The Comparative Molecular Epidemiology of GII.P7_GII.6 and GII.P7_GII.7 Norovirus Outbreaks in Victoria, Australia, 2012-2014.

The comparative molecular epidemiology of the related GII.P7_GII.6 and GII.P7_GII.7 noroviruses has not been examined in detail. ORF 1, ORF 2 and ORF 1/ORF 2 RT-PCR as well as sequencing and phylogeny analysis were carried out on faecal specimens from 873 gastroenteritis outbreaks in Victoria, Australia (2012-2014). There were 575 (66%) detected as positive for norovirus by means of ORF 1 RT-PCR and/or ORF 2 RT-PCR. Of these, 24 (4.2%) were GII.6 (ORF 2) outbreaks, 7 (1.2%) were GII.7 (ORF 2) outbreaks, and 1 outbreak (0.2%) involved both GII.6 (ORF 2) and GII.7 (ORF 2) noroviruses. The median age of patients identified with GII.6 (ORF 2) (84 years) was significantly different from that of patients identified with GII.7 (ORF 2) (39 years). ORF 2 GII.6 and ORF 2 GII.7 sequences were always associated with a GII.P7 ORF 1 sequence, and GII.P7 sequences fell into two clusters, with one corresponding to the GII.6 ORF 2 genotype and the other to the GII.7 ORF 2 genotype, thereby indicating that the ORF 1 has been evolving separately for the two viruses. Thus, two closely related noroviruses can have a markedly different incidence and epidemiology.