Norovirus

Noroviruses belong to the genus Caliciviridae (caliciviruses), which takes its name from the cup-shaped depressions (calix, lat. = calyx) that can be detected on the capsid surface. Human pathogenic noroviruses belong to a group of non-enveloped single-stranded RNA viruses with positive polarity, which include a number of enteroviruses, such as parechovirus and hepatitis A virus (Robilotti E et al. 2015).

The virus has a diameter of 35 to 39 nm. It has an icosahedral (twenty-surface) capsid and a genome of about 7.3 to 7.7 kB. Noroviruses are, like most single-stranded RNA viruses, noroviruses are highly variable in their genome sequence. This variability causes numerous different subtypes. Noroviruses show antigen drift as well as a seasonal antigen shift due to genetic recombination.

The virus is adsorbed by receptors on the epithelial cells of the gastrointestinal tract. From the positive-stranded genomic RNA, a polyprotein is first translated, but this only covers the area of non-structural proteins. After autocatalytic cleavage of this polyprotein, the RNA-dependent RNA polymerase is also released, which writes antigenomic RNA molecules of negative polarity from the genomic RNA. After several RNA replication cycles and the translation of the structural proteins VP1 and VP2, the new genomes are packaged again. The newly formed virus particles leave the cell (Schnitzler P et al. 2019).

Faecal-oral contamination, also droplet infection possible: The virus is usually transmitted faecal-orally by ingestion of contaminated food or water. There are indications that contamination of the environment by vomit can also be a source of infection. Aerosol formation during vomiting plays a role here, leading to surface contamination or oral uptake of fine droplets. Although virus excretion is possible in the pre-symptomatic phase, the massive release of infectious viruses only begins with the onset of clinical symptoms. An accumulation of norovirus infections can be observed in the winter months ("winter vomiting disease").

The prognosis is favourable. However, the infection can end fatally in old, very young or immunocompromised patients.

Noroviruses are the causative agents of diarrhoeal diseases: Noroviruses are responsible, along with rotaviruses (family of Reoviridae), for the majority of non-bacterially caused diarrhoeal diseases (so-called gastroenteritis) in children (about 30% of cases) and adults (about 50% of cases). Children < 5 years and older adults are particularly often affected. The virus is highly contagious; 10 infectious particles are sufficient to infect an individual. Infections can occur sporadically in the form of outbreaks in community facilities and clinics (Karst SM et al. 2016).

Incubation time: approx. 10-50 hours.

High resistance to environmental influences: Human noroviruses show the resistance to environmental influences typical for non-enveloped viruses. They can survive in water at 25 °C for several days to weeks: Noroviruses survive temperature fluctuations from -20 to +60 °C and showed their "survivability" on a contaminated carpet even after 12 days.

The pathogen is detected by antigen detection (EIA) or by RT-PCR in stool. For special epidemiological questions, sequencing of the viral RNA can be useful.

High standard of hygiene: Only a high standard of hygiene can interrupt the chain of infection. This means that patients must be isolated during outbreaks and, in addition to consistent hand and surface disinfection, increased protective measures such as the wearing of gloves and possibly suitable respiratory protection should be taken (see Norovirus infections below).

Obligation to notify: The obligation to notify in accordance with the Infection Protection Act applies to the heads of diagnostic laboratories in the case of direct virus detection and to doctors if the sick person is employed in food production and/or food preparation in restaurants in accordance with § 42 of the Infection Protection Act.

1. Karst SM et al (2016) Recent advances in understanding norovirus pathogenesis. J Med Virol 88:1837-1843.
2. Karst SM et al (2014) Advances in norovirus biology. Cell Host Microbe 15:668-680.
3. Karst SM et al. (2015) What is the reservoir of emergent human norovirus strains? J Virol 89':5756-5759.
4. Koromyslova A et al (2017) Human norovirus inhibition by a human milk oligosaccharides. Virology 508:81-89.
5. Robilotti E et al. (2015) Norovirus. Clin Microbiol Rev 28:134-164.
6. Schnitzler P et al (2019) Viruses with positive-stranded RNA genome. In: Hof H, Schlüter D, Dörries R, Ed. Duale Reihe Medizinische Mikrobiologie. 7th, completely revised and extended edition. Stuttgart: Thieme S 205-206