Page content: 5.1 Introduction | 5.2 Types of outbreaks | 5.3 Roles | 5.4 Steps in outbreak investigation

5.1 Introduction

An outbreak may be defined as the occurrence of a disease or health event in excess of the expected number of cases for a given time or place. Cases may be related to an apparent common source, (eg a particular food) or sometimes to a specific setting (eg an institution).

The majority of outbreaks which come to the attention of EHOs are of gastrointestinal illness. It is not always easy to recognise outbreaks. Some experience and judgement is needed to distinguish sporadic cases of common notifiable diseases such as Campylobacter, from a real increase in cases which signals an outbreak.

Why do we investigate outbreaks?

The reasons for investigating outbreaks are to:

• Prevent further disease transmission

• Provide information which can be used to control the outbreak

• Prevent similar occurrences in the future

• Identify populations at risk for a disease

• Evaluate the effectiveness of public health programs

• Characterise modes of disease transmission

• Train public health staff

• Fulfil legal obligations and duty of care

• Learn more about a disease including the impact of control measures

• Share knowledge/findings with other health professionals by documenting the outbreak investigations in reports or journal articles

Most of the information and the procedures in this section are directed towards the investigation of outbreaks involving a number of cases, but the principles also apply to the more common occurrence of one or two cases of "gastro" or "food poisoning", in which the public health response is typically more limited.

Be wary of accepting the case's supposition that the take-away meal which they consumed some hours before they became ill was the source of their illness. Single and clustered cases of gastroenteritis may sometimes be due to pathogens spread in food and water, but gastrointestinal pathogens (particularly viruses) are often spread from person to person by close contact, environmental sources or contaminated fomites, particularly when hygiene is poor.

Prompt action is needed to prevent further cases.

During an investigation of an outbreak, it is critical that clear notes be kept detailing all steps taken during the investigation. These notes should include the completed questionnaires, assessment of all potential risk factors, details of any control measures implemented and any other action taken including a summary of findings. Local government should keep their own records for future reference.

5.2 Types of outbreaks

Outbreaks of gastroenteritis allegedly related to food or food premises, and outbreaks of gastroenteritis in institutions, are the two most common types of outbreaks reported.

Some of these apparently widespread outbreaks may ultimately be shown to have a common source (such as a widely distributed food), while others will represent community-wide outbreaks due to agents (usually viral) spread person-to-person.

The common event outbreak occurs as a result of a common exposure at a defined time and place (eg an outbreak of gastroenteritis among people who attended an event such as a wedding reception or party). Such outbreaks are commonly reported, as illness amongst guests are recognised by the participants themselves, often leading to a report to local government or the Department of Human Services.

Outbreaks of gastroenteritis in institutions are most commonly caused by viral agents such as Norwalk like virus, which are spread person-to-person.

Identifying the source of an outbreak of illness is not always straightforward. Some outbreaks may only become apparent through an increase in notifications in one or more areas. Dispersed outbreaks, in which the cases have no immediately apparent association, such as a cluster of cases of the same serotype of Salmonella, are much more difficult to investigate. However, the cases may be due to a point source of infection, for example contaminated food from a particular take away food premises or a widely distributed contaminated food item from a particular manufacturer.

How an outbreak is investigated will depend on the nature of the outbreak. Rigid categorisation of outbreaks into discrete types with recipes for how to investigate them is not possible. In practice, almost every outbreak will be unique in some way, requiring a degree of flexibility in the approach to recognition and investigation.

Specific control and prevention strategies, according to the mode of transmission, are outlined in the following sections.

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5.3 Roles

5.3.1 Local Government

Outbreaks which are confined to a local government area are primarily the responsibility of that local government authority. Local government should regularly liaise with the Region throughout the investigation. Local government's contact point with DHS should firstly be with the Region. If the REHO is not available contact should then be with Disease Control Victoria.

The Department, through the regional office in the first instance, should be kept informed throughout the investigation to enable departmental briefing to be undertaken.

5.3.2 Department of Human Services

Outbreaks where cases are spread beyond council and/or regional boundaries, will usually be coordinated by Disease Control Victoria in conjunction with the Region. Regional EHOs will act as the contact point with local government and provide assistance and advice during the investigation. They will liaise between local government and Disease Control Victoria.

While this can be used as a general rule, there may be circumstances where the outbreak does not spread beyond a council boundary but Disease Control Victoria may coordinate the investigation. This will depend on the scale and the complexity of the outbreak. For example the size and/or the profile of the outbreak and actions required (eg closure of premises) may necessitate that the Department coordinate the outbreak.

Disease Control Victoria will provide advice on key epidemiological aspects of the outbreak (case definition, extent of outbreak, questionnaire development etc.) and advise on specific urgent investigations, interventions and any additional technical expertise.

Clusters of disease are identified periodically, through surveillance by Disease Control Victoria and also by MDU (see Section 3.2 and 3.3). Cases identified as part of a cluster, are usually followed up by a specific actioning officer within Disease Control Victoria.

