Facts about norovirus

Norovirus cause gastrointestinal illness to humans. Norovirus infection can cause vomiting, diarrhoea, and stomach pain. Less common symptoms are low fever, chills and headache. Vomiting can be sudden and frequent resulting in remarkable fluid loss. Death is rare but remains as a risk especially for elderly or persons with weakened immune system.



Recovery occurs usually in one or two days. The incubation period ranges between 12 and 48 hours. Sometimes, symptoms can be milder and last for a week but no long-term adverse health effects have been reported.

Noroviruses belong to the Caliciviridae family and they are well known as causing “winter-vomiting disease” or “stomach-flu” referring to their rapid spread in human populations especially during winter months. Noroviruses are relatively resistant in the environment: they can survive freezing as well as high temperatures (up to 60°C). The viruses survive long periods on different surfaces. Steam cooking of shellfish may allow them to survive. It is important to notice that the viruses can survive in up to 10 ppm chlorine, well in excess of levels routinely present in public drinking water systems (less that 2 ppm).

Transmission

Noroviruses are highly contagious and 10-100 viral particles may be sufficient to infect an individual. They are transmitted primarily through the faecal-oral route, either by consumption of contaminated food or water, or by spreading directly from person to person. Vomiting creates effectively aerosols with high content of virus particles, which enter the oral mucosa or contaminate surfaces. The virus survives long on different surfaces and thus, environment may serve as a source of new infections.  During one single outbreak of norovirus gastroenteritis, several modes of transmission usually occur. Even though the incubation period is relatively short (15-50h), since the infective dose is very small, and asympomatic shedding does occur, the origin of the outbreak is often difficult to confirm. For example, initially food or water borne transmission is often followed by secondary person-to-person transmissions to close contacts. Virus shedding usually starts with the onset of symptoms (mainly vomiting and diarrhoea) and may continue for 2 weeks after recovery.

As the immunity may only last a few months and is strain-specific, and given their genetic variability, infection can happen several times in a lifetime and affects individuals of all ages.  Susceptibility to infection is probably genetically determined. According to recent studies, persons of blood group 0 are at greatest risk for infection.

Outbreaks

Many different food items have been associated with norovirus outbreaks. Raspberries and oysters have caused several national and international outbreaks. In principle, any food item may become contaminated if handled by infected person or if washed or humidified with contaminated water. Norovirus infections spread effectively from person to person in community settings like hospitals, schools, day care centers and nursing homes. Several outbreaks have been recorded in cruise ships, which provide an ideal closed setting for the spread of infection.

Preventive measures 

• Proper hand washing after using the bathroom/toilet and always before start of food preparation;
• Isolation of sick patients, if possible;
• Careful environmental cleaning and disinfection of public places, including toilets, where vomiting has occurred. Disinfectants should have high virucidal effect;
• While cleaning up vomits it is advisable to use light masks to prevent infection through aerosols. Gloves should be used when cleaning and disinfecting the environment;
• Avoiding food handling when having gastrointestinal symptoms and staying away for at least 48 hours after the end of symptoms. For health care workers, intensified hand disinfection for 14 days with virucidal disinfectant is recommended after the end of symptoms.