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The Principles of Outbreak Epidemiology

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Title: The Principles of Outbreak Epidemiology


1
The Principles of Outbreak Epidemiology
  • By
  • Dr Abhinav Sinha MBBS, MD

2
Author
  • Currently doing MD in Community Medicine at the
    Department of Community Medicine, NSCB Medical
    College, Jabalpur, India.
  • Passed MBBS from the same institute with Gold
    Medals in Human Physiology and Biochemistry in
    2000.
  • Email- aspsm2000_at_yahoo.co.in

3
Interest Motivation
  • During the last two and a half years of my
    postgraduation, I had an opportunity to witness
    and investigate 3 or 4 of different kinds of
    outbreaks ranging from Food poisoning to
    Gastrointestinal to Pyrexia of Unknown Origin to
    Cholera. What aroused an interest in Outbreak
    Investigation was the scientific cum artistic way
    of handling such outbreaks. It is more of an art
    than a pure science. When I came to know about
    the Supercourse, I decided to contribute or
    rather share my little experience with the world
    in the field of Outbreak Epidemiology. I hope you
    will enjoy this so called lecture.

4
Learning Objectives
  • The main motives behind this lecture are to
    develop the basic concept in investigating an
    epidemic, the need to recognize the urgency
    behind it and to orient the students toward the
    art and science of outbreak investigation and
    epidemiology.

5
Performance Objectives
  • After going through this lecture, the students
    should be able to perform the initial
    investigation of many smaller outbreaks that
    occur so much frequently in their vicinity that
    may often pass unrecognized. Also, they should be
    able to differentiate the epidemic and the
    endemic fluctuations in the frequencies of a
    disease.

6
Definitions
  • Outbreak Sudden occurrence of an epidemic in
    relatively limited geographic area. While an
    outbreak is usually limited to a small focal
    area, an epidemic covers larger geographical
    areas has more than one focal point.
  • Outbreak Epidemiology Study of a disease cluster
    or epidemic in order to control or prevent
    further spread of the disease in the population.

7
Field Epidemiology
  • A definition has been proposed by
  • Goodman. The essential elements are
  • The problem is unexpected
  • An immediate response may be necessary
  • Epidemiologists must travel to work on location
    in the field
  • The extent of investigation is likely to be
    limited because of imperative for timely
    intervention

8
Objectives of OI
  • Primary- to control the spread of disease
  • To determine the causes of disease, its source
    mode of transmission
  • To determine who is at risk
  • To determine what exposures predispose to disease
  • To know magnitude of the problem

9
Objectives continued.
  • 6. To identify new agent
  • 7. To determine the effectiveness of control
    measures
  • 8. To identify methods for present future
    prevention control
  • 9. Research training opportunities
  • 10. Public, Political and legal concerns

10
Unique aspects of OI
  • There is a pressure urgency to conclude the
    investigations quickly which may lead to hasty
    decisions.
  • Data sources are often incomplete less
    accurate.
  • Decreased statistical power due to analysis of
    small numbers.
  • Publicity surrounding the investigation
    community members may have preconceived ideas.

11
The pace commitment of OI
  • There is often a strong tendency to collect what
    is essential in the field then retreat to
    home for analysis. Such premature departure
    reflects lack of concern by the public, makes any
    further data collection or direct contact with
    the study population difficult. Once home, the
    team loses the urgency momentum the sense of
    relevancy of the epidemic. Dont leave the field
    without final results recommendations.

12
Trigger events Warning Signals
  • Clustering of cases/deaths in time/space
  • Unusual increase in cases/deaths
  • Shift in age distribution of cases
  • High vector density
  • Acute hemorrhagic fever or acute fever with renal
    involvement/altered sensorium
  • Severe dehydration following diarrhea in patients
    above 5 years age
  • Unusual isolate

13
Diseases requiring investigations
  • Endemic diseases with epidemic potential
    malaria, cholera, measles, hepatitis,
    meningococcal meningitis
  • Even a single case of diseases for which
    eradication/elimination goals have been set
    polio, guineaworm and yaws
  • Rare but internationally important diseases with
    high case fatality rates yellow fever
  • Outbreaks of unknown etiology

14
General lines of action
  • The basic general lines of action during
    epidemics include Preparedness and
    Interventions (investigations). Success in
    dealing with an epidemic depends largely on the
    state of preparedness achieved in advance of any
    action. It would be an error to consider as an
    epidemic, a hitherto unrecognized endemic
    situation or a mere seasonal increase in the
    incidence of a disease.

