Step 5: Solve Point of Contamination and Source of the Food

Illustration representing the source and movement of outbreak by showing a farm, home, shopping cart and a restaurant.

Health officials use three types of data to link illnesses to contaminated foods and solve outbreaks: epidemiologic, traceback, and food and environmental testing. Health officials assess all of these types of data together to try to find the likely source of the outbreak. A contamination can happen anywhere along the chain of food production, processing, transportation, handling, and preparation. Check out this infographic about how health officials solve foodborne outbreaks.

What epidemiologic data is used to help find the likely source of the outbreak?

  • Patterns in the geographic distribution of illnesses, the time periods when people got sick, and past outbreaks involving the same germ.
  • Foods or other exposures occurring more often in sick people than expected
  • Clusters of unrelated sick people who ate at the same restaurant, shopped at the same grocery store, or attended the same event.

Key words:

Traceback: An investigation that starts with the sick people or restaurants and works its way back through the chain of food production to search for a common point between the people or places to find the contamination source

The Food Production Chain Everyone has a role to play in keeping food safe. Production Processing Distribution Restaurant Food service workers and customers Retail home cooks and consumers

Diagram tracing the movement of food from consumer back to retailers, restaurants, distributors, processors, and producers.

What is a traceback and how is this used to help find the likely source of the outbreak?

  • A traceback investigation that starts with the sick people or restaurants and works its way back through the chain of food production to search for a common point between the people or places here the contamination is likely to have happened.
  • The decision about when product tracing is needed, and for what food item(s), depends on the evaluation of the epidemiologic data.
  • Investigators may conduct environmental assessments at common points in food production facilities, farms, and restaurants to identify any food safety risks which may have led to the contamination.

How are food and environmental testing used to help find the likely source of the outbreak?

  • Investigators may collect suspect food items from a sick person’s home, a retail location, or in the food production environment to test and try and find the germ that caused illness
  • DNA fingerprinting may be used to link germs found in foods or production environments to germs found in sick people.

Why can’t investigators link a food to an illness?

It is not unusual to find no statistical association between an illness and any particular food item, even when all the clues point to foodborne transmission. Reasons include:

  • Public health officials learned of the outbreak so long after it occurred that they could not do a full investigation.
  • An initial investigation did not lead to a specific food hypothesis, so no analytic study was done.
  • There were too few illnesses to analyze with a full study.
  • The food was a “stealth food” – foods that people may eat but are unlikely to remember. Ex. garnishes, condiments on sandwiches, and ingredients in a food item such as the filling in a snack cracker.
  • Food testing did not find any pathogen related to the outbreak.
  • Food testing was not done.
  • There were not enough available staff or resources to do a full investigation.

What if no link is found between a specific food and illness?

When no statistical association between a food and illness is found, it does not mean the illness or outbreak was not foodborne. It means only that the source could not be determined. If the outbreak has ended, the source of the outbreak is declared unknown. If people are still getting sick, investigators must keep gathering information and studying results to find the food that is causing the illnesses.