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Epidemiological Investigation of Outbreaks

An outbreak is commonly defined as: The occurrence in a community or region of cases of an illness with a frequency clearly in excess of normal expectancy. The number of cases indicating presence of an outbreak will vary according to the infectious agent, size and type of population exposed, previous experience or lack of exposure to the disease, and time and place of occurrence. Therefore, the status of an outbreak is relative to the usual frequency of the disease in the same area, among the same population, at the same season of the year
Establish the existence of an outbreak

Epidemiological Investigation

Regardless of the communicable disease, the procedure for the investigation of a suspected outbreak is the same:

1. Establish the existence of an outbreak
Compare current information with previous incidence in the community during the same time of year to determine if the observed number of cases exceeds the expected. Compare available information about new cases with a predetermined definition of an outbreak.

2. Confirm the diagnosis
Analyze clinical histories of cases and have standard laboratory tests performed to confirm or reject the suspected diagnosis and to determine the type of agent associated with the illness (e.g., bacterial, viral, other).

3. Establish the case definition and count cases
Develop a case definition and establish methods for identifying and counting cases.

4. Relate the outbreak to time, place and person
Characterize the outbreak according to person, place or time by interviewing known or selected cases to determine common experiences, such as when they became ill (time), where they became infected (place) and who they are (person).

5. Determine who is at risk of becoming ill
Count cases and relate these counts to the appropriate population to find the group(s) at risk. Contact those who can provide information on the illness or about the environmental circumstances that contributed to the outbreak.

6. Formulate a tentative hypothesis
Formulate a tentative hypothesis to explain the most likely cause, source and distribution of cases.

7. Compare the hypothesis with the established facts
The hypothesis will direct the course of the investigation, and will be tested by the various data gathered during the investigation. Several successive hypotheses may be required.

8. Plan a detailed epidemiologic investigation
Determine from the collected data what other information is needed and what resources are available to test the hypothesis. Develop or obtain interview forms, gather sampling equipment, and alert people who will take part in the investigation.

Carry out the investigation. Interview people who are similar or who had similar experiences to the ill with respect to time or place but who are not ill (controls); gather appropriate community and environmental information; investigate potential sources of the responsible agent and factors that contribute to the outbreak; and obtain specimens and samples.

9. Prepare a written report
A written report should summarize the investigation and be prepared as soon as the investigation is completed. This report should include the various information listed in the example referenced (refer to “Report of an Investigation of an Outbreak”).

10. Implement control and prevention measures
Effective control measures should be developed using the evidence that has been uncovered. 

Use the information that has been collected during the investigation to control the current outbreak and also to prevent future problems in the community. Initiate or intensify surveillance of the disease and agent. If imminent danger exists, control measures should be initiated after a tentative hypothesis has been formed.

Management of Outbreaks in Institutions

All of the above considerations apply to the investigation of outbreaks in health-care or residential-care facilities. However, there are several additional items for consideration.

• Each facility should have a plan in place for routine surveillance and for outbreak investigation. The Regional Health Authority should be aware of this plan and be available to assist where required. In most facilities, the infection control practitioner is responsible for surveillance, and initiation and execution of outbreak investigations.

• The successful control of outbreaks within institutions requires a team approach by the facility staff and Regional Health Authority (usually the Medical Officer of Health or delegate).

• The Provincial Infection Control Nurse will provide training to health-care facilities and assist them in developing infection control policies and procedures. An integral part of the team, the Provincial Infection Control Nurse should be informed of and involved in the outbreak investigation.

• It may be difficult to recognize an outbreak in a health-care facility. Diarrhea, for example, is not uncommon in residents of personal care homes. The Regional Health Authority or other health-care jurisdiction should establish circumstances under which they are to be notified. Circumstances should include diseases which have the potential for involvement beyond the facility, and those where public health intervention or expertise may be required (e.g., foodborne illness, influenza).

• It is very important that food-borne outbreaks in institutions be recognized immediately since the potential for spread is great in these facilities. In these instances, early consultation with a public health inspector, Provincial Infection Control Nurse and staff within the RHA or other health-care jurisdiction responsible for communicable diseases is strongly recommended.


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