Associations between systemic inflammatory blood biomarkers and intramammary infection caused by <i>Staphylococcus aureus</i> in susceptible and resistant Holstein cows

Intramammary infections (IMI), especially in their subclinical form, are a major concern in dairy herds due to their impact on udder health and productivity. This study assessed the associations between IMI induced by Staphylococcus aureus and 33 hematobiochemical parameters (inflammation, liver function, metabolism, and oxidative stress) in Holstein cows without clinical signs of disease. Four hundred multiparous cows were selected from a commercial herd based on genetic (estimated breeding values for somatic cell count [SCC]) and phenotypic data (SCC and mastitis history), and classified as resistant (TOP, n = 200) or susceptible (BOTTOM, n = 200) to mastitis. At time 0 (T0), milk cultures identified 37 cows positive for IMI pathogens (27 for S. aureus, 4 for S. dysgalactiae, and 3 for S. uberis) that were bacteriologically monitored also at T1 and T2 (2 and 4 weeks after T0, respectively) and sampled for the hematochemical parameters. Statistical analyses focused on cows infected with S. aureus, the only group with adequate case numbers. Infected cows showed evidence of systemic inflammatory and immune responses, such as increase in serum amyloid A (T1; p = 0.0496) and neutrophil counts (T2; p = 0.0337). Markers of hepatic function were also altered, with higher gamma-glutamyl transferase (T2; p = 0.0125) and aspartate aminotransferase (T2; p = 0.0304) concentrations, suggesting liver involvement during infection. Beta-hydroxybutyrate (T1; p = 0.0172) levels were elevated in infected cows, reflecting energy imbalance. Overall, these findings underscored the value of blood biomarkers for the early detection of affected animals.