Complete loss of hearing is called deafness, and can occur in one or both ears. A deafness of both ears is called bilateral, and one ear is called unilateral. It may also happen that the dog has a hearing threshold greater than in normal animals in one or both ears, denominating this condition partial deafness (hearing loss) of one or two ears.
Causes of deafness in dogs and cats
The causes of deafness in pets can be divided into two main categories: conduction deafness and sensor neural deafness. Conductive hearing loss in dogs is observed when there are disturbances in the transmission of sound vibrations to the inner ear and the auditory pathway. Any defect or disease that affects the dog's external ear canal, tympanic membrane, auditory pathway and / or middle ear may be grounds for conduction deafness.
Sensor neural hearing loss occurs when there are abnormalities in the inner ear structures in cochlear nerve and / or the auditory pathway anywhere on their way to the cerebral cortex. The main causes include hereditary deafness, neuronal damage by ototoxic substances (antibiotics such as gentamicin or diuretics such as furosemide) or senile deafness.
Hereditary deafness has been reported in many breeds of dogs and cats. It is caused by degeneration of the inner ear structures and spiral ganglion neurons. This process occurs during postnatal maturation of the auditory system. The clinical signs are among the first few weeks and the first 2 months of life. Were associated with a greater predisposition to hereditary deafness in dogs with predominantly white robe, blue or speckled gray. The breeds most often affected are: Dalmatian, English Setter, Australian Shepherd, Collie and Shetland Sheepdog limit, but has been reported in at least 54 breeds of dogs, including the Gran Danish Rye s, Boxer, Bull Terrier and Cocker Spaniel, including.
It is assumed that hereditary deafness in dogs is predominantly autosomal dominant, however, in the Bull terrier is described an autosomal recessive inheritance.
Brief concepts about inheritance of deafness in dogs and cats
The legacy of disease, genetic abnormalities, or traits is described: a) by the type of chromosome that is the abnormal gene (chromosome autosomal or sex chromosome), and b) if the trait is dominant or recessive. Autosomal diseases (such as deafness) are inherited through the non-sex chromosomes and sex-linked diseases are inherited through one of the "sex chromosomes", the X chromosome (diseases are not inherited by through the Y chromosome). Dominant inheritance occurs when an abnormal gene from one parent is capable of causing disease even though the matching gene from the other parent is normal. The abnormal gene exerts control over the outcome of the gene pair.
Recessive inheritance occurs when both genes must be abnormal to produce disease. If only one gene in the pair is abnormal, then the disease manifests slightly hard of hearing or not manifest. In other words, the normal gene can replace the pair of gene function, so that said abnormal gene recessive acts. However, an animal with a single defective gene is called a carrier, indicating that the disorder can happen to the puppies. Both parents must be carriers for the puppy is deaf.
Chance of inheriting deafness in dogs and cats
In the case of autosomal dominant inheritance: if one parent
is a carrier and the other is normal, there is a 50% chance that each puppy inherits
the abnormal gene and therefore the dominant feature. In other words, if we
assume that in a litter with 4 cubs, one parent is a carrier of an abnormal
gene for deafness, the statistical expectation is: 2 normal and 2 puppies deaf.
This does not necessarily mean that puppies suffer hearing loss, but it does
mean that each puppy has a 50:50 chance of inheriting it. Puppies that do not
inherit the abnormal gene will not develop or pass deafness.
In the case of autosomal recessive: if both parents are
carriers of an autosomal recessive trait, there is a 25% chance that a puppy
inherits two abnormal genes and therefore manifest deafness, and 50% chance
that a puppy inherits only one abnormal gene (thus being carrier). In other
words, assuming that in a litter with 4 puppies, both parents are carriers (not
manifest hearing loss), the statistical expectation is: 1 puppy with two normal
chromosomes (normal), 2 puppies with one normal chromosome and one abnormal
(carriers, without hearing loss), and one puppy with 2 abnormal chromosomes
(deaf). This does not mean that this distribution will be observed
compulsorily, but it does mean that each of the puppies has 1 in 4 chance of
inheriting the disorder and a chance of being a carrier 50:50.
