Responding to Norovirus Outbreaks

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

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 may perform diagnostic testing

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

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

CDC helps with investigations and control of norovirus outbreaks by—

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

Using Kaplan Criteria for Suspected Norovirus Outbreaks

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

The Kaplan Criteria 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 outbreak was not caused by norovirus.

Detecting Norovirus in Food, Water, and Environmental Specimens

To recover and detect norovirus in food, laboratory methods must be adapted for each type of food. Such methods are generally available only in research and academic settings. So, no standardized, validated laboratory tests for detecting norovirus in food are available, except for shellfish.

Water can be tested for norovirus by processing large volumes of water (10-100L) through specially designed filters.

Swabs of objects, such as doorknobs and handrails, and surfaces, such as kitchen counters, can also be tested for norovirus. However, obtaining virus from swabs is highly variable. Results should be interpreted with caution and in the context of the available epidemiologic evidence.