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Norovirus is the cause of norovirus infection. Noroviruses (genus Norovirus) are a group of related, single-stranded RNA, nonenveloped viruses that cause acute gastroenteritis in humans. Noroviruses belong to the family Caliciviridae.
Norovirus was the first virus to present with gastroenteritis in human and was first identified from stool specimen. The illness caused by this virus was primarily given the term “winter vomiting disease”, due to its prominent gastrointestinal manifestations. An outbreak in 1968 at an elementary school in Norwalk lead to the identification of the virus. Norovirus is the leading cause of gastroenteritis among all age groups and is responsible for 64000 cases of diarrhea leading to hospitalization, 900,000 visits to the clinic among children of developed countries and 200,000 deaths among children below 5 in developing countries.
Norovirus is transmitted through person-to-person contact, food and water. Genotype GII.4 is mostly contact transmitted. Non-GII.4 genotypes such as GI.3, GI.6, GI.7, GII.3, GII.6 and GII.12 are mostly food-borne. Genogroup GI strains are more often transmitted through water. This is due to their higher stability in water compared to other strains of the virus.
Norovirus is among top ranks of food born viruses, globally. Transmission could occur in different stages of pre- and post-production of the food products. For instance, shellfish can be contaminated with fecal discharge in the water, fresh and frozen berries could be contaminated through water contaminated by sewage or contact during harvesting. Viral outbreaks through food-borne transmission can lead to a mixture of the viral strain and increased risk of genetic recombination. Studies show that about 7% of the foodborne outbreaks have a common source.
Norovirus also has a nosocomial transition, causing a major burden for health care services. Immunocompromised patients may develop numerous norovirus variations due to the chronic infection. This intra-host viral variation may lead to the appearance of variants not similar to any of the ones of previous outbreaks, thus can escape the herd immunity.
To date, animal norovirus strains have not been reported to infect human population, but there has been evidence of intra-species transmission. Human norovirus has been detected in the stools of pigs, cattle and dogs.
Norovirus could be classified into different genogroups and P (polymerase)-groups, then further into genotypes and P-types, based on diversity of amino acids of the complete VP1 gene and nucleotide diversity of the RNA-dependent RNA polymerase (RdRp) region of ORF1, respectively. The genus Norovirus consists of genogroups from GI to GX. Each of the genogroups, consist of 49 capsid genotypes (9 GI, 27 GII, 3 GIII, 2 GIV, 2 GV, 2 GVI and 1 genotype each for GVII, GVIII, GIX [formerly GII.15] and GX). There a few viruses classified into tentative new genogroups (GNA1 and GNA2) and genotypes (GII.NA1, GII.NA2 and GIV), of which only one sequence exists, awaiting additional sequences. Norovirus is classified into 60 P-types (14GI, 37 GII, 2 GIII, 1 GIV, 2 GV, 2 GVI, 1 GVII and 1 GX), based on nucleotide diversity of the RdRp region. Moreover, currently 2 tentative P-groups and 14 tentative P-types of Norovirus exists.
- Norovirus genome structure and protein coding regions: the genome is positive-sense single stranded RNA encoding three open reading frames (ORF). ORF1 encodes the nonstructural proteins. ORF2 and ORF3 encode the major capsid (VP1) and minor structural protein (VP2), respectively.
- Structural proteins: Norovirus consists of 90 dimers of VP1 and one or two copies of the VP2.
- VP1: This major structural protein encoded by ORF2, consists of 530–555 amino acids with calculated molecular weights of 58–60 kDa. The protein has two conserved domains and a central variable domain with antigenic characteristics defining the specificity of the strain. VP1 assembles into virus-like particles. VP1 has two major domains; 1) the shell domain (S) and 2) the protruding domain (P). The S domain is on the N-terminal (225 amino acids), containing the elements for icosahedron formation. The P domain is comprised of the remaining amino acids and has two subdomains of P1 and P2. The P domain contributes to the stability of the capsid and formation of protrusions on the virion. P2 has a hypervariable region which is thought to play a role in receptor binding, immune reaction and interactions of ABO blood group antigens associated with susceptibility to the viral infection.
- VP2: This minor structural protein encoded by ORF3, ranges from 208–268 amino acids with calculated molecular weights of 22–29 kDa. VP2 shows high sequence diversity among strains. The exact function of this protein in the virus is not yet known. It is suggested that VP2 might contribute in RNA genome packaging. VP2 is not necessary for viral particles assembly but it is necessary for the formation of an infection virus. 
- Nonstructural proteins
- p48 (p37)
- p22 (p20)
Norovirus has a cytoplasmic replication. It attaches to the host receptors and enters the cell through endocytosis. Since, it is a positive sense virus, replication and transcription follows the corresponding models for positive stranded RNA viruses. Translation occurs by leaky scanning, and RNA termination-reinitiation.
|Genus||Host Details||Tissue Tropism||Entry Details||Release Details||Replication Site||Assembly Site||Transmission|
|Norovirus||Humans; mammals||Intestinal epithelium||Cell receptor endocytosis||Lysis||Cytoplasm||Cytoplasm||Oral-fecal|
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