NCCID Disease Debriefs provide Canadian public health practitioners and clinicians with up-to-date reviews of essential information on prominent infectious diseases for Canadian public health practice. While not a formal literature review, information is gathered from key sources including the Public Health Agency of Canada (PHAC), the USA Centers for Disease Control and Prevention (CDC), the World Health Organization (WHO) and peer reviewed literature.

This debrief was prepared by Aleksandra K. Wierzbowski.  Questions, comments and suggestions regarding this debrief are most welcome and can be sent to

What are Disease Debriefs? To find out more about how information is collected, see our page dedicated to the Disease Debriefs.

What are important characteristics of Norovirus?

Norovirus belongs to a group of related single-stranded, non-enveloped RNA viruses previously described as “Norwalk-like viruses” (NLV) named after the first outbreak in Norwalk, Ohio or Small Round Structured Viruses (SRSVs) because of their morphologic features. They are part of the larger Caliciviridae family. To date there are five recognized norovirus genogroups; three (GI, GII, and GIV) are known to affect humans. Among them, genogroup GII is the most prevalent human genotype, responsible for 95% of all infections.  More than 25 different genotypes have been identified within these three human genogroups and new genotypes replace circulating dominant type every 2-3 years due to recombination and antigenic drift.

PHAC-Pathogen Safety Datasheet-Norovirus


CDC-Norovirus –Clinical Overview

CDC-Norovirus-Fact Sheet

Noroviruses are a cause of majority of acute gastroenteritis in humans. They are the leading cause of acute gastroenteritis in all age groups, and the leading cause of severe acute gastroenteritis among children less than 5 years of age.  In addition, they are the leading cause of foodborne illness and the leading cause of illness and outbreaks from contaminated food.

CDC-Burden of Norovirus Illness and Outbreaks


Norovirus infection causes predominantly a gastrointestinal illness of sudden onset (inflammation of the stomach or intestines) but can include neurological symptoms as well. The most common symptoms include diarrhea (watery, non-bloody), vomiting (more common in children), nausea, cramping and stomach pain.  Other symptoms may include fever, chills, headache, body and muscle aches, and fatigue.


CDC-Norovirus –Clinical Overview



The symptoms of norovirus infections can be severe and people may feel very sick. Diarrhea and vomiting can occur many times throughout the day, leading to dehydration (decease in urination, dry mouth and throat, and feeling dizzy), fatigue and weakness. Although unpleasant, most people get better within 1 to 3 days and have no long lasting complications from norovirus illness.

Dehydration is the most common complication and can be serious. Young children, older adults, pregnant women and people with underlying medical conditions are at a greater risk of dehydration and might require hospitalization and treatment with intravenous fluid.   In infants and young children it may present as tearless crying, unusual sleepiness or fussiness. Mortality rates are low, even in hospital outbreaks, however norovirus infection has been identified as a contributing factor to the death of frail, elderly individuals.

CDC-Norovirus –Clinical Overview

PHAC-Pathogen Safety Datasheet-Norovirus



The symptoms of norovirus infection usually develop within 48 to 72 hours but can start as soon as 12 hours after being exposed to the virus.

CDC-Norovirus –Clinical Overview

PHAC-Pathogen Safety Datasheet-Norovirus



The main reservoir of norovirus is human feces and vomit.  Noroviruses are extremely contagious because of their low infectious dose (less than 20 virus particles are needed to cause an infection), high shedding titre (billions of virus particles are shed during symptomatic phase of the illness),  prolonged asymptomatic shedding (the virus is present before symptoms of feeling sick commence and for 2 weeks or more after the symptoms resolve, and even longer in immune compromised individuals), ability to resist some disinfectants, and stability in the environment (it resists freezing temperatures, heating to 60ºC, and disinfection with chlorine or alcohol).

Anyone can get infected with norovirus and get sick. Norovirus illness may occur many times in one’s life due to many different genotypes of noroviruses and being infected with one type of norovirus does not protect against other types. In addition, one can also be infected with the same genotype more than once as the immunity wanes quickly.  Consequently, overall the population is highly susceptible to norovirus infection.

A person is most contagious when they are feeling sick and few days after they recover. Some people exposed to norovirus infection may be asymptomatic, however they may still shed the virus and spread the illness.

The transmission route for norovirus is through close person-to-person contact with infected persons, contaminated food or water, or contaminated surfaces or objects. The mode of transmission is fecal-oral.  This usually happens by:

  • having contact with someone who is infected with norovirus (for example, caring for, diapering a child or sharing food or eating utensils with someone with norovirus illness)
  • touching surfaces or objects contaminated with norovirus then putting your fingers in your mouth, or
  • eating food or drinking liquids that are contaminated with norovirus
  • Norovirus can spread quickly in closed places like daycare centers, long-term care facilities, schools, and cruise ships. In Canada, most norovirus outbreaks happen from November to April.

