Serum IgG concentrations in cohorts of pigs from uninfected farm A (square symbols) and infected farm C (diamond symbols) from six weeks to slaughter. Panel A, ELISA results using whole cell plate coating antigen; panel B, ELISA results using recombinant Bhlp29.7 as plate coating antigen. # $ ^ indicate p < 0.05 compared with 6 w, 10 w and 14 w within the same group respectively, while + indicates p < 0.001 compared to the same time point in the negative herd. COD represents calibrated optical density. Data are the means ± SD.