COMPARISON OF SEROLOGIC AND PROTECTIVE RESPONSES INDUCED BY 2 PASTEURELLA VACCINES

Vaccine development for the prevention of pneumonic pasteurellosis rem ains a critical issue for the feedlot industry. Most currently availab le 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 stimul ate rapid and high titres to key antigens is a key factor that will in fluence subsequent resistance to disease. The serologic and protective responses to a streptomycin-dependent, modified-live vaccine and a ki lled (bacterin-toxoid) vaccine against experimental pneumonic pasteure llosis were compared. Calves were vaccinated with a single injection o f either a test vaccine or phospate-buffered saline, challenged 14 d l ater by transthoracic injection with Pasteurella haemolytica, and euth anized 3 d post-challenge to evaluate the severity of pneumonia. On da ys 0, 7, and 14, serologic responses to various P. haemolytica antigen s, including cell-associated and soluble antigens, were determined by enzyme-linked immunosorbent assays, and anti-leukotoxin antibody level s were determined by leukotoxin neutralization. The bacterin-toxoid el licited 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 prepara tion. Lesion scores were significantly smaller (greater protection) in calves that received the bacterin-toxoid, but not the modified-live v accine, compared to controls.