Norovirus

Norovirus, also known as Norwalk virus, is a highly contagious virus and the most common cause of gastroenteritis worldwide. Often referred to as the winter vomiting bug, norovirus infection is characterised by symptoms such as non-bloody diarrhoea, vomiting, and stomach pain. It primarily affects the gastrointestinal tract and can lead to dehydration, especially in vulnerable populations such as the young, elderly, and immunocompromised individuals.

Transmission electron micrograph of Norwalk virus. The white bar = 50 nm.

Signs and Symptoms

Norovirus infection typically begins 12 to 48 hours after exposure. Common symptoms include nausea, vomiting, watery diarrhoea, and abdominal pain. Some may also experience fever, headache, muscle aches, and a general feeling of lethargy. Although most cases resolve within one to three days, the infection can occasionally lead to severe dehydration, particularly in high-risk groups. Chronic infection can occur in immunocompromised individuals, potentially leading to more serious gastrointestinal issues such as norovirus‐associated enteropathy and malabsorption.

Transmission

Norovirus is transmitted through the faecal-oral route, which includes direct person-to-person contact, consumption of contaminated food or water, and contact with contaminated surfaces. The virus is highly contagious, with as few as five virus particles capable of causing infection. Aerosolized particles from vomiting can also spread the virus, making environments like restaurants and cruise ships particularly susceptible to outbreaks.

Diagnosis

Diagnosis of norovirus is primarily based on clinical symptoms due to the lack of readily available confirmatory tests. In outbreak situations, public health agencies may perform tests such as polymerase chain reaction (PCR) assays for definitive diagnosis. These tests are highly sensitive and can detect even a small number of virus particles.

Prevention

Preventative measures include thorough hand washing with soap and water, disinfection of contaminated surfaces, and proper food handling practices. Alcohol-based hand sanitizers are less effective against norovirus due to its non-enveloped structure. Health care facilities should adhere to stringent cleaning protocols to prevent nosocomial infections.

Hand Washing and Disinfectants

Hand washing with soap and water is very important for reducing transmission. Surfaces contaminated by norovirus should be disinfected with a bleach solution or other effective disinfectants. Alcohol-based rubs are not recommended as they are less effective against this non-enveloped virus.

Health Care Facilities

In health care settings, preventing the spread of norovirus involves routine and terminal cleaning, patient cohorting, and isolation precautions. The CDC has published guidelines with specific recommendations for outbreak control in such environments.

Treatment

There is no specific antiviral treatment for norovirus. Management focuses on supportive care, primarily rehydration to counteract fluid loss from vomiting and diarrhoea. Oral rehydration solutions are preferred, but intravenous fluids may be necessary in severe cases. Antiemetics and antidiarrheals can help alleviate symptoms.

Virology

Noroviruses are non-enveloped, single-stranded RNA viruses belonging to the family Caliciviridae. The virus has a linear RNA genome of approximately 7.5 kilobases and is known for its genetic diversity, with at least seven genogroups identified. The virus's capsid structure, which can exhibit T=1 or T=3 symmetry, is very important for its infectivity and immune response evasion.

X-ray crystallographic structure of the Norwalk virus capsid

Epidemiology

Norovirus is responsible for approximately 685 million cases of disease and 200,000 deaths annually worldwide. It is particularly prevalent during the winter months and often occurs in outbreaks in enclosed environments like schools, nursing homes, and cruise ships. In the United States, norovirus accounts for about half of all foodborne disease outbreaks.

Annual Trend in Reports of Norovirus Infection in England and Wales (2000–2011). Source: HPA

Laboratory reports of norovirus infections in England and Wales 2000–2012. Source: HPA, NB Testing methods changed in 2007.

Human Genetics

Susceptibility to norovirus infection is influenced by an individual's histo-blood group antigens (HBGAs). For instance, individuals with non-functional fucosyltransferase FUT2 genes, known as non-secretors, have a reduced risk of infection with certain norovirus strains. This genetic variability explains the differences in susceptibility and immune response among different populations.

History

The name "Norwalk virus" originates from an outbreak in Norwalk, Ohio, in 1968. Since then, numerous outbreaks have been recorded globally. The virus was named "norovirus" by the International Committee on Taxonomy of Viruses (ICTV) in 2002. Despite its various names and nomenclature changes, norovirus remains a significant public health concern due to its high transmissibility and impact on vulnerable groups.

Self-assessment MCQs (single best answer)

What is the primary cause of gastroenteritis worldwide?

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Which of the following symptoms is NOT typically associated with norovirus infection?

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How is norovirus primarily transmitted?

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Which of the following is the most effective method for preventing norovirus transmission?

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Norovirus belongs to which family of viruses?

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Which of the following groups is NOT at higher risk for severe dehydration due to norovirus infection?

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What is the preferred method for rehydration in patients with norovirus infection?

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What is one of the main reasons norovirus is so highly contagious?

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Which genetic factor influences an individual's susceptibility to norovirus infection?

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In which type of environments are norovirus outbreaks particularly common?

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