Pain In Horses

Pain is defined as "a feeling of distress, suffering, or agony caused by stimulation of specialized nerve endings." The scientific term for the perception of pain is "nociception," with noci the Latin for "harm or injury."

Pain and its control are one of the largest topics in medicine. A quick literature search on only one database going back to 1986 yields 58,700 articles pertaining to some aspect of pain (99 of which related specifically to pain in horses). Over the last century, the effort to control pain and suffering in people has been great, but it has only been in the last two decades that a similar effort has been directed toward our animal companions.

In 1987, an entire issue of the Journal of the American Veterinary Medical Association was devoted to animal pain by publishing the Colloquium on Recognition and Alleviation of Animal Pain and Distress. The issue included 40 articles by 32 authors on 23 topics related to the understanding of pain and its alleviation.

In an extremely simplified explanation, pain is first identified by special receptors at nerve endings. Once the signal, be it a sharp nail in the foot or a gas pocket in the colon, is detected by the receptor, the message is sent via nerves to the spinal cord. The spinal cord functions as a relay switch, where signals are potentially modified and sent up the spinal system to the brain. It is in the brain that the pain signals are processed, perceived, and the agony begins. This is oversimplified, but gives enough foundation for us to later discuss mechanisms for "blocking" pain at the receptor, in the spinal cord, and in the brain.

Pain is generally divided into two broad classifications: somatic pain and visceral pain. Soma is Greek for "a body," and generally cutaneous (skin) is superficial somatic pain and musculoskeletal is deep somatic pain. Viscer is Latin for "the organs of the body cavity" and, therefore, visceral pain refers to pain originating from organs, i.e., the gastrointestinal system, kidneys, etc.

The Nuances Of Equine Pain

A friend of mine who is a physician said to me: "Your career as a veterinarian must be frustrating because your patients can't tell you where it hurts." But, in fact, once you learn the language, it's not too hard. For example, the subtle pointing of a foot as well as a greater frequency of shifting weight potentially indicates soreness somewhere in the leg. In the horse, somatic pain is usually related to muscle, tendon/ligament, or bone origin. Most pain is accompanied by or, a direct result of, inflammation; the hallmarks of inflammation are heat, pain, swelling, and redness. All soft-tissue structures (muscle and tendons) usually develop heat and swelling in response to inflammation, so looking for asymmetry and warmth in conjunction with sensitivity to palpation can help to localize the pain to a specific area. Reaction to manipulation such as pulling away, pinning of ears, and occasionally biting or aggressive posturing can be a clue that you are in the right area for the cause of the pain.

You have to be careful in interpretation though, as some horses just plain resent manipulation, and many athletic horses often carry some degree of sensitivity to the tendons and ligaments of the lower limbs.

Joint or bone pain can be more difficult. The use of hoof testers can apply pressure to the coffin bone or navicular area and might help localize pain to those areas. The hoof testers also are useful for the localization of foot abscesses.

With bucked shins or shin splints, the dorsal surface of the cannon bone is sensitive to palpation. Splints are usually identified as warm/hot painful bumps palpated on the splint bones. The remainder of the "hands-on" examination is looking for any other clues: resentment to manipulation of a limb or flexing of the neck from side to side. Identification of distention or "fill" in a joint can be an indication of a problem in that joint.

Lameness can generally be localized to a particular limb by evaluating the horse at the trot and looking for that ominous head-nod or unevenness of the hips. Once the lame leg has been identified, if there were no give-away clues on the palpation, the joints can be flexed in an effort to make what's painful hurt even more and hopefully identify a localized area. Once an area is identified, it can be further evaluated if necessary with radiographs and/or local anesthesia.

Visceral pain is most commonly related to colic in the horse. (Colic is Greek for "affecting the bowels.") The most common signs of abdominal pain in the horse are probably anorexia (a general sign of illness in all species), attempting to roll, looking at its belly, pawing, sweating, lying down excessively, and various elevations of heart rate or respiratory rates. These are all non-specific signs that often are associated with abdominal discomfort.

Horses, like people, differ greatly in their tolerance of pain. Visceral pain can originate from other organ systems, although that is much less common. Some horses with kidney stones or stones in the bile duct of the liver (horses don't have a gallbladder) might show abdominal pain.

The thoracic cavity, in cases of pleuritis (infection/inflammation within the thoracic cavity), can be intensely painful. Palpation in between the ribs of horses with pleuritis can elicit an extremely painful response. Many cases of pleuritis are so painful that horses will refuse to walk, often appearing like a horse with laminitis or founder.

