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Deafness: Breeds
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Congenital deafness can be inherited or result from damage (toxic or viral) to the developing fetus. An autosomal gene in cats causes white fur, blue eyes, and deafness; it is dominant with complete expression for white fur and incomplete expression for blue eyes and deafness. Deafness in this instance is due to cochleosaccular degenerative changes that are expressed in the first week of life. Merle and white coat colors are associated with congenital deafness in dogs and other animals. Dog breeds commonly affected include the Dalmatian, Australian Heeler, Catahoula, English Setter, Australian Shepherd, Boston Terrier, Old English Sheepdog, Great Dane, West Highland White Terrier, and Boxer. The list of affected breeds (now >48) continues to expand and may change due to breed popularity and elimination of the defect through selective breeding.
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Advice to clients faced with a deaf puppy or kitten varries based on breed, animal age, home environment, unilateral versus bilateral deafness, and other factors. Bilaterally deaf animals present a variety of liabilities and emotional land mines, more so for dogs than for cats. Deaf animals are at risk of injury or death from undetected dangers such as motor vehicles. When startled, they may reflexly bite, which is a special concern around infants and toddlers. Anxious or aggressive personalities may develop in deaf dogs from constantly being subjected to startle, and familiar family members and friends may be attacked without warning or cause. Not all deaf dogs develop these problems, and no data exist on prevalence rates for such events, but there is no way to predict which animals may or may not have these experiences.
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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.
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Australian Shepherds are not the only breed to be effected by this type of deafness. Any breed with a white or merle hair coat is at risk. The highest incidence is found in Dalmatians. Deafness ... occurs in white cats with blue eyes.
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Pigment-associated deafness has been reported since the last century,1,5,8-10,12 but the hereditary mechanisms are not yet fully understood. The merle gene, which is responsible for the pattern of dark and light hair in breeds such as the Collie and Shetland Sheepdog, is a simple autosomal dominant gene.7 Dogs that are homozygous for the merle gene are usually deaf and frequently solid white, blind, and sterile. Heterozygotes have an increasing tendency to be deaf as the percent of white in the coat increases. Although the merle geneis a dominant gene, the deafness associated with it is not inherited as a simple dominant (or recessive) disorder.
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Well-defined and publicized screening programmes and registries of dogs used for breeding will help to reduce the incidence of deafness. The Institute for Genetic Disease Control in Animals maintains an open registry for deafness, for all breeds. http://www.vetmed.ucdavis.edu/gdc/gdc.html
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