| Noroviruses(NoV) belong to the genus Norovirus in the family Caliciviridaes named human Caliciviridaes together with sapovirus.They are important cause of acute and sporadic nonbacterial gastroenteritis and highly contagious transmitted via faeces and vomit,either directly through contact with infected people or through objects touched by them, or indirectly via contaminated environmental surfaces.The main symptoms of NoV infections are diarrhea,abdominal pain,vomiting and a low-grade fever.Infection can occur at all ages.NoV can be divided in five genegroups(G)and further classified into genotypes or genetic clusters according to the sequence diversity of their capsids.NVs groupⅠcontains≥8 genotypes,groupⅡcontains 17 genotypes,and groupⅣcontains 1 genotype,of which viruses belonging to GⅠ,GⅡand GⅣcause infections in humans.Most prevalent in humans are NoV of GⅡ,and in this group, genotype GⅡ.4 has in recent years been identified as the cause of global epidemics.From time to time,new variants of this genotype appear and rapidly displace the resident dominant variant.In addition,GⅡviruses have been found in animals,which raises questions about their zoonotic potentialNVs have single-stranded,positive-sense RNA genomes of approximately 7.5 kb,which are was the first detected by Kapikian et al. (1972) and organized into three open reading frames(ORFs) that encode the nonstructural proteins(ORF1),The first ORF(ORF1) encodes a polypeptide with regions of similarity to helicase,cysteine proteinase, and RNA-dependent RNA polymerase(RdRp)-encoding regions of picornaviruses.ORF2 encodes a viral capsid protein(VP1),and ORF3 encodes a minor structural protein(VP2) associated with VP1 stability Lots of data show a new variant stain appeared and caused epidemic every one to two years.Such as 95/96US appeared in 1995 to 2001,Farmingtong Hills in 2002,Hunter in 2004,Sakai in 2004,2006a and 2006b in 2006NoV are divided into five distinct genogroups,and each genogroup can be divided into several genotypes.The classification of caliciviruses in genogroup and genotypes is based on the sequence of the capsid,partial or complete.According to a classification adapted from Green et al. and Ando et al.the genogroupⅠ(GⅠ),GⅡ,and GⅢare composed of seven, eight,and two genotypes,respectively.GⅣis represented by one strain, as is genogroupⅤ.This classification is in constant evolution with the discovery of new strains.Recently,Kageyama et al.proposed a classification with 31 genotypes(14 for GⅠand 17 for GⅡ).At the same time,Vinje et al.identified 15 genotypes in the GⅡ.Also,Zheng et al. defined 8 and 17 genotypes in the GⅠand GⅡ,respectivel.Lots of people suggest the rule of classification of NoV.this huge amount of genetic diversity is a direct result of the virally encoded RNA dependent RNA polymerase and strain recombination,appearance of NoV recombination shows insufficient of classification.A recombinant NoV can be defined as one that clusters with two distinct groups of NoV strains when two different regions(normally the capsid and polymerase) of the genome are subjected to phylogenetic analysis.However,genotyping based solely on the capsid sequence may not be sufficient with the growing identification of naturally occurring recombinant NoVs,particularly as the recombination breakpoint is close to or within the ORF1/2 overlap.Thus,recombinants would be missed by sequencing only the capsid region.Moreover,genotyping of recombinants has been further complicated as recombinants may cluster with a capsid genotype but the polymerase sequence does not cluster with polymerases from characterized strains.Recombination is a common event in RNA viruses.It has been reported in retroviruses,flaviviruses,enteroviruses,and to a much lesser extent, in negative-sense RNA viruses.RNA viruses show extremely high mutation rates,owing to lack of proofreading activity in their replicases.The genome of these viruses often undergoes recombination and segmentation; many genera of positive strand viruses show genome scale ordered RNA structure(GORS),that could play an important role in RNA virus replication and rapid evolution.RNA recombination is among the major driving forces of viral evolutionRecently,several naturally occurring recombinant NVs have been reported,the NoV strain Arg320 from Norfolk,USA with RdRp region like Lordsdale virus(GⅡ.4) and capsid region of the Mexican(GⅡ.3)strain; the NoV strain Mc37 from Thailand showed that its ORFI sequence has 97.2% nucleotide identity to that of Saitama U1 virus but only 71.3%and 67.9% nucleotide identity in ORF2 and ORF3,respectively.The NoV strain MD145-12 has hybrid genome comprising stretches from Lordsdale virus, Gifu96,SaitamaU1,U3,U4,U16,U17,and U25.GⅡb variants were detected for the first time in France in August 2000 Their RNA polymerases do not correspond to any known genotype,but their capsid genes are close to the capsid gene of viruses of three different genotypes,Hawaii,Toronto,Snow Mountain.However,detailed analysis of the clinical and serological features to establish what effects recombination has on clinical severity or viral antigenicity was not possible,because the number of virologically confirmed outbreak cases due to recombinant NVs was small.The study was conducted to investigate the molecular epidemiology, recombinant characterization of the sporadic community cases and progress of NoVs in Beijing in China.A total of 547 fecal specimens were collected during September 2007-February 2008 from adults,＞or=15 years of age suffering from acute gastroenteritis.332 specimens collected in September to October 2007 were subjected to ELISA at first and then RT-PCR to NoV,sapovirus,positive sample of Rota A tested in ELISA,Rota B,Rota C,Adenvirus,Picobirnavirus,torovirus,corovirus,the result the rate of Nov is 12.3%,Rota A 0.6%,Adenvirus 4.2%,PicobirnavirusⅠandⅡ1.5%, Rota B,Rota C torovirus,coronavirus zero.the rate of NoV higher than others virus tested and is the main cause in adult diarrhea,the rest 215 samples collected in November 2007-February 2008 was subjected to RT-PCR to NoV.All the specimens were sequenced and phylogenetic analysis for detection and characterization of NoVs.NoV infections were very common and is a predominant etiological factor in the adults.A majority(19.4% 106/547) of the patients suffered from diarrhea caused by NoV.3 for SVs. winter month seasonality supported NoV infections in beijing.The genotypes detected in this study were GⅡ/4,GⅠ/2,GⅠ/3,GⅠ/4,GⅡ/b,GⅡ/d, GⅡ/16,GⅡ/novell.Of these,NoVs GⅡ-4 was the most predominant genotype no matter ages(16.4%,90/547) or NoV infected people(84.9%90/106),followed by NoV GⅠ-3,GⅠ-2,GⅡb,GⅡd,GⅡ-16,GⅠ-4 and others 3G-Ⅱnovell.The Genogroup of NoVs is close to the predominance strains breaking out in worldwide,on the five groups of 15-24,25-34,35-44,45-60,＞60岁,rate of noros is 27.2%,20.6%,19.1%,16.3%,15.8%repectivly and NoVs more prone to age of 35-44,＞60 years,there is no statistics difference on the age and sex,It was interesting to note that we selected long fragment of sample random from the RNA polymerase region to the 3′end of the genome, eight different combinations of RdRp and capsid genes(GⅠ-3/GⅠ-8, GⅠ-2/GⅠ-6,GⅡ-6/GⅡ-14,GⅡ-4/GⅡ-3,GⅡ-b/GⅡ-13,) were identified. the result show Recombination of the NoV is a common case and genotype of norovirus is a genome diversity,we should not neglect it. |
PDF Full Text Request