Infectious Bronchitis Virus QX Field Progenitor Loses Nephropathogenicity After Attenuation into a Live Viral Vaccine

Introduction: Infectious bronchitis (IB) is a gammacoronavirus-induced respiratory disease in chickens. IB virus strain QX is additionally known for its detrimental nephropathogenicity. In this study we compared the phenotypic differences between an IB QX vaccine virus and its field progenitor with focus on viral replication and dissemination through the host. Materials and Methods: Specific pathogen free chickens were inoc-ulated intratracheally with either 10 3 EID 50 vaccine strain Nobilis IB Primo QX (MSD Animal Health) or field strain IBV-D388 and killed at 1 day intervals over an 8-day period. Histopathology was studied over time in the trachea, kidney, cloaca and gastrointestinal tract. Immunohistochemistry was used to demonstrate viral protein expression in these tissues. qPCR was performed on tracheal and cloacal swabs and kidney to reveal viral RNA loads. Results: IB Primo QX and IBV-D388 induced comparable tracheal le-sions, characterized by epithelial cell death and desquamation and heter-ophilic and lymphohistiocytic infiltration. Changes were earlier and more severe with IBV-D388, while tracheal viral RNA loads were comparable over time for both viruses. In contrast, degenerative and inflammatory le-sions and viral protein and RNA were only found after IBV-D388 infection in the kidneys. This renal viral presence was preceded by viral RNA detection in the cloaca and viral protein expression in the gastrointestinal tract, albeit without associated lesions in these tissues. Discussion: Dissemination to the kidneys by IBV-D388 might result from cloacal ascending infection. In contrast to its field progenitor, IB Primo QX was unable to cause renal infection, which likely considerably contributes to attenuation of its phenotype.