Surveillance for Norovirus Outbreaks

Noroviruses are responsible for over 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 spreads and, therefore, how best to prevent infection.

Figure 1: Outbreaks of Acute Gastroenteritis, 30 States, January 2007 through April 2010 (larger view)

Figure 1. (larger view)

Norovirus outbreaks occur throughout the year. But, over 80% of the outbreaks occur from November to April. Also, when there are new strains of norovirus, the number of outbreaks tends to increase periodically. This occurred in the winter of 2006 to 2007 with the Minerva strain. However, a new strain that appeared in 2009 (New Orleans) did not lead to more outbreaks (Figure 1).

Most outbreaks are caused when norovirus is spread from infected people to others. But, the virus can also spread by consuming contaminated food or water and touching things that have the virus on them. People can get norovirus illness many times during their lifetime. Outbreaks can affect people of all ages and in a variety of settings (Figure 2).

Norovirus in Long-Term Care and Other Healthcare Facilities

Chart: Setting of 1,518 Confirmed Norovirus Outbreaks, U.S., 2010-2011

Figure 2. (larger view)

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). Nearly two-thirds of all norovirus outbreaks reported in the United States occur in long-term care facilities.

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 foods. Outbreaks in these settings can be quite long, sometimes lasting months. Illness can be more severe, occasionally even fatal, in hospitalized or nursing home patients compared with otherwise healthy people.

Norovirus in Restaurants and Catered Events

Figure 3: Known Causes of Foodborne Illness Outbreaks, U.S., 2006-2008. Norovirus 51%; Bacteria 38%; Chemicals 6%' Parasites 1%; Other/Multiple 4%.

Figure 3. (larger view)

Norovirus is the leading cause of foodborne-disease outbreaks in the United States. Over half of all foodborne-disease outbreaks due to a known cause that were reported to CDC from 2006 to 2008 were attributed to norovirus (Figure 3).

Food can get contaminated with norovirus at any point when it is being grown, shipped, handled, or prepared. A variety of foods have caused outbreaks. But, foods that are eaten raw, such as leafy vegetables, fruit, and shellfish, are most commonly involved in norovirus outbreaks.
Contamination by infected food handlers (people who prepare or serve foods at a restaurant or other place) probably causes the most foodborne norovirus infections. (See Norovirus: Facts for Food Handlers.)

Norovirus outbreaks can also occur from fecal (stool) contamination of certain foods at their source. For example, oysters harvested from contaminated water and raspberries irrigated with contaminated water have caused norovirus outbreaks.

Norovirus on Cruise Ships

Passengers and crew on cruise ships are also affected by norovirus outbreaks (see Facts about Noroviruses on Cruise Ships). The virus is most often brought onto the ship by people who were infected before they boarded. Norovirus can be brought onto the ship in food that was contaminated before loading. Also, the virus can remain on surfaces in the ship from prior cruises. When the ship docks in other countries, norovirus can be brought on board in contaminated food or water or by passengers who become infected while ashore. Repeated outbreaks happen 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, for example, 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 to 2007.

CDC's Role in Norovirus Outbreaks

While most norovirus outbreaks are investigated by state and local public health authorities, CDC coordinates outbreaks that involve multiple states. Such outbreaks may result from contaminated food that is 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 National Outbreak Reporting System (NORS) collects reports from state and local health departments on all acute gastroenteritis outbreaks, including those spread by contaminated food or water and person-to-person contact. This information is used to determine where norovirus outbreaks commonly occur, how the virus is transmitted, and specific food or water sources involved. In turn, this helps with identifying the best ways to prevent and control norovirus illness and outbreaks.
CaliciNet is a national surveillance network that includes a database for public health laboratories to submit gene sequences from human caliciviruses (noroviruses and sapoviruses) identified from outbreaks. The information is used to link norovirus outbreaks that may be caused by common sources (such as food), monitor trends, and identify emerging norovirus strains.