Refining a ‘toolkit’ for objective assessment of pain and stress in ruminants

Abstract

Pain is a complex phenomenon, and to address pain management in animals, the presence of pain needs to be determined. Most studies assess the presence of pain based on the appraisal of either a single biomarker or a combination of a few parameters, which were often not examined simultaneously. In ruminants, measurements of cortisol and haptoglobin (Hp), and the assessment of behavioural changes have been the standard practice for this purpose. Nevertheless, there are still limitations in using these parameters, as they are also indicators of psychological stress, and can be misinterpreted as to indicate the presence of pain. Assessment of behavioural changes in ruminants can be difficult as prey animals conceal their experience of pain to avoid predators' attention.Therefore it is recommended that a reliable assessment tool be devised that includes a combination of various parameters to determine the presence of pain in ruminants in order to address pain management effectively.The primary aims of this study were to evaluate the conventional and contemporary biomarkers to determine the response of an animal to a noxious experience, and to refine a multi-parameter 'toolkit' that can be applied in assessing pain in ruminants. For these purposes, castration was used as a model of a common painful husbandry procedure that presumably evokes pain associated with the tissue injury.Merino cross lambs were assigned to two groups that were either surgically castrated or subjected to restraint stress (control). Castration was an open surgical technique without the provision of anaesthesia or analgesia. The conventional biomarkers estimated were plasma cortisol, Hp and betaendorphin (β-EP). The contemporary biomarkers measured were interleukin-6 (IL-6), substance P (SP), and prostaglandin E 2 (PGE 2 ). The peripheral leukocytes expression of IL-6 and proopiomelanocortin (POMC), the precursor of β-EP were also estimated. Behavioural changes were recorded to assess pain-related behaviour caused by the surgical procedure.Following treatment, the cortisol concentration in the castrated lambs was higher than control animals from ten minutes to two hours post-castration, indicative of the acute noxious experience associated with the surgical procedure. Surgical castration also caused a response in SP indicative of nociception and neurogenic inflammation, which was increased from 30 minutes and became significant eight hours following castration. The behavioural assessment showed that the castrated lambs demonstrated statue standing, a pain-like behaviour, from three to five hours post-castration, which were associated with the increase in SP, indicative of the pain sensitisation caused by the tissue injury. Increase in the systemic inflammatory mediators, the Hp and IL-6 were observed from ii day two, in which Hp remained significant until day four and resolved by day five. The IL-6 continued to increase over time, suggesting of an ongoing inflammatory response and inflammatory pain possibly until wound healing had occurre...