Epidemiology of exertional heat illness in Thoroughbred racehorses in temperate eastern Australia: The role of extrinsic (environmental) factors in disease causation

Abstract

A simple epidemiological model of disease causation is proposed for exertional heat illness (EHI) in Thoroughbred racehorses. The agent of disease causation that must be present for the condition to occur is strenuous exercise, producing substantial quantities of metabolic heat. This is stored during racing but must be dissipated rapidly in the post-race period to prevent core body temperature rising to a critical level and causing the clinical manifestations of EHI. Environmental factors are next in the epidemiological triad, and it is a common misconception that these are the direct cause of EHI. In fact, environmental conditions enable EHI by either diminishing the evaporative capacity of the environment or promoting internal heat gain. This article deals with the specific effects of the four thermal elements, separately and in combination, on individual thermo-effector mechanisms. The final component in the epidemiological triad is individual host factors. A critical premise of epidemiology is that conditions such as EHI may not occur randomly in a population but may be more likely to occur in some individuals due to the presence of certain factors that predispose them to the condition. For the purpose of assessing risk, it is not feasible to examine the balance between metabolic heat production and the intrinsic and extrinsic factors, which collectively determine each individual's heat stress response. Therefore, the measurement of environmental factors remains the only practical way of obtaining a credible risk assessment for EHI, so that effective countermeasures can be instigated and the welfare of our racehorses ensured.