How the CDC Investigates Foodborne Illness Outbreaks

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When is a foodborne illness outbreak officially a foodborne illness outbreak? The U.S. Centers for Disease Control (CDC) defines a foodborne illness outbreak as occurring when two or more persons are diagnosed with the same illness stemming from the same contaminated food or drink. Ultimately, it doesn't take much to be labeled a foodborne illness outbreak, but unfortunately, it generally takes more than two diagnoses to uncover and investigate the outbreak and its source.

Investigating Multi-State and Nationwide Outbreaks

Foodborne illness outbreaks are declared after appropriate investigations have been performed by the CDC and other agencies. Alongside local, state, and other federal agencies, information is gathered to determine how and/or why an outbreak occurred, create control measures, and prevent future outbreaks. The U.S. Food and Drug Administration (FDA) and the Food Safety and Inspection Service (FSIS) also pitch in to determine foodborne illness origins, inspect food and handling facilities, lead farm investigations, and announce food recalls. While foodborne illness outbreak investigations require swift action in the name of public health, there is a process that the CDC follows in every multi-state investigation.

The CDC's 7-Step Investigation Program

CDC outlines a seven-step program utilized in the detection and control of outbreaks, prevention of further infections, and determination of how future outbreaks can be prevented. With multiple agencies working on an outbreak at any given time, many of these investigative steps are often performed simultaneously.

Step One: Detecting a Possible Outbreak

In the continuous detection phase, the CDC and partner agencies ensure the regular completion of certain procedures, including:

  • Public Health Surveillance: Gathering reports of illnesses on a regular basis allows the CDC to determine the number of illnesses to expect based on area and time period. 
  • Monitoring Clusters and Outbreaks: Larger than expected groups of people with the same illness in the same area and time period are called clusters. Once subjects within the cluster are deemed to be related to the same illness, the group of illnesses is called an outbreak.
  • Detecting a Cluster or Outbreak, Formal and Informal Reporting: Informal reporting includes members of a community alerting the local health department to report a group of suspected food-related illnesses. While formal reporting requires that physicians and healthcare professionals report when certain infections are detected.

Step Two: Defining and Finding Cases

Typically, the first reported outbreak instances only account for a small portion of the outbreak numbers. By creating a case definition, the CDC is able to determine which people are included as part of the outbreak.

A case definition may include the following details:

  • The pathogen or toxin, if known
  • Certain symptoms typical of that pathogen or toxin
  • Time range for when the illnesses occurred
  • Geographic range, such as residency in a state or region
  • Other criteria, such as DNA fingerprint

When multiple case definitions exist for an investigation, the investigators now have the ability to find more illnesses related to a specific outbreak.

Step Three: Creating a Hypothesis of Likely Sources

Conducting Hypothesis-Generating Interviews and compiling case components allow investigators to create a hypothesis about the likely source of the outbreak.

Step Four: Testing the Hypothesis

Once a hypothesis is determined, it must be tested. Testing the hypothesis to determine the source of the outbreak is generally performed in one of two ways: analytic epidemiological studies and food testing.

Analytic Epidemiologic Study: In analytic epidemiological studies, comparisons are made between groups to determine the role of various risk factors in causing the problem including, but not limited to:

  • Frequencies of exposure to a specific food item
  • Strength of the statistical association
  • Dose-response relationships
  • The food’s production, preparation, and service
  • The food’s distribution

Food Testing: Food testing, on another hand, requires finding foodborne pathogens with the same DNA fingerprint of an unopened food product and from stool samples. Such testing can significantly increase the ability to find the source of an illness.

While researchers find this food testing data helpful, it is generally understood that it can also create unhelpful or confusing results for several reasons:

  • Food items with a short shelf life are often no longer available by the time the outbreak is known, so they cannot be tested.
  • Even if the actually suspected food is available, the pathogen may be difficult to detect. This is because the pathogen may have decreased in number since the outbreak or other organisms may have overgrown the pathogen as the food started to spoil.
  • The pathogen may have been in only one portion of the food. A sample taken from a portion that was not contaminated will have a negative test result. So, a negative result does not rule out this food as a source of illness or the cause of the outbreak.
  • Leftover foods or foods in open containers may have been contaminated after the outbreak or from contact with the food that actually caused the outbreak.
  • Some pathogens cannot be detected in food because there is no established test that can detect the pathogen in the suspect food.

Step Five: Finding Point of Contamination and Source of Food

Point of Contamination is discovered by performing environmental assessments to determine how the food was contaminated. Investigators typically focus on the following:

  • Interviews with those infected determining where the food came from, how it was handled and prepared, and what temperatures the food product was subject to during this process.
  • Investigating health practices and training methods of employees and state of the kitchen (i.e., cleanliness).
  • Health status of employees at time food was determined to be contaminated.
  • Revisit past inspection reports
  • Perform a "Traceback" investigation

Step Six: Controlling an Outbreak

Once a food is determined to be the source of an outbreak, control measures are issued accordingly and may include:

  • Specific measures to clean and disinfect all facilities that came in contact with the food such as restaurants and processing plants
  • Closing food facilities, restaurants, etc.
  • Recall food items
  • Informing the public of the problem, prevention measures, illness symptoms, etc.

Step Seven: Determining an Outbreak is Over

Once the number of new illnesses drops back to normal, the outbreak is considered to be over. Health officials continue to monitor the situation even after the outbreak has been deemed over to ensure illness cases do not reoccur and the contaminated food source is removed in its entirety from the food supply.

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