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Antimicrobial resistance and genotypic characterization of coagulase-negative staphylococci over the dry period

“Blanket” antimicrobial dry cow therapy (DCT) (where long acting antimicrobials are administered to all cows at dry off regardless of infection status) is practiced in many countries. A study was conducted to determine the prevalence of antimicrobial resistance (AR) in coagulase-negative staphylococci (CNS) before and after DCT. 752 cows were assigned to two groups: high risk (assumed to be infected by SCC level, n=360) and low risk (assumed not to be infected by SCC level, n=392). All high risk cows received DCT; low risk cows were further divided into two groups, one receiving DCT and one receiving no DCT. Quarter milk samples were collected at dry off and at calving for culture and antimicrobial susceptibility testing was conducted for CNS (n=460) CNS from high risk cows exhibited more AR than those isolated from low risk, untreated cows; CNS from multiparous cows were more likely to exhibit AR than those from heifers. These observations suggest that antimicrobial treatment exerts pressure on bacteria over time which may accumulate resistance.