The transmission of antimicrobial residues and occurrence of resistant bacteria after oxytetracycline treatment of veal calves in relation to animal keeping practices.

The use in animal husbandry of antimicrobials to treat bacterial infections remains unavoidable. However, the use of antimicrobials provokes antimicrobial resistance (AMR), therefore, there is a continuing effort to reduce veterinary antimicrobial use. A potential challenge is the transmission of residues and AMR from treated to untreated animals housed in the same environment. Taking this into consideration, alternative animal keeping practices should be explored. So far, there is little published research aiming to abate, especially the transmission of antimicrobial residues from treated to untreated animals and their environment. In this study, two animal husbandry practices were investigated, exploring the effect of physical separation on the transmission of oxytetracycline residues and the occurrence of antimicrobial resistance. Residue levels were analysed in internal (manure, urine, blood, and saliva) and external (wipes and hair) matrices. Additionally, antimicrobial resistance of Escherichia coli isolated from manure and the environment was assessed. When calves were housed in the same pen during treatment, residues were found not only in the treated animal but also observed in all tested matrices (internal and external) of the untreated pen-mate. When calves were physically separated during treatment to the end of the withdrawal period, no residues were detected in internal matrices of the untreated animals, indicating that transmission can be effectively prevented. Contamination of the pen and the exterior of the animals is also drastically reduced by the separation of animals during treatment and the withdrawal period. AMR results seem to indicate that tetracycline resistance levels return more or less to pre-treatment levels after the withdrawal period. In conclusion, it is recommended to physically separate calves for the duration of both treatment and the withdrawal period, in order to minimise and/or prevent transmission of oxytetracycline residues to untreated animals.