AUTHOR=Verdegaal Elisabeth-Lidwien J. M. M. , Howarth Gordon S. , McWhorter Todd J. , Delesalle Catherine J. G. 

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

JOURNAL=Frontiers in Veterinary Science

VOLUME=9

YEAR=2022

URL=https://www.frontiersin.org/journals/veterinary-science/articles/10.3389/fvets.2022.894146

DOI=10.3389/fvets.2022.894146

ISSN=2297-1769

ABSTRACT=<p>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<sub><italic>sk</italic></sub>) coincident with core temperature (T<sub><italic>c</italic></sub>) has not previously been investigated in horses exercising in the field. We investigated the suitability of monitoring surface T<sub><italic>sk</italic></sub> as a metric of the thermoregulatory response, and simultaneously investigated its relationship with T<sub><italic>c</italic></sub> using gastrointestinal (GI) temperature. We evaluated T<sub><italic>sk</italic></sub> in 13 endurance horses competing during four endurance rides over 40 km (<italic>n</italic> = 1) or a total of 80 km (<italic>n</italic> = 12) distance. Following each 40-km loop, the horses were rested for 60 min. T<sub><italic>sk</italic></sub> and T<sub><italic>c</italic></sub> 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<sub><italic>sk</italic></sub> over time (°C × minutes) using the trapezoidal method. The relationship between T<sub><italic>sk</italic></sub> and T<sub><italic>c</italic></sub> was assessed using scatterplots, paired <italic>t</italic>-test or generalized linear model ANOVA (delta T<sub><italic>sk</italic></sub>) (<italic>n</italic> = 8). Ambient temperature ranged from 6.7°C to 18.4°C. No relationship was found between T<sub><italic>sk</italic></sub> and T<sub><italic>c</italic></sub> profiles during exercise and recovery periods, and no significant difference between delta T<sub><italic>sk</italic></sub> results was detected when comparing exercise and rest. However, time to maximum T<sub><italic>sk</italic></sub> (67 min) was significantly reduced compared to T<sub><italic>c</italic></sub> (139 min) (<italic>p</italic> = 0.0004) with a significantly lesser maximum T<sub><italic>sk</italic></sub> (30.3°C) than T<sub><italic>c</italic></sub> (39°C) (<italic>p</italic> = 0.0002) during exercise. Net AUC T<sub><italic>sk</italic></sub> was 1,164 ± 1,448 and −305 ± 388°C × minutes during periods of exercise and recovery, respectively. We conclude that T<sub><italic>sk</italic></sub> monitoring does not provide a reliable proxy for the thermoregulatory response and horse welfare, most probably because many factors can modulate T<sub><italic>sk</italic></sub> without directly affecting T<sub><italic>c</italic></sub>. Those factors, such as weather conditions, applicable to all field studies can influence the results of T<sub><italic>sk</italic></sub> in endurance horses. The study also reveals important inter-individual differences in T<sub><italic>sk</italic></sub> and T<sub><italic>c</italic></sub> time profiles, emphasizing the importance of an individualized model of temperature monitoring.</p>