5.4 Steps in outbreak investigation

To determine the correct epidemiological response the following steps must be carried out when investigating an outbreak.

5.4.1 Information Gathering

Confirm the existence of the outbreak by talking to the person who first reported it.

Determine or obtain:

• Date, time and place of function or incident

• The number of people ill and number of persons "at risk"

• Symptoms and severity, ie if any cases have been hospitalised

• Onset dates and times for cases

• Duration of symptoms

• List of residents, guests, patrons

• Names and phone numbers of any contact person or organisers or contact details for all exposed persons

• Whether any staff were ill prior to the outbreak or worked while symptomatic

• Copies of menus if a function or if persons affected are residents of premises (eg hospital), obtain menus for at least 3 days prior to the onset of symptoms for the first case

• Illness in any person prior to the event or whether anyone vomited in a public place

• Circumstances which allegedly implicate a particular source.

5.4.2 Reporting an Outbreak

All suspected outbreaks of gastrointestinal illness must be reported immediately to the Regional Office who will notify Disease Control Victoria. If the REHO is not available, notification should be made directly to Disease Control Victoria. (See contact numbers). Disease Control Victoria will inform Food Safety Victoria, Environmental Health Unit and senior management within the Public Health Branch of any suspected outbreak reported via an internal DHS incident report.

In conjunction with the REHO, assess the progress in the investigation and determine the next appropriate steps to take. (These will usually involve formal investigations, control and clean-up measures.) Through a consultation process with local government, the Region and Disease Control Victoria a decision will usually be made about whether the remaining actions will be handled by local government or whether additional assistance is required from Disease Control Victoria.

Give the outbreak a name.

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5.4.3 Inspection

• Contact the person in charge of the premises/institution/organisation where the outbreak occurred and advise them that you are investigating a suspected outbreak. Ask them to stop selling/serving any product which is suspected to be the cause of the outbreak.

• Inspect/Investigate the site of the outbreak paying particular attention to the following:

• • Kitchen and food storage areas, food handling practices, food storage temperatures and details of preparation of foods on menus suspected as the possible cause of the outbreak including dates and times of preparation

  • Obtain samples of remaining food, water or ice for analysis (see Attachment 1 - Specimen Collection)

  • Obtain environmental swabs if appropriate (see Attachment 1 - Specimen collection)

  • Type of water source

  • A detailed map or plan of the premises or institution

  • Sewerage system/toilets recent problems

  • Cleaning methods and general infection control

  • Illness in staff members organise the collection of faecal specimens from ill staff (and occasionally all food handling staff regardless of whether they were ill or not)

  • Compliance with guidelines for exclusion of food handlers (See Attachment 6)

  • HACCP or other food safety documents

  • Staff training

  • Other potential sources eg swimming pools, recent functions, informal eating areas.

• Advise on immediate control measures to prevent further cases (see following sections). These may include sanitation procedures, boiling or disinfection of water, isolation of infected cases, restriction of movement of staff and patients within an institution and ensuring that ill staff do not work.

5.4.4 Questionnaires/Interviews

Use the gastro outbreak questionnaire (Attachment 13) with modifications as necessary, such as attaching a menu etc. Consult with Disease Control Victoria about any modifications to the questionnaire that you may think are necessary. Only structured questionnaires, where all the subjects are asked exactly the same questions, are likely to be of use in outbreak investigations.

Persons affected by an outbreak may forget specific details of recent meals and gatherings. It is therefore crucial to move swiftly to administer outbreak questionnaires. To accurately isolate the source of the outbreak it is necessary to administer the questionnaire to both affected and unaffected persons who attended the gathering implicated in the outbreak (cohort study) or it may be appropriate to interview healthy people from a similar demographic background (case control study). Disease Control Victoria can give advice regarding this issue.

Questionnaires must also be administered to food handlers where there is a food premises implicated in the outbreak. When the outbreak is institutional, administer questionnaires to staff, carers etc.

It is preferable that outbreak questionnaires are administered to the subject by an interviewer either face to face or by telephone. This enables the interviewer to discuss fully the circumstances surrounding the event and thereby assisting the recall process. Postal or self administered questionnaires generally provide less useful and sometimes incomplete data.

Ensure, that if more than one interviewer is involved, that all interviewers administer the questionnaire in a standardised fashion, so that interviewer bias is minimised.

During the interview, ascertain if the case has submitted a faecal sample for testing. If not, arrange for one to be taken if they are willing to provide it. This is especially important for people who still have symptoms, however, in some circumstances a faecal specimen should also be taken even if the case has recovered (see Attachment 1 - Specimen Collection).

If considered appropriate, contact local medical practitioners and hospitals to determine if other cases of similar illness are occurring in the community.

