OUTBREAK OUTBREAK INVESTIGATIONINVESTIGATION

Dr.Kedar KarkiDr.Kedar Karki

DefinitionDefinition• Occurrence of more cases of disease

than expected• Nosocomial outbreak-any group of

illnesses of common etiology occurring in patients of a medical care facility acquired by exposure of those patients to the disease agent while confined in such a facility.

Reasons to investigateReasons to investigate• Control/prevention• Research opportunities• Training• Public, political, or legal concerns• Program considerations

Control/preventionControl/prevention• Where are we in the outbreak?

– Goals will be different depending on answer(s)• Cases continuing to occur

– Goal: prevent further cases– Assess population at risk, implement control

measures• Outbreak appears to be coming to an end

– Goal: prevent future outbreaks– Identify factors contributing to outbreak,

implement measures to prevent similar events in the future

Research opportunitiesResearch opportunities• Gain additional knowledge• Opportunity to study natural history of the

disease• Newly recognized disease

– Define natural history– Characterize the population at risk

• Well recognized diseases– Assess impact of control measures– Usefulness of new epidemiology and

laboratory techniques

TrainingTraining

• Requirements of an epidemiologist!– Diplomacy– Logical thinking– Problem solving ability and

quantitative skills– Epidemiologic know-how– Judgment

• Pair a new epidemiologist with an experienced epidemiologist

Public, political, or legal Public, political, or legal concernsconcerns

• Sometimes override scientific concerns

• Health department needs to be responsible and responsive to public concerns– Even if the concern has little

scientific basis

Program considerationsProgram considerations• Outbreak of disease targeted by a

public health program– Reveal a weakness and opportunity

to change or strengthen the program• Identify population that may have been

overlooked• Failure of intervention strategy• Changes in the agent

• Events beyond scope of the program

Steps of an Outbreak Steps of an Outbreak InvestigationInvestigation

1. Preparation1. Preparation• Investigation

– Scientific knowledge• Review literature• Consult experts• Sample questionnaires

– Supplies• Consult with laboratory

– Equipment• Laptop, camera etc.

Preparation cont.Preparation cont.• Administration-assure personnel

resources, funding– Travel arrangements (orders)– Approval– Personal matters

• Consultation-make sure you know your role and its parameters– Lead investigator or just lending a hand?– Know who to contact when you arrive

2. Establish existence of an 2. Establish existence of an outbreakoutbreak

• Is an outbreak truly occurring?– True outbreak– Sporadic and unrelated cases of same

disease– Unrelated cases of similar unrelated

disease• Determine the expected number of cases

before deciding whether the observed number exceeds the expected number

Establish existence of an outbreak Establish existence of an outbreak cont.cont.

• Comparing observed with expected– through surveillance records for

notifiable diseases– hospital discharge data, registries,

mortality statistics– data from other facilities, states,

surveys of health care providers– community survey

3. Verify the Diagnosis3. Verify the Diagnosis

• Ensure proper diagnosis and rule out lab error as the bias for increased diagnosis– Review clinical findings, lab results

• Summarize clinical findings with frequency distributions– Characterize spectrum of disease– Verify diagnosis– Develop case definition

Verify the Diagnosis cont.Verify the Diagnosis cont.• See and talk with patients if at all possible

– Better understand clinical features– Mental image of disease and the

patients affected– Gather critical information

• Source of exposure• What they think caused illness• Knowledge of others with similar illness• Common denominators• Helpful in generating ideas for hypothesis about

etiology and spread

4a. Establish a case 4a. Establish a case definitiondefinition

• Case definition– Standard set of criteria for deciding

whether an individual should be classified as having the health condition of interest

– Includes clinical criteria and restrictions by time, place and person

– Must be applied consistently and without bias to all persons under investigation

– Must not contain an exposure of risk factor you want to test

4a. Establish a case definition 4a. Establish a case definition cont.cont.

• Classification– Definite (confirmed)

•Laboratory confirmed

– Probable•Typical clinical features without lab

confirmation

– Possible (suspected)•Fewer of the typical clinical features

4a. Establish a case definition 4a. Establish a case definition cont.cont.

