Hello and thanks for researching this very important question!
Parkinson's Disease in humans causes, among other things, a progressive muscle tremor when the body is at rest...any important distinction from other neuromuscular illnesses that produce muscle tremor when the affected limb/part is moving!
While there is not an actual Parkinsons-condition identified in dogs, there are a lot of conditions in canines that can cause a muscle tremor.
Excitement is mediated through the brain by chemicals such as serotonin and norepinephrine. It may be that your dog's system is either giving out too much excitatory chemicals, or he is especially sensitive to their presence. Very prolonged "firing" of nerves in the muscles can cause discomfort. Excited shivering can be considered normal in the absence of other symptoms of illness, but is ordinarily seen throughout o dog's lifetime.
Pain in the joints associated with injury, arthritis or other degenerative diseases can cause the excitatory system to become over-active, due to the body's response to stress.
Tumors growing in the brain and/or spinal cord can press on nerves that control muscle movement.
Degenerative conditions in the nerves and muscles themselves are often to blame in older dogs: these conditions are accompanied by reduction of muscle mass in the affected body part(s)...check your fellow for thinness and boniness over the head and along the length of the legs. This often occurs slowly over time and can be missed.
Occasionally, we still do not know what causes these symptoms in older pets ("Idiopathic Tremor")...in this way, there is similarity to Parkinson's only in that more research is needed over time to find the root cause(s).
To get a full picture of what has been happening to your dear, he would need to be evaluated by a Veterinary Neurologist. Your regular vet can provide you with a reliable referral in your area.
This would be especially important if other symptoms, such as lameness, incontinence, or generalized weakness develop.
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