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

Background: Exertional heat illness (EHI) is recognised in horses, but few reports have investigated its risk factors.

Objectives: To identify risk factors for EHI in racehorses participating in flat races in Japan.

Study Design: Descriptive epidemiology and retrospective unmatched case-control study.

Methods: Between 2005 and 2016, veterinary records of horses diagnosed with EHI after flat races were reviewed retrospectively and data of the months from April to September were used for a case-control study. For each case, three control horses were randomly selected from starts between April and September. Race records of horses and estimated wet-bulb globe temperature (WBGT) indexes at the local meteorological observatory closest to the racecourse were investigated. To identify risk factors for EHI, univariable and multivariable logistic regression analysis was used.

Results: Of 194 cases during the study period, 188 cases occurred between April and September. The highest incidence risk was in July (1.1 cases per 1000 starts, 95% confidence interval 0.84-1.45). In the final multivariable model, WBGT index, sex, race distance, age and bodyweight were associated with EHI. When WBGT index exceeded 28°C, the risk of EHI was considerably higher than <20°C (OR 28.5, 14.2-62.4, P<0.001). Compared with uncastrated males, geldings (OR 4.9, 1.8-13.3, p = 0.002) and females (OR 2.4, 1.5-3.7, P<0.001) were at high risk of EHI (P<0.01). Furthermore, races of >1600 m (OR 1.8, 1.2-2.8, P = 0.002), 4-year-old (OR 3.5, 1.6-7.9, P = 0.002) and ≥5-year-old (OR 3.9, 1.8-9.2, P = 0.001) horses and horses with low bodyweight (OR per 20 kg, 0.8, 0.7-1.0, P = 0.02) were associated with increased risk of EHI.

Main Limitations: The median straight-line distance between the racecourse and the local meteorological observatory was 14.2 km (range, 1.1-28.3 km). There was a lack of objective criteria of EHI due to the retrospective nature of the study.

Conclusions: We identified specific risk factors for EHI in racehorses. These results may be useful to the equine industry for reducing EHI occurrence in racehorses.