Skin readings not a reliable indicator of core temperature in Endurance horses – study

Monitoring the skin temperature of Endurance horses does not provide a reliable proxy for their core thermoregulatory response, researchers have found.

Researchers, writing in the journal Frontiers in Veterinary Science, believe the reason is most probably because many factors can influence skin temperature without directly affecting a horse’s core temperature.

They could find no correlation between the constantly monitored skin temperature and the core temperature in 13 Endurance horses competing in Australia.

The skin temperature of the horses in the study was continuously recorded every 15 seconds by an infrared thermistor sensor located in a modified belt. The core body temperature was similarly recorded every 15 seconds via a telemetric pill which made its way through the horse’s gastrointestinal tract.

Elisabeth-Lidwien Verdegaal and her fellow researchers said hyperthermia and heat stress have become increasingly challenging issues for a wide array of equine disciplines in the face of climate change, especially during field competitions.

An increase in core body temperature leading to hyperthermia may trigger a range of problems and, if unchecked, can lead to exertional heat illness. Clinical signs include neurological signs, varying from irritability, depression, impaired co-ordination and collapse, and may progress to heat stroke with multi-organ dysfunction and death.

From a physiological standpoint, it is essential to appreciate that a time lag exists between exercise-induced metabolic heat output and core body temperature, they said.

To identify a reliable proxy for thermoregulatory response in the field, a solid correlation must exist between that specific proxy and core temperature, despite the existence of a time lag.

The study team monitored the 13 mainly Arabian Endurance horses competing over distances of 40km, 80km or 100km, with each 40km loop followed by a 60-minute recovery period.

Following each 40km exercise loop, the sweat response of each horse was graded. In addition, the horses were immediately cooled for an average of 10 minutes by pouring buckets of tap water over their bodies and scraping it off.

The horses were allowed to drink water and eat hay during the 60-minute rest periods in a shaded area.

The ambient temperature during the exercise periods ranged from 6.7°C to 18.4°C.

The authors said they found no relationship between the skin temperature and core temperature profiles during exercise and recovery periods.

The typical time to maximum skin temperature, at 67 minutes, was significantly lower than the time to maximum core temperature, at 139 minutes. The skin maximum (30.3°C) was significantly lower than the core maximum (39°C) during exercise.

“We conclude that skin temperature monitoring does not provide a reliable proxy for the thermoregulatory response and horse welfare, most probably because many factors can modulate skin temperature without directly affecting core temperature,” they said. Those factors, such as weather conditions, apply to all field studies and can influence skin temperature results in Endurance horses.

The study also revealed important differences between individuals in terms of their skin and core temperature profiles, emphasizing the importance of an individualized model of temperature monitoring.

The researchers said that while monitoring skin temperature may be non-invasive and straightforward, the study results clearly show that it does not reliably estimate core temperature evolvement during Endurance events, since a correlation with core readings could not be identified.

Notably, a high skin temperature at a single point during a field exercise in cool ambient air temperatures did not identify the Endurance horses with an increased core temperature.

“Further research into core temperature monitoring in different equine sports and under differing weather conditions must be undertaken to create a baseline for further fine-tuning hot weather policies,” they said.

“Accordingly, veterinarians, trainers, and owners can be advised to continuously monitor core temperature to ensure the health and welfare of all horses.”

The study team comprised Verdegaal, Gordon Howarth and Todd McWhorter, all affiliated with the University of Adelaide; and Catherine Delesalle, with Ghent University in Belgium.

Verdegaal E-LJMM, Howarth GS, McWhorter TJ and Delesalle CJG (2022) Is Continuous Monitoring of Skin Surface Temperature a Reliable Proxy to Assess the Thermoregulatory Response in Endurance Horses During Field Exercise? Front. Vet. Sci. 9:894146. doi: 10.3389/fvets.2022.894146 

The study, published under a Creative Commons License, can be read here.