Many piglets are exposed to potentially painful husbandry procedures within the first week of life, including tail docking and castration, without the provision of either anaesthesia or analgesia. The assessment methods used to evaluate pain experienced by piglets are often affected by low specificity and practical limitations, prompting the investigation of alternative methodologies. The assessment of changes in facial expression following a painful event has been successfully applied to several species. The objective of this pilot study was to evaluate the utility of a Grimace Scale applied to neonatal pigs to evaluate pain evoked by tail docking and castration. Eight female piglets, sus scrofa domesticus (Landrace/Large White X synthetic sire line) underwent tail docking and 15 male piglets (75% Large White and 25% Belgian Landrace) were exposed to the castration procedure. Clear images of the faces of the piglets were collected immediately pre- and post-procedure. The images were used by experienced observers to identify Facial Action Units (FAU) which changed in individuals over this period and a scoring scale was depicted in a training manual. A set of randomly selected images were then combined in a scorebook, which was evaluated after training by 30 scorers, blind to the treatment. The scale for most FAU was used with a high level of consistency across all observers. Tail docking induced a significant change (P<0.05) only in the ‘orbital tightening’ Action Unit, whereas no change in any unit was observed in castrated piglets. In this initial stage of development, orbital tightening at least seems to have the potential to be applied to investigate painful conditions in neonatal pigs. Nonetheless, more studies are needed to assess its full effectiveness and to evaluate the influence of possible confounds (e.g. handling stress) on the observed changes in facial expressions.

There is a moral obligation to minimize pain in pigs used for human benefit. In livestock production pigs experience pain caused by management procedures, e.g. castration, and tail docking, injuries from fighting or poor housing conditions, “management diseases” like mastitis or Streptococcal meningitis, and at parturition. Pigs used in biomedical research undergo procedures which are regarded as painful in humans, but do not receive similar levels of analgesia, and pet pigs also experience potentially painful conditions. In all contexts, accurate pain assessment is a prerequisite in: a) the estimation of the welfare consequences of noxious interventions; and b) the development of more effective pain mitigation strategies. This narrative review identifies the sources of pain in pigs, discusses the various assessment measures currently available, and proposes directions for future investigation.

Tail docking of neonatal pigs is widely used as a measure to reduce the incidence of tail biting, a complex management problem in the pig industry. Concerns exist over the long-term consequences of tail docking for possible tail stump pain sensitivity due to the development of traumatic neuromas in injured peripheral nerves. Tail stumps were obtained post mortem from four female pigs at each of 1, 4, 8 and 16 weeks following tail amputation (approximately two-thirds removed) by a gas-heated docking iron on post natal day 3. Tissues were processed routinely for histopathological examination. Non-neural inflammatory and reparative epidermal and dermal changes associated with tissue thickening and healing were observed 1 to 4 months after docking. Mild neutrophilic inflammation was present in some cases, although this and other degenerative and non-neural reparative changes are not likely to have caused pain. Traumatic neuroma and neuromatous tissue development was not observed 1 week after tail docking, but was evident 1 month after tail docking. Over time there was marked nerve sheath and axonal proliferation leading to the formation of neuromata, which were either localized and circumscribed or comprised of multiple axons dispersed within granulation tissue. Four months after tail resection, neuroma formation was still incomplete, with possible implications for sensitivity of the tail stump.

Background: Volatile anesthetics such as isoflurane are widely used in clinical and research contexts. Concerns have been raised that the effects of these drugs on the central nervous system may result in long-term impairment after surgery or general anesthesia. Hence, this study aimed to detect how different isoflurane concentrations influence spatial learning and cell death in adult mice. Methods: Fifty-two C57BL/6 mice were randomly divided in four groups. Mice in three groups were exposed to different concentrations of isoflurane (1, 1.5, and 2%) for 1 h; the control group was not exposed to anesthesia. Five mice per group were killed 3 h after anesthesia to perform histopathologic and immunohistochemical analyses (hematoxylin-eosin staining; caspase-3 activation). Eight mice per group were used for behavioral tests (open field, T-maze spontaneous alternation, and water maze) on subsequent days. Results: There were no differences between groups in the T-maze spontaneous alternation test or in the open field (no confounding effects of stress or locomotion). The group anesthetized with 1% isoflurane performed worse in the water maze task on day 1 (550.4 ±162.78 cm) compared with the control group (400.1 ± 112.88 cm), 1.5% isoflurane (351.9 ± 150.67 cm), and 2% isoflurane (364.5 ± 113.70 cm; P ≤ 0.05) and on day 3 (305.0 ± 81.75 cm) compared with control group (175.13 ± 77.00 cm) and 2% isoflurane (204.11 ± 85.75 cm; P ≤ 0.038). In the pyramidal cell layer of the region cornu ammonis 1 of the hippocampus, 1% isoflurane showed a tendency to cause more neurodegeneration (apoptosis) (61.4 ± 26.40, profiles/mm) than the group with 2% of isoflurane (20.6 ± 17.77, profiles/mm; P = 0.051). Conclusion: Low isoflurane concentration (1%) caused spatial learning impairment and more neurodegeneration compared with higher isoflurane concentrations. Results for mice receiving the latter concentrations were similar to those of control mice.

Commercial pigs are frequently exposed to tail mutilations in the form of preventive husbandry procedures (tail docking) or as a result of abnormal behaviour (tail biting). Although tissue and nerve injuries are well-described causes of pain hypersensitivity in humans and in rodent animal models, there is no information on the changes in local pain sensitivity induced by tail injuries in pigs. To determine the temporal profile of sensitisation, pigs were exposed to surgical tail resections and mechanical nociceptive thresholds (MNT) were measured in the acute (one week post-operatively) and in the long-term (either eight or sixteen weeks post-surgery) phase of recovery. The influence of the degree of amputation on MNTs was also evaluated by comparing three different tail-resection treatments (intact, ‘short tail’, ‘long tail’). A significant reduction in MNTs one week following surgery suggests the occurrence of acute sensitisation. Long-term hypersensitivity was also observed in tail-resected pigs at either two or four months following surgery. Tail amputation in pigs appears to evoke acute and sustained changes in peripheral mechanical sensitivity, which resemble features of neuropathic pain reported in humans and other species and provides new information on implications for the welfare of animals subjected to this type of injury.