Recognising stress in horses.
Can you recognize stress in a horse during a veterinary exam? Your observations might not be as accurate as you think. Physiological stress parameter measurements could give more reliable information about equine stress—but they have to be interpreted correctly, according to Scottish researchers.
“We can only have confidence in scientific results if the data is accurate, but that requires validation and calibration of the equipment,” said Gemma Pearson, BVMS, Cert. AVP (EM), MScR, MRCVS, Horse Trust funded PhD student at the University of Edinburgh’s Royal (Dick) School of Veterinary Studies, in Scotland.
In their recent study, Pearson and her fellow researchers noted “a lot of individual variation” among equine veterinarians, as well as equitation scientists and non-equine behaviour scientists, about how stressed horses are when they’re undergoing veterinary procedures.
“They don’t agree on which horses are stressed, their level of stress, or how to manage that stress,” Pearson said during her presentation at the 15th conference of the International Society for Equitation Science (ISES), held August 19-21 2019, in Guelph, Ontario, Canada.
Because of that disagreement, it makes sense to use objective measurements for evaluating physiological signs of stress, she said. Common parameters include salivary cortisol (SC), heart rate (HR), heart rate variability (HRV), and surface eye temperature (SET). However, the measurement equipment for these parameters is subject to multiple sources of error or misinterpretation, and can yield widely varying data.
“Salivary cortisol is a good parameter, but it mostly shows changes over time rather than immediate stress states,” said Pearson. Furthermore, horses can produce coloured saliva when given special licking treats like molasses-based “lollipops” for horses, for example. These treats help horses associate veterinary exams with a more positive experience, but the resulting saliva production would render their cortisol readings invalid. “The test for cortisol is an ELISA test, so it’s based on optical density,” she explained.
HRV—the variations in cardiac rhythm—can give useful data about immediate stress, she said. But this parameter is particularly hard to measure reliably, in part because the equipment is mostly designed for humans. “Many of the equine waves get misinterpreted as the wrong kinds of waves,” she said. Some commercial equipment is designed to “correct” these kinds of errors, but the algorithms they use are unclear. “I contacted one manufacturer to ask about their algorithms but did not receive an answer,” said Pearson. Trying to calibrate the commercial equipment to the “gold standard”—ECG—was a complex process that was essentially unsuccessful, she added. “It seems the best way to be sure that all the data is accurate is to use an ECG and then manually correct each reading.”
With heart rate monitors Horse hair can also affect readings, interfering with the electrical signal, causing a reduced signal, leading to misinterpretations, according to Pearson. Clipping can help, but horses aren’t always clipped for this purpose. Furthermore, horses can have irregular heart rates—called arrhythmias which can lead to inaccuracies, even though many are harmless. “Fortunately some arrhythmias can be easy to pick up with a basic stethoscope,” she said. “Others, however, but can be intermittent and easy to miss. It’s important to actually check each horse before carrying out measurements. You don’t want to waste time collecting data on these cases.” For other irregularities, though, the scientists would need to evaluate the readings to detect such problems.
While HRV might be unreliable using any equipment other than an ECG, commercial heart rate monitors can still provide useful readings about heart rate alone, Pearson added.
As for SET, it requires “a lot of money” for a good camera as well as detailed calibration, taking into consideration temperature and relative humidity. Focus is critical, Pearson explained. “You have to be at the correct angle and at the correct distance, and like all cameras, it takes a second to focus,” she said. “That’s really hard if you’ve got a horse that is stressed and is moving his head around. You just can’t get data from that.” Furthermore, any wind or even breeze can skew the results. “In certain climates that means you have to do it completely indoors or just forget about it.”
Although Pearson’s research has revealed “weaknesses” in the collection of scientific data for stress parameters in horses, that doesn’t mean previous research is invalidated, she said. “We don’t need to throw out these earlier studies,” she explained, pointing to worthwhile trends and comparisons overall that remain robust. “But the way forward needs to include careful study designs and good validation and calibration of any equipment used. Hopefully this will help people think about the research they’re doing.”