EFFECT OF VACCINATION WITH LIVE OR KILLED PASTEURELLA-HAEMOLYTICA ON RESISTANCE TO EXPERIMENTAL BOVINE PNEUMONIC PASTEURELLOSIS

Abstract

Using 6-8 mo.-old beef calves, 3 experiments were conducted to compare the effect of vaccination with live or killed Pasteurella haemolytica on resistance to a transthoracic challenge exposure with the organism and to correlate serum antibody response with resistance. Calves were vaccinated twice at 1-wk intervals and were challenge-exposed 21 days after the 1st inoculation. Lung lesions were evaluated by a system, such that higher scores indicated the more severe lesions. Calves immunized with live P. haemolytica had lower lesion scores than calves vaccinated with saline solution or bacterin. In 2 of the experiments, the differences were significant (P < 0.05). In all experiments, calves vaccinated parenterally with a commercial P. haemolytica/P. multocida bacterin or with a formalin-killed P. haemolytica bacterin had lesion scores that were not significantly different (P > 0.05) than for control calves vaccinated with saline solution. Live and killed bacterial preparations induced a significant serum antibody response to P. haemolytica as measured by a quantitative fluorometric immunoassay. The antibody response to vaccination was not affected by preexisting titers to P. haemolytica. Serum antibody titers were not consistently as high for calves vaccinated with bacterins as for calves vaccinated with live organisms. Although high antibody titers correlated with low lesion scores when calves vaccinated with saline solution or live organisms were analyzed collectively, there was not a significant correlation between the 2 variables when calves, vaccinated with saline solution or with bacterin, were analyzed collectively. Although bacterins may induce a detectabel serum antibody response, they do not induce protection against transthoracic challenge exposure to P. haemolytica.