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Definition of an Outbreak An outbreak is commonly defined as:

The occurrence in a community or region of casesof an illness with a frequency clearly in excess of normal expectancy. The number of casesindicating 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 isrelative to the usual frequency of the disease in thesame area, among the same population, at thesame season of the year.1

Epidemiological InvestigationRegardless of the communicable disease, the procedurefor the investigation of a suspected outbreak is thesame:

1. Establish the existence of an outbreak

Compare current information with previousincidence in the community during the same timeof year to determine if the observed number of cases exceeds the expected. Compare availableinformation about new cases with a predetermined definition of an outbreak.

2. Confirm the diagnosis

Analyze clinical histories of cases and havestandard laboratory tests performed to confirm orreject the suspected diagnosis and to determinethe type of agent associated with the illness (e.g.,bacterial, viral, other).

3. Establish the case definition and count casesDevelop a case definition and establish methodsfor 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 selectedcases to determine common experiences, such as

when they became ill (time), where they becameinfected (place) and who they are (person).

5. Determine who is at risk of becoming ill

Count cases and relate these counts to theappropriate population to find the group(s) at

risk. Contact those who can provide informationon the illness or about the environmentalcircumstances that contributed to the outbreak.

6. Formulate a tentative hypothesis

Formulate a tentative hypothesis to explain themost likely cause, source and distribution of cases.

7. Compare the hypothesis with the established facts

The hypothesis will direct the course of theinvestigation, and will be tested by the various

data gathered during the investigation. Severalsuccessive hypotheses may be required.

8. Plan a detailed epidemiologic investigation

Determine from the collected data what otherinformation is needed and what resources areavailable to test the hypothesis. Develop or obtaininterview forms, gather sampling equipment, andalert people who will take part in theinvestigation.

Carry out the investigation. Interview people whoare similar or who had similar experiences to theill with respect to time or place but who are not ill(controls); gather appropriate community andenvironmental information; investigate potentialsources of the responsible agent and factors thatcontribute to the outbreak; and obtain specimensand samples.

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C O M M U N I C A B L E D I S E A S E M A N A G E M E N T P R O T O C O L M A N U A L

Epidemiological Investigationof Outbreaks

ManitobaHealthPublic Health

Communicable Disease Control Unit


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Summarize and interpret all of the informationthat has been collected and examine the results of laboratory tests that have been conducted.Construct epidemic curves, calculate rates,develop appropriate tables and charts, apply statistical tests to the data, and interpret the data.

On the basis of the available data and theappropriate calculations, accept or reject thehypothesis. For a hypothesis to be accepted, thepatterns of disease in the host must relate to the

nature of the agent, its source, its mode of transmission and the contributory factors thatallowed the outbreak to occur. If the hypothesis isrejected, a second hypothesis must be developedand additional information must be gathered.

9. Prepare a written report

A written report should summarize theinvestigation and be prepared as soon as theinvestigation is completed. This report shouldinclude the various information listed in theexample referenced (refer to Report of an

Investigation of an Outbreak).10. Implement control and prevention measures

Effective control measures should be developedusing the evidence that has been uncovered. Usethe information that has been collected during theinvestigation to control the current outbreak andalso to prevent future problems in the community.Initiate or intensify surveillance of the disease andagent. If imminent danger exists, control measuresshould be initiated after a tentative hypothesis hasbeen formed.

References:1. Chin, James (Editor). Control of Communicable

Disease Manual. American Public Health Association, Washington DC, 2000.

2. Greg, Michael B. (Editor).Field Epidemiology .Oxford University Press, New York, 1996.

Practical Considerations Each Regional Health Authority (RHA) or other

health care jurisdiction should carry out routinesurveillance to identify outbreaks early.

Each RHA or other health care jurisdiction shouldhave a plan for the investigation of outbreaks, basedon the above model. The key public health officialsshould be clearly identified. In Regional Health

Authorities, this will include, at a minimum, theMedical Officer of Health or designate and thecommunicable disease control contact person.Many investigations, including all suspected food-and water-borne outbreaks will require the servicesof the public health inspector.

Each RHA or health care jurisdiction shouldestablish direct contact with Cadham ProvincialLaboratory (CPL) to ensure that the appropriatespecimens are obtained, processed and resultsreported using an assigned outbreak code, asquickly as possible.

At minimum, a line-listing of cases and contacts

with other relevant data should be maintainedelectronically. The computer software, EPI INFO,is available in most Regional Health Authoritiesand is ideal for collecting and analyzing data fromoutbreak investigations.

