Estimating rectal temperature in exercising horses in a competition environment using infrared thermal imaging

Short-term intense and or prolonged medium intensity exercise in conditions of moderate-high environmental temperature or temperature and humidity can result in moderate to marked hyperthermia. This in turn may result in decreased exercise capacity, earlier onset of fatigue and increased risk of falls or injury. Early identification of moderate-severe hyperthermia is important to allow appropriate intervention, either before, during or after exercise. Infra-red thermal imaging (IRT) allows measurement of horse surface temperature non-invasively, however it is generally considered to be unreliable in outdoor environments. Collection of pilot data suggested that this modality warranted further investigation. IRT Images were obtained from horses during training and competition at the 2019 Ready Steady Tokyo Olympic equestrian test event (Wet Bulb Globe Temperature [WBGT] > 28 °C), during and after the cross-country at the 2019 Burghley Horse Trials CCI 5* (WBGT 12-15 °C), the 2019 Adelaide International Horse Trials CCI 4* (WBGT ∼ 20 °C) and during the 2020 Tokyo Olympic Games (held in 2021; WBGT > 28 °C). A total of 87 paired hindquarter IRT images and rectal temperature measurements were obtained. When all data from all 4 events were pooled the bias and limits of agreement were −0.3 ± 1.1 °C (n = 87), i.e. IRT underestimated rectal temperature. When the data from Ready Steady Tokyo 2019 and Tokyo 2020 were combined as both events took place in similar thermal environmental conditions) the bias and limits of agreement were −0.1 ± 0.8 °C (n = 45). For 29 paired hindquarter thermal images, where a second image was obtained within 15 s, there was no significant difference between the mean of the first and second images (Image 1: 38.9 ± 0.9 °C versus Image 2: 38.7 ± 0.8 °C; ; n = 29). In conclusion, IRT imaging in environmental conditions of high temperature or high temperature and humidity may be useful in identifying horses with moderate to severe hyperthermia.