We previously demonstrated that adaptation of an embryo-attenuated infectious bronchitis virus (IBV) Arkansas Delmarva Poultry Industry (ArkDPI)-derived vaccine to chicken embryo kidney (CEK) cells shifted the virus population towards homogeneity in spike (S) and nonstructural protein genes. Moreover, the typical Ark vaccine subpopulations emerging in chickens vaccinated with commercial Ark vaccines were not detected in chickens vaccinated with the CEK-adapted virus. In this study, chickens vaccinated with a low dose (1.6 × 10(3) EID50/bird, where EID50 is 50% embryo infectious dose) of CEK-adapted Ark vaccine at 5 days of age showed a significant reduction of IBV RNA in lachrymal fluids and decreased incidence of IBV RNA detection in tracheal swabs 5 days after challenge compared to unvaccinated challenged chickens. In a second experiment, 5-day-old chickens were vaccinated with 10(4) or 10(5) EID50/chicken of CEK-adapted Ark vaccine, and protection was compared to chickens vaccinated with 10(5) EID50/chicken of the commercial ArkDPI-derived vaccine from which the CEK-adapted virus originated. All vaccinated chicken groups showed a significant reduction of respiratory signs and viral load 5 days after Ark virulent challenge compared to unvaccinated challenged controls. No viral subpopulations different from the challenge virus were detected in chickens vaccinated with CEK-Ark after challenge. In contrast, IBV S1 sequences differing from the predominant population in the challenge virus were detected in several chickens vaccinated with the commercial Ark attenuated vaccine. From an applied perspective, the CEK-adapted IBV ArkDPI-derived vaccine is an improved and effective vaccine candidate with which to protect chickens against virulent Ark-type strains.
Keywords: Arkansas; coronavirus; genetic variation; infectious bronchitis virus; vaccine; virus adaptation.