Relieving Rectal Pain in Mares

Roman Skarda, DrMedVet., PhD, professor in the Anesthesia Section of the Department of Veterinary Clinical Sciences at The Ohio State University, has studied techniques of epidural and spinal analgesia in horses for 20 years. For the past seven years, he has focused on acupuncture and alternative methods of analgesia, and is currently using an experimental model to re-create rectal pain in horses. A balloon is placed into the rectum and gently inflated to induce pressure and pain. This allows for objective comparison of different methods of analgesia in conscious horses.

Butorphanol is a good analgesic drug; however, it can have adverse effects on the cardiovascular and respiratory systems. Ideally, an alternative method, such as electroacupuncture (EA), would match butorphanol's analgesic effects while minimizing adverse effects. Skarda says EA was chosen over traditional acupuncture because previous experiments indicated that EA is more effective at releasing endorphins into the spinal fluid of horses.

Each horse in this study was assigned to receive butorphanol IV, saline IV (as a placebo), or EA at various acupoints. Skarda describes the electrical charge component of EA as "crossing the spine in a zigzag fashion" from one to another in each needle pair. "An electric field along a substantial distance of the spinal cord is produced," Skarda explains, "to stimulate the release of endorphins from several spinal nerves and produce good analgesia."

The results indicated that butorphanol provided the best analgesia, with mares in this group tolerating the highest rectal pressures (200 to 220 mm Hg). EA also provided good analgesia, allowing mares to tolerate 140 to 200 mm Hg pressure with minimal changes in heart rate (HR), respiratory rate (RR), and arterial blood pressure (BP), internal indicators of pain. Increasing balloon pressure wasn't particularly painful, producing only minimal increases in HR, RR, and BP above baseline. In contrast, butorphanol itself produced moderate increases in HR, RR, and BP, which partially masked further increases from pain once the balloon was inflated. Still, EA and butorphanol both appear to be effective for rectal analgesia in horses, with EA possibly preferable in horses with compromised cardiovascular or respiratory systems.

Skarda plans to investigate different modes of EA to learn which produces the greatest release of endorphins, and whether it can increase a horse's threshold for pain.

Skarda, R.T.; Muir, W.W. III. American Journal of Veterinary Research, 64 (2), 137-144, 2003.

About the Author

Susan Piscopo, DVM, PhD

Susan Piscopo, DVM, PhD, is a free-lance writer in the biomedical sciences. She practiced veterinary medicine in North Carolina before accepting a fellowship to pursue a PhD in physiology at North Carolina State University. She lives in northern New Jersey with her husband and two sons.

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