NS3 blocking ELISA results of farm A. The relative signal of the mAb 1B3 is depicted after blocking with porcine sera. Blocking with FCS was used as a reference. Sera of unsuspicious sows and piglets from unaffected farms showed relative intensities >50%. Seroconverted sows that had given birth to APPV infected piglets and their CT-affected piglets have a strong serum blocking activity with signal intensities <30%. The specificity of the ELISA reaction was validated by IF staining of cell expressing APPV-NS3H (Additional file 3). The ELISA intensities of all sows and boars of farm A are shown to demonstrate the mixed reactivity within this affected herd. In addition, the ELISA values of 14 sows, which had a litter with well-documented APPV positive CT cases, are presented separately. The sows with APPV associated CT-litters exhibit a stronger reactivity in the blocking ELISA on average. Using a cut-off value of 50% the seroprevalence in farm A reached 35% (60/170).