A young pup will run and play without missing a step. He may play too hard and be sore for a day or two, and that's normal. When a dog shows ongoing signs of weakness or pain in his back legs, though, the condition can be much more serious.
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Degenerative myelopathy is a disease that affects primarily older dogs; it is one possible cause of back leg weakness. In DM, the spinal cord begins to deteriorate, and the fibers that carry the signals from the brain to the legs disintegrate. Dogs with DM slowly develop weakness in their back legs and eventually lose control of them. Degenerative myelopathy is not a painful disease, but dogs with DM eventually lose their mobility. There is currently no definitive test for DM until after a dog has died. There is also no effective conventional treatment, though holistic medicine provides herbal substances that appear to relieve symptoms. If your older dog begins to exhibit weakness in the hind legs, talk to your veterinarian about the possibility that your dog has degenerative myelopathy.
A second common reason for back leg weakness is injury to the spinal cord or back. Spinal cord injuries, often as a result of trauma, disc rupture or stroke, account for 2 percent of all dog veterinarian visits. If the spinal cord or back is damaged, bruised or inflamed, the back legs may become weak or unstable. If your dog has experienced any kind of trauma and has weakness in his back legs, visit your veterinarian as soon as possible.
Your dog's leg weakness may be related to Cushing's disease, which is caused by the overproduction of cortisol in the adrenal glands. Cushing's disease generally develops in dogs older than 6 years but can develop in younger dogs. If your dog has back leg weakness accompanied by hair loss, a pot-bellied appearance, and increased appetite and thirst, you can suspect Cushing's disease; visit your veterinarian for tests.
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Dogs with diabetes may exhibit signs of back leg weakness, although this symptoms is more common in diabetic cats. Diabetes is more common in female dogs than in males and tends to be more prominent in golden retrievers, German shepherds, miniature schnauzers, keeshonden and poodles, although all breeds can be affected. Overweight dogs or dogs eating diets high in sugars and grains are more at risk than dogs of healthy weights on higher-protein diets. If your dog is experiencing weakness in his back legs along with other common symptoms of diabetes, a simple blood test from your vet can determine if diabetes is the cause.
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The cause of your dog's back leg weakness may be pain from arthritis. As dogs age, especially large dogs, the hip and knee joints begin to deteriorate and the cushioning in the joints disintegrates. An X-ray can generally confirm the presence of arthritis. Luckily, several medications -- such as the anti-inflammatory nonsteroidals Rimadyl and Metacam -- are available to help manage canine arthritis, improving the quality of life for a dog with painful, weak joints.