Norwalk and Noroviruses
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The Disease


What is it?

Norwalk virus causes viral gastroenteritis, which is also known as acute nonbacterial gastroenteritis, food poisoning, food infection, stomach flu, and winter vomiting disease. A Norwalk virus infection results in illness 24 to 48 hours after exposure, and symptoms last from 12-48 hours. Symptoms are generally mild and self-limiting. Serious illness is rare, and is usually associated with dehydration and/or electrolyte imbalance in elderly, disabled, or immunocompromised individuals. Illness is characterized by the abrupt onset of vomiting and/or non-bloody diarrhea; abdominal cramps are common. 25-50% of patients report headache, nausea, malaise, muscle aches and low-grade fever. It is important to note that there are other organisms such as rotaviruses which also cause viral gastroenteritis. Unlike rotaviruses, Norwalk virus infection is more frequent in older children and adults than in infants and toddlers.

 


How do you get it?

Noroviruses enters the body by the oral route and travels to the small intestine through the stomach. Virus replication occurs in the mucosal epithelium of the small intestine. This results in the broadening and flattening of the villi, damaging mucosal epithelial cells. Noroviruses do not appear to infect the cells of the large intestine. The exact binding site that Noroviruses attach to the epithelial cells is still being investigated. It is most likely that Noroviruses bind to a very specific cellular surface protein, which is not being expressed in animal models or cell lines.

New research suggests that there is a connection between Norovirus infection and a person’s blood type. A, B, and O phenotypes are determined by the presence or absence of carbohydrate antigens on glycolipids and glycoproteins found on the surface of mucosal epithelia as well as RBC’s. In this study, individuals with type O phenotype were found to be most susceptible to Norovirus infection. Individuals who express type B antigen (B and AB phenotypes) were at decreased risk of infection. When these B and AB phenotypes were infected, they were more likely to be asymptomatic.

 


Laboratory Diagnosis

Identification of Noroviruses can be made using electron microscopy since Noroviruses have a very characteristic morphology. This technique is only effective using stool specimens from infected patients within the early stages (within 72 hours) of illness, when large amounts of virus (106 – 107) are shed. Virus samples should but kept at 4°C and not be frozen, as this destroys the morphology of the virus.

Reverse transcription polymerase chain reaction (RT-PCR) can be used to detect the presence of Norovirus nucleic acid such as segments of ORF1 which codes for the RNA-dependent RNA polymerase. This detection method is very sensitive (102 – 104 viral particles/mL) and can be used to detect multiple strains of Noroviruses. Due to the specificity of this test and the variability between virus strains, there is always a chance that the RT-PCR primers will not recognize certain strains of Noroviruses.

Serological assays can also be used to detect the presence of Norovirus antigens. Genetically engineered strains of baculoviruses express Norovirus capsid proteins that self-assemble into virus-like particles. These particles can be used to generate Norovirus-specific antibodies from animals for use in immunoassays. Serum specimens from patients must be drawn at the proper times, and testing is restricted to the limited antigenic types (expressed in baculoviruses) available. For IgG assays, acute-phase specimens are taken with the first 5 days of symptoms and convalescent-phase specimens are taken from the third to sixth week after resolution of symptoms.

 


Treatment/Recovery

Antibiotics are ineffective against Norwalk virus or any other virus for that matter. Maintaining adequate fluid intake to replace losses from vomiting or diarrhea is important. The use of oral rehydration solutions (ORS) may be recommended by physicians. These are available in pharmacies without a prescription.

 


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