• Early in investigation may use a “loose” case definition – Better to collect more than necessary so

you don’t need to make repeat visits– Identify extent of problem and

population affected– Generating hypotheses

• Later when hypotheses are sharpened investigator may “tighten” case definition

4b. Identify and count 4b. Identify and count casescases

• Target health care facilities where diagnosis likely to be made– Enhanced passive surveillance e.g. letter

describing situation and asking for reports– Active surveillance e.g. phone or visit facility

to collect information

• Alerting the public– Media alert to avoid contaminated food

product and seek medical attention if symptoms arise

4b. Identify and count cases cont.4b. Identify and count cases cont.

• OB population restricted and large proportion of cases are unlikely to be diagnosed e.g. on a cruise ship– Survey entire population

• Always ask case-patients if they know of any others ill with the same symptoms

4b. Identify and count cases cont.4b. Identify and count cases cont.

• Information to be collected about every case– Identifying information

• Re-contact if additional questions come up• Notification of lab results and outcomes of

investigation• Check for duplicate records• Map geographic extent

– Demographics• Provide “person” characteristics for defining

population at risk

4b. Identify and count cases cont.4b. Identify and count cases cont.• Information to be collected about every

case cont.– Clinical findings

• Verify case definition met• Chart time course• Supplemental date e.g. deaths

– Risk factor information• Tailored to specific disease in question

– Reporter information• Id of person making report

4b. Identify and count cases cont.4b. Identify and count cases cont.

• Collection forms– Standard case report form– Questionnaire– Data abstraction form

• Line listing– Abstraction of selected critical

items from above forms– Contains key information

5. Perform Descriptive 5. Perform Descriptive EpidemiologyEpidemiology

• After collection of data characterize the outbreak by:–Time–Place –Person

TimeTime• Epidemic curve

– Histogram of the number of cases by their date of onset

– Visual display of the outbreak’s magnitude and time trend

– Where you are in the time course of the outbreak

– Future course?– Probable time period of exposure– Helps in development of questionnaire focusing

on that time period– Common source vs. Propagated

PlacePlace• Geographic extent of problem

• Clusters or patterns providing important etiologic clues

• Spot maps – Where cases live, work or may

have been exposed

PersonPerson• Determine what population at risk

• Usually define population by host characteristics or exposure

• Use rates to identify high-risk groups– Numerator = number of case– Denominator = number of people at risk

Develop HypothesesDevelop Hypotheses

• Hypotheses should address–Source of the agent

–Mode of transmission•Vector or vehicle

–Exposure that caused disease

Develop HypothesesDevelop Hypotheses• Generating the hypothesis

– What do you know about the disease?• Reservoir, transmission, common vehicles and

known risk factors

– Talk to several case-patients• Use open ended questions• Ask lots of questions

– Talk to local health department staff– Use descriptive epidemiology e.g. epi

curve

7. Test Hypotheses7. Test Hypotheses• Evaluate the credibility of your

hypotheses– Compare with established facts

• When clinical, lab, environmental and/or epi data undoubtedly support hypothesis

– Use analytic epidemiology to quantify relationships and explore the role of chance• Cohort studies• Case control studies

7. Test Hypotheses cont.7. Test Hypotheses cont.• Cohort

– Small, well defined population– Contact each attendee and ask a series of

questions– Ill Vs not ill – Look for source exposure

• Attack rate is high among those exposed• Attack rate is low among those not exposed• Most of the cases were exposed, so that the exposure

could explain most, if not all, of the cases• Relative risk = measure of association between

exposure and disease

7. Test Hypotheses cont.7. Test Hypotheses cont.

• Case-control – Population not well defined– Case patients and comparison

group (controls) questioned about exposure(s)

– Compute measure of association = Odds Ratio• Quantify relationship between exposure and

disease

8. Refine hypotheses and 8. Refine hypotheses and do additional studiesdo additional studies

• Epidemiologic– When analytical epi unrevealing need to

reconsider your hypotheses– Go back and gather more information– Conduct different studies

• Laboratory– Additional tests

• Environmental studies

9. Implement Control 9. Implement Control /Prevention Measures/Prevention Measures

• Implement control measures as soon as possible

• May be aimed at agent, source, or reservoir

• Short or long term

10. Communicate the 10. Communicate the FindingsFindings

• Orally within facility/community– Local health authorities and persons

responsible for implementation of control and prevention measures

• Written reports (consider publication) for planning, record of performance, legal issues, reference, adding to knowledge base