AUTHOR=Werner Thomas , Käsbohrer Annemarie , Wasner Barbara , Köberl-Jelovcan Sandra , Vetter Sebastian G. , Egger-Danner Christa , Fuchs Klemens , Obritzhauser Walter , Firth Clair L. 

TITLE=Antimicrobial resistance and its relationship with antimicrobial use on Austrian dairy farms

JOURNAL=Frontiers in Veterinary Science

VOLUME=10

YEAR=2023

URL=https://www.frontiersin.org/journals/veterinary-science/articles/10.3389/fvets.2023.1225826

DOI=10.3389/fvets.2023.1225826

ISSN=2297-1769

ABSTRACT=<p>The aim of this study was to investigate the prevalence of ESBL/AmpC-producing <italic>E. coli</italic> and the resistance pattern of commensal <italic>E. coli</italic>, as well as the link between the use of antibiotics (AMU) and the occurrence of resistance in <italic>E. coli</italic> on Austrian dairy farms. AMU data from 51 farms were collected over a one-year period in 2020. Fecal samples were collected from cows, pre-weaned and weaned calves in 2020 and 2022. Samples were then analyzed using non-selective and selective agar plates, <italic>E. coli</italic> isolates were confirmed by MALDI-TOF analysis. Broth microdilution was used for antimicrobial susceptibility testing. The AMU of each farm was quantified as the number of Defined Daily Doses (nDDD<sub>vet</sub>) and Defined Course Doses (nDCD<sub>vet</sub>) per cow and year. Cephalosporins (mean 1.049; median 0.732 DDD<sub>vet</sub>/cow/year) and penicillins (mean 0.667; median 0.383 DDD<sub>vet</sub>/cow/year) were the most frequently used antibiotics on these farms, followed by tetracyclines (mean 0.275; median 0.084 DDD<sub>vet</sub>/cow/year). In 2020, 26.8% of the <italic>E. coli</italic> isolated were resistant to at least one antibiotic class and 17.7% of the isolates were classified as multidrug resistant (≥3 antibiotic classes). Out of 198 <italic>E. coli</italic> isolates, 7.6% were identified as extended-spectrum/AmpC beta-lactamase (ESBL/AmpC) producing <italic>E. coli</italic>. In 2022, 33.7% of <italic>E. coli</italic> isolates showed resistance to at least one antibiotic and 20.0% of isolates displayed multidrug resistance. Furthermore, 29.5% of the samples carried ESBL/AmpC-producing <italic>E. coli</italic>. In 2020 and 2022, the most frequently determined antibiotic resistances among commensal <italic>E. coli</italic> isolates were to tetracyclines, sulfonamides and penicillins. In addition, pre-weaned calves had the highest resistance rates in both years. Statistical analyses showed a significant association between low and high use AMU classifications for penicillins (in nDDD<sub>vet</sub>/cow/year) and their respective resistance among commensal <italic>E. coli</italic> isolates in 2020 (<italic>p</italic> = 0.044), as well as for sulfonamide/trimethoprim (<italic>p</italic> = 0.010) and tetracyclines (<italic>p</italic> = 0.042). A trend was also noted between the total amount of antibiotics used on farm in 2020 (by nDDD<sub>vet</sub>/cow/year) and multidrug resistances in commensal <italic>E. coli</italic> isolated on farm that year (<italic>p</italic> = 0.067). In conclusion, the relationship between AMU and antimicrobial resistance (AMR) on dairy farms continues to be complex and difficult to quantify.</p>