Dynamics of intramammary infections and their association with quarter-level somatic cell score, milk yield, and milk...

Intramammary infections are often associated with elevated SCC, a key indicator of udder inflammation and milk quality. This condition leads to substantial economic losses, primarily through its direct impact on milk production. In addition to reducing milk yield, IMI can negatively affect milk components such as fat, protein, and lactose. Consequently, the main objective of this longitudinal study was to estimate the association of IMI caused by these microorganisms on the SCS, milk yield (kg/d), and milk components at the quarter level. A secondary objective was to describe the quarter-level incidence and elimination rates of IMI caused by microorganisms other than Staphylococcus spp. A convenience sample of 5 dairy herds using an automated milking system was selected and visited every 2 wk for milk sample collection. Daily quarter milk yield data were extracted from the automatic milking system software. The determination of SCC and milk composition parameters was performed by Lactanet. Bacteriological culture was performed according to the National Mastitis Council guidelines. The different types of colonies were counted and identified using MALDI-TOF. For the first objective, we used generalized linear models with robust variance to estimate the incidence rate and elimination rate of IMI. For the second objective, given the hierarchical structure of the data, a 4-level linear mixed model with herd, cow, and quarter as random intercepts was built with either the test-day SCS, milk yield, lactose, protein, or fat as the outcome. The most prevalent non-Staphylococcus microorganisms in primiparous cows were Aerococcus viridans, Streptococcus dysgalactiae, and Corynebacterium bovis with mean prevalences (95% CI) of 3.8% (3.3%, 4.4%), 0.9% (0.7%, 1.3%), and 0.9% (0.7%, 1.2%), respectively. Similarly, for multiparous cows, the 3 most prevalent pathogens were C. bovis, A. viridans, and S. dysgalactiae, with mean prevalences (95% CI) of 5.2% (4.8%, 5.5%), 3.7% (3.4%, 4.0%), and 2.8% (2.5%, 3.1%), respectively. For both primiparous and multiparous cows, the incidence rate of S. dysgalactiae was low, at 0.1 and 0.2 new intramammary infections/100 quarter-weeks, respectively. However, quarters infected with S. dysgalactiae exhibited a lower elimination rate compared with infections caused by other microorganisms. A similar pattern was observed for S. uberis infections in multiparous cows. In primiparous cows, quarters infected by S. dysgalactiae produced 1.3 kg/d less compared with healthy quarters. In multiparous cows, milk yield was not significantly associated with IMI status; however, quarters infected with S. dysgalactiae produced 0.7 kg/d less than healthy quarters, though this difference was only numerical. Regarding milk composition, quarters infected by S. dysgalactiae in primiparous cows had a 0.2% reduction in lactose content, with no significant differences observed for protein or fat content. In multiparous cows, S. uberis infections resulted in a 1.0% reduction in fat content, whereas S. dysgalactiae infections led to a 0.2% reduction in lactose content. Intramammary infections caused by S. dysgalactiae had a moderate impact on milk yield, particularly in primiparous cows. Although the effects on milk composition were minimal, the reductions in milk production underscore the importance of managing infections to mitigate economic losses in dairy herds.