Dominance status assessed using a food competition test influences plasma cortisol levels and cell mediated immunity in growing pigs

B.M. Hjarvard¹, O.N. Larsen², H.R.J. Madsen1 and K.H. Jensen¹

¹Department of Animal Health, Welfare and Nutrition, Danish Institute of Agricultural Sciences, Research Centre Foulum, Tjele, Denmark
²Institute of Biology, University of Southern Denmark, DK-5230 Odense M, Denmark

The pig is used in human medicine due to its physiological similarities with humans. Hence, the purpose of this study was to investigate whether rank, a psychosocial factor, could account for endocrine and immune differences among group members in a stable social environment and thus present a natural stress model.

Subjects were female growing pigs (n=36) housed in groups of four. Rank was assessed using a food-competition-test with precedence to the feeder as the determining variable. Endocrine and immune functions were evaluated by measuring plasma cortisol and cell immune phenotypes. The percentage of cells marked for TLR4, SLA II, CD4+ and CD8+ molecules was determined by flow cytometry. Plasma cortisol was determined by radioimmunoassay.

Mean growth was higher in both high (HR) and middle ranking (MR) pigs than in low ranking (LR) pigs. Plasma levels of cortisol related to rank (p<0.05) and were lower in MR pigs than in HR and LR pigs. The percentage of SLAII expressing cells varied with rank (p<0.05) and was highest in HR pigs. The percentage of CD4+ and CD8+ lymphocytes decreased over the experimental period. The combined effect of these changes was a decrease in CD4:CD8 ratio that tended to relate to rank (p<0.1). The decrease was most pronounced in low ranking (LR) pigs. The percentage of TLR4 marked cells showed no relationship with rank.

The rank-related inverse relationship between percentage of SLA II expression and difference in CD4:CD8 ratio suggests an interconnection between variables affecting the immune system. Thus, rank seems to affect immune and endocrine variables differently. The relationship between psychosocial factors and cellular immune system may have important implications for disease progression and for the management, treatment and selection of pig subjects for studies in which immunological variables are of interest.

Paper presented at Measuring Behavior 2005 , 5^th International Conference on Methods and Techniques in Behavioral Research, 30 August - 2 September 2005, Wageningen, The Netherlands.

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