Deafness, and the coat colours that cause it

Why colour and hearing are connected

The inner ear contains a layer of cells, the stria vascularis, that depends on melanocytes — the same pigment-producing cells that colour skin and hair — to maintain the chemistry the hearing hair cells need.

Genes that remove pigment do not target the coat specifically. They reduce or eliminate melanocytes wherever they would have gone, the inner ear included. Without them the structure degenerates in the first weeks of life and hearing is permanently lost.

That is why the same three genetic patterns keep appearing: the dominant white gene in cats, merle in dogs, and the piebald and extreme white spotting seen in Dalmatians, Bull Terriers, English Setters and Australian Cattle Dogs.

Albinism is a different mechanism and does not carry the same risk — an albino animal makes melanocytes but cannot make pigment in them, so the inner ear still works.

Deafness can affect one ear or both, and one-sided deafness is the case people almost never notice.

Which animals

White cats with two blue eyes have a high rate of deafness — commonly cited around 65 to 85 per cent. A white cat with two orange eyes is much less often affected, and an odd-eyed white cat is frequently deaf on the blue side only, which is exactly the pattern the pigment explanation predicts. It affects any white cat, including the Khao Manee, Turkish Angora and Turkish Van, and a white domestic cat with no pedigree at all.

Double merle dogs — the offspring of two merle parents — are frequently deaf, blind or both, and that pairing is avoidable.

Dalmatians are the most-studied breed, with roughly a fifth to a quarter affected in one or both ears.

Others with a raised rate include Bull Terriers, English Setters, Australian Cattle Dogs, Catahoulas, Jack Russells, Boston Terriers, Dogo Argentinos and the white bulldog types.

Deafness also arrives with age, which is far more common than the congenital kind and much less discussed. An older animal that has become unresponsive or started sleeping through things is often losing its hearing rather than becoming stubborn.

The BAER test

Brainstem auditory evoked response is the only way to know. It measures the electrical response of the brain to a click played into each ear, so it tests each side separately and does not depend on the animal's cooperation or on interpreting behaviour.

It takes minutes, needs no anaesthesia in most animals, and can be done from about five weeks old.

Home tests — clapping, keys, calling from behind — cannot detect one-sided deafness at all, because an animal with one working ear responds normally. It may be poor at working out where a sound came from, and that is the only clue.

Breeders of the affected breeds should be BAER testing litters, and in Dalmatians it is standard. Ask for the result rather than an assurance.

Living with a deaf animal

Deaf animals live full, ordinary lives, and most owners describe the adjustment as smaller than they expected.

Hand signals work as well as voice, and often better — dogs read gesture more naturally than speech. Train exactly as you would otherwise, with a visual marker such as a thumbs-up in place of a clicker or a spoken good.

Get attention with vibration through the floor, a hand near the shoulder, or a torch flicked on and off.

Wake them by touching the body gently and consistently rather than the head, and teach that being woken is followed by something good. A startled deaf animal can snap, and this is the single most useful thing to get right around children.

Traffic is the real danger. A deaf animal cannot hear a car or be called back, so a secure garden and a lead or a long line outside it are not optional.

A vibrating collar — not a shock collar — is a useful recall cue in a safe enclosed space.

And tell people. A tag or a harness saying the animal is deaf explains a lot to a stranger who has just been ignored.