The time-dependent levels of viral replication in the different tissues determined by real-time RT-PCR presented in Fig. 5 are consistent with the tissue histopathology results. Compared to rH120 and rH120-S1/YZ groups, one day after inoculation with the QX-like strain, rIBYZ, the viral load in all organs began to rise rapidly, except at 5 dpi in the trachea. The viral load in tracheal samples in the rH120-S1/YZ group increased and peaked at 5 dpi. The level of viral RNA was significantly lower than the rH120 group both at 1 dpi and 7 dpi (Fig. 5A). The viral load in lung samples from all infection groups increased starting at 3 dpi, and the viral RNA in the lungs of the rH120-S1/YZ group was significantly lower compared with both the rH120 and rIBYZ groups (Fig. 5B). The viral load in the kidney of all IBV-inoculated groups was detected at 1 dpi. In contrast to the rH120 and rH120-S1/YZ groups, the viral growth in kidney of rIBYZ-infected chickens exhibited an extremely significant increase at 3 dpi and peaked at 5 dpi. The viral RNA copies of the rH120-S1/YZ group peaked at 1 dpi, after which they gradually decreased (Fig. 5C). No viral load was detected in the control samples at any of the time points.
Fig. 5 Viral load in the different organs at 1, 3, 5, and 7 dpi of SPF chickens infected with the recombinant strains rH120, rIBYZ, and rH120-S1/YZ at 1-day-of age. The bars indicate means ± standard deviations. ***Indicates an extremely significant difference at P ≤ 0.001; **Indicates a highly significant difference at 0.001 < P ≤ 0.01; *Indicates a significant difference at 0.01 < P ≤ 0.05.