Tracheal populations of Mycoplasma gallisepticum after challenge of bacterin-vaccinated chickens.

Chickens vaccinated once or twice with inactivated oil-emulsion vaccine against Mycoplasma gallisepticum (MG) or left unvaccinated were challenged intratracheally with the R strain of MG. The population of MG organisms was determined by enumerating tracheal cultures periodically up to 28 weeks postchallenge (PC). The number of organisms in the respiratory tract increased rapidly after 4 days PC, and the number tended to decrease after 4 weeks PC. Tracheal populations of MG varied considerably among individual chickens. Bacterin-vaccinated chickens had numerically lower populations of MG in their tracheas up to 8 weeks PC, but the differences were small and considered of little practical significance.