Drugs And Pain

When you feel stiff and sore, you can grit your teeth and work through the pain; or, you can depend on medication like aspirin, ibuprofen, or even a stronger analgesic. Like you, the horse suffers from joint inflammation and painful movement. Your horse relies on you to make the decision that determines his comfort level. Either he bears the pain, or you choose to apply a remedy.

To help a horse in pain, treatment relieves the sensation so the horse can recover. The equine practitioner diagnoses the horse's condition and treats pain with drugs proven effective on horses.

But how do veterinarians and horse owners know what is effective on horses' pain? For that you need to understand the types of pain-relieving drugs. The results of recent studies performed at Louisiana State University School of Veterinary Medicine by Steven G. Kamerling, RPh, PhD, are focused on determining how certain drugs alleviate the pain of specific lamenesses.

Pain Relievers

Kamerling reported, "Pain signals imminent or on-going tissue injury. It underlies lameness, and can be considered a perform-ance-impairing condition. Thus, if pain is present in a performing horse, its elimination should restore normal performance to that animal."

Pain-relieving drugs include analgesics and anesthetics. The analgesics block pain, with various types matching the extent of pain.

Narcotic analgesics include opiates (drugs derived from the opium poppy plant) and opioids, usually synthetic drugs such as Darvon and Demerol. Through selective pain relief, the narcotic analgesics control pain.

Kamerling noted, "Opioids act to selectively depress pain-sensitive cells." By acting on receptors in the brain and spinal cord, they produce analgesia while leaving other sensations intact.

The group of non-narcotic analgesics include the NSAIDs (non-steroidal anti-inflammatory drugs). Examples are phenylbutazone, ketoprofen, and flunixin meglumine.

Local anesthetics block an area. Here the effect is non-selective, and the animal loses sensation in the treated area. "In sufficient doses, local anesthetics will depress all sensory, motor, and autonomic nerve fibers," Kamerling explained. "They can also render smooth, skeletal, and cardiac muscle inexcitable and unresponsive."

In the process of blocking cells, local anesthetics interrupt the transmission of pain signals. Examples of these numbing pain relievers include procaine, lidocaine, and mepivacaine.

As in humans, drugs in horses promote recovery by aiding the body's natural defenses. Relieving pain is part of that recovery. Both anesthetics and analgesics can be considered performance-restoring drugs; the narcotic analgesics fall into the category of performance-enhancing drugs.

Research Into Ketoprofen

Studies in pharmacokinetics help scientists understand how the horse's body absorbs, distributes, metabolizes, and eliminates drugs. Through a complex chemical process, a drug binds with cells and produces an action. The process varies according to each drug's properties, how it concentrates in plasma and tissue, and how it is broken down and eliminated from the body.

Non-steroidal anti-inflammatory drugs are prescribed for arthritis patients, with human medicine using the drug ketoprofen. A drug of the same family as ibuprofen, this NSAID also alleviates pain and inflammation in the horse. Ketoprofen acts directly at the level of the tissues, like another NSAID, phenylbutazone.

Kamerling and Dr. Jane Owens recently studied the pharmacokinetics of ketoprofen in horses. The study compared the drug's action in healthy horses with horses afflicted by acute synovitis, induced for the experiment. The researchers compared the concentrations of ketoprofen in the healthy joints and the inflamed joints to see how the joint inflammation affected the extent and duration of action of the drug.

The LSU research indicated that the drug penetrates more into inflamed joints. After a single intravenous dose, ketoprofen was recovered from the joints' synovial fluid and plasma.

According to Kamerling and Owens, "Ketoprofen synovial fluid and ketoprofen plasma concentrations in horses with acute synovitis reached equilibrium at one hour post-administration. In the horses with acute synovitis, synovial fluid concentrations at three hours tended to be higher than the corresponding plasma concentrations."

In comparison, healthy horses reached equilibrium at three hours after treatment. Human patients have shown similar results of increased penetration of the NSAID into inflamed joints. This means that ketoprofen reaches its main site of action (the joint) faster when inflammation is present.

