Physiological responses of horses to a treadmill simulated speed and endurance test in high heat and humidity before and after humid heat acclimation.

This study examines the physiological responses of horses to a high heat and humidity exercise test, both before and after a period of acclimatization to these conditions. The results show that acclimatization leads to improvements in exercise tolerance and a reduction in the risk of heat-related disorders.

Study Design and Methodology

• The study used five horses of different breeds and subjected them to a period of acclimation to high heat and humidity conditions.
• The process involved training the horses on a treadmill for 80 minutes on 15 consecutive days at a temperature of 30 degrees Celsius and 80% relative humidity.
• The training consisted of a mix of long duration low-intensity exercise, medium duration medium intensity exercise, and short duration high intensity exercise.
• Between training sessions, the horses were kept at a temperature of around 11 degrees Celsius and 74% relative humidity.
• The horses’ physiological responses were measured before and after the acclimation period during a simulated Competition Exercise Test (CET), which is designed to represent the Speed and Endurance Test of a three-day event.

Results of Acclimation

• The study found no significant change in maximal oxygen uptake (VO2PEAK) following the acclimation period.
• However, four of the five horses were able to complete a significantly greater amount of Phase D in the CET after acclimation, indicating improved endurance.
• Resting body temperatures were also found to be significantly lower after acclimation.
• During exercise, both metabolic heat production (M) and heat dissipation (HD) were significantly lower after acclimation, while heat storage (HS) was significantly higher.
• A lower tail-skin temperature for a given pulmonary artery temperature and a decrease in total fluid loss were associated with higher heat storage after acclimation.
• No change was observed in plasma volume following acclimation.
• In addition, the onset of sweating occurred at lower body temperatures following acclimation.

Conclusion

• These results confirm that horses can physiologically adapt to high heat and humidity over a period of 15 days, which is consistent with a heat and humidity acclimation response.
• The changes observed were similar to those seen in humans and other species, showing thermal acclimation and increased heat tolerance, which lead to improved exercise tolerance.
• The study concludes that a 15-day acclimation period can be beneficial for horses from cooler and/or drier climates that are expected to compete in hot and humid conditions to mitigate the decline in exercise tolerance and reduce heat-related health risks.