A porcine pain face –identifying visible characteristics of pain in pigs

Clinician and pain researchers agree that there is a need for measures of pain. In pigs, being a species of prey, pain may be concealed and thus difficult to assess. However, identification of pain is necessary to enable its alleviation. Several physiological and behavioural parameters have been used in this respect during the last decades, however with only little success. A useful method must be able to distinguish pain-related changes from behaviours related to e.g. drowsiness or diseases. The pain face, i.e. the description and measurement of changes in facial expression related to pain, have been considered as such a method in several species including man.

Topical capsaicin crème has previously been used for the experimental induction of a short action, reversible inflammatory type pain in horses and man. In this study, capsaicin was used aiming to develop a repeatable and reliable model for pain induction and the description of a porcine pain face. Following the dermal application of capsaicin, six pigs were video recorded. Each pig served as their own control, and was filmed with and without noxious stimulus in separate trials. All trials were performed with and without an observer and with the crème applied on the left respectively the right shoulder, in total six recordings per pig. Based on characteristic facial expressions in other species, qualitative changes of eye region, ear basis, snout and cheeks were described by two external assessors and a drawing of a porcine pain face was made. Subsequently, 126 still images were evaluated blindly to assess the presence or absence of these features. Finally, an ethogram was constructed to describe certain gross pain behaviours observed.

The facial expressions of the porcine pain face comprises an angled appearance of the “eyebrows”, lowered ears held back or in an asymmetrical manner, wrinkling of the snout and tension of certain muscles around the mouth and cheeks. In the blinded evaluation, an “angled eye” was significantly (P=0.004) correlated to the application of a noxious stimuli whereas the number of wrinkles of the snout were not (P=0.8). Among the gross behaviours assessed, “up and/or attentive positioning” of the ears were correlated (P=0.02) to control trials.

The facial expressional changes observed in the pigs during noxious challenge were subtle. However, this study indicates that a porcine pain face can be identified and may be developed to constitute a method for the assessment of pain. Further development is needed to establish the optimal concentration of the crème for various ages of animals and methods to obtain video recordings from standardized positions.

Disclosure of Interest: None Declared.