Every horse owner is familiar with pain. Horses get colic, get a scratch on their cornea, or go lame. We know something is wrong because they act painful. But what exactly is pain?
The International Association for the Study of Pain (IASP) defines pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.” The IASP adds, “The inability to communicate verbally does not negate the possibility that an individual is experiencing pain and is in need of appropriate pain-relieving treatment.”
Pain involves the sensory reception of a noxious stimulus that evokes a response, usually a withdrawal from the noxious stimulus. This is a protective mechanism that helps to keep us and animals from becoming injured by whatever noxious stimulus we just experienced. There is an emotional component to pain as well. We learn that a certain stimulus will cause pain and therefore we avoid it in the future. Our bodies are rich with pain receptors, called nociceptors. These are unmyelinated nerve endings that are present in vast numbers in the skin, joint capsules, muscles, and internal organs, basically anywhere pain can be felt. The main body of these cells is located within the spinal cord. The nociceptors are stimulated and release neurotransmitters that send signals through the spinal cord, up to the brain, are processed, and then the evasive action is transmitted back down the spinal cord and is initiated.
There are several types of nociceptors. Mechanosensitive nociceptors respond to pressure. Thermal nociceptors respond to temperature. Polymodal nociceptors respond to pressure, temperature, and chemicals released by injured cells. Inflammation causes many of these chemical messengers that cause pain to be released. This is how pain and inflammation are related. Adaptive pain is helpful pain. It produces responses and behaviors that avoid the painful stimulus and promote healing and recovery. If adaptive pain is not appropriately managed, physical changes occur in the central nervous system (spinal cord and brain), leading to maladaptive pain. Maladaptive pain is unhelpful pain that is out of proportion to the actual tissue damage and persists long after the injured tissues have healed. Maladaptive pain is very difficult to control. The changes to the central nervous system make the nervous system more sensitive to stimuli, even in the face of treatment and healing. The longer pain goes unmanaged, the more likely it is to undergo the change from adaptive to maladaptive pain and the harder it is to manage. This is true for both acute and chronic pain. With this basic understanding of the neurophysiology of pain, it is clear why some cases of what should be easily controlled pain are more difficult to manage than expected.
Treating pain can take many forms. Traditionally medications are used. Other methods to treat pain include rest, hydrotherapy, electrostim therapy, acupuncture, extracorporeal shock wave therapy, laser therapy, and ultrasound therapy. This article will focus on medications.
There are many different pain medications available today. The common pain medications we use in our equine patients belong to the classes of steroidal, non-steroidal, and opiate pain relievers. They come in many different forms such as intravenous and intra-articular preparations, oral medications, and topical ones as well. Each medication has a specific function and appropriate use of it will ensure that you are safely and effectively treating your horse.
Inflammation produces a host of mediators that can cause direct stimulation of pain receptors or cause tissue damage that in turn stimulates pain receptors. The pathway of inflammation is mediated by arachidonic acid that is further broken down into two inflammatory mediators. The first is prostaglandin that is produced from arachidonic acid by cyclooxygenase. The second is leukotrienes that are produced from arachidonic acid by 5-lipoxygenase. Steroids prevent the production of arachidonic acid and therefore prevent further inflammation by preventing the production of prostaglandins and leukotrienes. This blocks the entire cascade, also preventing the production of good prostaglandins that protect the lining of the stomach. Steroids do not directly act on pain receptors. They stop the production of chemicals that stimulate those receptors. Steroids are most commonly used to treat inflammation of joints and are injected into the affected joint to minimize unwanted side effects when given systemically. Topical steroids are used to decrease inflammation and pain in certain ocular conditions. It is imperative that you do not treat a corneal ulcer with steroids. Use steroids in the eye only under the advice from a veterinarian. Common steroids used for pain management include methylprednisolone, triamcinolone, and prednisolone. Steroids should be used only when recommended by your veterinarian.
