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Why Are Dogs Deaf?

There are four main reasons why a dog cannot hear. The reasons are: loss of hearing due to old age, illness, injury and deaf at birth (congenital deafness). The first three causes of deafness are individual to each dog and not hereditary. Congenital deafness may be unique to an individual or it may be inherited. Non-inheritable congenital deafness may be the result of a drug toxicity or illness of the dog's pregnant mother.

Congenital deafness means that during the development of the dog in the womb, the tissues needed to form an entire working ear and its nerves did not form normally. The outer parts of the ear are present and the puppy looks normal. Inside the ear, it is another story. The blood supplies to nerve tissue in the inner ear degenerate shortly after birth. This means that the sounds that reach the pup's outer ear, eardrum, middle ear and inner ear cannot be sent on to the brain To understand why some dogs are deaf and others in the same litter hearing, a few more details are needed about how dogs develop inside their mother's womb. Shortly after conception, the fertilized egg begins to develop into a variety of cells that will form the precursors to all the cells a dog will ever need to be a dog. Some of these cells are called melanocytes. These cells will change into two important types of cells - those that form the nervous tissue and those that form the cells that give colour to hair, skin, eyes, etc. If there are not enough melanocytes, a dog will potentially not have enough nerves to do certain work or enough pigment in the skin, hair, eyes, etc. The lack of melanocytes can be partial (the dog has some hearing) or complete (the dog has no hearing). One or both ears may be affected.

To complicate the story, there are different kinds of "white" that are controlled by different genes in dogs and some genes control where the white is located (pattern of colour on the dog). Genes for coat colour that are intricately linked to deafness are the merle genes and piebald genes. Merle genes produce the dappled or merle effect of multicoloured individual hairs. Piebald genes affect the amount and distribution of white on the dog's coat. How these coat colour genes interact to produce deaf dogs is complicated and not fully understood. Many years of observation of a variety of dog breeds indicate that there is a connection between the amount and distribution of white on the dog and congenital deafness. This does not mean all deaf dogs have white heads nor does it mean that dogs with externally dark coloured ears or heads will have hearing.

The inheritance of deafness is related to the genetic "package" that the dog's parents bring to the litter. If the parents have hearing and overly white heads, for their breed, and/or they are both merle dogs and/or they are both very white piebald, or they are congenitally deaf due to lack of pigment, the probability of producing deaf pups is much higher. If a dog is deaf due to illness, injury or because nature made a mistake during the development of the dog in the womb, then pups produced from that dog are no more likely to be deaf than any other pup.

Author: Heather Pate, member of DDEAF
Deaf-initely Dogs, Spring 2002 Edition
The official newsletter of Deaf Dog Education Action Fund
www.ddeaf.org