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The strains in this genus form a distinct clade within the family Caliciviridae (Figure 4.Caliciviridae).
See discussion under family description.
Full-length norovirus sequences are available for more than 100 strains. The genome of human noroviruses is organized into three major ORFs while murine norovirus (MNV) possess a fourth ORF (ORF4) that encodes a virulence factor (VF1) (McFadden et al., 2011). ORF1 encodes the non-structural polyprotein. ORF2 encodes the major structural capsid protein (VP1) and overlaps by 14–17 nucleotides with ORF1 resulting in a −2 frameshift of ORF2. ORF3 overlaps by one nucleotide with ORF2 in a −1 frameshift and encodes a minor capsid protein (VP2). Based on phylogenetic analysis of VP1 amino acid sequences, at least 10 norovirus genogroups (labelled G followed by a roman numeric e.g., GI, GII, etc.) are currently recognized (Chhabra et al., 2019). Examples of representative viruses that form the norovirus genogroups are shown in Figure 4.Caliciviridae. GI viruses can be subdivided into nine genotypes, GII into 27 genotypes (Chhabra et al., 2019, Kroneman et al., 2013), GIII into three genotypes, GIV into two genotypes, GV into one, GVI into two, and GVII, GVIII, GIX, and GX each into 1 genotype.
Noroviruses have been detected in several mammalian species but also in bats, sea lions and harbor porpoise (Green 2013, Wu et al., 2016). Human noroviruses belong to GI, GII, GIV, GVIII and GIX (Chhabra et al., 2019). Porcine (three GII genotypes), bovine and ovine noroviruses (GIII) have been described and GV consist of viruses infecting mice and rats. Of all noroviruses, murine norovirus can be grown in established cell lines (e.g., RAW 264.7 cells), while recently successful replication has been reported for human noroviruses in stem cell-derived nontransformed human enteroid monolayer cultures (Ettayebi et al., 2016). Several mammalian species can be experimentally infected with human noroviruses. Recombination events, along with point mutations within the norovirus genome, are well-documented forces that drive norovirus evolution and possibly herd immunity. Norovirus recombination most frequently occurs between the junction of ORF1 and ORF2 (Bull et al., 2007).
Currently only one species is recognized. However, the range of host species and the nature and extent of diversity of the noroviruses will need further characterization to delineate species criteria.
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