The effect of social isolation, gender and familiarity with the experimental procedure on tests of porcine nociceptive thresholds

OBJECTIVE: To investigate the effects of habituation and isolation on mechanical nociceptive thresholds in pigs at the pelvic limbs and at the tail.STUDY DESIGN: Prospective randomized multifactorial study.ANIMALS: Thirty-two healthy castrated male (experiment 1), and 12 castrated male and 12 female (experiment 2) Danish Landrace × Yorkshire pigs, weighing 63.5 ± 0.8 kg and 55.4 ± 0.6 kg (the mean ± SD, experiment 1 and 2, respectively).METHODS: Mechanical nociceptive thresholds were quantified with a von Frey anesthesiometer applied to two distinct anatomical regions (tail and pelvic limbs). Pigs receiving the mechanical challenge in the pelvic limbs were tested inside a cage, whereas pigs exposed to stimuli at the tail region were tested in an open arena. For both experiments, the effect of familiarity to the procedure was evaluated by comparing thresholds of nociception in habituated versus naïve pigs. The presence of a companion animal was also evaluated in pigs receiving stimuli at the pelvic limbs.RESULTS: Pigs tested inside the cage were affected by the habituation to the procedure as indicated by the increase in willingness and time spent by the animals in the test cage. This effect was reflected in the lower mechanical nociceptive thresholds (medians with 25-75 percentiles) recorded for familiar pigs compared with naïve animals [495 g (302-675) versus 745 g (479-1000), respectively; p = 0.026]. Mechanical nociceptive thresholds measured at the tail of the pigs in the open arena were not affected by the familiarity of the animals with the experimental procedure.CONCLUSIONS AND CLINICAL RELEVANCE: The current results reiterate the value of habituation in research involving animal behaviour. Further characterization of the methodology is needed to allow its application in the evaluation of clinical conditions in pigs.