Distribution of Pasteurella haemolytica and Pasteurella multocida in the bovine lung following vaccination and challenge exposure as an indicator of lung resistance.

Experimental calves were vaccinated with virulent strains of Pasteurella haemolytica or Pasteurella multocida or with phosphate-buffered saline solution either by an aerosol method or by subcutaneous injection. Calves were subsequently challenge exposed by intrapulmonic inoculation of the homologous virulent Pasteurella species. Sections obtained from the resulting pulmonic lesion were stained, using a fluorescent antibody technique, to determine relative number, location, and integrity of the challenge organism. The resistance of the calf to challenge exposure, as determined by other factors, was compared with the capacity of the components of the lung to engulf or destroy pasteurellae. Calves vaccinated with an aerosol of the bacterium were most resistant to challenge exposure; most bacteria were engulfed or degraded by the phagocytic cells. Vaccination by subcutaneous injection was less effective in inducing resistance. Tissue sections from these calves contained many more extracellular intact bacteria and fewer intracellular intact or degraded bacteria than were seen in the sections of calves vaccinated by the aerosol method. The control calves were the least resistant; bacteria seen in tissue sections from these calves were numerous, predominantly extracellular, and intact. A group of nonvaccinated calves experimentally inoculated with infectious bovine rhinotracheitis virus 5 days before intrapulmonic challenge exposure with P haemolytica developed severe pulmonic lesions. The lesions were larger and more invasive and contained many more extracellular bacteria than did the lungs of calves in control groups. As in other nonvaccinated calves, there were few intracellular bacteria; however, unlike in other calves, the extracellular bacteria were seen in large numbers, particularly in alveolar lumens.