Experimentally induced pneumonic pasteurellosis: dose-response relationships and protection against natural reinfection in calves.

Calves were inoculated intranasally with 2 X 10(6.2) tissue culture infective doses of infectious bovine rhinotracheitis virus, followed in 7 days by intratracheal inoculations with 1 of 4 challenge doses of pathogenic Pasteurella haemolytica. Severity and duration of the ensuing clinical signs of respiratory tract disease were correlated with the challenge dose of bacteria. Calves given 1 X 10(6) colony-forming units (CFU) of bacteria did not develop reliable clinical evidence of disease, whereas those given 1 X 10(8) CFU or 1 X 10(10) CFU of bacteria developed clinical signs of pneumonic pasteurellosis within 12 to 24 hours of bacterial challenge. Severity of clinical signs was equal at the 10(8) and 10(10) doses of bacteria, but duration of clinical signs was greater in calves given the 10(10) dose. Calves given 1 X 10(12) CFU of bacteria developed relatively severe respiratory tract disease in excess of what was necessary for positive clinical detection. Positive correlations were found between the bacterial challenge dose and the height and duration of increased rectal temperature, amount and duration of increases in ocular and nasal discharges, and the subjective evaluation of depressed attitude and appetite. Correlations were not found between challenge dose and respiratory rate or character, or between challenge dose and complete blood cell count. Convalescent calves were resistant to naturally occurring pneumonic pasteurellosis, which caused severe disease in nontreated calves. Adverse effects of P haemolytica were not observed after the first 4 to 15 days after bacterial administration; however, the bacteria were isolated from nasal secretions of convalescent calves 89 to 116 days after bacterial inoculation.(ABSTRACT TRUNCATED AT 250 WORDS)