Guidelines for conducting investigations of suspected Norovirus gastroenteritis outbreaks
Conduct a preliminary investigation
Because characteristics of an outbreak are not apparent until some investigative activities are conducted, the following actions are recommended to be undertaken within one working day after notification of an outbreak is received:
- Inspection of the facility by a registered sanitarian
- Administration of the Hypothesis Generating Questionnaire for Gastroenteritis Outbreaks (available on dshs website, see references) to 5-6 ill persons
- Collection of stool or vomitus specimens from a minimum of two and a maximum of 12 ill persons
- Evaluation of findings from the sanitary inspection and the Hypothesis Generating Questionnaires
If the evaluation indicates that the outbreak meets the epidemiologic profile of a Norovirus outbreak, whether or not any clinical specimens have tested positive for a Norovirus, then it is recommended that the investigation be limited as described below. Of course, there may be reasons why you would choose to conduct a thorough investigation; the decision on whether or not this is warranted is to be made by the local or regional health department. Please note: The type of investigation recommended does not include the development and administration of a comprehensive questionnaire to both ill and well persons at risk of exposure. It also, consequently, does not involve any statistical evaluation of results, such as odds ratios and p-values, except the descriptive statistics noted below.
Investigate a suspected Norovirus outbreak
A questionnaire based on the Hypothesis Generating Questionnaire that includes information specific to the outbreak, such as a calendar and building floor plans, would be a suitable investigative tool. Administration of a questionnaire by health department personnel always yields more reliable information than questionnaires administered by staff at a facility, self-administered questionnaires, or illness reports generated from nursing or clinic logs. However, it is not always feasible to have health department personnel conduct these activities. Questions on illness history should use the CDC’s definition of diarrhea, which is > 3 loose stools in a 24-hour period.
- Identify the pathogen: Identification of Norovirus in stool specimens from a minimum of two ill persons is necessary to identify the causative agent of the outbreak. Because some stool specimens are likely to test negative for Noroviruses even for persons with illness caused by a Norovirus, specimens from several but not more than 12 different persons should be collected. Stool specimens should be collected no later than 24 hours following resolution of illness, as virus particles are usually shed only briefly (CDC, 2001). However, some people do shed virus particles for a week or even longer, so it may be worthwhile to attempt getting specimens late in an outbreak if it is not possible to get them earlier (CDC, 2001). Note: All stool specimens should also be tested for aerobic enteric bacterial pathogens and enteric bacterial toxins, because it cannot be determined that an outbreak is caused by a Norovirus until laboratory confirmation is received. It might also be appropriate to test specimens for parasites.
Note on environmental specimen testing: Most institutions keep trays of food from all meals served for 72 hours. Food or other environmental specimens might be collected by a sanitarian during an inspection. Testing of food or other environmental specimens for the presence of Noroviruses is generally not recommended unless an item or items is epidemiologically linked to cases of illness in an outbreak. Such a link can only be clearly established by conducting a thorough investigation that includes questionnaires administered to both ill and well persons.
Please contact an epidemiologist and/or the laboratory at dshs before you submit outbreak-related specimens. Staff members will coordinate specimen collection, shipping, testing, and result reporting. Laboratory staff always appreciate advance notification on samples being sent for testing; such notice enables them to make sure they have the media, personnel, and other resources on hand to test the specimens appropriately for the outbreak and as quickly as possible. They cannot conduct miracles on specimens that are too small, maintained at a temperature that destroys the pathogens, or collected too late in an outbreak.
- Characterize the outbreak: The following information about a suspected Norovirus -caused outbreak should be collected, if at all possible:
The ill population:
The place of the outbreak:
- The number of persons at risk (e.g., nursing home population, usually residents and staff counted separately), the number of persons ill with symptoms of gastroenteritis, the percentage of at-risk persons who became ill
- Any suspected source of an outbreak (e.g., foodhandlers ill > 24 hours before other persons became ill, a specific meal associated with gastroenteritis in a number of persons > 24 hours later)
- The average (mean and median) incubation period of illness, the range of incubation periods, if known
- The symptoms expressed, the number and percentage of persons expressing each symptom
- The average (mean and median) duration of illness, the range of illness duration
- The number and percentage of ill persons who sought medical care, the number and percentage of ill persons who suffered serious sequellae (e.g., IV rehydration, hospitalization, death)
- The number of persons providing stool and/or vomitus specimens, the number of stool and/or vomitus specimens that tested positive for Norovirus, the number of stool and/or vomitus specimens tested for bacterial pathogens and/or toxins and results of these tests, and the viral strain identified if known
- The average age of ill persons, the range of ages; the approximate percentage of total cases in each age group:
< 1 yr, 1–4 yr, 5–19 yr, 20–49 yr, ≥ 50 yr
- The gender distribution of illness (percent female cases, percent male cases)
The time frame of the outbreak:
- The total population in the institution, the number of buildings and the number of these buildings housing ill persons, or the number of persons attending an event, as appropriate
- Any unusual occurrence that might have contributed to the outbreak (e.g., crowding, construction, other common illness)
- Any food or other environmental specimens that tested positive for Noroviruses and the viral strain identified if known
- The date of illness onset for the first ill person and the last ill person, the peak date of illness, the dates of any subsequent illness onsets
- An Epi-curve
- Implement control measures: The most important control measure for Norovirus outbreaks is thorough and frequent handwashing. In institutions, maintaining vigilance in infection control practices is important. Disinfection with a 1:50 bleach solution is recommended for appropriate surfaces and articles. Recommending a glove policy in the kitchen of a facility might be appropriate, at least for a period of time, and ill foodhandlers should be excluded from any food preparation until they are well. It might be appropriate to isolate ill persons in their rooms, or stop movement of residents and staff members from buildings experiencing illness to buildings not experiencing illness. If activities are identified as possibly contributing to the spread of an outbreak, staff or other appropriate persons should be informed of the possibility of such a link and educated about ways to modify or substitute for these activities.
Conclude the investigation and report the outbreak
Please compile your findings and send them to the DSHS central office. Even reports that do not have all of the desired information are valuable to DSHS and to the CDC. You may complete the CDC's Viral Gastroenteritis Outbreak Investigation form included herein, or DSHS central office staff will complete it and then submit it to CDC.
CDC. Norovirus Activity - United States, 2002. MMWR 2003;52(03):41-45.
CDC. “Norwalk-Like Viruses”: Public Health Consequences and Outbreak Management. MMWR 2001;50(RR09)1-18.
Hall JA, Goulding JS, Bean NH, Tauxe RV, Hedberg CW. Epidemiologic profiling: Evaluating foodborne outbreaks for which no pathogen was isolated by routine laboratory testing: United States, 1982-9. Epidemiol Infect 2001;127:381-387.
Kaplan JE, Feldman R, Campbell DS, Lookabaugh C, Gary GW. Frequency of a Norwalk-like pattern of illness in outbreaks of acute gastroenteritis. Am J Pub Health 1982;72:1329-1332.