Objectives
For the planning of prevention and control measures against bovine mastitis, infection pathways, durations of intramammary infections as well as transmission rates of contagious pathogens have to be considered. Still, little is known about these parameters for several mastitis pathogens (e.g. Strep. dysgalactiae). Additionally, most studies investigating intra-mammary infection dynamics were carried out in herds experiencing udder health problems. These are not necessarily representative of all herds that need data to plan cost-effective interventions. Therefore, we investigated the duration of infections, transmission rates and the potential infection pathways for several mastitis pathogens in a dairy herd with an acceptable udder health situation.

Materials and methods
One Swedish dairy herd with approximately 200 cows, a clinical mastitis incidence of 1.6 cases/ 100 cows/ month and a geometric mean bulk tank SCC of 195,000 somatic cells/ml was visited 10 times in 14-day intervals. At each visit, foremilk samples were taken from all lactating quarters. These underwent standard microbiological examination and consecutive identification of isolates with MALDI-TOF MS. Quarters were considered infected with Staph. aureus, Strep. dysgalactiae or Strep. uberis when 100 cfu/ml were detected. For Staph. epidermidis or Staph. haemolyticus a cut-off of 500 cfu/ml was used. Strain typing with Random amplified poly-morphic DNA PCR was carried out for all isolates of the named species to distinguish ongoing from new infections. Infection durations were estimated using the Life Table method taking censoring into account and excluding all quarters treated with antimicrobials (n=6). Transmission rates were calculated using Poisson regression models.

Results
The average quarter level prevalence of each of Staph. epidermidis, Staph. haemolyticus, Staph. aureus, Strep. dysgalactiae and Strep. uberis was below 2% throughout the study. A dominating strain was detected for Staph. aureus and Strep. dysgalactiae (each causing 83 and 48% of the observed infections). The estimated daily mean quarter-level transmission rates for the dominant strains of these two pathogens were 0.029 (95% CI: 0.0153 - 0.0492) and 0.009 (0.0042 - 0.0158) cases/ quarter-day, respectively. Furthermore, three strains of Strep. uberis caused together 49% of all infections. In contrast, of the two non-aureus staphylococci no strain caused more than 4% of all infections. The average infection duration of Staph. aureus was 80 days, of Strep. dysgalactiae and Strep uberis more than 84 and 70 days, and of Staph. epidermidis and Staph. haemolyticus 28 and 22 days, respectively.

Conclusions
Most infections with Staph. aureus, Strep. dysgalactiae and Strep. uberis originated from contagious transmission. Once infected with these patho-gens, many quarters were positive over long periods. This indicates that also in this herd with an unremarkable udder health situation, an improved milking hygiene could have contributed to a further reduction in the preva-lence of these organisms.
In contrast, infections with Staph. epidermidis and Staph. haemolyticus were not transmitted from quarter to quarter. The short observed durations of infections of these organisms indicate that it might not be justified to treat intramammary infections with these with antimicrobials (e.g. at drying off).
Acknowledgements: This work was funded by FORMAS (grant numbers: 2019-02276; 2019-02111).