Heat stress results from your horse not losing heat by sweating efficiently enough to cool himself down. A typical British summer day won’t affect your horse if you’re working him at a leisurely pace, but if temperatures are high and the exercise is strenuous, then he’s at risk of developing it.
Sometimes known as heat exhaustion, heat stress strikes for a number of reasons, such as a raised ambient temperature. To remove excess muscular heat after intensive exercise, your horse sweats, pulling the heat from his interior to the skin surface, to help to reduce his overall body temperature. Horses normally sweat efficiently over a large surface area, but heat stress develops when they can’t dissipate heat fast enough. There is ongoing interest as to whether some horses are genetically more prone to the condition, but not enough research has been done to confirm this.
Sweating helps your horse to cool down after serious exercise