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Responding to Norovirus Outbreaks

Prompt investigation and reporting are critical for identifying the cause of norovirus outbreaks and the primary way the virus was spread. Collection of appropriate specimens also depends on rapid response.

Definition of a Norovirus Outbreak

An outbreak of norovirus is defined as an occurrence of two or more similar illnesses resulting from a common exposure that is either suspected or laboratory-confirmed to be caused by norovirus.

Typical Responsibilities During an Outbreak

State, local, and territorial health departments:

  • serve as the lead agencies in most investigations of norovirus outbreaks
  • interview patients, collect stool specimens, and sometimes perform diagnostic testing

CDC helps investigate and control norovirus outbreaks by:

  • providing epidemiologic consultation and tools
  • testing specimens and providing other laboratory support
  • coordinating multi-state outbreak investigations as needed

Food regulatory agencies (FDA, USDA, and state authorities):

  • Collaborate with health departments when a link between contaminated food and illness is identified
  • Perform food testing for specific foods, such as shellfish and produce
  • Coordinate recalls of foods involved in outbreaks.

Using Clinical and Epidemiologic Criteria for Suspected Norovirus Outbreaks

When it is not possible to get laboratory confirmation of norovirus, health departments can use clinical and epidemiologic criteria to determine if the outbreak was likely caused by norovirus.

The original criteria proposed by Kaplan et al are:

  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 found.

When all four criteria are present, it is very likely that the outbreak was caused by norovirus. However, about 30% of norovirus outbreaks do not meet these criteria. If the criteria are not met, it does not mean that the outbreak was not caused by norovirus.

Recently, an alternate set of clinical criteria proposed by Lively et al have been identified that are more specific for norovirus and more often available during outbreak investigations than the Kaplan criteria. These are:

  1. A greater proportion of cases with vomiting than with fever,
  2. Bloody diarrhea in less than 10% of cases, and
  3. Vomiting in greater than 25% of cases.

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