An investigation into environmental variables influencing post‐race exertional heat illness in thoroughbred racehorses in temperate eastern Australia

Abstract

Exertional heat illness (EHI) is a syndrome that occurs when metabolic heat production from muscular contraction exceeds the rate at which it can be dissipated. Core body temperature rises to critical levels, causing hyperthermia and central nervous system dysfunction. Best practice for the prevention of EHI centres around early detection, rapid response and aggressive cooling. Advance planning enables risk mitigation measures. The more that is known about EHI in horses, the better prepared those in the positions of responsibility can be to anticipate events in which the risk of EHI may be elevated. This prospective, observational study investigated environmental risk factors associated with the occurrence of EHI. From 2014 to 2018, the number of horses exhibiting the symptoms of post-race EHI was recorded at 73 convenience sampled race meetings. Of the 4809 starters, the signs of EHI were recorded for 457. Thermal environmental data were measured and included ambient temperature, radiant heat, vapor pressure (humidity) and wind speed (WS). Mixed linear regression models were computed to assess the associations between the occurrence and incidence of post-race EHI and the four thermal environmental variables. The analysis showed that vapor pressure and WS had the largest effects on the occurrence of post-race EHI. The major limitations were that the race meetings selected were convenience sampled according to attendance by the first author and individual horse data were not available. EHI is influenced by a complex interaction of independently acting environmental variables, but warm, windless, humid days are most likely to result in the cases of EHI.Keywords ambient temperature; environmental variables; exertional heat illness; radiant heat; thoroughbred racehorses; vapour pressure; wet-bulb globe temperature; wind speed Abbreviations DB, Tdb ( C), dry-bulb air temperature; EHI, exertional heat illness; EHIRS, exertional heat illness race status (i.e. did at least one horse show post-race symptoms of EHI? [yes/no]); MRT ( C), mean radiant temperature; PEHI, percentage of horses showing the signs of post-race EHI; Tg ( C), globe temperature; Twb ( C), wet-bulb temperature; VP, vapour pressure (also referred to as absolute humidity, kPa); WBGT ( C), wet-bulb globe temperature; WS, wind speed (m/s)

Is Continuous Monitoring of Skin Surface Temperature a Reliable Proxy to Assess the Thermoregulatory Response in Endurance Horses During Field Exercise?

“…With respect to cooling down approaches, the mechanism by which the thermoregulatory systems are challenged greatly depends on how the loss of heat counteracts MH production through non-evaporative pathways as well as evaporative methods. The evaporative exchange of heat of the skin with the environment depends on the thermal gradient between T sk by local skin perfusion and its immediate environment, including vapor pressure, airflow, and solar radiation, especially during field exercise ( 17 , 49 , 75 ). At the end-of-exercise period, the cooling of sport horses is standard, especially in endurance and 3-day eventing competitions.…”

“…Endeavors to assess the thermal environmental variables were limited to BOM and HOBO recordings of the T a and the RH, with the HOBO device placed at one location. Other essential external variables, such as wind speed, were not included in the T a measurements ( 17 ). The current study involved only one type and location of the wearable T sk sensor based on IR technology.…”

“…Recently, the prevalence of EHI in racehorses has been reported: two studies from Japan state prevalence of 0.09% during summer with a clear increase over the past few years ( 15 , 16 ). Moreover, a study in Eastern Australia focused on selected EHI cases post-exercise at the racetrack and suggested an EHI incidence of up to 9.5% during hot summer months ( 17 ). The latter study used the four severity levels of EHI reported by Brownlow et al ( 4 ) and concluded that 96% of horses could be categorized as Level 1.…”

“…Environmental conditions are the dominant risk factor in heat stress events, and EHI cases are expected to further increase in prevalence due to global warming ( 1 , 4 , 15 – 17 ). That worrying reality drives the ongoing efforts of research groups worldwide to develop reliable approaches to monitor and safeguard thermoregulatory wellbeing in horses ( 2 , 18 – 21 ).…”

The Use of Percutaneous Thermal Sensing Microchips to Measure Body Temperature in Horses during and after Exercise Using Three Different Cool-Down Methods

“…Without appropriate cool-down intervention, this can progress to exertional heat illness (EHI) [2,3]. Although EHI can occur at lower ambient temperatures, and during a short duration of exercise [4], the chances of developing EHI increase with prolonged exercise, especially under hot and humid conditions which limit the physiological thermoregulatory capacity in horses [5][6][7]. Early detection of EHI immediately post-exercise is important as it can rapidly develop into severe illness causing endotoxaemia, neuronal injury, and/or heat stroke [2,3].…”