Heat stress occurs when the horse is unable to cool itself sufficiently to prevent a rise in body temperature. Cooling is normally effected mainly by evaporation of sweat from the skin surface and to a lesser extent from the lung surfaces via expired air. High ambient temperature, especially if combined with high relative humidity, may reduce the capacity for evaporative cooling. The source of heat is normally from exercising muscles, the harder and faster the work or the longer it’s duration, the more waste heat is generated.
Signs include heavy sweating which may give way to reduced or no sweating. Rapid respiration, panting, flared nostrils and rapid heart rate >60 bpm. Skin is hot to the touch. In more severe cases there may be muscle weakness, head down posture, stumbling, swaying, and collapse. Rectal temperature in the range 40o to 42o centigrade or even higher.
Treatment for mild to moderate cases includes promptly removing all tack then hosing the entire body and head followed by scraping and repeating the hosing. Apply the hose especially to the great veins between the hind legs and jugular. Move the horse into shade or a breezy spot or near a fan to increase evaporation and provide drinking water at ambient temperature (not too cold). More severe cases and any horse which has collapsed will require urgent veterinary intervention with intravenous fluids. The prognosis for any horse which has collapsed and is unable to rise is guarded. Affected horses should be provided with oral electrolytes once they are drinking well.
Prevention is far better than treatment and revolves around appropriate fitness conditioning for the work involved, rest in a cool environment when horses are becoming stressed, adequate dietary electrolytes and avoiding work during the hottest time of day. When travelling, park floats in the shade if possible and consider travel during cooler time of day or night. Attention to weather forecasts and forward planning can help avoid or reduce the risk.