Researchers measured horses’ facial sensitivity to touch, pressure, and heat. The results could help diagnose cases of equine idiopathic headshaking and improve welfare.
Researchers measured horses’ facial sensitivity to touch, pressure, and heat to find out what they’re feeling. They used handheld devices—including a pressure reader designed for horse faces—to check each horse’s sensitivity and facial nerve functions.
“Knowing the normal sensitivity values of the face would provide a tool for veterinarians to diagnose alterations in sensitivity,” said Kata O. Veres-Nyéki, DrMedVet, Dipl. ECVAA, PhD, MRCVS, of The Royal Veterinary College, in Hatfield, the U.K., formerly of the Vetsuisse Faculty at the University of Bern, in Switzerland.
“Acute injuries of the face are probably obvious, but chronic pain conditions might be overlooked otherwise,” Veres-Nyéki said. “Using the quantitative sensory testing methods, we can not only detect alterations but also follow up the efficacy of analgesic (pain relief) treatments in a noninvasive (harmless) way.”
Testing Touch, Pressure, and Heat Thresholds in Equine Faces
Veres-Nyéki and her fellow researchers tested facial sensitivity in 34 healthy Warmblood mares, geldings, and stallions ranging in age from 1 to 23 years old. They used von Frey filaments—a kind of thin, flexible stick—to test tactile thresholds (how sensitive horses are to light touch such as from a hair or a fly). To test heat thresholds, they pressed a handheld thermode (medical heating device) gently against the horse’s facial skin. The temperature gradually increased from 30 degrees C (86 degress F) up to a maximum of 55 degrees C (141 degrees F). And to test pressure thresholds, they developed a handheld algometer with a silicon tip that applies gradually increasing pressure against the horse’s face.
In all three tests, the horses were free to move their heads, and the test stopped when the horse reacted in any way to the contact (such as by moving the head away, twitching the skin, or blinking reactively), Veres-Nyéki said.
Comparing to Humans, Other Horses
Horses seem slightly less sensitive than humans when it comes to facial contact, the researchers reported. But Veres-Nyéki explained that this might be because humans can say when they feel the sensation, whereas horses must physically react to the stimulus—which might mean they feel it and tolerate it before reacting.
And although men have higher thresholds than women for pressure and heat, sex didn’t seem to affect sensitivity thresholds in horses.
Age, however, did. Veres-Nyéki said horses generally had increasingly higher thresholds for all three kinds of contact—touch, pressure, and heat—as they got older. “It is not known why would that happen, but it is suspected to be caused by decreased innervation density (basically, the age-related ‘damage’ of the nervous system),” she said.
Pinpointing Facial Regions to Diagnose Headshaking
The study allowed the scientists to pinpoint precise places on the horse’s face for the most reliable results of each test. Specifically, they recommended testing tactile sensitivity on the nostril, pressure sensitivity at the temporomandibular joint (on each side of the jaw), and heat sensitivity at the supraorbital foramen (the forehead just over the eye). These sites gave the most consistent results and are innervated by the trigeminal nerve, she said.
Facial sensitivity testing of individual horses might help diagnose, in particular, cases of equine idiopathic headshaking, a neuropathic pain condition, Veres-Nyéki said. By performing these field tests and comparing measurements with the healthy horse threshold data provided in the researchers’ open-access study, veterinarians can detect possible facial nerve sensory abnormalities, she said.
Keeping Facial Sensitivity in Mind
Clipping facial hair for testing isn’t necessary, as the team’s results showed only insignificant differences when testing sites were shaved versus unshaved. Even so, the scientists did note trends suggesting that unshaven horses were more sensitive to touch, underlining the importance of leaving clipping horses unclipped (except when medically necessary) so as to not deprive them of their full level of sensitivity.
The study has practical applications in equitation, as well, Veres-Nyéki added. For example, trainers might consider using gentler equipment on younger horses due to their lower sensitivity thresholds.
“The equipment used on the face should not provoke pain in the first place, but as more force is applied to them over smaller surfaces, they might cause pain in the horses,” she said.