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Norovirus Fact Sheet

Synopsis of Norovirus
Noroviruses, previously known as "Norwalk-like viruses", is a group of related, single-stranded RNA, nonenveloped viruses which cause acute gastroenteritis in humans. Currently, there are at least four norovirus genogroups (GI, GII, GIII, and GIV), which, in turn, are divided into at least 20 genetic clusters. Only the common cold is reported more frequently than viral gastroenteritis as a cause of illness in the United States. Humans are the only known reservoir for noroviruses; although some noroviruses are present in swine, cattle, and mice. The primary mode of transmission is fecal-oral spread and usually occurs through consumption of a fecally contaminated source, either food or water. And, because the infectious dose is low (about 100 viral particles), low-level contamination can lead to outbreaks. Noroviruses are found worldwide and are a leading cause of acute gastroenteritis. The CDC estimates that 23 million cases of acute gastroenteritis can be attributed to norovirus infection (about 1/3 of all cases).
  • Transmission:
    • Fecal-oral route
      • Consumption of fecally contaminated food or water
        1. Outbreaks frequently associated with consumption of cold foods, such as salads, sandwiches, and bakery products
        2. Shellfish from contaminated waters
        3. Sewage contamination of wells and recreational water (most common source of outbreaks)
    • Direct person-to-person spread
  • Environmental and fomite contamination
  • Risk factors:
    • All age groups can be affected
    • Elderly, immunocompromised patients, and infants may be severely affected and may even be fatal if combined with other serious underlying medical conditions
    • Some patients may be genetically more susceptible
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Clinical Signs and Symptoms
Animals:
  • Cases are rare
Humans:
  • Incubation: 12 to 72 hours
  • Asymptomatic infections may occur in as many as 30% of infections
  • Clinical signs and symptoms:
    • Vomiting
    • Watery, non-bloody diarrhea
    • Abdominal cramps
    • Nausea
    • Low-grade fever (occasionally)
    • Dehydration
  • Immunity appears to be stain-specific and lasts only a few months, therefore, individuals are likely to be repeatedly infected throughout their lifetime.
  • Evidence also suggests that susceptibility to infection may be genetically predetermined, with people of O blood group being at greatest risk for severe infection.
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Diagnosis and Treatment
Diagnostic Samples:
  • In humans:
    • Stool and emesis samples
    • Rectal swabs
    • Blood
Differential Diagnosis:
  • Differential diagnoses include the following:
    • Staphylococcus aureus
    • Bacillus cereus
    • Heavy metal poisoning
    • Camphyobacter spp.
    • Clostridium perfringens
    • Escherichia coli
    • Shigella spp
    • Vibrio cholerae
Clinical Diagnosis:
  • Clinical diagnosis is often based on the combination of symptoms, particularly the prominence of vomiting, little fever, and the short duration of illness.
Laboratory Tests:
  • Laboratory diagnosis is often difficult
  • Real time reverse transcriptase polymerase chain reaction (method of choice)
  • Direct and immune electron microscopy of fecal specimens
  • Specific antibodies in acute-and convalescent-phase blood samples
  • Enzyme-linked immunosorbent assay for detection of virus in stools (under development)
Treatment:
  • In humans:
    • No antiviral therapies exist
    • Supportive care
  • Replacement of lost fluids
  • Correction of electrolyte disturbances
  • Possibly intravenous parenteral therapy
  • Bismuth subsalicylate can reduce gastrointestinal symptoms
  • Antiperistaltic agents can control diarrhea
  • No vaccines currently available
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Econcomic Consequences and Disease Eradication
Economic Consequences:
  • Medical costs associated with treatment
Disease Prevention and Eradication:
  • Prevention:
    • Proper handling and cooking of foods
    • Thorough cooking of foods destroys the virus
    • Avoid uncooked shellfish, such as oysters and clams
    • Wash raw vegetables thoroughly
    • Frequent hand washing
    • Drink only bottled water in areas that appear to have polluted water
    • Exclude people with gastroenteritis from the kitchen
    • Always dispose of sewage in a sanitary manner
    • Paid sick leave
  • Prevention for food handlers:
    • Standard hygiene and control measures
    • Rigorous food handling procedures
    • Exclusion of ill food handlers
    • Disinfection of all working surfaces
  • Viability:
    • Noroviruses are relatively resistant to environmental challenge
    • Able to survive freezing, temperatures as high as 60° C, up to 10 ppm chlorine
Tularemia and Bioterrorism:
  • Norovirus is highly contagious - as few as 100 viral particles may be sufficient to infect an individual
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Outbreaks
  • 2006: Carrabba's Restaurant in Lansing Michigan was the source of an outbreak that sickened more than 400 people. The Barry-Eaton District Health inspectors found six critical violations, four repeat violations, and a number of non-critical violations. The source of the outbreak was determined to be sick employees.
  • 2005: The largest recent outbreak occurred among Hurricane Katrina evacuees in Houston. 1,169 evacuees out of 6,500 who visited the Reliant Park Medical Clinic presented with acute gastroenteritis. Norovirus was confirmed in about 50%.
  • 2005: Michigan Blimpie's was the source of an outbreak that sickened approximately 125 people.
  • 2004: Las Vegas Flamingo Hotel was the source of a major outbreak that sickened more that 1,200 guest and workers.
  • 2002: There were 25 reported outbreaks aboard cruise ships, with approximately 2,700 passengers becoming ill from the virus
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Sources and Related Articles
Sources:
  1. CDC Respiratory and Enteric Virus Branch, 2005:
    1. Norovirus Technical Fact Sheet. Available at http://www.cdc.gov/ncidod/dvrd/revb/gastro/noro-factsheet.pdf.
    2. Norovirus and Food Handlers. Available at http://www.cdc.gov/ncidod/dvrd/revb/gastro/noro-foodhandlers.pdf.
    3. Norovirus in Healthcare Facilites. Available at http://www.cdc.gov/ncidod/dhqp/id_norovirusFS.html.
  2. CIDRAP, 2006. Norovirus. Published on the CIDRAP Website. Available at http://www.cidrap.umn.edu/cidrap/content/fs/food-disease/causes/noroview.html.
    3. References:
  3. CDC, 2005. Norwalk-Like Viruses: Public Health Consequences and Outbreak Management. MMWR. 50 (RR-9). Available at http://www.cdc.gov/ncidod/dvrd/revb/gastro/rr5009.pdf.
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