Antimicrobial susceptibility of mastitis pathogens isolated from North American dairy cattle, 2011-2022.

A total of 10,890 bacterial isolates of Streptococcus dysgalactiae, Streptococcus uberis, Staphylococcus aureus and Escherichia coli isolated as etiological agents from dairy cows with mastitis by 29 veterinary laboratories across North America between 2011 and 2022 were tested for in vitro antimicrobial susceptibility by broth microdilution to ampicillin, cefoperazone, ceftiofur, cephalothin, erythromycin, oxacillin, penicillin-novobiocin and pirlimycin according to CLSI standards. Using available clinical breakpoints, antimicrobial resistance among S. dysgalactiae (n = 2406) was low for penicillin-novobiocin (0% resistance), ceftiofur (0.1%), erythromycin (3.2%) and pirlimycin (4.6%). Among S. uberis (n = 2398), resistance was low for ampicillin (0%) and ceftiofur (0.2%) and moderate for erythromycin (11.9%) and pirlimycin (18.4%). For S. aureus (n = 3194), resistance was low for penicillin-novobiocin (0%), ceftiofur (0.1%), oxacillin (0.2%), erythromycin (0.7%), cefoperazone (1.2%) and pirlimycin (2.8%). For E. coli (n = 2892), resistance was low for ceftiofur (2.8%) and cefoperazone (3.4%) and moderate for ampicillin (9.2%). Overall, the results indicate that mastitis pathogens in the United States and Canada have not shown any substantial changes in the in vitro susceptibility to antimicrobial drugs over the 12 years of the study, or among that of the proceeding survey from 2002-2010. The data support the conclusion that resistance to common antimicrobial drugs among mastitis pathogens, even to drugs that have been used in dairies for mastitis management for many years, continues to remain low.

Revision of fluoroquinolone breakpoints used for interpretation of antimicrobial susceptibility testing of canine bacterial isolates.

The fluoroquinolone antimicrobial agents, enrofloxacin and marbofloxacin, were US Food and Drug Administration (FDA) approved in the United States for use in dogs in 1988 and 1999, respectively. There have been many advances since then concerning the pharmacokinetic-pharmacodynamic (PK-PD) evaluation of fluoroquinolones, and there are data available on the susceptibility of targeted pathogens since the original approval. Using this information, the Clinical and Laboratory Standards Institute (CLSI) Veterinary Antimicrobial Susceptibility Testing Subcommittee (VAST) revised its antimicrobial susceptibility testing breakpoints. The previous breakpoints (used in older editions of CLSI standards) for enrofloxacin in dogs were susceptible (S), ≤ 0.5 µg/mL, intermediate (I) 1-2 µg/mL, and resistant (R) ≥ 4 µg/mL. The new breakpoints are S ≤ 0.06 µg/mL for a dose of 5 mg/kg, 0.12 µg/mL for a dose of 10 mg/kg, 0.25 µg/mL for a high dose of 20 mg/kg, and R ≥ 0.5 µg/mL. The breakpoints of 0.12 and 0.25 µg/mL represent a new susceptible-dose dependent (SDD) category. For marbofloxacin, previous breakpoints were S, ≤ 1 µg/mL, I 2 µg/mL, and R ≥ 4 µg/mL. The new breakpoints are S ≤ 0.12 µg/mL for a dose of 2.8 mg/kg, 0.25 µg/mL for a dose of 5.5 mg/kg (SDD), and R ≥ 0.5 µg/mL. The new breakpoints will be published in the next edition of CLSI-Vet01(S) and deviate considerably from the prior breakpoints. Laboratories are encouraged to revise their testing standards. These changes will likely reduce the unnecessary use of these fluoroquinolones in dogs.