Studies on Pasteurella multocida. IV. Immunofluorescence detection of the organisms in spleen and lungs of turkeys vaccinated with live oral vaccines

Oral vaccination of turkeys with live avirulent strains of P. multocida (M-2283 and CU strain) resulted in the local as well as systemic dissemination of the organisms. The persistence of P. multocida in the lungs and splenic tissues of these vaccinated turkeys was demonstrated by the indirect immunofluorescence technique. All of the tissues examined up to the first week post vaccination were P. multocida positive. Cryostat sections of lungs from birds vaccinated with the avirulent strain M-2283 were negative at 2 weeks post vaccination, while the spleen continued to be positive up to the third week post vaccination. In contrast to the group of turkeys vaccinated with strain M-2283, lung and spleen cryostat sections from turkeys vaccinated with strain CU remained positive up to the fourth week post vaccination. Tissues examined thereafter were negative for the presence of P. multocida from both groups.The major immune mechanism in the defense against fowl cholera is still unknown. If local immunity is primarily responsible, the CU strain may be the better vaccine strain as it persists in the lungs for 2 weeks longer than the M-2283 strain. However, if systemic immunity is chiefly responsible for immunity, both strains could protect equally, since they persist in the spleen for approximately the same period of time.