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Health Scoop

Canine Degenerative Myelopathy and Genetic
Testing in Australian Terriers

Jerold S Bell DVM, Dept. Clinical Sciences, Cummings School of Veterinary Medicine at Tufts University

Degenerative myelopathy (DM) is a specific genetic disease causing While all pathologically confirmed DM affected dogs are homozygous
spinal cord degeneration in older dogs. It has a complex mode of for the sod1 mutation, the vast majority of homozygous “at risk” dogs
inheritance with the effect of more than one gene pair having to com- in confirmed DM breeds will never develop clinical DM. This is because
bine to cause a clinically affected dog. Affected dogs begin to show they do not carry the other as-of-yet unidentified genetic mutation(s)
signs of the disorder usually between 8 and 14 years of age. These necessary for DM development. In all breeds that are pathologically
signs include; weakness of the hind legs, scuffing of the hind toes, confirmed with clinical DM, the frequency of homozygous “at risk”
crossing of the hind legs, and progressing to dragging of the hind testing dogs is exponentially greater (up to 45X) than the frequency
legs. The progression is slow, occurring over 6 months to 2 years. At of clinical disease in the breed. Many clinically affected DM breeds
the advanced stage of the disease dogs can lose urine and bowel con- test 14% to 50% homozygous “at risk” but have less than 1% clinically
trol and begin to show signs of weakness in the forelimbs. DM is not a affected dogs.
painful disease but one of loss of motor (muscular) function. There is
no treatment that alters the clinical progression of DM. Affected dogs Due to the high frequency of the sod1 mutation but the low frequen-
eventually have to be euthanized due to the severe clinical signs. A di- cy of clinical disease in confirmed DM breeds, it is recommended
agnosis of DM can only be confirmed through a pathological examina- to not use the results of the sod1 mutation in making breeding
tion of the spinal cord. DM has only been confirmed in 17 dog breeds, decisions unless there are confirmed DM affected close relatives
and Australian Terriers are not a confirmed breed. (and therefore a high risk of carrying the other required mutation(s)
for clinical disease). If breeders attempt to eliminate the sod1 muta-
There are many diseases that can mimic DM - especially those that tion, or only breed dogs carrying the sod1 mutation to normal testing
cause hind end lameness or weakness. These include; hip arthritis, dogs, breed gene pools will shift and contract causing unnecessary re-
lumbosacral stenosis/cauda equine syndrome, intervertebral disc striction of breed gene pool diversity. In breeds where pathologically
disease, spinal lymphoma or other cancers that can grow in the spinal confirmed DM does not exist, the sod1 mutation results should never
canal, fibrocartilaginous embolism, discospondylitis, spinal trauma, be used to make breeding decisions as they are not related to disease
and any other disease that can cause muscle weakness or hind limb in the breed. Without a pathological conformation of DM in the breed
lameness. there is no reason to test Australian Terriers for the sod1 mutation.

In 2008, researchers at the University of Missouri identified a recessive In the breeds where clinical DM exists the sod1 test is an excellent
mutation in the sod1 gene that is homozygous “at risk” (carrying two test to rule out a clinical diagnosis. Dogs that do not test homozygous
copies) in all pathologically confirmed DM affected dogs. (The only “at risk” do not have DM and have a different disease process causing
exception to this is a different mutation in the sod1 gene that is found their clinical signs. A sod1 test result cannot confirm a clinical case of
only in the Bernese Mountain Dog.) Dogs that do not have two DM.
mutated copies of the sod1 gene will not develop DM. A genetic
test for the sod1 mutation is available from several dog DNA testing The experts researching DM have never confirmed a pathological
laboratories including the University of Missouri (OFA testing). diagnosis in the Australian Terrier. If an Australian Terrier is sus-
pected of having clinical signs referable to DM, they should be worked
The sod1 mutation is an ancient mutation in the dog genome and is up for other treatable disorders and possibly examined by a veterinary
the most frequent mutation identified in the genetic screening of neurologist. If an Australian Terrier is suspected of having DM and
mixed-breed and purebred dogs. Mars/Genoscoper testing finds that eventually has to be euthanized, the dog should undergo a complete
the sod1 mutation frequency is 7.77% in all mixed-breed dogs tested, necropsy including a microscopic spinal cord evaluation. This is the
and 5.41% in all purebred dogs tested. The mutation has been found only way to definitively diagnose what is going on with the dog. This
in over 120 breeds, but the clinical disease has only been patho- is important for the owner, the breeder, and the breed. If you feel that
logically confirmed in 17 breeds. a dog you own or have bred has or had DM, please contact me. It is
possible for any breed to have clinical DM, however over 25 years of
As DM is a complexly inherited disease, clinically affected dogs must research has shown the sod1 mutation to be common, but clinical DM
have a mutation in another (yet unidentified) gene or genes in addi- to be limited to a small number of breeds.
tion to being homozygous for the sod1 mutation. If a breed does not
carry the other causative mutation(s) the presence of the sod1 muta- I will be addressing DM as well as genetic disorders that occur in the breed
tion is of no consequence to the breed. Some breeds - both those with during the breed health seminar at the National Specialty in Asheville, NC
clinical DM and those without clinical DM - have sod1 mutation fre- at 9 AM on May 24th.
quencies of 50% to 90%. The gene frequency of the sod1 mutation
in the Australian Terrier has not been published by any testing This article can be reproduced with the permission of the author;
agency, but the frequency is irrelevant if it is not associated with jerold.bell@tufts.edu
clinical disease.

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