Abstract
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.