Susceptibility of specific pathogen-free lambs to concentrations of Pasteurella haemolytica serotype A2 in aerosols.

A 100-fold reduction in the numbers of organisms in an aerosol of Pasteurella haemolytica used to infect specific pathogen-free lambs did not alter the number of cases of pneumonia which resulted. In a separate experiment a further 10-fold reduction in the number of organisms in the aerosol did not cause fewer cases of pneumonia.