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This spring has been a bit of a stop-start affair. The sun trying to warm us up despite cold winds pushing down from the north for much of April. Hopefully, by June we will be basking in southern breezes! It always amazes me that they survive at all, let alone have the strength to sing in cold early mornings. Maybe it’s their way of celebrating life as much as claiming territory and finding a mate. One morning, as I prepared to cycle into the Sherborne surgery, I paused to listen to some birdsong. It was perched up in a cherry tree, so small it was hidden by a bunch of blossoms. The sound was so complex it entranced me for several minutes. Afterwards, I started thinking about the special sense that allowed me the experience. The concept of a dog’s hearing is also frequently mentioned in the consulting room.
Our ability to hear, like vision, is often taken for granted until it begins to fail. At least as humans, we can boost our abilities with high-tech hearing aids and glasses. Unfortunately, in veterinary medicine, we can only address medical conditions of the outer ear that may cause deafness. It is not uncommon for a pet owner to tell me they think their dog may be deaf. Although, we have yet to invent a canine hearing aid or cochlea implant!
A dog’s hearing, similar to vision, depends on a linked system. This transmits sound from the outer ear through to the auditory cortex of the brain. On the way, the sound waves have to be converted into nerve impulses that are sent up the auditory nerve. The connections on the way to the cortex are complex. They involve cross-overs to the other side of the brain and switchbacks so both sides receive impulses from each ear. In the middle of this, is our old friend the Reticular formation. This is responsible for waking us up when the alarm clock rings. It links with areas of the brain that govern our emotions, giving meaning to what we hear. Hence my pleasure at the sound of the dawn chorus. Or for dogs, the wag of a tail in response to a friendly voice.
So, what causes deafness in dogs and cats? Again, a parallel with vision exists. Anything that upsets message transmission from the outside to the sensory part of the brain, can cause hearing impairment. In older dogs and cats, degenerative changes in the inner ear and brain can lead to “selective” deafness. My (dear departed) Border terrier, Trilby, seemed as deaf as a post in later years. Interestingly, a dog’s hearing often hasn’t degenerated enough to miss the rustle of a crisp packet though! We learned this with Trilby!
Of course, it is entirely possible to lose the ability to hear certain frequencies and retain others. Although it’s funny that the human voice is one of the first to go, in both man and animals! This may be due to the structure of the cochlea. The snail-like organ in our inner ear acts as the transducer. It turns sound waves into nerve impulses. Its coiled shape saves space, a bit like a spiral staircase. Different frequencies are detected at different locations by specialised pigmented cells. If parts of the staircase lose their treads, gaps appear in our hearing.
For many dog and cat breeds, lack of skin pigmentation is associated with deafness, present at birth. A white coat and blue eyes have been associated with deafness for many years. Common in Persian cats but Turkish Vans and many other pure and out-bred cats can be affected. Lack of, or complex, coat pigmentation is also associated with issues in a dog’s hearing. Inherited deafness in dogs, is particularly common in:
Inherited deafness is obviously a concern to owners and breeders, particularly of those breeds primarily affected. The ear canals of puppies open at about 2 weeks old, so responses to sound start after that. Although it’s difficult to recognise deafness as behaviour is similar to normal pups. There is, however, a test for hearing in dogs and cats that can be performed from 5 weeks old. It is called the Brainstem Auditory Evoked Response (BAER) test. Picking up signals in the nerve pathways, it can detect hearing in each ear independently. It does so by using a series of clicks to stimulate responses in the auditory system. Similar to an ECG, electrodes pick up the signals and display them for the audiologist to interpret.
The BAER test can also be performed in older dogs, although sedation may be needed. This is not usually necessary in young puppies as they sleep between feeds. They tend to remain quiet for the duration of the test, which takes 10-15 minutes. There are a number of testing centres in the UK, none are very close (Cheltenham and Worcester are closest). The Mobile BAER Testing Clinic visits the South West. They have a Facebook page and website that describes their service and booking information (tel. 07912 027332).
Unfortunately, congenitally deaf puppies and kittens cannot have their hearing restored. Breeding can be planned to limit the spread of the defective genes responsible. Essentially, only animals with hearing in both ears should be bred. Due to the complexity of the genes involved, however, deaf offspring are still possible.
Deafness in older dogs and cats is commonly associated with chronic ear and skin conditions. It can cause problems such as:
Caught early, medication can arrest progression but in some cases, surgery to remove chronically affected tissue is necessary. Although this in itself can adversely affect hearing, it allows the patient to have a pain and infection-free life. As you might expect, there is plenty of information about deafness in dogs on the internet. As always, be careful what you read but the social effects of deafness on canine behaviour. It is described well by the Association of Pet Behaviour Counsellors (APBC).
