The public health response to outbreaks of norovirus is determined by state or territory legislation, local reporting requirements and available resources. Public health agencies will seek to determine the nature of the outbreak to define their role in the investigation.
In institutional outbreaks that are suspected to be person-to-person spread, it may be more appropriate for the facility to manage the outbreak with advice or support from the PHU or Environmental Health Officers (EHO), as required. On the other hand, public health agencies will need to rapidly investigate and manage outbreaks of food- or waterborne disease, as intervention can reduce human illness, and they can have important policy implications for government.
7.1 Assessment of a suspected outbreakNotifications or reports of an outbreak received by a PHU will need initial assessment. This may be done by public health nurses or public health officers. Initial information collected should include:
- the time the outbreak began
- the total number of cases and unaffected people to calculate the proportion of people affected
- symptoms and duration of illness
- type of outbreak setting
- if food is implicated
- results of any laboratory tests that may have been done.
- the time and date of when the illness began
- duration of illness
- information on key symptoms
- whether laboratory tests have been conducted
- any key exposures related to illness.
- develop an epidemic curve to determine if the outbreak is suggestive of a point-source or person-to-person spread
- review symptoms and duration of illness to determine if the illness is consistent with norovirus
- follow up and review any initial laboratory tests that may have been conducted
- develop a preliminary case definition for cases in the outbreak to assist with counting further infections
- review information available, in particular the type of outbreak setting, characteristics of cases and attack rate of illness.
7.2 Criteria for starting an investigationWhen to start an investigation on a suspected outbreak of norovirus will be determined by the:
- type of outbreak setting
- number of affected persons
- severity (morbidity, e.g. hospitalisation and mortality history) of cases
- suspicion of a foodborne source.
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7.3 Action when outbreaks are detectedThere should be clear processes for action where outbreaks are detected. Levels of investigation can range from a case series and site inspection to large analytical epidemiological studies involving collection of information on hundreds of patients and people who do not have gastroenteritis (controls).
The steps and the order in which an outbreak is investigated may differ depending on the nature of the outbreak. Public health agencies can use the flow chart in Figure 3 as a guide to investigating outbreaks.
In any reported outbreak, preliminary collection of information such as data on time, person and place, number of cases and the identification of the ‘at risk population’ is important in order to determine the mode of transmission and severity of illness. It is also important to confirm the diagnosis where possible as this has implications on the management of the outbreak.
A further investigation may not be needed for each outbreak, especially if the outbreak appears typical for person-to-person spread. In outbreaks where the clinical or epidemiological picture suggests a bacterial agent or where morbidity or mortality is concerning or suggestive of a common food source, further investigation may be indicated. An investigation may also be necessary if there is an ongoing suspected source of infection or the investigators have a hypothesis that they may want to test.
In both person-to-person and foodborne outbreaks, EHO play an important role in the investigation and management of norovirus outbreaks. Their role may involve the following:
- Investigate the outbreak and identify the source of illness, in collaboration with the PHU.
- Facilitate the collection of specimens (both environmental and clinical) for transport to a laboratory and subsequent testing.
- Provide advice on control and preventive measures.
- Inspect the premises to review environmental health and infection control concerns.
- Consider whether formal action under legislation is appropriate in the event of food being identified as the source of the outbreak.
PDF printable version of Figure 3: Flow chart – Investigation and management of gastroenteritis outbreaks (PDF 172 KB)
Figure 3: Flow chart – Investigation and management of gastroenteritis outbreaks
Two or more cases of diarrhoea or vomiting in a 24 hour period that are epidemiologically related in time, place or person
- Establish the existence of an outbreak
- Conduct a site visit
- Confirm the diagnosis
- Form outbreak team
- Define cases and establish the number of cases
- Collect time, person and place data and develop an epicurve to determine the mode of transmission
- Determine the at risk population
- Provide advice on infection control measures
If suspected food – or waterborne
From an outbreak investigation team who will:
- Obtain necessary information, such as menus of items consume, to expedite the investigation
- Develop and text hypothesis
- Conduct an analytical study involving both ill and well people
- Conduct environmental investigation to establish the source of infection, including testing of food and environmental samples where appropriate
- Institute the necessary control and prevention measures
- Write an outbreak report
- From an outbreak investigation team who will:
- Conduct environmental investigations
- Institute the necessary control and preventive measure.
