How to go about the investigation of an outbreak of communicable disease to take prevention and control measures

1. OUTBREAK
INVESTIGATION
DR MALLAPPA O MBBS, MD-CM, PGDHFWM
DISTRICT SURVEILLANCE UNIT
SHIVAMOGGA

2. INTRODUCTION
• A set of procedures to identify cause, people affected, circumstances and mode
of spread and other relevant factors involved in the epidemic and to take action
• An outbreak/epidemic is the occurrence of cases in excess of what would
normally be expected in a defined community, geographical area or season
• An outbreak/epidemic may occur in a restricted geographical area, or may extend
over, several neighboring areas, whole country or several countries. It may last
for a few days or weeks, or for several years
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3. TERMINOLOGY
• Epidemic: occurrence of more than expected cases in a given area or a population group over a particular period of
time. Usually, the cases are presumed to have a common cause or to be related to one another in some way.
Epidemic is usually applied to situations involving larger numbers of people over a wide geographic area
• Outbreak: Epidemic limited to localized increase in the incidence of disease (such as village, town, or closed
institution)
• Cluster: is an aggregation of cases in a given area over a particular period without regard to whether the number of
cases is more than expected
• A single case of a communicable disease long absent from a population, or caused by an agent not previously
recognized in that community or area, or the emergence of a previously unknown disease, may also constitute an
outbreak and should be investigated
• Outbreaks may be detected while analyzing surveillance data, reporting by health facility/ medical practioners,
affected persons, local leaders, or by keeping rumour registry or through media
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4. DECIDING WHETHER TO INVESTIGATE A POSSIBLE
OUTBREAK
• depends on a variety of factors related to the health problem,
• Number affected is large, the disease is severe (with high risk of hospitalization,
complications, or death) and when the outbreak has the potential to affect others
unless prompt control measures are taken
• Occurrence of a new or rare disease or a change in the pattern of disease in an
area
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5. STEPS OF AN OUTBREAK INVESTIGATION
1.Prepare for field work
2.Establish the existence of an outbreak
3.Verify the diagnosis
4.Define and identify cases
5.Perform descriptive epidemiology
6.Develop hypotheses
7.Evaluate hypotheses/ perform additional
studies as necessary
8.Implement control and prevention measures
9.Communicate findings
10.Follow up recommendations
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6. PREPARE FOR FIELD WORK
• Scientific and investigative issues
• Appropriate scientific knowledge about the disease
• Plan of action, necessary supplies, the proper lab protocol
• Management and operational issues
• Most investigations are conducted by a team rather than just one individual
• Team members should know their expected roles and responsibilities
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7. ESTABLISH THE EXISTENCE OF AN OUTBREAK
• Epidemic is more cases than expected in a given area or among a specific group
of people over a particular period of time
• The expected number is usually the number from the previous few weeks or
months, or previous few years and may be based on outpatient records, IHIP,
community survey…
• The excess may not necessarily indicate an outbreak – Seasonal Variations,
notification variations (change in the notification procedure, case definition,
increased interest due to public awareness), sudden increase in population size
(resort areas, college towns), diagnostic variations, misdiagnosis
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8. VERIFY THE DIAGNOSIS/ ENSURE PROPER
DIAGNOSIS OF REPORTED CASES
• This is important to ensure that the disease has been properly identified, since
control measures are often disease-specific; and to rule out laboratory error
• Visit one or more patients with the disease and collect appropriate specimens
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9. DEFINE AND IDENTIFY CASES
• Construct a case definition - a standard set of criteria for deciding whether an
individual should be classified as having the health condition of interest
• A case definition includes clinical criteria (signs and symptoms) and restrictions
by time, place, and person. The criteria must be applied consistently to all
persons under investigation.
• different categories of a case definition, such as confirmed, probable, and suspect
cases.
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10. PERFORM DESCRIPTIVE EPIDEMIOLOGY (FIND CASES
SYSTEMATICALLY AND RECORD INFORMATION)
• Cases that report may be small in number and may not represent the total cases, so a survey must be
conducted to look for additional cases to determine the true geographic extent and the populations
affected.
• Describe key characteristics of those persons in terms of time, place, and person.
• All cases must be line listed with demographic info, clinical features, and risk factors
• With information on date of onset or hours of onset construct an epidemic curve to depict the time
course of an epidemic
• Epidemic curve – magnitude, mode of spread, stage of epidemic, can be used for evaluation (time
taken to identify, whether intervention measures working)
• Assessment by place provides information on geographic extent, source of exposure, etiologic clues
etc
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12. DEVELOP HYPOTHESES
• Generate hypotheses considering the following
o what is known about the disease itself
o What is the agent's usual reservoir
o How is it usually transmitted?
o What vehicles are commonly implicated?
o What are the known risk factors?
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13. EVALUATE HYPOTHESES
• Hypotheses are evaluated using a combination of environmental evidence,
laboratory science, and epidemiology
• From an epidemiologic point of view, hypothesis is evaluated in one of two ways:
either by comparing the hypotheses with the established facts or by using analytic
epidemiology to quantify relationships and assess the role of chance.
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14. COMMUNICATE FINDINGS
• The final task is to summarize the investigation, its findings, and its outcome in a
report
• A written report that follows the usual scientific format of introduction, background,
methods, results, discussion, and recommendations should be send to concerned
authority.
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15. IMPLEMENT CONTROL AND PREVENTION MEASURES
• The primary goal is control of the outbreak and prevention of additional cases.
• In practice control and prevention activities are implemented as early as possible.
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16. FOLLOW UP RECOMMENDATIONS/ MAINTAIN
SURVEILLANCE
• Once control and prevention measures have been implemented, outbreak
situation should be monitored. Active surveillance should be continued as it is
essential for:
• Deciding if outbreak is over and for this surveillance may be continued for at least
two incubation periods of concerned disease following onset in the last case
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