What is Norovirus?

Noroviruses are a family of very contagious viruses which cause digestive system illnesses in people.  Prior to 2002 these viruses were referred to as Norwalk-like (or more often Norwalk) viruses.   First recognized as a cause of gastrointestinal disease during an  outbreak in a school in Norwalk, Ohio in 1968, this family of viruses frequently causes illnesses - sometimes involving large numbers of people.  The symptoms of a norovirus infection are diarrhea and vomiting and victims can be quite ill, suffering frequent bouts of diarrhea and vomiting which can last from 1 to 4 days.  Other symptoms include nausea, abdominal cramps, headache, low grade fever and muscle ache.  Dehydration can occur and this complication can be particularly serious for the very young or the elderly.  Otherwise healthy people normally fully recover within a few days.

The illness is sometimes referred to as ‘stomach flu” despite the fact that it is unrelated to influenza ( the ‘flu’).  Suffering an episode of norovirus infection does not convey lasting immunity.

How is a norovirus infection transmitted?

A norovirus infection is transmitted via a fecal / oral route.  The virus is present in both feces and vomit of people suffering from norovirus infection and poor sanitation can spread the virus from these sources to other people.  A person who is infected with norovirus, and who is insufficiently careful about hand washing, can contaminate surfaces or food or alternatively the virus can find its way into drinking water.  A healthy person might become infected when they consume contaminated food or water or, by touching a contaminated surface and subsequently transferring the virus to their mouth, a person can become infected.

The incubation period – the time between exposure and the onset of symptoms - is usually in the range of 10 to 60 hours but most often between 24 to 48 hours.  People who are infected can transmit the illness to others during the period when they are suffering symptoms and for at least a few days – and perhaps for as long as 2 weeks - after the symptoms subside.

Bouts of illness can occur at any time but they are more frequent during the winter.

How hardy is the virus?

Noroviruses are very hardy and can survive on hard surfaces for up to 12 hours.  Experience shows that resolving an outbreak of norovirus infections often requires aggressive cleaning.  In 1977 over 500 passengers on a US-based cruise ship came down with what was later determined to be norovirus infections.  Despite efforts to thoroughly clean the ship between voyages, passengers on each of the next 5 voyages also came down with the illness.

How common are norovirus infections?

Norovirus infections are very common.  Although there are other pathogens which also cause food-borne illness public health authorities believe that many outbreaks of “food poisoning ‘are caused by norovirus.  The American government estimates the norovirus causes over 20 million illnesses each year in the US.  The Canadian Public Health Agency has followed hundreds of outbreaks every year for the past 5 years.

Outbreaks of norovirus infection often occur in hospitals, senior citizen complexes, military barracks, schools, day care centres, summer camps, holiday resorts and other locations where people work or live in high density situations or where it is likely that infected people are being cared for.

In October 2006 a Norovirus outbreak occurred in a student residence at Mt Allison University in Sackville New Brunswick.  The outbreak caused about 300 students to become ill over a period of several days.  Despite efforts to isolate those who were ill, the outbreak then spread to St Francis Xavier University in Antigonish Nova Scotia.  But, on the alert for the illness, University administrators responded promptly to the initial cases and limited the outbreak to two dozen or so students.  During this period there were no reported cases of serious gastrointestinal illness in Dalhousie University residences.  We did however experience two cases of what was likely norovirus infection later in the fall.  Again prompt action - including isolation of the ill students and aggressive cleaning - prevented the outbreak from spreading. 

How can one prevent norovirus infection?

There are no antiviral medications which are used in treating people suffering from norovirus infections.  Nor is there a vaccine available to protect against the illness.

Prevention focuses on good personal hygiene.  Frequent and effective hand washing ( link), keeping ones hands away from the nose and mouth, careful washing of fresh fruits and vegetables and avoiding,when possible avoiding handing food with one’s hands all help reduce the risks of becoming infected.  In some cases use of alcohol-based hand sanitizers may also help lessen the risk.