Parasympathetic tone activity, heart rate, and grimace scale in conscious horses of 3 breeds before, during, and after nociceptive mechanical stimulation.
Authors: Flores Jorge Eduardo Mendoza, Terrazas Angelica, Lara Sagahon Alma V, Aleman Monica
Journal: Journal of veterinary internal medicine
Summary
# Editorial Summary Assessing pain responses in conscious horses has proven challenging, particularly regarding parasympathetic nervous system activity during nociceptive events. Mendoza and colleagues investigated how parasympathetic tone, heart rate, and facial grimacing change in response to mild mechanical pressure on the metacarpus in 30 Friesians, 30 Quarter Horses, and 30 Warmbloods, using a prospective design with baseline habituation followed by controlled nociceptive stimulation over three days. During nociceptive stimulus, parasympathetic tone decreased significantly across all breeds—most markedly in Quarter Horses, intermediately in Warmbloods, and mildly in Friesians (P<0.001)—whilst heart rate and grimace scale scores increased uniformly (P<0.001) but normalised rapidly post-stimulus. Notably, Friesians required substantially higher pressure to produce an aversive response, suggesting breed-related differences in pain tolerance and stress reactivity. These findings indicate that parasympathetic withdrawal represents a measurable physiological marker of pain and stress in horses, with breed-specific variations that practitioners should consider when assessing discomfort in clinical or training settings; integrating parasympathetic tone monitoring alongside traditional indicators like heart rate and facial expression may refine pain evaluation protocols, particularly when working with Friesians and other breeds demonstrating attenuated stress responses.
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Practical Takeaways
- •Breed differences in pain response and tolerance exist: Friesians show less physiological reaction to nociceptive stimuli than Quarter Horses and Warmbloods, which may influence handling, training, and pain management decisions
- •Monitor parasympathetic tone, heart rate, and facial grimace together as complementary pain indicators—one measure alone may not capture full pain response, particularly in breeds with higher pain tolerance
- •Environmental stress (equipment placement, proximity of stimuli) triggers parasympathetic withdrawal in some breeds independent of actual painful stimulus, affecting baseline assessment and interpretation of pain responses
Key Findings
- •Parasympathetic tone activity (PTA) decreased significantly (P<0.001) during nociceptive stimulus, with marked decrease in Quarter Horses, intermediate in Warmbloods, and mild in Friesians
- •Heart rate and horse grimace scale both increased significantly (P<0.001) during stimulus in all breeds but returned to baseline post-stimulation
- •Friesians required higher pressure (P<0.05) to elicit aversive response compared to other breeds, suggesting greater pain tolerance
- •Stress from device placement alone decreased PTA in Quarter Horses and Warmbloods (P<0.01) but not in Friesians