Norovirus is the leading cause of illness and outbreaks from contaminated food.  Most of these outbreaks occur in the food service settings like restaurants where, infected food workers are frequently the source of the outbreaks by handling ready-to-eat foods, such as raw fruits and vegetables or cooked food with their bare hands before serving them and especially when not washing their hands properly after using the bathroom.

Some foods like shellfish particularly oysters as well as many fruits and vegetables can be contaminated at their source, before they are harvested by sewage contaminated waters.  Drinking and recreational waters can also be contaminated with norovirus and lead to waterborne outbreaks.

Furthermore, a person who is infected by eating contaminated food or drinking contaminated water can spread the virus to household members through direct contact or by touching and contaminating objects and surfaces.

It is possible for norovirus to spread through ingestion of aerosolized vomit but there is no evidence showing that people can get infected by breathing in the virus.

CDC-Norovirus –Clinical Overview

PHAC-Pathogen Safety Datasheet-Norovirus



The virus is commonly identified from stool or vomitus specimens of infected individuals but can also be used identified from environmental surfaces.  The organism can then be identified by direct microscopic visualization, immunoassay or PCR identification of the sample.

Real-Time PCR Assays

The most widely used diagnostic assay for detecting norovirus is the real-time reverse transcriptase-polymerase chain reaction (RT-qPCR).  This assay detects the genetic material (RNA) of the virus during acute phase of the illness to 2 weeks after the person recovers.

Enzyme Immunoassays (EIAs)

Several Food and Drug Administration (FDA) approved Enzyme Immunoassays (EIAs) for detecting norovirus during outbreaks are available however these EIAs are currently not sensitive enough (<50%) for diagnosing individual cases.

Genetic characterization

Genetic characterization of norovirus strains found in stool and environmental samples has been very useful in epidemiologic investigations by linking cases to each other, and suggesting a common source.

PHAC-Reference Diagnostic Tests

CDC-Norovirus-Laboratory Diagnosis

Norovirus is diagnosed through a specimen of vomitus or diarrhea (stool). A testing method, called the PCR test, is used to identify norovirus.   


Norovirus infection is highly contagious, and anyone can become infected more than once. To help prevent getting infected or infecting others:

  • Wash hands thoroughly, especially after using the toilet or changing a diaper
  • Avoid contaminated food and water, including food that may have been prepared by someone who was sick
  • Wash fruits and vegetables before eating
  • Cook seafood thoroughly
  • Dispose of vomit and fecal matter carefully, to avoid spreading norovirus by air, soak up material with disposable towels, using minimal agitation, and place them in plastic disposal bags
  • Disinfect virus-contaminated areas with a chlorine bleach solution, wear gloves
  • Stay home from work, especially if your job involves handling food, children should stay home from school or child care.
  • Avoid traveling until signs and symptoms have ended

 PHAC-Pathogen Safety Datasheet-Norovirus

CDC-Norovirus-Prevention and Control


There is currently no vaccine to prevent norovirus infections, although several vaccines are in development.

PHAC-Pathogen Safety Datasheet-Norovirus

CDC-Norovirus-Prevention and Control


Since there are many different genotypes of norovirus, people can get infected many times during their lifetime. It is possible to develop immunity to specific types, but it is not known how long immunity lasts. This may explain why so many people of all ages get infected during norovirus outbreaks. Also, whether someone is susceptible to norovirus infection or not may be determined in part by his or her genetic predisposition.

PHAC-Pathogen Safety Datasheet-Norovirus 

CDC-Norovirus-Prevention and Control


There is no specific therapy to treat people with gastroenteritis caused by norovirus infection. The disease is usually self-limited. The treatment is supportive and includes replacing fluids lost from vomiting and diarrhea and correcting electrolyte disturbances with oral rehydration fluids or intravenously in cases of severe dehydration.

Older children and adults may benefit from antimotility agents but these are not recommended for children under 3 years old. Adults can also be treated with antiemetic agents.  Antibiotics are of no benefit in treating norovirus infections.

PHAC-Pathogen Safety Datasheet-Norovirus




Norovirus is the most common viral cause of epidemic gastroenteritis worldwide; it is also a common cause of endemic diarrhea in community settings.   Norovirus illness can happen year-round, but outbreaks are more common in fall and winter months. The illness is easily spread in group settings where people are in close contact, like schools, hospitals, childcare facilities, nursing homes, cruise ships, and passenger trains.



WHO-Norovirus- Epidemiology


PHAC-Norovirus in Canada


In the United States, norovirus has become the most common cause of gastroenteritis in adults and children since introduction of the rotavirus vaccination.

CDC-Burden of Norovirus Illness and OutbreaksBack to top

What is happening with current outbreaks of Norovirus?

In 2018, a total of 172 cases of gastrointestinal illness linked to raw oyster consumption have been reported in three provinces: British Columbia, Alberta and Ontario.  No deaths have been reported. Individuals became sick between mid-March and mid-April 2018.

