Short communication: Selection of extended-spectrum β-lactamase-producing Escherichia coli in dairy calves associated with antibiotic dry cow therapy-A cohort study

摘要

Antimicrobial residues in milk have been discussed asa possible selector for Enterobacteriaceae that produceextended-spectrum β-lactamases (ESBL) in dairy herds.Such residues are found in waste milk after antibiotictreatment of mastitis, but antibiotic dry cow therapymight also lead to antibiotic residues in colostrum andin milk during early lactation. While it is known thatfeeding of waste milk selects ESBL bacteria in calves,this was not investigated for colostrum yet, which is supposedto contain much lower antibiotic concentrationsthan waste milk. In this observational prospective casestudy on 2 farms, we hypothesized that blanket dry cowtreatment with β-lactams would have more selective(here: increasing) effects on ESBL concentrations thanselective (here: individually chosen) antibiotic dry cowtherapy. Thus, we compared concentrations of ESBLproducingEnterobacteriaceae in feces of calves (n = 50)at 2 dairy farms with different management of antibioticdry cow therapy. Considerably higher concentrationsof ESBL-producing Escherichia coli were observed inblanket antibiotic dry cow therapy on d 3 of the calf’slife (7.6 vs. 5.3 log cfu/g of calf feces). Both farms usednarrow-spectrum penicillin combined with aminoglycosidesfor drying off, and the majority of ESBL isolates(93%) were co-resistant to aminoglycosides. No wastemilk was fed to calves and no calf was treated withβ-lactam antibiotics or aminoglycosides during the first3 d of life, thus differences were most likely associatedwith different frequency of antibiotic dry cow therapyon farms (19 of 25 mother cows on farm A, 9 of 25 onfarm B). Even though the presumable selection effectof antibiotics used for drying off decreased within thenext 3 wk, this result further emphasizes the need forthe reduction and prudent use of antibiotic dry cowtherapy on farms.