It is usually impossible to determine the cause of congenital deafness unless a clear problem has been observed in a breed or carefully planned breedings are performed. In affected breeds, deafness has often been long-established but kept hidden from outsiders to protect reputations. Hereditary deafness can potentially result from any of several mechanisms: autosomal dominant or recessive, X-linked, mitochondrial, or polygenic; in most instances the mechanism is unknown. Incomplete penetrance, where not all aspects of a deafness syndrome are expressed in an affected individual, frequently complicates an understanding of the mode of inheritance. No known X-linked or mitochondrial deafness has been reported in dogs or cats. With a few known exceptions, hereditary deafness is usually associated with pigmentation patterns, where increasing amounts of white in the hair coat increase the likelihood of deafness.
Two pigmentation genes 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 or extreme piebald gene (Bull Terrier, Samoyed, Greyhound, Great Pyrenees, Sealyham Terrier, Beagle, Bulldog, Dalmatian, English Setter). Not all breeds with these genes have been reported to be affected with deafness.
The merle (dapple) gene (M) produces a mingled or patchwork combination of dark and light areas (Little, 1957; Searle, 1968). This gene is dominant so that heterozygous dogs (Mm) show the pattern, which is considered desirable in many breeds. However, when two dogs with merle are bred, 25% on average 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. Experienced breeders of these dogs know not to breed merle to merle. Heterozygous merles can also be deaf, with the likelihood of deafness increasing with increasing amounts of white in the hair coat. 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 piebald (sw) pigment genes, such as the Dalmatian, is less clear. These genes affect the amount and distribution of white areas on the body (Little, 1957; Searle, 1968). The canine piebald genes are recessive, but individuals in breeds such as the Dalmatian are homozygous, so all dogs within the breed express the pigment pattern. Deafness in Dalmatians does not appear to be dominant since deaf puppies result from hearing parents. It does not appear to be a simple recessive disorder: we have repeatedly bred pairs of deaf Dalmatians from our research colony and obtained many bilaterally hearing puppies, when all should have been deaf if the disorder was recessive. These findings might be explained by a polygenic cause, the presence of two different autosomal recessive deafness genes, or a syndrome with incomplete penetrance. Suggestions have been made for two different recessive genes, either of which can cause deafness, or two recessive genes where both are required to cause deafness (Hewson-Fruend, 1990), or a recessive multifactorial gene with incomplete penetrance (Greibrokk, 1994). Deafness is still clearly linked to the extreme piebald gene in Dalmatians. In this breed, the underlying coat color is black (B) or liver (b, simple recessive). The extreme piebald gene (sw) covers the color with white, and the dominant ticking gene (T) opens the spots through the white. In Dalmatians with a patch, the sw gene does not completely suppress the underlying coat color; the sw gene is only weakly expressed. Patched Dalmatians have been shown to have significantly lower deafness rates (Strain et al., 1992), but a patch is not allowed in the breed standard. Conversely, blue-eyed Dalmatians, where the normal brown iris pigment is suppressed, are significantly more likely to be deaf (Strain et al., 1992; Greibrokk, 1994). Blue eyes are allowed in the breed standard of the United States, but not in Canada or Europe. Dalmatians that are the offspring of one bilaterally hearing parent and one unilaterally deaf parent are twice as likely to be deaf (unilaterally or bilaterally) as dogs that are the offspring of two bilaterally hearing parents (Strain, 1992b). Efforts through breedings to reduce blue eyes in Norwegian Dalmatians reduced the prevalence of deafness (Greibrokk, 1994).
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 (Wilkes & Palmer, 1992). Vestibular 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 (Igarashi et al., 1972).
Numerous references report that most congenital deafness in dogs is autosomal recessive. However, the available data suggest that this is not true for most breeds.
Dr. George M. Strain
Professor of Neuroscience
School of Veterinary Medicine
Associate Vice Chancellor of Research
Office of Research & Graduate Studies
Louisiana State University
Baton Rouge, Louisiana 70803
Visit Dr. Strain's website for a complete discussion of Deafness in Dogs & Cats. We are grateful to Dr. Strain for allowing us to present some of his research and valuable material here.
We are grateful to Dr. Strain for allowing us to present some of his research and valuable material here.