Dogs may be congenitally deaf or may acquire deafness later in life from trauma to the head, infections in the ear, or because of cochlear degeneration.
Congenital deafness in dogs may be an inherited condition or may occur due to damage to the unborn puppy’s ears caused by viral infections or ingestion of toxic substances by the pregnant mother. Merle patterns and white color of the fur are found to be associated with congenital deafness, not only in dogs, but in many other animals also. Dalmatians, Catahoula, Australian Shepherd and Heeler, Old English sheepdog, West Highland White Terrier, Boston Terrier, English Setter, Boxer, Great Dane are some of the breeds that are usually affected by congenital inherited deafness.
There are about fifty similarly affected breeds. While continuous inbreeding may add to the list of breeds affected by the inherited form of deafness, selective breeding, as in the case of Cocker Spaniels, may help overcome this trait. Deafness used to be a common trait of Cocker Spaniels, but it is no longer a regular occurrence in this breed.
Chronic inflammation of the external ear canal may result in blockages that may cause deafness. Damage to the middle ear, the hearing organ called cochlea, or to the other parts of the inner ear may render the dog deaf. Trauma to the head, especially involving the temporal bone around the dog’s inner ear is another cause. Exposure to very loud noises such as a gunshot may also impair hearing. Damage to the myelin sheath that protects the auditory nerves is a neurologic reason for deafness. Use of certain drugs such as aspirin and antibiotics containing aminoglycosides may result in loss of hearing. Tumors in the brain stem or in the inner parts of the ear and gradual cochlear degeneration due to old age are two other reasons. Deafness may affect either one ear or both the ears, and in varying degrees of severity.
Since dogs may respond to a number of stimuli such as smell, movement and touch, it is rather difficult to diagnose deafness in them. Careful observation is necessary to distinguish between their responses to other stimuli and the response to sounds or the lack of it. Puppies and dogs that live in the company of other dogs learn to take their cue from the responses of other dogs, making it nearly impossible to make out their deafness. Dogs have to be evaluated individually, and puppies should be about 3-4 weeks old, before deafness can be diagnosed by observation.
Deafness in dogs is often detected accidentally when the dog fails to respond to calls or other sounds in the environment. Difficulty in waking up a dog by making a noise, or by calling out its name, its lack of alertness towards a nearby source of noise, or the dog not perking up at a distant noise etc., may indicate total or partial deafness. Other usual symptoms are hyperactivity, barking excessively or in a peculiar voice, confusion, or lack of any response to vocal commands and lack of movement of the pinnae. In gradually developing deafness, often seen in senior dogs, the dog’s response to the owner’s commands may decrease over time until it no longer reacts to its name being called by the owner.
If the dog is deaf in only one ear, it is hard to recognize it, as its hearing is compensated by the good ear. If it is suspected, the veterinarian may conduct electronic tests to diagnose it. While the otoscope allows a clear view of the external ear, x-rays, CT scans and MRI scans can help in the diagnosis of abnormalities in the middle and inner ear. Neurologic tests are also used to identify the cause. Puppies belonging to the breeds which have a high probability of congenital deafness are electronically tested for hearing at birth.
Acquired deafness resulting from infections in the middle and inner ear are usually resolved by antibiotic therapy. If the dog is deaf because of any blockages present in the external ear, medical and surgical treatments may help restore hearing. Deafness due to blockages is usually partial deafness. If severe trauma or continuous high decibel noise or certain viral infections have resulted in deafness, the dog may be able to recover partial hearing following the treatment, or it may remain deaf in spite of the treatment. It may not be possible to reverse acquired deafness caused as a side effect of certain drugs, or as result of ingesting toxic substances.
By preventing carriers of the deafness trait from breeding, congenital deafness due to hereditary factors may be successfully removed from a given breed.