Subclinical inflammation may have multiple welfare implications in pigs

Clinical disease impacts directly upon animal welfare. However, the majority of subclinical disease remains undetected. Sickness behaviour is mediated by a systemic pro-inflammatory response. Although inflammatory markers are upregulated in sub-clinical pig disease, evidence for behavioural alterations remains lacking. We propose that mild sickness behaviour within clinically ‘healthy’ herds could have negative implications for livestock management. We aimed, therefore, to identify behaviour associated with subclinical inflammation in pigs free from clinical disease. Our sample population comprised pigs (n=120, 60 kg) with/without tail lesions from four UK farms (one without any tail biting history and three with current outbreaks). Baseline behavioural analysis and a novel object test were performed in-situ and a comprehensive immunochemical assessment was conducted on serum collected at a single sampling point. Data were analysed using random-intercept nested multilevel models. Correlations were observed between several inflammatory markers and behaviours associated with sickness. High C-reactive protein (CRP) was associated with low physical activity (P=0.046), social responsiveness (P=0.037), interaction time with a novel object (P=0.045) and feeding (P=0.045). Systemic inflammation was associated with tail biting and being tail bitten. Pigs with low Hb (P=0.032) and high levels of IFN-γ (P=0.046) and CRP (P=0.024) were more frequently tail-bitten, regardless of whether they had a tail lesion. The behavioural profile of ‘biters’ was suggestive of hyperactivity rather than sickness behaviour. However, tail biting was associated with pro-inflammatory cytokine upregulation (IFN-γ: P=0.028; IL-1β: P<0.001; IL-6: P=0.018). Low Hb (P<0.001), high CRP (P<0.001), and high latency to interact with a novel object (P=0.028), were all observed in pigs with healed tail-lesions, indicative of secondary sub-clinical infection. These results highlight an association between systemic sub-clinical inflammation and altered behaviour which may have wider welfare implications. Reductions in activity and environmental interest, associated with an upregulated inflammatory response may, for example, help to explain the variability in the success of management efforts (e.g. enrichment provision) to prevent/manage detrimental behaviours such as tail-biting. Furthermore, sub-clinical inflammation within our sample appeared to have multiple links with tail-biting (both as a trigger and following lesion infection). Quantification of inflammatory (especially CRP) and behavioural markers of sub-clinical inflammation, therefore, offers potential as an advanced means of identifying welfare risk in asymptomatic pig herds.