Epidemiology 101: Outbreak Investigation

Steps in Investigating an Outbreak

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Victor Torres/Stocksy United

Outbreaks are frequently reported in the news. What exactly is an outbreak, and why is it important to investigate it? What do epidemiologists (scientists who study the spread of diseases) do when investigating an outbreak?

What is an outbreak?

According to the CDC, an “outbreak” is the occurrence of more cases of disease than normally expected within a specific place or group of people over a given period of time.

There are all sorts of outbreaks that occur. These can range from food poisoning to Ebola to Enterovirus.

What is the difference between an outbreak, and epidemic, and a pandemic?

An “outbreak” and an “epidemic” essentially mean the same thing to an epidemiologist, but the term “epidemic” has a more serious connotation than “outbreak” and is used less frequently to avoid the perception of a crisis situation. "Outbreak" is often used to refer to a more limited geographic area. A “pandemic” refers to a disease epidemic that is widespread and often global.

Another term used in epidemiological investigations is “cluster,” which refers to a group of cases in a specific time and place that may or may not be greater than normal. Investigations of disease clusters are used to determine the normal or expected rate of disease. A disease with a high background rate in a given place and time is said to be “endemic.”

Why is it important to investigate outbreaks?

Investigation of outbreaks is necessary to understand and ultimately control and prevent the spread of diseases. By understanding how certain diseases are transmitted and studying trends of diseases, epidemiologists can use collected information to identify sources of infections and make recommendations for stopping their spread.

Investigation of outbreaks is particularly important when the disease in question is particularly severe or has high rates of transmission. The study of outbreaks can facilitate the development of new vaccines and drugs, changes in human behavior, as well as legislation for the improvement of public health.

What steps are needed in an outbreak investigation?

The CDC has provided a list of 10 steps used by epidemiologists for investigating outbreaks. They implement these methods to optimize the speed and accuracy of outbreak investigations in order to ensure the safety and health of the population.

  1. Prepare for field work. Investigators should be familiar with the disease and have a plan of action which includes lists of supplies, division of tasks among team members, and administrative and travel arrangements.
  2. Establish the existence of an outbreak. An outbreak is defined as the occurrence of more cases of disease than normally expected within a specific place or group of people over a given period of time. To establish that an outbreak is real (that is, more cases than expected), an investigator can examine health department surveillance records, hospital records, and other disease registries. If this information is unavailable, other options include interviews with doctors or people within the community.
  1. Verify the diagnosis. An investigator will need to review clinical findings and lab tests in order to verify the diagnosis, as well as determine the specific nature of the disease. For example, in infectious disease outbreaks, additional lab tests may be necessary to determine the specific strain of microbe implicated in the outbreak.
  2. Define and identify cases. The investigator is responsible for establishing what constitutes a case. A case definition will usually include information about the disease, characteristics of the patients, information about the location, and a specific range in time. By doing so, investigators can eliminate an excess of false-positives. To identify cases, it is important to have open communication with personnel from healthcare facilities and other relevant facilities or people who will be on the radar for observing potential cases.
  3. Describe and orient the data in terms of time, place, and person. An investigator will understand more about the outbreak by compiling a comprehensive description of its trends over time, place, and kinds of people (age, race, sex, etc.) affected by the disease.
  4. Develop hypotheses. The hypothesis is an educated guess about the source of the disease, mode of transmission, and/or exposures that caused the disease, based on available information.
  5. Evaluate hypotheses. The credibility of the hypotheses can be evaluated by looking at the facts or by crunching numbers to get actual statistics on available information.
  6. Refine hypotheses and carry out additional studies. Additional studies may include lab tests or environmental studies, among other methods of evaluation.
  7. Implement control and prevention measures. Control and prevention methods are usually aimed toward the source of the disease, but may also include interrupting transmission or limiting exposure.
  8. Communicate findings. Findings of the investigation should be communicated to local health authorities who are responsible for implementing control measures. In addition, a written report provides a legal record of the findings and contributes to public health awareness.


CDC: Steps of an Outbreak Investigation




PHE - Public Health England: The steps in outbreak investigation including the use of relevant epidemiological methods


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