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Norwalk
Virus Group
Norwalk virus is a virus,
the prototype of the Norwalk virus family or the noroviruses. This is a
family of unclassified small round structured viruses (SRSVs) which may
be related to the caliciviruses. They contain a positive strand RNA
genome of 7.5 kb and a single structural protein of about 60 kDa. The
27-32 nm viral particles have a buoyant density of 1.39-1.40 g/ml in
CsCl. The family consists of several serologically distinct groups of
viruses that have been named after the places where the outbreaks
occurred. In the U.S., the Norwalk, Ohio and Montgomery County strains
are serologically related but distinct from the Hawaii and Snow
Mountain strains. The Taunton, Moorcroft, Barnett, and Amulree strains
were identified in the U.K., and the Sapporo and Otofuke strains in
Japan. Their serological relationships remain to be determined.
Nature of Acute
Disease
Common names of the illness caused by the Norwalk and Norwalk-like
viruses are viral gastroenteritis, acute nonbacterial gastroenteritis,
food poisoning, (incorrectly) food poisoning, and (most commonly in
American English) stomach flu.
Nature of Disease
The disease is self-limiting, mild, and characterized by nausea,
vomiting, diarrhea, and abdominal pain. Headache and low-grade fever
may occur. The infectious dose is unknown but presumed to be low.
Diagnosis of Human
Illness
Specific diagnosis of the disease can only be made by a few
laboratories possessing reagents from human volunteer studies.
Identification of the virus can be made on early stool specimens using
immune electron microscopy and various immunoassays. Confirmation often
requires demonstration of seroconversion, the presence of specific IgM
antibody, or a four-fold rise in antibody titer to Norwalk virus on
paired acute-convalescent sera.
Associated Foods
Norwalk gastroenteritis is transmitted by the fecal-oral route via
contaminated water and foods. Secondary person-to-person transmission
has been documented. Water is the most common source of outbreaks and
may include water from municipal supplies, well, recreational lakes,
swimming pools, and water stored aboard cruise ships.
Shellfish and salad ingredients are the foods most often implicated in
Norwalk outbreaks. Ingestion of raw or insufficiently steamed clams and
oysters poses a high risk for infection with Norwalk virus. Foods other
than shellfish are contaminated by ill food handlers.
Relative Frequency
of Disease
Only the common cold is reported more frequently than viral
gastroenteritis as a cause of illness in the U.S. Although viral
gastroenteritis is caused by a number of viruses, it is estimated that
Norwalk viruses are responsible for about 1/3 of the cases not
involving the 6-to-24-month age group. In developing countries the
percentage of individuals who have developed immunity is very high at
an early age. In the U.S. the percentage increases gradually with age,
reaching 50% in the population over 18 years of age. Immunity, however,
is not permanent and reinfection can occur.
Course of Disease
and Complications
A mild and brief illness usually develops 24-48 h after contaminated
food or water is consumed and lasts for 24-60 hours. Severe illness or
hospitalization is very rare.
Target Populations
All individuals who ingest the virus and who have not (within 24
months) had an infection with the same or related strain, are
susceptible to infection and can develop the symptoms of
gastroenteritis. Disease is more frequent in adults and older children
than in the very young.
Food Analysis
The virus has been identified in clams and oysters by radioimmunoassay.
The genome of Norwalk virus has been cloned and development of gene
probes and PCR amplification techniques to detect the virus in clinical
specimens and possibly in food are under way.
Persons can
reduce their chance of getting infected by frequent handwashing, prompt
disinfection of contaminated surfaces with household chlorine
bleach-based cleaners, and prompt washing of soiled articles of
clothing. If food or water is thought to be contaminated, it should be
avoided.
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Viral
Gastroenteritis
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Gastroenteritis means
inflammation of the stomach and small and large intestines. Viral
gastroenteritis is an infection caused by a variety of viruses that
results in vomiting or diarrhea. It is often called the "stomach flu,"
although it is not caused by the influenza viruses.
Many different
viruses can cause gastroenteritis, including rotaviruses, noroviruses,
adenoviruses,type 40 or 41, sapoviruses, and astroviruses. Viral
gastroenteritis is not caused by bacteria (such as Salmonella
or Escherichia coli) or parasites (such as Giardia), or
by medications or other medical conditions, although the symptoms may
be similar. Your doctor can determine if the diarrhea is caused by a
virus or by something else.
The main
symptoms of viral gastroenteritis are watery diarrhea and vomiting. The
affected person may also have headache, fever, and abdominal cramps
("stomach ache"). In general, the symptoms begin 1 to 2 days following
infection with a virus that causes gastroenteritis and may last for 1
to 10 days, depending on which virus causes the illness.
The viral
gastroenteritis is contagious. The viruses that cause gastroenteritis
are spread through close contact with infected persons (for example, by
sharing food, water, or eating utensils). Individuals may also become
infected by eating or drinking contaminated foods or beverages.
Food may be
contaminated by food preparers or handlers who have viral
gastroenteritis, especially if they do not wash their hands regularly
after using the bathroom. Shellfish may be contaminated by sewage, and
persons who eat raw or undercooked shellfish harvested from
contaminated waters may get diarrhea. Drinking water can also be
contaminated by sewage and be a source of spread of these viruses.
Viral
gastroenteritis affects people in all parts of the world. Each virus
has its own seasonal activity. For example, in the United States,
rotavirus and astrovirus infections occur during the cooler months of
the year (October to April), whereas adenovirus infections occur
throughout the year. Norovirus outbreaks can occur in institutional
settings, such as schools, child care facilities, and nursing homes,
and can occur in other group settings, such as banquet halls, cruise
ships, dormitories, and campgrounds.
The most
important of treating viral gastroenteritis in children and adults is
to prevent severe loss of fluids (dehydration).
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