Development of a Piglet Grimace Scale to Evaluate Piglet Pain Using Facial Expressions Following Castration and Tail Docking: A Pilot Study

Abstract

Facial expressions are increasingly being used to assess pain in non-human species, including rodents, horses, and lambs. The development of these species-specific grimace scales has allowed for more rapid pain detection, which can lead to better animal welfare if intervention promptly occurs. For grimace scales to ever be used as a stand-alone measure of pain, it is important they correlate with established pain assessment tools, such as behavioral analysis. This preliminary study aimed to determine whether piglets exhibit pain grimacing and if these facial expressions correlate with their behavior. It also assessed and compared the behavior of boar piglets given an analgesic and topical anesthetic prior to surgical castration and tail docking to piglets that did not receive anything for pain relief. Five-day-old male Yorkshire piglets (n = 19) from four pens were randomly assigned, within their pen, to one of five possible treatments: meloxicam (0.4 mg/kg, intramuscularly) + EMLA^® cream, meloxicam (0.4 mg/kg, intramuscularly) + non-medicated cream, saline (intramuscularly) + EMLA^® cream, saline (intramuscularly) + non-medicated cream, or no treatment prior to surgical castration and tail docking. Piglet behaviors were video recorded for 8 h immediately after castration, as well as for 1 h at 24 h pre- and post-castration. Their individual behaviors were scored continuously for the first 15 min of every hour of video collected. Facial images were also captured across all time points. A Piglet Grimace Scale (PGS) was developed and used by two observers blinded to treatment, time, and procedure to score over 600 piglet faces. All piglets displayed significant behavioral changes up to 7 h post-castration when compared to baseline, and the use of meloxicam and EMLA^® cream was not associated with a reduction in painful behaviors. Significantly higher PGS scores were noted at 0, 3, 4, and 5 h post-castration when compared to PGS scores at 7 h and there was no effect of treatment. PGS scores significantly correlated with piglet behavioral activity. The results suggest that the PGS may have utility for pain evaluation in neonatal pigs.

Keywords: analgesia; animal welfare; grimace scale; pain; piglet; refinement.

Figure 1

Piglet Grimace Scale with descriptions for each of the three facial action units (FAUs) employed: ear position, cheek tightening/nose bulge, and orbital tightening. FAUs are scored based on whether it is absent (score of 0), moderately present (score of 1), or obviously present (score of 2), with the exception of orbital tightening, which is scored on a 2-point scale of absent (score of 0) and present (score of 1).

Figure 2

Comparison of active (walking, playing, suckling, nosing, etc.) and inactive (lying, sleeping, isolated, awake inactive, etc.) behaviors of piglets at all time points, pre- and post-castration (±SE) are represented by the bar graph. Active behaviors decreased significantly following castration, independent of treatment given, and eventually returned to baseline levels after approximately 7 h. Proportion (±SE) of Piglet Grimace Scale (PGS) scores within each pain category across all treatment types are represented by the line graph. Observers (n = 2) were unaware of piglet treatment or time point when scoring. PGS scores significantly correlated with piglet behavioral activity.