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Lesson 6: Investigating an Outbreak
Summary, References, and Websites
Outbreaks occur frequently. Not every outbreak comes to light, but of those that do, public health agencies must decide whether to handle them without leaving the office, or spend the time, energy, and resources to conduct field investigations. The most important reason to investigate is to learn enough about the situation to implement appropriate control and prevention measures. Other reasons include taking the opportunity to advance knowledge about the disease, agent, risk factors, interventions and other scientific issues; responding to public, political, or legal concerns; evaluating a health program’s effectiveness and weaknesses; and to provide training.
Outbreaks are almost always unexpected events. Sometimes they are the subject of media attention and public concern, so investigators feel pressured to work quickly to find answers. When multiple agencies are involved, coordination and communication become even more essential but are more complicated than usual. Often the investigation takes place in the field, far from the conveniences and routines one counts on in the office. Under these circumstances, it is essential to have a systematic plan for conducting the investigation.
The steps listed in Table 6.2 comprise one such plan. Note that the order of the steps is conceptual, and investigators may decide that a different order is best suited for any given outbreak. To summarize, these are the steps of an outbreak investigation:
- Planning for field work, establishing the existence of an outbreak, and verifying the diagnosis are usually the first steps, sometimes done in that order, sometimes done in reverse order, sometimes done simultaneously. (Steps 1–3)
- After the diagnosis has been confirmed investigators create a workable case definition, then go out and look for additional cases. Information about these cases is organized either in a line listing or in a computer database that allows staffers to check for duplicate records, update records as additional information comes in, and perform descriptive epidemiology. (Steps 4-6)
- Descriptive epidemiology — organizing the data by time, place, and person — is essential for characterizing the outbreak, identifying populations at risk, developing hypotheses about risk factors, and targeting control/prevention strategies. An epidemic curve — a histogram of number of cases by time of onset of illness — provides a handy visual display of the outbreak’s magnitude and time trend. (Step 6)
- Hypotheses, based on what is known about the disease, descriptive epidemiology, and what others have postulated, must be developed before conducting any kind of epidemiologic study (what are you going to study if you don’t know what you are looking for?). (Step 7)
- While not every outbreak requires an analytic study, those that do are usually addressed by either a cohort study or a case-control study. Both types of study attempt to identify associations between exposures (risk factors or causes) and the disease of interest. In a cohort study, best suited for an outbreak in a well-defined population such as guests at a wedding, investigators usually attempt to enroll everyone, determine exposures and outcomes, calculate attack rates, and compare attack rates with a risk ratio or relative risk to identify associations. In a case-control study, which is well suited for outbreaks without a well-defined population, investigators usually enroll all of the case-patients plus a sample of persons who did not get ill, then ask about exposures and compute an odds ratio to look for associations. (Step 8)
- If needed, hypotheses can be refined and re-evaluated. In many investigations, while the epidemiologists are conducting their epidemiologic investigations, environmental health specialists and laboratorians are conducting studies and tests of their own. Ideally, this multidisciplinary approach points to a single conclusion. (Steps 9 and 10)
- While implementing control and prevention measures is listed as Step 11, it is the primary goal of most outbreak investigations and usually occurs early in the investigation. Such measures can be implemented as soon as any link in the chain of disease transmission that is susceptible to intervention can be identified. If the source and mode of transmission is known, disease control measures need not wait. However, there is no guarantee that these measures will work, so continued surveillance is essential. (Steps 11 and 12)
- Finally, communicating what was found and what should be or was done in a written report provides key public health, scientific, and legal documentation. (Step 13)
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