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 (degrees C x 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 degrees C to 18.4 degrees 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 degrees C) than T-c (39 degrees C) (p = 0.0002) during exercise. Net AUC T-sk was 1,164 +/- 1,448 and -305 +/- 388 degrees C x 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.