Frequency of non-aureus staphylococci and mammaliicocci species isolated from quarter clinical mastitis: a six-year retrospective study

: Non-aureus staphylococci and mammaliicocci (NASM) are the most frequently isolated bacterial group from bovine milk samples. Most studies focus on subclinical mastitis caused by NASM, however NASM can cause clinical mastitis (CM) as well. We evaluated retrospective data from 6 years (2017-2022) to determine the species and frequency of NASM isolated from quarter bovine CM. The data comprised of microbiological results from quarter CM samples routinely submitted to Quality Milk Production Services (QMPS), Cornell University, NY, US, for microbial identification by MALDI-TOF MS. A total of 9,909 microbiological results from 410 dairy herds were evaluated. Our results showed that 29 distinct NASM species were identified, with the 8 most prevalent NASM species being Staphylococcus chromogenes, S. haemolyticus, S. simulans, S. epidermidis, S. sciuri (now Mammaliicoccus sciuri), S. agnetis/S. hyicus, S. borealis, and S. xylosus. The NASM distribution remained similar among seasons, but the frequency of NASM CM cases was higher during the summer. Our results showed different patterns of variations in the isolation frequency over time, depending on the bacterial species: increasing or decreasing trends, cyclic fluctuations, and except for S. borealis, a significant seasonality effect for our study's most prevalent NASM was observed. This study showed that S. chromogenes remains the most frequent (43%) NASM species identified from bovine CM, followed by S. haemolyticus (18%), and S. simulans (12%).