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.

Is the Piglet Grimace Scale (PGS) a Useful Welfare Indicator to Assess Pain after Cryptorchidectomy in Growing Pigs?

“…There is a lack of repeatability in stress/fear/pain studies in domestic animals, including pigs [35], in which case the absence of a clear pain assessment tool applicable across pigs of different age and weight arguably factors in. Existing literature reported results from experiments on 5-day-old piglets [9,11,12], minipigs [36], or straight-eared cross-breeds [37]. The original PGS describes the FAU "ear position" as "the ears are drawn back from forward (baseline) position" [12]; however, our observations failed to record unequivocal backward ear movements, which could be explained by the conformation of our older pigs.…”

“…Several anesthetic drugs are still not approved by Italian legislation, including buprenorphine. Therefore, we could not include a blinded treatment group that received this opioid, even if it has been reported to be highly effective in alleviating castration-associated pain behaviors and facial grimacing in piglets [12,21]. For both ethical and welfare reasons, a control group undergoing general anesthesia without surgery was not included in this study.…”

Optimization of analgesia for piglet castration under isoflurane anaesthesia with parenteral butorphanol, meloxicam or intratesticular lidocaine

“…Some of the pain specific behaviour was also observed in not castrated piglets of the litter and therefore we conclude that the scoring system used was not appropriate to properly detect differences in postoperative pain between the groups. A newly available method to score postoperative pain, the piglet grimace scale, is an alternative that could be implemented in further investigations (Viscardi et al, 2017).…”

The Utility of Iris-pupillary Area Ratio as a Non-invasive Index of Stress in Primates

“…One of the potential applications of I/P ratio analysis could be in laboratory animal welfare, where currently the grimace scale is used to measure the degree of pain/stress. [38][39][40][41][42][43] I/P ratio analysis could be integrated to grimace scale scores to refine the efficiency and sensitivity of progressively detecting animal in pain/stress. Although this study was focused on primates, we see the general principle of this work is applicable to all species.…”