SUMMARY.

We investigated the significance of differing proportions of specific subpopulations among commercial Arkansas (Ark) Delmarva poultry industry (DPI) vaccines with regard to vaccination outcome. Two ArkDPI-derived vaccines that contain a higher proportion of viruses with S1 genes that become selected during replication in chickens exhibited more rapid establishment of those selected subpopulations in chickens, produced significantly higher viral loads in tears, and induced higher antibody responses compared with two other ArkDPI vaccines with lower proportions of viruses that become selected in chickens. The presence of higher proportions of selected subpopulations was also associated with a significantly higher incidence of respiratory signs early after vaccination and in some cases more severe tracheal lesions. However, one of the ArkDPI-derived vaccines with a lower proportion of selected subpopulations, despite producing a lower viral load in tears, also induced a higher incidence of respiratory signs later after vaccination and more severe tracheal lesions. Furthermore, one of the ArkDPI-derived vaccines with a higher proportion of selected subpopulations, despite producing a higher viral loads in tears, resulted in less severe tracheal damage. These discrepancies suggest that infectious bronchitis virus (IBV) load in tears may not always predict degree of tracheal damage and that phenotypic characteristics other than S1 may also be involved in severity of vaccine reactions following ArkDPI vaccine administration. We observed lower antibody responses to the vaccines that produced lower viral loads, which might contribute to the persistence of Ark serotype IBV vaccines observed in commercial flocks.

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