About Norovirus

The Centers for Disease Control and Prevention (CDC) estimates that noroviruses cause 23 million cases of acute gastroenteritis annually, making noroviruses the leading cause of gastroenteritis in the United States (CDC, 2006; Fankhauser, et al., 2002; Mead, et al., 1999).

Of viruses, only the common cold is reported more often than viral gastroenteritis (norovirus) (Benson & Merano, 1998).

Nature has created an ingenious bug in norovirus. The round blue ball structure of norovirus is actually a protein surrounding the virus’s genetic material. The virus attaches to the outside of cells lining the intestine, and then transfers its genetic material into those cells. Once the genetic material has been transferred, norovirus reproduces, finally killing the human cells and releasing new copies of itself that attach to more cells of the intestine’s lining.

Norovirus (previously called “Norwalk-like virus” or NLV) is a member of the family Caliciviridae. The name derives from the Latin for chilice—calyx—meaning cup-like, and refers to the indentations of the virus surface.

The family of Caliciviridae consists of several distinct groups of viruses that were first named after the places where outbreaks occurred. The first such outbreak occurred in 1968 among schoolchildren in Norwalk, Ohio (Glass, et al., 2000).  The prototype strain was identified four years later, in 1972, and was the first virus identified that specifically caused gastroenteritis in humans (Glass, et al., 2000).

Other discoveries followed, with each strain name based on the location of its discovery-location, e.g., Montgomery County, Snow Mountain, Mexico, Hawaii, Parmatta, Taunton, and Toronto viruses. A study published in 1977 found that the Toronto virus was the second most common cause of gastroenteritis in children (Middleton, Szmanski & Petric, 1977).

Eventually this confusing nomenclature was rsolved, first in favor of calling each of the strains a Norwalk-like virus, and then simply, a norovirus – the term used today.

How is norovirus transmitted?

Noroviruses are transmitted primarily through the fecal-oral route, and fewer than 100 norovirus particles are said to be needed to cause infection (MMWR, 2001, June 1).
Transmission occurs either person-to-person or through contamination of food or water.  Foodborne norovirus transmission can occur when food is contaminated by an infected food handler (Caceres, et al., 1998; MMWR, 2001, June 1).

Noroviruses are recognized as causing over half of all foodborne illness outbreaks.  CDC statistics show that food is the most common vehicle of transmission for noroviruses; of 232 outbreaks of norovirus between July 1997 and June 2000, 57% were foodborne, 16% were spread from person-to-person, and 3% were waterborne (CDC, 2006, August 3).

The virus is shed in large numbers in the vomit and stool of infected individuals, most commonly while they are ill.  Some individuals may, however, continue to shed norovirus long after they have recovered from the illness (Patterson, 1993).  Aerosolized vomit has also been implicated as a mode of norovirus transmission (Marks, et al., 2000).

As noted by the CDC in its Final Trip Report, “noroviruses can cause extended outbreaks because of their high infectivity, persistence in the environment, resistance to common disinfectants, and difficulty in controlling their transmission through routine sanitary measures” (MMWR, 2001, June 1).

Norovirus Diagnosis

Diagnosis is often based on the combination of symptoms, particularly the prominence of vomiting, little fever, and the short duration of illness.  If a known norovirus outbreak is in progress, public health officials may obtain specimens from ill individuals for testing in a lab.

Norovirus can be identified in a lab under an electron microscope. A reverse transcriptase polymerase chain reaction test (RT-PCR assay) also can detect norovirus in food, water, stool samples, and on surfaces. These tests isolate and replicate the suspected virus’ genetic material for analysis (Mayo Clinic, 2007).

Treatment for Norovirus Infection

No specific treatment is available for norovirus. In most healthy people, the illness usually is self-limiting and resolves in a few days.

Outbreaks among infants, children, elderly, and immunocompromised populations may result in severe complications among those affected.  Death may result without prompt measures (Mayo Clinic, 2007, April 5).

The replacement of fluids and minerals such as sodium, potassium and calcium – otherwise known as electrolytes – lost due to persistent diarrhea is vital. This can be done either by drinking large amounts of liquids, or intravenously (Mayo Clinic, 2007, April 5).

Preventing Norovirus Infection

Common settings for norovirus outbreaks include restaurants and events with catered meals (36%), nursing homes (23%), schools (13%), and vacation settings or cruise ships (10%) (CDC, 2006, August 3).  Proper hand washing is the best way to prevent the spread of norovirus.

The good news about norovirus is that it does not multiply in foods as many bacteria do. In addition, thorough cooking destroys this virus.  To avoid norovirus, make sure the food you eat is cooked completely.

Shellfish (oysters, clams, mussels) pose the greatest risk and any particular serving may be contaminated with norovirus; there is no way to detect a contaminated oyster, clam, or mussel from a safe one.  Shellfish become contaminated when their waters become contaminated (e.g. when raw sewage is dumped overboard by recreational and/or commercial boaters).  Shellfish are filter feeders and will concentrate virus particles present in their environment. With shellfish, only complete cooking offers reliable protection; steaming does not kill the virus or prevent its transmission (Kirkland, et al., 1996).

Raw vegetables should be washed thoroughly before being served, and travelers should drink only boiled drinks or carbonated bottled beverages without ice when in areas that have polluted water sources.

Immunity to calciviruses is not permanent and re-infection can occur.  Infection may also spread from person-to-person. So, if you or another household member suffer a bout of gastroenteritis, be absolutely sure to wash hands with soap and warm water after toilet visits, before preparing or eating food, and after caring for the sick.  Exclude persons with gastroenteritis from the kitchen, and always dispose of sewage in a sanitary manner.

Recently, outbreaks of norovirus have become synonymous with cruise ships (CDC, 2006, August 3; MMWR Weekly, 2002, December 13).  Healthcare facilities also experience a high incidence of norovirus outbreaks.  The CDC has published information regarding the prevention of norovirus outbreaks on cruise ships and in healthcare facilities on its website (CDC, 2006, December 21; CDC, 2007).


* Bravo! Cucina Italiano Norovirus Outbreak
* Campylobacter Outbreak – Washington State Penitentiary
* Carrabba’s Restaurant Norovirus Outbreak
* Las Vegas Flamingo Norovirus Outbreak
* Michigan Blimpie’s Norovirus Outbreak
* Regent Beverly Wilshire Hotel Norovirus Outbreak
* Si Casa Flores Norovirus Outbreak


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