A DM carrier is a dog that has one normal copy and one mutated copy of the gene linked to degenerative myelopathy, a progressive spinal cord disease. On a DNA test result, this is labeled as A/N (or N/DM), meaning your dog inherited the mutation from one parent but not the other. Carriers are far less likely to develop the disease than dogs with two copies of the mutation, but the result has real implications for both your dog’s health and any breeding plans.
What Degenerative Myelopathy Actually Is
Degenerative myelopathy (DM) is a disease that gradually destroys the nerve fibers in a dog’s spinal cord, similar to ALS in humans. It typically appears in older dogs, usually after age 8, and starts with weakness and loss of coordination in the hind legs. Over months, affected dogs progressively lose the ability to walk and eventually lose control of bladder and bowel function. There is no cure and no effective treatment to stop its progression.
The disease is tied to a mutation in the SOD1 gene. This gene normally produces a protein that protects cells from damage. When both copies of the gene carry the mutation, that protective function breaks down in the spinal cord over time.
How DNA Test Results Are Categorized
The Orthopedic Foundation for Animals (OFA), which runs the most widely used DM screening test, reports results in three main categories:
- Normal (N/N): Two normal copies of the gene. The dog will not develop DM from this mutation and cannot pass it to offspring.
- Carrier (A/N): One mutated copy and one normal copy. The dog carries the mutation but is unlikely to develop clinical disease.
- At-Risk (A/A): Two mutated copies. The dog is at risk of developing degenerative myelopathy.
A fourth result, Equivocal, occasionally appears when testing is inconclusive and needs to be repeated.
What Carrier Status Means for Your Dog’s Health
The most important thing to understand: being a carrier does not mean your dog will develop DM. The disease follows a largely recessive inheritance pattern, meaning two copies of the mutation are typically needed for the full disease to develop. The OFA notes that they have confirmed DM in a few carrier dogs, so the risk is not completely zero, but it is rare.
Cornell University’s veterinary college notes that when carriers do develop symptoms, the progression tends to be significantly slower than in dogs with two mutated copies. In practical terms, a carrier dog is very unlikely to face the mobility loss and decline that defines degenerative myelopathy. You do not need to manage your carrier dog any differently than a dog that tested normal.
Why Carrier Status Matters for Breeding
Where carrier status becomes critical is in reproduction. A carrier dog will pass the mutated copy to roughly half of its puppies. The outcomes depend entirely on what the other parent contributes:
- Carrier bred to Normal (A/N × N/N): Roughly 50% of puppies will be carriers, 50% will be normal. No puppies will be at-risk.
- Carrier bred to Carrier (A/N × A/N): Roughly 25% normal, 50% carriers, and 25% at-risk.
- Carrier bred to At-Risk (A/N × A/A): Roughly 50% carriers, 50% at-risk. No puppies will be normal.
The safest breeding strategy is pairing a carrier with a normal-tested dog. This avoids producing any at-risk puppies while still allowing valuable dogs to contribute to the gene pool. Completely removing all carriers from breeding programs would dramatically shrink the genetic diversity of many breeds, which creates its own health problems. The OFA encourages owners of carrier and at-risk dogs to seek genetic counseling before making breeding decisions.
Breeds Where DM Carrier Status Is Common
The SOD1 mutation is not rare. A large study published in the Journal of Veterinary Internal Medicine genotyped over 33,000 dogs across more than 200 breeds and found the mutation widespread. In some breeds, the majority of dogs carry at least one copy.
Wire Fox Terriers had the highest frequency of the mutated gene, at 94%. Pembroke Welsh Corgis came in at 79%, and Boxers at 72%. Cavalier King Charles Spaniels were at 68%. German Shepherd Dogs, one of the breeds most commonly associated with DM, had a 37% mutation frequency across more than 6,400 dogs tested. Chesapeake Bay Retrievers (37%), Kerry Blue Terriers (34%), Rhodesian Ridgebacks (28%), and Pugs (32%) also showed high rates.
In these breeds, carrier results are extremely common. A carrier Pembroke Welsh Corgi or Boxer is the norm, not the exception. This is precisely why responsible breeders test and plan pairings carefully rather than excluding every dog that carries one copy.
Some breeds have very low rates. Golden Retrievers, Rottweilers, Doberman Pinschers, and Labrador Retrievers all had mutation frequencies below 7%. A carrier result in these breeds is less common and worth noting for breeding purposes, but it still carries the same low health risk to the individual dog.
What to Do With a Carrier Result
If your dog tested as a DM carrier and you have no plans to breed, the result is largely reassuring. Your dog is very unlikely to develop the disease, and no special monitoring or lifestyle changes are needed.
If you are breeding your dog, the result means you should test any potential mate before pairing. Breeding a carrier to a normal dog is considered safe. Breeding two carriers together creates a one-in-four chance of producing at-risk puppies, which is avoidable with testing. Many breed clubs now include DM screening in their recommended health testing protocols for exactly this reason.
Keep in mind that having two copies of the mutation (A/A) makes a dog at-risk but does not guarantee disease. Not every at-risk dog develops symptoms during its lifetime. Other genetic and environmental factors likely play a role. Still, the goal of testing is to reduce the odds where possible, and knowing your dog’s carrier status is the foundation of that effort.