The ALS (Lou Gherig’s disease) that strikes people and the degenerative myelopathy that strikes dogs are similar in that they both affect the spinal cord. But they start — and play out — differently. ALS is a disease of the neurons — the cells that make up the nervous system (including the brain). Degenerative myelopathy, on the other hand, is a disease of the axons — long, thread-like parts of the neurons via which impulses (messages) travel from one cell to another to elicit movement of the limbs and other body parts. If you were to look under a microscope, you’d see the disease in different spots in the two species.
The disease also originates in different places in dogs’ bodies and people’s bodies. In other words, they’re not just different under the microscope but in the way they strike the anatomy. In people, the condition often starts in the brain itself. In dogs, it tends to start in the spinal cord, in the back.
That difference affects the way the disease looks — the clinical presentation. “With people, it’s more weakness than wobbliness,” veterinary neurologist Casey Neary, DVM, DACVIM, explains. With dogs, it’s more wobbliness than weakness — certainly at the beginning, which is why dogs start to walk funny, scuffing their toes and so on.
When the condition begins to manifest itself in dogs, the problem spreads first from the spinal cord toward the back legs and the tail and then toward the front ones — and the brain. That’s why the back legs go first, not performing the way they should and then becoming fully paralyzed. After that, the front legs become affected. After they become fully paralyzed, the degeneration spreads to the brain. “I’ve seen two corgis in my career like that, whose disease spread to the brain before they were euthanized,” Dr. Neary says. “They had difficulty swallowing, problems in moving their tongues. Usually by that point, even in small-breed dogs, euthanasia is recommended. When a dog can’t walk or even swallow — those are major issues with regard to quality of life.” (People with ALS usually die when the respiratory system stops working. Pets rarely are allowed to reach the point at which they suffer with more and more difficulty breathing.)
Interestingly, while the disease does not share the exact same etiology in dogs and people and does not “look” exactly the same in the two species as it progresses, its manifestation is similar enough that research into cause and cure in one species can potentially prove helpful with treatment in the other. “To be able to look at dogs with nervous systems similar to ours is only going to help us study this condition and see the response to therapeutics,” Dr. Neary says. “There’s a lot to learn here with regard to comparative medicine,” meaning medicine and medical therapies that translate from one species to another.
Indeed, Tufts is already on it. In a pilot study begun about a year ago, neurology researchers at the Cummings School of Veterinary Medicine at Tufts University tried a gene-silencing therapy on a small number of dogs. The therapy had been developed by a neurologist for people at the University of Massachusetts Medical School, who approached Tufts about the work. It’s actually an engineered virus given as a spinal fluid injection, which is meant to silence the mutated gene responsible for causing this degenerative disease.
The therapy had already shown promising results in mice, and it was hoped that it would show good results in dogs, too. As of this writing, none of the dogs treated showed obvious benefit, so the investigators are tweaking the drug and planning to start treating additional dogs in the near future. If the research bears fruit, the treatment can then be tried in people. Stay tuned.