PWCCA's position on the new DNA test for DM:
"A recommendation for breeders is to take into consideration the DM test
results as they plan breeding programs. The breeder should not over-emphasize
this test result. Instead, the test should become one factor among many others
in obtaining a balanced breeding program. This approach over generations should
reduce the prevalence of the gene while continuing to maintain good qualities."
Degenerative Myelopathy
Joan R. Coates DVM, MS, Diplomate ACVIM (Neurology)
Associate Professor Veterinary Neurology and Neurosurgery
In 1973, Dr. Averill first described a degenerative disease of unknown cause affecting the spinal cord in the older German Shepherd Dog. Degenerative (progressive) myelopathy (disease of the spinal cord) is a disease of the spinal cord causing progressive weakness in the hind limbs. Though most commonly reported in German Shepherd Dogs, high disease prevalence also exists in other breeds, such as Cardigan and Pembroke Welsh Corgis, Rhodesian Ridgebacks, Chesapeake Bay Retrievers and Boxers. While the cause has been unknown, an increasing number of cases reported in families of pure bred dogs have raised concerns for an underlying genetic predisposition.
What is degenerative myelopathy?
Degenerative myelopathy is a progressive disease of the spinal cord in older dogs. The disease has an insidious onset typically between 8 and 14 years of age. It begins with a loss of coordination (ataxia) in the hind limbs. The affected dog will wobble when walking, knuckle over or drag the feet. This can first occur in one hind limb and then affect the other. As the disease progresses, the limbs become weak and the dog begins to buckle and has difficulty standing. The weakness gets progressively worse until the dog is unable to walk. The clinical course can range from 6 months to 1 year before dogs become paraplegic. If signs progress for a longer period of time, loss of urinary and fecal continence may occur and eventually weakness will develop in the front limbs. Another key feature of DM is that it is not a painful disease.
What is the cause of degenerative myelopathy?
The exact cause of DM is unknown. We feel that genetics play an important role
in the disease since it is common in certain breeds of dogs and follows
stereotyped pattern with age of onset and clinical signs. Various immune mediated and nutritional
theories have been investigated but no definitive cause has been found.
We do know that the disease begins with the
spinal cord in the thoracic (chest) region.
If we look under the microscope at that area of the cord from a dog that
has died from DM, we see degeneration of the white matter of the spinal
cord. The white matter contains fibers
that transmit movement commands from the brain to the limbs and sensory
information from the limbs to the brain.
This degeneration consists of both demyelination (stripping away the
insulation of these fibers) and axonal loss (loss of the actual fibers), and
interferes with the communication between the brain and limbs.
How is degenerative myelopathy diagnosed?
Degenerative myelopathy is a diagnosis of elimination. We look for other causes of the weakness using diagnostic tests like myelography and MRI. When we have ruled them out, we end up with a presumptive diagnosis of DM. The only way to confirm the diagnosis is to examine the spinal cord under the microscope when a necropsy (autopsy) is performed. There are characteristic degenerative changes in the spinal cord typical for DM and not some other spinal cord disease.
What else can look like degenerative myelopathy?
Any disease that affects the dog’s spinal cord can cause similar signs of loss of coordination and weakness. Since many of these diseases can be treated effectively, it is important to pursue the necessary tests to be sure that the dog doesn’t have one of these diseases. The most common cause of hind limb weakness is herniated intervertebral disks. The disks are shock absorbers between the vertebrae in the back. When herniated, they can cause pressure on the spinal cord and weakness or paralysis. Short-legged, long back dogs are prone to slipped disks. A herniated disk can usually be detected with X-rays of the spine and myelogram or by using more advanced imaging such as CT scan or MRI. Other diseases we should consider include tumors, cysts, infections, injuries and stroke. Similar diagnostic procedures will help to diagnose most of these diseases. If necessary, your veterinarian can refer you to a board certified neurologist who can aid in diagnosing degenerative myelopathy. A directory to a neurologist near you can be found at www.acvim.org under the “Find a specialist near you” link.
How do we treat degenerative myelopathy?
There are no treatments that have been clearly shown to stop or slow progression of DM. Although there are a number of approaches that have been tried or recommended on the internet, no scientific evidence exists that they work. The outlook for a dog with DM is still grave. Efforts are being made to improve quality of life such as good nursing care, physical rehabilitation, pressure sore prevention, monitoring for urinary infections, and ways to increase mobility through use of harnesses and carts.
How do we find the gene responsible?
The goal in
dealing with hereditary diseases is to identify the gene responsible. Drs.
Gary Johnson and Joan Coates at the Animal Molecular Genetics Laboratory of the
University of Missouri and Drs. Claire Wade and Kerstin Lindblad-Toh at the
Broad Institute of MIT/Harvard and their colleagues have identified a DNA
mutation that is a major risk factor for development of DM in dogs. We
are close to developing a DNA test for DM that will aid pet owners and breeders. Genes contain the genetic code that programs
everything about an animal from the color of their coat to disease predisposition.
Genetic disease occurs when a mutation
interferes with the ability of a gene to function normally. Genes are found within chromosomes in the
command centers of individual cells.
With the completion of the first draft of the canine gene map, we have used
that map to find the gene(s) responsible for DM.
How can I help?
If you have dog that might be affected, please visit our website, www.caninegeneticdiseases.net or contact us directly. We can help you in determining whether or not your dog is affected. In return, we would ask your help in collecting the samples and information necessary to continue understanding the genetic cause of this disease. Your ongoing support will be necessary to achieve our goal.
Any information provided to us will be kept
strictly confidential.
|
Liz
Hansen Animal
Molecular Genetics Laboratory (573)884-3712 |
Dr.
Joan R. Coates Department
of Veterinary Medicine & Surgery (573)882-7821 |