OUTBREAK INVESTIGATION

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OUTBREAK INVESTIGATION
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Definition

• 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.

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Reasons to investigate

• Control/prevention
• Research opportunities
• Training
• Public, political, or legal concerns
• Program considerations

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Control/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

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Research 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

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Training

• 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

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Public, political, or legal concerns

• Sometimes override scientific concerns
• Health department needs to be responsible and
  responsive to public concerns
• Even if the concern has little scientific basis

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Program 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

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Steps of an Outbreak Investigation
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1. Preparation

• Investigation
• Scientific knowledge
• Review literature
• Consult experts
• Sample questionnaires
• Supplies
• Consult with laboratory
• Equipment
• Laptop, camera etc.

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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

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2. Establish existence of an outbreak

• 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

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Establish existence of an outbreak 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

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3. 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

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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

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4a. Establish a case definition

• 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

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4a. Establish a case definition cont.

• Classification
• Definite (confirmed)
• Laboratory confirmed
• Probable
• Typical clinical features without lab
  confirmation
• Possible (suspected)
• Fewer of the typical clinical features

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4a. Establish a case definition cont.

• Early in investigation may use a loose case
  definition
• Better to collect more than necessary so you
  dont need to make repeat visits
• Identify extent of problem and population
  affected
• Generating hypotheses
• Later when hypotheses are sharpened investigator
  may tighten case definition

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4b. Identify and count cases

• 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

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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

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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

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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

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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

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5. Perform Descriptive Epidemiology

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

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Time

• Epidemic curve
• Histogram of the number of cases by their date of
  onset
• Visual display of the outbreaks 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

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Place

• Geographic extent of problem
• Clusters or patterns providing important
  etiologic clues
• Spot maps
• Where cases live, work or may have been exposed

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Person

• 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

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Develop Hypotheses

• Hypotheses should address
• Source of the agent
• Mode of transmission
• Vector or vehicle
• Exposure that caused disease

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Develop 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

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7. 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

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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

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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

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8. Refine hypotheses and do 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

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9. Implement Control /Prevention Measures

• Implement control measures as soon as possible
• May be aimed at agent, source, or reservoir
• Short or long term

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10. Communicate the Findings

• 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