It is a requirement that outbreaks be reported tothe Director of Comminicable Disease Controlusing the outbreak reporting form (seeFormssection of this manual).

If media interest is likely, the RHA or other healthcare jurisdiction should establish one contactperson as quickly as possible. Ideally, the MedicalOfficer of Health will be the contact if the outbreak is occurring within the community. Assistance isavailable from the communications staff atManitoba Health.

Refer to the Protocol to Investigate Food BorneIllness and Suspected Food Borne Illness if

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food/water appears to be the source of an outbreak.This manual includes a section on investigation of outbreaks, with samples of forms which may beused to collect and summarize information.

The RHA or other health care jurisdiction may request assistance from Manitoba Health with theinvestigation of an outbreak. Where required, theManitoba Provincial Outbreak Response Plan(ORP) will be implemented (refer to Manitoba Provinvial Outbreak Response Plan (ORP) in the

intro section of this manual).Management of Outbreaks in Institutions All of the above considerations apply to the

investigation of outbreaks in health-care orresidential-care facilities. However, there are severaladditional items for consideration.

Each facility should have a plan in place forroutine surveillance and for outbreak investigation. The Regional Health Authority should be aware of this plan andbe available to assist where required. In most

facilities, the infection control practitioner isresponsible for surveillance, and initiationand execution of outbreak investigations.

The successful control of outbreaks withininstitutions requires a team approach by thefacility staff and Regional Health Authority (usually the Medical Officer of Health ordelegate).

The Provincial Infection Control Nurse willprovide training to health-care facilities andassist them in developing infection controlpolicies and procedures. An integral part of the team, the Provincial Infection ControlNurse should be informed of and involvedin the outbreak investigation.

It may be difficult to recognize an outbreak in a health-care facility. Diarrhea, forexample, is not uncommon in residents of

personal care homes. The Regional Health Authority or other health-care jurisdictionshould establish circumstances under whichthey are to be notified. Circumstancesshould include diseases which have thepotential for involvement beyond the facility,and those where public health interventionor expertise may be required (e.g., food-borne illness, influenza).

It is very important that food-borneoutbreaks in institutions be recognizedimmediately since the potential for spread is

great in these facilities. In these instances,early consultation with a publichealth inspector, Provincial InfectionControl Nurse and staff within the RHA orother health-care jurisdiction responsible forcommunicable diseases is strongly recommended.

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ExampleReport of an Investigation of an Outbreak Reason for Investigation Include a statement describing how the outbreak

was identified (i.e., unusual number of cases).

Investigation Present a general summary characterizing the

outbreak by person, place and time.

Include the date the investigation began andindividuals interviewed. State the number of cases ill and number of

individuals at risk. If the number of cases is small (e.g., 25 or

less), a table may be constructed such thatrelevant data (e.g., age, gender, date of onset)are presented for each individual.

If the number of cases is large, the data may be summarized and presented in tabular orgraphical format (e.g., age distribution,

epidemic curve). Compare the observed number of cases with

that expected for the same populationduring the same time period.

Present a clinical description of cases Describe the symptoms and physical

findings, including the results of laboratory examinations.

If a table of individual cases is included,incorporate the more important clinical andlaboratory findings in the table; otherwise,summarize the findings numerically.

Present a summary of the epidemiologic data and findings.

State pertinent information describing thedistribution of cases which would indicatethe source (e.g., outbreak confined tocustomers of a particular restaurant).

If an outbreak is localized to a group, such asan institution or in connection with a dinner, provide the total population at risk (total population potentially exposed).

Present in tabular form any associatedindividual characteristics (e.g., age, history of

previous attack, immunization) with thecases. Where possible, compare these casecharacteristics with unaffected individuals.

Present in tabular form, the association of cases with various environmental factorssuch as water, milk supply, foods, etc.Compare these cases with the generalpopulation when possible.

Summarize results of an investigation of commonfood and drink supplies

If the outbreak investigation required data

from the water or milk supply, summarizethe findings of the engineer, public healthinspector or veterinarian.

Summarize the results of bacteriologicexamination of any other food suspected asthe source.

Provide details of preparation, handling andstorage of suspected foods, brands of thesefoods, and places from which they werepurchased.

State information as to illnesses or infections

among those handling the suspected food.Discussion as to Source State reasons for suspecting or excluding milk,

water, and each article of food or other possiblemedium of infection.


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