Deafness in Dogs
Congenital Deafness

There are two categories of hearing loss or deafness in dogs.  Conduction deafness and sensorineural deafness.  Conduction deafness occurs when there is a failure of proper transmission of sound vibration to the inner ear and auditory (of or relating to hearing) nervous system.  It is more commonly caused by sever otitis external/media (otitis externa being inflammation of the outer ear and ear canal;  otitis media being middle ear infection).  In comparison sensorineural deafness results from abnormalities of the inner ear structures., auditory nerve, and/or central auditory pathways in the brain stem, thalamus and cerebrum.  Sensorineural deafness may be caused by inherited deafness, ototoxic substances causing neural damage, and age-related deafness due to cochlear (the cochlea is a spiral-shaped cavity that is the auditory portion of the inner ear) degeneration.

Congenital deafness can be inherited or result from damage to the developing foetus, caused by either viruses or toxins.  Congenital deafness is often associated with pigmentation disorders, such as a white coat colour and blue eyes.

Congenital deafness has been noted in numerous canine breeds.  The list of affected breeds (more than 48) continuous to expand and may change due to breed popularity and elimination  of the defect through selective breeding.  Congenital/hereditary deafness is frequently seen in Dalmations, Autstralian Heelers, English Setters, Catahoulas, and Australian Shepherds.  Dalmations however have the highest prevalence of deafness of all the canine breeds, with an overall incidence of 15 to 30%. In Dalmations the congenital deafness is consistent with an autosomal recessive, multifactorial gene with incomplete penetrance.  An increased prevalence was found in female Dalmations, especially in those with two blue eyes.  There seems to be no difference in the prevalence of hearing loss between offspring of deaf mothers and offspring of deaf sires.

Congential sensorineural deafness is a permanent condition that is usually present from birth.  It may also develop within a few weeks post-natally (occurring in the period immediately after birth).  Some animals may be affected unilaterally (deafness in only one ear).

The diagnosis of congenital deafness requires careful observation of the dog's response to sound.  This involves a clinical evaluation of hearing loss, followed by a behavioural evaluation.  The response to visual tactile (touch), and olfactory (smell) stimuli must be differentiated from the response to sound.  Unilateral deafness may be difficult to detect, expect by electrodiagnostic procedures.  Further diagnostics include otoscopic (ear) examination, radiography and neurological examination.  In congenital deafness these diagnostic procedures reveal normal anatomic structures despite deafness.  The Brain stem auitrory evoked responses (BAER) is an electodiagnostic test and is a very useful test in assessing hearing in puppies of breeds prone to congenital deafness.  This test provides a quantitative way to determine the type (conduction vs sensorineural) and degree (partial vs complete) of deafness present, as well as the symmetry (unilateral vs bilateral) of dysfunction.  It is advisable that puppies should be at least 6 weeks of age, prior to performing the BAER test for inherited deafness, in order to avoid erroneous conclusions.  The BAER test does however require specialized equipment and may only be available at specialist referral practices.

Removal of identifiable carriers of hereditary deafness from the breeding program is necessary.  The BAER test may be used to identify both unilaterally and bilaterally (both ears) affected dogs, which can then be eliminated from the breeding program.  Other elimination procedures include determination of the mode of inheritance of the deafness in dogs trait by study of pedigree or by test mating.

For more information on other inherited defects in dogs:

Dog health conditions - A chart of many inherited dog health conditions

Canine Leukocyte Adhesion Deficiency  - CLAD

Defects of the vertebral column in dogs - Congenital conditions

Congenital Portosystemic - Shunts in dogs

Defects of the Nervous System of Dogs

Bladder problems - Ectopic Ureter in Dogs

Congenital conditions of the Skin Eyes and Ears of dogs

Dog Hernias

Tracheal Collapse

Return from Deafness in dogs to Veterinary Articles

Veterinary articles supplied by Dr S Strydom and published with kind permission of DIA Publishing - publishers of KUSA Dogs in Africa Magazine

Dr Sunelle Strydom qualified as a veterinarian in 2004 at the Faculty of Veterinary Science, University of Pretoria.  She has a passion for writing and the sharing of knowledge to promote animal health and welfare.  She has written several articles for KUSA Dogs in Africa as well as for Vra vir Faffa on the Landbouweekblad website at www.landbou.com

References

1. Braund K G. 2003. Neuropathic Disorders.  Braund's Clinical Neurology in Small Animals:  Localization, Diagnosis and Treatment.  Available at www.ivis.org as at 22 May 2011
2. The Merck Veterinary Manual (website).  Available at http://www.merckvetmanual.com/mvm/index.jsp as at 21 May 2011
3. Ettinger S J and Feldman E C. 2000.  Textbook of Veterinary Internal Medicine.  W.B. Saunders Company, USA
4. The Free Dictionary by Farlex (website).  Available from http://medical-dictionary.thefreedirectory.com/ as accessed on 22 May 2011

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