The Application of 3D Landmark-Based Geometric Morphometrics towards Refinement of the Piglet Grimace Scale

Simple Summary Piglet welfare on commercial farms is negatively affected by the routine husbandry procedure of surgical castration. Objective identification of pain is necessary to improve on-farm piglet welfare and for the development of proper pain management. The Piglet Grimace Scale (PGS) is a method of pain assessment using an ordinal scale that quantifies changes in facial expression induced by acute pain. This study evaluated the application of 3D landmark geometric morphometrics in refining existing PGS methods. To determine if these methods could objectively identify alterations in pig facial shape through landmark analysis, piglet images were taken using a multi-camera rig. Images were then used to construct 3D facial models using photogrammetry (structure from motion). Neither established nor refined PGS methods were able to detect pain-induced facial expressions. Further research is warranted to refine the PGS. Proper methods of assessment to objectively identify pain are essential for welfare improvements of piglets undergoing surgical castration on commercial farms. The Piglet Grimace Scale (PGS) is used to identify changes in facial expressions caused by acute pain in piglets undergoing tail docking and castration. However, subjective scoring methods are a concern for the validation of the PGS. The objectives of this study were to evaluate and refine the PGS through 3D landmark geometric morphometrics. Male piglets (n = 88) were randomly allocated to one of two treatments: castration and sham-castration. Piglet facial images were taken at four time points (before treatment, immediately post-treatment, 1 h post-treatment, and 4 h post-treatment) using a photogrammetry rig. Images were scored by four raters using five facial action units (FAUs): orbital tightening, ear position, temporal tension, lip contraction, and nose bulge/cheek tension. Three-dimensional facial models were generated and landmarked denoting 3 FAUs (orbital tightening, lip contraction, and nose bulge/cheek tension). Results suggest that orbital tightening and ear position may be reliable FAUs for the PGS. However, neither the PGS nor 3D landmark-based geometric morphometrics were able (both p > 0.10) to reliably identify facial indicators of pain in piglets undergoing castration.