15
Preparedness
  • Identify a nodal officer at state/district level
  • Strengthen routine surveillance system
  • Constitute rapid response teams
  • Train medical other health personnel
  • Prepare a list of laboratories
  • List the high risk pockets
  • Establish rapid communication network
  • Undertake IEC activities
  • Ensure availability of essential supplies
  • Setup inter-departmental committees

16
Investigations
  • Recognition response to a request for
    assistance
  • Check initial information
  • Formulate a plan of action
  • Prepare for field work
  • Confirm the existence of epidemic
  • Verify the diagnosis
  • Identify count cases/exposed persons
  • Orient data in terms of person, place time

17
Investigations continued..
  • Choose a study design
  • Collect specimens for lab analysis
  • Conduct environmental investigations
  • Formulate test hypotheses
  • Implement control measures
  • Conduct additional studies
  • Prepare a written report
  • Communicate the findings

18
Step 1 Recognition Response
  • If the local health officials request assistance,
    the regional epidemiologist should try to acquire
    as much information about the disease and the
    population at risk as possible. It is also
    important to find out why the request is coming
    need extra hands?, unable to uncover the details
    of the disease in question?, share the
    responsibility?, or legal or ethical issues?

19
  • Step 2 Check initial information As soon as
    the initial information on an outbreak reaches,
    the regional health coordinator must determine
    whether the information is correct.
  • Step 3 Formulate a plan of action The plan
    should be based on situational analysis taking
    technical, economical political factors into
    account.
  • Step 4 Prepare for field work Identify the
    team members assign responsibilities.

20
Composition of typical field team
21
Step 5 Confirm the existence
  • Are there cases in excess of the baseline rate
    for that disease setting? The excess frequency
    should be found out with Epidemic Threshold
    Curve. The periodic frequency for previous 3
    years is plotted on a graph. Another graph at
    mean 2SD level is superimposed on it. Any
    fluctuations beyond these 2 graphs should be
    treated as epidemic fluctuations (method of
    moving averages).

22
Step 6 Verify the diagnosis
  • The initial report may be spurious arise from
    misinterpretation of the clinical features. This
    involves a review of available clinical lab
    findings that supports the diagnosis. Do not
    apply newly introduced, experimental or otherwise
    not broadly recognized confirmatory tests at this
    stage. 15-20 of the suspected cases may be lab
    confirmed.

23
Step 7 - Identify count cases
  • Identify additional cases not known or reported
    initially. The case definition must be precise
    but not too exclusive.. Persons who meet the case
    definition should be line-listed. Also,
    identify the population at risk or the exposed
    persons, places where the cases live, work have
    traveled to, the possible exposures that might
    have lead to the disease.

24
  • Search for the source of infection The main
    purpose here is to eliminate, terminate or
    isolate the source. The steps involved are
    identify the time of disease onset, ascertain the
    range of incubation periods look for the source
    in time interval between the maximum the
    minimum IPs. In outbreaks with person-to-person
    transmission, all the contacts of the index case
    are to be searched (contact tracing).

25
Step 8 Compile Orient data
  • Identify when patients became ill (time), where
    patients became ill (place) what
    characteristics the patients possess (person).
    The earlier one can develop such ideas, the more
    pertinent accurate data one can collect.
  • (a) Time The epi-curve gives the magnitude of
    outbreak, its mode of spread the possible
    duration of the epidemic. The unit of time on
    X-axis are smaller than the expected incubation
    period of the disease.

