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Norovirus: For health professionals

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What health professionals need to know about norovirus

Noroviruses (previously called Norwalk-like viruses) are a group of highly contagious small structured ribonucleic acid (RNA) round viruses belonging to the family Caliciviridae. They are the most common cause of sporadic gastroenteritis across all age groups. Although commonly referred to as 'stomach flu', norovirus illness is not related to the influenza virus.

Noroviruses cause approximately 90% of all outbreaks of epidemic gastroenteritis. They are an important source of foodborne outbreaks globally. Most outbreaks occur in:

Outbreaks occur throughout the year, but have a distinct winter seasonality. Norovirus persists in the environment, so outbreaks can last a long time. Some have lasted over 3 months. Humans are the only known reservoir for noroviruses.

Clinical symptoms

The incubation period for a norovirus infection is 24 to 48 hours (range 12 to 72 hours). Infections are usually self-limiting and mild to moderate in severity. Symptoms often begin suddenly. The main symptoms include:

In children, vomiting is more prominent than diarrhea, whereas in adults, diarrhea usually predominates. Gastrointestinal symptoms typically last 48 to 72 hours with rapid recovery. Dehydration is the most common complication.

Other symptoms may include:

Severe manifestations

People at higher risk of severe manifestations are:

Fever is more common in these people, and illness may last several days longer.

Neurologic complications, such as benign convulsions and encephalitis have been reported in children.

Chronic diarrhea is the most commonly reported chronic sequelae among immunocompromised individuals. It can last for many months, leading to wasting or failure to thrive. Other post-infectious sequelae may include dyspepsia, constipation and reflux.

Transmission

Norovirus infections are most communicable during the acute stage of the disease, but people can continue to shed the virus for 2 to 3 weeks after symptom resolution. Transmission only requires a small inoculum (<100 viral particles).

Asymptomatic individuals can also shed infectious virus particles up to 3 weeks after exposure. In immunocompromised individuals, viral shedding in stool can persist for months following infection.

Secondary transmission of norovirus infections within households, daycare centres or nursing homes is common.

Noroviruses can spread through the fecal-oral route, including:

Vomitus-oral transmission can also occur if virus particles become aerosolized and:

Noroviruses are able to survive relatively high levels of chlorine and temperatures ranging from freezing to as high as 60°C.

Norovirus is the leading cause of foodborne disease outbreaks. Outbreaks of norovirus can be difficult to contain given the small inoculum required for transmission and its environmental stability.

Diagnosis

Norovirus is usually diagnosed on clinical suspicion in a patient with acute onset of vomiting and/or watery diarrhea. The diagnosis of norovirus is usually presumptive in such patients. The likelihood of norovirus is higher in the setting of an outbreak or during the winter in temperate regions.

Confirming the diagnosis with stool testing is generally not necessary, although it may be useful in immunocompromised patients with severe or persistent symptoms.

Laboratory testing can confirm the diagnosis of norovirus infection using reverse transcriptase–polymerase chain reaction (RT-PCR) testing on a stool sample.

In patients with a positive norovirus test but atypical symptoms, such as dysentery/bloody diarrhea or voluminous watery stools, other pathogens should be ruled out before attributing them to norovirus. These would include bacterial causes of diarrhea, including cholera in endemic regions.

More than 1 similar illness resulting from a common exposure should raise suspicion for a norovirus outbreak and prompt involvement of public health officials for laboratory testing and epidemiologic investigation.

Asymptomatic shedding of norovirus has diagnostic implications, as diarrhea due to another cause in an asymptomatic carrier may be misattributed to norovirus infection. In addition, asymptomatic shedding has epidemiologic implications. For example, asymptomatic food handlers can transmit infection to others, as viral loads on the hands of asymptomatic and symptomatic food handlers during outbreaks are similar.

Treatment

Most patients begin to recover on their own after 1 to 2 days without specific medical treatment. There are no vaccines or medicines that will prevent a norovirus infection. Patients can be re-infected after having the virus.

Patients should drink plenty of liquids to replace fluids lost from vomiting and diarrhea. This is particularly important for:

In severe cases, patients may need to be hospitalized and given fluids intravenously.

Prevention and control

People can help prevent the spread of norovirus by practicing good hygiene, such as:

Isolating people who are sick until 24 to 72 hours after symptom resolution can also help reduce the spread of infection.

Surveillance

The Public Health Agency of Canada estimates that each year about 4 million people in Canada get sick from the food they eat. Norovirus causes more than 1 million cases of non-travel related foodborne illness each year.

Outbreaks of norovirus are nationally notifiable. Report any outbreaks that meet the national outbreak case definition to the federal government.

Canada uses different surveillance systems to monitor outbreaks of norovirus. These include:

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