"The Genetic Connection" A Guide to Health Problems in Pure Bred Dogs

Lowell Ackerman,DVM,PhD,Dipl,ACVD

AAHA Press

DEAFNESS

Deafness is a loss of hearing, which can be complete or partial. Inherited deafness is a sensorineural deafness resulting from degeneration of inner ear structures and neurons of the spiral ganglion, with clinical signs apparent from a few weeks to a few months of age (619). Predominantly white, merle (e.g., American foxhound, collie, dappled dachs-hund, harlequin Great Dane, Norwegian dunker-hound, Old English sheepdog, Shetland sheepdog), or piebald (beagle, bulldog, hull terrier, Dalmatian, English setter, Great Pyrenees, greyhound, Samoyed, Sealyham terrier) coat coloring predis-poses dogs to inherited deafness.

Interestingly, deaf dogs are not born that way; they lose their hearing between 3 and 4 weeks after birth (1011). The mode of inheritance is predomi-nantly autosomal dominant (619). Recessive inher-itance, however, has been described in the bull terrier (1012), Doberman pinscher (85), pointer (1004), and rottweiler (83). In the Dalmatian, deaf-ness is attributable to an autosomal recessive mul-tifactorial gene with incomplete penetrance (386). The condition in Dalmatians seems to be slightly more prevalent in females (1138).

Deafness is most commonly reported in Dalmatians, English setters, Australian shepherds, border collies, and Shetland sheepdogs (619) but has also been reported in many other breeds (85, 461, 533, 1011, 1012). Breeds affected by congeni-tal deafness are as follows, with the breeds having the highest incidence in italic type:

Akita

American cocker spaniel

American Staffordshire terrier

Australian blue heelers

Australian cattle dog

Australian shepherd

Beagle

Border collie

Boston terrier

Boxer

Bull terrier

Catahoula leopard dog

Collie

Dappled dachshund

Dalmatian

Doberman pinscher

Dogo Argentino

English bulldog

English setter

English springer spaniel

Foxhound

Fox terrier

German shepherd dog

Great Dane

Great Pyrenees

Greyhound

Ibizan hound

jack Russell terrier

Kuvasz

Maltese

Miniature pinscher

Miniature poodle

Norwegian dunkerhound

Old English sheepdog

Papillon

Pointer

Rhodesian ridgeback

Rottweiler

St. Bernard

Schnauzer

Scottish terrier

Sealvham terrier

Shetland sheepdog

Shropshire terrier

Siberian husky

'Thy poodle

Walker American foxhound

West Highland white terrier

Whippet

Prevalence is highest in the Dalmatian, in which 8% are bilaterally deaf and approximately 22% are unilaterally deaf (454). In a British study, 5.3% were bilaterally deaf and 13.1% were unilat-erally deaf (1139). The heritability of deafness in Dalmatians has been calculated as 0.21 (298). The prevalence is as high as 75% in all white Norwegian dunkerhounds; and in dappled dachshunds, 18% are bilaterally deaf and 36%, unilater-ally deaf (101 1). The prevalence of deafness in the bull terrier, English setter, English cocker spaniel, and Australian cattle dog is one-half to one-third that of the Dalmatian. (1011).

The merle gene (M) is dominant, so heterozygous dogs (Mm) still retain the pattern. Breeding two merles (Mm), however, may produce genotypic homozygotes (MM) that may be white in coloration and have an increased incidence of deafness, blind-ness, and sterility. The chances of deafness increase with the amount of white in the coat (1011). The piebald (sp) and extreme piebald (sw) genes affect the amount and location of white coloration. The inheritance of this form of deafness does not seem to be dominant and likely involves more than one recessive gene pair, or incomplete penetrance

(1011). In the Doberman pinscher, neither the merle nor the piebald gene is involved and deafness in the breed is inherited as an autosomal recessive trait (1125).

Because deaf dogs don't usually make good pets (and definitely shouldn't be used for breeding), all breeding animals of breeds at risk should have hearing tests and pups should he tested when they are weaned at 6 to 8 weeks of age and before they are sold. Whereas dogs that are deaf in one ear can still make good pets, they should be neutered so they will not contribute genetically to future gen-erations. Dogs that are deaf in both ears make poor pets because they are difficult to teach and because they are easily startled.

You can tell subjectively if a dog can hear by shaking keys, clapping your hands, or otherwise trying to attract the dog's attention while out of sight. The definitive way to test, however, is known as BAER (Brainstem Auditory-Evoked Response) testing, which is completely painless and can detect any loss of hearing in one or both ears. This test-ing is available from veterinary schools and referral centers. Ideally, every dog of a susceptible breed and color pattern should be BAER tested before breeding, and all breed susceptible pups should be BAER tested before being sold.

Studies have shown that prevalence is strongly associated with parental hearing status (1139), so breeding stock should be selected from families in which deafness is not a problem. Because the defective gene for type I Wardenburg syndrome in humans has been identified, it is hoped that a DNA test for inherited deafness will eventually become available for dogs, too. The Institute for Genetic Disease Control in Animals (GDC) maintains an open registry for deafness in all breeds.