Jar. Conlon et al., COMPARISON OF PROTECTION OF EXPERIMENTALLY CHALLENGED CATTLE VACCINATED ONCE OR TWICE WITH A PASTEURELLA-HAEMOLYTICA BACTERIAL EXTRACT VACCINE, Canadian journal of veterinary research, 59(3), 1995, pp. 179-182
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.