Development of an aerosolized Mannheimia haemolytica experimental pneumonia model in clean-catch colostrum-deprived calves.

Mannheimia haemolytica is an important cause of bovine respiratory disease (BRD). BRD is usually a multifactorial disease with host factors and viral infections influencing pathogenesis. Previous studies that have attempted to experimentally induce pneumonia using aerosolized M. haemolytica alone have produced inconsistent results, yet an aerosol model would be useful to study the details of early infection and to investigate the role of innate defences in pathogenesis. The objective of these studies was to develop and characterize an aerosolized M. haemolytica disease model. In an initial study, conventionally raised calves with higher levels of antibody against M. haemolytica leukotoxin developed acute respiratory distress and diffuse alveolar damage, but did not develop bronchopneumonia, following challenge with M. haemolytica serotype 1. Clean-catch colostrum-deprived calves challenged with 1 × 1010 colony forming units of M. haemolytica serotype 1 consistently developed bronchopneumonia, with elevations in rectal temperature, serum haptoglobin, plasma fibrinogen, and blood neutrophils. Mannheimia haemolytica serotype 1 was consistently isolated from the nasal cavities and lungs of challenged calves. Despite distribution of aerosol and isolation of M. haemolytica in all lung lobes, gross lesions were mainly observed in the cranioventral area of lung. Gross and histologic lesions included neutrophilic bronchopneumonia and fibrinous pleuritis, with oat cells (necrotic neutrophils with streaming nuclei), and areas of coagulative necrosis, which are similar to lesions in naturally occurring BRD. Thus, challenge with M. haemolytica serotype 1 and use of clean-catch colostrum-deprived calves with low or absent antibody titres allowed development of an effective aerosol challenge model that induced typical clinical disease and lesions.