“Treated and untreated cows housed side by side in tie-stalls and their respective risk of harboring E. coli resistant to antimicrobials” v1

Parenteral antimicrobialtreatment results in the excretion of antimicrobial-resistant bacteria. Dairycows are commonly housed side by side in tie-stalls and often receiveantimicrobial treatment. However, studies investigating treated cows as sourceof colonization of neighboring cows with resistant bacteria are scarce.Antimicrobial resistance (AMR) in cows (treated and untreated) in tie-stalls wasinvestigated to assess their respective risks of carrying resistant bacteria.Furthermore, we analyzed associations of farm management with AMR. Case-controlstudy: For isolation of indicator Escherichia (E.) coli, rectal swabsamples were taken. Cows were sampled depending on treatment history and proximity to one another (cow A: recently treatedparenterally; cow B: untreated, next to cow A; cow C: untreated, atconsiderable distance from all treated cows). Antimicrobial susceptibility wastested by microdilution. Associations of AMR with exposure to cow A, treatments,and management were analyzed using generalized mixed-effects logistic models. Susceptibilitydata on 571 isolates from 131 dairy farms were obtained. Almost no differencein proportions of resistant E. coli was observed between cows B and C (B:53.4%; C: 57.2%; P=0.52). Untreated cows had lower odds of carryingresistant E. coli than treated cows (B: OR 0.44, P<0.001; C:OR 0.54, P=0.007). Non-pansusceptibility of isolates was associated withantimicrobial treatment (1 treatment: OR 2.11, P=0.001; ≥2: OR 1.76, P=0.043).Using manure on forage crops was associated with higherodds of pansusceptibility (OR 2.01, P=0.004). For daily practice, withregard to the risk of AMR transmission, results of this study do not provideevidence for the need to separate treated cows from others during treatment intie-stalls.