Investigation of an Outbreak

Question 1

How do we know about an outbreak?

Answer 1

1. From health workers
2. General population/citizens
3. Media
4. Routine analysis of public health surveillance data
   – Data analysis shows an increase over normal background level

Question 2

Why should we investigate outbreaks?

Answer 2

1. Control and prevention
2. Severity and risk to others
3. Research opportunities
4. Training opportunities
5. Program considerations
6. Public, political, or legal concerns

Question 3

Describe the steps in control and prevention of an outbreak?

Answer 3

1. Control the outbreak at hand and prevent future outbreaks
2. Assess its extent and the characteristics of the population at risk
3. Design measures to prevent additional cases
4. Investigate further to identify its source and use that information to develop measures that will prevent future outbreaks
5. Strike a balance between instituting control measures and conducting further investigation

Question 4

Describe the research opportunities in outbreaks?

Answer 4

1. Each outbreak offers a unique opportunity to study the natural history of the disease in question
   - including the agent, mode of transmission, and incubation period.
2. Learn more about the impact of control measures and the usefulness of new epidemiological and laboratory techniques

Question 5

Investigating an outbreak of a disease may highlight what?
Program considerations?

Answer 5

1. Populations at risk that have been overlooked
2. Failures in the program’s intervention strategy
3. Changes in the agent causing the disease
4. Events beyond the scope of the program

Question 6

What are the steps in investigating an outbreak?

Answer 6

1. Prepare for field work
2. Establish the existence of an outbreak
3. Verify the diagnosis
4. Define and identify cases
5. Describe and orient the data in terms of time, place and person
6. Develop hypotheses
7. Evaluate hypotheses
8. Refine hypotheses and carry out additional studies
9. Implementing control and prevention measures
10. Communicate findings

Question 7

How do you prepare for field work?

Answer 7

1. Research the disease and gather supplies and equipment
   – Review the diseases natural history
   – Remember the iceberg phenomenon and potential disease reservoirs
2. Make necessary administrative and personal arrangements for such things as travel

Question 8

How do you establish an existence of an outbreak?

Answer 8

Determine if the observed number of cases is greater than the expected number of cases

Question 9

What is an outbreak or epidemic?

Answer 9

The occurrence of more cases of disease than expected in a given place or among a specific group of people over a particular period of time

Question 10

What is a cluster?

Answer 10

Grouping of cases in a given place over a particular period of time, without regard to whether the number of cases is more than expected

Question 11

In what ways does ‘excess’ number of cases not necessarily indicate an outbreak?

Answer 11

1. Reporting procedure changes
2. Changes in case definition
3. Improvements in diagnostic procedures
4. Increased interest because of media awareness
5. Increased awareness of health practitioners
6. Seasonal population changes

Question 12

How do you verify the diagnosis?

Answer 12

1. Confirm that the right diagnosis has been made
2. be certain that the increase in diagnosed cases is not the result of laboratory error
3. Review clinical findings
4. Summarize clinical and laboratory findings

Question 13

How do you define and identify cases?

Answer 13

1, establish a case definition
2. collect key information
3. analysis - time, person, place
4. establish - causative pathogen and its source

Question 14

What is a case definition?

Answer 14

a standard set of criteria for deciding whether a person should be classified as having the disease or condition

Question 15

Name the types of case classifications?

Answer 15

1. suspected case
2. probable case
3. confirmed case

Question 16

What is a suspected case?

Answer 16

A case that is classified as suspected for reporting purposes
- Some diseases require laboratory confirmation for diagnosis regardless of clinical symptoms, others are diagnosed based on epidemiologic data
- Suspect cases may be dropped when case definition is tightened over the course of the investigation

Question 17

What is a probable case?

Answer 17

A case that is classified as probable
for reporting purposes
- There may be supportive laboratory results or inconclusive laboratory results

Question 18

What is a confirmed case?

Answer 18

A case that is classified as confirmed for reporting purposes; usually laboratory-confirmed as a case

Question 19

What key information do you collect when defining and identifying cases?

Answer 19

1. Identifying information
   – Name, address, phone
2. Demographics
   – Age, sex, race, occupation
3. Clinical
   – Date of onset, symptoms and signs
4. Risk Factors
   – Helps to focus investigation to the specific disease in question

Question 20

What is analysis?

Answer 20

Characterize the outbreak by person, time and place

Question 21

How do you analyze time?

Answer 21

Plot an epidemic curve
- Depicts time course of the outbreak by drawing a histogram of number of cases by their date of onset

Question 22

Name the types of epidemic curves?

Answer 22

1. point source
2. continuous common source
3. propagated

Question 23

What is a point source epidemic?

Answer 23

Shape – a steep up slope, a peak and a gradual down- slope
* Interpretation - people are exposed to the same source over a relatively brief period

Question 24

What is a continuous common source epidemic?

Answer 24

Shape - curve will have a plateau instead of a peak
* Interpretation - people are exposed to the same source over an extended period

Question 25

What is a propagated epidemic?

Answer 25

Shape - a series of progressively taller peaks
* Interpretation - person-to-person spread

Question 26

How do you analyse place?

Answer 26

– Provides geographic extent of problem
– May demonstrate clusters or patterns that provide important etiologic clues
– Spot map useful for illustrating where cases live, work or may have been exposed

Question 27

How do you analyse person?

Answer 27

– Determine the populations at risk by
characterizing the outbreak by person
– Define populations by personal characteristics
(Examples: age, race, sex, or medical status)
– Assess for exposures (Examples: occupation, leisure activities, use of medications, tobacco, drugs)

Question 28

Hypotheses may be based on?

Answer 28

1. Interviews with affected people
2. Consultation with health officials
3. Descriptive epidemiology

Question 29

A hypothesis should address what?

Answer 29

1. Causative agent
2. Mode(s) of transmission
3. Exposure(s) that caused the disease
   Note: Outlier cases (ones you think might not have anything to do with the others) can sometimes provide important clues

Question 30

How do you evaluate the hypotheses?

Answer 30

Compare hypotheses with the established facts using analytic epidemiology

Question 31

What tools do we use to evaluate hypotheses?

Answer 31

1. Cohort studies
   * Relative risk (ratio of attack rates) can be calculated
   * Best for analyzing an outbreak in a small well-defined population
2. Case-control studies
   * Odds ratio can be calculated
   * population in an outbreak is not well defined

Question 32

How do you refine/reconsider hypotheses?

Answer 32

1. Hypotheses not confirmed: consider new vehicles or modes of transmission
2. Conduct additional epidemiologic studies
   – Laboratory e.g serology
   – Environmental

Question 33

How do you implement control and prevention measures?

Answer 33

1. interrupting transmission or exposure
   - Isolation of symptomatic individuals
   - Quarantine
   - Instruct avoidance of exposure areas
2. reducing susceptibility
   - immunizations
   - Chemoprophylaxis
3. Universal Precautions
   - Using personal protective equipment

Question 34

How do you communicate your findings?

Answer 34

1. an oral briefing
2. a written report