26
  • (b) Place It provides major clues regarding the
    source of agent and/or nature of exposure. Spot
    maps show a pattern of distribution of cases.
  • (c) Person Examine characters such as age, sex,
    race, occupation or virtually any other character
    that may be useful in portraying the uniqueness
    of case population.

27
Step 9 Choose a Study Design
  • The design (Case-control, Cohort Case-cohort)
    is chosen based on size availability of the
    exposed population, the speed with which the
    results are needed the available resources. The
    study design that is chosen will then dictate the
    appropriate analysis hypothesis testing.

28
Step 10 Perform Lab analysis
  • It consists of collecting testing appropriate
    specimens. To identify the etiologic agent, the
    collection need to be properly timed. Examples of
    specimens include - food water, other
    environmental samples (air settling plates), and
    clinical (blood, stool, sputum or wound) samples
    from cases controls.

29
Step 11 Environmental Investigation
  • A study of environmental conditions the
    dynamics of its interaction with the population
    etiologic agents will help to formulate the
    hypothesis on the genesis of the epidemic. Such
    actions assist in answering How? And Why?
    questions.

30
Step 12 Formulate Test Hypothesis
  • As soon as the preliminary data indicate the
    magnitude severity of the outbreak, a
    hypothesis should be made regarding time, place
    and person the suspected etiological agent the
    mode of transmission. Risk specific attack rates
    are calculated compared relative risk/odds
    ratio is calculated.

31
Important points
  • 1. Rare disease assumption The OR RR
    approximate each other if the attack rates is
    less than 5 but the attack rates are much higher
    in outbreaks.
  • 2. To correct for multiple comparisons, the most
    effective approach is to lower the p-value
    according to the number of comparisons made.

32
Step 13 Control measures
  • Simultaneous to data collection hypothesis
    formation, steps should be taken to contain the
    epidemic. These measures depend upon knowledge of
    etiologic agent, mode of transmission other
    contributing factors. Protective measures are
    necessary for patients (isolation
    disinfection), their contacts (quarantine) and
    the community (immunization, etc).

33
Step 14 Additional studies
  • Because there may be a need to find more
    patients, to define better the extent of the
    epidemic, or because a new lab method or case
    finding method may need to be evaluated, the
    epidemiologists may want to perform more detailed
    carefully executed studies.

34
Step 15 Prepare Written Report
  • The final responsibility of the investigative
    team is to prepare a written report to document
    the investigations, findings and the
    recommendations. The written report should be
    submitted, in a standardized format, to the
    public health authorities including the ministry
    of health remain confidential until it has been
    given official permission.

35
Step 16 Communicate findings
  • Communicating the investigative findings clearly
    is essential. All public health officers will
    benefit if the experience acquired by the
    investigative team is shared by the publication
    of an account of the outbreak. As a rule, the
    epidemiologist informs those who reported the
    first cases of the epidemic first.

36
Step 17 Post-epidemic Measures
  • The efficacy of control measures should be
    assessed day by day during the outbreak, a final
    assessment being made after it has ended. This
    will provide a logical basis for post-epidemic
    surveillance preventive measures aimed at
    avoiding the repetition of similar outbreaks.

37
Bibliography
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  • Bres P. Public Health Action in Emergencies
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    WHO. 1986.
  • Diane M Dwyer and Carmela Groves. Outbreak
    Epidemiology. Infectious Disease Epidemiology.
    119-147.

38
  • Epidemiologic Surveillance Outbreak
    Investigation. Textbook of Epidemiology and
    Biostatistics in Preventive Medicine. W B
    Saunders Publication. 43-53.
  • Internet website www.cdc.gov/excite/classroom/out
    break.html
  • Johan Giesecke. Detection and Analyses of
    Outbreaks. Modern Infectious Disease Epidemilogy.
    Edition 1. Arnold Publishers. Co-published by
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39
  • Kulkarni A P Baride J P. Investigation of an
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  • Michael B Gregg. The principles of an epidemic
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40
  • Outbreak investigation and control. Training
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41
  • Reingold Arthur L. Outbreak Investigations a
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