You are right to be alarmed if you see your dog shivering. It is not normal for a dog to tremble and shake; it can mean something is significantly wrong. There are several reasons why your dog might be shivering or shaking, and some of them need to be addressed quickly.
Cold Shivers
The simplest explanation – and easiest to fix – is that your dog is cold. The answer – warm him up! If bringing him indoors near a heater and covering him with a blanket doesn’t quickly stop the shivers, he may be hypothermic, and a prompt call to your veterinarian is in order. (Next time you take him outside on a cold day, put a jacket on him!)
Medical Reason for Shaking
Shaking can also be caused by pain, seizures, or by other medical conditions. If you suspect one of these is the cause, again, it’s time for a prompt call to your veterinarian. There are effective medications for pain and seizures, and your vet will be able perform diagnostic tests to determine if there are internal medical conditions causing this level of distress.
Anticipation Shivers
Sometimes dogs shiver because they are very happily anticipating something – like going for a walk, greeting a loved friend who is approaching, or chasing a ball that you are about to throw. No response (on your part) is called for unless they become more exuberant than you like. If that’s the case, you can employ behavior modification to tone down their happy excitement.
Behavioral Shivering
Behavioral shaking is most likely caused by stress and/or fear. Begin by calmly comforting your stress-shaking dog. No, you won’t be reinforcing his fear; you cannot reinforce emotion. While you comfort, try to determine what’s stressing him and implement management to reduce or eliminate his exposure to the stressor(s). Then seek the assistance of a qualified force-free behavior professional to help change his association with stressors that can’t be eliminated. Your behavior professional may also suggest you talk to a veterinary behaviorist or behaviorally knowledgeable veterinarian about anti-anxiety medications to help your dog cope with his stressful world.
Whatever the cause of your dog’s shaking, you can and must help him. Figure out the cause and do the right thing as quickly as you can!
Might be good to add an attendant note regarding the trembling frequently seen in older dogs, especially their legs — this is generally understood as normal in the absence of known stressors or medical conditions, right?