What is pain? Do animals feel pain? While these may be interesting philosophical questions, on the practical level, they are obvious to horse owners. Pain is what I get after a really long trail ride. My horse is obviously in pain when he is limping around on three legs. Pain is the indicator built into an animal’s body to identify a problem, whether sickness or injury. Pain is triggered by some nerve impulse that is strong enough to then trigger higher nerves in the brain and cause a reaction. This can be demonstrated by gently striking your thumb with a hammer. Then hit it a little harder. Pain is also chemical. When injury has occurred at the tissue level where cells are being damaged, chemicals are released that trigger a lower threshold for nerve firing. This is like the opposite of Novocain at the dentist. The tissues get more sensitive instead of less. These chemicals also trigger increased blood flow to the area so that it appears red and warm and the tissue becomes leaky to the outflow of fluid thereby causing swelling. Swelling, redness, pain and heat are all signs of inflammation. Hit your thumb very hard and then wait until the next day. That is inflammation! Inflammation is caused by the chemicals that are released in response to injury (even injury caused by a virus or bacteria as they destroy tissue).
The chemicals that are responsible for inflammation and pain are called catecholamine, one of which is called prostaglandin. Prostaglandins are used in normal healthy places in the body like the lining of the stomach and the kidney to maintain normal blood flow. They also lower the pain threshold of nerves. So, when there is injury of any kind to a cell in a tissue, the pain is caused by the release of prostaglandins to let the rest of the body know there is a problem. “Hello, immune system, white blood cells and all you guys! Come fix this!” The problem is that most of the time the response of the body is too good, too big and the response can hurt and slow down healing. The way to treat the pain and inflammation has been to develop drugs that block the pathway to release prostaglandins. This involves the enzyme cyclooxygenase (COX), which has different pathways to produce various prostaglandins. One pathway (COX 1) produces prostaglandins that are important in blood flow to the gut and kidneys. Another pathway (COX 2) produces prostaglandins that are responsible for pain. Most drugs that have been developed for pain (Aspirin,Phenylbutazone, Banamine, Tylenol) inhibit all the pathways of the COX enzyme. This is why you get a decrease in pain but can have an upset stomach or worse with too much of these drugs. Newer drugs have been developed that target only the COX 2 pathway, but in humans they have been linked to heart disease. We have not seen these problems in animals, but to date, no COX 2 inhibitors have been approved for horses.
Other drugs that decrease pain and inflammation are steroids. They are powerful anti-inflammatories that can have a long term affect on tissue that is under chronic stress of inflammation. These are used mostly to treat arthritis and immune system problems. These are usually injected at the target tissue, like the joint capsule. This method bypasses the body’s other organs. Another way to bypass the body’s organs is to apply the drug topically. Recently, a cream called Surpass has become available that absorbs into the target joint through the skin. The use of this type of medication may decrease the amount of systemic medications used and help protect the body’s sensitive organs.