Vaccine development for the prevention of
pneumonic pasteurellosis remains a critical issue for the feedlot industry. Most currently available Pasteurella
vaccines are formulated to stimulate immunity by either providing an adequate antigenic mass in the administered dose, or by relying on subsequent production of
antigens by in vivo growth of live organisms. The ability of these different types of
vaccines to stimulate rapid and high titres to key
antigens is a key factor that will influence subsequent resistance to disease. The serologic and protective responses to a
streptomycin-dependent, modified-live
vaccine and a killed (
bacterin-
toxoid)
vaccine against experimental
pneumonic pasteurellosis were compared. Calves were vaccinated with a single injection of either a test
vaccine or
phosphate-buffered saline, challenged 14 d later by transthoracic injection with Pasteurella haemolytica, and euthanized 3 d post-challenge to evaluate the severity of
pneumonia. On days 0, 7, and 14, serologic responses to various P. haemolytica
antigens, including cell-associated and soluble
antigens, were determined by
enzyme-linked
immunosorbent assays, and anti-
leukotoxin antibody levels were determined by
leukotoxin neutralization. The
bacterin-
toxoid elicited significantly greater serologic responses compared to controls for all
antigens. The modified-live
vaccine elicited a significantly greater response compared to controls for a whole-cell
antigen preparation. Lesion scores were significantly smaller (greater protection) in calves that received the
bacterin-
toxoid, but not the modified-live
vaccine, compared to controls.