Antimicrobial resistance in Escherichia coli isolates from calf feces and associations with herd-level factors on Swiss dairy farms.

The objective of this study was to evaluate possible associations between the practice of feeding waste milk from cows treated with antimicrobials (WMA) to calves and antimicrobial resistance (AMR) to at least one drug, or to 3 or more antimicrobial classes (multidrug resistance [MDR]) of commensal Escherichia coli in calf feces on Swiss dairy farms. This study was a repeated cross-sectional study. A total of 58 farms were enrolled, including 30 on which WMA was fed to calves (WMA farms) and 28 on which no WMA (i.e., only saleable milk or milk replacer without antimicrobials) was fed to calves (control farms). The farms were visited 4 times at intervals of ∼3 mo between January 2022 and February 2023. At each visit, feces from 1 to 3 calves (30-60 d old, selected in decreasing age order) were collected and pooled. Three E. coli isolates per sample were randomly selected, and MIC were determined by broth microdilution. Herd-level antimicrobial use (AMU) estimates were calculated for the month before each sampling. Data from 601 E. coli isolates obtained from 204 visits on 57 farms were included. Among the pooled fecal samples from the WMA farms, 45.9% were from calves that had not received any WMA. The proportions of all isolates showing AMR and MDR were 47.1% and 24.1%, respectively. The proportion of MDR isolates was 27.2% on WMA farms and 20.6% on control farms, but the odds of MDR did not significantly differ (WMA farms: odds ratio [OR] 2.3, 95% CI 0.7-8.0). Similarly, the odds of MDR did not significantly differ between isolates from pooled samples from calves on WMA farms which had received WMA and isolates from unexposed calf samples on WMA and control farms (exposed calves: OR 1.6, 95% CI 0.5-5.5). A low frequency of calves' exposure to WMA on WMA farms may have contributed to the lack of observed significant association between the feeding of WMA to calves and AMR outcomes in calves' fecal E. coli on small dairy farms in this study.