Noroviruses are pathogens which can cause vomiting and diarrhoea in humans. These viruses belong to the virus family Caliciviridae and can be divided into so-called genogroups and genotypes based on genetic characteristics. In humans, noroviruses of genogroups I and II play the most significant role. They almost exclusively infect humans, meaning that animals play no role in their transmission. Within these genogroups, the virus changes continuously and new genotypes and virus variants develop regularly which thereafter have the potential to spread all over the world. Noroviruses are widespread and are among the most common pathogens causing gastrointestinal infections in Germany. In the environment, these viruses are very stable and can remain infectious there for days or even weeks. By contrast, sufficient heating of foods while cooking or frying them inactivates the pathogens.

Noroviruses can cause disease in people of all ages, although children under the age of five and older people over 70 are particularly often affected. Typical symptoms include severe vomiting and diarrhoea, which can lead to dehydration, commonly accompanied by headache and sometimes fever. Intake of just a few virus particles (10-100) can be sufficient to cause an illness. The first symptoms tend to start around 24 hours after infection (incubation period of between 6 and 50 hours) and typically last one to two days before recovery. After symptoms subside, people often still excrete large virus amounts, which is conducive to further transmission of the virus. Most norovirus illnesses happen during the winter months and disease outbreaks are common in communal facilities such as nurseries, hospitals and retirement homes. You can find further information about norovirus infections on the website of the Robert Koch Institute at www.rki.de/en.

Noroviruses are typically transmitted via the faecal-oral route, meaning through intake of virus-containing excretions via the digestive tract. Infected persons excrete noroviruses through stool or vomit. Large amounts of norovirus can be expected to be excreted in stool for at least two days after illness symptoms have subsided, but longer durations are also possible. After excretion, the norovirus is highly stable and can remain on objects and surfaces such as toilets or door handles for days or even weeks. This presents a risk of infection for other people. Food which has been contaminated with noroviruses and is not heated before consumption can also be a source of infection and illness.

Noroviruses are predominantly transmitted through direct contact with diseased people or their excretions and well as indirectly via contaminated surfaces. However, infection can also occur following consumption of contaminated foods. Foods can become contaminated with noroviruses in a variety of different ways. These include foods which have been treated or prepared by infected persons without proper adherence to general kitchen hygiene rules. Virus contamination can also occur through people whose illness symptoms have already subsided, but who are still excreting noroviruses immediately after being ill.

While noroviruses do not multiply in foods, they remain infectious for a prolonged period of time. Deep-freezing foods does not inactivate noroviruses. Their presence cannot be identified by the smell or the taste of the foods. If the contaminated food is then consumed raw or without being sufficiently heated, norovirus illness can be the result. Contaminated foods in community catering, on cruise ships or at larger events (buffets) can also lead to larger disease outbreaks.

A special role in norovirus transmission can play food that may come into contact with human wastewater during its production. One example is shellfish and oysters, which, under certain conditions during growing, can accumulate different viruses. If thereafter not properly heated or not cooked at all, they can lead to norovirus illnesses. Certain plant-based foods can also come into contact with human wastewater if they are improperly farmed, or can be contaminated with noroviruses by infected persons during harvesting. For example, frozen berries which were consumed raw or not thoroughly heated before eating have been identified as the cause of norovirus outbreaks in the past.     

There is currently no vaccination against noroviruses. Therefore, hygiene is the most important insurance against contracting and spreading norovirus infections. Thorough hand-washing with soap lowers the risk of becoming sick as well as of contaminating surfaces and foods with the virus.

It is also particularly important that foods are treated and prepared hygienically. Fruit, vegetable, fresh herbs, and salads should be carefully washed before consumption. Additionally, certain risk foods such as oysters and frozen berries should always be evenly and thoroughly heated before consumption in order to inactivate potentially present noroviruses. Short cooking or warming, for example in the microwave, might not be sufficient.

Infected persons should not prepare foods for others during or shortly after illness, as the norovirus can still be excreted in stool for a prolonged duration following the subsidence of disease symptoms.