DNA Diagnosis of Degenerative Myelopathy in Dogs
New DNA test may help 'dog'inose degenerative myelopathy The signs were subtle at first. Osten, a collie mix, would use his front legs to push himself to a sitting position before standing, and when he did stand, he just seemed slightly unsteady. An X-ray revealed he had some bone spurs on his spine and everyone thought that was all there was to it. But after another six months, the once scarcely perceptible signs were now more obvious.
He had trouble getting up, but not all the time. Sometimes his gait was unsteady and he stumbled or crossed his rear legs when walking. The nails of his hind feet often scraped the ground. Back at the veterinarian, who checked his proprioception (that is, his ability to perceive where his own body parts are or sense of orientation) by placing a rear foot topside down on the ground.
In the summer, Osten passed the placement test, immediately changing his foot so the bottom was on the ground. By autumn, he was failing. He could no longer feel his foot to know it was in the wrong position. That was when his owner first heard the words degenerative myelopathy, also known as DM.
DM is a progressive spinal cord disease causing weakness and paralysis in the hind limbs that eventually moves forward toward the front limbs. It usually begins between eight and 14 years of age, with the dog wobbling, knuckling over, or dragging one or both hind feet when walking. During the next few months to a year, the hind legs become weaker and less coordinated until he can no longer walk. DM causes the loss of both nerve fibers and their insulating myelin sheaths (surrounds the core of a nerve fiber and facilitates the transmission of nerve impulses) that are vital for proper nerve conduction of impulses between the brain and limbs.
Osten was referred to a neurologist, who explained that DM was a "diagnosis of exclusion," meaning that DM was diagnosed by a process of elimination of other conditions. Any disease that affects the dog's spinal cord can cause similar signs, and because many of these diseases are treatable, it's important to test for them. Complete testing includes radiographs, CT scan, MRI and a myelogram. Besides costing thousands of dollars, they are not without risk. In fact, Osten's owners feel he never really made it back to the point he was before the myelogram. The tests were all negative, and Osten was diagnosed with DM. Shortly afterward Osten could no longer walk on his own.
Had Osten's signs appeared a few months later, his owners may have had other options. Researchers have recently identified a DNA mutation that is a major risk factor for development of DM in dogs. The test, just available this summer, can identify whether a dog has zero, one or two copies of the mutated gene. Those with two copies of the mutated gene are at higher risk of developing DM, although not all such dogs are affected with DM. The mutated gene has been found in more than 40 breeds so far, but it's only been associated with DM in the boxer, Chesapeake Bay Retriever, German Shepherd, Rhodesian Ridgeback, Standard Poodle, and Cardigan and Pembroke Welsh Corgis. Because these are the only breeds for which an association has been shown, they are the only ones for which the DNA test, now available through the Orthopedic Foundation for Animals ($65; www.offa.org/dnatesting/), is officially offered.
Currently, there is no effective treatment for DM, and in time, the paralysis moves to the front limbs. Fortunately, DM is painless and many dogs find new mobility with the help of a cart. You can see Osten in his cart here.
Join the DM Support group for dogs at www.mzjf.info.
Researchers want to know why some dogs with two copies of the mutated gene don't develop DM. One possibility is that such dogs have other modifier genes that counteract the DM genes. To this end they are offering free DNA testing for dogs that meet the following criteria:
1) Any dog with a presumptive diagnosis of DM made by a veterinarian.
2) Any dog, healthy or not, that is 10 years of age or older from the breeds listed below ONLY:
Boxer
Chesapeake Bay Retriever
Pembroke Welsh Corgi
Rhodesian Ridgeback
For more information go to www.caninegeneticdiseases.net.
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