Behavioural, endocrine and immune consequences of mixing in weaned piglets
Introduction
Several practices occurring in modern pig husbandry can affect the animal’s health and welfare. In growing or adult pigs, mixing of unfamiliar animals induces fighting (Rushen and Pajor, 1987, Moore et al., 1994, Otten et al., 1997), elevates heart rate (Otten et al., 1997, de Jong et al., 2001), plasma catecholamines (Otten et al., 1997, Ruis et al., 2001), body temperature (de Jong et al., 1999) and plasma cortisol (Moore et al., 1994, Deguchi and Akuzawa, 1998, Ruis et al., 2001) and reduces growth rate (Rundgren and Löfquist, 1989). Social reorganisation can also have immune consequences. It affects skin response to antigens (Moore et al., 1994), increases neutrophils/lymphocytes blood ratio (Moore et al., 1994, Ruis et al., 2001) and suppresses peripheral blood mononuclear cell proliferation (Hessing et al., 1995, Deguchi and Akuzawa, 1998, de Groot et al., 2001). Physiological consequences are usually transient, but there could be long-lasting behavioural alterations in defeated animals (Ruis et al., 2001).
In commercial piggeries, weaning often combines several stressful events such as a sudden change of diet, a move to a new housing environment, the mother-young link disruption and mixing with unfamiliar piglets. These sudden changes induce behavioural, endocrine and immune alterations (Blecha et al., 1985, Puppe et al., 1997, Carroll et al., 1998, Worobec et al., 1999) and are followed by a transient reduction of growth and digestive disorders (Madec et al., 1998, Carroll et al., 1998). Different stress factors involved in weaning including new housing environment (Metz and Gonyou, 1990, Puppe et al., 1997, Patience et al., 2000) and social reorganisation (Friend et al., 1983, Blecha et al., 1985, Puppe et al., 1997) have been studied. Studies showed that mixing piglets at weaning increases plasma cortisol concentrations and agonistic behaviour (Friend et al., 1983, Blecha et al., 1985, Puppe et al., 1997), but they failed to show any effect of social environment on other behavioural variables or on immune function (Blecha et al., 1985, Puppe et al., 1997). At weaning, the physiological effects of social reorganisation could be masked by the much more important effects of the diet change. In other words, the lack of effect of mixing would not mean that it is not perceived as a stressful event. The aim of this experiment was to evaluate the reactivity of piglets to mixing by dissociating social reorganisation from weaning in itself. For this purpose, we investigated the influence of mixing 5 days after weaning, when most of the acute physiological adjustments of weaning have disappeared (Metz and Gonyou, 1990, Puppe et al., 1997, Carroll et al., 1998). The response to mixing was assessed by measuring salivary cortisol, blood lymphocyte proliferation, interferon-γ production and behavioural activity.
Section snippets
Experimental housing and animals
Eight sows (Large White×Landrace) were chosen at farrowing and piglets were weaned at 28 days of age. Six healthy female pigs per litter were chosen at weaning according to weight (8.9±0.6 kg). Litter-mates were moved to a post-weaning experimental room and housed together in the same pen. The room consisted of twelve 1.2×1.3 m pens, with 90 cm high wooden partitions between the pens, preventing visual and physical contact with other pigs. Artificial light was provided between 08:00 and 18:00.
Physiological responses
Time-related variations of salivary cortisol are presented in Fig. 1. There was a significant treatment×time interaction (P<0.01). Analysis within each treatment group showed that the effect of time tended to be significant in the C group (P<0.06) and was significant in the M group (P<0.001). M pigs had significantly increased salivary cortisol concentrations in 1 h (P<0.01) and 3 h (P<0.05) after the beginning of the stress in comparison to C group. Cortisol concentrations returned to levels
Discussion
Relocation and social mixing were perceived as a stressful event, as shown by increased salivary cortisol levels. However, this elevation was modest and levels came back to control values within 8 h. This result is in accordance with previous studies in which social conflict did not induce long-term changes in plasma cortisol levels in pigs (Blecha et al., 1985, Deguchi and Akuzawa, 1998, Ruis et al., 2001, de Groot et al., 2001). Mixing can have different consequences whether the animal becomes
Acknowledgements
The authors thank Nathalie Le Floc’h, Isabelle Oswald, Philippe Pinton and Anne-Marie Mounier for their technical help and useful discussions, and Dr. Pierre Neveu for his critical review of the manuscript.
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