Causes of Deafness



Article located at:  Pet Wave
Causes of Deafness
For a dog to hear, the cells and nerves that transmit sound, as well  as the areas of the brain that interpret and respond to sound, must be  intact and fully functional. There are a number of causes of deafness in  dogs. Conduction deafness is caused by diseases that destroy the outer  ear canal, rupture the ear drum (tympanum) or interfere with the  function of the tiny bones (ossicles) in the middle ear. These include  infections of the inner, middle or outer ear (otitis interna, media or  externa), cancer or other tumors, wax or other debris blocking the ear  canal and other ear canal disorders.
Deafness can be caused by degeneration of the nerves responsible for  sensing sound, especially in older dogs. Senile deafness usually  develops gradually and rarely progresses to complete lack of hearing.  Most old dogs with hearing loss still can hear high-pitched sounds, such  as a whistle or siren. Hearing loss is often not detected in geriatric  dogs until it is accompanied by declining vision.
Deafness can also be congenital, which means it is present at birth.  Congenital deafness is cause by some developmental defect in the cells,  nerves and/or tissues responsible for conduction and interpretation of  sound. It may or may not have a genetic component. Breeds most commonly  affected include Dalmatians, Border Collies, English Setters, Bull  Terriers, Collies, Old English Sheepdogs and Rottweilers. Congenital  deafness is most commonly seen in very young dogs and is frequently  associated with incomplete pigmentation of the haircoat, especially on  the head and around the ears. Dogs with predominantly white, piebald or  merle coats have an increased risk of congenital deafness, especially if  their heads are entirely or largely white. Merle genes produce the  dappled or merle effect of multicolored individual hairs. Piebald genes  affect the amount and distribution of white on the dog’s coat. How these  coat color genes interact to produce deafness is complicated and not  well understood. If these dogs have pigmented cells in their inner ears,  they usually can hear normally.
The inheritance of deafness is related to the genetic “package” that  the dog’s parents bring to the litter. If the parents can hear but have  overly-white heads for their breed, and/or if they are both merle,  piebald or mostly white in color, and/or if they are congenitally deaf  from a heritable condition, the probability of producing deaf puppies is  increased. On the other hand, if a dog is deaf due to illness, injury  or because nature made a mistake during the dog’s development in the  womb, the puppies produced by that dog are no more likely to be deaf  than any other puppy produced by normally hearing parents.
Cancer, infection, inflammation, trauma or other physical damage to  nerves or nerve endings, especially in the middle or inner ear, can also  cause deafness. A number of topical and systemic drugs and other toxins  have been responsible for causing deafness in dogs, especially when  applied topically to an ear with a ruptured or otherwise damaged ear  drum. These include certain antibiotics (aminoglycosides,  chloramphenicol, erythromycin), antiseptics (chlorhexidine, ethanol),  diuretics (furosemide), anti-cancer drugs (cisplatin) and heavy metals  (lead, mercury, arsenic). Repeated exposure to loud noises, such as  gunfire, can contribute to hearing loss as well.
Dogs with the white, piebald or merle gene should not be bred to one  another, because the risk of deaf offspring is quite high. Any ear  infections should be treated promptly until they are fully resolved.  Blue-eyed dogs of breeds that are predisposed to deafness are best kept  out of the breeding population.
 Special Notes

Hereditary deafness is a complex genetic condition that is difficult  to eliminate from predisposed breeds, even with careful breeding. The  chances of inherited deafness are greatly increased if two deaf dogs are  mated. However, two dogs with completely normal hearing can still  produce deaf offspring. If you suspect that your dog has a hearing  problem, consult your veterinarian so that he or she can determine  whether partial or total deafness is involved and what, if anything, can  be done about it.

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