Characterisation of short and long-term changes in mechanical nociceptive thresholds in pigs following tail injury

To date, studies on pain arising from tail docking and tail biting in pigs have mainly focussed on non-evoked behaviours. The development of neuromas and possible abnormal neural activity prompted us to target changes in nociceptive thresholds. We investigated the feasibility of pressure algometry applied to the tail of unrestrained pigs to quantify sensitisation following simulated tail biting and neonatal tail docking. Mechanical nociceptive thresholds (MNT) were determined in 3 tail regions (1: proximal; 2: intermediate; 3: distal) with a Pressure Application Measurement device (PAM). MNT were measured in pigs of 9 (n=41) and 17 weeks of age (n=67) at baseline and one week following resection. To evaluate the effect of time since injury, a subsample of pigs was tested at 8 (n=24), and 16 (n=38) weeks post-resection. Severity of injury was assessed using 3 treatments: ‘Intact’ (sham-resection); ‘short tail’ (2/3 of tail removed); ‘long tail’ (1/3 of tail removed). MNT were also recorded in 17 week-old pigs tail-docked at 3 days of age (n=23) according to the 3 treatments. The PAM induced clear withdrawal responses (response rate 95.2 %). Pre-resection MNT were higher in region 1 vs. 2 and 3 (P<0.05). Higher pre-resection MNT were recorded in 17 vs. 9 week-old pigs (P<0.05). Intra-individual and intra-site variability in MNT was 30% with a fair-to-good level of consistency (Intra-class Correlation Coefficient). One week post-resection, MNT were lower in resected (P<0.05), but not intact tails. Similarly, MNT were lower than intact values at 16 weeks following resections at 9 weeks of age (P<0.05). For resections at 17 weeks of age, MNT were lower in long vs. intact at 8 weeks after resection (P<0.05), but no significant difference was observed at 16 weeks following surgery (Two-way Repeated Measures ANOVA for all comparisons). No difference in MNT was recorded across treatments 17 weeks after neonatal docking. The PAM was an effective method for the assessment of MNT in intact and resected tails. Tail injury resulted in acute sensitisation. Long lasting effects were seen up to 16 weeks post-resection performed at 9 weeks, but were not apparent 16 weeks after neonatal docking or resections performed at 17 weeks. The current results provide novel information on the welfare compromise due to tail damage early or later in the life of pigs and inform assessment of the trade-off between tail docking and the risk of tail biting.