Herd-level factors associated with Streptococcus uberis, Staphylococcus aureus, and Streptococcus agalactiae mastitis: Dairy herd data from 2013, 2018, and 2023

Streptococcus uberis is an important cause of bovine intramammary infection (IMI), primarily transmitted via environmental pathways, while some strains show potential for contagious transmission. This retrospective observational study aimed to determine and evaluate the herd characteristics associated with 1) increased proportional morbidity (PM) of S. uberis IMI, 2) increased PM of Staphylococcus aureus IMI, and 3) presence of Streptococcus agalactiae in a herd. Data consisted of Finnish national dairy herd improvement records and quarter-level milk sample qPCR analyses for 2013, 2018, and 2023. S. uberis and S. aureus PM were estimated through bootstrapping, and herds were classified as having high PM if the 95% CI lower bound exceeded 19% (1 754 herds) and 22% (1 534 herds), respectively. Herds were classified as S. agalactiae-positive if any sample tested positive (2 726 herds). High S. uberis PM was associated with increasing herd size, higher herd-average parity, and an increased proportion of multiparous cows treated for mastitis. High S. aureus PM was associated with decreasing herd size, a pipeline milking system, lower herd-average parity, and udder diseases as a common culling reason. Positive S. agalactiae herd-status was associated with increasing herd size, automatic milking system, and lower herd-average parity. Compared to the two more contagious pathogens, S. aureus and S. agalactiae, high S. uberis PM was less related to the milking system, while increasing herd size increased the odds of having high S. uberis PM, which could suggest that the transmission route of S. uberis might have been more environmental than contagious.