Biology 103
2005 Final Paper
On Serendip

Around five years after we got Mickey, we began to notice that he frequently was not responsive to his name being called. We thought that perhaps he was just a bit defiant, but after various home made tests, it seemed clear that he was losing his hearing. If his back was turned, Mickey rarely responded when pots and pans were banged together. He never sang along anymore when I practiced the violin. It seemed crazy that such a young dog would be going deaf, but the veterinarian was able to verify it. She covered Mickey's eyes and brought him into the back of her office where various sick dogs barked and whined. He didn't appear to hear any of them.

Congenital deafness in dogs can be caused by many of the same reasons as in humans. It can be genetic, or acquired through a number of possibilities. Drug toxicity (also called ototoxicity), the negative effect of the dispensation of a drug or chemical, can directly or indirectly damage cochlear hair cells. This may result in hearing loss or total deafness. Aminoglycoside antibiotics (including gentamicin, kanamycin, neomycin, tobramycin and others), which are sometimes the only treatment for a life-threatening infection in dogs, are the most common drugs which cause ototoxicity. Ear cleaning solutions which contain chlorhexidine and other less common chemicals may also cause deafness. These solutions have since been taken off the market. Drug toxicity can also be vestibulotoxic and disturb the dog's sense of balance, giving it a head tilt and sometimes causing it to walk in circles. (1) Like aminoglycoside antibiotics and chlorhexidine, general anesthesia may also cause deafness. While the causes of this possibility have yet to be established, it may be that after receiving general anesthesia, the dog's body sends blood away from the cochlea to shield other critical organs, or that the positioning of the dog's jaw constricts the arterial supply and keeps it from reaching the cochlea. Deafness sometimes occurs when a dog has gone under anesthesia for an ear or teeth cleaning. (1)

Two additional possibilities for acquired hearing loss are noise trauma and ear infections. Depending on the volume of a noise, temporary or permanent hearing loss can be caused. The middle ear has small muscles which "reflexly contract to reduce sound transmission into the inner ear in response to loud sounds and prior to vocalization), which helps in sustained or continuous noise." Percussive noises (gun fire, explosions), though, are too quick for the middle ear muscles to protect the inner ear, and hair cells and support cells are disrupted. Infections of the middle ear (otitis media) or inner ear (otitis interna) can also cause deafness. Both types of infection can leave behind "crud" which blocks sound transmission. In the case of otitis media, the body can clear out this "crud" and hearing can gradually improve. Otitis interna, however, will result in permanent nerve deafness if it is not treated right away. (1)

More common than acquired deafness in dogs is inherited deafness, caused by an autosomal dominant, recessive or sex-linked gene. Deafness usually develops in the first few weeks of life when the ear canal is still closed. It occurs when the blood supply to the cochlea degenerates and the nerve cells of the cochlea die. (2) Congenital deafness has been found in over 85 breeds of dog, with high prevalence in the Australian Cattle Dog, the Australian Shepherd, the Bull Terrier, the Catahoula Leopard Dog, the Dalmatian, the English Cocker Spaniel and the English Setter. (3) If you are familiar with dog breeds, you may notice that these breeds, along with most of the other breeds who suffer from congenital deafness, have some white pigmentation in their coats. It has been suggested that the cause of the degeneration of the blood supply to the cochlea is associated with the lack of pigment producing cells (melanocytes) in the blood vessels. (2)

Inherited congenital deafness in dogs is associated with two pigmentation genes, the merle gene and the piebald gene. The merle (dapple) gene causes dogs to have coats with a mingled or patchwork combination of light and dark areas. The merle gene is dominant. Dogs affected with the merle gene will have the pigmentation pattern on their coats. When two dogs with heterozygous merle genes are bred, 25% of their puppies will not have the merle pigmentation pattern, but a solid white coat and blue irises. They are often deaf and/or blind, and sterile. Pigmentation has been disrupted and has produced deaf dogs. (2) The piebald and extreme white piebald pigment genes are less well-understood. The Dalmation, a breed with a 29.9% deafness prevalence (10), has the extreme white piebald pigment gene, which affects the amount and distribution of white areas on the body of the dog. The genetic pattern of deafness in Dalmations has lead to a great deal of confusion. Deaf puppies have resulted from hearing parents, so deafness does not appear to be autosomal dominant. Pairs of deaf Dalmations have been bred and produced bilaterally hearing and unilaterally hearing puppies. If deafness was recessive, all of the puppies would have been deaf. It is, however, possible that there is a multi-gene cause for deafness in dogs with the piebald pigment genes, such as the existence of two different autosomal recessive deafness genes. (5)

