“In the last 10 years, we have been alarmed by heat waves due to climate change and global warming,” said Elisabeth-Lidwien (E.J.M.M.) Verdegaal, MVM, DVM, Dipl. RDVS, ECEIM, a Dutch, European, and Australian registered specialist in equine internal medicine, senior lecturer equine medicine, and joint PhD candidate at Ghent University, in Belgium, and the University of Adelaide, in Australia.

“The sudden unexpected increase in hot ambient temperatures results in a significantly increased risk of heat stress because horses have not been able to acclimatize,” she said. “The global warming aspect underlines that we need to be prepared to prevent exertional heat illness.”

Swallowing the Smart Pill—Literally

To better understand how horses’ body temperatures increase during exercise, Verdegaal and her fellow researchers used a “smart” thermometer that horses swallow as a pill. It transmits real-time data about the animal’s core body temperature, providing a method more accurate than taking periodic rectal temperatures, which usually lag behind and are slightly lower than the core body temperature, she explained.

The team tested 13 endurance horses and 12 Standardbred trotters as they raced during cool winter months in Australia at moderate speeds for 40, 80, or 120 kilometers (25, 50, and 75 miles) or at intense speeds for 1,540 meters (5,000 feet).

Sprinters Heat Up Most Post-Race, Endurance Horses Reach Max Heat Near Race End

The researchers discovered that the endurance horses generally reached their peak core temperatures—averaging 39.0 degrees C (102.2 F)—toward the end of each 40-kilometer leg of the race, Verdegaal said. The trotters were the hottest—averaging 38.8 C (101.8 F)—within the first hour or more after they stopped racing. It took at least 30 minutes for all the trotters’ temperatures to peak and, on average, this occurred 40 minutes after exercise. Nearly half the trotters still had temperatures above 39 C (102.2 F) well over an hour after finishing the race—despite mild maximum outdoor temperatures of 18 C (64.4 F).

“The finding that trotting horses reached their maximum temperature at a mean time of 30 minutes post-exercise (including 10 minutes of cooldown) is very significant,” Verdegaal told The Horse. “Post-exercise cooling should be commonplace, as well as continuous monitoring for up to 60 minutes. Those horses should not be transported earlier than that, especially in warm or hot weather circumstances.”

Heart Rates: Unreliable Indicators of Heat Levels

The horses’ heart rates—as measured by a smart monitor they wore throughout and after exercise—didn’t accurately reflect internal heat levels, said Verdegaal. For example, many of the horses were still in a state of hyperthermia—meaning their core body temperatures were significantly elevated, exceeding 39 C (102.2 F) (mean 38.7 ± 0.4 C; range 38.0 – 39.6 C)—even though their heart rates had dropped below 64 beats per minute (bpm). That’s concerning, she added, given the Fédération Equestre Internationale (FEI) recognizes 64 beats per minute as the threshold for allowing endurance horses to continue their races. Other governing bodies hold lower thresholds, such as 60 beats per minute, she said.

“Veterinarians involved in endurance exercise are advised to not only rely on the heart rate recovery to 60 or 64 bpm as a measure of (body temperature) recovery,” Verdegaal said. “Veterinarians in the racing industry should monitor horses (in particular, their core body temperature via the GI pill) up to 60 minutes post-exercise, especially during heat waves.”

Heat Response Is an Individual Phenomenon

The team also confirmed that horses have a wide range of individual variation in how they heat up and cool down with exercise, said Verdegaal. In other words, each horse is unique in the way his or her body responds to exercise, with some heating up more than others and reaching peak body temperatures at different moments during or after exercise.

In addition, they confirmed that horses cool down at different rates, suggesting generalized protocols for avoiding heat stroke might not be ideal for all horses.

“We think our horses are unique, and our research confirmed that the heat load balancing during and post-exercise showed substantial individual variation,” Verdegaal said.

The discovery conflicts with what standardized laboratory testing with horses on treadmills in controlled settings has suggested, she added.

“I was surprised,” said Verdegaal. “I would have expected some individual variation in the body temperature recordings during field exercise, but not as significant as what we saw. If you look at the figures (scatterplots) of the individual horses, you will notice that not one horse is the same.”

Even so, the findings are consistent with what researchers have noted previously about human athletes and their body heat responses to exercise, she said.

“For the horse owner, this means that they will have to determine their own horses’ needs for training, hydration, cooling, and acclimatization,” said Verdegaal.

Tools such as the gastrointestinal thermometer pill used in the study might become more commonplace in the competition world as owners strive to keep track of their individual horses’ thermal responses, she explained.

“The equine industry is responsible for the horse’s welfare, and accurate monitoring equipment enables identifying horses with high temperatures,” Verdegaal said. “Continuous monitoring gives us the tools to prevent severe heat stress in horses, including endurance and racehorses. The gastrointestinal pill has been a reliable method to monitor body temperature in exercising horses and, moreover, to identify horses with increased body temperatures. Hence, I hope that the GI pill will be used during horse training and competitions.”

The study, “Continuous Monitoring of the Thermoregulatory Response in Endurance Horses and Trotter Horses During Field Exercise: Baselining for Future Hot Weather Studies,” was published in Frontiers in Physiology in August 2021.