Porcine pain face – identifying visible characteristics of pain in pigs

The sensation of pain is an important evolutionary adaptation, vital for the survival of an organism. Not only does it enable identification and subsequent avoidance of potentially harmful threats, it also allows for special tending of the affected and possibly injured area. However, the suppression of pain may also constitute an essential evolutionary adaptation, especially in prey animals, which tend not to show such vulnerability for as long as they can possible disguise it. In pigs, pain may thus be difficult to identify due to the stoic nature of this species. However, alleviation of pain requires it to be acknowledged. Numerous methods have been evaluated as possible means of pain assessment in pigs, but the development of a precise and reliable, objective and efficient pain assessment tool is however yet to await. In humans, facial behavioural changes associated with pain is considered to be a reliable and consistent method for pain evaluation, and a good alternative when patients cannot convey their pain through spoken words. The concept of pain face has recently been further studied in e.g. non-human primates, horses, rodents and rabbits, and the method seems to be valuable also in these individuals unable to verbally communicate. However, to our knowledge, a pain face has not yet been developed in pigs. Through video recordings of six pigs, following the application of topical Capsaicin creme inducing a transient burning pain sensation, facial expressions of pain were evaluated in accordance with a previously developed protocol for pain face in horses. The pigs served as their own control, and were filmed without any noxious stimulus, prior to noxious challenge. Topical Capsaicin was thereafter applied, during separate trials, to an area on the left respectively the right shoulder. All trials were performed twice, with and without an observer in the room. The films were evaluated by two external assessors, and a drawing of a porcine pain face was produced based on these findings. Subsequently, a blinded evaluation was performed based on a number of still images from each film, to assess the presence or absence of these pain face features. Furthermore, an ethogram was constructed regarding certain gross pain behaviours observed. The evaluation of various gross pain behaviours revealed no significant behavioural changes as a consequence of topical administration of the Capsaicin, and gross behaviours indicative of discomfort or pain were infrequently and only occasionally observed. The facial expressions of the porcine pain face seem to comprise an angled appearance of the eyes (P = 0.004), lowered ears held back or in an asymmetrical manner, wrinkling of the snout and possibly also tension of certain muscles around the mouth and cheeks. Furthermore, ears held in an upright position, turned forward in an attentive manner, were less frequent during pain induction (P = 0.02) than during the control trials. The facial expressional changes observed in the pigs during noxious challenge may have been rather subtle. Moreover, various concentrations of Capsaicin need to be evaluated in relation to the age and weight of the pigs under investigation. The findings of this study does however indicate that also in pigs, a pain face can be identified – and that it may constitute a possible future method for the assessment of porcine pain.