Pilot Study: Soluble LPS/IgG Milk Complexes in Relationship to Early Lactation Acute Mastitis in Dairy Cows

Prevalence of LPS in Gram-negative bacterial udder infections determines mastitis severity and disease prognosis. This pilot study explores the notion that milk-soluble (s) LPS/IgG complex levels in dairy cows link mastitis severity to intramammary Gram-negative infections during early lactation. Milk, within a single herd, was analysed from (i) 34 early lactating cows with acute mastitis and (ii) milk selected from peak lactation cows displaying either healthy (SCC < 100 × 10³ cells/mL, <i>n</i> = 146) or subclinical mastitis (SCC > 150 × 10³ cells/mL, <i>n</i> = 135) characteristics. Milk was assessed for (i) sLPS/IgG using an "in-house" ELISA, (ii) udder inflammation using LDH activity, and (iii) bacterial presence applying on-farm and standard microbiological laboratory techniques. Mean milk sLPS/IgG absorbances in acute mastitis cows were higher than those detected in healthy and subclinical mastitis cows, with mean differences of 0.35 (95% CI, 0.28 to 0.42) and 0.36 (95% CI, 0.28 to 0.44), respectively. On day 1 of acute mastitis, sLPS/IgG levels in milk containing only Gram-positive bacteria ranged from OD 0.04 to 0.14 (median = 0.1). In contrast, sLPS/IgG levels ranging from OD 0.27 to 1.42 (median = 0.58) and from 0.02 to 1.67 (median = 0.21) were detected in milk containing only Gram-negative bacteria or both Gram-negative and Gram-positive bacteria (i.e., polymicrobial), respectively. Furthermore, differential milk sLPS/IgG absorbance profiles (observed during the testing period days 1-3) were observed in cows with acute mastitis caused by Gram-positive, Gram-negative or polymicrobial infections. Our preliminary findings support the notion that milk sLPS/IgG complexes provide a link between mastitis severity and intramammary Gram-negative infections in dairy cows during early lactation.