Use of a fluorometric immunoassay to determine antibody response to Pasteurella haemolytica in vaccinated and nonvaccinated feedlot cattle

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Abstract
A retrospective study of the antibody response to P. haemolytica was conducted by using sera from 368 feedlot cattle divided among 5 experiments. In 3 experiments, live vaccines or a bacterin were administered to some of the cattle; others were left as nonvaccinated controls. In 2 experiments, cattle were not vaccinated. Clinical signs of disease with subsequent recovery developed in 48.0% of the cattle, 10.3% of the cattle died. Vaccination had no apparent effect on morbidity or mortality. At the time of purchase, 78% of the cattle had low antibody titers (< 25) as measured by a quantitative fluorometric immunoassay. In most groups of cattle (vaccinated and nonvaccinated), there was a significant rise in mean antibody titers between the time of purchase and days 28-32 in the feedlot. The antibody titers at the time of shipment and days 28-32 and the ratio of the 2 antibody titers were compared with the health status of cattle. The antibody ratios were significantly greater for cattle that became sick and then recovered compared with those of cattle that remained healthy. Although significance could not be established, antibody titers at the time of shipment were higher for cattle that remained healthy compared with cattle that became sick and then recovered.