Occurrence of mastitis pathogens in cow's milk samples from the Hanover region, north-western Germany - an overview of routine laboratory data from 2005 to 2023.

Mastitis remains the most frequent disease in dairy cows, affecting both animal welfare and the profitability of dairy farms. In the present investigation, we retrospectively analysed 102,179 cow's milk samples, which were object of routine diagnostics at the Department of Milk Hygiene, Institute for Food Quality and Food Safety, University of Veterinary Medicine Hannover, from 2005 to 2023, regarding the occurrence of major mastitis pathogens. On average, approx. 48% of all tested samples were bacteriologically positive. Differences in the occurrence of positive findings were described regarding the season, the submitter, the reason for submission, as well as the udder quarters. Generally, the 13 most common groups of pathogens accounted for 94.0% of all positive samples. Non-aureus staphylococci and mammaliicocci (23.6%) were detected most frequently, followed by Streptococcus uberis (14.8%), Escherichia coli (13.2%), Corynebacterium spp. (9.5%), Staphylococcus aureus (6.7%), yeast (6.1%), and Enterococcus spp. (5.6%). While a slight decline was recorded for most of these pathogen groups, S. uberis and E. coli showed a significant upward trend throughout the investigation period. Moreover, a constantly high percentage of samples contaminated with more than three different colony forms was found. Monitoring long-term development of the mastitis pathogen spectrum as well as its antibiotic resistance patterns is of utmost importance to enable adaptation of hygiene and treatment strategies. This includes appropriate training of the treating veterinarians with regard to sterile milk sampling.