Development and application of an aerosol challenge method for reproduction of avian colibacillosis.

We have developed a reliable aerosol challenge method for reproduction of avian colibacillosis. This method involves intranasal administration of infectious bronchitis virus (IBV) to 1-day-old birds followed by three aerosol administrations of .E. coli at 3 to 4-day intervals. In four separate experiments there was no significant difference in virulence for a single isolate of .E. coli (E3) whereas four other field isolates of .E. coli ranged from highly virulent to avirulent. We also observed that the dominant .E. coli population isolated from 1-day-old birds in a flock of broiler breeders was highly virulent but that the dominant .E. coli population isolated from birds in the same flock 2 weeks later was avirulent. The dominant .E. coli population isolated from 1-day-old birds in this flock was sensitive to all of the antimicrobial agents tested whereas the dominant .E. coli population in the 2-week-old birds was multiresistant. A previous study has shown that it is likely that the changed resistance pattern of the dominant .E. coli population in the flock was linked to the administration of antimicrobial agents.