Exertional heat illness: a review of the syndrome affecting racing Thoroughbreds in hot and humid climates

Abstract

Metabolic heat produced by Thoroughbred racehorses during racing can rapidly elevate core body temperature (1°C/min). When environmental conditions are hot and humid, the normal physiological cooling mechanisms become ineffective. The heat accumulated may exceed a critical thermal maximum (estimated to be 42°C), which may trigger a complex pathophysiological cascade with potentially lethal consequences. This syndrome has been labelled exertional heat illness (EHI). EHI is described in humans, but has not been well documented in Thoroughbred racehorses. The clinical signs described in racehorses would suggest that the pathophysiological events affecting the central nervous (CNS) and gastrointestinal systems are similar to those described in humans. Clinical signs are progressive and include signs of endotoxaemia and increasing levels of CNS dysfunction. Initially, horses that may be mildly irritable (agitated, randomly kicking out) may progress to unmanageable (disorientation, severe ataxia, falling) and ultimately convulsions, coma and death. Currently, the approach to treatment is largely empirical and involves rapid and effective cooling, administration of drugs to provide sedation, administration of non-steroidal anti-inflammatory drugs to ameliorate the effects of endotoxaemia and glucocorticoids to stabilise cell membranes and reduce the effects of inflammation on the CNS. This review provides an overview of the current knowledge about EHI in Thoroughbred racehorses, suggests a likely pathophysiology of the syndrome in horses based on the current literature on heat illness in humans and horses, and outlines current treatment strategies being used to treat racehorses with clinical signs of EHI.

Risk factors for exertional heat illness in Thoroughbred racehorses in flat races in Japan (2005–2016)

“…Veterinary records of horses diagnosed with EHI after races during the study periods were reviewed retrospectively. Cases were defined as horses diagnosed with EHI by official JRA veterinarians according to at least one of the following clinical signs at the racecourse after races: slow recovery (prolonged increase of respiratory or heart rates); irritable behaviour, such as head‐shaking, kicking out in a random fashion and restlessness; ataxia and a reluctance to move; or collapse . The frequency of each clinical sign was calculated by dividing the number of each case by the number of total cases with exact binomial confidence intervals of 95%.…”

“…This can elevate core body temperature by 1°C/min without heat dissipation , leading to hyperthermia that exceeds a critical thermal maximum. Therefore, high‐intensity exercise during races in hot and humid environments, where the rate of heat dissipation is low, can be associated with exertional heat illness (EHI), defined as an exercise‐induced heat stress .…”

“…In the equine industry, the WBGT index is used successfully to quantify environmental heat loads and contributes to the management of equestrian competitions . In some countries, a hot weather policy addresses the welfare of Thoroughbred racehorses and recommends that race organisers should take measures to prevent EHI in high WBGT conditions, when the WBGT index exceeds 28°C . However, no evidence has shown an association between WBGT index and EHI occurrence in horse competitions.…”

In this issue – July 2016

“…An overview of the current knowledge about exertional heat illness (EHI) in Thoroughbred racehorses is presented, suggesting a likely pathophysiology of the syndrome in horses based on the current literature on heat illness in humans and horses. It also outlines current treatment strategies being used to treat racehorses with clinical signs of EHI …”

“…It also outlines current treatment strategies being used to treat racehorses with clinical signs of EHI. 5 Metabolic heat produced by Thoroughbred racehorses during racing can rapidly elevate core body temperature (1 C/min). When environmental conditions are hot and humid, the normal physiological cooling mechanisms become ineffective.…”