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

To investigate whether horses were able to acclimate to conditions of high temperature and humidity, 5 horses of different breeds were trained for 80 min on 15 consecutive days on a treadmill at 30 degrees C and 80% RH. Train ing consisted of a combination of long duration low-intensity exercise, med ium duration medium intensity exercise and short duration high intensity ex ercise. Between training sessions the horses were maintained at 11 +/- 3 de grees C and 74 +/- 2% RH. Before (PRE-ACC) and after acclimation (POST-ACC) the horses undertook a simulated Competition Exercise Test. (CET), designe d to represent the Speed and Endurance Test of a 3-day event, at 30 degrees C/80%RH. Maximal oxygen uptake ((V) over dot O-2PEAK) was not changed following accl imation (PRE-ACC 141 +/- 8 ml/min/kg bwt vs, POST-ACC 145 +/- 9 ml/min/kg b wt [STPD], P>0.05). Following acclimation, 4 of the 5 horses were able to c omplete a significantly greater amount of Phase D in the CET (PRE-ACC 6.3 /- 0.3 min vs. POST-ACC 7.3 +/- 0.3 min, P<0.05; target time = 8 min). Resting body temperatures (pulmonary artery [T-PA], rectal [T-REC] and tail -skin [T-TSK] temperatures) were all significantly lower following acclimat ion, During exercise, metabolic heat production (M) and heat dissipation (H -D), for the same exercise duration, were both significantly lower followin g acclimation (P<0.05), although heat storage (H-S) was significantly highe r (P<0.05). The higher heat storage following acclimation was associated wi th a lower T-TSK for a given T-PA and a decreased total fluid loss (% bodyw eight, P<0.05). Plasma volume was not changed following acclimation. The re lationship of sweating rate (SR) to T-PA or T-TSK On either the neck or the gluteal region was not significantly altered by acclimation, although the onset of sweating occurred at a lower T-PA or T-TSK following acclimation ( P<0.05). The horses in the present study showed a number of physiological adaptation s to a period of 15 days of exposure to high heat and humidity consistent w ith a humid heat acclimation response. These changes were mostly similar to those reported to occur in man and other species and were consistent with thermal acclimation and an increased thermotolerance, leading to an improve d exercise tolerance. It is concluded that a 15 day period of acclimation i s beneficial for horses from cooler and or drier climates, that have to com pete in hot humid conditions and that this may redress, to some extent, the decrement in exercise tolerance seen in nonacclimated horses and reduce th e risk of heat related disorders, such as heat exhaustion.