Background: Infectious bronchitis virus (IBV) is one of the leading causes of mortality and morbidity in chickens.\nThere are numerous serotypes and variants, which do not confer cross protection resulting in failure of currently\nused IBV vaccines. Although variant IBV isolates with major genetic differences have been subjected to comparative\nstudies, it is unknown whether minor genetic differences in IBV variants within a serotype are different in terms of\npathogenesis and eliciting host responses. Two Massachusetts (Mass) variant IBV isolates recovered from commercial\nlayer flocks in the Western Canadian provinces of Alberta (AB) and Saskatchewan (SK) were compared genetically and\nevaluated for their pathogenicity, tissue distribution and ability to recruit and replicate in macrophages.\nResults: Although whole genome sequencing of these two Mass IBV isolates showed low similarity with the M41\nvaccinal strain, they had an identical nucleotide sequence at open reading frames (ORFs) 3a, 3b, envelop (E),\nmatrix (M), 5a and 5b. The rest of the ORFs of these 2 IBV isolates showed 99.9% nucleotide similarity. However,\nupon experimental infection, we found that the IBV isolate originating from AB was different to the one that originated\nin SK due to higher tracheal lesion scores and lower lung viral replication and lower genome loads in cecal tonsils.\nNevertheless, both IBV isolates elicited host responses characterized by significant macrophage recruitment to\nthe respiratory tract and there was evidence that both IBV isolates replicated within tracheal and lung macrophages.\nConclusions: Overall, this study shows that Mass variant IBV isolates, although possessing minor genetic variations, can\nlead to significant differences in pathogenicity in young chickens. Further studies are required to investigate the\npathogenicity of these two Mass variant IBV isolates in laying hens.
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