Abstract
Elaboration of heat-labile toxin (PMT) is an important virulence factor in some isolates of Pasteurella multocida from rabbits. Previously, we reported that immunization with inactivated PMT (IPMT) stimulated protective immunity to challenges from PMT. To test the hypothesis that immunization with a commercial swine vaccine containing IPMT stimulates similar protective immunity, groups of five rabbits were inoculated twice intramuscularly (i.m.), 10 days apart, with 0.5 ml of sterile saline or a commercial swine P. multocida bacterin-toxoid (BT). In addition, a group was inoculated intranasally with 5 μg of IPMT. Serum and nasal lavage samples were taken on days 0, 7, 14 and 21 after initial immunization and assayed by ELISA for anti-PMT antibody. Serum IgG and nasal lavage IgA were detectable by day 14 in BT and IPMT-immunized rabbits, but not in the saline controls. Groups of similarly inoculated rabbits were then challenged intranasally with 28 μg of PMT 21 days after initial immunization, and necropsied 7 days later, along with control challenged and non-challenged rabbits. Histological lesion severity was graded on a numerical scale. Nonimmunized and saline, challenged controls developed more severe pneumonia, pleuritis, nasal turbinate atrophy and testicular atrophy than IPMT and BT-immunized rabbits. The results confirm the hypothesis that immunization with a commercial swine P. multocida BT confers protective immunity in rabbits against challenges from PMT.