Evaluation of Substance P as a New Stress Parameter in Horses in a Stress Model Involving Four Different Stress Levels

SIMPLE SUMMARY: The public perception of animal welfare in equestrian sports depends on factors associated with training methods and presentation of horses at equestrian events. Regarding this, correctly buckled nosebands have played an important role in recent years. Furthermore, methods were required to objectify stress and anxiety reactions in horses. Besides cortisol measurements, ridden horse ethograms were developed to quantify adverse reactions and pain-related behavior in the ridden horse. The main aim of the study was to evaluate substance P (SP) as a new stress parameter mainly addressing emotional stress reactions. Other objectives were to establish reference values for SP in horses and to compare concentrations to the established stress parameter cortisol. A ridden horse ethogram was used to simultaneously evaluate obvious adverse reactions. Plasma concentrations of SP showed large interindividual variations, whereas they stayed within more narrow ranges in individual horses. While cortisol showed a linear increase with the four applied stress levels (level 1: horses ridden with loose noseband, level 2: tight noseband, level 3: loose noseband and overground endoscope, level 4: tight noseband and overground endoscope), SP showed no correlation and may therefore be not suitable to quantify emotional stress in horses in the present minor stress model. ABSTRACT: Stress has a significant impact on equine welfare. There are some studies on the stress response in horses ridden with tight nosebands, but little is known about other stress parameters than cortisol, which potentially could address an emotional component. In this study, blood samples of a total of 74 warmblood horses were used to establish reference values for plasma substance P (SP) concentrations. Moreover, 16 of these warmblood horses were included in a stress model. Four different stress levels (level 1: horses ridden with loose noseband, level 2: tight noseband, level 3: loose noseband and overground endoscope, level 4: tight noseband and overground endoscope) were applied to evaluate SP as a potential stress parameter in horses. Blood samples were taken at rest (t0) and directly after inducing stress (noseband tightening, insertion of endoscope; t1), as well as after 20 min of riding at all gaits (t2). A ridden horse ethogram was applied and showed that horses in the tight noseband group resorted to other stress-related behavioral issues than horses with loose nosebands. Serum cortisol showed a linear increase concurrent with the increase in stress levels with a significant difference between level 1 and level 4 (p = 0.043), proving that stress factors were adequate to evaluate the stress response, whereas SP did not show a correlation with the stress levels. Furthermore, concentrations of SP differed widely between horses but stayed within more narrow limits in the individual horse. As a conclusion, SP might not be a reliable stress parameter in horses in the applied minor stress model.