Not all of the above are always related to pain, and sometimes are related to behavioral or training problems. As with humans, each horse is an individual and has a different tolerance and way of manifesting pain. Some horses are very stoic and might be hard to "read" as to what hurts and how much.

Drugs And Pain

Drugs that affect the perception of pain do so in three general ways. The local anesthetics are used in the diagnosis of lameness or for aiding in the repair of lacerations or other surgical procedures. Local anesthetics work by blocking the movement of sodium (essential for nerve conduction) in nervous tissue locally where they have been injected. This action blocks or disrupts the transmission of the pain signal by the nerve from the receptor picking up the signal to the spinal cord (and therefore the brain).

An interesting note is that cocaine was discovered in 1860 and first used as a local anesthetic in 1884. It has been replaced over the years with numerous other compounds that don't possess the human abuse potential. The drugs lidocaine, bupivacaine, and mepivacaine are the most commonly used local anesthetics in veterinary medicine today.

The next common classification of drugs used to alleviate pain are the non-steroidal anti-inflammatory drugs (NSAIDs). Aspirin (acetylsalicylic acid) is a NSAID. It was first produced in 1853 by Charles Gerhardt, a German chemistry professor. By 1899, acetylsalicylic acid was identified as having relieving properties for rheumatism and was marketed that year by Bayer. Aspirin, unfortunately, is very rapidly eliminated by the horse's body and therefore has never served as an effective pain reliever in that species. Other NSAIDs commonly used in the horse today are phenylbutazone (Bute), flunixin meglumine (Banamine), and ketoprofen (Ketofen).

Another classification name for this family of drugs (NSAIDs) is the anti-prostaglandins. This name alludes to their general mechanism of action. During the cellular process of inflammation, chemicals are produced that have many effects on the body. One such group of chemicals is called the prostaglandins. Certain prostaglandins act as mediators to stimulate pain receptors locally at the site of inflammation. The NSAIDs inhibit the production of prostaglandins, and therefore moderate pain at that level.

Phenylbutazone and Banamine are very potent drugs and generally are very effective in reducing pain. One potential complication is the development of gastrointestinal ulcers.

One of the prostaglandins that is inhibited by the NSAIDs plays a very important role in maintaining the health of the gastrointestinal tract--it offers a "protective" effect. The ulcer complication seems to be, to some degree, a horse-by-horse sensitivity since there are horses with chronic arthritis which have been given phenylbutazone daily for years and have had no problems. But, there are horses which have been on an NSAID for a short period of time (days to weeks) which develop a fairly severe gastrointestinal problem.

A degree of concern is probably warranted, and NSAIDs used as judiciously as possible--always seeking the lowest dose and frequency that achieves the desired effect. In addition, any horse which is on a NSAID that develops signs of anorexia, depression, or colic should be evaluated promptly.

Brain Pain

Now we are into the brain and the drugs that control pain there. These drugs are referred to as centrally acting analgesics. Such drugs are classically the narcotics. Reports of the use of opium (morphine was first derived from the opium poppy) for the control of pain date back to the 12th Century; its use to produce sleep dates back to the 3rd Century BC in Asia Minor. Alcohol in earlier times was also used as a centrally acting analgesic. It was recommended by the naval surgeon Moyle in 1693 to administer alcohol prior to surgery, and a successful Caesarean section was reported in 1879 under the influence of banana wine.

Centrally acting analgesics commonly used in the horse today include xylazine (Rompun), detomidine (Domosedan), butorphanol (Torbogesic), pentazocine (Talwin), and occasionally morphine. Most of these have both sedative and analgesic effects. These drugs alter the way the brain processes the pain signal and therefore work at a "central" level.

Most pain medications will work well at alleviating the pain, but do very little for alleviating the true problem. In addition, severity and persistence of pain are often used as a diagnostic tool. For example, uncontrollable or poorly controllable abdominal pain is often a strong indication for surgical exploration of a horse with colic.

Many pain medications available today are extremely potent and can potentially "mask" the pain of a severe colic or musculoskeletal injury. It is therefore extremely important that any pain medication given to the horse be recorded as to the kind, specific dose, route of administration, and time(s) administered for use by the attending veterinarian evaluating the horse in pain.

About the Author

Michael Ball, DVM

Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.

Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from TheHorse.com. Learn More

Free Newsletters

Sign up for the latest in:

From our partners