The Norwalk virus was first identified as
the cause of a primary school outbreak of vomiting/diarrhea
in Norwalk, Ohio in the early 1970’s. Similar viruses
were responsible for subsequent outbreaks in Montgomery
County and Hawaii. These morphologically similar viruses
were named after their geographic origins and are collectively
referred to as small round structured viruses (SRSV) or
more recently as Noroviruses.
Early attempts to replicate noroviruses in
cell culture failed. To date, noroviruses are still considered
non-cultivatable in any tissue culture or animal system.
Early characterization of noroviruses was based on surface
morphology observed from electron microscopy of fecal samples
obtained from infected individuals.
The genome of Norwalk virus has been sequenced
from a library of cDNA obtained from stool samples of human
volunteers. The genome consists of a single positive ~7.6
kb strand of RNA which is polyadenylated at its 3’
terminus. The genome was found it contain 3 open reading
frames. The first open reading frame (ORF1) encodes a polyprotein
precursor to an RNA-dependent RNA polymerase. The second
open reading frame (ORF2) encodes a structural capsid protein.
The function of the third open reading frame (ORF3) is unknown,
but is suspected to encode a minor structural protein of
||In 1999, a research group using X-ray crystallography
to characterize the protein coat of the Norwalk virus
which is 27 to 32 nm in diameter. The capsid of Norwalk
virus is constructed using 180 copies of a ~60 kD capsid
protein, organized into 90 dimers. The capsid protein
contains two principal domains linked by a flexible
hinge. The S domain near the NH2–terminus is involved
in forming of the icosahedral shell, and the P domain
at the COOH-terminus forms the protrusion emanating
from the shell. Noroviruses display antigenic and genetic
diversity, which is likely the result of a variable
region within the P domain of the Norwalk capsid protein.
Outbreaks of Noroviruses frequently occur in institutional
settings such as schools and nursing homes, where infection
is passed person to person. In these cases, the virus is introduced
through common-source exposure (e.g. contaminated food or
drink) and spread through person-to-person transmission. Crowded
quarters and reduced levels of hygiene may be a factor in
the spread of Noroviruses within these institutions.
Norovirus outbreaks also occur in restaurants, banquet halls,
cruise ships, dormitories, and campgrounds, where contaminated
food and/or water is consumed. Contamination may occur at
the food source, such as when oysters or fruits are treated
with contaminated water and harvested. The virus is capable
of surviving temperatures of 60°C for 30 minutes, therefore
thorough cooking of food and boiling of water is recommended.
Food that has been handled by infected individuals after cooking
can also become contaminated and cause outbreaks. Frequent
hand washing with soap is essential, especially after using
the washroom and before handling food and drink.
One of the most common causes of Norovirus outbreaks is consumption
of raw shellfish (oysters and clams) that has been contaminated
with the virus. Shellfish from waters exposed to raw sewage
will concentrate virus particles from the water since they
are filter feeders. Other potential sources of contaminated
food include fruits, sandwiches, eggs, salads, bakery products,
and semi-liquid foods (e.g. cake frosting, sauces, salad dressings).
Potential sources of contaminated water include municipal
water supplies, recreational lakes, swimming pools, and water
stored on cruise ships. Contaminated water will introduce
the virus to beverages, ice, fruits, and salad ingredients.
The low infectious dose of Noroviruses along with its high
concentration in human stool create a situation where only
a few virus particles are needed to start a large outbreak.
Asymptomatic shedding has also been reported creating a situation
where individuals such as food handlers could be spreading
the virus without displaying any symptoms of illness. The
virus particles are also environmentally stable, able to withstand
freezing, heating up to 60C, chlorine levels <= 10 ppm,
stomach acid, and pH levels between 5 and 10.
Norwalk and Noroviruses are very contagious and transmitted
primarily through the fecal-oral route. The majority of outbreaks
are caused by introduction of contaminated food. Consumption
of raw oysters was responsible for several outbreaks, including
one in Australia in August 1996, one in Louisiana in December
1996, one in Florida in January 1995, and two in northwestern
Florida and Louisiana in November 1993. Outbreaks caused by
infected water sources are caused by human waste contamination
of water supplies.
Noroviruses can also be spread by secondary person-to-person
transmission through sharing food, water, and/or utensils.
The virus can be transmitted by close contact with an infected
individual and/or touching contaminated surfaces. This method
of transmission extends the duration of an outbreak, as virus
particles infect new individuals after the initial exposure
event. In environments such as cruise ships, enclosed living
quarters and shared resources promote person-to-person transmission.
Arrival of new susceptible passengers every few weeks will
allow an outbreak to propagate over several successive cruises
unless major corrective actions are taken.
In some outbreaks, there is evidence that Noroviruses are
spread by the airborne route. Virus particles contained in
vomit are released in aerosol form and are capable of contaminating
surfaces over a large radius.
Norwalk and Noroviruses are a common cause of vomiting and
diarreal illness each winter and has often been referred to
as "stomach flu" or "Winter Vomiting Disease".
Norwalk virus infections have been linked to outbreaks of
vomiting and/or diarrhea in institutions such as child-care
centres and long term care facilities as well as on cruise
ships, camps, schools and households.
The virus does not discriminate. The symptoms of Norwalk
virus infection may be more severe in older individuals, young
children and those with underlying medical conditions who
may become dehydrated due to the vomiting and diarrhea.