Evaluation of protective immunity induced by experimental subunit and commercial vaccines against pneumonic pasteurellosis in cattle

The objective of this study was to evaluate the magnitude of the immune response and protective efficacy of Pasteurella haemolytica Al vaccines in calves using 3 experimental subunit vaccines (ESV), and 3 commercial vaccines (CV) available in the USA. The 3 ESV were: (a) culture supernatant (CS) containing leukotoxin (Lkt), lipopolysaccharide (LPS), and capsular polysaccharide (CP); (b) sodium salicylate extract (SSE) containing iron regulated outer membrane proteins (IROMPs), LPS, and CP, (c) and a combination (CB) of the above two. The 3 CV were: (a) One Shot (SmithKline Beecham, West Chester, PA); (b) Presponse (Langford Laboratories, Guelph, Ontario, Canada); and (c) Once PMH (BioCor Inc., Omaha, NE). Vaccine efficacy was defined in terms of reduction in clinical scores and pneumonic lesion scores (PLS) after intrapulmonic challenge with live log phase P. haemolytica Al organisms. The results indicate that the CS vaccine elicited antibodies against both Lkt and CP, while the SSE vaccine elicited antibodies against IROMPs and CP. Calves immunized with the CB vaccine showed increased levels of antibodies against IROMPs, Lkt, and CP. The results in the calves vaccinated with CV indicate that One Shot elicited antibodies against Lkt, CP, while Presponse and Once PMH elicited antibodies against CP. There was significant correlation between lung and serum antibodies against Lkt, CP and IROMPs in both ESV and CV animals. Animals that received the CB vaccine had significantly lower mean PLS as compared to CS, SSE, and control groups. The animals which received CS vaccine had mean PLS significantly lower than that of the control group. Animals that received One Shot had significantly lower mean PLS as compared to the control group. A strong negative correlation existed between serum antibody levels against Lkt and PLS in both vaccine studies, but high antibody levels against. CP did not correlate consistently with reduced PLS.