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 ORF2 genotype has been identified. The GII.e genotype first emerged in 2008. In 2012 the epidemic strain GII.e/GII.4 (2012 and 2012-like) became the predominant norovirus strain associated with norovirus outbreaks. Over the period 2008-2012 changes occurred in both the GII.e region and its ORF2 recombinant form, and both changes were needed for the epidemic strain to emerge.
Nucleotide sequencing of the ORF1 region of GII.e specimens showed two forms, the 2008-2010 form and the altered 2012 form. No GII.e outbreaks occurred in 2011.
Nucleotide sequencing of the ORF2 region of GII.e specimens showed that in 2008-2009, all the ORF2 sequences corresponded to the GII.4 (2007) variant, in 2010 all the ORF2 sequences corresponded to GII.4 (2012-like) and in 2012 the majority of the ORF2 sequences corresponded to GII.4 (2012) and GII.4 (2012-like). It was notable that a significant increase in the prevalence of the epidemic strain GII.e/GII.4 (2012 and 2012-like) did not occur until the norovirus underwent significant genetic changes in ORF1 after 2010.
The evidence indicates that the development of the 2012 GII.e/GII.4 (2012 and 2012-like) epidemic strains was due to evolutionary change rather than a novel recombination event. The results also support the notion that ORF1 is critical in determining the virulence of a norovirus strain.