Pigs in pain-Porcine behavioural responses towards mechanical nociceptive stimulation directed at the hind legs.

Objectives In recent years, pigs are used increasingly for biomedical translational research and often considered superior to rodent models. However, only very few pain assays for quantification of behavioural nociceptive responses are available for pigs. This experiment is part of a larger project aiming at developing pain assays for pigs, and the present aim was to examine behavioural responses towards mechanical cutaneous stimulation applied to the caudal part of the metatarsus on the hind legs of pigs using an IITC Electronic von Frey Anesthesiometer. Methods Nine slaughter pigs (bodyweight 54 ± 0.7 kg) were subjected to handheld nociceptive mechanical stimulation using the electronic von Frey anesthesiometer (max 1000 g). The animals were kept in one group and tested while slightly fixated in a testroom, to which they had been habituated. Each nociceptive test consisted of 4 single stimulations. Results The animals responded behaviourally after single stimulations of 12-1000 g (8% censored) and had a mean threshold for leg movements of 540 ± 85 g. Neither the type of behavioural responses (four categories from slight leg movements to kicking) nor the mechanical threshold was affected by successive stimulations (517 ± 126, 413 ± 84, 598 ± 97 and 499 ± 122 g, for first to fourth stimulation, respectively) or differed between right and left leg (522 ± 86 g vs. 492 ± 64 g, respectively). No correlations between mechanical thresholds and behavioural responses were found. Conclusion The results show that pigs respond behaviourally to mechanical von Frey stimulation directed at the hind legs, and suggest that responses were not affected by successive stimulations.