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Norovirus

Synonym: Norwalk virus, winter vomiting bug
German: Norovirus

1 Definition

Noro viruses are positive single stranded RNA viruses of the caliciviridae family. The human-pathogenic strains cause the norovirus gastroenteritis in humans.

2 History

The norovirus was originally named after Norwalk, a city in Ohio. In 1968 there was an outbreak of gastroenteritis in an elementary school of that city. In 1972 the virus was imaged for the first time using an electron microscope. It was officially named norovirus in 2002.

3 Classification

Noroviruses can be separated in to 5 different genogroups (GI – GV). Humans are effected by viruses from the groups GI, GII and GIV. These groups can be further subdivided into various genotypes. The majority of norovirus infections in adults are caused by the genotype 4 of the genogroups II (GII.4). The viruses display a strong ability of antigendrifting as well as season antigenshifting. This results in the formation of numerous variants of the virus.

4 Morphology

Noroviruses are single stranded RNA viruses with a diameter of 35-39 nm. They possess an icosahedral capsid and display as a blurred, circular structure in the electron microscope. The norovirus genome has a size of approximately 7,3 to 7,7 kB and has many variable gene sequences. The viruses have a high tenacity against chemical and physical environmental influences. They can survive in a temperature range from -20°C up to +60°C.

Similar to the influenza virus, noroviruses change through antigendrift and seasonal antigenshift caused by genetic recombination of different viral strains. As of 2016 multiple different human pathogenic virus strains within the norovirus genus are distinguished:

  • Species Norwalk-virus
  • Species human norovirus alphatron
  • Species human norovirus saitama

The species Norwalk-virus can be further subdivided into subgroups:

  • Subtype Desert-Shield virus
  • Subtype Hawaii virus
  • Subtype Lordsdale virus
  • Subtype Mexiko virus
  • Subtype Norwalk virus
  • Subtype Snow-Mountain virus
  • Subtype Southampton virus
  • Subtype Wilkinson virus

Additional classified species of the norovirus are:

  • Species bovines norovirus CH126
  • Species bovines norovirus Jena
  • Species murines norovirus
  • Species norovirus of the pig (porcine norovirus)
  • Species norovirus of the oyster

5 Epidemiology

Noroviruses are not limited to a specific region and can be found world-wide. The natural reservoir of the human pathogenic virus strains is the human. The seasonal summit of infections is typically during the months from October to March. However, cases can occur year-round because of traveling. Infections present typically as endemics in public institutions (kindergartens, hospitals, nursing homes). The high resistance and infectiousness of the virus make it very difficult to control.

6 Pathophysiology

After transmission the noroviruses reproduces mainly in the small intestine. After roughly 1-2 days the first symptoms become present. These symptoms of an acute gastroenteritis include: Nausea, vomiting, liquid diarrhea and stomach pain. Sometimes there is also a loss of taste. These symptoms are maintained for about 1-3 days. Further signs of sickness can be generalized weakness, muscle pains, headache headaches, coughing and sub febrile temperatures. Normally the disease heals up completely after this timeframe and leaves no impairments.

The general course of illness is rather mild. Only in exceptional cases can an in-patient treatment be necessary. Very young and very old patients, as well as immunosuppressed patients are at an increased risk for a severe course of the illness. Isolated cases can lead to death, in particular if dehydration is not treated properly.

7 Vaccination

As of 2016 there was no vaccine for the norovirus available in Germany. Vaccination with recombinant virus particles (VLP) is currently in the stage of clinical testing. Oral or nasal administration of the VLP was able to induce a specific immune response and alleviate the symptoms but not prevent the infection. It is questionable if it can provide a dependable prevention of infections with newly arising virus strains. This is due to the same problematic as known from vaccinations against flu viruses.

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