Opsonic activity of serum and whey from cows immunized with the ferric citrate receptor

The effects of immunizing dairy cows with the ferric citrate receptor, FecA, on the opsonic activity of serum and whey were measured in a phagocytosis assay. Fifteen cows were assigned to five blocks of three cows based on date of expected parturition. Cows within a block were randomly assigned to one of three treatments: 1) FecA immunization, 2) immunization with a commercially available Escherichia coli J5 bacterin, and 3) unimmunized controls. Cows were challenged at approximately 21 DIM by intramammary infusion of E. coli 727 into one mammary quarter. Escherichia coli 727 were opsonized for the phagocytosis assay with either 10% heat-inactivated serum or 50% heat-inactivated whey collected from each cow at calving, immediately before challenge and 7 d after challenge. Cows immunized with FecA or the E. coli J5 bacterin had increased IgG titers against FecA and E. coli 727 compared with unimmunized control cows. However, sera and whey collected from cows immunized with FecA did not enhance opsonization of E. coli 727 compared with sera and whey from control cows. Immunization with the E. coli J5 bacterin increased opsonization of sera greater than immunization with FecA. Immunoglobulin M antibody titer against E. coli 727 in whey and phagocytic indexes were positively correlated. The phagocytic index of whey immediately before challenge and 7 d after challenge were negatively associated with peak bacterial counts in mammary quarters challenged with E. coli 727. Results of the current trial suggest that the immune response resulting from immunization with FecA did not enhance opsonization and in vitro phagocytosis of E. coli 727.