Estimation of epidemiological cut-off values for eight antibiotics used for treatment of bovine mastitis caused by Streptococcus uberis and Streptococcus dysgalactiae subsp. dysgalactiae.

Interpretive criteria for antimicrobial susceptibility testing are lacking for most antimicrobials used for bovine streptococcal mastitis. The objectives of this study were to determine (tentative) epidemiological cut-off ((T)ECOFF) values for clinically relevant antibiotics used for treatment of bovine mastitis, and to estimate the proportion of acquired resistance (non-wild-types) in Streptococcus dysgalactiae subsp. dysgalactiae and Streptococcus uberis. A total of 255 S. uberis and 231 S. dysgalactiae subsp. dysgalactiae isolates were obtained in Denmark and Norway from bovine mastitis. The isolates were tested for susceptibility to 10 antibiotics using broth microdilution. In accordance with the European Committee on Antimicrobial Susceptibility Testing (EUCAST) standard operating procedure, additional published MIC distributions were included for the estimation of ECOFFs for cloxacillin, cephapirin, lincomycin and tylosin, and TECOFFs for amoxicillin, benzylpenicillin, cephapirin and oxytetracycline. The proportion of non-wild-type (NWT) isolates for the beta-lactams was significantly higher in the Danish S. uberis (45-55%) compared to the Norwegian isolates (10-13%). For oxytetracycline, the proportion of NWT was significantly higher in the Danish isolates, both for S. uberis (28% vs. 3%) and S. dysgalactiae (22% vs. 0%). A bridging study testing in parallel MICs in a subset of isolates (n = 83) with the CLSI-specified and the EUCAST-specified broths showed excellent correlation between the MICs obtained with the two methods. The new ECOFFs and TECOFFs proposed in this study can be used for surveillance of antimicrobial resistance, and - for antimicrobials licensed for streptococcal bovine mastitis - as surrogate clinical breakpoints for predicting their clinical efficacy for this indication.

Ten years later: still a high prevalence of MRSA in slaughter pigs despite a significant reduction in antimicrobial usage in pigs the Netherlands.

OBJECTIVES: In 2005, 39% of pigs and 81% of the slaughter batches at Dutch slaughterhouses were MRSA positive. The objective of the present study was to investigate whether the 50% reduction of antimicrobial usage in finishing pigs in 2014 compared with 2009 in the Netherlands has led to a lower MRSA prevalence among Dutch slaughter pigs. METHODS: Nasal swabs from eight slaughter batches of on average 10 animals at seven slaughterhouses were taken and cultured using method 1, which was used in 2005, and method 2, using high-salt pre-enrichment. Suspected isolates were confirmed by PCR for two Staphylococcus aureus-specific DNA fragments and the mecA gene. A subset of MRSA isolates were further investigated using spa typing, multiple-locus variable number of tandem repeat analysis (MLVA) and antimicrobial susceptibility testing. RESULTS: Using methods 1 and 2, we found 461 of 558 (83%) and 552 of 558 (99%) of the pigs to carry MRSA in their nares, respectively. All 56 slaughter batches were MRSA positive. All MRSA isolates belonged to the livestock-associated MLVA complex 398, had a non-WT phenotype for tetracycline and spa type t011 predominated. CONCLUSIONS: A very high prevalence of nasal MRSA carriage was found in Dutch slaughter pigs and therefore the reduction in antimicrobial usage at the national level has not yet had an effect on the MRSA carriage rate of pigs entering the slaughterhouse. Therefore, there is still an increased risk of MRSA carriage for personnel working at pig slaughterhouses, particularly those having contact with living animals. Method 2, using high salt pre-enrichment, detected more MRSA-positive pigs and is currently the preferred method for screening of MRSA in livestock in the Netherlands.