Emergence of GII.e as a major ORF 1 norovirus genotype and its associated ORF 2 GII.4 variant forms

The noroviruses are a major cause of outbreaks of gastroenteritis. The norovirus genotype "GII.e", identified by ORF (Open Reading Frame) 1 nucleotide sequencing, appears to be an obligatory recombinant, in that no unique GII.e ORF 2 genotype has been identified. In 2012 GII.e norovirus became the predominant ORF 1 genotype in norovirus outbreaks in Victoria, Australia, and the current study documents changes in the ORF 1 region of GII.e norovirus since it first emerged in 2008, as well as in the ORF 2 genotypes associated with GII.e norovirus. GII.e norovirus underwent significant genetic change in ORF 1 between 2010 and 2012 and this genetic change corresponded to a significant increase in the prevalence of the virus. Nucleotide sequencing of the ORF 2 region of GII.e specimens showed that in 2008-2009, all the ORF 2 sequences corresponded to the GII.4 (2007) variant, in 2010 all the ORF 2 sequences corresponded to the GII.4 (2012-like) variant and in 2012 all the ORF 2 sequences corresponded to the GII.4 (2012) variant, the GII.4 (2012-like) variant, or the GII.4 (2009-like) variant. The evidence indicated that the development of the 2012 GII.e epidemic strains was due to evolutionary change rather than a novel recombination event. The results also support the notion that ORF 1 is critical in determining the virulence of a norovirus strain.