About Norovirus
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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 resolved, 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).