is a genus in the family Caliciviridae
, a group of nonenveloped, icosahedral viruses that have a single-stranded, positive sense RNA genome. The viral genome is 7.5 to 7.7 kilobases in length and contains three open reading frames (ORFs) (). ORF1 encodes a large polyprotein that is post-translationally cleaved into six nonstructural proteins that are involved in viral replication, including a N-terminal protein (designated p48 for Norwalk virus), a NTPase, a “3A-like protein” (designated p22 for Norwalk virus), a VPg, a viral protease, and a viral polymerase (Green 2007
). ORF2 encodes the major structural protein (VP1) that forms the capsid, and ORF3 encodes a minor structural protein, VP2. The viral capsid contains 180 copies of VP1 protein and a few copies of VP2, and in vitro
expression of the VP1 gene leads to the spontaneous formation of virus-like particles (VLPs). Virus replication takes place through a negative sense strand intermediate from which both positive sense genomic RNA and a positive strand subgenomic RNA containing ORFs 2 and 3 are produced (). NoVs cannot be cultivated in vitro
, with the exception of murine strains (Duizer 2004
; Wobus 2004
). NoV VLPs that are expressed from ORFs 2 and 3 are antigenically and morphologically similar to intact NoV particles. Thus, the VLPs are often used as a surrogate in laboratory studies and VLPs are under evaluation as a vaccine candidate (see below).
Figure 1 Norovirus genomic organization and replication strategy. Norovirus is a positive strand RNA virus with three open reading frames (ORFs) encoding a large polyprotein from which non-structural proteins are produced by proteolytic cleavage and two structural (more ...)
NoVs are antigenically and genetically diverse. The lack of in vitro
cultivation systems precludes classification of NoVs into distinct serotypes. Instead, NoVs are classified using genetic analysis. Currently there are five genogroups. Within a genogroup, NoVs are further divided into genotypes, or genetic clusters (Zheng 2006
). Genogroup is designated by a capital “G” and a Roman numeral, and genotype is designated by an Arabic numeral. The prototype NoV strain, Norwalk virus, belongs to genogroup I and genotype 1, and is designated as GI.1. Different criteria for separating strains into genotypes have been proposed (Katayama 2002
; Zheng 2006
). At the Third International Calicivirus Conference held in Cancun, Mexico, in 2007, there was general agreement among attendees to accept the methods proposed by Zheng et al (2006)
that require analysis of the entire VP1 capsid amino acid sequence for identification of a new genotype rather than analysis of shorter sequences, as originally proposed by Katayama et al. (2002)
. Using these criteria, there are at least 33 NoV genotypes, with 9, 19, 2, 2, and 1 genotypes belonging to genogroups I through V, respectively (). Genogroup and genotype are often inferred by analysis of shorter capsid sequences, or by analysis of other portions of the genome (e.g., viral polymerase). The latter approach can lead to misclassification because of the propensity of NoVs to recombine, most commonly near the junction of the first and second ORFs (Bull 2007
). Thus, a strain may have nonstructural genes that were originally associated with one genotype and a VP1 gene associated with a second genotype.
Figure 2 Classification of noroviruses into genogroups and genotypes. The Norovirus genus is divided into 5 genogroups (GI-GV). Each genogroup has from 1 to 19 different genotypes. GI.9 is a newly designated genotype, and its prototype strain is NoV/IF2036/2003/Iraq (more ...)
NoV strains that infect humans are found in genogroups I, II and IV. NoV genogroups III and V contain strains that infect cows and mice, respectively. Porcine strains are found in genogroup II (II.11, II.18, and II.19) and strains that infect feline and canine species are found in genogroup IV (IV.2). NoVs have tended to be species specific, although human NoV strains have been found in beef and porcine species by RT-PCR (Mattison 2007
), and a human GII.4 strain is able to infect gnotobiotic pigs and calves (Cheetham 2006
; Souza 2008
). However, to date no infections with animal strains have been identified in people.