- Write an outbreak report
7.3.1 Initial respons
Establish existence of an outbreakFollowing review of line-list information regarding a suspected outbreak, public health agencies need to decide whether an outbreak is actually occurring. This can sometimes be difficult to decide, particularly for community-wide outbreaks, as it is difficult to determine the background rate of gastroenteritis. In determining whether an outbreak is occurring, it is important to consider what constitutes the population at risk, which should include the total number of cases and unaffected people, including staff.
Conduct site visitA site visit by an EHO may be useful and can be decided locally, in conjunction with the institution and depending on the situation. A site visit may assist with specimen collection, ensuring infection control measures are in place and inspecting the kitchen facilities to rule out foodborne contamination.
Confirm a diagnosisPublic health agencies should discuss with the institution what specimens have been collected, the laboratory they were sent to and any available results. If specimens have not been collected, investigators should advise the institution about collecting specimens and the type of testing to be requested. For optimal detection of norovirus in faecal specimens using ELISA tests, it is necessary to collect at least six samples from individual patients in outbreak settings . For PCR-based testing, three or more specimens are necessary to have adequate sensitivity to confirm norovirus. Where a public health agency is conducting a major investigation, it is best if multiple specimens are collected and sent to a reference laboratory. This is particularly important if there is a need for more advanced testing, such as gene sequencing.
PHU staff should liaise with the laboratory regarding specimen collection and transport. The laboratory should be advised as early as possible that specimens will be arriving. A request form must be completed by a medical officer and if possible a copy of the results should be sent to the public health agency, and a copy going to each case’s treating medical officer.
For outbreaks in institutional settings, it may be easier for affected people to see their general practitioner and to submit a faecal specimen for testing to a specified laboratory. The treating medical practitioner should liaise with the PHU as to the type of testing required. The PHU will need to confirm with the institution and the laboratory about the number of clinical specimens to be taken in a given outbreak.
Once public health agencies confirm that norovirus is the causative agent of an outbreak, it is important that they communicate the results back to people and institutions that are affected. There is no need for further collection of faecal specimens once norovirus is confirmed as the cause of an outbreak, unless the nature of cases’ illness changes. For more information on collecting faecal specimens during outbreaks see Appendix 3.
As norovirus is predominantly spread directly from person-to-person and is highly infectious, the major emphasis will be placed on infection control rather than investigation into outbreaks of suspected viral gastroenteritis. Instituting immediate infection control measures is a priority in managing person-to-person outbreaks and should not be delayed while waiting for laboratory confirmation. The ability to respond to an outbreak will depend on the nature of the institution or setting of the outbreak. Hospitals are likely to have dedicated Infection Control Practitioners (ICP) and will probably manage the outbreak. On the other hand, institutions such as some ACFs, childcare centres, schools and others are less likely to have ready access to technical assistance in relation to infection control and will look to public health agencies for advice. For more information on infection control in relation to norovirus see Chapter 8.
Key features of outbreaks that might indicate foodborne transmission are where many people become ill at the same time indicating a common exposure, or where a common event involves a meal. However, with norovirus outbreaks it can be difficult to distinguish between person-to-person and foodborne transmission due to the highly infectious nature of the virus . For foodborne outbreaks of norovirus there may be considerable numbers of secondary cases. Epidemiological data and microbiological evidence will give an indication of the true number of foodborne cases .
Most foodborne norovirus outbreaks are likely to originate from infected food handlers contaminating ready-to-eat foods with faeces or vomit during harvesting, transport, preparation or serving. In Australia, foodborne norovirus outbreaks are most likely to occur because of food handlers preparing or serving food while symptomatic. Many states and territories have their own ‘protocols’ or ‘guidelines’ document for investigating outbreaks of foodborne illness. Details in the documents will vary considerably; some jurisdictions will have information on very general principles whereas others will have more detail. Nevertheless, the steps outlined below are intended as a general guide and public health agencies may refer to local documents as well as these.
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Form an outbreak management teamPublic health agencies should form an Outbreak Management Team (OMT) to investigate and manage all aspects of a suspected foodborne outbreak of norovirus. The team may include a variety of personnel, including public health nurses, medical officers, public health officers, epidemiologists, laboratory scientists, infection control nurses and EHOs. If the outbreak is in an institution, a representative of that institution may also be part of the team. The OMT team leader, who is usually an epidemiologist or public health officer, should clearly define the roles and responsibilities of team members, along with activities required to investigate and contain the outbreak.
Formulate a case definitionThe case definition may be reviewed and updated during the course of the investigation. A sensitive case definition should be developed initially so it will capture most of the cases, while recognising there will also be some false positives, i.e. cases which are not truly cases. As more information becomes available during the investigation, the case definition can be expanded or refined to ensure that the definition is as specific as needed. The working case definition should be appropriate and fit the criteria described in Chapter 6.
