Antimicrobial susceptibilities in dairy herds that differ in dry cow therapy usage

摘要

Intramammary infusion of antimicrobials at the endof lactation (dry cow therapy; DCT) is a central partof mastitis control programs and is one of the major indicationsfor antimicrobial use in dairy cows. However,with increasing focus on prudent use of antimicrobialsand concerns about emergence of antimicrobial resistance,the practice of treating every cow at the end oflactation with DCT is in question. This cross-sectional,observational study determined the minimum inhibitoryconcentrations (MIC) of 10 antimicrobials for coagulase-negative staphylococci (CNS), Staphylococcusaureus, Streptococcus dysgalactiae, and Streptococcusuberis isolates from milk samples from dairy cows withsomatic cell counts >200,000 cells/mL in herds thathad been organic for >3 yr (n = 7), or had used eitherampicillin-cloxacillin DCT (n = 11) or cephaloniumDCT (n = 8) in the preceding 3 yr. The organic herdswere certified under the United States Department ofAgriculture National Organic Program, meaning thatthere was no blanket DCT, and minimal use of antimicrobialsin general, with a loss of organic status ofthe animal if treated with antimicrobials. Breakpoints(where available) were used to categorize isolates asresistant, intermediate, or susceptible to antimicrobials.The MIC distributions of isolates from differentherd types were compared using binomial or multinomiallogistic regression. Of 240 CNS isolates, 12.9, 0.8,7.1, 32.6, and 1.2%, were intermediate or resistant toampicillin, cephalothin, erythromycin, penicillin, andtetracycline, respectively. Of 320 Staph. aureus isolates,29.0, 2.5, 1.2, and 34.9% were intermediately resistantor resistant to ampicillin, penicillin, erythromycin, andoxacillin, respectively. Of 184 Strep. uberis isolates,1.1, 25.0, 1.6, and 1.6% were intermediately resistantor resistant to erythromycin, penicillin, pirlimycin, andtetracycline, respectively. Generally, the MIC of CNSand streptococcal isolates from organic herds werelower than isolates from herds using DCT. However,the differences in MIC distributions occurred at MICbelow clinical breakpoints, so that the bacteriologicalcure rates may not differ between isolates of differingMIC. Bimodal distributions of MIC for ampicillin andpenicillin were found in Staph. aureus isolates from organicherds, suggesting that isolates with a higher MICare a natural part of the bacterial population of thebovine mammary gland, or that isolates with higherMIC have persisted within these organic herds from atime when antimicrobials had been used. Given theseobservations, further work is required to determine ifexposure to DCT is causally associated with the riskof elevated MIC, and whether reduction or removal ofDCT from herds would reduce the risk of elevated MICof mastitis pathogens.