Development of long-term antimicrobial resistance patterns of bovine mastitis pathogens in north-western Germany from 2005 to 2023

The spread of antimicrobial resistances is a global threat. This study provides a retrospective, explorative analysis of bovine mastitis samples from 2005 to 2023, regarding the antimicrobial resistance patterns of the corresponding pathogens. 41.8% of the tested samples were positive for mastitis pathogens. Regarding the occurrence of antimicrobial resistances, 18,623 tests were analysed in total, comprising the 13 most frequently identified bacterial pathogens. The highest percentage of total antimicrobial resistant samples was detected for Proteus spp. (55.2%), Bacillus spp. (44.3%) and Citrobacter freundii (40.8%), while the lowest percentage was found for Staphylococcus aureus (7.1%), Trueperella pyogenes (6.1%), and Streptococcus dysgalactiae (3.1%). Furthermore, S. uberis, S. dysgalactiae, and T. pyogenes showed an overall linear decrease of antimicrobial resistant samples from 2005 to 2023. Trends of increased resistances were also identified for individual pathogen-antimicrobial combinations. To date, long-term, large-scale studies like this have rarely been conducted, but they are of particular importance in identifying trends in the shift of antimicrobial resistances among mastitis pathogens. The results of such routine diagnostic data must be viewed with caution, as they cannot be readily extrapolated to the entire dairy population. Nevertheless, these findings emphasize the necessity of unambiguous pathogen identification, including susceptibility testing, prior to antimicrobial administration. Alternative therapy methods also become increasingly important, as well as proper farm management and hygiene to prevent infections from the beginning.