Dogs and cats generally hear very well, but that isn't always the case.
They may be born or become deaf, or become hard of hearing.
Dogs and cats use their hearing to engage with their environment, to sense potential hazards, and to communicate with other animals and of course with people.
With good care, deaf animals can continue to lead quite happy lives.
There are four major types of deafness in animals:
1. Conductive hearing loss: is caused by the failure of transmission of sound energy into the outer and middle ears, often due to inflammation, polyps or growths in the ear canal, or build-up of material such as wax in the external ear canal. This is potentially reversible if whatever is obstructing the ear canal can be removed. This may require flushing the ear out (usually under anaesthetic) or surgery.
2. Sensorineural hearing loss: occurs due to a failure of transmission of sound from the cochlear nerve due to loss of cochlear hair cells.
3. Central-mediated hearing loss: results from a failure to process sound at the level of the brain, for example due to brain disease.
4. Presbycusis: is age-related hearing loss associated with degeneration of the inner ear structures such as the cochlear, for which other causes of hearing loss are not found.
The most common types of deafness in dogs and cats are congenital, associated either with a white coat and blue eyes in cats, or piebald and merle genes in dogs.
Some breeds have a higher rate of deafness than others.
This includes the Dalmatian, Australian cattle dog, bull terrier, English setter, English cocker spaniel, Jack Russell terrier and Boston terrier.
Hearing loss can occur in one or both ears, though it is much easier to detect if both ears are affected.
Hearing loss isn't all bad news.
Animals that are born deaf, and those that lose their hearing in the first few weeks of life, can adapt very well.
They rely on their other senses to navigate the world, although it is recommended that when outdoors, deaf dogs are supervised at all times as they cannot hear auditory warnings signalling potential hazards (like the sound of an approaching vehicle).
Deaf cats should be kept strictly indoors for the same reasons.
Acquired deafness is typically gradual in onset, but it can be sudden - for example in the case of some brain diseases, or secondary to toxicity from ear medication.
Sudden-onset hearing loss is associated with more dramatic signs, as animals don't have time to adapt.
These animals may show signs of distress, including loud vocalisation or sudden loss of response to verbal cues.
Age-related hearing loss tends to be gradual. Often owners notice that their pet "sleeps in" as they aren't roused by signs that might normally wake them, like the doorbell or the sound of the fridge opening.
Hearing loss isn't all bad news.
I've had canine patients with terrible noise phobias that resolve in their senior years when they become hard of hearing.
Dogs and even cats are capable of learning to read and respond to hand signals if these are clear and used consistently by the owners.
Deafness is definitively diagnosed via brainstem auditory evoked response (BAER) testing, usually only available at specialist facilities and requiring a general anaesthetic.
It is recommended that animals with congenital hearing loss are not bred.
If you are concerned that your dog or cat has experienced loss of hearing, consult your veterinarian.
Dr Anne Quain BVSc (Hons), MANZCVS (Animal Welfare), Dip ECAWBM (AWSEL) is a lecturer at the Sydney School of Veterinary Science and a practising veterinarian
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