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Structure of Norwalk Virus (Norovirus)
- The virus is non-enveloped, round, 27-nm particles with a ‘ragged’ outer edge but lacking a definite surface structure.
- It has a buoyant density of 1.33–1.41 g/cm3, an inability to propagate in vitro.
- Characteristically, Norwalk virus possesses a single capsid protein.
- The genome is a nonsegmented, single-stranded, positive polarity RNA genome.
- Ten prominent spikes and 32 cup-shaped depressions can be seen on the virion by microscopy.
- The structure of the capsid protein is organized into two domains joined by a flexible hinge.
- The inner shell (S) domain is composed of the N-terminal 225 residues and is involved in the formation of the icosahedral capsid shell.
- The protruding (P) domain forms prominent structures extending from the surface of the shell and is formed from the C-terminal half of the protein.
- The P domain is further organized into two subdomains (P1 and P2), and it has been suggested that these structures may be involved in binding to cellular receptors and may also be the determinants of strain specificity.
Genome of Norwalk Virus (Norovirus)
- RNA genome (monopartite, linear ssRNA(+) genome of 7.3 to 8.3 kb) of the Norwalk virus is organized into three major open reading frames (ORF1, 2, and 3) with a polyadenylated 3´-end.
- ORF1 encodes a large polyprotein, with three regions containing nonstructural protein motifs.
- The ORF2 encodes the major capsid protein.
- ORF3 encodes a basic protein.
- VPg is covalently linked to the 5´-end region of the positive-strand RNA genome.
Epidemiology of Norwalk Virus (Norovirus)
- Human caliciviruses have a worldwide distribution.
- Norwalk viruses are the most common cause of nonbacterial gastroenteritis in the United States, causing an estimated 21 million cases annually.
- Norwalk viruses are now established as the most important cause of epidemic nonbacterial outbreaks of gastroenteritis worldwide.
- National surveillance and diagnosis by EM of outbreaks of nonbacterial gastroenteritis in the UK have shown that Noroviruses (Norwalk virus) are a more common cause of infective gastroenteritis than Salmonella or Campylobacter (Food Standards Agency 2000).
- There has also been an increasing prevalence of Norwalk viruses in recent years, with a strong suggestion of a winter seasonality culminating with a major epidemic in the UK in January 2002.
- Similarly, since October 2002 several states in the USA have reported an increase in outbreaks of Norovirus infection.
- This virus originally caused a community-wide outbreak in Norwalk, Ohio.
- Subsequent surveillance and investigations of outbreaks in many parts of the world identified the potential of Noroviruses for causing epidemic gastroenteritis in semiclosed or communitywide populations, for example, families, healthcare institutions, holiday locations including cruise ships, educational establishments, and the catering industry.
- Outbreaks occur among children and adults, but rarely among neonates or very young children.
Replication of Norwalk Virus (Norovirus)
- Norwalk virus enters the body predominantly via the oral route.
- Virions are acid-stable, consistent with an ability to survive passage through the stomach.
- The virion causes infection first by binding to the cell receptor on the cell membrane and enter the cell.
- Receptor binding triggers conformational change which results in the release of viral RNA into the cell cytoplasm.
- VPg is removed from the viral RNA.
- Positive stranded RNA serves both as genomic and mRNA for these viruses and is translated into a large polypeptide known as non-capsid viral protein.
- Subsequently, the viral protein is utilized by the viral enzymes protease to form capsid protein of the progeny as well as several non-capsid proteins including the RNA polymerase.
- RNA polymerase initiates the synthesis of progeny RNA genomes.
- The infecting viral RNA is copied and the complementary strand serves as a template for the synthesis of new plus strands.
- The subgenomic RNA serves as a template for the translation of both the capsid and terminal ORF proteins.
- Replication is followed by packaging of plus strands into virions and maturation involves several cleavage events.
- The progeny virion assembly occurs by a coating of genomic RNA with the capsid protein in the cell cytoplasm called encapsidation.
- The release of progeny virions occurs by the lysis of the cell.
Pathogenesis of Norwalk Virus (Norovirus)
- The virus enters the body predominantly via the oral route.
- Virions are acid-stable, consistent with an ability to survive passage through the stomach and replication occurs in the jejunum.
- As few as 10 virions will initiate disease in humans.
- Damage to the intestinal brush border prevents proper absorption of water and nutrients and causes watery diarrhea.
- Partial flattening and broadening of villi with disorganization of the mucosal epithelium.
- Lamina propria infiltrated with mononuclear cells and vacuolization of mucosal epithelium.
- Crypt cell hyperplasia is common.
- Dilatation of the rough and smooth ER with an increase in multivesicular bodies in mucosal epithelial cells.
- Microvilli were significantly shortened and amorphous electron-dense was present in the expanded intercellular spaces.
- Shedding of the virus may continue for 2 weeks after symptoms have ceased.
- Immunity is generally short-lived and may not be protective.
Clinical manifestations of Norwalk Virus (Norovirus)
- Norwalk viruses cause gastroenteritis in adults.
- The illness in symptomatic cases typically begins after an incubation period of 24–48 hours.
- The illness is characterized by sudden onset of nausea, vomiting, which can be projectile and severe.
- Low-grade fever and diarrhea usually occur, the latter being relatively mild.
- In contrast to bacterial gastroenteritis, diarrheal stools do not contain blood, mucus, or white cells.
- Fecal leukocytes are absent.
- Other symptoms- mild abdominal pain, malaise, and headache.
- Vomiting may arise from a decrease in gastric motility, giving rise to a reflux action into the stomach.
- Gastric emptying is delayed, and malabsorption of fat, D-xylose, and lactose has been observed.
Lab Diagnosis of Norwalk Virus (Norovirus)
- Virus isolation by the transmission of Norwalk virus in chimpanzees, in which serological responses and excretion of Norwalk virus antigen in stools were described.
- The use of RT-PCR for detection of the genome in stool or emesis samples.
- Immunoelectron microscopy can be used to concentrate and identify the virus from the stool.
- The addition of an antibody directed against the suspected agent causes the virus to aggregate, thereby facilitating recognition.
- ELISA tests have been developed to detect the virus, viral antigen, and antibody to the virus.
- Both ELISA and RIA are the serodiagnostic tests frequently used to detect specific antibodies to Norwalk virus in the serum.
Treatment, Prevention, and Control of Norwalk Virus (Norovirus)
- No specific treatment is available for the Norwalk virus.
- Bismuth subsalicylate may reduce the severity of the gastrointestinal symptoms.
- No vaccine is available against the virus.
- Outbreaks may be minimized by handling food carefully and by maintaining the purity of the water supply.
- Careful hand washing is also important.
- Norwalk virus is resistant to heat (60° C), pH 3, detergent, and even the chlorine levels of drinking water.
- Contaminated surfaces can be cleaned with a 1:50 to 1:10 dilution of household bleach.
- Decontamination of all potentially infected surfaces in the kitchens and associated rest and toilet facilities is essential.
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