Thirst and rehydration in dehydrated horses

Body fluid homeostasis is crucial for maintaining normal physiologic function. During exercise, especially in endurance horses, a substantial loss of body fluid by sweating can occur. Replacement of this fluid deficit by drinking is important to restore body fluid homeostasis. The primary stimulus for thirst is an increase in plasma osmolality or, more specifically, an increase in plasma sodium concentration. However, horses typically do not drink adequately to replace sweat fluid losses, even when water or other rehydration fluids are readily available. This phenomenon of incomplete restoration of body fluids, reflected by persisting body weight loss after exercise, has been termed “involuntary dehydration”. The objective of my research was to investigate involuntary dehydration and factors affecting thirst and voluntary water intake in horses dehydrated by endurance exercise. Horses are often transported long distances within a few hours after completion of an endurance competition. Therefore, this study focused on manipulating rehydration during the initial hour following exercise in order to enhance recovery prior to transport. The following hypotheses were tested: The magnitude of “involuntary dehydration” in horses after the first hour of recovery from endurance exercise is affected by: (i) volume of water initially offered after exercise-induced dehydration; (ii) type of rehydration solution (water vs. saline) initially offered; and (iii) temperature of rehydration fluid.; Limiting the volume of the initial drink of water (to 4 l, 8 l, or an unlimited amount during the first 5 minutes of recovery) had no significant effect on persisting body weight loss after the first hour of recovery. In contrast, initially offering a saline solution (0.45% or 0.9% NaCl solution during the first 5 minutes of recovery) resulted in greater total fluid intake and attenuated of the magnitude of involuntary dehydration than when plain water was initially offered. Next, providing rehydration fluid at near ambient temperature (20°C) resulted in greater voluntary fluid intake by the end of the initial 60-minute recovery period, in comparison to offering fluid at cool (10°C) or warm (30°C) temperatures.; In conclusion, sodium content and temperature of rehydration fluid affected voluntary fluid intake and involuntary dehydration in horses dehydrated by endurance exercise. Thus, an initial drink of salt water (0.9% NaCl) at temperature near 20°C immediately after exercise (or at rest periods during prolonged exercise) appears to be a good strategy for enhancing rehydration of endurance horses, especially during competition under conditions of high heat and humidity.