Pain is the unpleasant sensation that develops with the stimulation of specialized sensory nerve endings, called pain receptors. Pain most often develops from damage, irritation or inflammation of tissues or structures of the body. Pain is a protective mechanism. It causes the animal to react and to move away from the source of the stimulus.

Pain receptors are present in many tissues of the body, including the skin, the covering of the bones (periosteum), the walls of arteries, the surfaces of joints, the lining tissues of the chest and abdomen, the cornea and tissues around the eye, and the meninges of the brain and spinal cord.

Types of stimuli that excite the pain receptors include mechanical forces, such as stretching, tearing or fracturing of tissues; thermal stimuli, such as cold or heat; and chemical substances. The chemical substances that stimulate pain receptors are tiny molecules produced in the body when tissues are damaged or inflamed. They include chemicals like serotonin, histamine, prostaglandins, bradykinin, and various enzymes. These chemicals are all mediators of inflammation in the body.

Once a pain receptor is stimulated, information on the pain is transported back to the brain, where the sensation is perceived. Both fast and slow pain fibers exist in the body. The fast transmission of pain signals allows the animal to be rapidly warned about impending damage to some area of the body, and allows them to react quickly. The slow transmission of pain information allows the pain sensation to continue and provides a mechanism for chronic pain to develop.

Pain may be perceived only at the site of the stimulation of the nerve receptors, or may be referred to a nearby area on the body. For example, compression of nerve roots just outside the spinal cord of the neck may result not only in neck pain, but also lameness in the front leg. Pain caused by inflammation within the kidney may be detected as pain along the upper back.

The threshold for perceiving pain is determined by the sensitivity of the pain receptors. Some animals appear to have a higher threshold for pain than others. In general, cats seem to be more stoic with regards to pain than are some dogs. In animals pain thresholds and sensation are difficult to measure because they do not speak. In people various methods have been developed for detecting and measuring pain, but these tests are not often applicable in animals.

Causes of Pain

Any cause of tissue damage or inflammation is potentially capable of causing pain. Examples of such causes include the following:

Trauma - fractures, sprains, dislocations, wounds, muscle tearing, blunt forceful injuries

Exposure to heat – flame, hot water, heating devices

Exposure to extreme cold – frostbite, cold surfaces, cold weather

Tissue inflammation – from infections, from pathologic conditions or diseases, from physical changes

Necrosis of tissue – death of the tissue

Ischemia – loss of blood supply to the tissue

Stretching of tissue – especially of round or hollow organs

Spasm of tissues – especially of muscles or muscular organs

Organs That May Manifest Pain or Become Painful

Bones

Joints and ligaments

Muscles

Skin and soft tissues under the skin

Tissues of the mouth

Portions of the brain and spinal cord

Tissues within and around the eye

Certain components of the ear and ear canals

Certain structures within the chest, particularly the esophagus and tissues lining the chest (pleura)

Many abdominal organs, including those of the gastrointestinal tract, urinary tract, reproductive tract, and the tissues lining the abdomen (peritoneum)

Tissues near the anus and tail

External genitalia

What to Watch For

Manifestations of pain are highly variable in animals. Some signs are obviously related to pain, while others are more subtle. The individual personality of the animal and its tolerance for pain also affect the clinical manifestations of pain. Animals in pain often present with a wide variety of signs. Some signs that are associated with pain include the following:

Altered behavior – quieter than normal, avoidance of other animals or people, hiding, aggressive behavior, fear biting, mental dullness and depression, agitation, restlessness, pacing

Altered movement or gait – lameness, reluctance to move, reluctance to get up, wobbliness, abnormal carriage or use of one or more legs, stiffness

Vocalization – meowing, howling, moaning, groaning, absence of purring

Decreased or lack of appetite (anorexia)

Increased respiratory rate

Increased heart rate

Shock, collapse