DM

Degenerative Myelopathy

You'll notice a lot of other breeder don't even test for this.  I understand this test is fairly new and some people don't hold much confidence in it.  I've done some research and this is something that needs to be continually updated as the research is done. We started testing for DM but with the information I have seen, I most likely won't be testing any longer until more information becomes available. Sometimes we breeders feel organizations develops testing (even obedience testing) becomes available just for profit.

The inheritance mode(s) of the other gene(s) involved with DM are unknown at this time, but current information suggests that another gene may be required to create symptoms of disease. This gene is also suspected to be a simple recessive, which may be a "switch" which turns on the expression of the disease, causing symptomatic DM. The current test should not be used to remove any dogs from breeding. The test result is only a guideline, not a certainty, as to a dog's DM status. Dogs have been reported who have developed DM even when they have been tested as Carrier or Normal. These diagnoses of DM were confirmed by necropsy and examination of the spinal cord. In one case, the dog had a different mutation at the SOD1 locus, which confirms that more than one form of DM exists, and more than one gene is involved. One of the dogs tested Carrier but confirmed as having DM

DM is an inherited condition, and is polygenic in inheritance mode. This means that several genes, together with perhaps some environmental factors, combine to create the disease in dogs. Currently, there is a DNA-based test for DM. It looks at only one of the genes that causes DM. The gene it identifies is a simple recessive. The test can be ordered online through OFA. A list of Clear dogs, and those Carrier and At Risk dogs whose owners have released the information is available online through the OFA's online database. Results of the test are reported as Clear, Carrier, or At Risk. Dogs are listed as At Risk, rather than Affected, because having this gene alone does not cause the disease. Greater than 90% of Fox Terriers have tested At Risk or Carrier, yet there has never been a single case of DM reported within that breed, which lives a very long time. An At Risk result is an indicator that the dog may develop DM at some point, but it is not a certainty. Dogs testing At Risk can still be bred; owners should take into consideration the dog's overall value as a breeding animal, and the test status of any mates when doing breedings.

 

From the OFA site...

Guidelines for Breeding dogs who are Carrier or At Risk for DM

Owners with dogs testing as Carriers (A/N), or At-Risk (A/A) are strongly encouraged to share these results with their attending veterinarian and seek genetic counseling when making breeding decisions. 

The “A” (mutated) allele appears to be very common in some breeds. In these breeds, an overly aggressive breeding program to eliminate dogs testing A/A or A/N might be devastating to the breed as a whole because it would eliminate a large fraction of the high quality dogs that would otherwise contribute desirable qualities to the breed. Nonetheless, DM should be taken seriously. It is a fatal disease with devastating consequences for the dog, and can be a trying experience for the owners that care for them. A realistic approach when considering which dogs to select for breeding would be to treat the test results as one would treat any other undesirable trait or fault. Dogs testing At-Risk (A/A) should be considered to have a more serious fault than those testing as Carriers (A/N). Incorporating this information into their selection criteria, breeders can then proceed as conscientious breeders have always done: make their breeding selections based on all the dog’s strengths and all the dog’s faults. Using this approach and factoring the DM test results into the breeding decisions should reduce the prevalence of DM in the subsequent generations while continuing to maintain and improve upon positive, sought after traits.

We recommend that breeders take into consideration the DM test results as they plan their breeding programs; however, they should not over-emphasize the test results. Instead, the test result should be one factor among many in a balanced breeding program.

Order OFA DNA Test Kit(s)

 



Testing

The Orthopedic Foundation For Animals has a DNA test that can be purchased to see if their dog carries the mutated genes for DM. The test is for any dog but is only recommended for certain breeds. The test consists of a cheek swab (using something similar to a Q-tip to swab the inside of the cheek to submit for testing).

The test checks for the mutated gene that will tell if your dog may be affected by degenerative myelopathy, if it is a carrier, or if it is unaffected.

The results are:

  Genetics

Breeding risks for degenerative myelopathy can be calculated using the Punnett Square:

 

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.

 


Affected brain
Degenerative myelopathy is a devastating disease causing progressive paralysis in a large number of dog breeds. New research has identified a gene that is associated with a major increase in risk of the disease.

What causes Degenerative Myelopathy?

Degenerative myelopathy 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.

DM vs Normal

In the section of a spinal cord from a dog who has died of DM (Left), the degeneration is seen as a loss of the blue color at the edges (arrows) compared with the spinal cord from a normal dog which is blue througout (Right).

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. Recent research has identified a mutation in a gene that confers a greatly increased risk of developing the disease.

How is degenerative myelopathy clinically 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 degenerative changes in the spinal cord characteristic for DM and not typical for 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 American College of Veterinary Internal Medicine website 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. The discovery of a gene that identifies dogs at risk for developing degenerative myelopathy could pave the way for therapeutic trials to prevent the disease from developing. Meanwhile, the quality of life of an affected dog can be improved by measures 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.

 

http://www.offa.org/dnatesting/dm.html