Altered circulating levels of serotonin and immunological changes in laying hens divergently selected for feather pecking behavior

The aim of this study was to investigate the changes in immunological parameters as well as changes with respect to plasma levels of serotonin and tryptophan in lines selected for and against feather pecking (FP) behavior [high FP (HP) line and low FP (LP) line] for 5 generations. The hens from the HP line had a higher plasma serotonin level than those from the LP line (0.059 VS. 0.037 mu mol/L, F-2,F-27 = 0.031, P < 0.05). The plasma level of tryptophan was, on average, 67.30 mu mol/L and did not differ between the lines (68.3 vs. 66.3 mu mol/L, F-2,F-28 = 0.36, P > 0.05). The HP line had a higher response to infectious bursal disease virus vaccination after 1 wk post-vaccination compared with the control and LP lines. The number of white blood cells (P < 0.0001) and the expression of MHC class I molecules on CD4 (P < 0.02), CD8 beta (P < 0.006) and on B cells (P < 0.03) were highest in the LP line compared with the control and HP lines. Selection for or against FP, therefore, changes the number of white blood cells and the expression of MHC class I molecules on T and B cells, which may influence the health status of the birds.