Understanding Chronic Pain
Photo: Anne M. Eberhardt/The Horse
All of us suffer from aches and pains from time to time, and many of us think this is simply a fact of life and a natural consequence of the aging process. We buck up, march on, and when needed take an over-the-counter pain reliever. So shouldn't our horses do just the same?
Not according to a wide variety of equine practitioners, surgeons, and specialists, including William W. Muir III, DVM, MS, PhD, Dipl. ACVA (anesthesia), ACVECC (emergency and critical care), who is currently the chief medical officer for the Animal Medical Center in New York City.
"(According to the Farm Animal Welfare Council), all animals are entitled to five freedoms: freedom from hunger and thirst; freedom from discomfort; freedom to express normal behavior; freedom from fear and distress; and freedom from pain, injury, and disease," says Muir. "Studies have shown that chronic pain can modify the nervous system, can become an actual disease, and cause distress."
Simply put, chronic pain can't be ignored. It leads to suffering and distress and negatively impacts both a horse's physiology and his mental health.
In this article we'll look at some sources of chronic pain in horses, common clinical signs, and traditional and alternative means of controlling pain. Pain management in horses, even for mildly painful conditions, is important because in addition to depression and stress, uncontrolled pain can negatively affect appetite, the immune system, and tissue healing, and it can increase the horse's risk for developing gastric ulceration and colitis (inflammation of the colon).
Causes of Chronic Pain
Scientists (both veterinary and human) used to think there was only one "type" of pain--that which results from an unpleasant stimulus that pain-specific nerves transmit from the injury site to the central nervous system (brain and spinal cord). In turn, the central nervous system sends back a reflex signal, causing the individual to move the injured body part away from the unpleasant stimulus. A classic example of this pain mechanism in people is when someone burns a finger. Scientists now know there are multiple "pain sensing" systems in mammals' bodies involving nerves and the central nervous system, and that various mechanisms can impact these systems to increase or decrease pain intensity.
The pain referred to in the classic finger-burning example is "nociceptive" pain, in which a noxious stimuli (heat) stimulates the pain response. Other types of pain include inflammatory and neuropathic pain, among others. Inflammatory pain occurs secondary to tissue injury and is thought to amplify pain (i.e., cause hypersensitivity) and essentially protect the injured area from further damage while the body heals. Neuropathic pain occurs as a result of nerve trauma and is perceptible even after the painful stimulus is gone.
Musculoskeletal injuries, even old injuries that have healed, are among the most common causes of chronic pain in horses. Osteoarthritis, for example, is the painful erosion of articular cartilage--that which lines the ends of bones in a joint. This irreversible condition causes inflammation and pain in the soft tissues surrounding the joint as well as the layer of bone that lies directly below the affected articular cartilage. Laminitis is another extremely painful condition that can become chronic; it occurs when the hoof wall and the coffin bone separate due to the mechanical failure of the connecting structures, called laminae. Laminitis can have rapid onset (acute laminitis), but some horses with underlying medical disorders such as pituitary pars intermedia dysfunction (PPID, known as equine Cushing's disease) suffer milder, repeated bouts of laminitis. Back pain, navicular disease, old tendon or ligament injuries, gastric ulcers, eye problems (such as moon blindness or corneal ulcers), and even some respiratory conditions such as inflammatory airway disease and recurrent airway obstruction (heaves) can also cause chronic pain or distress.
How to Recognize Pain
Recognizing a horse in severe pain--such as a colicky horse thrashing violently in his stall--is usually easy. But it's possible to miss other extremely painful conditions such as a fracture or acute laminitis.
"Horses are able to 'cover' their pain due to the presence of a built-in pain suppression response that mammals have, which is called 'stress-induced analgesia,' or SIA," explains Ann Wagner, DVM, MSc, Dipl. ACVP, ACVA, a professor in anesthesia at Colorado State University's (CSU) College of Veterinary Medicine & Biomedical Sciences. "Essentially, SIA explains how a racehorse that suffers a serious leg injury during a race can keep galloping despite the jockey's efforts to pull it up."
So if even extremely painful events might be missed, how can owners identify subtle signs of pain in their steeds?
"Even in human medicine, where patients are able to provide a self-assessment of pain, recognition and successful management of pain in people is challenging," Wagner notes. "In animals, recognizing and alleviating pain is even more complex, requiring both behavioral and physiological changes to be carefully evaluated. Although several different pain scales exist in equine medicine, there is no 'gold standard,' and not all pain scales provide consistent results."
Nora Matthews, DVM, Dipl. ACVA, a professor of anesthesia at Texas A&M University's College of Veterinary Medicine, adds, "The pain scales we have are designed for specific types of pain such as orthopedic pain or colic, but the equine industry needs a pain scale that can provide a good assessment of all types of pain."
Wagner encourages owners to take the time to assess their horses' behavior daily and be aware of the more subtle signs of pain. These can include lameness; restlessness; head-lowering; teeth-grinding; flaring of nostrils; sweating; rigid posture; head turning toward the flank; kicking at the abdomen; reluctance to be handled; flight behaviors; and aggression. Other signs of chronic pain are weight loss; shifts in social behavior such as allodynia (e.g., a horse that normally enjoys being petted now shows aversion to it); changes in eating, drinking, and sleeping patterns; and decreased response to stimuli.
