Significance of viral subpopulations within Ark Serotype Infectious Bronchitis Vaccines

Abstract

As a result of spontaneous mutations and/or recombination occurring within the viral genome, infectious bronchitis virus (IBV) exists as multiple serotypes and genotypes that poorly cross protect. Molecular analysis of many field isolates in the United States (US) has revealed that the most frequent IBV isolates are Arkansas (Ark) DPI isolates despite extensive use of Ark DPI derived vaccines in the field. The factors behind the predominance of Ark DPI IBV isolates are unclear. Analysis of ArkDPI derived vaccines currently used in the US revealed that they are heterogeneous and that some minor viral subpopulations within these vaccines efficiently replicate in the upper respiratory tract of chickens and are selected as early as three days post vaccination. The proportion of these minor subpopulation differed among the Ark vaccines. We analyzed the significance of these viral subpopulations within the Ark serotype vaccines. We found that Ark vaccines containing high proportions of the selected subpopulations resulted in higher viral loads, more severe respiratory signs and tracheal pathology but also a higher immune response. In addition we found that temporal evolution occurs after Ark vaccine administration, resulting in some of the minor subpopulations persisting longer in the respiratory tract than others. These subpopulations iii were also more readily transmitted to contact non-vaccinated birds than others, further suggesting that these subpopulations have better fitness? When given simultaneously with IBV Mass serotype vaccine, we found interference with Ark vaccine virus replication as compared to when the Ark vaccines were administered alone, a factor that may contribute to poor immune response induction by some Ark vaccines. Persistence of Ark vaccine viruses may also give these viruses ample time to mutate and/or recombine and time for selection to occur resulting in generation of more virulent variants.