An attenuated infectious bronchitis virus (TW2575/98) vaccine strain was successfully developed after 75 serial passages in embryonated chicken eggs. However, the in ovo vaccination for disease control was not applied in practice because this vaccine strain is highly pathogenic to chicken embryos (CEs) causing early death, dwarfing and other harmful effects. We compared the differences in virus replication, pathological changes, and tissue tropism between the wild virus and attenuated vaccine strain in CEs inoculated with different viral titer levels, i.e. 0.1, 1 and 10 EID[Formula: see text]/egg. The wild virus caused dwarfing effect at high titer inoculation, whereas the attenuated vaccine strain caused the dwarfing effect only at a lower viral inoculation accompanied by the earlier infection establishment and embryonic death at high and medium titers. There were no significant differences in the pathological changes in CEs infected by both wild and attenuated strains. Detected by immunohistochemistry, the viral antigens of both strains could be found mainly at the epithelium of the chorioallantoic membrane, lung parabronchus, renal tubules and some in the spleen and heart serosa. These findings indicated that the early embryonic death and dwarfing is not related to the change in cell/tissue tropism of the vaccine strain, rather on the early infection establishment and viral load. We suggest that the vaccine strain inoculated titer could be adjusted to an optimal low level for in ovo vaccination to overcome the poor hatching rate for its higher virulence to chicken embryos.
Establishment of reverse genetics system for infectious bronchitis virus attenuated vaccine strain H120
