Causes

What causes infection with a norovirus? How is it spread?

Noroviruses are found in the stool or vomit of infected people. The viruses are very contagious and can spread easily from person to person. People can become infected with the virus in several ways, including

• through direct contact with another person who is infected (for example, caring for or diapering an ill child, sharing food or eating utensils with an ill person);
• touching surfaces or objects contaminated with a norovirus (such as door handles); and
• eating food or drinking water that has been contaminated.

People exposed to the virus usually develop symptoms of illness within 24 to 48 hours, but symptoms can occur as soon as 12 hours after exposure. People infected with a 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.

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

Symptoms

What are the symptoms of a norovirus?

The most common symptoms of norovirus illness are

• nausea,
• vomiting,
• diarrhea, and
• stomach cramps.

Symptoms can also include

• 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 24 to 72 hours, and people normally recover within one to two days. 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 long are people infectious?

People infected with a 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 practices, including frequent hand washing, are very important during this period.

Risks

Are there certain foods that 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.

Are 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.

Are noroviruses dangerous? Are they more 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.

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 rather than the cause of death.

Norovirus illnesses affect all age groups and occur throughout the year but are more common in winter months. Norovirus illness can recur throughout a person’s lifetime.

Treatment

How is a norovirus diagnosed and what is the treatment?

A norovirus is diagnosed through a laboratory test on the stool of an infected person.

There is no vaccine or antiviral to prevent getting sick with a norovirus and antibiotics are not effective in treating the illness (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 a foodborne illness. 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.

Prevention

How can norovirus infections be prevented?

Proper hygiene and safe food handling and preparation practices are key to reducing the risk of all food borne illnesses including noroviruses.

• Wash your hands thoroughly with warm water and soap after using the washroom and before preparing food.
• Make sure that you wash your hands thoroughly with warm water and soap after changing diapers. Dispose of dirty diapers properly.
• If your child is sick with a norovirus, keep him or her home from daycare or school, and reduce contact with other children. Do not allow children who are ill and infants in diapers in areas where food is prepared.
• Ensure your children wash their hands thoroughly, especially if they have persistent diarrhea. This is particularly important for children who are still in diapers.
• Carefully wash fruits and vegetables with clean water.
• Keep raw foods away from other foods while shopping, storing and preparing foods.
• Avoid raw shellfish, including oysters. Cook thoroughly before eating. .
• Make sure to thoroughly clean any vomit and/or feces with soapy water and disinfect with a bleach solution immediately after illness.
• Immediately remove and wash clothing or linens that may be contaminated with the virus.
• After handling foods in the kitchen, especially raw foods such as meat and fish, thoroughly clean and sanitize all surfaces used for food preparation with a kitchen sanitizer (following the directions on the container) or use a bleach solution (add 5 ml household bleach to 750 ml of water), and rinse with water.

If you think you are infected with a norovirus or any other foodborne or waterborne illness, do not prepare food for other people.

Surveillance

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

Yes. The Public Health Agency of Canada works with the provinces and territories to track outbreaks of norovirus across the country.

Are norovirus outbreaks common in Canada?

About 300–400 outbreaks of norovirus are reported to the National Enteric Surveillance Program at the Public Health Agency of Canada each year. Only the common cold occurs more often.

Outbreaks occur more frequently during the fall and winter months. Many outbreaks go unreported. The Public Health Agency of Canada expects to see an increase in reports of norovirus outbreaks because these outbreaks were made nationally notifiable in 2009. This means that provinces and territories report cases of norovirus to the Public Health Agency of Canada.

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.

A national outbreak occurs when illness is linked in two or more provinces or territories.