Efficacy of an outer membrane protein of Pasteurella haemolytica A2, A7 orA9-enriched vaccine against intratracheal challenge exposure in sheep

The outer membrane proteins (OMP) were extracted from the P. haemolytica A2 , A7 and A9 to determine their potential as immunogens and their capability for cross-protection. Sixty lambs of approximately 9 months old were divid ed into four main groups. Animals in Group 1 were vaccinated with 2 ml vacc ine containing 100 mu g/ml of 1:he outer membrane proteins of P. haemolytic a A2. Animals in Group 2 were similarly vaccinated with the OMPs of P. haem olytica A7 while Group 3 with OMPs of P. haemolytica A9. Animals in Group 4 were unvaccinated control. During the course of the study, serum was colle cted to evaluate the antibody levels toward each OMP. There appeared to be good immune responses. However, high antibody levels did not necessarily re sult in good protection of the animals, particularly against cross-infectio n with P. haemolytica A9 in animals vaccinated with the OMPs of P. haemolyt ica A2. It seemed that the antibody responses were more specific toward the homologous challenge but generally did not cross-protect against heterolog ous serotype challenge. However, the OMPs of P. haemolytica A7 produced goo d in vivo cross-protection and excellent correlations when good antibody re sponses against all serotypes led to successful reductions of the extent of lung lesions following homologous and heterologous challenge exposures. Th us, the OMPs of P. haemolytica An was effective in protecting animals again st homologous and heterologous infection by live P. haemolytica A2, A7 and A9. (C) 2000 Elsevier Science B.V. All rights reserved.