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Food Protection Connection: Understanding Norovirus

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(reprinted from Dietary Manager, June 2010)

Many of us have heard of the dreaded “cruise ship virus,” and if you have ever been ill on a ship you probably have an intimate understanding of its impact. Infamous and ever popular on the foodborne illness list, Norovirus (NV) is the most common cause of gastroenteritis in people of all ages with almost 50 percent of all foodborne gastroenteritis outbreaks and 35 percent of all sporadic gastroenteritis outbreaks accountable to it. The Food and Drug Administration (FDA), suggests Norovirus is the most significant diarrheal virus in terms of work/education days lost because it impacts all ages.

In order to know how to control any pathogen, one needs to understand what makes that pathogen grow, survive, and transmit. What is Norovirus? Put on your biology hats and think back…Norovirus is a single stranded, non-enveloped RNA virus belonging to the genus Norovirus and family Caliciviridae. Norovirus is a seasonal virus peaking in the winter months between December and March. Norovirus is also known as “Norwalk-like Virus,” “Small Round-structured Virus,” and “Winter Vomiting Virus.”

After exposure or ingestion of NV, incubation time is about 12-77 hours before onset of symptoms. The clinical illnesses that NV creates are acute onset of vomiting (often explosive and projectile), diarrhea (often explosive), headache, and cramps, and an illness duration lasting 24-60 hours. While pre-symptomatic shedding of the virus may occur, shedding of the virus in stool can continue for two weeks following recovery. Norovirus does not age discriminate as it can have a serious impact on healthy individuals of all ages, resulting in hospitalization and even intensive care. It has been associated with serious complications and even fatalities among elderly and immune-compromised individuals.

To add insult to injury, the secondary attach rate for NV is greater than 50 percent in areas where a large population is enclosed within a static environment, such as schools, institutions, military operations, hotels, busses, recreational camps, and -- of course -- cruise ships. So, if one person is sick in these closed situations, they can infect almost 50 percent or more of the people around them, hence the strict protocols on cruise ships when a Noro-like virus is suspected.

Transmission of Norovirus

Norovirus is very easily transmitted. It can be shed in fecal material at levels up to 1 trillion viral particles per gram of feces. One projectile vomiting incident can potentially contaminate the surrounding environment with 30,000,000 viral particles. We know that one person carrying and spreading NV can be shedding a multitude of viral particles, but how much exposure does a healthy person need to acquire the virus and be infected themselves? According to Teunis & Moe, 2008, the infectious dose of NV is estimated to be as low as 1-10 viral particles.

According to data collected by the Centers for Disease Control and Prevention (CDC), the rate of foodborne outbreaks associate with NV is growing exponentially. Learning more about the virus itself and teaching control and clean-up methods to the industry will hopefully begin to control this trend.

Interestingly enough, of the 76 outbreaks of this virus reported to the CDC between 1991 and 2000 (for which data was complete and there were implicated foods), 43percent of the outbreaks had some type of produce or fruit associated with them, including salads (26 percent) and produce/fruit (17 percent). Why might this be? The answer lies in understanding how Norovirus lives and moves around the environment.

NV can be transmitted through several routes: fecal-oral; airborne inhalation of droplets; person-to-person; and environment-to-person. Breaking these routes of transmission is key to stopping the spread of the virus.

Norovirus Rates
Selected Published Foodorne Norovirus Outbreaks Associated With Different Settings

Norovirus Rates

Recurring outbreaks in cruise ships, hotels, and hospitals have shown that contaminated fomites are the most likely factor responsible for sustaining a succession of outbreaks. Caul, 1994, concluded that hands and contaminated surfaces play an important role in the transmission of NV, in addition to possible aerosol inhalation. Hands are important in facilitating the NV transmission either by direct fecal-oral transfer or by transfer to foods that are eaten without further cooking.

