Abstract
BackgroundInfectious bronchitis virus (IBV) is one of the leading causes of mortality and morbidity in chickens. There are numerous serotypes and variants, which do not confer cross protection resulting in failure of currently used IBV vaccines. Although variant IBV isolates with major genetic differences have been subjected to comparative studies, it is unknown whether minor genetic differences in IBV variants within a serotype are different in terms of pathogenesis and eliciting host responses. Two Massachusetts (Mass) variant IBV isolates recovered from commercial layer flocks in the Western Canadian provinces of Alberta (AB) and Saskatchewan (SK) were compared genetically and evaluated for their pathogenicity, tissue distribution and ability to recruit and replicate in macrophages.ResultsAlthough whole genome sequencing of these two Mass IBV isolates showed low similarity with the M41 vaccinal strain, they had an identical nucleotide sequence at open reading frames (ORFs) 3a, 3b, envelop (E), matrix (M), 5a and 5b. The rest of the ORFs of these 2 IBV isolates showed 99.9% nucleotide similarity. However, upon experimental infection, we found that the IBV isolate originating from AB was different to the one that originated in SK due to higher tracheal lesion scores and lower lung viral replication and lower genome loads in cecal tonsils. Nevertheless, both IBV isolates elicited host responses characterized by significant macrophage recruitment to the respiratory tract and there was evidence that both IBV isolates replicated within tracheal and lung macrophages.ConclusionsOverall, this study shows that Mass variant IBV isolates, although possessing minor genetic variations, can lead to significant differences in pathogenicity in young chickens. Further studies are required to investigate the pathogenicity of these two Mass variant IBV isolates in laying hens.
Highlights
Infectious bronchitis virus (IBV) is one of the leading causes of mortality and morbidity in chickens
Molecular characterization of the mass IBV isolates, 15AB-01 and 15SK-02 For the isolate 15AB-01, total of 525,998 and 908,860 read sets were obtained from the direct allantoic fluid sample and layered virus sample, respectively from the Illumina MiSeq workflow
For isolate 15SK-02, a total of 4,355,902 and 3,731,578 reads were obtained from the direct allantoic fluid sample and layered virus sample, respectively, of which, a total of 3,146,584 and 3,152,622 reads were aligned with the IBV reference genomes
Summary
Infectious bronchitis virus (IBV) is one of the leading causes of mortality and morbidity in chickens. Two Massachusetts (Mass) variant IBV isolates recovered from commercial layer flocks in the Western Canadian provinces of Alberta (AB) and Saskatchewan (SK) were compared genetically and evaluated for their pathogenicity, tissue distribution and ability to recruit and replicate in macrophages. Identification of new variants and/ or serotypes of IBV have shown a wide variation of tissue tropism including urinary [3,4,5,6,7,8,9,10], gastrointestinal [6, 9, 11, 12], oviduct [9, 13] and bursa of Fabricius [11, 14]. IBV can replicate in the testes of cockerels [19]
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