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Steps of an Outbreak Investigation

In investigating an outbreak, speed is essential, but getting the right answer is essential, too. To satisfy both requirements, epidemiologists approach investigations systematically, using the following 10 steps:

  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. Implement control and prevention measures
  10. Communicate findings

The steps are presented here in conceptual order. In practice, however, several may be done at the same time, or they may be done in a different order. For example, control measures should be implemented as soon as the source and mode of transmission are known, which may be early or late in any particular outbreak investigation.

Step 1: Prepare for Field Work

Before leaving for the field, you should:

  • Research the disease and gather the supplies and equipment you will need
  • Make necessary administrative and personal arrangements for such things as travel, and 
  • Consult with all parties to determine your role in the investigation and who your local contacts will be once you arrive on the scene.

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Step 2: Establish the Existence of an Outbreak

One of your first tasks as a field investigator, or disease detective, is to verify that a suspected outbreak is indeed a real outbreak. Some will turn out to be true outbreaks with a common cause, some will be unrelated cases of the same disease, and others will turn out to be unrelated cases of similar but unrelated diseases. Before you can decide whether an outbreak exists (i.e., whether the observed number of cases exceeds the expected number), you must first determine the expected number of cases for the area in the given time frame.

How, then, do you determine what is expected? Usually you can compare the current number of cases with the number from the previous few weeks or months, or from a comparable period during the previous few years. The sources of these data vary:

  • For a notifiable disease (one that, by law, must be reported), you can use health department surveillance records.
  • For other diseases and conditions, you can usually find data from local sources such as hospital discharge records, death (mortality) records, and cancer or birth defect registries.
  • If local data are not available, you can make estimates using data from neighboring states or national data, or you might consider conducting a telephone survey of physicians to determine whether they have seen more cases of the disease than usual. You could even conduct a survey of people in the community to establish the background level of disease.

Even if the current number of reported cases exceeds the expected number, the excess may not necessarily indicate an outbreak. Reporting may rise because of changes in local reporting procedures, changes in the case definition, increased interest because of local or national awareness, or improvements in diagnostic procedures. For example, if a new physician, infection control nurse, or health care facility is reporting cases more consistently than they were reported in the past, the numbers would go up even though there might be no change in the actual occurrence of the disease. Finally, particularly in areas with sudden changes in population size, such as resort areas, college towns, and migrant farming areas, changes in the number of reported cases may simply reflect changes in the size of the population.

Whether or not you should investigate an apparent problem further is not strictly tied to your verifying that an epidemic exists (that is, that the observed number is greater than the number expected). As noted earlier, other factors may come into play, including, for example, the severity of the illness, the potential for spread, political considerations, public relations, and the availability of resources.

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Step 3: Verify the Diagnosis

In addition to verifying the existence of an outbreak early in the investigation, you must also identify as accurately as possible the specific nature of the disease. Your goals in verifying the diagnosis are two-fold. First, you must ensure that the problem has been properly diagnosedthat it really is what it has been reported to be. Second, for outbreaks involving infectious or toxic-chemical agents, you must to be certain that the increase in diagnosed cases is not the result of a mistake in the laboratory.

Verifying the diagnosis requires that you review the clinical findings (the symptoms and features of illness) and laboratory results for the people who are affected. If you are at all uncertainty about the laboratory findings (e.g., if they are inconsistent with the clinical findings), you should have a laboratory technician review the techniques being used. If you expect a need for specialized laboratory work (e.g., special culturing or DNA analysis), you should begin obtaining the appropriate specimens, isolates, and other laboratory material from a sufficient number of patients as soon as possible.

Finally, you should visit several of the people who became ill. If you do not have the clinical background to verify the diagnosis, a doctor or other qualified clinician should do so. Regardless of your background, though, you should see and talk to some of these people to gain a better understanding of the disease and those affected by it. In addition, you may be able to gather critical information by asking such questions as, What were their exposures before becoming ill? What do they think caused their illness? Do they know anyone else with the disease? Do they have anything in common with others who have the disease? Conversations with patients are very helpful in generating hypotheses about the cause, source, and spread of disease.

Back to Top | Step 4: Define and Identify Cases


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