5.4.5 Sampling

Laboratory activities should be targeted. It is not necessary to test all specimens for all pathogens. It is important to contact the laboratory to alert them to the investigation of an outbreak. Identify a contact person and discuss with the laboratory, their contribution to the investigation and any recommendations they may have. If large numbers of specimens are being submitted, special scheduling of these specimens for processing may need to be made.

The laboratory can offer:

• Expert advice

• Recommendations on type and number of specimens that should be collected

• Specific requirements for collection of certain specimens

• Specimen collection kits and containers

• Estimated time frame for results

Interpretation of results:

A positive result

• From an epidemiologically implicated person or item, strongly suggests that the person or item was the source or vehicle of infection

• From a person or item without epidemiological association, does not suggest or prove that the person or item was the source or vehicle

A negative result

• Does not rule out an association

• Indicates only that pathogen was not found in specimen collected

Reasons for a negative result:

• Implicated item no longer available

• Not all units/batches/parts were contaminated

• Specimen source was not uniformly contaminated

• Intermittent contamination/carriage

• Specimen size too small

• Competitive micro-organisms outgrew pathogen

• Item/source not tested for pathogen

• Inappropriate processing, handling or storage diminished; injured or inactivated pathogen

• Laboratory methods were inappropriate or inadequate

• Possibility of a false negative

Laboratory investigations are:

• Generally used to support a diagnosis or hypothesis, not to make it

• Only as good as the specimens collected

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5.4.6 Clean Up and Control Measures

Advise proprietor of immediate control measures to take depending on suspected source and whether it appears the outbreak is food or water-borne or passed person to person.

If outbreak is in an institution (eg nursing home, hospital, provide a copy of "Controlling an Outbreak of Gastroenteritis - Guidance for Institutions" (Attachment 3) to the person in charge.

If point source outbreak ie. food or water-borne spread, explain and hand out to proprietor, a copy of "Food or Water-borne outbreak - Guidance for Food Premises" (Attachment 2. See also Section 6.).

If it is unclear how the outbreak was spread, use both guidelines (Attachments 2 & 3).

Consider re-swabbing surfaces to check validity of clean up. (check with lab).

If applicable hand out 'procedures for cleaning up vomit and other body fluids'. (See Guidelines for the Control of Infectious Diseases 1997 (The Blue Book) page 225).

5.4.7 Interpretation of Laboratory Results and Data Analysis

Disease Control Victoria will assist in the collation, analysis and interpretation of the data collected in the questionnaires, along with other descriptive and microbiological data. Epi Info 6.04 is the computer software package used for analysis of data on disease outbreaks.

The results of epidemiological studies should not be considered in isolation. Combining epidemiological results with consideration of the results of laboratory testing, environmental investigations and HACCP/food safety plan evaluations is the best approach. Conclusions about a source of disease should also take into account:

• Whether symptoms experienced by patients were consistent with those commonly produced by the aetiological agent

• Whether the organism/toxin was isolated from the cases

• Whether the implicated vehicle (eg food, utensils, animal etc) had the same organism or toxin as found in specimens from the cases

• Information on methods of food processing, preparation and storage and whether these provided opportunities for contamination, survival and growth of the organism.

In practice, most investigations will not have complete data in all these areas. However, the investigation should aim to ascertain as much detail as possible.

Disease Control Victoria, the Region and local government will cooperatively interpret the results and develop any new control strategies which may be necessary or to confirm or modify precautionary measures that were initiated on the basis of preliminary hypotheses.

5.4.8 Monitoring

After control measures are put into place, the situation should be monitored to determine the effectiveness of those measures and the need for further intervention.

5.4.9 Debriefing

Within a reasonable time frame of the outbreak concluding, all parties involved in the outbreak should meet to debrief and determine cooperative outcomes or future actions. Debriefs tend to focus on the success or failure of human lines of communication developed during an incident, rather than on technical or scientific issues relating to the incident. This debrief will be coordinated by DHS.

5.4.10 Report

An outbreak investigation report can be used to justify public health actions at the time, as a useful debrief for improving handling of future outbreak investigations and has value as a teaching tool.

The Outbreak Report Form Summary in Attachment 14 should be completed by the organisation who has the lead role in the investigation, for each outbreak. If local government is required to complete the report, assistance can be provided by DHS. The completed report should be forwarded to the Regional EHO and Disease Control Victoria after the investigation is completed but no later than 4 weeks.

A more detailed report as outlined in Attachment 15 will be required to be completed for certain outbreaks (eg when large numbers of people are affected, or when significant health consequences result). Disease Control Victoria will determine when such a report needs to be completed and by whom.

5.4.11 The Media

The media may become aware of an outbreak officially through a Departmental media release or unofficially through other sources (eg the general public).

The Department of Human Services has a Media Unit which handles all Departmental media enquiries. If local government receive enquiries in relation to outbreaks, advice from the Department can be obtained to ensure that any information provided is accurate and consistent with any Departmental media releases.

Guidelines for the investigation of gastrointestinal illness index