Do Different Emotional States Affect the Horse Grimace Scale Score? A Pilot Study

The Horse Grimace Scale (HGS) is a facial-expression-based pain coding system that incorporates six Facial Action Units, independently scored. The aim of this pilot study was to determine whether HGS score changed in response to emotional states other than pain. Seven riding school horses were exposed to different experimental conditions that are known to elicit different emotional states (new environment, grooming, anticipation of food reward, fear). As a control, videos were also recorded with the horse undisturbed in their home environment. Videos were recorded in each experimental condition and individual frames were captured from HD video files. Thirty-five pictures were scored using HGS by three experimental design blind observers. The results showed that the HGS score did not change in any of the experimental conditions when compared with control. In all the conditions but one (fear), the mean HGS score was low (≤2 on a maximum score of 12), but not zero; the scores of stiffly backward ears and prominent strained chewing muscles tended to be higher in fear than in other conditions. Although these are preliminary results as the number of tested horses is limited, HGS seems to be a specific tool to assess pain in horses. Changes in ear posture and tension in chewing muscles tend to be present in negative emotional states such as fear; therefore, a larger scale study is needed to collect data from the same horses in positive and negative situations, including pain.