How Dogs Feel Pain
The sensation of pain is transmitted by way of the nervous system, a network of billions of nerve cells, or neurons. (See Chapter 6 for more information about the nervous system.) One of the jobs of this system is to interpret sensory information acquired through smell, hearing, sight, taste, and touch. When your dog cuts himself on a barbed-wire fence, for example, the sensation stimulates numerous nerve endings beneath the skin. These nociceptors transmit a pain signal through sensory neurons in the spinal cord. This reaction releases a neurotransmitter that relays the pain signal from one neuron to another until it reaches the part of the brain where pain perception occurs. The signal then travels to the cerebrum, at which point your dog realizes that he's in pain and yelps.
The perception of pain is unique to each individual, human or dog. (That's one of the reasons pain is difficult to manage.) Factors that affect each dog's individual response to pain include age, gender, health status, and breed differences. For instance, young dogs have a lower tolerance for sudden pain, but they're less sensitive to the emotional stress or anxiety that accompanies anticipated pain (such as having the anal glands expressed). Healthy dogs tend to tolerate pain better than sick dogs, but sick dogs may be less likely to respond obviously to pain because they don't feel like making the effort. A stoic working or herding breed may show less response to pain than an excitable or sensitive breed.
Nociceptors are the free nerve endings of neurons that have their cell bodies outside the spinal column. They look like the branches of small bushes. Different nerves transmit information about different sensations, such as pain, cold, heat, and pressure.
The response to pain can be involuntary or voluntary. For instance, when a groomer's nail clippers cut into the quick, the painfully sensitive blood vessel that feeds the nail, the dog reflexively jerks his paw back. That's an involuntary response. A voluntary response is based on experience. A dog who's had his nails clipped too short in the past remembers the pain and jerks his paw back before the clippers even touch the nail.
Types of Pain
The sensation of pain can originate in the skin, bones, joints, muscles, or internal organs. Each causes a different type of pain. For instance, pain caused by injury to the skin or superficial tissues is called cutaneous pain. Minor cuts, burns, and lacerations are examples of cutaneous pain. A broken bone or sprained joint produces somatic pain, which originates from ligaments, tendons, bones, blood vessels, and nerves. Nociceptors located within body organs or body cavities produce visceral pain, such as a stomachache.
When humans are in pain, they usually don't hesitate to let other people know about it. A person may yell, shake the affected limb, limp, or otherwise express the feeling of injury. Dogs are a bit more subtle. A few individual canines don't mind letting you know that they've been hurt or don't feel well, but most dogs try to hide pain. It's an evolutionary response dictated by thousands of years of predatory knowledge: The weak don't survive. (Or maybe they secretly pass it on that pain means a trip to the veterinarian.) While they're generally not as secretive as cats, dogs will make a pretty good effort to keep you from knowing that they're not in tip-top condition. You have to be alert and observant to figure out that your dog isn't feeling his best.
Signs of Pain
Recognizing and finding the source of pain in dogs is a challenge. Dogs can't say where it hurts or how much it hurts, so you and your veterinarian must rely on your knowledge and observations of your dog's normal behavior.
Knowing what's normal is one of the most important ways you can recognize and assess pain in your dog. Once you really start to pay attention to your dog's regular actions and habits, anything that's abnormal will jump out at you.
The early signs of pain are subtle. They might include eating less, failing to greet you at the door when you come home from work, or not wanting to be groomed when normally that's a pleasurable experience. More obvious signs of pain include limping, reluctance to move, squinting or pawing at the eyes, crying out or whining when touched, or even snapping when touched. Any unexplained abnormality in your dog's routine behavior or activity level is significant and warrants a visit to the veterinarian. Common signs of pain you should watch for include the following:
Changes in personality or attitude, such as a normally quiet and docile dog becoming aggressive or an aggressive dog becoming quiet
Abnormal vocalizations, such as whining or whimpering, especially when a painful area is touched or the dog is forced to move
Licking, biting, scratching, or shaking of one area
Piloerection, a reflex of the muscles at the base of the hair shafts that causes the hair to stand on end
Changes in posture or movement, such as limping, holding a paw up, or tensing the abdominal and back muscles to produce a tucked-up appearance
Changes in activity level, including restlessness, pacing, lethargy, or reluctance to move
Loss of appetite
Changes in facial expression, such as dull eyes or pinned ears
Changes in bowel movements or urination, such as straining
Your veterinarian will check your dog's heart rate, respiratory rate, and body temperature. All of these tend to increase in the presence of pain. The veterinarian may check a blood sample for elevations in glucose, corticosteroid, and catecholamine concentrations.
Diagnosing the Pain
To find the source of the pain, your veterinarian will probably begin by palpating your dog's body, examining it by hand to check the condition of the organs and search for painful lumps or bumps. He may put pressure on the trigger points along the spine and check the range of motion of the legs by extending and flexing the joints to look for signs of discomfort. Once he knows where the pain is, your veterinarian can try to figure out what's causing it and how to treat it.
When there isn't an obvious cause for pain, such as a surgical wound, for instance, or a broken bone, sophisticated diagnostic techniques can help. These include analysis of the cerebrospinal fluid, radiographs of the spine using dye (myelography), measurements of the electrical activity in the muscles (electromyography), and brain imaging with computed tomography (CT) or magnetic resonance imaging (MRI) scans.
If your dog balks at taking a certain pain medication, ask if there's another way to give it. Some drugs can be made into syrups that are applied to the gums so the medication is absorbed through the mucous membranes. Others can be compounded into something tasty, such as peanut butter, to make them more palatable.