Cortisol release, heart rate, and heart rate variability in transport-naive horses during repeated road transport.

The research analyzes the stress response in horses that are repeatedly exposed to road transport. The study examines various physiological measures, such as cortisol secretion and heart rate, to determine how horses acclimatize to stress over time.

Research Objective and Methodology

• The researchers sought to examine the stress response in horses repeatedly exposed to road transportation. They focused on transport-naive horses, i.e., those unaccustomed to transport.
• They used various markers to assess the stress response in the animals, including salivary cortisol concentrations, fecal cortisol metabolites, RR interval (time gap between heartbeats), and heart rate variability (HRV).
• The study involved a sample of eight horses which were transported four times on a standardized journey of 200 km. The cortisol levels and heart rates were measured before and after each transport.

Research Findings

• The study observed that horses always experienced an increase in salivary cortisol levels in response to transport, indicating the presence of stress. However, the degree of cortisol release decreased with each subsequent transport suggesting that the horses were able to adapt to the stress of transportation over repeated exposures.
• The concentration of cortisol metabolites in the horses’ feces also increased significantly after the first transport, from 55.1 ng/g before to 161 ng/g after. However, this cortisol level did not elevate further in the following transits.
• Akin to cortisol levels, the RR interval diminished when the horses were loaded for transport and even further during the journey. Again, this decrease was less pronounced in subsequent transports, indicating a reduced stress response among horses over repeated journeys.
• The HRV variable, that details changes in the time intervals between heartbeats, faced a short climb, indicating sympathetic activation which usually accompanies stress responses. However, there were no consistent changes noted for other heart rate variability measures.

Conclusion

• The research concluded that horses show a transport-induced stress response which diminishes with subsequent transports. This supports the idea of animal habituation, where exposure to a stressor over time can reduce the sensitivity of the stress response, reducing the associated physical manifestations.
• However, even as the horses seemed to acclimatize to the transportation, an elevated cortisol secretion remained detectable, indicating some level of persisted stress.