Outbreak Investigation Methods

Question 1

Outbreak investigation steps (10)

Answer 1

1) Prepare for field work
2) Determine the existence of an outbreak
3) Confirm the diagnosis
4) Identify and count cases
5) Tabulate data and perform descriptive epidemiology
6) Consider implementing control measures
7) Develop and test hypotheses (analytic epidemiology)
8) Plan for additional studies
9) Implement and evaluate control measures
10) Communicate finding

Question 2

Outbreak investigation step 1: prepare for field work

Answer 2

• Investigation needs
- Supplies
- Laboratory
- Expertise
• Administrative needs
- Team leader
- Procedures
• Logistics and dynamics
- Where to go/Who to meet
- What is your role

Question 3

Outbreak investigation step 2: determine existence of outbreak

Answer 3

• Outbreak, epidemic, and cluster - Grouping of cases in a given place or time is a cluster ; No difference between an outbreak and epidemic (Both describe an increase in cases over what is expected)

• Real vs. Artifact: changes in surveillance methodology can result in the appearance of an outbreak when there really isn’t one

Question 4

Outbreak investigation step 3: confirm diagnosis

Answer 4

• Ensure proper diagnosis - Do all individuals have the same disease? ; Laboratory error?
• Examine initial case-patients
• Review medical records

Question 5

Outbreak investigation step 4: identify and count cases

Answer 5

• Who is a case?
• Index Case: refers to the first case in a disease outbreak
• Sometimes recognized/known, sometimes not
• Need a working case definition to identify and count cases!
• Essential to standardize the case definition to find cases no matter where they are and who the investigators are

Question 6

Index case definition

Answer 6

First case in disease outbreak

Question 7

Working case definition: 4 components

Answer 7

Clinical presentation
Who/person
Where/place
When/time

Question 8

Categories of certainty in working case definition (3)

Answer 8

Confirmed
Probable
Possible

Question 9

Common symptoms of an outbreak (7)

Answer 9

• Fever
• Nausea
• Diarrhea
• Vomiting
• Headache
• Rashes
• Stomach pain

Question 10

What allows you to begin establishing the case definition

Answer 10

Typical signs and symptoms reported in outbreak

Question 11

Importance of consistent case definition: AIDS example

Answer 11

Expansion of surveillance case definition resulted in huge spike in number of reported cases

Question 12

Outbreak investigation step 5: tabulate data and perform descriptive epi

Answer 12

• Organize case information identified in Step 4 into a line listing
• Perform descriptive epidemiology:
Time: Epidemic curve
Place: Orient to areas of concern (Spot map)
Person: Describe case group ( Numerators and denominator)

Question 13

Epidemic curve definition

Answer 13

Graphical depiction of number of cases of illness by date of illness onset
Usually a bar graph

Question 14

How can epidemic curve help in an outbreak (2)

Answer 14

• Can provide information about the outbreak
-Pattern of spread (e.g., Did person-to-person transmission occur?)
- Magnitude
- Time trend
-Exposure and/or period of incubation

• Different shapes reveal the type of outbreak
•-Common Source: all cases are exposed to agent from same source( Point source vs Continuous source vs Intermittent )
- Propagated source: secondary cases exposed to primary cases

Question 15

Outbreak investigation step 6: consider implementing control measures

Answer 15

• Target to:
-Eliminate the source
- Interrupt transmission
- Reduce susceptibility

Question 16

Outbreak investigation step 7: develop and test hypotheses

Answer 16

• Review descriptive epidemiology (Explain outliers)
-Generate Hypotheses
• Analytic epidemiology options - Cohort study, Case-control study
• Compare results with hypothesis

Question 17

Outbreak investigation step 8: plan for additional studies

Answer 17

• Improve quality of numerator and
denominator data
• Environmental studies
• Laboratory studies

Question 18

Outbreak investigation step 9: implement and evaluate control measures

Answer 18

• Evaluate impact of control measures taken at Step 6
• Modify as needed
• Active surveillance to monitor for new cases

Question 19

Outbreak investigation step 10: communicate findings

Answer 19

• Why?
-Document findings
- Justify recommendations
- Document for legal purposes

Methods
• Written report
• Oral presentation to stakeholders
• Publication in public health bulletins, journals

Question 20

Point source epidemic: curve description

Answer 20

Single sharp rise and fall on epi curve

Question 21

Continuous source epidemic curve description

Answer 21

Curve flattened and more spread out

Question 22

Intermittent source epidemic curve description

Answer 22

Repeated gaps in case onset dates

Question 23

Propagated source epidemic curve description

Answer 23

Secondary cases exposed to primary cases
Subsequent peaks higher than initial peak