The molecular epidemiology of norovirus outbreaks in Victoria, 2016.

Noroviruses are a leading cause of outbreaks of gastroenteritis. This study examined the incidence and molecular characteristics of norovirus outbreaks in Victoria, Australia in 2016. Norovirus was detected in 52.4% of gastroenteritis outbreaks surveyed and the year was notable in that there was no significant temporal peak in norovirus outbreaks. Norovirus Open Reading Frame (ORF)2 (capsid) genotypes were successfully identified in 84 of 110 norovirus outbreaks and included GI.3, GI.6, GI.9, GII.2, GII.3, GII.4, GII.6, and GII.17. Norovirus GII.4 was the most common ORF2 genotype detected (55.9%). Other relatively common ORF2 genotypes included GII.2 (19.0%), GII.17 (9.5%), GI.3 (7.1%) and GII.3 (4.8%). The GII.P4_NewOrleans_2009/GII.4_Sydney_2012 recombinant emerged as an important new GII.4 form. The study also confirmed the identity of three ORF1/ORF2 recombinant forms as follows: GII.P16/GII.2, GII.P16/GII.3 and GII.P16/GII.4. Statistical analysis indicated GII.4 (ORF2) was much more common in healthcare settings than in non-healthcare settings. The study indicates 2016 was a transition year in Victoria, Australia, in that the previous norovirus epidemic strain had diminished to the point where it was no longer dominant but as yet no replacement epidemic strain had become obvious.