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About Noroviruses Posted By: WZZM 13 Source of Article: http://www.wzzm13.com/news/story.aspx?storyid=101156&catid=2 ·
Updated:11/7/2008 2:18:04 PM - Posted:
11/7/2008 2:11:27 PM Noroviruses
(genus Norovirus, family Caliciviridae) are a group
of related, single-stranded RNA, nonenveloped
viruses that cause acute gastroenteritis in humans. Norovirus was recently
approved as the official genus name for the group of viruses provisionally
described as " Noroviruses are
named after the original strain " Clinical Presentation The incubation
period for norovirus-associated gastroenteritis in
humans is usually between 24 and 48 hours (median in outbreaks 33 to 36
hours), but cases can occur within 12 hours of exposure. Norovirus infection
usually presents as acute-onset vomiting, watery non-bloody diarrhea with
abdominal cramps, and nausea. Low-grade fever also occasionally occurs, and
vomiting is more common in children. Dehydration is the most common
complication, especially among the young and elderly, and may require medical
attention. Symptoms usually last 24 to 60 hours. Recovery is usually complete
and there is no evidence of any serious long-term sequelae.
Studies with volunteers given stool filtrates have shown that asymptomatic
infection may occur in as many as 30% of infections, although the role of asymptomatic
infection in norovirus transmission is not well
understood. Virus Transmission Noroviruses are transmitted primarily through the
fecal-oral route, either by consumption of fecally
contaminated food or water or by direct person-to-person spread.
Environmental and fomite contamination may also act
as a source of infection. Good evidence exists for transmission due to aerosolization of vomitus that
presumably results in droplets contaminating surfaces or entering the oral
mucosa and being swallowed. No evidence suggests that infection occurs
through the respiratory system. Noroviruses are highly contagious and as few as 10 viral
particles may be sufficient to infect an individual. During outbreaks of norovirus gastroenteritis, several modes of transmission
have been documented; for example, initial foodborne
transmission in a restaurant, followed by secondary person-to-person
transmission to household contacts. Although presymptomatic
viral shedding may occur, shedding usually begins with onset of symptoms and
may continue for 2 weeks after recovery. It is unclear to what extent viral
shedding over 72 hours after recovery signifies continued infectivity. Immunity to Norovirus Mechanisms of
immunity to norovirus are unclear. It appears that
immunity may be strain-specific and lasts only a few months; therefore, given
the genetic variability of noroviruses, individuals
are likely to be repeatedly infected throughout their lifetimes. This may
explain the high attack rates in all ages reported in outbreaks.
Recent evidence also suggests that susceptibility to infection may be
genetically determined, with people of blood group O being at greatest risk
for severe infection. Disease burden of Norovirus Gastroenteritis CDC estimates
that 23 million cases of acute gastroenteritis are due to norovirus
infection, and it is now thought that at least 50% of all foodborne
outbreaks of gastroenteritis can be attributed to noroviruses.
