Response of broiler-type chickens to live Pasteurella multocida--duration of immunity and minimum dose

Broiler-type chickens were exposed to avirulent Pasteurella multocida by stick-wing once or twice, and groups were challenged with pathogenic P. multocida at 20, 50, or 80 weeks postexposure. Immunity was not lower when chickens were challenged at 80 weeks rather than at 20 and 50 weeks postexposure. Various doses of avirulent P. multocida were administered by stick-wing or subcutaneously in the back of the neck. Results of challenge at 20 weeks postexposure indicated that protection was reduced when exposure dose by either route was lower than 6.1 X 10(4) viable organisms.