GI noroviruses are relatively rare and systematic studies of the molecular epidemiology of GI norovirus outbreaks are lacking. The current study examined the molecular virology of GI norovirus outbreaks in Victoria, Australia (2002-2010). Of 1,617 norovirus outbreaks identified, 69 (4.3%) were associated with GI norovirus alone, 1,540 (95.2%) with GII norovirus alone and 8 (0.5%) with GI + GII. Some differences between GI and GII outbreak epidemiology were found. GI outbreaks peaked in the 2-month period November/December whereas GII outbreaks peaked in the 2-month period September/October and GI norovirus outbreaks were significantly more common in non-healthcare settings (37.7%) than GII outbreaks (9.5%). ORF 1/ORF 2 genotypes found in the 69 outbreaks involving GI norovirus alone were: GI.2/GI.2, 7 outbreaks; GI.2/GI.6, 18 outbreaks; GI.3b/GI.3, 14 outbreaks; GI.4/GI.4, 21 outbreaks; GI.8/GI.8, one outbreak; GI.d/GI.3, four outbreaks; and GI.e/GI.13, one outbreak. The current study appears to be the first to have identified the recombinant form, GI.2/GI.6. Whereas GI.2/GI.6 and GI.3b/GI.3 outbreaks occurred with equal frequency in both healthcare and non-healthcare settings, GI.4/GI.4 occurred predominantly in healthcare settings. GI ORF 1/ORF 2 genotypes found in the eight outbreaks involving GI + GII norovirus were GI.2/GI.6, GI.3b/GI.3, and GI.4/GI.4, indicating GI genotypes in GI + GII outbreaks were similar to those found in outbreaks involving GI alone. Apparent differences in the evolution of different GI genotypes were noted. GI.2/GI.2, GI.2/GI.6, and GI.4/GI.4 strains tended to undergo periodic shifts in nucleotide sequence whereas various GI.3b/GI.3 strains tended to circulate simultaneously.

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Objectives: The molecular and epidemiological features of community-based norovirus-associated sporadic gastroenteritis incidents (NASGIs) are poorly understood. This study examined these features and compared the findings with studies of community-based and institutional norovirus-associated gastroenteritis outbreaks (NAGOs). Methods: Fecal specimens from NASGIs and NAGOs that occurred in Victoria, Australia (2002–2007) were tested for norovirus by reverse transcription-polymerase chain reaction methodology. Norovirus genotype was determined by nucleotide sequence analysis. Results: 106 community-based NASGIs, 116 community-based NAGOs and 902 institutional NAGOs were identified. The mean age and gender ratio of individuals associated with community-based NASGIs and community-based NAGOs were similar but differed from that found for institutional NAGOs. Although GII.4 was the predominant genotype associated with all three incident types, the mix of genotypes was similar for community-based NASGIs and community-based NAGOs but that for institutional NAGOs was different. All three incident types had a similar seasonal periodicity due to the pronounced seasonal periodicity of GII.4 incidents. Conclusions: The molecular and epidemiological features of noroviruses associated with community-based NASGIs and community-based NAGOs are similar but are different from those found for institutional NAGOs.

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