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Norovirus: a Common Virus with Olympic proportions

Karen Michael, MPH, PhD

February 16, 2018

Norovirus has been in the news recently due to an outbreak among security guards and athletes at the South Korean Olympic Village. Norovirus is commonly associated with illness aboard cruise ships but has recently been found to be responsible for widespread illness in other venues, including a recent outbreak affecting hundreds of diners at two Seattle-area restaurants.

What is Norovirus?

Norovirus is a virus that causes acute gastrointestinal distress (ie: diarrhea and vomiting). In the US, it is the leading cause of acute gastrointestinal distress overall and the number one cause of foodborne outbreaks.1 Norovirus is incredibly easy to “catch” with a low infectious dose requiring only 18 viral particles to induce infection.2 The Centers for Disease Control and Prevention (CDC) estimate that 5 billion “doses” are present in each gram of feces during peak symptoms.3 Symptoms and shedding can begin in as little as 12 hours after infection.4 This period of time is shorter than many other enteric and respiratory pathogens such as rotavirus and influenza.

You are most contagious while you feel sick and are showing symptoms. However, you can be contagious before you show signs of illness and for up to 2 weeks after recovery but this is usually limited to 3 days after recovery. Contagious in this context means shedding of the virus into the environment around you.

Norovirus and the environment

Due to the levels of virus shed in feces and vomitus, it is incredibly easy to spread norovirus. It is also very difficult to control. Something as simple as a toilet flush, vomiting or not washing your hands effectively when sick with norovirus can easily lead to environmental contamination and subsequent transmission (ie: from the source to the environment to the next person). You can pick up the virus just being in the same space as someone who is shedding it or catch it directly from the ill person. Contaminated food can act as another source of the virus.

How can I tell if norovirus is in my environment?

Norovirus is invisible to the naked eye, so you can’t tell by looking at a surface or food item if it is contaminated. There are methods that test for the presence of the virus, but most of these are for use on body fluids such as feces and vomitus. The test methods to determine if a surface is contaminated with the virus can be hard to compare across studies and sites, depending largely on collection method. In some cases, food can be tested, but there are a number of major limitations to this type of testing. As with the surface testing, the amount of virus is considerably lower than would be found in feces or vomitus. This makes detecting the virus very difficult and can lead to false positives and false negatives. Another factor is that norovirus is incredibly hard to grow in a lab, so in vitro studies are complicated to do and interpret.

How do I reduce my risk of norovirus?

Because environmental contamination is so extensive with norovirus, anything that interrupts the pathway from the environment to your body will help control the spread of the disease. Frequent and effective hand washing, with soap and water, is the best way to reduce risk of transmission. Alcohol based sanitizers are largely ineffective against norovirus. Routine surface decontamination of high-touch and food preparation surfaces with bleach is another method that will reduce the bioburden of norovirus. This lowers the risk of cross contamination and transmission of the virus to other surfaces.

After an outbreak, high level environmental disinfection usually occurs. This is a heightened version of the normal disinfection procedures that occur in public settings. While continuing this level of cleaning would reduce the risk of future outbreaks, it is often times hard to achieve and maintain even in clinical settings.

The statistically likely event of norovirus spreading anywhere people congregate is a simple reality. Travel can also be associated with an increased risk of infection due to a change in sleeping and eating behaviors. There is no vaccine for norovirus and infection does not make you immune to future infection. Vigilant hand washing, routine surface disinfection and careful food prep are ways that you can reduce the risk of norovirus disease.

  1. CDC. Burden of Norovirus Illness and Outbreaks. (2017)
  2. Teunis, P. F. et al. Norwalk virus: how infectious is it? J Med Virol 80, 1468-1476, doi:10.1002/jmv.21237 (2008)
  3. CDC. Updated norovirus outbreak management and disease prevention guidelines. MMWR 60, 1-18 (2011)
  4. CDC. Norovirus in Healthcare Facilities Fact Sheet. (2011)

About the Author

Karen Michael, MPH, PhD

Industrial Hygienist and Bacteriologist

Karen Michael, MPH, PhD, is an industrial hygienist who specializes in microbial hygiene in clinical, residential, and commercial settings. Her practice at Veritox® focuses on environmental contamination with microbiological pathogens, infection prevention and biosafety. Specifically, her expertise deals with collecting and analyzing microbiological data that have no regulatory exposure limits.

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