Degenerative Myelopathy in Dogs: DM Testing and What Results Mean for Breeding

DM is fatal, incurable, and increasingly prevalent — but DNA testing gives breeders the tools to make informed decisions

Degenerative Myelopathy (DM) is a progressive neurological disease that affects the spinal cord in dogs. It begins with weakness in the hindquarters and progresses to complete paralysis, then loss of bladder and bowel control, and eventually loss of the ability to breathe independently. There is no effective treatment and no cure. Most affected dogs are euthanized within 1-3 years of diagnosis.

Understanding DM genetics is essential for any breeder working with at-risk breeds.

What Is Degenerative Myelopathy?

DM bears a striking resemblance to Amyotrophic Lateral Sclerosis (ALS or Lou Gehrig's disease) in humans — both involve the progressive degeneration of motor neurons in the spinal cord. In dogs, the disease typically begins with mild wobbling or weakness in the rear legs, progressing through:

• Stumbling and scuffing of rear paws
• Difficulty rising from a lying position
• Progressive weakness — dogs walk with a swaying hindquarters
• Loss of proprioception (the dog does not know where its feet are)
• Paraplegia — inability to use the rear legs at all
• Ascending paralysis to the thoracic limbs
• Loss of bladder and bowel control
• Loss of respiratory muscle function in terminal stages

Average time from first symptoms to paralysis: 6-12 months. Average time from paralysis to euthanasia: an additional 6-18 months.

The Genetic Mutation: SOD1

The primary genetic mutation associated with DM in most breeds is in the SOD1 gene (superoxide dismutase 1). Two variants have been identified:

• SOD1A — The original mutation found across many breeds
• SOD1B — A second mutation found in Bernese Mountain Dogs and some other breeds

A dog must have two copies of the mutation (one from each parent) to be at risk for developing DM. However — and this is critical — not all dogs with two copies of the mutation develop DM. The mutation confers risk, not certainty. Other genetic and environmental modifiers likely play a role.

Dogs with one copy (carriers) do not develop DM but can pass the mutation to offspring.

Breeds With High DM Prevalence

DM has been documented in dozens of breeds, but prevalence is notably high in:

• German Shepherd Dogs — DM was first documented extensively in this breed
• Pembroke Welsh Corgis — Very high carrier frequency
• Boxers
• Chesapeake Bay Retrievers
• Rhodesian Ridgebacks
• Bernese Mountain Dogs (often SOD1B)
• Golden Retrievers
• Labrador Retrievers
• German Wirehaired Pointers
• Wire Fox Terriers

In some Corgi populations, carrier frequency exceeds 50% — meaning the majority of the population carries at least one DM mutation.

DNA Test Results: What They Mean

DNA tests for DM report results as:

Clear (N/N) — No copies of the tested SOD1 mutation. Dog will not develop DM from this mutation and cannot pass it to offspring.

Carrier (N/DM) — One copy. Dog will not develop DM. Can pass the mutation to 50% of offspring.

At Risk (DM/DM) — Two copies. Dog may develop DM during its lifetime. Importantly, not every DM/DM dog develops the disease — but they are at significantly elevated risk.

How to Interpret "At Risk" Results

The term "At Risk" rather than "Affected" is intentional. The SOD1 mutation is necessary but may not be sufficient for DM to develop. Incomplete penetrance means some DM/DM dogs live long lives without developing the disease.

However, from a breeding standpoint, an "At Risk" (DM/DM) result should be treated seriously:

• Do not breed At Risk dogs to Carrier dogs (produces 50% At Risk offspring)
• Do not breed two At Risk dogs together (all offspring At Risk)
• Breeding an At Risk dog to a Clear dog produces all Carriers — which is manageable but adds DM to the breeding population

Best practice: work toward clear lines by breeding Clear to Clear when possible, or at minimum, Clear to Carrier (producing 50% clear, 50% carrier, no At Risk).

The Challenge of High Carrier Frequency

In breeds like the Pembroke Corgi, the DM mutation is so widespread that breeding exclusively Clear to Clear dogs would eliminate the majority of the breeding population, including dogs with excellent health, temperament, and structure in every other respect.

This is why some breed clubs recommend Clear to Carrier as the minimum standard: it prevents At Risk puppies while preserving genetic diversity. Over generations, selective breeding moves the population toward a higher proportion of Clear dogs.

What to Ask a Stud Dog Owner

For any at-risk breed, ask the stud dog owner for:

1. DM DNA test results (SOD1A and SOD1B where applicable)
2. Which testing laboratory was used
3. Whether he is Clear, Carrier, or At Risk

Know your own dog's DM status before the breeding, and make sure the combination of the two dogs does not produce At Risk offspring.

Summary

Degenerative Myelopathy is a fatal, progressive spinal disease linked to SOD1 mutations. Dogs with two copies of the mutation are At Risk for developing DM, though not all will. Carriers will not develop DM but pass the mutation to half their offspring. Never breed Carrier to Carrier or At Risk to Carrier — it produces At Risk puppies. Work toward Clear-to-Clear or Clear-to-Carrier breedings. DM is one of the most important health tests in several popular breeds and should be a non-negotiable part of your screening process.