Stress responses of horses transported long-distance by road to and from a jumping competition in a tropical climate

Long-distance road transport is known to be stressful for horses, with most research conducted in temperate climates. This study investigated horses' stress responses during long-distance transport before and after a jumping competition in a tropical environment. Ten healthy horses (aged 7.9 ± 1.9 yr) were studied. Blood cortisol concentrations, hematology, and heart rate (HR) variability (HRV) were monitored during outbound (OTJ) and inbound journeys (INJ). Cortisol concentrations increased 5 min after unloading and remained elevated for up to 180 min in the OTJ (P < 0.05 to 0.001); during the INJ, levels returned to baseline and remained stable after 60 min (P < 0.01 to 0.001). White blood cell and neutrophil counts were higher in the OTJ than the INJ in the 60 to 180 min after unloading (P < 0.05). Beat-to-beat (RR) intervals decreased during 120 min of transport (P < 0.05 to 0.0001) and 60 min after unloading (P < 0.05) in OTJ and INJ. Parasympathetic nervous system index decreased over 30 min during travel (P < 0.01) and 30 min after unloading (P < 0.05) in OTJ, while slightly decreasing at 60 to 90 min into the INJ. HR and sympathetic nervous system (SNS) index increased at 30 to 60 min during travel (P < 0.05 to 0.0001) and 30 min after unloading (P < 0.001) in the OTJ, while they increased at 60 to 90 min during travel in the INJ. The standard deviation of the averages of RR intervals in 5-min segments (SDANN) and the mean of the standard deviations of RR intervals in 5-min segments (SDNNI) showed greater values during the OTJ than the INJ 330 to 360 min after unloading (P < 0.01 to 0.001). Stress index increased at 30 to 120 min of travel in both the OTJ and INJ (P < 0.05 to 0.001). High-frequency (HF) band power increased, with a concurrent decrease in the low-frequency (LF) band and LF/HF ratio at 90 to 440 min of travel (P < 0.05 to 0.0001 for all variables). The standard deviation 2/standard deviation 1 ratio was reduced at 120 to 600 min of travel (P < 0.05 to 0.0001). Correlations between environmental parameters and HRV variables were mainly observed during the INJ. In summary, horses primarily experienced stress during the initial hours of travel and immediately after unloading. Stress responses differed between journeys and were partly influenced by environments and housing conditions. This study highlights welfare considerations concerning horse transport for equestrian events in tropical climates.