Pain in animals: Anatomy, physiology, and behaviors

Pain, an adaptive but unpleasant sensation, is the most common symptom of numerous diseases in humans and animals. Although animal patients express this symptom frequently, a lack of communication abilities hinders its recognition by veterinary physicians, thereby leading to unsatisfactory management of the symptom. On the other hand, pain itself has its own neurological mechanisms, regardless of the disease that causes it. Thus, a physician may need to know the mechanisms underlying pain development in order to properly manage the symptom in a particular disease. In this review, we attempt to provide a brief introduction to the anatomical, physiological, and neurological basis of pain transmission and sensation. Although most knowledge about these mechanisms comes from studies in humans and laboratory animals, it is generally applicable to pet, farm, or zoo animals. In addition, we summarize pain behavior in several pet, farm, and laboratory animals for its proper identification. This information will help to identify and manage pain, and thus improve welfare, in animals.