JRT DEAFNESS
A REPORT ON A STUDY

A E Marshall DVM, PhD and Amy Koterbay MS

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Several members of the JRT Association donated dogs to the College of Veterinary Medicine, Auburn University, for study of deafness as seen in this breed. Ms. Amy Koterbay was awarded the Master of Science Degree for her work and resulting thesis on these dogs. Amy is now enrolled in the School of Veterinary Medicine in Edinborough, Scotland

This is a report to the JRT Association about the findings, some observations and some opinions resulting from this work.

Ten Jack Russell Terriers, ranging in age from 8 weeks to 2 years, were donated. Two had normal hearing, but were excessively aggressive [the reason for the donation]. These two dogs were used as the normal controls against which the abnormals were compared. Seven dogs donated were bilaterally deaf, of these: 5 were 2 to 3 months of age, and 2 were adults. One of the dogs donated was unilaterally deaf.

Every bilaterally deaf dog that we tested in this study was observed to have a solid white haircoat, the only pigment being small grey spots usually located on the skin of the belly region.

After the usual physical examination, each dog was tested for auditory function using the BAER (Brainstem Auditory Evoked Response). After humane euthansia the complete inner ear and brain of each dog was removed for study. The cochlear part of both inner ears were prepared for microscopic study by cutting into thin sections, placed onto microscope slides and stained. Observations and descriptions were made of the cochlea, the spiral ganglion, and the temporal (auditory) cortex of the gross brain.

Observations on the deaf ears included:
1). Consistently, scala media was collapsed with the spiral membrane (Reissner's Membrane)
being collapsed against the spiral organ. Hair cells were absent, or where a few were present there were signs of degeneration. The stria vascularis was abnormal in structure, showing only one of the three cell layers normally present. The spiral ganglion was normal in appearance and cell count. Grossly, the auditory (temporal) cortex of the brain was smaller than normal or was absent in the deaf dogs.
The external and middle ear structures were all normal.

Explanation / "Translation":

It is appropriate to briefly review a description of the ear (of the dog). Some illustrations of the anatomy of this region are included. The ear (of the dog) consists of three major regions: external ear, middle ear, and inner ear (cochlea).

The external ear consists of the pinna and the ear canal. At the end of the ear canal is the tympanic membrane (ear drum) which separates the canal from the middle ear space.

The middle ear is an air space containing three small bones (malleus, incus and stapes) and some small muscles attaching to them. The malleus is attached to the tympanic membrane, and the stapes is in the oval window, which is the connection to the inner ear space. The incus lies between the malleus and stapes, thus serving as the connection between the two.

The inner ear consists of three chambers: scala vestibuli and scala tympani which are connected and also contain a fluid called perilymph. The scala media is between them and contains a fluid called endolymph. Scala media also contains a basement membrane on which the auditory receptor cells, or hair cells, sit. One wall of the scala media is made up of a structure called the stria vascularis, which consists of three cell layers and many blood vessels. The stria vascularis produces the endolymph and is responsible for it's chemical content. Endolymph is the fluid which nourishes and provides the proper environment in which the hair cells function. One of the three cell layers of the stria vascularis is of melanocyte origin. These cells must be present and/or functioning properly for the endolymph to be produced and/or to have the correct chemical content. Without the presence of endolymph or the correct ionic content, hair cells die and degenerate.

Melanocytes are the pigment cells of the animal's body. They originate from a specific site in the embryo (the neural crest), and very early in embryonic development these cells migrate from this site to various "assigned" locations in the animal's body. Migration to the skin is what produces the skin and haircoat color. A defective migration of these cells result in the piebald, mearle, spotted or patched haircoat patterns. This also means that in some individual animals melanocytes may not end up in their proper location on the stria vascularis of the inner ear.

We observed in the bilaterally deaf terriers the stria vascularis had only one cell layer present. One of the missing layers was the melanocyte layer. We believe that the other changes seen (collapsed spiral membrane, collapsed scala media, missing and/or degenerate hair cells, and a reduced auditory cortex) was a result of this defective stria vascularis.

The unilaterally deaf Terrier that we examined was donated due to fight injuries. BAER and microscopic observations of the cochlear structure of the deaf ear showed the same changes seen in the bilaterally deaf dogs.

One donated dog had apparently lost his hearing after a year of age. Microscopic appearance of the inner ears of this dog was not like those observed in the other deaf dogs. It showed degenerative changes which are typical of ototoxic drug injury. Some classes of antibiotics, and other drugs are known to produce injury, thus deafness, to the inner ear. The medical history of this dog was not known.

Observations of the deaf Jack Russell Terriers is almost exactly like the changes described for the deaf Dalmatian. From this, we concluded that the cause of deafness in these dogs is very likely the same origin or mechanism as described for the Dalmatian.

An opinion, (mine): How does a breeder prevent or avoid the deafness trait? Pick color.
Test and cull. Remove from the breeding population any dog that exhibits deafness whether unilateral or bilateral, since this individual is displaying the trait of defective melanocyte migration to the inner ear. I don't know how one could select dogs that show the piebald haircoat and not have the possibility of failed melanocyte migration to the inner ear. I suspect with much culling and a careful selection process, it may be accomplished.

There is a lot more to this story, but this is the essence of our observations.

In conclusion: Our sincere thanks to the individuals who donated their dogs for this study. With this study Amy Koterby secured her dream of studying to become a veterinarian, and hopefully we are able to give the JRT Association some information of value about this trait.

7/25/02