Abstract
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.
Highlights
Tissue and nerve injuries caused by surgical procedures lead to acute peripheral sensitisation of the affected area, with a possible involvement of central pain-processing pathways
Neuropathic pain refers to ‘pain arising as a direct consequence of a lesion or disease affecting the somatosensory system[10], and its physiological manifestation has been investigated in rodent models; it is yet unknown if a similar phenomenon occurs in pigs following tail amputation and this is an important question in the current debate on animal welfare issues
Tail amputation is an event frequently observed in pig production as the result of tail docking or tail biting
Summary
Tissue and nerve injuries caused by surgical procedures lead to acute peripheral sensitisation of the affected area, with a possible involvement of central pain-processing pathways. Neuropathic pain refers to ‘pain arising as a direct consequence of a lesion or disease affecting the somatosensory system[10], and its physiological manifestation has been investigated in rodent models; it is yet unknown if a similar phenomenon occurs in pigs following tail amputation and this is an important question in the current debate on animal welfare issues The aetiology of this particular type of neuropathic pain is uncertain; persistent ectopic discharge of axons within the neuroma has been suggested as the most plausible mechanism[11]. In addition to its effects on the damage and reorganization of peripheral nerve endings, deep tissue injury can cause alterations in peripheral and spinal dorsal horn neuron activity, which can retrogradely affect the function of peripheral sensory afferent nerves (e.g. modulate excitability) and/or feed forward ascendingly to supraspinal centres[13] This altered activity in spinal and supraspinal regions of the pain processing pathways represents a potential source of sustained pain, such as phantom limb pain experienced by amputees[14]. Due to the traumatic nature of the injury, it is highly relevant to investigate its short- as well as long-term consequences on the pain sensitivity of the animals, in order to provide novel information on the underlying mechanisms and to appraise the associated welfare impairment
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