Antimicrobial susceptibility in E. coli and Pasteurellaceae at the beginning and at the end of the fattening process in veal calves: Comparing 'outdoor veal calf' and conventional operations.

Animal husbandry requires practical measures to limit antimicrobial resistance (AMR). Therefore, a novel management and housing concept for veal calf fattening was implemented on 19 intervention farms (IF) and evaluated regarding its effects on AMR in Escherichia (E.) coli, Pasteurella (P.) multocida and Mannheimia (M.) haemolytica in comparison with 19 conventional control farms (CF). Treatment intensity (-80%) and mortality (-50%) were significantly lower in IF than in CF, however, production parameters did not differ significantly between groups. Rectal and nasopharyngeal swabs were taken at the beginning and the end of the fattening period. Susceptibility testing by determination of the minimum inhibitory concentration was performed on 5420 isolates. The presence of AMR was described as prevalence of resistant isolates (%), by calculating the Antimicrobial Resistance Index (ARI: number of resistance of one isolate to single drugs/total number of drugs tested), by the occurrence of pansusceptible isolates (susceptible to all tested drugs, ARI=0), and by calculating the prevalence of multidrug (≥3) resistant isolates (MDR). Before slaughter, odds for carrying pansusceptible E. coli were higher in IF than in CF (+65%, p=0.022), whereas ARI was lower (-16%, p=0.003), and MDR isolates were less prevalent (-65%, p=0.001). For P. multocida, odds for carrying pansusceptible isolates were higher in IF before slaughter compared to CF (+990%, p=0.009). No differences between IF and CF were seen regarding the prevalence of pansuceptible M. haemolytica. These findings indicate that easy-to-implement measures to improve calf management can lead to a limitation of AMR in Swiss veal fattening farms.