Emergence of GII.Pg norovirus in gastroenteritis outbreaks in Victoria, Australia

The ORF 1 GII.Pg genotype represents an obligatory recombinant comprising the ORF 1 GII.Pg genotype and a number of ORF 2 genotypes. The emergence, incidence, and molecular features of GII.Pg norovirus have never been considered in detail and are the subject of the current study. Over the period 2002-2013, GII.Pg norovirus was detected in 16 outbreaks in Victoria, Australia. It was first identified in 2009 and thereafter was detected at low level in each year of the study. GII.Pg norovirus outbreaks occurred in both healthcare and non-healthcare settings and could be found in individuals with a broad range of ages. The seasonality of GII.Pg norovirus outbreaks was significantly different from that of all other (non-GII.Pg) norovirus outbreaks. For the 15 GII.Pg norovirus outbreaks where ORF 2 sequencing data were available, two ORF 2 recombinant genotypes were found: GII.1 in 5 (33%) outbreaks and GII.12 in 10 (67%) outbreaks. The ORF 1 phylogenetic tree shows that the GII.Pg ORF 1 genotype fell into two distinct groups. The ORF 2 phylogenetic tree indicates that the GII.1 and GII.12 clusters each corresponded to one of the groups in the ORF 1 tree. This indicates the two recombinant forms were evolving in parallel and not one from the other. Analysis of age data indicates the GII.1 and GII.12 recombinant forms circulated in different ways in the community. J. Med. Virol. 88:1521-1528, 2016.