Residues of dry cow antimicrobials in colostrum and extended-spectrum and AmpC β-lactamaseproducing Escherichia coli in feces from young dairy calves

The duration of excretion of dry cow antimicrobials is hard to predict and therefore the level of antimicrobial residues in colostrum can differ considerably between cows. The feeding of colostrum with antimicrobial residues to newborn calves may lead to antimicrobial resistance in calves. The aim of this study was to quantify the prevalence and residue level of dry cow antimicrobials in colostrum from dairy cows. Additionally, the association between antimicrobial residues in colostrum and the prevalence of extended spectrum and AmpC β-lactamaseproducing Escherichia coli (ESBL/AmpC-EC) in colostrum and in the feces from young dairy calves was evaluated. The work described in this paper consists of 3 studies. In the first study, in 75 of 118 colostrum samples antimicrobial residues were found, in 50 of 88 β-lactam treated cows, and in 25 of 30 cows treated with a combination of an aminoglycoside and a β-lactam. In cows that had been dried off with a combination of an aminoglycoside and a β-lactam, in significantly more samples antimicrobial residues were found. In 38 of the colostrum samples tested, antimicrobial concentrations (mainly cloxacillin) exceeded the Maximum Residue Limit. The prevalence of antimicrobial positive colostrum samples presented was much higher than earlier described. In the second study 28 dairy cows from 8 farms were dried off with either cloxacillin or a combination of neomycin and benzylpenicillin. Because in study 1 the tendency of a negative association was found between the use of teat sealants and the presence of antimicrobials in colostrum, it was hypothesized that with the striping of the teat sealant much of the antimicrobial residues was removed. This hypothesis could not be confirmed by comparing antimicrobial concentrations in the first streams of the first milking and the remainder of that milking. The 22 clocaxilin treated cows were followed up for 6 subsequent milkings, during which a decrease in the concentration of antimicrobial residues was found. In the third study, 87 cows from 10 herds were included of which 58 cows were dried off with cloxacillin while 29 cows were dried off without antimicrobials. Cows were compared with respect to antimicrobial residues in the first 5 milkings after calving and the occurrence of ESBL/AmpC-EC in the feces from their calves, collected at d 1, d 7 and d 14 of life. Significantly higher percentages of ESBL/AmpC-EC positive samples were found in feces samples collected at d 7 and d 14 compared with the d 1 samples. No significant differences were found in the prevalence of ESBL/AmpC-EC positive fecal samples between calves born from cows that had been dried off without antimicrobials and those that had been dried off with cloxacillin. Given the fact that cloxacillin does not select for ESBL/AmpC-EC, this was not unexpected. This study showed that colostrum from cows dried off with antimicrobials often contains antimicrobial residues. Although no effect of these residues on the prevalence of ESBL/AmpC-EC in fecal samples of calves was found in this study, we recommend to minimize antimicrobial use at drying off and to respect withdrawal recommendations.