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Diversity of selected staphylococci and streptococci isolated from milk samples in a dairy cow herd with low prevalences
Woudstra, S.; Wente, N.; Zhang, Y...
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Background and objective: Most mastitis cases world-wide are caused by staphylococci and streptococci. Farm-specific strategies for the prevention and control of intramammary infections with these can be developed based on the results from the regular microbiological analysis of milk samples from clinical and subclinical mastitis cases. The dominant organisms on a farm determine if the focus should be on improving milking practices or the hygiene in the cows ́ environment. This approach is based on the traditional categorization into contagious pathogens (e.g. Staphylococcus (Staph.) aureus) that are mainly transmitted during milking, and environmental pathogens (e.g. Streptococcus (Strep.) uberis) that have their reservoirs in the environment. The diversity of a bacterial species (i.e. number of different strains) in a herd is indicative of the sources of infections. If the diversity is low, a common source of infections is assumed (contagious transmission or an environmental hotspot). When the diversity is high, independent infection events are most probable. For some pathogens like Strep. dysgalactiae, it remains unclear if they spread rather contagiously or if cows become infected from environmental sources. Additionally, for some species (e.g. Strep. uberis) the traditional categorization has been challenged in recent decades. Furthermore, most studies have investigated the diversity of mastitis-causing pathogens in herds that are experiencing mastitis outbreaks or an unsatisfactory ud- der health situation. Therefore, the objective of the present study was to describe the diversity of selected staphylococci and streptococci isolated from milk samples in a herd with low prevalences.
Material and methods: From June to October 2020, one Swedish dairy cow herd was visited 10 times with 14-day intervals. At each visit, quarter foremilk samples from all lactating cows were collected aseptically according to DVG guidelines and transported immediately to the laboratory [1]. From each sample, 10μl were streaked onto esculin blood agar. Microbial growth was evaluated after 24 and 48 hours incubation at 37°C. Preliminary species identification was based on morphological and biochemical characteristics, and final species identification conducted by MALDI-TOF MS. For all isolates identified as Staph. aureus, Staph. epidermidis, Strep. dysgalactiae or Strep. uberis, strain typing based on RAPD-PCR was conducted. All isolates with the same banding pattern were considered to be the same RAPD-type. A quarter was considered infected if at least 100 colony-forming units per milliliter were detected and infections ended when at least two consecutive samples were negative for the respective strain type.
Results: In total, 8056 milk samples from 263 cows were collected. From 82 (1.0%), 91 (1.1%), 106 (1.3%) and 114 (1.4%) of all samples Staph. aureus, Staph. epidermidis, Strep. dysgalactiae and Strep. uberis were isolated, respectively. For Staph. aureus, five different RAPD-types were found. Of all Staph. aureus infections (n=48), 81% (n=39) were due to the same strain. All other strains caused maximum 8% of all Staph. aureus infections. For Staph. epidermidis, 45 different strains were isolated. None of these strains caused more than 5% of all infections (n=55). Seven different strains of Strep. dysgalactiae were isolated. One of these caused 50% (n=18) of all (n=36) observed infections. Two other strains were iso-lated from 19% (n=7) and 11% (n=4) of the Strep. dysgalactiae infections. All other strains caused maximum 8% of all Strep. dysgalactiae infections. The Strep. uberis isolates belonged to 18 different RAPD-types. 24% (n=10) of all Strep. uberis infections were caused by the same RAPD-type and another 15% (n=6) and 12% (n=5) were caused by two other RAPD- types. All other Strep. uberis strains caused less than 10% of all infections.
Conclusions: For Staph. aureus, Strep. dysgalactiae and Strep. uberis, dominating strains were isolated. This confirms the contagious nature of Staph. aureus even in this herd with a low occurrence and indicates that also Strep. uberis and Strep. dysgalactiae infections can spread mainly contagiously or via environmental hotspots. In contrast, no dominating strain was found for Staph. epidermidis. Therefore, most infections with Staph. epidermidis seemed to have occurred due to independent infection events.
Acknowledgements: This work was funded by FORMAS (Swedish Research Council for Sustainable Development, Stockholm, Sweden; grant numbers: 2019-02276 and 2019- 02111).
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Affiliation of the authors at the time of publication
University of Copenhagen, Copenhagen, Denmark;
University of Applied Sciences and Arts Hannover, Hannover, Germany.
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