Download Understanding Epidemiology

Understanding Epidemiology
The Steps to Conduct an
Outbreak Investigation
Source: Center for Disease Control and Prevention (CDC) Excite Program
http://www.cdc.gov/excite/classroom/outbreak/steps.htm
Levels of Disease Occurrence

Endemic
◦ The natural or “expected” level of a disease
among a specific group of people or in a
specific area.

Epidemic
◦ Also known as an outbreak; cases of a disease
in excess of the normal level.

Pandemic
◦ An epidemic that spans multiple countries and
continents.
What is an Outbreak?

Also known as an epidemic.

Defined as “more cases of a particular
disease than expected in a given area, or
among a specific group of people, over a
particular period of time.”
How do Outbreaks Occur?

Usually involves three conditions:
1)
2)
3)
Introduction of a new pathogen, or a
change in the virulence (severity) of a
known pathogen.
Adequate number of exposed and
susceptible persons.
Effective means of transmission
between the source and susceptible
persons.
How are Outbreaks Uncovered?

Most commonly occurs in two ways:
1) Local health departments will receive phone
calls from a health provider or citizens that
know of “several cases” of a particular
disease.
2) Public health surveillance allows
epidemiologists to track patterns of
diseases in a community based on reports
submitted by local doctors, laboratories, and
other sources.
Outbreak Investigations

Also known as “field epidemiology.”

The action-oriented practice or
application of epidemiology to control
and prevent health problems.

Goal = Solve a pressing health issue by
instituting some control to safeguard the
health of a community.
Why Investigate an Outbreak?

Represent a breakdown in public health.
◦ This provides an opportunity to learn what
went wrong.
Control and prevention purposes.
 Research and training opportunities.
 Program considerations for health
agencies.
 Public, political, and legal concerns
because of the severity and risk to others.

Challenges

Cause or source of outbreak is often
unknown.

Fear among population affected.

Limited access to laboratory testing for
sample analysis.

Limited time to plan a study.

Limited control over the situation.
Steps to Outbreak Investigations

Just because the following slides present
the steps in conceptual order, the truth is:
◦ Many of these steps can be done at the same
time.
◦ Many of these steps can be done in different
orders.
◦ The steps are highly dependent upon the
outbreak situation.
Steps to Outbreak Investigations





Step 1 – Prepare.
Step 2 – Establish
existence of an
outbreak.
Step 3 – Verify diagnosis.
Step 4 – Define and
identify cases.
Step 5 – Describe data
in terms of person, place,
and time





Step 6 – Develop
hypotheses.
Step 7 – Evaluate
hypotheses.
Step 8 – Refine
hypotheses and conduct
additional studies.
Step 9 – Implement
prevention measures.
Step 10 – Communicate
findings.
Step 1: Prepare

Research suspected disease.

Have appropriate supplies and equipment.

Develop a TEAM organization among all
involved parties.
Step 2: Establish Existence of an
Outbreak

Are there really more cases than
normal?

Compare current disease numbers with
numbers from the past few weeks and
months.

Sources for this data:
◦ Notifiable diseases reports.
◦ Disease registries.
◦ Neighboring local and state health
departments.
Step 3:Verify Diagnosis

Requires that you determine the specific
nature of the disease.

This involves comparing clinical findings
(symptoms) and laboratory results from
the people affected.

Also, talk to the cases to gather more
information about the disease.
Step 4: Define and Identify Cases

Develop a case definition:
◦ Clinical information about the disease.
◦ Characteristics about the people who are
affected.
◦ Information about the location or place.
◦ Specification of time during which the
outbreak occurred.

Identify and count cases to form a list of
those affected.
Step 5: Describe Data in Terms of
Person, Place, and Time

This is known as “descriptive
epidemiology.”

Characterizing by time using an epidemic
curve.

Characterizing by place using a spot map.

Characterizing by person to determine
the population most at risk.
Step 5: Describe Data in Terms of
Person, Place, and Time
Example of an epidemic curve used to characterize a disease outbreak by time.
Step 5: Describe Data in Terms of
Person, Place, and Time
Example of a spot map used to characterize a disease outbreak in terms of place.
Step 6: Develop Hypotheses

Realistically, you may have already
generated hypotheses before this point.

However, with the information collected
throughout the investigation, you can now
form more specific hypotheses about:
◦ Source of the infectious agent.
◦ Mode of transmission.
◦ Exposures and risk factors.
Step 7: Evaluate Hypotheses

Two ways to evaluate:
1) Compare your hypotheses with established
facts.
o
Often used when the cause of the outbreak is
clear.
2) Use “analytic epidemiology,” which utilizes
study methods to actually tests your
hypotheses.
o
Often used when the cause of the outbreak is
still unclear.
Step 8: Refine Hypotheses and
Conduct Additional Studies

It is important to consider what questions
remain unanswered.

Utilize other types of studies to gain
more information, such as laboratory or
environmental studies.

This could provide information about the
disease, its mode of transmission, host
factors, and specific characteristics about
the infectious agent.
Step 9: Implement Preventative
Measures

Realistically, this should be done as soon
as possible to prevent further illness.

These control measures should be aimed
at specific links in the chain of infection.

Sometimes control measures can be
aimed at interrupting transmission or
exposure to an infectious agent.
Step 10: Communicate Findings

Provides a blueprint for action.

Provides an opportunity to explain what
you did, what you found, and what should
be done about the situation.

Most often occurs in two ways:
1) Oral briefing for local health authorities .
2) Written report.
Resources

Center for Disease Control and Prevention (CDC). Excite: Epidemiology
in the Classroom. “How to Investigate an Outbreak – Steps of an
Outbreak Investigation.”
http://www.cdc.gov/excite/classroom/outbreaks/steps.htm.