Current Treatment Options
Matthews presented a comprehensive review of pain-relieving drugs at the 2009 American Association of Equine Practitioners (AAEP) Annual Convention. In her presentation she described currently available drugs for managing horses' pain, including non-steroidal anti-inflammatory drugs (NSAIDs); opioids such as morphine or butorphanol; the α-2 agonists (e.g., xylazine); dissociative drugs such as ketamine; and local anesthetics such as lidocaine.
While some of these drugs are extremely important for providing "balanced anesthesia" in horses before, during, and after surgery, most of them simply are not useful for managing chronic pain, largely because they are not available in easy-to-administer oral formulations.
"We need more options for treating chronic pain in horses," says Matthews. "New formulations of existing drugs, such as oral drugs that owners can give to their horses or transdermal patches, and long-lasting formulations are needed."
Non-steroidal anti-inflammatories are considered the mainstay for chronic pain management and include such staples as phenylbutazone (Bute), firocoxib (Equioxx), and the topical product 1% diclofenac sodium (Surpass). Two other non-NSAID drugs that might be suitable for managing pain in specific cases (upon the advice of a veterinarian) are gabapentin and tramadol. Gabapentin is an anticonvulsant drug (generally used to treat neuropathic pain in humans and small animals) that can also be used to treat neuropathic pain caused by an injury or disease affecting the equine nervous system. The second drug, tramadol, is a narcoticlike pain reliever related to codeine. Although it has been used successfully in humans to treat moderate to severe pain, such as back pain, studies on the efficacy of tramadol in horses have yielded conflicting results.
So before owners get excited about havingother options to treat chronic pain, Matthews notes, "Further study is needed regarding effectiveness, dosing, and the availability of (each) drug after oral administration, how these drugs are metabolized, and potential side effects."
Alternative Pain Treatment Options
We've stressed pain's negative impacts and the importance of treating a painful animal immediately and appropriately to minimize his discomfort. Unfortunately, even with the use of pharmaceutical drugs, some horses' pain cannot be adequately controlled. In these cases some veterinarians might consider alternative therapies.
"There are a large number of alternative therapies available for horses that are used as adjuncts to more traditional regimens to help control chronic pain," relays Muir.
The list of complementary and alternative therapies is extensive and includes massage, hydrotherapy, chiropractic, static magnets, nutritional supplements, prolotherapy (repeated injections of an irritant solution, such as hypertonic glucose, with a goal of reducing pain), extracorporeal shock wave therapy, and acupuncture, among many others. However, not all modalities have been properly studied, so currently researchers and veterinarians cannot make any firm recommendations on treatments and whether they work. Of the available alternative therapies, acupuncture is commonly used in equine practice to help manage chronic pain caused by musculoskeletal pain due to lameness in the feet or back.
One study presented at the 1997 AAEP Annual Convention assessed using acupuncture on pain caused by special lameness-inducing shoes in five horses. Researchers from California State Polytechnic University's Department of Animal and Veterinary Sciences judged pain based on heart rate, which has previously been shown to be a reliable indicator of pain in the horse. When the shoes were manipulated to induce pain and lameness, the horses' heart rates increased. The heart rates were subsequently measured every 15 seconds during and after acupuncture treatment, and a significant decrease in heart rate was noted in four of the five horses. Based on this study, the authors concluded, "there is evidence to suggest that acupuncture has merit as an alternative source of therapy in the alleviation of equine pain."
A second study conducted by researchers at the University of Pennsylvania (also presented at the 1997 AAEP Convention) evaluated acupuncture use for treating horses with chronic back pain. To test their theory that it would be useful for this purpose, the researchers performed various forms of acupuncture (i.e., laser stimulation, traditional dry needling, or injection of saline with or without methylprednisolone acetate) on 350 horses with back pain. Each horse received one weekly treatment for eight weeks. In total, 263 (75%) horses responded positively and were able to perform at an acceptable level following treatment, prompting the researchers to conclude that "acupuncture may be a useful form of treatment for chronic back pain in horses."
Both studies' authors did indicate, however, that more research in this field would be beneficial.
Researchers also have studied the effects of chiropractic and manipulation on conditions such as back pain. At the 2010 AAEP Convention, Kevin Haussler, DVM, DC, PhD, of CSU, presented results from his and colleagues' research suggesting spinal manipulation poses notable pain relief for horses with chronic neck and back pain.
"Over time, the nervous system of a horse experiencing chronic pain actually changes, which makes it much more difficult or impossible to effectively treat," warns Muir. Thus, recognizing and treating pain--even low-grade aches and pains--with your veterinarian's help and advice before the nervous system's "wires" are irreversibly altered--is essential. Adequate pain relief will improve your horse's health and quality of life and can lessen the likelihood that he'll become aggressive or develop other undesirable pain-related behaviors.
About the Author
Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.