Degenerative myelopathy (DM) is a complex and poorly understood disease in dogs, typically diagnosed through a process of elimination. A definitive diagnosis can only be confirmed via necropsy after a dog’s death. DM is generally a late-onset condition, with symptoms appearing in older dogs. According to a PhD scientist from Embark, research on DM is still in its early stages, and not all genetic markers for the disease have been identified. Even dogs that test “clear” for DM can still develop the condition, highlighting the limitations of current testing methods.

Studies suggest that only about 27% of dogs with two copies of the suspected DM gene develop the disease, while the remaining 73% do not. Alarmingly, over 65% of dogs diagnosed with DM test negative for the gene, raising questions about the accuracy of existing tests. Anecdotal evidence from dog forums and social media includes cases where breeders tested their dogs at multiple laboratories and received conflicting results, or where dogs certified as DM-clear were later euthanized due to DM symptoms. The lack of oversight in testing also raises concerns, as there’s no way to verify which dog was actually swabbed, further undermining the reliability of results.

Inaccurate DM testing poses significant risks, including the potential for unnecessarily culling dogs and narrowing the gene pool based on faulty data. Conversely, a false sense of security from a “clear” result could lead to overbreeding affected dogs, particularly popular studs. In Europe, many German Shepherd breeders opt not to test for DM due to inconsistent results and lack of accountability. While some European and American breeders do test, kennel advertisements often only highlight “clear” results, with suggestions that other results are intentionally omitted in the U.S. and elsewhere. There are also claims that some owners of prominent studs misrepresent their dogs’ DM status, and in cases where dogs are exported through multiple hands, DM information may be concealed.

Adding to the complexity, some researchers propose that DM in German Shepherds may be a distinct disease, potentially requiring different tests altogether. Despite these uncertainties, many puppy buyers demand DM test results, often misunderstanding terms like “DM clear,” “DM carrier,” and “DM affected.” This has led some breeders to use test results as a marketing tactic rather than a reliable health indicator. To mitigate risks, responsible breeders can offer contracts guaranteeing replacement puppies for serious health issues like DM.

While I test my dogs for DM, I believe the focus should extend beyond testing. Similar to hip dysplasia, environmental factors such as injuries or nutritional deficiencies may trigger DM in genetically predisposed dogs. Until science provides clearer answers, breeders and buyers should approach DM testing with caution, recognizing its current limitations and the need for ongoing research.

More about DM.

This is a small excerpt from the article/lecture given at the WSAVA by Jerold S Bell DVM, Dept. of Clinical Sciences, Tufts Cummings School of Veterinary Medicine, N. Grafton, MA USA jerold.bell@tufts.edu

(Lecture presented at the World Small Animal Veterinary Association 2021 Virtual World Congress 13 Nov. 2021)

GENETIC COUNSELING

Degenerative myelopathy is not a treatable disease, and no intervention will alter its fatal outcome. The high gene frequency of the sod1 variant across breeds and the infrequent clinical presentation of DM in any breed causes much confusion in genetic counseling. With public acceptance of direct-to-consumer multiplex panel genetic testing for dogs, all panel tests include sod1 variant test results regardless of their relevance to individual breeds and dogs. In all breeds (and mixed-breed dogs) a DM “at risk” result places a significant and unnecessary emotional burden on owners who believe that their family member will develop DM and die from the disease – which is highly unlikely to occur. A decision to euthanize a dog due to a DM “at risk” test result when differentials include common treatable diseases such as disc disease and musculoskeletal disease are unacceptable and borders on malpractice.

The greatest issue with the misuse of sod1 genetic test results (both in breeds with and without confirmation of affected DM dogs) is where breeders are devastating their gene pool diversity by selecting against the sod1 variant. In many breeds, heterozygous carriers and homozygous “at risk” dogs represent the majority of the breed. The only situation where sod1 test results should be considered in making treatment and breeding decisions is when there are close relatives confirmed with clinical DM (and therefore a high probability of carrying other unidentified but necessary mutations for clinical disease).