The Effect of Aerosolized Bacterial Lysate on the Development of Pneumonia in Cattle

Bovine respiratory disease (BRD) is a significant problem affecting the health, welfare and productivity of beef cattle. Disease most commonly occurs shortly after arrival to the feedlot following a number of stressors including weaning, transportation, co-mingling, processing, and ration changes. This leads to stress and viral infections which contribute to alterations in immune responses and a high incidence of bacterial pneumonia. Control of BRD often relies on mass administration of antimicrobial medication to high-risk groups. There is increasing concern about antimicrobial use in food production and a need for alternative control strategies. The innate immune system responds rapidly to stimuli, and induced responses can protect against a number of different pathogens. Stimulation of the innate immune system was investigated as a potential target for intervention to prevent BRD. For proof of concept, respiratory innate responses were stimulated via delivery of aerosolized bacterial lysate. Pulmonary cellular and humoral responses and systemic responses were characterized. Lysate administration triggered cytokine production, neutrophil infiltration, and alterations in bronchoalveolar lavage fluid, and caused a mild and transitory systemic inflammatory reaction with no significant lung lesions. When delivered to clean-catch colostrum-deprived Holstein calves prior to aerosol Mannheimia haemolytica challenge, aerosolized bacterial lysate provided mild protection as evidenced by reduced neutrophilia early after infection and a tendency for reduced lung lesions at necropsy. However, lysate administration prior to challenge did not reduce nasal or lung M. haemolytica numbers. Unexpectedly, in high-risk feedlot steers, administration of bacterial lysate following arrival increased BRD. Lysate-treated cattle had increased serum haptoglobin and body temperature and reduced weight gain over the first 28 days. Lysate administration increased nasal Mycoplasma bovis and increased mortality due to M. bovis. The combined findings of these studies provide insight into the mechanisms behind increased susceptibility to respiratory disease in newly arrived feedlot cattle and suggest that stress and virus-induced pro-inflammatory responses may contribute to pathogenesis. Furthermore, this study suggests that pre-existing inflammation contributes to the development of BRD and that mortalities due to M. bovis occurring in the second month are influenced by factors initiated in the immediate post-arrival period.