1. Understanding Epidemiology
The Steps to Conduct an Outbreak Investigation
Source: Center for Disease Control and Prevention (CDC) Excite Program

2. 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.

3. 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.”

4. How do Outbreaks Occur? Usually involves three conditions:
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.

5. How are Outbreaks Uncovered?
Most commonly occurs in two ways:
Local health departments will receive phone calls from a health provider or citizens that know of “several cases” of a particular disease.
Public health surveillance allows epidemiologists to track patterns of diseases in a community based on reports submitted by local doctors, laboratories, and other sources.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. Step 1: Prepare Research suspected disease.
Have appropriate supplies and equipment.
Develop a TEAM organization among all involved parties.

12. 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.

13. 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.

14. 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.

15. 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.

16. Step 5: Describe Data in Terms of Person, Place, and Time
Example of an epidemic curve used to characterize a disease outbreak by time.

17. 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.

18. 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.

19. Step 7: Evaluate Hypotheses
Two ways to evaluate:
Compare your hypotheses with established facts.
Often used when the cause of the outbreak is clear.
Use “analytic epidemiology,” which utilizes study methods to actually tests your hypotheses.
Often used when the cause of the outbreak is still unclear.

20. 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.

21. 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.

22. 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:
Oral briefing for local health authorities .
Written report.

23. Resources
Center for Disease Control and Prevention (CDC). Excite: Epidemiology in the Classroom. “How to Investigate an Outbreak – Steps of an Outbreak Investigation.”