The Horse Grimace Scale (HGS): past, present and future challenges

Pain in horses is a critical welfare concern leading to considerable suffering and distress. Using valid assessment methods to identify whether a horse is in pain is of a clear importance also for the use of analgesic treatment; however, reliable pain recognition is still difficult as many currently available scales are non-specific, inconsistent and/or impractical when applied by equine clinicians. Recently, a facial-expression-based coding system was developed as a method to assess pain in horses: the Horse Grimace Scale (HGS) [1]. The HGS includes six Facial Action Units (FAUs): stiffly backwards ears, orbital tightening, tension above the eye area, prominent strained chewing muscles, mouth strained and pronounced chin, and strained nostrils. Each FAU is scored on a 3-point scale (0-2), with zero indicating that the assessor is confident that the action unit is not present, one indicating that it is moderately present, and two indicating that the assessor is confident that the action unit is obviously present. The HGS has been demonstrated to be a potentially effective tool for assessing pain associated with surgical castration [1,2] and acute laminitis in horses at rest [3,4]; thus confirming that it is pain-specific [5], non-invasive and easy to learn. Furthermore, in horses with acute laminitis, HGS scores were positively correlated with changes in other potential pain and distress biomarkers (miR-23b-3p, miR-145-5p and miR-200b-3p) [4]. For all these reasons, the Horse Grimace Scale could be a promising scale to be applied by equine clinicians in everyday practice to detect pain in horses. However, there are still limitations and challenges to the use of HGS in everyday practice that should be addressed. A review of the studies of HGS, starting from the development to most recent unpublished results related to the training of new assessors will be presented, highlighting the conceptual and methodological issues we have faced, as well as discussing the advantages and future challenges of using facial expression as method of pain assessment in horse practice.