Norovirus

Norovirus, also known as Norwalk virus, is the most common cause of gastroenteritis. The infection, sometimes referred to as the winter vomiting disease, is characterized by non-bloody diarrhea, vomiting, and stomach pain. Fever or headaches may also occur. Symptoms usually develop 24 hours after being exposed, and recovery typically occurs within one to three days. Complications are uncommon, but may include dehydration, especially in the young, the old, and those with other health problems. The virus is usually spread by the fecal–oral route. This may be through contaminated food or water or person-to-person contact. It may also spread via contaminated surfaces or through air from the vomit of an infected person. Risk factors include unsanitary food preparation and sharing close quarters. Diagnosis is generally based on symptoms. Confirmatory testing is not usually available but may be performed by public health agencies during outbreaks. Prevention involves proper hand washing and disinfection of contaminated surfaces. There is no vaccine or specific treatment for norovirus. Management involves supportive care such as drinking sufficient fluids or intravenous fluids. Oral rehydration solutions are the preferred fluids to drink, although other drinks without caffeine or alcohol can help. Hand sanitizers based on alcohols tend to be ineffective against noroviruses due to their being non-enveloped, although some virus genotypes are more susceptible. Norovirus results in about 685 million cases of disease and 200,000 deaths globally a year. It is common both in the developed and developing world. Those under the age of five are most often affected, and in this group it results in about 50,000 deaths in the developing world. Norovirus infections occur more commonly during winter months. It often occurs in outbreaks, especially among those living in close quarters. In the United States, it is the cause of about half of all foodborne disease outbreaks. The virus is named after the city of Norwalk, Ohio, where an outbreak occurred in 1968.

Signs and symptoms Norovirus infection is characterized by nausea, vomiting, watery diarrhea, abdominal pain, and in some cases, loss of taste. A person usually develops symptoms of gastroenteritis 24 hours after being exposed to norovirus. General lethargy, weakness, muscle aches, headaches, and low-grade fevers may occur. The disease is usually self-limiting, and severe illness is rare. Although having norovirus can be unpleasant, it is not usually dangerous, and most who contract it make a full recovery within two to three days. Norovirus can establish a long-term infection in people who are immunocompromised, such as those with common variable immunodeficiency or with a suppressed immune system after organ transplantation. These infections can be with or without symptoms. In severe cases, persistent infections can lead to norovirus‐associated enteropathy, intestinal villous atrophy, and malabsorption.

Virology

Transmission Noroviruses are transmitted directly from person to person (62–84% of all reported outbreaks) and indirectly via contaminated water and food. Transmission can be aerosolized when those stricken with the illness vomit or by a toilet flush when vomit or diarrhea is present; infection can follow eating food or breathing air near an episode of vomiting, even if cleaned up. The viruses continue to be shed after symptoms have subsided, and shedding can still be detected many weeks after infection. Vomiting, in particular, transmits infection effectively and appears to allow airborne transmission. In one incident, a person who vomited spread the infection across a restaurant, suggesting that many unexplained cases of food poisoning may have their source in vomit. In December 1998, 126 people were dining at six tables; one person vomited onto the floor. Staff quickly cleaned up, and people continued eating. Three days later, others started falling ill; 52 people reported a range of symptoms, from fever and nausea to vomiting and diarrhea. The cause was not immediately identified. Researchers plotted the seating arrangement: more than 90% of the people at the same table as the sick person later reported becoming ill. There was a direct correlation between the risk of infection of people at other tables and how close they were to the sick person. More than 70% of the diners at an adjacent table fell ill; at a table on the other side of the restaurant, the infection rate was still 25%. The outbreak was attributed to a Norwalk-like virus (norovirus). Other cases of transmission by vomit were later identified. In one outbreak at an international scout jamboree in the Netherlands, each person with gastroenteritis infected an average of 14 people before increased hygiene measures were put in place. Even after these new measures were enacted, an ill person still infected an average of 2.1 other people. A US Centers for Disease Control and Prevention (CDC) study of 11 outbreaks in New York State lists the suspected mode of transmission as person-to-person in seven outbreaks, foodborne in two, waterborne in one, and one unknown. The source of waterborne outbreaks may include water from municipal supplies, wells, recreational lakes, swimming pools, and ice machines. Shellfish and salad ingredients are the foods most often implicated in norovirus outbreaks. Ingestion of shellfish that has not been sufficiently heated – under 75 °C (167 °F) – poses a high risk for norovirus infection. Foods other than shellfish may be contaminated by infected food handlers. Many norovirus outbreaks have been traced to food that was handled by only one infected person. From March to August 2017, in Quebec, Canada, there was an outbreak of norovirus that sickened more than 700 people. According to an investigation by Canada's CFIA Food Control Agency, the culprit was frozen raspberries imported from Harbin Gaotai Food Co Ltd, a Chinese supplier. Canadian authorities subsequently issued a recall on raspberry products from Harbin Gaotai. According to the CDC, there was a surge in norovirus cases on thirteen cruise ships in 2023, which marks the highest number of outbreaks since 2012.

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