Horse owners are usually most familiar with this class of drugs. It includes phenylbutazone, flunixin meglumine (Banamine), ketoprofen (Ketofen), firocoxib (Equioxx), aspirin, and meclofenamic acid (Arquel). The NSAIDs block the cyclooxygenase enzyme, interrupting formation of thromboxane, prostacyclin and the prostaglandins from arachidonic acid. Recent research has also shown that some of the NSAIDs act on pain receptors in the central nervous system and block pain in the same way as drugs like morphine. There are two types of cyclooxygenase (COX) enzymes, COX-1 and COX-2. The products of the action of COX-2 on arachidonic acid are the bad prostaglandins that result in inflammation. The products from the action of COX-1 are actually protective against inflammation and maintain the health of the gastrointestinal tract, the renal tract, and proper platelet function, among other functions. Blocking the products from the action of COX-1 is detrimental to your horse’s health. Older NSAIDS are nonspecific cyclooxygenase inhibitors. Phenylbutazone, flunixin meglumine, aspirin, ketofen, and meclofenamic acid are all nonspecific COX inhibitors. Side effects from these medications include gastrointestinal ulceration and renal disease (from decreased blood flow), impaired platelet function tract, and diarrhea and protein loss from mucosal damage to the gastrointestinal tract. Firocoxib is a newer NSAID that is a COX-2 inhibitor. This helps to minimize the side effects seen with the older NSAIDS. Many horses are on long term NSAIDS for chronic pain and minimizing side effects. Phenylbutazone, flunixin meglumine, aspirin, and firocoxib are available in both injectable and oral forms. Ketoprofen is injectable only. Meclofenamic acid is an oral product. One NSAID is available as an ointment. Surpass, diclofenac sodium, is available in a preparation that allows it to be applied to the skin and is absorbed locally where the inflammation is. There is minimal to no systemic absorption of the drug and therefore no adverse side effects.
The nervous system contains receptors that respond to opiates. Stimulation of these receptors causes euphoria, sedation, pain relief, hallucinations, and in humans, addiction. There are three types of opiate receptors, “mu,” “kappa,” and “sigma.” Opiates work in several ways to produce their effects. Receptors can be stimulated (agonized) or blocked (antagonized). Some opiates will only stimulate receptors and are called “pure agonists.” Others can block some receptors and stimulate others. These are called “mixed agonist/antagonists.” The most commonly used opiate pain medication in horses is butorphanol. It is mixed agonist/antagonist. Fentanyl is another opiate used in veterinary medicine. It is available in adhesive patches that slowly release the medication over an extended period of time. Unfortunately, fentanyl patches are not effective in the horse. Opiates are controlled substances and should be administered by a veterinarian only.
Xylazine and detomidine are alpha-2 agonist sedatives that also have analgesic properties. Analgesia is produced by binding to receptors in the spinal cord, thus inhibiting the release of neurotransmitters. These sedatives are useful for producing sedation and mild analgesia when your horse undergoes minor procedures that may cause mild discomfort such as teeth floating and suturing small lacerations, but are not suitable for long term pain management.
Many horses suffer from acute or chronic pain that requires careful treatment. The goal is to alleviate the pain as quickly as possible with the least amount of side effects. For horses with chronic pain, it is often necessary to try different medications to see which one is most effective for that particular horse. Many horses with chronic pain are still able to perform, but need to be medicated. Make sure that you check with your veterinarian to make sure that you are not violating any association rules when you mediate a performance horse that is competing. There are mandatory withdrawal times for certain medications and a new rule that restricts treatment to just one NSAID at a time. These rules are in place for the welfare of your horse.
Pain can be, well, a pain. Veterinary medicine has made tremendous strides in helping to reduce and control pain in our equine patients. Careful, judicious use of the correct medications at the correct time can greatly improve your horse’s comfort whether he is recovering from an injury or surgery, keeping comfortable in retirement, or easing the aches and pains of competition.
Registered 2011 by Equestrian Collections
Author: Sallie S. Hyman, VMD, DACVIM, CVA
Information in this article is for educational purposes only and is not a substitute for evaluation by an equine professional. In particular, all horse owners should seek advice and treatment from a licensed veterinarian, such as TEVA, for their horses' medical care.
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