Opioids have a powerful analgesic effect in horses. However, this drug class—especially systemic morphine—is known to diminish intestinal motility, depress respiration, and affect a horse’s excitement level. Tramadol exerts minimal interference with cardiac, respiratory, and gastrointestinal systems but is known to elicit tachycardia, hypertension, and seizures at high doses.
To address these two analgesics’ shortcomings, researchers examined epidural administration of morphine and tramadol to see how the drugs alter clinical and behavioral signs.
The study included eight horses ages 4-7 in a crossover design that split them into three groups. Each group received all three treatments with a 15-day interval between each trial:
- Tramadol at 1 mg/kg (TG).
- Morphine at 0.2 mg/kg (MG).
- Normal saline (CG) administered via an epidural catheter.
Study Results
Researchers evaluated heart rate, respiratory rate, rectal temperature, intestinal motility scores of each quadrant, latency to defecate (time between epidural treatment and passage of manure), and behavioral alterations.
The results demonstrate:
- Heart and respiratory rates did not change significantly with epidural administration of morphine or tramadol.
- Body temperature increased with morphine and saline infusions, but the authors believe this was a result of climatic conditions rather than opioid interference with temperature regulation.
- Epidural administration of morphine had a significant effect on intestinal motility and latency to defecate, potentially due to a blockade of opioid receptors in the myenteric nervous system that reduces cholinergic tone. These effects were temporary, and despite decreased motility, the horses did not experience abdominal discomfort.
- Epidural tramadol did not impact intestinal motility scores or latency defecation. This is a notable improvement over oral and intravenous tramadol, which are both reported to cause intestinal discomfort.
Final Thoughts
In summary, epidural administration of tramadol had little adverse effect on gastrointestinal motility, heart or respiratory rates, or behavior. Morphine elicited temporary decreases in gastrointestinal motility but did not cause clinical or behavioral changes. Epidural administration of both opioids appears to be safe and effective for controlling pain in horses.
Reference
Leite AmS, Bertonha CM, Valadao CAA, et al. Morphine or Tramadol Administration by Epidural Route in Horses – Clinical Evaluation. Acta Scientiae Veterinariae 2024, vol. 52: 1965; DOI: 10.22456/167909216.140962
