A Biological Transition Around 200 × 103 Somatic Cells/mL in Dairy Cows: Inflammatory and Oxidative Changes

Somatic cell count (SCC) is routinely used as an indicator of mammary gland health, but the SCC level at which coordinated inflammatory and systemic changes become apparent remains uncertain. This study aimed to characterize biological changes across increasing SCC and identify the SCC range associated with a coordinated transition. The study included 580 clinically healthy Polish Holstein–Friesian cows without detectable intramammary infection (IMI), classified according to milk SCC as <100, 100–199, 200–399, or ≥400 × 103 cells/mL. Milk was analyzed for composition, inflammatory cytokines, and mammary innate-defense proteins, while blood was assessed for metabolic and hepatic indicators and oxidative-status markers, including malondialdehyde (MDA), total antioxidant status (TAS), superoxide dismutase (SOD), glutathione peroxidase (GPx), and glutathione reductase (GluRed). Relatively little variation occurred between the two SCC groups below 200 × 103 cells/mL, whereas SCC ≥ 200 × 103 cells/mL was associated with increased pro-inflammatory activity, significant changes in mammary innate-defense proteins, oxidative and metabolic changes, lower body condition, and reduced milk yield. Interleukin-8 showed the strongest independent association with SCC ≥ 200 × 103 cells/mL. Independently estimated breakpoints for 12 inflammatory, innate-defense, and oxidative variables converged within 198–228 × 103 cells/mL. Among the evaluated thresholds, 200 × 103 cells/mL provided 88.0% sensitivity and 82.0% specificity for discrimination of the inflammatory phenotype. These findings identify the region around 200 × 103 cells/mL as a coordinated biological transition rather than a universal diagnostic threshold for mastitis or intramammary infection.