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NORWALK-LIKE VIRUSES: |
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NORWALK-LIKE VIRUSES: |
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of Norwalk Virus |
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of viral structure |
Extracted in part from CDC page on Norwalk viruses:
ORIGINS: Named for an outbreak of gastroenteritis in a school in Norwalk, Ohio, in 1968.
The CDC estimates at least 50% of all foodborne outbreaks of gastroenteritis can be attributed to Norwalk-like viruses, and that 23 million cases of acute gastroenteritis are due to norovirus infection.
SIGNS AND SYMPTOMS: The incubation period is 24 to 48 hours, but onset of symptoms can occur within 12 hours of exposure.
It usually presents as acute-onset vomiting, watery non-bloody diarrhea with abdominal cramps and nausea. Low-grade fever also occasionally occurs. Vomiting is more common in children.
Note that, contrary to public pronouncements, these symptoms of Norwalk-like viral infection are NOT “flu-like symptoms”. Most importantly, the major signs of vomiting and diarrhea are not typical signs of the flu. Furthermore, according to news reports, Mississippi Cruise patients lack other "flu-like" symptoms: significant fever, pronounced body aches and pains, chills, moderate to severe fatigue, unproductive cough and chest discomfort. (See this page contrasting cold versus flu symptoms.)
Dehydration is the most common complication. Symptoms usually last 24 to 60 hours.
TRANSMISSION: Transmitted primarily through the fecal-oral route, either by consumption of fecally contaminated food or water or by direct person-to-person spread.
Norwalk-like viruses are highly contagious and as few as 10 viral particles may be sufficient to infect.
Initial foodborne transmission in a food service setting, etc. The disease can them be followed by secondary person-to-person contacts.
Shedding of infectious particles usually begins with onset of symptoms and may continue for 2 weeks after recovery.
People of blood group O being at greatest risk for severe infection.
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 3% were waterborne Most foodborne outbreaks are likely to arise though direct contamination of food by a food
handler immediately before its consumption, especially cold foods, 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. Waterborne outbreaks in community settings have often been caused by sewage contamination of
wells and recreational water. (Normal drinking water chlorination does not kill the virus.) IDENTIFICATION: “Norwalk-like viruses”are single-stranded RNA, non-enveloped viruses
which cause acute gastroenteritis in humans. Kaplan's epidemiologic criteria criteria for identifying probable Norwalk gastroenteritis: 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. PREVENTION: Prevention of foodborne norovirus disease is based on the provision of safe
food and water. Norwalk-like viruses are able to survive freezing, temperatures as high as 60 ̊C,
survive in up to 10 ppm chlorine, well in excess of levels routinely present in public water
systems. Relatively simple measures can prevent transmission, such as 1) correct handling of cold foods 2) strict hand washing after using the bathroom and before handling food items 3) paid sick leave, may substantially reduce foodborne transmission of Norwalk-like virus 4) isolate infected persons, especially preventing spread via droplets from vomitus