Antimicrobial consumption on dairy herds and its association with antimicrobial inhibition zone diameters of non-aureus staphylococci and Staphylococcus aureus isolated from subclinical mastitis

The main objectives of this study were to quantify the consumption of antimicrobials on a convenience sample of dairy herds and to determine the association between herd-level antimicrobial consumption and inhibition zone diameters (IZD) of non-aureus staphylococci and Staphylococcus aureus isolates from subclinical mastitis cases. Also, the association between the IZD of non-aureus staphylococci and Staph. aureus isolates within a herd was studied. Antimicrobial consumption data on 56 Flemish dairy farms were obtained between 2013 and 2014 by so-called garbage can audits and expressed as antimicrobial treatment incidence (ATI), with the unit of ATI being the number of defined daily doses animal (DDDA) used per 1,000 cow-days. The average total ATI in adult dairy cattle for all active substances was 18.73 DDDA per 1,000 cow-days and ranged from 6.28 to 42.13 DDDA between herds. The ATI of critically important (for human health) antimicrobials was 6.91 DDDA per 1,000 cow-days; that is, 37% of total antimicrobial consumption. The average ATI for intramammary therapy of (sub)clinical mastitis, intramammary dry-cow therapy, and systemically administered therapy was 5.20, 6.70, and 6.73 DDDA, respectively. The IZD of 239 non-aureus staphylococci and 88 Staph. aureus isolates originating from milk samples from cows with subclinical mastitis collected on selected dairy herds were determined using Kirby-Bauer disk diffusion and ranged between 6 and 42 mm. Because only a limited number of clinical breakpoints (Clinical and Laboratory Standards Institute) and epidemiological cut-off values (European Committee on Antimicrobial Susceptibility Testing) are available for mastitis-causing bacteria in bovine, IZD were used as a proxy for antimicrobial resistance. Inhibition zone diameters of non-aureus staphylococci for cefquinome, a critically important β-lactam antibiotic, were negatively associated with the ATI of critically important β-lactam for systemically administered therapy and positively with the ATI for intramammary therapy of (sub)clinical mastitis of critically important β-lactam antimicrobials. Only for neomycin was a positive association between the IZD of non-aureus staphylococci and Staph. aureus isolates within the same herd observed.