Infectious Bronchitis Viruses with a Novel Genomic Organization

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Abstract
A number of novel infectious bronchitis viruses (IBVs) were previously identified in commercial poultry in Australia, where they caused significant economic losses. Since there has been only limited characterization of these viruses, we investigated the genomic and phenotypic differences between these novel IBVs and other, classical IBVs. The 3′ 7.5 kb of the genomes of 17 Australian IBV strains were sequenced, and growth properties of 6 of the strains were compared. Comparison of sequences of the genes coding for structural and nonstructural proteins revealed the existence of two IBV genotypes: classical and novel. The genomic organization of the classical IBVs was typical of those of other group III coronaviruses: 5′-Pol-S-3a-3b-E-M-5a-5b-N-untranslated region (UTR)-3′. However, the novel IBV genotype lacked either all or most of the genes coding for nonstructural proteins at the 3′ end of the genome and had a unique open reading frame, X1. The gene order was either 5′-Pol-S-X1-E-M-N-UTR-3′ or 5′-Pol-S-X1-E-M-5b-N-UTR-3′. Phenotypically, novel and classical IBVs also differed; novel IBVs grew at a slower rate and reached lower titers in vitro and in vivo and were markedly less immunogenic in chicks. Although the novel IBVs induced histopathological lesions in the tracheas of infected chicks that were comparable to those induced by classical strains, they did not induce lesions in the kidneys. This study has demonstrated for the first time the existence of a naturally occurring IBV genotype devoid of some of the genes coding for nonstructural proteins and has also indicated that all of the accessory genes are dispensable for the growth of IBV and that such viruses are able to cause clinical disease and economic loss. The phylogenic differences between these novel IBVs and other avian coronaviruses suggest a reservoir host distinct from domestic poultry.