Trembling and Shivering in dogs might look strange or uncomfortable, but it is not an uncommon problem, particularly in small dogs and (some say!) white dogs. There are several types of trembling and several possible underlying causes.
Types of Movement Shivering - Involuntary (sometimes voluntary) Slight shaking as a result of being cold, frightened, or excited. Shivering in these circumstances is not a health concern.
Tremors & Trembling - involuntary shaking with slight range of movement and rapid motion. Trembling is often a sign of a health issue.
Shaking - Always voluntary (on purpose) and always a large muscle movement, shaking is a dog's only way to itch areas of out reach of paws or mouth or to remove water or other materials from their coat.
Muscle weakness - Elderly animals often will tremble due to muscle weakness. Sometimes underweight dogs or dogs with low muscle tone due to poor breeding or poor nutrition will display benign trembling as well. This harmless form of trembling is most common in the legs or jaws of older dogs and isn't something to be concerned about unless it is accompanied by additional signs of discomfort or poor health.
Cold - when you notice your dog shivering or trembling you should first move the dog to a warmer area or clothe the dog with a sweater or cover with a blanket. Even if the temperature feels comfortable to you, a dog may not regulate temperature like your body does due to age, health, or short fur. If you notice your dog trembling with its whole body, first try warming the dog.
Fear - Many fearful dogs tremble or quiver when intimidated or frightened. Is the dog shivering because of a high anxiety situation in your home, or in anticipation of an event (like crating, or being forced to go outside in the rain) that it doesn't like? Even if you don't perceive an anxiety provoking circumstance for the dog, your dog might. For example, a rescue dog mistreated by a male may withdraw and shiver when a similar person is nearby. A dog traumatized by a thunderstorm might react with trembling when barely audible thunder is in the distance.
Illness - Persistent trembling or even continued voluntary shaking is always something to ask your vet about at the next regular visit. If your dog exhibits any other behavior changes or absolutely any signs of discomfort or ill health you should make a veterinary appointment immediately- as the trembling may be a first symptom of an oncoming illness.
Many dogs, mostly small breeds, tremble or shake for no apparent reasons. There are uncommon diseases and illnesses that can cause quivering and trembles but in most cases trembling dogs test healthy. Thus, no one has really been able to explain why some healthy dogs shake and shiver even when they are warm and have no reason to be nervous. If your dog shivers excessively or suddenly starts shaking it may be cause to consult your veterinarian (as sudden onset can be a symptom of pain or a nervous disorder) but if full body shivering or a foot or leg that trembles is something you've noticed from puppyhood then your dog's shaking is probably nothing to worry about.