COMPARISON OF PROTECTION OF EXPERIMENTALLY CHALLENGED CATTLE VACCINATED ONCE OR TWICE WITH A PASTEURELLA-HAEMOLYTICA BACTERIAL EXTRACT VACCINE

Three groups of calves (15-18 per group) were injected twice at a 3-we ek interval with 2 doses of phosphate buffered saline (PBS, CONTROL gr oup), 2 doses of PRESPONSE, a Pasteurella haemolytica Al bacterial ext ract vaccine (PRESPONSE-2 group) or 1 dose of PBS followed by a 2nd va ccination with 1 dose of PRESPONSE (PRESPONSE-1 group), Three weeks af ter the 2nd vaccination, the calves were challenged intratracheally wi th P, haemolytica Al, Calves were evaluated clinically for 3 days prio r to challenge and for 5 days after challenge, Six days postchallenge, calves were either euthanized or sent to slaughter and the lungs were evaluated for percent pneumonic tissue. There was a significant effec t of single or double application of vaccine on clinical scores (P = 0 .0409), Percent pneumonic tissue at necropsy was significantly affecte d by vaccine group (P = 0.014), Calves in the CONTROL group had signif icantly higher percent pneumonic tissue after arcsine transformation ( 45.30%) than calves in any group receiving PRESPONSE, regardless of va ccination frequency (25.18% and 25.78%, for calves receiving 2 doses o r 1 dose of PRESPONSE, respectively), Both serum toxin neutralizing an d direct agglutinating titers were negatively correlated with percent pneumonic tissue, Most importantly, 1 dose of PRESPONSE was as efficie nt as 2 doses at eliciting a protective immune response. It is conclud ed that the presence of P. haemolytica as a natural commensal in the u pper respiratory tract of the calf can effectively prime the animal, a nd allow the animal to respond in an anamnestic nature to only 1 dose of this vaccine.