Noroviruses - Fact Sheet

Noroviruses are a group of viruses that cause gastroenteritis (illness that usually includes diarrhea and/or vomiting) in people. This illness—gastroenteritis—is often called stomach flu, although it is not related to the flu (or influenza) which is a respiratory illness caused by the influenza virus. The term norovirus was approved as the official name for this group of viruses in 2002. Previously, they were called Norwalk-like viruses, as norovirus was first identified as a virus in 1972 after an outbreak in Norwalk, Ohio.

Noroviruses are commonly found throughout North America and are very infectious. Norovirus illnesses occur throughout the year but are more common in winter months and affect all age groups. There is no vaccine or medicine that will prevent a norovirus infection and norovirus illness can recur throughout a person’s lifetime.

• What causes infection with noroviruses?
• What are the symptoms of illness caused by noroviruses?
• How long do the symptoms last?
• How can norovirus infections be prevented?
• How long are people infectious?
• What is the treatment for norovirus infection?
• Are noroviruses dangerous?
• Have people died of noroviruses in Canada?
• Does the Public Health Agency keep track of noroviruses across the country?
• How many outbreaks do we see in Canada in a year?
• What's the difference between a case and an outbreak?

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What causes infection with noroviruses?

Noroviruses are found in the stool or vomit of infected people. They are very contagious and can spread easily from person to person. People exposed to the virus usually develop symptoms of illness within 24 to 48 hours, but symptoms can occur as early as 12 hours after exposure. People infected with norovirus can be contagious from the moment they start feeling ill to at least three days after they have recovered. Some people may be contagious for as long as two weeks after recovery.

People can become infected with the virus in several ways, including: through direct contact with another person who is infected (e.g. caring for or diapering an ill child, sharing food or eating utensils with an ill person), by touching surfaces or objects contaminated with norovirus (like door handles) or by eating food or drinking water that has been contaminated.

Most foodborne outbreaks of norovirus illness likely occur when food is contaminated by food handlers who have the virus, especially if they do not wash their hands properly after using the bathroom. Some foods can be contaminated at their source (e.g. shellfish such as oysters may be contaminated by sewage in water before they are harvested). Waterborne outbreaks are often caused by sewage contamination of drinking water (e.g. from wells) and recreational water.

The virus is able to survive relatively high levels of chlorine and varying temperatures. Noroviruses can survive on practically any surface including door handles, sinks, railings and glassware. On hard surfaces in the environment, they have been found to survive for up to 12 hours. On contaminated carpet, noroviruses have been found to survive for up to 12 days.

Noroviruses are easily spread in environments where people are in close contact, and outbreaks often occur in group settings such as schools, hospitals, child care facilities, nursing homes, cruise ships and passenger trains.

What are the symptoms of illness caused by noroviruses?

The symptoms of norovirus illness include nausea, vomiting, diarrhea and stomach cramps. Sometimes, people may have a low-grade fever, chills, headache, muscle aches and fatigue. The illness often begins suddenly, about 24 to 48 hours after exposure and the infected person may become very sick with frequent vomiting and/or diarrhea. In general, children experience more vomiting than adults.

How long do the symptoms last?

In most healthy people, acute diarrhea and vomiting usually last eight to twelve hours, and people normally recover in 48 hours. Symptoms may last longer in some people.

Most people feel better within one or two days, with symptoms resolving on their own and no long-term health effects occurring after illness.

How can norovirus infections be prevented?

There are various prevention methods you can follow to reduce the risk of norovirus infections:

• Wash hands frequently, especially after using the bathroom or changing diapers and before eating or preparing food. This is very important if you have recently been ill or in contact with an ill person.
• Do not eat raw or undercooked oysters or other shellfish.
• Wash fruits and vegetables thoroughly before eating.
• Thoroughly clean contaminated surfaces immediately after an episode of illness with hot, soapy water and then disinfect the area using a household disinfectant.
• Immediately remove and wash clothing or linen that may be contaminated with stool or vomit (use hot water and soap).
• Flush or discard any vomit and/or stool in the toilet and make sure that the surrounding area is kept clean.
• If you do become infected, avoid contact with others until you are well again and avoid preparing food while you have symptoms and for three days after you recover.

How long are people infectious?

People infected with norovirus are contagious from the moment they begin feeling ill to at least three days after recovery. Some people may be contagious for as long as two weeks after recovery. Good hygiene standards, including frequent hand washing, are very important during this period.

What is the treatment for norovirus infection?

There is no preventative treatment (no vaccine or antiviral medication) for noroviruses and they cannot be treated with antibiotics because antibiotics fight bacteria, not viruses.  However, healthy people normally recover within one or two days, with symptoms resolving on their own.

Those suffering from illness should drink plenty of fluids to prevent dehydration – a possible serious health effect of norovirus infection. The most helpful fluids for protecting against dehydration are oral rehydration fluids. These products are sold as pre-mixed fluids and are commonly found in drug stores. Other drinks that do not contain caffeine or alcohol can also help with mild dehydration; however, these drinks may not replace the nutrients and minerals lost during illness.

Young children, the elderly and people with other illnesses are at greatest risk for dehydration. Symptoms of dehydration in adults and children include a decrease in urination, a dry mouth and throat and dizziness upon standing. A dehydrated child may cry with few or no tears and be unusually sleepy or fussy. Severe dehydration can be serious and the ill person may require re-hydration in a hospital. If you think you or someone under your care is dehydrated, contact your healthcare provider.

Are noroviruses dangerous?

Generally, noroviruses cause unpleasant but self-limiting, gastrointestinal symptoms such as diarrhea and vomiting. More severe illness, including dehydration, is likely to occur in the very young, the elderly and those with weakened immune systems.

Have people died of noroviruses in Canada?

Deaths have occurred in long-term health care facilities during norovirus outbreaks, but it is difficult to say to what extent norovirus caused death in already frail or sick people. It could be considered a contributing factor but not the cause of death.

Does the Public Health Agency of Canada keep track of noroviruses across the country?

The Agency monitors for gastrointestinal disease cases and outbreaks through national surveillance programs. Norovirus outbreaks have been nationally notifiable since 2009, and are reported to the Public Health Agency of Canada through National Notifiable Diseases.

How many outbreaks do we see in Canada in a year?

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. Many outbreaks go unreported. The Public Health Agency of Canada expects to see additional norovirus outbreak reporting in the future, as norovirus outbreaks have been made nationally notifiable. Only the common cold occurs more often.

What's the difference between a case and an outbreak?

A case refers to illness in one person whereas an outbreak refers to two or more people linked by a common exposure within a specific time frame.