Case findingAppropriate clinical specimens should be collected from cases, including ill staff, in order to confirm a diagnosis and identify the causative agent. Refer to Appendix 3 on specimen collection and notifying the laboratory about the outbreak. An attempt should be made to identify additional persons who meet the case definition. It may be possible to obtain a guest list or list of attendees from functions, social events or patron booking details if a food premises or restaurant is the place where exposure occurred.
Conduct hypothesis generating interviewsPublic health staff should conduct hypothesis generating interviews of cases to obtain food histories to identify a potential food vehicle or source of infection that may be associated with the illness and can guide further investigation.
Determine source of infectionTo determine if the outbreak is related to contaminated food at the source or at the point of service by an infected food handler, food handlers should be asked about gastrointestinal illness prior to and during the outbreak. They should also be asked about their role in food preparation before and during the outbreak, particularly if they were ill at the time. It may be necessary to collect faecal specimens from food handlers, especially if they are ill with gastrointestinal symptoms at the time of the investigation. Information should also be obtained about people vomiting in the food premises prior to the first onset date of illness in a case.
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Initiate environmental investigation and food samplingIt is essential that an EHO inspects catering or food premises if an outbreak of suspected foodborne gastroenteritis has occurred. Public health agencies may refer to local guidelines and consult with environmental health or local government if a site visit or food testing is to take place. The EHO may assist public health agencies in their investigation by carrying out the following activities at the site visit:
- sampling of suspect foods and the environment
- obtaining a copy of the menu or list of foods that were served
- collecting information on the preparation, storage and distribution of cooked food
- assessing potential for cross contamination from raw to cooked foods
- checking for illness among staff and staff absenteeism records
- interviewing food handlers
- reviewing and reinforcing general hygiene issues and food control systems
- implementing control measures where necessary.
Carry out trace-back of food productsFood production and distribution has become increasingly complex, which requires multiple agencies to collaborate to identify the origins of foods during outbreak investigations . Trace-back is where food safety agencies follow the production and distribution chain of a food to identify where it was grown, processed, packaged, transported and sold at retail. Trace-back of foods is usually only required where foods have become contaminated well before the point of food preparation. For example, Australian and New Zealand outbreaks due to norovirus-contaminated oysters were shown to have originated from Japan and Korea, requiring international trace-back [44, 78]. The main aim of trace back is to determine the source and distribution of the implicated food item and identify potential sites of contamination along the food chain. External agencies are likely be involved in the trace-back, at local, national and international levels .
Analytical studyAn analytical epidemiological study may be necessary to test a hypothesis or a range of hypotheses. The study uses a comparison group that enables quantitative statistical associations between exposure and illness to be calculated. Case control and cohort studies are the two most frequently used epidemiological study designs in a foodborne outbreak investigations. For more information on how to conduct analytical studies in foodborne disease outbreaks see information provided by:
- the World Health Organization <http://www.who.int/foodsafety/publications/foodborne_disease/fdbmanual/en/index.html>
- the Centers for Disease Control and Prevention in the United States <http://www.cdc.gov/foodborneoutbreaks/>.
7.4 Monitoring the outbreakOngoing surveillance during the outbreak is conducted to identify new cases and update the status of existing cases. In an institutional outbreak caused by person-to-person transmission, a representative from the institution will update the initial line-list with new information and send it to the PHU. Changes in the outbreak control measures may be indicated from review of these data. If new cases continue to be identified and the nature of the illness changes then a new organism causing infection or a different mode of transmission must be considered. In foodborne investigations, it is critical to continue to monitor for cases to ensure that the contamination has been removed and there is no longer a risk to public health.
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7.5 Declare that the outbreak is overThe point at which an outbreak can be declared over depends on the nature of the outbreak. The OMT should decide when an outbreak is over. There appears to be no consistency as to when an outbreak of norovirus should be declared over. One recommendation is that outbreaks in residential facilities can be said to be over when seven days have elapsed since resolution of symptoms in the last case. Others have recommended declaring that an outbreak is over when two incubation periods for the organism have passed since the end of symptoms in the last case.
As a general guide, norovirus outbreaks can be declared over if no new cases have occurred in 72 hours from the onset of symptoms of the last case. Although norovirus has been detected in faeces many weeks after a person has recovered, it is not known if that person remains infectious [59, 60].