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.
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.