Thermoregulation in exercising horses: Aspects of temperature monitoring during field exercise

Joint Degree Program between the School of Animal and Veterinary Science, University of Adelaide, and Faculty of Veterinary Medicine, Ghent University, BelgiumHyperthermia is an ongoing welfare and performance issue for all horses exercising in racing and other competitive sport events. At present, little is known about the influence of core body temperature evolvement on hyperthermia in real time during different types of exercise performed in field conditions such as racing and endurance events. Consequently, it is becoming increasingly important to establish appropriate policies regarding the detection and prevention of all types of heat stress. To achieve this, a detailed view of the variability of equine thermoregulation during field exercise and recovery is essential. To date, the vast majority of thermoregulatory studies have been conducted in indoor laboratory conditions using a treadmill and subjecting horses to specific standardized exercise tests. However, this approach cannot successfully reflect real-time field conditions. Hence, there is a need to accurately and reliably monitor equine core body temperature responses to avoid potential harm due to increasing heat load. Chapter 1 provides a review of current research into thermoregulation, hyperthermia and exertional heat illness (EHI) in exercising horses. In addition, several temperature monitoring methods in horses are described along with some relevant methods in human athletes. However, no studies have investigated the promising continuous monitoring method involving a telemetric gastrointestinal pill (GI) that can be applied in the field in all conditions. Chapter 2 outlines the scientific aims of the thesis. Chapter 3 describes a study designed to evaluate the efficacy of continuous monitoring of core body temperature using the novel telemetric GI pill during real-time field exercise for the first time. The results showed that the continuous recording of the GI core temperature in exercising horses in the field using the GI pill was non-invasive, practical and accurate. Temperature fluctuations experienced during exercise and recovery are reliably recorded, and tendencies toward EHI will be easily observed during field exercise. Importantly, the GI pill has proven to be a more accurate and precise tool to monitor core thermal response when compared with serial Tre measurements in the field. Summary 194 Chapter 4 describes the application of this novel thermoregulation monitoring method in detail. The study involved measurements conducted in both endurance horses and trotters in order to compare exercise types in real-life competitions in the field. Not only were the core body temperatures (Tc) continuously monitored during exercise and recovery, the thermoregulatory responses to the different exercise intensities were also compared. The findings of this study reported real-time temperature evolvement during real-time competition in the field. More specifically, endurance horses reached peak temperature at 75% of completion of 40 km of exercise. However, trotters reached peak temperature always during recovery. In addition, the Tc in endurance horses returned to baseline within 60 minutes into recovery while in 30% of trotters, Tc was still higher than 39°C at the end of recovery. Since endurance horses are considered as ‘fit to continue’ competition when the heartrate (HR) is 60 beats per minute or below, the finding that the mean Tc was still 38.8 ± 0.4°C at a HR of 60 bpm is of importance. Overall, the study showed that horses have very individual thermoregulatory responses which require highly accurate monitoring no matter what type of exercise is performed in the field. Chapter 5 investigated the usefulness of monitoring skin temperature in endurance horses. A large array of skin temperature methodologies recently used in the field is reviewed, mainly pre- and post-exercise at time points, In this study, to evaluate if skin temperature could be used as a proxy for core temperature, the skin temperature was continuously monitored and evaluated using an infrared monitor during a real-life endurance competition. The skin temperature was compared to the GI temperature and importantly, there was no correlation between skin and GI temperature.Thesis (Ph.D.) -- University of Adelaide, School of Animal and Veterinary Sciences, 202

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