The effects of stressful exercise on leukocytes in cattle with experimental pneumonic pasteurellosis.

Seven yearling bulls were treated with stressful exercise and intrabronchial Pasteurella haemolytica A1. Group 1 bulls (nos. 1-4) underwent treadmill exercise and, 24 days later, intrabronchial instillation of P. haemolytica A1. Group 2 bulls (nos. 5-7) underwent treadmill exercise, followed 30 min later by intrabronchial P. haemolytica A1. Blood lactic acid values were raised (p less than 0.05) by treadmill exercise only, but plasma cortisol was raised (p less than 0.05) by treadmill exercise and by P. haemolytica A1 infection. Neutrophils in bronchoalveolar lavage (BAL) differed from control values 24 h after treadmill exercise, and 1 h and 4 h after P. haemolytica A1 infection. Respiratory disease was more severe and the gross lung lesions were larger in group 2 bulls than in group 1 bulls. P. haemolytica A1 was recovered from the livers, spleens and mesenteric lymph nodes of group 2 but not group 1 bulls, suggesting that group 2 bulls had experienced bacteraemia. Decreased neutrophils in BAL fluid from group 2 bulls at 1 h and 4 h after infection suggests that exercise transiently inhibited neutrophil egress from the blood to the alveoli; BAL neutrophils peaked at 1 h and 4 h after infection in group 1 bulls but declined at 24 h. We conclude that group 2 bulls were made more susceptible to experimental pneumonic pasteurellosis by stressful exercise.