Validation of the Horse Grimace Scale

Abstract

Recognising signs of pain in animals can help to ensure appropriate and effective treatment. The grimace scale has been developed for a variety of species, including domestic horses (Equus caballus), to showcase the facial expressions associated with different levels of pain. These scales are designed to be easily usable by anyone, with minimal training; however, to date this has not been validated. Our study investigated the use of the Horse Grimace Scale (HGS) by people with varying levels of experience with horses, including veterinarians, riders and owners, trainers, farriers, physical therapists, and people with no experience with horses (control group). If the HGS is a universal tool, there should be no difference in the results between groups of people. Survey participants were asked to watch six videos of horses, 3 of which were post-castration and 3 which had no pain experience, and assess if they thought pain was present and identify which facial features they thought were indicators of pain. They were then subject to a short training session using still images highlighting the facial expressions and features which indicate signs of pain. The participants then assessed another six videos of different horses, again half post-castration and half control animals. Training did result in improved agreement between groups.

However, the HGS scores of both treatment and pain-free horses increased 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, thus assuming pain must exist. It is important to note that these results do not necessarily reflect accuracy in HGS scoring, only agreement between groups. We suggest that the HGS may require more than minimal training to be an effective tool, despite assertions in the literature to the contrary.

Furthermore, the previous experience a person has with horses, and their chosen profession, may create bias when assessing pain in horses, regardless of training. This has ramifications for the reliability of grimace scales.