Effects of antimicrobial exposure on the antimicrobial resistance of Escherichia coli in the digestive flora of dairy calves.

Veal calves are often identified as reservoirs for antimicrobial resistant Escherichia coli (E. coli). This production is closely linked with dairy production, as young calves - mostly males - are collected from dairy farms to enter the fattening process. The aim of this prospective study was to explore the factors on dairy farms that favour the selection of antimicrobial resistance (AMR) in the digestive E. coli strains of young calves and to assess whether the resistance levels and selection pressure were the same for males and females. The exposure of calves to antimicrobials was investigated through three factors: antimicrobial treatment of calves; feeding of calves with milk from cows treated with antimicrobials; and the consumption of colostrum from cows treated with antimicrobials at dry-off. The study design involved 100 dairy farms. A calf of each sex was selected from birth on each farm. Information on the calves' exposure to antimicrobials was collected daily and calves were sampled (rectal swab) two weeks after birth, then seven weeks after birth for females only. Laboratory analyses included culture on two distinct media: a non-selective medium (identifying dominant flora) and a medium containing ceftiofur to select the extended-spectrum beta-lactamase (ESBL) phenotype. Susceptibility testing was performed on an E. coli strain from each medium. Generalised linear models were used to assess associations between the resistance of E. coli strains and antimicrobial exposure. A set of 280 swabs from healthy calves were analysed. In dominant flora, high levels of resistance (>60 %) were identified for streptomycin, tetracycline and amoxicillin but AMR levels were low (3 %) for critically important antimicrobials (3rd- and 4th-generation cephalosporins and fluoroquinolones). For females staying in dairy farms, a marked decrease in resistance was observed for almost all antimicrobials between the age of 15 days and 7 weeks. A selective medium revealed an ESBL phenotype for 20.7 % of the calves. Whether for AMR or antimicrobial exposure, no significant difference was found between male and female calves. The antimicrobial treatment of calves was associated with an increased resistance of E. coli from dominant flora for amoxicillin (OR = 2.9), gentamicin (OR = 4.6), florfenicol (OR = 5.0) and trimethoprim-sulfonamide (OR = 5.6). The consumption by calves of milk from cows treated with antimicrobials was also associated with an increased resistance to amoxicillin (OR = 2.6), gentamicin (OR = 4.0), tetracycline (2.6) and trimethoprim-sulfonamide (OR = 2.2). In contrast, the models did not reveal any association between AMR and consumption of colostrum from cows treated with antimicrobials at dry-off.