If you have concerns that your dog may have lost its hearing, there are a few ways you can test for deafness. When still in the liter, a deaf puppy may play or bite more aggressively than others because it does not hear that the other puppies are yelping in pain. A deaf puppy may also not wake up at feeding time unless it is bumped by a littermate. Later in life, you will notice that the deaf dog does not respond when it is called or a noise is made. When it is sleeping, far away, or not looking at you, it will not acknowledge that you are calling for it. Home tests for deafness can include rattling a can of coins or keys, squeaking a toy, turning on a vacuum cleaner or ringing the doorbell. The BAER test (Brainstem Auditory Evoked Response) is the only 100% reliable test for deafness in a dog. The test is not painful and can be performed on dogs at least six weeks of age. In the BAER test, a computer records the electrical activity of the brain in response to sound stimulation, measuring the same range of hearing as in human infants. It does not measure the full range of canine hearing, but it is able to determine if a dog has hearing within the normal human range. (4)

Deafness in dogs cannot be cured or treated, but it can be dealt with. Because dogs that are bilaterally deaf can often startle easily and are difficult to train, many are euthanized as puppies. (6) This solution is cruel and absolutely unnecessary. Dogs can be trained to respond to hand signals for approval, punishment and activities such as eating and going outside. The greatest challenge is getting a deaf puppy's attention. Until a signal connected with "attention" is created, the deaf puppy has no "name". Technology can be of help in communicating with the deaf dog. Flashlights are useful in the evening hours, but are of limited use during daylight. During the day, a laser pointer can be used as a way of getting the dog's attention. (7) The possibility of developing hearing aids for dogs where residual auditory function remains has been researched. Dr. A.E. Marshall at Auburn University in Alabama placed human hearing aids in a collar-mounted container, and led a plastic tube from the aid that terminated in a foam plug placed in the ear canal. Smaller dogs tolerated the presence of a foam plug in the ear better than large breeds. These units, however, are generally not favored by veterinarians. (8)

In the case of my own dog, Mickey, my research has led me to believe that his deafness is genetic, not acquired. According to CavalierHealth.org, progressive hereditary deafness is prevalent in Cavalier King Charles Spaniels. Hearing loss generally begins during puppyhood and progresses until the dog is completely deaf, usually between the ages of three and five years, when we noticed Mickey could not hear. (9) I am personally shocked at the suggestion of many websites that a deaf puppy should be euthanized. None of my research indicates that a deaf dog has an inferior quality of life to that of a dog with full hearing. Mickey seems to not notice at all that he cannot hear. He goes along with his happy dog life, eating, sleeping and licking, and communicates with us as well as any other dog I have ever met. Just as is the case with humans, no animal should be selected for life over another according to their health. People all over the world are willing to love all dogs, blind, deaf, healthy and sick. Every person and animal should be allowed to love and to give love in return.

WWW Sources

1)Causes of Sudden Onset of Deafnessa>, from the Louisiana State University website
2)Genetics of Deafness in Dogs, from the Louisiana State University website
3)Dog Breeds With Reported Congenital Deafness, from the Louisiana State University website
4)Frequently Asked Questionsa>, from the Deaf Dog Education Action Fund website
5)Congenital Deafness and its Recognition, from the Louisiana State University website
6)What is Deafness?, from the Canine Inherited Disorders Database
7)Why the Deaf Dog Barks, from the Click and Treat website
8)What About Hearing Aids?, from the Louisiana State University website
9)Deafness is Hereditary and Progressive in Cavalier King Charles Spaniels, from the CavalierHealth.org website
10)Breed-Specific Deafness Prevalence In Dogs (percent), from the Louisiana State University website
11)How Well Do Dogs and Other Animals Hear?, from the Louisiana State University website



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