Associations between antimicrobial treatment modalities and antimicrobial susceptibility in Pasteurellaceae and E. coli isolated from veal calves under field conditions.

Antimicrobial consumption, with bovine respiratory disease as main indication, is higher in the veal calf industry compared to other livestock production branches. The aim of the present study was to investigate possible associations between antimicrobial drug use and resistance in Pasteurellaceae and indicator Escherichia (E.) coli from veal calves under field conditions in a prospective trial. Over a period of one year, nasopharyngeal and rectal swabs were collected from 2587 animals on 12 and 43 farms, respectively. Antimicrobial susceptibility testing was performed on 346 Mannheimia (M.) haemolytica, 1162 Pasteurella (P.) multocida and 2138 E. coli. Drug use was quantified as treatment incidence for each farm based on the used daily dose methodology (TIUDD), separately for group and individual treatments, and for antimicrobial classes. In multivariable mixed logistic regression analyses, risk factors could be identified for reduced susceptibility to certain antimicrobial classes. Group treatment was generally associated with higher rates of not susceptible (NS) M. haemolytica and P. multocida and non-wildtype (non-WT) E. coli. Individual treatment was associated with less NS and non-WT isolates. Age and entry protocol were important confounders with younger animals showing higher rates of NS and non-WT strains. The present findings suggest that, under field conditions, targeted individual treatment of calves can reduce the development of antimicrobial resistance compared to oral group treatment. For the different microorganisms, risk factors for resistance were partially different. This demonstrates that indicator organisms like E. coli do not necessarily reflect the associations observed in respiratory pathogens.