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In vitro penicillin sensitivity of Streptococcus ssp. isolated from quarter milk samples of Bavarian dairy cows (2018 - 2019)
Huber-Schlenstedt, R.; Macias...
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Objective: The objective was to evaluate the in vitro antimicrobial resistance against penicillin of Streptococcus (S.) uberis, S. agalactiae, S. dysgalactiae, and S. canis of routine quarter milk samples.
Material and methods: In 2018 and 2019 the Bavarian Animal Health Services received quarter milk samples from whole dairy herd tests as well as individual cows. The laboratory used standard culture methods in accordance with guidelines of the German veterinary association to diagnose infections in quarter milk samples. The identification of Streptococci was based on colony morphology, hemolysis, esculin hydrolysis, camp factor as well as gram strain. Further differentiation of esculin-positive Streptococci was conducted with an in-house method that utilized Enterococcus selective agar plates and a disc test against Penicillin and Rifampicin. Strains of unclear results were identified by MALDI-TOF-MS (microf- lexTM MALDI BiotyperTM, Bruker Daltonik, reference database V.3.3.1.0., Bruker Daltonik GmbH, Bremen Germany). A sub-set of aforementioned Streptococci was tested for sensitivity to common antibiotics using with the method of MIC (minimum inhibitory concentration) (mastitis 3 plat, Merlin Diagnostika GmbH). Breakpoints for sensitivity are based upon CLSI Vet01-A4 (2015) and CLSI Vet01S 5th edition. As a breakpoint for penicillin for the indication “mastitis cow” does not exist, an older breakpoint (CLSI M31-A2; AVID, 1998) was used instead. Furthermore, intermediate results were reported as resistant. Results were summarized by descriptive statistics.
Results: A total of 854,782 quarter milk samples from 218,336 cows of >7,000 individual dairy farms were analyzed in the laboratory in 2018-2019. These included samples from subclinical (based on California Mastitis Test (CMT), n= 213,174) and clinical cases (n= 17,642). Mastitis pathogens were found in 18.7 % (n= 159,900) of all submitted sam- ples.
S. uberis was the most commonly isolated mastitis pathogen of clinical samples (2018: 35.0%, n= 2,566; 2019: 34.4%, n= 2,280). Additionally, S. uberis was the second most commonly isolated pathogen in samples of subclinical cases in both years (2018: 23.9%, n= 12,178; 2019: 23.1% n= 9,778) - only surpassed by Coagulase Negative Staphylococci (2018: 29.5%; 2019: 30.4%).
Of 7,345 S. uberis tested, exceedingly few isolates (n=47) had a breakpoint >0.125 for penicillin (2018: 0.8%, n= 33; 2019: 0.4%, n= 14). The vast majority had a breakpoint <0,125 for penicillin and thus was considered susceptible to penicil- lin in vitro. All tested isolates of S. agalactiae (n= 900) and S. canis (n= 355) had a breakpoint <0.125 for penicillin - i.e., none were considered resistant to penicillin. Only five (0.2%) of 2,684 tested S. dysgalactiae were considered resistant to penicillin as they had a penicillin breakpoint above >0.125 (2018: 0.1%, n= 1,444; 2019: 0.2%, n= 1,240).
Conclusion: Only sporadic Streptococci isolates had a breakpoint >0.125 and were resistant to penicillin in vitro. The vast majority of tested Streptococci had a breakpoint <0.125 for penicillin. Therefore, penicillin should continue to be the first choice for antibiotic therapy of S. uberis and esculin-neg- ative Streptococci in Bavaria (Southern Germany).
Funding Source: Bavarian Ministry for Food, Agriculture and Forestry and the Bavarian Joint Founding Scheme for the Control and Eradication of contagious Livestock (Bayerische Tierseuchenkasse).
Keywords: Mastitis, streptococci ssp., penicillin sensitivity.
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Affiliation of the authors at the time of publication
Bavarian Animal Health Service, Poing, Germany
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