Degenerative Myelopathy
Degenerative Myelopathy, commonly abbreviated DM, is a progressive disease of the spinal cord that causes weakness, and eventually inability to walk in the rear legs. It has long been know to exist in German Shepherds, Corgis and over 50 breeds. It has long been suspected in collies, and this last year has been confirmed in a collie by Collie Health Foundation and University of Missouri, after confirmatory autopsy of a collie believed to be affected. In its early stages DM is commonly mistaken for “arthritis”, “old age”, “hip dysplasia” or “spine problems”. Dr. Joan Coates and Dr Gary Johnson’s research at University of Missouri, in collaboration with Dr. Claire Wade and Dr. Kirsten Lindbald-Toh, have identified a gene that is associated with a major increase in risk of DM. The genetic mutation discovered is equivalent to the most common inherited form of the human disease, ALS (Amyotrophic Lateral Sclerosis), also known as Lou Gehrig’s Disease.
Degenerative myelopathy typically occurs in older dogs, 7 to 14 years of age. It is a relentlessly progressive disease of the spinal cord. The first signs are loss of coordination (ataxia) and weakness in the hind legs. One rear leg is often worse then the other. The disease starts as trouble rising in the rear legs and weakness, and progresses to wobbly rear legs. Over time the rear legs become weaker, buckle, and have trouble standing or walking. Eventually, the disease progresses, over months to a couple years, to complete paraplegic. Lastly, fecal and urinary incontinence occurs, with front leg weakness. Amazingly this disease is not painful.
The mutation identified as a risk factor for Degenerative Myelopathy has incomplete penetrance, which means that not all at risk dogs will develop the disease. Only a postmortem biopsy can truly diagnose DM. The test for this mutation is included in the Optimal Selection panel available at a reduced price for Collie Health Foundation members.
Click here for more information on the Optimal Selection test!
Diagnosis and Treatment
Unfortunately there is no single test to confirm DM in a live patient. The genetic test developed by University of Missouri can only confirm if a dog is at risk for the disease. Not all dogs with the affected genes will develop the disease. The test can be most valuable to rule in or out DM. If the test is negative, the dog does NOT have DM. If it is positive for both mutations, the dog is at risk, and further testing needs to be done to diagnose the disease.
Many diseases that affect the dog’s spinal cord can cause similar signs, and some of these diseases can be treated effectively. It is important to pursue necessary tests to be sure that the dog does not have other spinal diseases such as herniated disc, tumors, cysts, infections, injuries, and stroke. Usually your veterinarian would need to refer you to a board certified veterinary neurologist for a thorough neurologic exam and possibly further testing. Collies can and have been affected by all these diseases, and treatment can vary from medical to surgical intervention. Some of these other diseases have an excellent prognosis with appropriate treatment.
It is important to confirm the diagnosis, as Degenerative Myelopathy has no proven treatment, and the prognosis is grave. Many have tried physical therapy, diets, supplements, and alternative medicine in an attempt to slow this progressive disease. More recently, stem cell therapy has been used in some affected German Shepherds which has subjectively slowed the disease. This is currently being done by ReGena Vet Laboratories with Dr. Richard Vulliet. Dogs affected with DM need a lot of nursing care and walking assistance (cart or assistance harness). Eventually euthanasia is usually elected when the quality of life is considered poor.
Genetic Results and their Meaning
Mutant/Mutant – These dogs carry two copies of the mutation and are at increased risk of developing the disease. Not all M/M dogs will develop DM. These dogs will pass one copy to all their offspring.
Mutant/Normal – These dogs carry one copy of the mutation and will not be affected by DM. They will pass a copy of the mutation to 50% of their offspring.
Normal/Normal – These dogs do not carry any copies of the mutation and will not be affected by DM. It will not pass the mutation to any of their offspring.
The Science
Degenerative Myelopathy starts with degeneration of the white matter of the spinal cord in the thoracic (chest) area. White matter is responsible for transmitting motor (movement) signals from the brain to the limbs and sensory information from the limbs to the brain. Because it starts in the thoracic region, the rear legs are affected first. Over time it spreads throughout the spinal cord and affects the front legs. The degeneration involves both demyelination (loss of the insulation material on the nerves) and axonal loss (loss of nerve fibers).
The risk mutation identified by Dr. Joan Coates and Dr Gary Johnson’s research at University of Missouri, in collaboration with Dr. Claire Wade and Dr. Kirsten Lindbald-Toh codes for the protein SuperOxide Dismutase 1, or SOD1. In humans, mutations of SOD1 account for approximately 20% of inherited (or familial) ALS cases and up to 5% of spontaneous ALS cases. While we do not know exactly the mechanism of this disease, there is evidence mutant SOD1 is misfolded and may lead to the formation of amyloid plaques on the spinal cord, causing the damage seen in ALS.
Differences between a normal spinal cord and a spinal cord of a dog affected by degenerative myelopathy. The white matter degeneration is depicted by regions of pallor where there has been loss of nerve fibers.
References
Awano, T., Johnson, G.S., Wade, C.M., Katz, M.L., Johnson, G.C., Taylor, J., Perloski, M., Biagi, T., Baranowska, I., Long, S., March, P.A., Olby, N.J., Shelton, G.D., Khan, S., O’brien, D.P., Lindblad-Toh, K., & Coates, J.R. (2009). Genome-wide association analysis reveals a SOD1 mutation in canine degenerative myelopathy that resembles amyotrophic lateral sclerosis. Proceedings of the National Academy of Sciences, 106, 2794 – 2799. https://www.pnas.org/doi/10.1073/pnas.0812297106
Zeng, R., Coates, J. R., Johnson, G. C., Hansen, L., Awano, T., Kolicheski, A., Ivansson, E., Perloski, M., Lindblad-Toh, K., O’Brien, D. P., Guo, J., Katz, M. L., & Johnson, G. S. (2014). Breed distribution of SOD1 alleles previously associated with canine degenerative myelopathy. Journal of Veterinary Internal Medicine, 28 (2), 515–521. https://doi.org/10.1111/jvim.12317
Wang, L. Q., Ma, Y., Yuan, H. Y., Zhao, K., Zhang, M. Y., Wang, Q., Huang, X., Xu, W. C., Dai, B., Chen, J., Li, D., Zhang, D., Wang, Z., Zou, L., Yin, P., Liu, C., & Liang, Y. (2022). Cryo-EM structure of an amyloid fibril formed by full-length human SOD1 reveals its conformational conversion. Nature Communications, 13 (1), 3491. https://doi.org/10.1038/s41467-022-31240-4
