The virulence and protective efficacy for chickens of Pasteurella multocida administered by different routes

The relative virulence for chickens of five strains of Pasteurella multocida was evaluated. Twenty groups, each of ten chickens, were inoculated with a standard dose of 10(5) of each of five strains by the intramuscular (Im), intravenous (Iv), intratracheal (Itr) or conjunctival (Co) routes, The highest mortality occurred in the groups dosed Im and Iv, followed by Itr inoculation. The relative virulence of each strain did not change when inoculated by the different routes. The most virulent strain, VP161, caused 100% mortality by all except the Co route. The least virulent strain, VP17, caused a single mortality by the Iv route, but gave a high level of protection to birds inoculated by both the Im and Iv routes, when challenged by intramuscular injection with (VP161). There was no protection against Im challenge in the birds inoculated by the Itr or Co routes. Serum antibody levels measured by ELISA correlated with the level of protection against virulent challenge for groups inoculated Im or Iv, but not Itr. Western blots of pooled sera from each group did not show any specific antigen recognition that might explain the observed differences in protection. Inoculation with strain VP17, (both Im and Itr) also gave a high level of protection to birds challenged with strain VP161 by intratracheal instillation. (C) 2000 Elsevier Science B.V. All rights reserved