Noroviruses are considered one of the most common causes of virus-related infections of the gastrointestinal tract worldwide. They are responsible for a large proportion of outbreaks of acute viral diarrhoeal diseases in community settings such as schools, hospitals, and retirement and nursing homes.
Noroviruses are non-enveloped RNA viruses and belong to the Caliciviridae family. They are distributed worldwide and are the most common cause of viral intestinal infections. Infection occurs via stool or vomit either directly through person-to-person contact or indirectly via contaminated objects and food. Humans are the only known reservoir for human pathogenic noroviruses. Norovirus infections occur more frequently in the winter months.
Due to their high stability in the environment and the low infectious dose, norovirus outbreaks with a large number of infected persons occur time and again, especially in community facilities such as schools, kindergartens, hospitals and senior residences.
Noroviruses are very contagious and are easily transmitted from person to person. They are present in large quantities in the stool and vomit of ill persons. Even small amounts (10 - 100 virus particles) can lead to illness. The cause of further spread is often insufficient hand hygiene.
The following transmission routes are known:
- Direct contact with stool or vomit of an infected person.
- Contact with virus-containing droplets suspended in the air, which can be produced during explosive vomiting
- Contact with surfaces (e.g., doorknobs) or objects contaminated with noroviruses
- Eating or drinking food that has been contaminated with noroviruses
Food can be contaminated by people shedding norovirus during processing or by washing or watering with water containing the virus. Frozen soft fruits in particular are considered a possible source of infection in this context. Mussels are a frequent cause of norovirus outbreaks worldwide, as they can filter out the virus particles from faecally contaminated seawater.
The incubation period for norovirus infections is approximately 6 to 50 hours. Persons suffering from norovirus gastroenteritis are highly contagious as long as they show symptoms of illness. After clinical symptoms resolve, contagiousness persists for at least 2 days. If careful hygiene in the toilet area and thorough hand hygiene are observed, there is no longer any relevant risk of infection after this period, but it should be noted that excretion in small quantities can continue for 7 to 10 days, and in exceptional cases for several weeks.
The disease usually begins with violent vomiting and diarrhoea; other symptoms that may occur are abdominal pain, headache, muscle aches and fever. The symptoms usually last 12-48 hours. Asymptomatic courses are also possible with norovirus infections. These cases, if hand hygiene is inadequate, often transmit the disease via food. In principle, a norovirus infection is a self-limiting disease, but there is a risk of dehydration (dehydration, exsiccosis), especially in children, older people or people who are weakened due to an underlying disease. In this case, immediate medical attention is required!
There is no antiviral therapy available against noroviruses. Treatment is symptomatic due to the sometimes considerable loss of fluids and electrolytes. During the acute phase of the disease, patients should stay in bed and drink sufficient fluids (water, tea). Special electrolyte mixtures from the pharmacy can be used to compensate for both fluid and mineral salt loss.
There is no vaccine that protects against norovirus. The most important preventive measure is personal hand hygiene (thorough hand washing before eating and after each visit to the toilet). To prevent the spread of noroviruses in communal facilities (e.g. schools, kindergartens), hospitals, retirement homes and kitchens, the general hygiene rules must be consistently observed. Especially in community catering and gastronomy, people should be aware that certain foods such as imported frozen berries or shellfish (e.g. oysters) may be contaminated with noroviruses. Consumption of raw or undercooked shellfish is a common cause of norovirus infections worldwide.
Measures to be taken during outbreaks
In outbreaks, it is important to quickly identify the source of infection. If contaminated food or drink is a possible cause, immediate action must be taken to eliminate the effect of this source. If there is evidence of a norovirus outbreak, control measures should be initiated immediately, even before laboratory diagnostic confirmation. To avoid faecal-oral transmission, comprehensive hygiene measures must be initiated (wearing gloves and protective gowns, segregation of sick persons, additional cleaning of toilets, intensified hand hygiene, frequent disinfection of bed linen).
In communal facilities such as hospitals and old people's homes, patient, resident and staff movements within wards should be restricted as far as possible in order to minimise the spread between individual wards and areas of the facility. Sick staff should be excused from work even in the case of minor gastrointestinal symptoms and should not resume work until at least 2 days after the clinical symptoms have ceased, with careful attention to hand hygiene.
Stool or vomit from the sick person are suitable for testing for noroviruses. Stool samples are usually analysed for the diagnosis of norovirus. Ideally, stool samples should be taken during the symptom phase, as the viral load is highest during this infection phase. Stool containers should be filled to at least one-third full. The filled vessel must be stored in an over-vial for safe transport. If short-term storage of stools is necessary (e.g., while collecting specimens during an outbreak), the vessels should be kept refrigerated.
Refrigeration is not required during short-term transport. In principle, the same instructions regarding transport and storage apply to vomit as to stool specimens. In the event of an outbreak, it is advisable to examine no more than 5 stools from acutely ill patients. Once the diagnosis confirms the presence of norovirus infection, no further testing is required.
- Detection of viral RNA by polymerase chain reaction (RT-PCR).
- Typing of noroviruses by nucleic acid sequencing and database analysis within the viral RNA polymerase region
- Electron microscopy