The effects of three metaphylactic drugs on the nasal prevalence of Pasteurellaceae throughout the feeding period

Bovine Respiratory Disease (BRD) is a substantial cause of morbidity in cattle feedlots. Antimicrobial metaphylaxis is widely used to prevent disease and improve cattle growth performance, but its effects on the isolation of BRD-associated bacteria are not well defined. The aim of this study was to quantify the effects of three common metaphylactic drugs on the isolation frequency of members of the Pasteurellaceae throughout the feeding period. Upon feedlot arrival, cattle at high risk of developing BRD (n = 246) were randomized into four treatment groups: control, florfenicol, ceftiofur, and tulathromycin. Deep nasal swabs were collected upon arrival, approximately 1 month on feed, and prior to harvest (∼245 days on feed). Bacteriological culture was performed on all swabs, and colonies with morphology consistent with BRD-associated bacteria were subjected to MALDI-TOF MS for final confirmation. Nasal shedding prevalence over time was modeled with GEE multinomial logistic regression. An overall metaphylaxis effect was observed (p < 0.01). Pasteurellaceae nasal shedding prevalence did not differ significantly between experimental groups at one month on feed. By contrast, florfenicol-treated cattle had a greater proportion of samples without isolation than ceftiofur-treated cattle (10% versus 2%, p < 0.05). At the end of the feeding period, tulathromycin- and florfenicol-treated cattle had the least proportion of Pasteurellaceae isolates (28% and 32%, respectively, p < 0.05). Taken together, our findings highlight the differing effects of metaphylactic drugs in decreasing the nasal prevalence of bacterial pathogens in high-risk feedlot cattle.