Noroviruses spread from person to person, through contaminated food or water, and by touching contaminated surfaces. Outbreaks can happen to people of all ages and in a variety of settings.
Noroviruses are responsible for about half of all reported outbreaks of gastroenteritis (vomiting, diarrhea, and stomach cramping caused by inflammation of the stomach and intestines). While the vast majority of norovirus illnesses are not part of a recognized cluster, outbreaks provide important information on how the virus is spread and, therefore, how best to prevent infection. Norovirus outbreaks occur throughout the year, but over 80% of them occur during November–April. In addition, norovirus outbreaks tend to increase periodically when new strains of the virus appear. The virus can be spread through food, water, by touching things that have the virus on them, as well as directly from person to person. There is no long-lasting immunity to norovirus; thus, outbreaks can affect people of all ages and in a variety of settings (Figure 1).
Norovirus in Long-Term Care and Other Healthcare Facilities
Healthcare facilities, including nursing homes and hospitals, are the most commonly reported settings for norovirus outbreaks in the United States and other industrialized countries (see Norovirus in Healthcare Settings). The virus can be introduced into healthcare facilities by infected patients--who may or may not be showing symptoms--or by staff, visitors, or contaminated food products. Outbreaks in these settings can be quite long—sometimes lasting months—and illness can be more severe, occasionally even fatal, in hospitalized or nursing home patients than for otherwise healthy persons.
Norovirus-contaminated Food in Restaurants and Catered Events
Norovirus is recognized as the leading cause of foodborne-disease outbreaks in the United States. Nearly half of all illnesses associated with foodborne-disease outbreaks reported to CDC during 2006–2007 were attributed to norovirus (Figure 2). Food can become contaminated with norovirus at any point when food is grown, shipped, handled, or prepared. A variety of foods have caused outbreaks, but foods that are eaten raw, such as leafy vegetables, fruits, and shellfish, are most commonly identified as causing foodborne-disease outbreaks of norovirus.
Contamination by infected food handlers (those who may prepare or serve foods at a restaurant or other place) probably causes the most foodborne norovirus infections. (See Norovirus: Facts for Food Handlers.)
However, norovirus outbreaks can also occur from fecal (stool) contamination of certain food products at the source. For example, oysters harvested from fecally contaminated growing waters and raspberries irrigated with sewage-contaminated water have all caused norovirus outbreaks.
Norovirus on Cruise Ships
Passengers and crew aboard cruise ships are also affected by norovirus outbreaks (see Facts about Noroviruses on Cruise Ships). Virus generally is introduced by persons who were infected before coming on board. Virus can also be introduced from food items contaminated before loading or can persist from norovirus-contaminated environmental surfaces from prior cruises. Virus may also be acquired when ships dock in other countries; the virus can be brought on board in contaminated food or water or by passengers who become infected while ashore. Repeated outbreaks happen again as a result of environmental persistence (incomplete cleaning) or infected crew, particularly if control measures have not been consistent and thorough.
Norovirus in Schools and Other Institutional Settings
Norovirus outbreaks occur in a range of other institutional settings, including, but not limited to, schools, child care centers, colleges, prisons, and military encampments. Norovirus outbreaks on university campuses have led to campus closures. Norovirus was the most common cause of gastroenteritis in U.S. Marines during Operation Iraqi Freedom and a common cause of outbreaks among British troops deployed to Iraq during 2002–2007.
CDC's Role in Norovirus Outbreaks
While most norovirus outbreaks are investigated by state and local public health authorities, CDC provides coordination of norovirus outbreaks that involve multiple states. Such multi-state outbreaks may result, for example, from a contaminated food that has been widely distributed to various states or from an event that involves participants from multiple states. When requested, CDC also provides technical consultation and assistance to state and local public health agencies during norovirus outbreaks. CDC is also working to improve state and local laboratory capacity and can provide diagnostic support, if requested, to help confirm and report norovirus outbreaks.
National surveillance systems for norovirus outbreaks coordinated by CDC include the following:
* The National Outbreak Reporting System (NORS) collects reports from state and local health departments on all enteric (intestinal) disease outbreaks, including those spread by contaminated food, water, and person-to-person contact. Data from NORS are used to evaluate trends in norovirus outbreaks and to help identify important settings, foods, and contamination pathways where prevention efforts should be focused.
* CaliciNet is a national surveillance network, through which public health laboratories contribute to a database of gene sequences from human caliciviruses (noroviruses and sapoviruses) identified in clinical specimens collected during outbreaks. Over time, as more states participate, CaliciNet may find links to help identify multi-state outbreaks, identify the emergence of new viruses, and, in the future, perhaps indicate potential norovirus contamination of food during production or processing.
Data Sources
Zheng DP, Widdowson MA, Glass RI, Vinjé J. Molecular epidemiology of genogroup II-genotype 4 noroviruses in the United States between 1994 and 2006. J Clin Microbiol. 2010;48(1):168–77.
CDC. Surveillance for foodborne disease outbreaks --- United States, 2006. MMWR. 2009;58(22):609–15. [Surveillance for Foodborne Disease Outbreaks --- United States, 2006]
CDC. Surveillance for foodborne disease outbreaks --- United States, 2007. MMWR. 2010;59(31):973–9. [Surveillance for Foodborne Disease Outbreaks --- United States, 2007]
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CDC Data & Statistics | Feature: Surveillance for Norovirus Outbreaks
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