109 Use of the Horse Grimace Scale to evaluate markers of stress in horses involved in equine assisted activities

Abstract

Much like humans, horses possess the ability to generate a wide range of facial behaviors. These expressions are a form of communication used to convey substantial amounts of information to other members of the herd, including stress. A variety of studies have developed ethograms to quantify the degree of facial behaviors expressed as a means of assessing the welfare of the horse. This is particularly important in working horses, such as those involved in equine assisted activities (EAA). The objective of this study was to use the Horse Grimace Scale (HGS) to assess the stress response of horses involved in EAA by numerically scoring facial muscular tension. The HGS is a scale that focuses on 6 specific facial action units to evaluate the degree of pain and distress experienced by a horse. Each unit was given a score of 0 (not present), 1 (moderately present), or 2 (obviously present). The highest possible score was 12, indicating the greatest degree of pain. Horses trained in EAA (EAA, n = 6) were paired with a veteran with posttraumatic stress disorder (PTSD) for a 30-min weekly lesson or assigned as astanding non-EAA trained control (CON, n = 6) for the duration of 8 wks. Images of the profile of the horses were taken at 3 time points during the lesson: 10 min, 20 min, and 30 min after the start of the session. After the sessions, images were organized and redacted if necessary and scored by 4 independent raters who had received training on the use of the HGS. Score data were analyzed using a negative binomial mixed model, repeated measures ANOVA with fixed effects of treatment (EAA vs CON), session, and time point, and their interactions and a random effect of horse. Intraclass correlation coefficient for inter-rater agreement was 0.446 (P = 2.17e-59). Statistically different means were separated using Tukey's method. Significance was set at P ≤ 0.05. Scores were higher (P = 0.0178) in wk 4 (5.2 ± 0.4, mean ± SE) than wk 1 (3.7 ± 0.4) among CON horses. No other significant differences were found. Participation in EAA did not appear to have an adverse effect on horses as defined by pain and distress scores on the HGS.