Heat stress in the horse

Ratification Date: 10 Dec 2021

Policy

All associations involved in horse competitions must have a policy/procedure for reducing the risk of heat stress in horses. This policy should include protocols for cooling horses after exercise, ensuring the resources are available for appropriate cooling, monitoring of environmental conditions and reduction in exercise intensity of the competition, or cancellation of competitions when the weather conditions are too extreme.
All owners or those responsible for horses working in hot and/or humid conditions should take appropriate steps to prevent heat stress
In addition, suitably trained staff, preferably veterinarians, must be available to examine horses at competitions, particularly in the period after exercise to monitor, prevent and treat heat stress.

Background

Heat stress can occur in horses when they are exercising in hot conditions. Heat stress occurs when the capacity of the thermoregulatory system to dissipate heat is exceeded. Exercise results in the generation of heat; during racing horses produce enough heat to increase the body temperature by 1°C for every minute of exercise.¹ Heat dissipation relies on evaporation of sweat thus in conditions of high heat and humidity the capacity for evaporation and the efficiency of sweating is reduced.²The presentation of heat stress depends upon the intensity and duration of exercise and climatic conditions.

Heat stress in endurance horses is well recognised, with horses losing significant volumes of sweat resulting in dehydration and electrolyte imbalance.³ Clinical signs include elevated rectal temperature, depression, dehydration, increased respiratory rate, slow capillary refill times, decreased pulse pressure, intestinal stasis, muscle spasms and cramps, which can progress to exertional rhadbomyolysis, renal failure, liver dysfunction and laminitis without immediate and appropriate treatment including cooling and fluid therapy. Exclusion of potentially infectious zoonotic diseases, where the presenting symptoms mimic those of a heat exhausted horse, is important to fulfil Biosecurity, Public Health and WH&S obligations.⁴

Eventing horses also commonly suffer heat stress in particular after the cross-country phase of competitions.⁵

Exercise-induced heat stress syndrome in Thoroughbred (TB) racehorses in Australia, Exertional Heat Illness (EHI) differs from heat stress in endurance horses and signs relate more to hyperthermia than to dehydration and electrolyte imbalance.⁶The brain is particularly sensitive to heat stress and the clinical signs of EHI include abnormal mentation with irritability, head shaking and random kicking, progressing to disorientation, unpredictable lunging forward, ataxia and collapse. Signs of endotoxaemia develop with hyperaemic mucous membranes and slow capillary refill times, and aggressive treatment is required.⁶

All horses competing and working in hot, humid conditions should be appropriately cooled after exercise and monitored carefully for signs of heat stress or EHI. Ideally a designated shaded area with misting fans should be available for all horses post-exercise, with ample hoses, mobile spray units or large volumes of ice water and a dedicated team of people hosing/pouring preferably ice cold water (0-12°C) over the entire horse.⁵ Water will warm up on the horse’s skin and scraping may allow for rapid replacement with cold water. If sufficient hoses and water are available continuous hosing is ideal. For horses in heat stress/collapse cold water should be continuously applied with no breaks for scraping. To facilitate handling of horses with altered mentation, sedation is recommended. Use anti-inflammatories in a hydrated horse as well as sedatives as appropriate.⁶

Heat stress policies are available from Equestrian Australia and Racing Associations from each state, in addition the FEI has produced a detailed supporting document, see references below. These policies include environmental conditions at which competitions should be altered or cancelled.

The Wet Bulb Globe Temperature (WBGT) Index is the best measure of the potential for heat stress in horses and includes temperature, humidity, wind speed, sun angle and cloud cover.⁵ WBGT information is published on the Bureau of Meteorology (BOM) website under their Thermal Comfort Observations webpages http://www.bom.gov.au/info/thermal_stress/

Typically, no changes are recommended for WBGT readings less than 28, some precautions at 28-30, additional precautions at 30-32, 32-33 is hazardous and over 33 cancellation of all competitions is recommended. Measurement of vapour pressure is an additional tool for monitoring the potential for heat stress.⁷

The early recognition and treatment of heat stress is critical as the disease can result in death if left untreated.⁶ Rapid cooling is the mainstay of treatment in combination with sedation and circulatory support.

References

Hodgson DR, Davis RE and McConaghy FF. Thermoregulation in the horse in response to exercise. British Vet Journal 1994;150:219-235.
Che Muhamed AM, Atkins K, Stannard SR et al. The effects of a systematic increase in relative humidity on thermoregulatory and circulatory responses during prolonged running exercise in the heat. Temperature 2016;3:455-464. http://dx.doi.org/10.1080/23328940.2016.1182669
Carlson GP. Medical problems associated with protracted heat and work stress in horses. Comp Cont Educ Pract Vet 1985;7:S542-S550.
Ball MC, Dewberry TD, Freeman PG, Kemsley, Poe, I. Clinical review of Hendra virus infection in 11 horses in NSW, Australia. Aust V. J 2014;92(6):213-8. doi: 10.1111/avj.12170
Elliott C. Clinical insights: Preparing for the Tokyo Olympics—Ensuring health and welfare of equine athletes in the face of heat and humidity. Equine Vet J. 2021;00:1–3. DOI: 10.1111/evj.13446
Brownlow MA, Dart AJ and Jeffcott LB. Exertional heat illness: a review of the syndrome affecting racing Thoroughbreds in hot and humid climates et al in 2016 Aust Vet J 2016;94:240–247 doi: 10.1111/avj.12454
Brownlow MA and Brotherhood JR. An investigation into environmental variables influencing post-race exertional heat illness in thoroughbred racehorses in temperate eastern Australia. Aust Vet J 2021 doi: 10.1111/avj.13108