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

Elisabeth-Lidwien J M M Verdegaal; Gordon S Howarth; Todd J McWhorter; Catherine J G Delesalle

doi:10.3389/fvets.2022.894146

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

Front Vet Sci. 2022 May 27:9:894146. doi: 10.3389/fvets.2022.894146. eCollection 2022.

Authors

Elisabeth-Lidwien J M M Verdegaal^{1

2}, Gordon S Howarth^{1

3}, Todd J McWhorter¹, Catherine J G Delesalle²

Affiliations

¹ Equine Health and Performance Centre, University of Adelaide, Adelaide, SA, Australia.
² Department of Translational Physiology, Infectiology and Public Health, Research Group of Comparative Physiology, Faculty of Veterinary Medicine, Ghent University, Merelbeke, Belgium.
³ Gastroenterology Department, Women's and Children's Hospital, Adelaide, SA, Australia.

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.

Keywords: competition; endurance; exercise; hyperthermia; metabolic heat; performance; skin surface temperature; thermoregulation.