Persistence of infectious bronchitis virus (IBV) in farms with chickens immunized with Massachusetts vaccines alone or combined with 793B serotype.

Infectious bronchitis virus (IBV) remains a major challenge for poultry production due to its genetic diversity and rapid evolution under vaccination pressure. This study evaluated how changes in vaccination strategy affect IBV lineage dynamics in commercial broiler farms using a sentinel-bird model. Six farms were monitored over 2 consecutive years. During the first survey (Y1), broilers were vaccinated with a combination of Massachusetts (GI-1) and 793B (GI-13) strains, whereas during the second survey (Y2) vaccination was restricted to Massachusetts alone. Non-vaccinated sentinel birds placed within vaccinated flocks were sampled at different time-points for IBV detection, genotyping, and serology. Overall IBV detection reached 65%, with GI-1, GI-13, and GI-16 lineages identified. In Y2, after withdrawal of the 793B vaccine, a significant expansion of the field strain GI-16 was observed (P < 0.0001), with widespread distribution across farms, while GI-13 continued to circulate despite cessation of its use. GI-1 was primarily associated with respiratory tissues (P = 0.003), whereas GI-16 showed increased detection in lymphoid tissues and older birds. Mixed infections and elevated antibody titres indicated ongoing viral circulation. These findings indicate that changes in vaccination strategy are associated with alterations in IBV lineage distribution, highlighting the importance of aligning vaccination strategies with circulating field strains and supporting continuous molecular surveillance.