Neurodegenerative disease of the spinal cord, the canine equivalent of human amyotrophic lateral sclerosis (ALS) due to SOD1 mutations. The classic variant (exon 2 of the SOD1 gene, described in 'all breeds') produces a dysfunctional superoxide dismutase 1 and degeneration of the long spinal tracts, with progressive paralysis beginning in adulthood. It is the best-known form of canine degenerative myelopathy (DM) and is inherited in an autosomal recessive manner with age-dependent penetrance.

Applies to 'all breeds' in the commercial test; the classic SOD1 variant is present in many canine breeds. Carrier frequency is high in some breeds (Pembroke Welsh Corgi, Boxer, Poodle, Bernese Mountain Dog, etc.) and low or very low in others. The clinical incidence of DM is lower than the frequency of homozygotes, owing to the incomplete penetrance.

- Ataxia and paresis beginning in the pelvic limbs, in adults (from middle age, depending on the breed)
- Decreased proprioception with preserved reflexes initially
- Progressive neurogenic muscle atrophy
- Ascending paralysis that can reach the thoracic limbs
- Preservation of urination/defecation initially; no evident pain

Canine degenerative myelopathy was described decades ago in breeds such as the German Shepherd and the Pembroke Corgi. In 2009, Awano and colleagues identified by GWAS the exon 2 variant of SOD1 associated with most cases in breeds such as the Pembroke Welsh Corgi, the Boxer and the German Shepherd; the variant is known as the classic one and is present in many breeds. Additional SOD1 variants were subsequently identified in specific breeds (e.g. the exon 1 variant of the Bernese Mountain Dog) and it was recognised that the penetrance of the classic allele is incomplete.

- Test breeding animals before mating, especially in breeds with a high carrier frequency (Pembroke Corgi, Boxer, etc.)
- Do not mate two carriers: 25% risk of homozygotes in each litter
- A carrier can be mated with a clear dog; offspring intended for breeding must be tested and clear animals preferentially selected
- Remember that a homozygote will not necessarily develop DM (incomplete penetrance): do not base euthanasia decisions solely on the genotype
- In breeds with two SOD1 variants (e.g. Bernese Mountain Dog) test both and consider the combined risk
- Communicate the status to the buyer and the veterinarian

Differential diagnosis with other causes of chronic myelopathy in adults (disc herniation, spinal compression, tumours, discospondylitis, ischaemic myelopathy). MRI helps to exclude compression; CSF is non-specific. Homozygosity for SOD1 is compatible with DM, but does not exclude other diagnoses and does not predict clinical onset. In the Bernese Mountain Dog, remember the second SOD1 variant (exon 1) and the need for combined testing. Supportive management: physiotherapy, mobilisation, prevention of pressure ulcers; there is no curative treatment.

1. Awano T et al. 2009, análisis de asociación del genoma que revela una mutación de SOD1 en la mielopatía degenerativa canina (PMID 19188595)
2. Coates JR et al. 2010, mielopatía degenerativa canina (PMID 20732599)
3. OMIA:000261 Mielopatía degenerativa canina