The Principles of Outbreak Epidemiology

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The Principles of Outbreak Epidemiology

• By
• Dr Abhinav Sinha MBBS, MD

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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

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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.

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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.

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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.

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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.

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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

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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

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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

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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.

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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.

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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

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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

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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.

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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

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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

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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

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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?

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• 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.

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Composition of typical field team
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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).

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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.

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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.

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• 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).

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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.

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• (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.

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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.

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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.

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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.

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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.

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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.

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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).

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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.

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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.

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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.

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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.

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