Increase of Escherichia coli inoculum doses induces faster innate immune response in primiparous cows

The objective of the current study was to evaluate the dynamics of infection and the immunological response to varying numbers of Escherichia coli injected into the mammary glands of primiparous cows during the periparturient period. Primiparous cows have been shown to be more resistant to intramammary E. coli challenge, and an increase of the inoculum dose by 2 log(10) units induced a more rapid clinical response and clearance of the organisms. Recognition of lipopolysaccharide (LPS) is a key event in the innate immunity response to gram-negative infection and is mediated by the accessory molecules CD14 and LPS-binding protein (LBP). Primiparous cows were inoculated with 1 x 10(4) (Group A; n = 8) or 1 x 10(6) (Group B; n = 8) cfu E. coli P4:O32 in their 2 left quarters during the periparturient period. Clinical examination and analysis of blood and milk parameters, including IL-8, complement fragment 5a (C5a), LBP, and soluble CD14 (sCD14), were performed from d -4 to d +3 relative to infection. Primiparous cows in Group B initiated a more rapid clinical response following intramammary infection (IMI), resulting in typical clinical signs and changes in blood and milk parameters approximately 3 h earlier compared with primiparous cows in Group A. Based on average milk production in the noninfected quarters on d +2 postinoculation, all heifers reacted as moderate responders. Distinct differences in the kinetic patterns of rectal temperature, somatic cell count (SCC), IL-8, C5a, LBP, and sCD14 were observed between both groups during the early phase of inflammation. Both C5a and IL-8 increased before cellular influx into the infected glands, followed by increases in sCD14 and LBP. In conclusion, primiparous cows are able to clear an intramammary E. coli infection efficiently. Moreover, increasing the inoculum dose induces a more rapid inflammatory reaction, mainly because of early activation of the innate host immune response.