How easily does NV transfer from surface to surface? De Wit, et al., 1992, examined the fecal contamination levels of the hands after handwashing. Using E. coli as an indicator, De Wit found that 25 percent of sampled individuals had the presence of E. coli after stools and handwashing. Handwashing decreased the presence of E. coli counts on hands, but did not eliminate it. De Wit, et al., concluded that infected individuals shedding at high levels will have a high hand contamination level, even with proper hygiene practiced. Rheinbaben et al. (2000) performed a study of hand transmission of viruses using a bacteriophage. When a door handle was contaminated with the bacteriophage, 14 out of 14 people were contaminated by touching that same door handle one after another. Additionally, when a hand was contaminated with the bacteriophage, 13 out of 14 people were contaminated by shaking hands with the contaminated hand, and shaking hands with one of those 13 individuals contaminated an additional six people.

J. Barker (et al., 2004, J. Hosp. Inf., 58, 42-49) studied the transfer of Norovirus from contaminated fecal material, using fingers and cloths as contact media to other environmental surfaces. Fingertips were contaminated by pressing the fingers onto contaminated toilet tissue for 10 seconds, and then drying the fingers for 15 seconds at room temp. The results indicated that NV is consistently transferred through contaminated fingers to surfaces and from the surfaces to other hand-contact surfaces, including; faucets, door handles, and telephone receivers. They found that the NV could sequentially transfer to at least seven different environmental surfaces. Secondary transfer of NV (from contaminated surfaces to clean fingers, to other surfaces) can transfer sequentially to four different surfaces.

NV can be found in soils throughout the environment from flood waters, septic tank malfunctions, and other surface and groundwater contamination events. Viral survival rate in soil can give us an idea under what conditions NV will survive. Sobsey, et al. (1989) reported that at least 12 weeks were required for a 99 percent reduction rate of viral particles in soil at 41ºF. Rzezutka, A., et al., (2004) found that winter field conditions of 59ºF viral decay took 92 days and in summer field conditions of 80-91ºF decay was reduced to 1.2 days. Notably, moisture levels during these times of year reduced from 25 percent to 2 percent as temperature rose. Moisture and temperature matter when it comes to survival of NV.

The stability of NV in water is also important to consider when determining what factors effect the survival of the virus. NV will survive freezing conditions. It will additionally survive exposure to acidity levels below pH 3. The presence of feces or other viral aggregation in the water will prolong NV’s survival. NV will survive in salt water for >6 days at room temperature and in dechlorinated water for 25 days at 39.2ºF. NV is inactivated by boiling at 212ºF. In water, NV is definitely impacted by microbial antagonism, sunlight, warmer temperatures, and the presence of oxygen.

Most important, to control foodborne outbreaks of NV, one must understand the stability and survival of NV in food. As with water, NV will survive freezing conditions. It will survive heating to 140ºF for 30 minutes and is inactivated by boiling. NV is impacted by warmer temperatures, sunlight, and oxygen. It will additionally survive in food at acidity levels below pH 3. It will additionally survive moist conditions vs. dry conditions. NV will actually survive on produce longer than the shelf life of the produce itself under refrigerated temperatures.

Settings of 348 outbreaks of gastroenteritis reported to CDC during January 1996 - November 2000

Outbreaks of Gastroenteritis

Source: Fankhauser RL, Noel JS, Monroe SS, Ando T, Glass RI. Molecular epidemiology of "Norwalk-like viruses" in outbreaks of gastroenteritis in the United States. J. Infect Dis 1998; 178: 1571-8: and CDC, unpublished data, 1997-2000

So why would 43 percent of outbreaks of NV have produce or fruit associated with them? Cool, dark, moist, and anaerobic conditions are conditions that NV likes the most. And where do we store our produce? In cool, dark, moist, low oxygen refrigerators. These conditions reduce viral decay and enhance survival rates of the virus. To help reduce the rate of NV with produce, FDA recommends that you wash and dry produce as much as possible without direct human contact and avoid dark, anaerobic storage conditions.

