

The incidence of deafness in Dalmatians

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.).

Significantly, 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 (Steel 1995).

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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