Deafness may be congenital (present at birth) or acquired as a result of infection, trauma, or degeneration of the cochlea (the organ of hearing).
Deafness present at birth can be inherited or result from toxic or viral damage to the developing unborn puppy. Merle and white coat colors are associated with deafness at birth in dogs and other animals. Dog breeds commonly affected include the Dalmatian, Bull Terrier, Australian Heeler, Catahoula, English Cocker Spaniel, Parson Russell Terrier, and Boston Terrier. The list of affected breeds (now approximately 100) continues to expand and may change due to breed popularity and elimination of the defect through selective breeding.
Acquired deafness may result from blockage of the external ear canal due to longterm inflammation (otitis externa) or excessive ear wax. It may also occur due to a ruptured ear drum or inflammation of the middle or inner ear. Hearing usually returns after these types of conditions are resolved.
Permanent causes of acquired deafness include trauma to the hard portion of the temporal bone that surrounds the inner ear, loud noises (for example, gunfire), conditions in which there is a loss or destruction of myelin (the fatty material that surrounds some nerve cells), drugs toxic to the ear (for example, aminoglycoside antibiotics or aspirin), anesthesia, tumors involving the ear or brainstem, and degeneration of the cochlea in aged dogs. Deafness in one ear or partial hearing loss, or both, is possible in some of these instances. Degeneration of the cochlea in aged dogs is the most common cause of acquired deafness.
Diagnosis of deafness requires careful observation of the animal’s response to sound. The response to touch, smell, and objects that can be seen must be differentiated from the response to sound. In young animals or in animals kept in groups, deafness may be difficult to detect, because the individual being evaluated will follow the response of others in the group. If the animal is observed on its own, after an age when responses to sound are predictable (about 3 to 4 weeks for dogs), then the deafness may be detected.
The primary sign of deafness is failure to respond to a sound, for example, failure of noise to awaken a sleeping dog, or failure to alert to the source of a sound. Other signs include unusual behavior such as excessive barking, unusual voice, hyperactivity, confusion when given vocal commands, and lack of ear movement. An animal that has gradually become deaf, as in old age, may become unresponsive to the surroundings and refuse to answer the owner’s call.
Deafness in one ear is difficult to detect, except by careful observation or by electronic diagnostic tests that a veterinarian can perform. Examination of the external ear using an otoscope (an instrument that allows a veterinarian to see into the ear canal), x-rays, computed tomography (CT scan), magnetic resonance imaging (MRI), and neurologic examination may reveal the cause, especially in cases of acquired deafness. Electronic testing is useful in assessing hearing in puppies of breeds prone to deafness at birth.
Deafness due to blockage of the external ear canal usually responds to appropriate surgical or medical treatment. This deafness is usually not complete. Deafness due to bacterial infections of the middle and inner ear may respond to antibiotic treatment. If deafness is due to persistent intense noise or trauma, recovery is unlikely. Recovery from deafness caused by drugs that are toxic to the ear is rare.
Hereditary deafness may be eliminated from a breed by removal of identifiable carriers from the breeding program.
Deaf dogs do not appear to experience pain or discomfort due to the condition. However, caring for a dog that is deaf in both ears requires more dedication than owning a hearing dog. These dogs are more likely to be startled, which can lead to biting. These dogs are also less protected from certain dangers, such as motor vehicles.
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