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Noroviruses Summary Document

Virology
Name: Noroviruses (NoV)
Synonyms: Norwalk virus (NV); Norwalk-like virus (NLV); Caliciviruses; Small round structured viruses (SRSV); epidemic viral gastroenteritis, acute infectious non-bacterial gastroenteritis, winter vomiting disease
Description: Noroviruses are a group of related, single-stranded RNA, non-enveloped viruses that cause significant gastroenteritis in humans
Classification: Virus
Family/Genus: Caliciviridae/Norovirus
Envelop present: No
Size: 26 to 35-nm
Shape: Spherical
Genome: single-stranded(+)-RNA
Human Genogroups: GI, GII, GIV.  GI and GII cause the majority of illness in humans.  GIII is not known to cause illness in humans.  There is no evidence of zoonotic transmission
Epidemiology
Incidence/Prevalence: Specific data are not available for the United States.  Estimated to cause 23 million cases of acute gastroenteritis in the United States each year.  It is also estimated that at least 50% of foodborne illness outbreaks are caused by noroviruses.

Attack rates generally do not differ significantly between are or gender groups.  Norovirus gastroenteritis outbreaks are usually characterized by high attack rates
Geographic distribution: Worldwide
Temporal distribution: Outbreaks occur throughout the year in the United States, with the lowest frequency during the winter months.
Reservoir: Man
Infectious dose: <100 viral particles (highly infectious)
Mode(s) of transmission: Consumption of fecally-contaminated food or water and by person-to-person spread or contact with contaminated fomites.  Most frequently associated with outbreaks following the ingestion of raw (and less frequently steamed) shellfish
Incubation period: 24 to 48 hours
Period of communicability: Unknown
Outbreak definition: Provisional diagnosis of infection with noroviruses can be made if the following criteria are met (Kaplan's criteria):
  1. Bacterial or parasitic pathogens are not detected
  2. Vomiting occurs in more than 50% of the cases
  3. Mean or median duration of illness ranges from 12 to 60 hours
  4. Incubation period is 24 to 48 hours
Clinical characteristics
Symptoms: Generally produces a mild, self-limiting gastrenteritis characterized by nausea,, vomiting, diarrhea, abdominal cramps, headaches, chills, myalgia and sore throat.  Bloody diarrhea is not usually associated with gastroenteritis caused by noroviruses.  Low-grade fever sometimes occurs.  Vomiting usually occurs more frequently than diarrhea in children; diarrhea usually occurs more frequently than vomiting in adults.
Duration of illness: 12 to 60 hours
Treatment: Usually gastroenteritis associated with noroviruses is self-limited.  Oral or intravenous rehydration may become necessary if severe vomiting or diarrhe persist.  Deaths from norovirus-associated gastroenteritis have been documented, but are considered rare.  Usually deaths are associated with underlying disease in elderly populations.
Susceptibility and resistance: High degree of susceptibility in all populations.  Immunity to infection is poorly understood.  Strain-specific immunity may last a few months.  It is not believed that long-term immunity is conferred with infection.  Repeat infections can occur.
Control and Prevention
Hygienic practices:
  1. Effective hand washing with soap and running water for at least 20 seconds
  2. Prompt disposal or disinfection of contaminated materials
  3. Isolation of infected persons for at least 48 hours after the resolution of the last symptom
Food protection:
  1. Hygienic processing of food items
  2. Exclusion of infected food handlers from food preparation and service
Potable water supplies:
  1. Prevention of cross-contamination with non-potable water supplies
  2. Strict adherence to prescribed upkeep and maintenance of drinking and recreational water systems, including acceptable halogen levels, filtration subsystems and distribution systems.

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