Among the 232
outbreaks of norovirus illness reported to CDC from
July 1997 to June 2000, 57% were foodborne, 16%
were due to person-to-person spread, and 3% were waterborne; in 23% of
outbreaks, the cause of transmission was not determined. In this study,
common settings for outbreaks include restaurants and catered meals (36%),
nursing homes (23%), schools (13%), and vacation
settings or cruise ships (10%). Most foodborne outbreaks of norovirus
illness are likely to arise though direct contamination of food by a food
handler immediately before its consumption. Outbreaks have frequently been
associated with consumption of cold foods, including various salads,
sandwiches, and bakery products. Liquid items (e.g., salad dressing or cake
icing) that allow virus to mix evenly are often implicated as a cause of
outbreaks. Food can also be contaminated at its source, and oysters from
contaminated waters have been associated with widespread outbreaks of
gastroenteritis. Other foods, including raspberries and salads, have been
contaminated before widespread distribution and subsequently caused extensive
outbreaks. Waterborne
outbreaks of norovirus disease in community
settings have often been caused by sewage contamination of wells and
recreational water. Diagnosis of Norovirus Human. In the
last 10 years, diagnosis of norovirus as cause of
outbreaks of acute gastroenteritis has improved with the increasing use of
the reverse transcriptase polymerase chain reaction (RT-PCR). Currently,
state public health laboratories of 47 states have the capability to test for
noroviruses by (realtime)
RT-PCR. RT-PCR detects the norovirus RNA and can be
used to test stool and emesis samples, as well as environmental swabs in
special studies. Identification of the virus can be best made from stool
specimens taken within 48 to 72 hours after onset of symptoms, although good
results can be obtained by using RT-PCR on samples taken as long as 5 days
after symptom onset. Virus can sometimes be found in stool samples taken as
late as 2 weeks after recovery. Older methods
for diagnosis include direct and immune electron microscopy of fecal
specimens, and detection of a fourfold increase of specific antibodies in
acute- and convalescent-phase blood samples. Several commercially available
enzyme-linked immunosorbent assays for detection of
virus in stools have been developed but await evaluation further evaluation
regarding sensitivity and specificity. Sequencing of norovirus strains found in clinical and environmental
samples has greatly helped in conducting epidemiologic investigations by
linking cases to each other and to a common source and by differentiating
outbreaks that were mistakenly connected. Sequences can be entered into CaliciNet, a recently developed sequence database on the
basis of the PulseNet model. In the next years CaliciNet will be further implemented to be able to help
to determine links (e.g., norovirus contaminated
foods) between outbreaks across the In addition to
microbiological techniques, several epidemiologic criteria have been proposed
for use in determining whether an outbreak of gastroenteritis is of viral
origin. Kaplan's criteria for this purpose are as follows:
1) a mean (or median) illness duration of 12 to 60 hours, 2) a mean
(or median) incubation period of 24 to 48 hours, 3) more than 50% of people
with vomiting, and 4) no bacterial agent previously found. Although quite
specific, these criteria are not very sensitive, and therefore the
possibility of a viral etiology should not be discarded if the criteria are
not met. Environmental.
Methods to recover and detect virus in food need to be adapted for each food
substance; these have been only rarely used, with the exception of assays to
detect virus in shellfish. Drinking water can be tested for noroviruses by using RT-PCR when large (10-100 L) volumes
of water are processed through specially designed filters. Management of Norovirus Infection No specific
therapy exists for norovirus gastroenteritis.
Symptomatic therapy consists of replacing fluid losses and correcting
electrolyte disturbances through oral and intravenous fluid administration. Prevention Prevention of foodborne norovirus disease is
based on the provision of safe food and water. Noroviruses
are relatively resistant to environmental challenge: they are able to survive
freezing, temperatures as high as 60°C, and have even been associated with
illness after being steamed in shellfish. Moreover, noroviruses
can survive in up to 10 ppm chlorine, well in
excess of levels routinely present in public water systems. Despite these
features, it is likely that relatively simple measures, such as correct
handling of cold foods, strict hand washing after using the bathroom and
before handling food items, and paid sick leave, may substantially reduce foodborne transmission of noroviruses.
Prevention of norovirus disease spread via droplets from vomitus (person to person transmission) should focus on
methods to limit transmission including isolation precautions (e.g., cohort
sick patients in a healthcare facility) and environmental
disinfection. Surveillance of Norovirus Infection in the CDC currently
does not conduct active surveillance to monitor outbreaks of gastroenteritis
caused by noroviruses. Outbreaks are reported to
CDC's Viral Gastroenteritis Section, Respiratory and Gastroenteric
Viruses Branch, Division of Viral Diseases, National Center for Immunization
and Respiratoiry Diseases (NCIRD) when states send
specimens for testing or sequencing, or outbreaks are reported directly by
states to the electronic database (eFORS)
maintained by the Foodborne Diarrheal Diseases
Branch. For further
details please email calicinet@cdc.gov CDC |
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