On Saturday May 19, 2012, I had to put my dog Malcolm down. Malcolm was a Boxer and was ten when he died. When he was about five, he began to suffer from an arrhythmia of the heart. But it was not until several years ago that he began to acutely suffer from the Canine Degenerative Myelopathy that ultimately resulted in the complete and sudden paralysis of his back legs.

Canine Degenerative Myelopathy is similar to Amyotrophic Lateral Sclerosis or Lou Gehrig's Disease in humans. It is a neurodegenerative disorder resulting from an inherited genetic mutation which has become common in a number of dog breeds. DM causes a degeneration of the upper motor neurons in the spinal cord. Dogs affected with DM begin to show the disorder late in life, between eight to twelve years of age. An affected dog will begin to suffer from a lack of agility in the back limbs or in all limbs.

Malcom, for example, began to have difficulty going up or down the stairs several years ago. As his disorder progressed, he became unable able to jump or to stand up on his back legs or to stretch forward into a "downward facing dog" position. His muscle tone in his back legs deteriorated. We began to see the outline of his lower spine through his coat even though he had grown somewhat overweight from his inability to get the quality of activity he once enjoyed.

At first we attributed this inability to his heart arrhythmia which slowed him down and could cause him to pant excessively at times. But in retrospect, this arrhythmia could have been caused by his DM, although I have found nothing to support this hypothesis. As Malcolm turned ten, he experienced a frightening episode on a walk. His back legs failed him entirely and, panicked by this, flailed on the ground until I could calm him down. Within minutes he regained use of his back legs but only enough to get him back to the car. I had to lift him up into the back seat. That was the last walk on a leash that Malcolm ever took.

After that last walk, Malcolm's began to experience the classic symptoms of advanced DM. He dragged his feet constantly while walking, his nails scraping along the floor. He stood with his back legs close together and wobbled a lot. If he was standing funny, if he was standing on the tops of his toes rather than on his pads, he was slow to realize it. Once, I accidentally stepped on his toes and he was not even aware that I had done so. Indeed, despite being an older dog with a heart condition, Malcolm's spirits were never extinguished even at the end. He could not run or play like a young dog, but he never lost his passion for protecting the house from the evils of the UPS guy or attacking the lawn tractor.

Malcolm's end came shockingly and abruptly on a Saturday evening. He was standing up, begging for sausage at the dinner table. Twenty minutes later we found him unable to stand up in the kitchen. He also lost control of his bladder. Unlike on his walk just months prior, he did not regain control of his back legs. We rushed him to the vet. He had no reflexes at all in his back legs and could not feel the vet pinching his skin on his lower back along his spine. Malcolm was paralyzed, afraid, panicked, and wanted desperately to get up and walk out of the clinic, but was never in any pain. The Vet offered us little hope for recovery given his age. We put him to sleep.

Malcolm was a pure-bred boxer. Other dog breeds that are commonly affected by Degenerative Myelopathy are the Siberian Husky, Cardigan Welsh Corgi, Soft Coated Wheaten Terrier, German Shepherd, Bernese Mountain Dog, Rhodesian Ridgeback, Miniature Poodle, Wirehaired Fox Terrier, Golden Retriever, Standard Poodle, Kerry Blue Terrier, Pug, Pembroke Welsh Corgi, Chesapeake Bay Retriever and the American Eskimo Dog. (1)

Degenerative Myelopathy's genetic mutation is in exon 2 of SOD1 in the dog's genome sequence. This exon should be G but had transitioned to A. As genomes come in pairs, a dog with the genetic mutation can have either half or both of the genome pair mutated. Thus, a dog can be a carrier of the mutation or affected by the mutation depending on the presence of the mutation in its parents. A simple series of Punnett Squares can illustrate this where (DM) indicates the presence of the mutated genome.

Carrier Parent A (DM) Carrier Parent A
Non-Carrier Parent B Carrier Offspring A Non-Carrier Offspring C
Non-Carrier Parent B Carrier Offspring B Non-Carrier Offspring D

Affected Parent A (DM) Affected Parent A (DM)
Non-Carrier Parent B Carrier Offspring A Carrier Offspring C
Non-Carrier Parent B Carrier Offspring B Carrier Offspring D

Carrier Parent A (DM) Carrier Parent A
Carrier Parent B (DM) Affected Offspring A Carrier Offspring C
Carrier Parent B Carrier Offspring B Non-Carrier Offspring D

Affected Parent A (DM) Affected Parent A (DM)
Carrier Parent B (DM) Affected Offspring A Affected Offspring C
Carrier Parent B Carrier Offspring B Carrier Offspring D

Affected Parent A (DM) Affected Parent A (DM)
Affected Parent B (DM) Affected Offspring A Affected Offspring C
Affected Parent B (DM) Affected Offspring B Affected Offspring D

There is a cheek swab test that has recently become available that can identify this DNA mutation. As this disorder continues to be studied, reputable breeders are now beginning to screen for DM. However, the rates of DM in some breeds, Boxers and Pembroke Welsh Corgis in particular, have been discovered to be so high it may be prohibitive to "exclude carriers from breeding (and is) recommended that carriers be bred to non-carriers. Breeding of carriers of other breeds, or affected dogs of any breed, should be avoided."(1)(2)

There is no cure for Canine Degenerative Myelopathy. Dogs who suffer from the effects of the disorder usually have to be euthanized just like Malcolm had to be in the end.(3) Malcolm's death broke my heart. But, as my wife Virginia says, "They all break your heart in the end." I know that all old dogs must pass on so that young dogs, full of the vigor of youth, can take to life's stage. This is The Way of all things. I just hope that, since the canine genome has been mapped, these kind of disorders can be more readily identified and maybe even treated in the future. (4)

References
(1) http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=omia&dopt=Detailed&list_uids=506
(2) http://omia.angis.org.au/OMIA263/9615/
(3) http://vet.sagepub.com/content/46/4/684.full
(4) http://www.genome.gov/17515860


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