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  • Writer's pictureGina White

Canine Degenerative Myelopathy Interviewing the Experts

Updated: Dec 14, 2022

by Gina White - Zwinger Der Meer Küste


As an owner and breeder of German Shepherd Dogs for over

thirty years, I have been asked many health-related questions

regarding the breed I am so enamored by.

In the past several years, aside from hip and elbow dysplasia, the

biggest concern from GSD owners is the occurrence of

degenerative myelopathy, or DM. For those not familiar with

DM, it is a progressive degenerative spinal cord disease,

typically occurring in dogs over 8, which is characterized by

hind limb paralysis and an eventual loss of other motor

functions.

Since first described in the early 1970s, DM was understood to be a degeneration of the spinal cord due to an unknown cause. But in 2009, Dr. Joan Coates, a veterinary neurologist and associate professor of veterinary medicine and surgery in the MU College of Veterinary Medicine, and her peers, identified

the mutation in the gene superoxide dismutase 1 (SOD1) as the primary cause of DM. Mutations in SOD1 are associated with

some forms of human amyotrophic lateral sclerosis (ALS), also

known as Lou Gehrig’s disease, which is adult in onset, causing

muscle weakness and eventually respiratory failure therefore

sharing many traits of the canine expression of this mutation. Dr.

Coates begins, "The SOD1 mutation is mostly recessive (2

copies); however, some carriers in certain breeds like German

Shepherds get DM. The mutation is a risk factor with higher risk

in dogs with 2 copies."

In other words, not all dogs that have the mutation will develop

DM so the mutation test, although beneficial to breeders and

owners alike is currently a test for risk.

Dr. Coates goes on to say the SOD-1 mutation is an ancestral or

very old mutation, probably present since the first GSD, Horand

(Hektor) was registered by the SV in 1899 but so many years

later, it would be impossible to say for sure. When asked why

the disease only seems to be on the radar in recent decades she

says, "I believe the gene mutation has been present since the

breed's inception. In my opinion and I have no statistical or

empirical evidence to support it, but given that DM is a late

onset disease and dogs are living longer due to better lifestyle

and healthcare, may be one reason the 'known' incidence of

disease is increasing."

Dr. Daniel Kelleher, DVM, who practices reproductive medicine

at Broadview Veterinary Hospital in Dover, NH, also added in

his knowledge regarding DM. 'I think DM has been there all

along as well but now it's something that is actively tested for"

he states. "I think the notion of bad breeding is sort of a

misconception since breeders didn't know how to detect the

problem earlier on, so how could they have known"? 


Dr. Kelleher adds that since it is known to have a genetic basis,

he believes that outside sources are unlikely to play a major role.

"My understanding is that the disease is autosomal recessive

with incomplete penetrance. This means that one gene from each

parent is required for a dog to develop symptoms (phenotype)

consistent with having two copies of the gene (genotype)."

He goes on with a brief overview of Mendelian Genetics. "The

probability of each puppy inheriting two copies of the gene from

affected parents is 100%. The probability of each puppy in a

litter born to parents that are carriers (heterozygous) is 25%

chance to have both affected copies (homozygous), 25% chance

to be clear, and 50% chance to be carriers (heterozygous). Dogs

without both affected copies will therefore be negative for

symptoms." 

Dr. Kelleher explains that the test is not uniform across breeds,

even for those that do share similar SOD1 mutations.  "This is

because there are specific locations along the gene called alleles

that are variable among breeds.  In other words, the test does not

apply well to all breeds." He does believe, however, that in his

experience it is most accurate in predicting the risk of DM in

Corgis and GSDs.

"I believe genetic testing is in most cases a complement to other

pre-breeding health testing," he says. ";For example, it would be

ill-advised to breed a dog with a high PennHip distraction index

that is also a DM carrier. The DM carrier piece is less important

but when combined with another disease process (hip dysplasia)

that has a multifaceted genetic basis, you can use it to optimize

decision making."  


Robert Westra, Associate Medical and Veterinary Market

Development Director at Neogen, another expert in the study of

DM, reiterates that one mutated copy of the SOD1 gene from

each parent is the factor for concern. "Having only one copy

creates a carrier who will not get the disease (per current

thinking). Because the mutation has recessive inheritance, a dog

that only inherits one copy of the mutation is not typically at risk

to develop clinical signs."

Many breeders who are not familiar with the mechanism of

inheritance, are ONLY breeding non-carriers to other non-

carriers which is of great concern, given the limiting of the

German Shepherd gene pool.

"Through genetic testing, we encourage the identification of

‘carriers’ not to eliminate them from the breeding population,

but rather to maintain them in any program by pairing them to

an appropriate ‘clear’ mate so the positive features and traits

within the ‘carrier’ have the potential to be conserved," he

answered. "At the same time, this breeding strategy will not

produce any ‘at-risk’ pups."

Dr. Westra counters, "It is possible that by selectively breeding

for certain other attributes, that some breeders may be including

affected dogs (homozygous) or carriers (heterozygous) in their

programs unintentionally." He mentions this is especially

plausible if testing is not performed prior to breeding."

There are other mutations we deal with that are more pertinent

to this question than DM," he adds. "These mutations are found

in such high frequency in a specific breed that if an aggressive

breeding strategy is adopted to eliminate the mutation, too many

dogs would be disqualified from breeding. That may lead to a

genetic bottleneck and may have deleterious consequences to the

breed."

In closing, Dr. Westra concluded that although the SOD 1 gene

mutation is responsible for the vast number of symptomatic

cases, there may be other genes that could play a role. "Other

breeds where this variant occurs but is not associated with DM

risk likely have genetic factors protecting them from this

disease. Because there is still so much to understand about

genetics we cannot say that a similar neurologic condition

arising from an unknown mutation does not exist."

So it is important to remember that even in breeds like the

German Shepherd, where DM association is recognized, many

dogs testing at-risk from the variant will live long lives and

never develop symptomatic disease.


 

Special Thank You To The Following:


Interviews:

Dr Joan R. Coates

Dr. Dan Kelleher

Dr. Robert Westra


https://embarkvet.com/breeders/resources/canine-genetics-

for-dog-breeders/health-spotlight/dm/

https://www.scientificamerican.com/article/mind-control-by-

cell/

https://www.caninejournal.com/degenerative-myelopathy/

http://vhc.missouri.edu/small-animal-hospital/neurology-

neurosurgery/facts-on-neurologic-diseases/degenerative-

myelopathy/

https://scitemed.com/article/2792/scitemed-cmt-2019-00105

https://www.sciencedaily.com/releases/2009/01/09012117412

4.htm

https://pubmed.ncbi.nlm.nih.gov/26432396/



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