Understanding Pain

By studying your horse's behavior, you can recognize a horse which suffers either the acute or chronic pain of inflammation. Indicators of acute pain can include behavior changes in alertness, increased or decreased movement, posture, appetite, and social interactions.

Chronic pain might be displayed as anxiety or depression. Kamerling noted, "Pain of a chronic nature is more likely to be viewed in terms of distress, discomfort, or frank suffering by the animal. It may be more difficult to assess behaviorally than acute pain."

Types of pain respond differently. The treating veterinarian chooses the appropriate drug to match the pain the horse communicates.

Because horses express pain through their behavior, the horse owner observes each animal to detect its condition. Horses do vary in their pain thresholds, or how they tolerate pain. Like a person, a horse might feel pain without demonstrating evidence of it.

Humans and animals can suppress pain by affecting nerve fibers. Rubbing or scratching an area near an injury can lessen pain.

"Working through pain" occurs through the body's natural pain relief by endorphins. Released from the pituitary gland during exertion or other stress, these hormones reduce the sensation of pain.

"The measurement of pain threshold in inflamed tissue is different from that in normal tissue," noted Kamerling. "In fact, some drugs (e.g., aspirin) do not appreciably alter normal pain perception yet raise the pain threshold in the presence of inflammation. Pain experienced by the equine athlete can be acute or reflect chronic inflammatory tissue injury."

Testing And Relieving Laminitis Pain

The chronic pain of laminitis continues to concern horse owners. Here the suffering horse confronts owners and veterinarians with tough decisions about how to provide a level of comfort.

In another recent study, LSU researchers compared two analgesics--ketoprofen with the commonly used phenylbutazone to relieve the pain of laminitis. Ketoprofen had not been evaluated in horses with laminitis.

This study of seven laminitic animals assessed each horse's lameness on a five-level scale. Within each grade, examiners added a plus or minus to note the severity of the score. Examiners graded each equine subject immediately before testing right and left hooves.

Researchers assigned 16 areas on the surface of each forefoot. Examiners applied a calibrated electronic hoof tester to the areas, which measured the amount of compressive force associated with the horse's reaction. Horses with chronic hoof pain proved sensitive to such compression.

Kamerling and Owens reported how a hoof tester was used to record the extent and duration of analgesia. They evaluated the horse's expression of pain relief prior to treatment and at three-, six- and 24-hour increments.

They said, "The force in Newtons required to produce a hoof withdrawal response by the laminitic horse was designated the hoof compression threshold (HCT)." They added that examiners stopped the pressure when the horse responded.

To assign responses to each area, researchers established a HCT of five grades. These ranged from Grade 0 of no response to hoof compression, to Grade 4 of the horse pulling away to put its foot on the ground.

The study took into account how horses accepted pain. Kamerling and Owens noted, "To minimize variance in pain thresholds over experimental sessions, all data were expressed as percent change from baseline for each horse at each post-treatment time within a given session."

The study resulted in ketoprofen showing an increase in the HCT. Compared to the responses of horses treated with phenylbutazone, the horses treated with ketoprofen showed a reduction in hoof pain. Kamerling and Owens reported that results "suggest that ketoprofen at the dosage rate of 1.65 times the recommended therapeutic dose (or 3.63 mg/kg) was more potent than phenylbutazone in alleviating chronic pain and lameness in horses." They added that the laminitic horse might benefit from the increased dosage rate, which did not appear to be associated with toxicity in this study.

Research such as that at LSU continues to explore means of reducing the distress of equine athletes by better understanding drugs and how best to use them for horses with specific problems. Studies of drugs such as ketoprofen have shown better ways in which it can reduce pain, heat, swelling, and loss of function--the effects of inflammation that affect performance.

About the Author

Charlene Strickland

Award-winning writer Charlene Strickland lives in Bosque Farms, N.M. She has published 8 books and over 600 magazine articles, and is a member of the International Alliance of Equestrian Journalists.

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