Origin and evolution of Georgia 98 (GA98), a new serotype of avian infectious bronchitis virus.

We previously identified GA98, a new serotype of infectious bronchitis virus (IBV), which is closely related to the DE072 serotype of IBV genetically, but not antigenically. Herein, we analyzed the 421bp sequence of a hypervariable region (HVR) (position 114-534, counting from the ATG start site) of the S1 subunit of GA98 IBVs to further examine the evolution of these viruses. These viruses were isolated between the years 1997 and 2000. Phylogenetic analysis of the deduced amino acid sequence on that region indicated that GA98 isolates from different regions of Georgia were the result of a single introduction of the S1 gene of the DE072 serotype progenitor. Most of the mutations were nonsynonymous and had become fixed in a progressive manner. The evolutionary and mutation rates in the HVR was calculated as 2.5 and 1.5% per year, respectively. This new serotype of IBV appears to be evolving very fast compared with other serotypes of IBV. We further determined the complete coding sequence of the S1 gene of seven isolates obtained from one selected region in North Georgia. Together with virus neutralization data, it appears that GA98 arose from immune selection caused by DE072 vaccine use. Reasons for this conclusion are discussed.