How stress alters immune responses during respiratory infection.

Fall-weaned calves entering the feedlot experience a variety of psychological and physical stressors, including maternal separation, transportation, social mixing, restraint, and dietary changes. Mixing calves from multiple sources also exposes them to respiratory pathogens at a time when maternal immunity has waned. Using an experimental bovine respiratory disease (BRD) challenge, we analyzed the effects of specific stressors on clinical disease and immune responses following bovine herpes virus (BHV-1/IBR) infection of naïve calves. Transportation stress was compared to either abrupt weaning plus transportation or transportation following a two-step weaning process. Transportation alone significantly (P < 0.05) increased BHV-1 shedding in nasal secretions despite elevated interferon-gamma production in the upper respiratory tract. In contrast, abrupt weaning and transportation, significantly (P < 0.05) increased serum haptoglobin on day 3 post-infection (PI) and blood leukocyte tumor necrosis factor α secretion on day 5 PI. These systemic responses were reduced by instituting a two-step weaning process 4 days prior to transportation and BHV-1 infection. In conclusion, these observations are consistent with earlier studies implicating weaning and transportation as stressors contributing to BRD severity and mortality. Current studies also revealed that different stressors or combination of stressors have distinct effects on host responses to viral infection in naïve calves.