The outbreak of norovirus and gastrointestinal illnesses linked to raw oysters is considered over as of May 17, 2018 although it is still being monitored Public Health Agency of Canada.

PHAC-Public Health Notice — Outbreak of norovirus and gastrointestinal illnesses linked to raw oysters

PHAC-Epidemiological information: Outbreak of norovirus infections linked to raw oysters

British Columbia  

As of April 26, 2018, 132 cases of norovirus have been identified.The investigation has identified that most people were infected after consuming oysters harvested in south and central Baynes Sound, BC.

BCCDC-Outbreak of norovirus associated with raw BC oysters

BCCDC-Outbreak of norovirus associated with raw BC oysters


As of April 26, 2018, 15 cases of norovirus have been identified.Most people were infected after consuming oysters harvested in south and central Baynes Sound, BC.

Alberta Health-Update: Albertans reminded to cook shellfish to reduce risk of illness


As of April 26, 2018, 25 cases of norovirus have been identified.The investigation has identified that most people were infected after consuming oysters harvested in south and central Baynes Sound, BC.

PHO-Norovirus outbreak after consuming oystersBack to top

What is the current risk for Canadians from Norovirus?

Risk factors for becoming infected with norovirus include:

  • Eating in a place where food is handled with unsanitary procedures
  • Attending preschool or a child care center (young)
  • Living in close quarters, such as in nursing homes (elderly)
  • Staying in hotels, resorts, cruise ships or other destinations with many people in close quarters
  • Having contact with someone who has norovirus infection

Norovirus outbreaks are common in Canada therefore, therefore it is not unlikely to be exposed to or infected by norovirus.

About 300–400 outbreaks of norovirus are reported to the National Enteric Surveillance Program at the Public Health Agency of Canada each year. Outbreaks occur more frequently during the fall and winter months.

Certain foods increase the risk of getting sick with a norovirus.

Norovirus outbreaks have been linked to shellfish, such as oysters harvested from contaminated waters. The virus is also spread through contaminated or untreated drinking water.

Some people more likely than others to get very sick from a norovirus.

Infection can cause severe vomiting and diarrhea, which could lead to dehydration. This is more likely in the very young, older adults and those with weakened immune systems.

Noroviruses can be dangerous for some people.

Generally, noroviruses cause unpleasant symptoms such as diarrhea and vomiting. More severe illness, including dehydration, is more common in the very young, the elderly and those with weakened immune systems.

PHAC-Norovirus-RisksBack to top

What measures should be taken for a suspected norovirus case or contact?


Case definitions have been established for norovirus and are defined as:

Confirmed case:

Laboratory confirmation of an infection with or without symptoms

Probable case:

Acute clinical illness in a person who is epidemiologically linked to a confirmed case

Clinical illness:

Two or more episodes of vomiting in a 24 hour period or 3 or more episodes of diarrhea in a 24 hour period or 1 or more episode of both in a 24 hour period when non- Infectious causes have been ruled out

Community outbreak:

Two or more cases of clinical illness compatible with norovirus that can be epidemiologically linked to one another (i.e. associated by exposure with onsets within a 1-3 day period), at least one of which is laboratory confirmed

Institutional outbreak:

Two or more cases of clinical illness compatible with norovirus that are epidemiologically linked in an institutional setting, such as acute care or long term care facility


Place on contact precautions and encourage fluids to maintain hydration


Monitor for symptoms and educate regarding signs and symptoms of infection and methods to prevent transmission

PHAC-Norovirus-Case Definitions

CDC- Key Infection Control Recommendations



In Canada, outbreaks of norovirus are nationally notifiable since 2007.

PHAC-Notifiable Diseases

PHAC-Canada Communicable Disease Reports   (Please note this is an archived link)

Provincial and Territorial

Please refer to the NCCID’s Notifiable Disease Database for detailed information of norovirus reporting in different provinces and territories in Canada.  This site also provides links to official provincial and territorial legislations, regulations and guidelines.

NCCID-Notifiable Diseases Database


In the US, healthcare providers are not required to report individual norovirus cases, however health care providers should report all possible outbreaks of acute gastroenteritis, including suspected outbreaks of norovirus, to the appropriate state, local or territorial health department.



Norovirus infections are notoriously difficult to prevent and control, owing to their low infectious dose, high shedding titre, and environmental stability. Guidelines for the prevention and control of norovirus gastroenteritis outbreaks in healthcare settings as well as in the community are available.

For community they focus on notification of public health, hand hygiene, symptomatic persons, environmental cleaning, hotels and cruise ships.

For acute and long term care facilities and they focus on surveillance, education, hand hygiene, contact precautions, accommodations, environmental cleaning, patient/resident care and transfer, as well as guidelines for visitors and staff.

PHAC-Norovirus-Infection and Prevention

Clinical Microbiology-Infection Control for Norovirus

Guidelines for management of norovirus infection across continuum of care