Antibiotic susceptibility of mastitis pathogens in Finnish dairy cows in 2022-2023: concerning changes in the penicillin susceptibility of Streptococcus uberis.

BACKGROUND: Intramammary infections are the most common reason for antibiotic use in dairy cattle. A multiplex PCR has become the mainstay in determining the causative mastitis pathogens in Finland. However, this method does not allow the monitoring of antibiotic resistance among mastitis pathogens. The aim of this study was to assess antibiotic susceptibility among common bacterial pathogens isolated from mastitis milk specimens in Finland. Specimens were collected on dairy farms from cows suffering from mastitis and analysed with the PathoProof™ Mastitis Complete-16 PCR assay. Specimens with moderate or large amounts of Staphylococcus aureus, Streptococcus agalactiae, Streptococcus uberis, Streptococcus dysgalactiae, Escherichia coli, Klebsiella sp. and non-aureus staphylococci (NAS) were cultured and susceptibility testing was performed with the broth microdilution method. Susceptibility results were categorised as wild type (WT) and non-wild type (NWT) according to epidemiological cut-off values. Resistance to penicillin among S. aureus and NAS was assessed based on phenotypic beta-lactamase production. Only one bacterial isolate was included per species and farm. RESULTS: In total, 502 bacterial isolates from 220 farms were tested for susceptibility. The most common bacterial species isolated was S. aureus (n = 126), followed by NAS (n = 124), S. uberis (n = 79), S. dysgalactiae (n = 74) and E. coli (n = 70). The proportions of NWT isolates were generally low, with the notable outlier being S. uberis, for which 42% (95% CI: 31.5-52.8) of the isolates was classified as NWT to penicillin. Furthermore, 38% (95% CI: 29.9-46.7) of the NAS isolates produced beta-lactamase. Only 4% (95% CI: 1.7-9.0) of the S. aureus isolates produced beta-lactamase, and no methicillin resistance was detected. All E. coli were classified as WT to fluoroquinolones, and no extended-spectrum beta-lactamase (ESBL)-producing isolates were discovered. CONCLUSIONS: Antibiotic susceptibility among bacteria in clinical mastitis is favourable. However, the proportion of Streptococcus uberis isolates classified as NWT to penicillin has increased over the past ten years, and continued surveillance is warranted.