Effects of early life and current environmental enrichment on behaviour, affective state and immunity in pigs

Abstract

The main aim of this thesis was to investigate whether and how early life and current environmental enrichment affect behaviour, affective state, and immunity (which may influence health) in pigs. First of all, the effects of environmental enrichment on natural (auto)antibodies (NA(A)b) in relative healthy pigs and in pigs co-infected with PRRSV and A. pleuropneumoniae were studied.  We found that environmental enrichment per se has an impact on immunity in pigs, by affecting NA(A)b levels and their changes in response to challenges and infection. Thereafter we aimed to further investigate the effects of early life environmental enrichment and a (mis)match in early life and current housing on behaviour, affective state, and immunity in pigs. To this aim, 30 litters of pigs were housed in either barren or enriched pens from birth. The coping style of the pigs was assessed by the backtest, and pigs were classified as reactive (LR) or proactive (HR). At 28 days of age, pigs were weaned, and in total 192 pigs were selected and regrouped in 32 new pens containing 6 non-littermate pigs each. All pigs were equally regrouped by taking sex, coping style and body weight into account. Housing treatment (barren vs. enriched) for each pig was kept the same as before weaning. At 47 days of age, half of the groups of pigs experienced a switch in housing type, and the other half did not. Thus, after the switch, there were four housing treatment groups: barren-barren (B1B2), barren-enriched (B1E2), enriched-enriched (E1E2) and enriched-barren (E1B2), n=8 pens per group. In the home pen observations, environmental enrichment increased time spent on exploring, chewing and play, and decreased damaging behaviours directed at pen mates, such as tail biting and ear biting, and pen-directed exploring and chewing. Behaviour of pigs that switched from barren to enriched pens or vice versa reflected not only their current environment, but also their early life housing conditions. Effects of early life and current enrichment on most behaviours were in opposite direction. The effects of (lack of) enrichment after the switch were therefore more pronounced in pigs that had experienced a different early life condition.   This might reflect frustration in pigs that switched from enriched to barren pens, and a ‘catching up’ effect in pigs that went from barren to enriched pens. For some behaviours this effect of the switch was affected by the coping style of the pigs.  For instance, the LR pigs that switched from enriched to barren pens showed more damaging behaviours directed at their pen mates than all other pigs. Enriched pigs gained more weight and had a higher feed intake in the first weeks post-weaning, suggesting that they were better able to cope with the weaning transition. Barren housed pigs had a lower body weight than enriched pigs just before the switch, after which growth was mainly determined by actual housing, with enriched kept pigs having a higher feed intake and final body weight. Barren housed pigs were expected to have a more negative affective state than enriched pigs, and even more so if they originated from an enriched early life environment. To investigate whether housing experiences in early life, and a (mis)match with current housing conditions, would have long-term effects on affective state, pigs were exposed to an attention bias test (ABT) and successive negative contrast test (SNC). In the ABT, current housing, but not early life housing, affected behaviour in a coping-style-dependent manner, as E2-HR pigs paid attention towards the threat more frequently, were more likely to utter high-pitched vocalisations and walked more compared to (part of) the other groups. This unexpected effect could be related to the larger contrast between the pigs’ home pen and the test environment for enriched housed pigs. The behaviour in the ABT may therefore have reflected a short-term effect of the test, rather than the long-term effect of housing, on affective state.  In the SNC, B1B2 pigs generally had a lower probability and higher latency to get the reward than other pigs. HR pigs ran overall slower than LR pigs. Reward downshift increased the latency and reduced the probability to get to the reward, but only in pigs exposed to barren conditions in early life, which thus were more sensitive to reward loss than pigs from enriched early life housing. To assess the effects of early life and current housing conditions on innate and adaptive immune competence, 64 pigs were primary  and secondary immunized with the same dose of KLH-TNP. Blood samples were collected, and IgM and IgG antibody responses and leukocyte subpopulations were measured. We found that both early life and later life enrichment, and, notably, a switch in housing conditions influenced specific antibodies and leukocyte subpopulations in pigs. Regarding the relative frequencies of leukocyte subsets, the immune system seemed to respond to a change in housing condition, in either direction, rather than to the long-term effects of housing per se. Lastly, the relationships between immunity and behaviour and/or affective states were explored, indicating that play behaviour in the home pen and growth were found to relate to immunity to some extent. To conclude, this thesis confirms that environmental enrichment has positive effects on the behaviour of pigs. It also indicates that switching from enriched to barren conditions is more detrimental for pigs’ behaviour than housing in barren pens throughout. Effects of environmental enrichment on behaviour in tests of affective state were equivocal. Both early life and current housing, and a switch in housing conditions influenced specific antibody levels and the frequencies of leukocyte subpopulations in pigs, and the immune system seems to be alerted by changes in the environment.  The results of this thesis underline the importance of enrichment for pigs, as well as a fit between their early and later life housing conditions.