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White-linked Deafness in Australian Shepherds

By Pam Bethurum, ASCA Educational Coordinator

In the very early years of the breed and conformation showing, it was not uncommon to see dogs in the show ring and being bred that had excessive white trim. These dogs had white stifles carrying up into the body color, white body splashes coming up from the belly, excessive collars and white on the ears. If you look at the first ASCA Yearbook, you will find pictures of some of the early IASA and ASCA Specialty winners, some of which had excessive white.

 

As the Breed grew in popularity and numbers, most of the conscientious breeders were very stringent on their breeding practices in regard to white.  Those puppies with white ears, white over eyes, white stifles, etc., were removed from the breeding population. Most of the really excessive Irish pattern dogs were eliminated from the gene pool and the occurrence of pups with excessive trim became fewer and fewer. With the growing popularity of our breed, more and more new breeders and kennels, and the breed's recognition by AKC, we are beginning to once again see more dogs in the show ring with full white ears, etc. This article is a discussion on white-linked deafness caused by the Irish spotting gene.

 

First we will lightly touch on the basics of the Irish spotting gene, known in genetics as the S locus. The S locus controls the white trim appearing on the muzzle, forehead star or blaze, chest, belly, one or more feet, and the tail tip. It acts independently of the merle gene (M or m). Depending on the number of modifiers of this gene, you can have a puppy with little to no white trim, to one completely white with only small patches of color, or even completely white, making it visually indistinguishable from a merle white. Most everyone who is involved in breeding Australian Shepherds is aware of the dominant merle white (MM) and the effect that this gene has in causing blindness, deafness and other physical defects. Merle whites can be produced when to normal heterozygous (Mm) merle dogs are bred together. You can also have excessively white marked (S locus) puppies produced from the mating of two heterozygous merles (Mm), from a heterozygous (Mm) merle to a tri or bi (mm) or even from to tri or bi (mm) parents. These puppies can be completely or partially deaf due to the lack of pigment in the inner ear. The S locus has at least for alleles. They are listed here in order of dominance:

S      Self, or completely colored body.
St     Irish spotting, varying amounts of white including one or more of the following: blaze, collar, and stockings.
Sv     Piebald spotting, larger amounts of white including body splashes.
Sw     Extreme piebald spotting, pure white.

These alleles are then affected by modifying genes that act upon them. They are called "plus modifiers" which allow more heavily pigmented areas and "minus modifiers" which cause more white. Each allele in effect will overlap each other in appearance, depending on the number and type of modifying genes a puppy has. So this is why we can see a full range of white from none at all to a completely white puppy. (See diagram 1 for examples of each allele.)

White Deafness- Diagram 1

In order to fully understand why a puppy can be deaf due to lack of white pigment in the inner ear, we need to understand the basics of how sound is transmitted from the ear to the brain stem. The following is a very basic discussion on the path that sound follows through the ear. The ear consists of three parts: the external ear, the middle ear and the inner ear. Sound enters through the external ear opening as air vibrations which are funneled through the ear canal or external auditory meatus (see diagram 2) to the ear drum (tympanic membrane). White Deafness- Diagram 2The eardrum then vibrates and transmits the energy of the sound waves to the middle ear. The middle ear consists of the auditory ossicles (malleus, incus, and stapes) which we as laymen would call the "bones" of the ear, or the hammer, anvil and stirrup. The ossicles act as a transformer and convert the large amplitude, low force vibrations at the eardrum to low amplitude, large force vibrations at the end of the stirrup. The vibrations are then transmitted to the inner ear. The inner ear consists of three segments: cochlea, vestibule and semicircular canals. The cochlea is coiled like a snail's shell and is filled with fluid. Within the basilar membrane of the cochlea is the organ of Corti. It contains hair cells which act as receptors. The vibrations pass from the stirrup (stapes) to the cochlea. The vibrations pass through the cochlea, vibrating the membrane. The organ of Corti and hair cells are located on its membrane, causing the hair cells to move. The hair cells convert the mechanical energy to electrical nerve energy which passes along the cochlear nerve and on through the network of nerves to the brain stem. This is where the lack of pigment comes into play in causing deafness. In order for the hair cell to convert the mechanical energy into electrical or nervous energy, the hair cell must contain a pigment cell. If there is not pigment cell, the hair cell cannot convert the mechanical energy and the sound path ends before reaching the brain stem. The degree of conduction deafness depends upon the extent of the lack of pigment in the inner ear. Some pigment would allow partial hearing and total lack of pigment would cause total deafness. This could occur in one or both ears. Therefore a puppy produced from a merle to non-merle breeding could, in fact, be deaf! 

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 also occurs in white cats with blue eyes.

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