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

Hyperthermia is a performance and welfare issue for exercising horses. The thermoregulatory stressors associated with exercise have typically been estimated by responses in the laboratory. However, monitoring surface skin temperature (T(sk)) coincident with core temperature (T(c)) has not previously been investigated in horses exercising in the field. We investigated the suitability of monitoring surface T(sk) as a metric of the thermoregulatory response, and simultaneously investigated its relationship with T(c) using gastrointestinal (GI) temperature. We evaluated T(sk) in 13 endurance horses competing during four endurance rides over 40 km (n = 1) or a total of 80 km (n = 12) distance. Following each 40-km loop, the horses were rested for 60 min. T(sk) and T(c) were continuously recorded every 15 s by an infrared thermistor sensor located in a modified belt and by telemetric GI pill, respectively, and expressed as mean ± SD. The net area under the curve (AUC) was calculated to estimate the thermoregulatory response to the thermal load of T(sk) over time (°C × minutes) using the trapezoidal method. The relationship between T(sk) and T(c) was assessed using scatterplots, paired t-test or generalized linear model ANOVA (delta T(sk)) (n = 8). Ambient temperature ranged from 6.7°C to 18.4°C. No relationship was found between T(sk) and T(c) profiles during exercise and recovery periods, and no significant difference between delta T(sk) results was detected when comparing exercise and rest. However, time to maximum T(sk) (67 min) was significantly reduced compared to T(c) (139 min) (p = 0.0004) with a significantly lesser maximum T(sk) (30.3°C) than T(c) (39°C) (p = 0.0002) during exercise. Net AUC T(sk) was 1,164 ± 1,448 and −305 ± 388°C × minutes during periods of exercise and recovery, respectively. We conclude that T(sk) monitoring does not provide a reliable proxy for the thermoregulatory response and horse welfare, most probably because many factors can modulate T(sk) without directly affecting T(c). Those factors, such as weather conditions, applicable to all field studies can influence the results of T(sk) in endurance horses. The study also reveals important inter-individual differences in T(sk) and T(c) time profiles, emphasizing the importance of an individualized model of temperature monitoring.