Learning Objective
Importance of investigating reported outbreaks Steps in the investigation of an outbreak or epidemic Describe epidemic curvesWhen the occurrence of adisease within an area is clearly in excess of the expected level for a given time period, it is called an epidemic. Public health officials often use the term outbreak, which means the same thing, because it is less provocative to the public. When an epidemic spreads over several countries or continents, affecting a large number of people, it is called a pandemic.
Why Investigate Possible Outbreaks?
Control/prevention Research opportunities Training Public, political, or legal concerns“Once a decision is made to investigate an outbreak, three types of activities are generally involvedThe epidemiologic investigation.The environmental investigation. The interaction with the public, the press, and, in many instances, the legal system.”
Step 1: Prepare for Fieldwork
Investigation Administration ConsultationStep 2: Establishing the Existence of an Outbreak
Is this an epidemic or cluster of cases? Does the observed number exceed the expected number of cases?For a notifiable disease use health department surveillance records.For other diseases and conditions, find existing data locally—hospital discharge records, mortality statistics, cancer or birth defect registries.If local data are not available, apply rates from neighboring cities or national data, or, alternatively, conduct a telephone survey of physicians to determine whether they have seen more cases of the disease than usual.Finally, conduct a survey of the community to establish the background or historical level of disease.
Step 3: Verifying the Diagnosis
to ensure that the problem has been properly diagnosed to rule out laboratory error as the basis for the increase in diagnosed cases. summarize the clinical findings with frequency distributions
Why?
Diseases can be misdiagnosed. Case may not be actual case, but rather suspected case. Information from non-cases must be excluded from the case information used to confirm the presence or absence of an epidemic.Step 4a: Establishing a Case Definition
A case definition is 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 and--particularly in the setting of an outbreak investigation--restrictions by time, place and person. Apply them consistently and without bias to all persons under investigation.To be classified as confirmed, a case usually must have laboratory verification. A case classified as probable usually has typical clinical features of the disease without laboratory confirmation. A case classified as possible usually has fewer of the typical clinical features.
Step 4b: Identifying and Counting Cases
Direct case finding at health care facilities where the diagnosis is likely to be made: physicians , clinics, hospitals, and laboratories. In some outbreaks, public health officials may decide to alert the public directly, usually through the local media. Identifying contacts to case-patients . Conduct a survey of the entire population.The following items of information should be collected about every case:identifying information• demographic informationclinical informationrisk factor informationreporter information
Develop a line listing:
Step 5: Performing Descriptive EpidemiologyCharacterize an outbreak by time, place, and person.
Why?
Provide a comprehensive description of an outbreak by portraying : - its trend over time, - its geographic extent (place), and - the populations (persons) affected by disease . Assess description of the outbreak to develop causal hypotheses ( in light of what is known ) : - usual source, - mode of transmission, - risk factors and - populations affected, etc.
Time
What is the exact period of the outbreak? What is the probable period of exposure? Is the outbreak likely common source or propagated?Place
What is the most significant geographic distribution of cases? Place of residence? Workplace? What are the attack rates?Person
What were the age and gender specific attack rates? What age and gender groups are at highest and lowest risk of illness? In what other ways do the characteristics of the cases differ significantly from those of the general population?Interpreting an epidemic curve
When analyzing an epidemic curve, it is important to consider the following factors to assist in interpreting an outbreak: the overall pattern of the epidemic the time period when the persons were exposed if there any outliersPoint Source
The graph above illustrates an outbreak of gastrointestinal illness from a single exposure. While there are outliers to this dataset, it is clear that there is an outbreak over a limited period of time, and the shape of the curve is characteristic of one source of exposure
Continuous Common Source
The data above is from the well-known outbreak of cholera in London that was investigated by the "father of epidemiology," John Snow. Cholera spread from a water source for an extended period of time. Note that the typical incubation period for cholera is 1--3 days that the duration of this outbreak was more than 1 month.Propagated (Progressive Source)
The graph above illustrates an outbreak of measles. The graph shows a single common source (the index case), and the cases appear to increase exponentially. Measles is caused by person-to-person contact. Its incubation period is typically 10 days but may be 7--18 days.Common Source Questions
Is the outbreak from a single source or common source? Does the disease spread from person to person? If there is a continued exposure to a single source?Propagated Source Questions:
Is the outbreak from multiple sources or exposures? Is the outbreak airborne, behaviorally or chemically caused , and does it involve multiple events? Are the sources of infection from inapparent sources? Is there a vector or animal reservoir involved?Duration of an epidemic
Is determined by: The number of susceptible persons who are exposed to a source of infection and become infected. Period of time over which susceptible persons are exposed to the source. Minimum and maximum incubation periods for the disease.Step 6: Developing Hypotheses
All aspects of the investigation should be addressed: - the source of the agent, - the mode of transmission, (vehicle &vector), - the exposures that caused the disease, - any additional time, person, and place factors.Step 7: Evaluating Hypotheses
Either by comparing the hypotheses with the established facts, or by using analytic epidemiology to quantify relationships and explore the role of chance.Step 8: Refining Hypotheses and Executing Additional Studies
Epidemiologic studies Laboratory and environmental studiesRetrospective cohort
Foodate
Did not eat
Ill
well
AR
Ill
well
AR
RR
meat
29
17
63
17
12
59
1.07
spinach
26
17
60
20
12
62
0.97
potato
23
14
62
23
14
62
1.00
salad
13
11
54
28
19
60
0.90
Ice cream
43
11
80
3
18
14
5.71
Case control
ExposureCase
Control
Total
Ate at A restaurant
Yes
30
36
66
No
10
70
80
Total
40
106
146
OR= (70 x 30) / (10 x36) = 5.83
Step 9: Implementing Control and Prevention Measures
Aim to apply control measures at the weak link or links in the chain of infection.An outbreak might be controlled by : - destroying contaminated foods, - sterilizing contaminated water, or - destroying mosquito breeding sites.“ An infectious food handler could be removed from the job and treated.”
In other situations, control measures might be aimed at: - interrupting transmission or - reducing the exposure (such as having nursing home residents with a particular infection “cohorted,” or put together in a separate area to prevent transmission to others).
In some outbreaks, control measures might be directed at: - reducing the susceptibility of the host, - Immunization , - chemoprophylaxis for travelers.