Genus: Norovirus


Genus: Norovirus

Distinguishing features

The strains in this genus form a distinct clade within the family Caliciviridae (Figure 4.Caliciviridae).

Virion 

See discussion under family description.

Genome organization and replication 

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.

Antigenicity

See discussion under family description.

Biology

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).

Species demarcation criteria

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. 

Member species

Exemplar isolate of the species
SpeciesVirus nameIsolateAccession numberRefSeq numberAvailable sequenceVirus Abbrev.
Norwalk virusNorwalk virusUS/1968M87661NC_001959Complete genomeNV
Norwalk virusLordsdale virusGII/UK/1995X86557Complete genomeLV
Norwalk virusMaryland virusGII/US/1987AY032605Complete genomeMV
Norwalk virusJena virusGIII/DE/1980AJ011099Complete genomeJV
Norwalk virusAlphatron virusGIV/NL/1998AF195847Partial genomeAV
Norwalk virusmurine norovirus 1GV/US/2002AY228235Complete genomeMNV1
Norwalk virusViseu virusGVI/PT/2007GQ443611Partial genomeVV
Norwalk virusdog norovirusGVII/HK/2007FJ692500Complete genomeCaV
Norwalk virusChiba-040502 virusGVIII/JP/2004AJ844470Partial genomeChV
Norwalk virusSapporo-HK299 virusGIX/JP/2007KJ196290Complete genomeSaV
Norwalk virusbat norovirusGX/CN/2010KJ790198Partial genomeBtRsV

Virus names, the choice of exemplar isolates, and virus abbreviations, are not official ICTV designations.