In order to control NV, one needs to interrupt the human transmission pathway. Proper handwashing and prohibiting bare hand contact with ready-to-eat foods is critical. Removing food workers with active vomiting and/or diarrhea is also critical. You must reduce airborne transmission via fans and vents. Treat all clean-up materials as infectious waste. Protect cleaning staff with barriers such as face masks, gloves, and aprons, and dispose of materials used in clean up. Next, disinfect the area appropriately. Finally, good thorough handwashing should follow any NV clean up. Use of many instant hand sanitizers is not effective in destroying NV. They are effective against other viruses, but not NV.

Cleaning up after a NV outbreak should be done with care. The entire area must be properly cleaned and disinfected, not just the location of the contamination event. J. Barker (et al., 2004, J. Hosp. Inf., 58, 42-49) found that when a cloth soaked only in a detergent solution, without a disinfectant, was used to clean-up a Norovirus contaminated area, the cloth actually helped spread Norovirus to other environmental surfaces. For this reason, cloths used to clean-up an area contaminated by Norovirus should be disposed, or treated as infectious material, and the initial cleaning should be immediately followed by thorough disinfection of the area. You must disinfect environmental surfaces, especially restrooms, as they are reservoirs for NV, even if a vomiting incident did not occur in the restroom. Reduce airborne transmission by turning off fans and vents until the area has been disinfected. In situations where the source of the contamination is unknown, it is suggested that all environmental surfaces be cleaned and disinfected.

J. Barker (et al., 2004, J. Hosp. Inf., 58, 42-49) also looked at the effectiveness of cleaning agents in eliminating NV contamination. Detergent-based cleaning with a cloth to produce a visibly clean surface consistently failed to eliminate NV contamination of the environmental surface, and actually helped in spreading the NV contamination to other surfaces. Fecal soiling reduced the effectiveness of a combined hypochlorite/detergent formulation at 5000 ppm of available chlorine in cleaning NV contaminated environmental surfaces. Although there was a reduction in NV, using this disinfectant/detergent to clean NV fecally contaminated surfaces, NV could still be detected in 28 percent of the surfaces. The only effective method found to clean NV fecally contaminated surfaces was to first clean the area with a cloth soaked in a detergent, and then follow with the combined hypochlorite/detergent. (Note: combining chlorine bleach and soap should not be done within a facility and is dangerous to your health; stable hypochlorite/detergent chemical should be purchased.)

Choosing a clean-up chemical may seem challenging. Some recommendations follow:

  1. Cleaning with a detergent followed by Hypochlorous acid at 188 ppm/1 min for stainless steel/tiles or fecally contaminated surfaces.
  2. Cleaning with detergent, a fresh water rinse, and followed by chlorine bleach solution (from 5.25 percent Sodium hypochlorate) at 5000ppm/1 minute for melamine surfaces or fecally contaminated surfaces. Note: Chlorine at this ppm may be harmful to some equipment.
  3. Heat at 158ºF/5 min or 212ºF/1min for carpets and upholstered materials.
  4. Soap and warm water for handwashing for 1 minute followed by 20 seconds of rinsing and use of a disposable paper towel.

Understanding the survival and transmission of NV outside of the host can help you develop effective control measures to interrupt the human transmission pathways and stop the continuation of an outbreak. Should you find yourself in the unfortunate situation of having to deal with a NV outbreak, don’t panic. Take precautions; cautiously and meticulously clean up the entire surrounding environment of the contamination event. Contact your local or state regulatory agency for assistance with clean up measures.

Most regulatory agencies will assure you that regardless of whether it is Norovirus, Salmonella, or any other foodborne illness pathogen, three things will greatly reduce your chances of having a foodborne outbreak in your facility: Have a stringent employee health policy that is strictly enforced (keep ill workers away from food); have a strong hand hygiene program in your facility; and have no bare hand contact with ready-to-eat foods. The National Advisory Committee for Microbiological Criteria for Foods concluded that when these three strategies are combined and utilized properly, the transmission of fecal-oral pathogens can be controlled.


More Info

by Melissa Vaccaro, MS, CHO
Melissa Vaccaro, MS, CHO is a Food Program Specialist for the PA Department of Agriculture and an Executive Board Member for the Central Atlantic States Association of Food and Drug Officials (CASA). Contact her at