Validation of the Horse Grimace Scale: Exploring the effects of training and previous experience

Abstract

limited number of validated pain assessment tools for horses to date can complicate welfare decisions. The Horse Grimace Scale (HGS) uses different elements of facial expression (facial action units) to score the degree of pain experienced by horses. However, the potential impact of previous equine knowledge on the use of the HGS, as well as the effectiveness of training in the use of the scale, has not yet been explored.

My study used an anonymous online survey to investigate the effect of training as an equaliser of scoring between participant groups with different previous knowledge and experience of horses, comprised of veterinary professionals, equine physical therapists, horse owners and riders, animal welfare inspectors, and a participant control group. Before and after they received training, participants visually rated the HGS using short video clips of horses 24 hours after castration (treatment subjects), and horses not known to be in pain (non-treatment subjects).

Participant group, exposure to training and the condition (non-pain versus 24 h post castration) of the horse had a small but significant effect on scoring of the HGS. All participant groups scored pain in treatment horses (24 h post castration) higher than non-treatment (pain free) horses both pre and post training. Training resulted in HGS scores of both treatment and pain free horses to increase. Contrary to expectations, the control group (limited or no exposure to horses) scored treatment horses at a higher HGS score than other participants groups, although this difference reduced post training. The reason for the increase in HGS for pain free horses after training remains obscure but may arise from participant biases, with participants more likely to score pain in horses after training explicitly conducted to evaluate pain. Participant agreement (ICC) was strong particularly for ears back before and after training. However, this does not necessarily reflect accuracy of HGS scoring. In sum, my results indicate that training led to decreased uncertainty of participants when scoring HGS scores, training resulted in increased HGS ratings whether the horse was expected to be in pain or not, and the nature of the training used in this study was not sufficient for raters to score the HGS accurately