Outbreak Investigation Basics

[starttext]Epidemic or outbreak is a significant rise of level of disease above the normal endemic level (expected level of disease in community). The steps in outbreak investigation are:

1. Prepare for fieldwork in advance: Literature review, decide role, team, equipments

2. Establish the existence of an outbreak:

  • Compare with previous weeks, months or years of same geographic area and time period
  • Excess may not be outbreak. It may be due to population change, media attention, improved screening, change in case definition, etc.

3. Verify the diagnosis, if possible

4. Define, identify and collect data on all cases

a. Case definition:

  • Must include personal information, place information, time information and clinical information
  • Types:
  • Possible (Suspected): Broadest definition
  • Probable
  • Confirmed (Definite): Tightest definition (A clinically compatible case that is not laboratory confirmed and is not epidemiologically linked to laboratory-confirmed cases)
  • Broad definition is needed in the beginning, so that cases aren't missed. It can be tightened later.

b. Identify all possible cases: surveillance data

c. Collect data on all possible cases: in relation to person, place, time and clinical features

5. Perform descriptive epidemiology

  • Person
  • Place (Spot map)
  • Time (Epidemic curve)

6. Develop a hypothesis

  • Include: What is known about likely pathogen, who is at risk, likely exposures, mode of transmission
  • Should be testable

7. Evaluate hypothesis

  • Comparative method: all cases having same symptoms, lab confirmation, etc.
  • Analytic method: attack rates, case fatality rates, relative risks, odds ratio, statistical significance testing (chi-square, p-value)

8. Refine hypothesis and execute additional studies if required

9. Immplement control and prevention measures

10. Communicate findings and prepare a report

A very good pictorial guide: http://dse.healthrepository.org/bitstream/123456789/80/1/Obi%20Guide.jpg

Some terms:

  • Index case: First person to come to attention to public health authorities
  • Primary case: Acquiring disease from an exposure
  • Secondary case: Acquiring disease from exposure to primary case

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