Recognizing pain in animals is a challenge because animals cannot communicate the same way people do. However, there are some species-specific behaviors that can indicate pain and help us recognize it. For example, animals that are natural predators, such as dogs, behave differently when in pain than do prey animals, such as rabbits and horses. Prey animals tend to hide their pain, making recognition of pain even more difficult in these species.
In evaluating pain in animals, veterinarians consider vital signs, behavioral changes, pain scales, and the animal's history.
Vital signs such as heart rate, respiratory rate, and blood pressure may be used to assess responses to an acute painful stimulus, particularly during surgery or after severe trauma. Measurement of vital signs can also be used to assess pain in some situations (such as horses with colic). However, measurements of vital signs do not differentiate between sources of pain, such as pain from surgery or another cause. In addition, vital signs may be normal in animals experiencing chronic pain. These indicators are not specific enough to distinguish pain from other stresses such as anxiety, fear, or physical responses to certain medical conditions (such as anemia). In other words, an animal can still be in pain even if its heart rate and breathing rate are normal.
Recognizing pain-induced behaviors is difficult or impossible without knowing the normal behaviors of a particular species or breed. Behavioral changes associated with pain may be subtle and not easily recognized during routine checkups or examinations in animals. Many animals mask their pain with normal behaviors. For instance, dogs may wag their tails and greet people in spite of being in pain. In addition, behavioral changes in response to pain might be very different from the typical responses associated with people who are in pain. A cat sitting quietly in the back of the cage after surgery may be in pain; however, a caregiver might not recognize the pain if he or she expected to see more active signs of pain, such as pacing, agitation, or meowing.
In general, signs of chronic pain are less obvious and harder to recognize than signs of traumatic or surgical pain. Criteria that can be used to evaluate chronic pain (for example, lack of activity, decreased appetite, weight loss, and lack of grooming) are not specific signs of pain. Instead, these signs point only to an underlying problem that needs to be diagnosed. Your observations about changes in your pet's attitude or interaction with family members are essential to help your veterinarian evaluate chronic pain. Response to treatment, such as increased activity after administering a pain-relieving drug, may reveal a relationship between pain and the behavioral changes.
A pet with cancer pain may have some signs of acute pain (such as in response to tumor growth or after surgery, radiation, or chemotherapy) and some signs of chronic pain. If your pet has cancer, you should be alert for behavioral changes associated with both acute and chronic pain.
A pain scale is one tool that veterinarians can use to rate an animal's pain. A pain scale is a questionnaire that includes the following characteristics: species, breed, age, gender, environment and rearing conditions, cause of pain (such as trauma or surgery), body region affected (such as the abdomen or muscle), and the duration and intensity of pain. Pain scales that consider species-specific behaviors are likely to be more accurate than generic scales that rely heavily on subjective assessment and interpretation. Even if the severity of pain is correctly estimated, determining how well the individual animal is coping with the pain may be difficult. Current methods assess the effects of physical pain, but do not evaluate the mental or psychological effects of pain that an animal may experience.
Last full review/revision July 2011 by Peter W. Hellyer, DVM, MS, DACVA