Example of heat stress CPR took the above video at a race meet in 2014 where temperatures were very high.
Summer is always a concerning time of year for us, with many race meets being held in extreme weather conditions. High temperature, high humidity, lack of air movement, poor ventilation, dehydration and exposure to direct sunlight all increase the danger of serious heat and sun related problems horses are no exception, especially when they are expected to perform at intense levels, just as racehorses are.
Racehorses suffering heat stress can collapse, sometimes resulting in death. During the Summer we expect to see more and more horses collapsing on the track than we do over the cooler months. While the heat may not always be the cause of death in racehorses, we believe it can certainly play a factor.
Heat stroke vs heat stress
Heat stress is not to be confused with heat stroke, which occurs over a short period of time – usually if a horse is unfit. Heat stress (which is often also referred to as heat exhaustion) more often than not results from electrolyte and protracted fluid loss during exhaustive exercise. Exhaustive exercise relates to exercise in high ambient temperature, with a lack of sweating and poor conditioning. These conditions, along with high humidity prevent horses from dissipating internal heat from their body.
How does it happen?
Exercising horses flex and string their muscles with each stride, and with that comes an accumulation of heat from the metabolism of working muscles. Heat buildup can cause muscles to fatigue and at higher temperatures; muscles require more oxygen for energy utilisation. A horse’s need for oxygen can exceed the amount available via respiration if they become too hot – causing heat stress.
Signs of Equine Heat Stress
Heat stress and loss of muscle control can lead to serious accidents for both horse and rider.
Signs of heat stress in horses:
Elevated heart rate with a slow recovery rate
Irregular heart beat (thumps)
Profuse or no sweating
Some outcomes of heat stress are falls, seizures, heart damage/attacks and muscle and kidney problems.
The most concerning conditions for horses racing in the heat are the high humidity days. Humid weather slows down the ability to shed heat from the body and horses can often struggle to sweat enough to stay ahead of the heat buildup in their body.
Of course these conditions differ from horse to horse based on their level of fitness, weight, how far they’ve travelled on the day, the length of the race and the conditions they are used to working in.
Hot weather policy
The racing industry continues to defend racing horses in hot weather conditions as they have a hot weather policy in place. However, this policy allows for horses to be raced in an ambient temperature of up to (and occasionally above) 38 degrees celcius.
The policy relies mainly on the web bulb globe temperature (WGBT), which measures:
The WGBT temperature is meant to then determine whether or not the temperature is suitable to race in.
CPR has attended race meets on hot days to measure the WGBT to find that the reading is well and truly into the ‘high risk conditions’ – a reading of 33 and over.
Comparison to other equine sports
Equestrian Australia National Showhorse rules – at 39° competition is to be suspended (however the level of intensity in showing cannot be compared to horse racing).
Equestrian Australia National Dressage rules – suggest using the FEI standard of suspending activities when the WBGT Index reaches or exceeds 32° or 33°.
HRCAV rules state cross country must be called off at 36° – XC is the most similar to horse racing in terms of impact.
Dr David Marlin warns that the BOM forecast is based on temperatures in the shade.