Step 5: Confirm the Outbreak Source

Health officials use three types of data to confirm the source of a multistate foodborne outbreak: epidemiologic, traceback, and food and environmental testing. Once the analysis of epidemiologic data indicates a suspected source for the outbreak, traceback and food or environmental testing data are used to confirm whether the food is the source.

In addition to helping confirm which food is causing the outbreak, officials use traceback and testing information to help find exactly where contamination occurred. Contamination can happen anywhere along the chain of food production, including during growing, harvesting, processing, transportation, handling, or preparation. Knowing where contamination occurred can help public health officials take actions to stop the outbreak.


During multistate foodborne outbreaks, traceback is usually conducted by state and federal food regulatory authorities and involves determining the movement of food through the food production and distribution chain. Traceback usually starts by following a suspected food consumed by a sick person back through the points of distribution, processing, and production to determine the source of the product or its ingredients. When a food reported by several sick people share a common point in the chain of food production, it can confirm the food is the source of the outbreak and suggests that contamination happened at or before the common point. For example, if several people bought and ate different brands of salad that were packed at a common facility, this suggests that the salads were contaminated at the facility or that the facility received a contaminated salad ingredient.

Food and environmental testing

Finding the outbreak strain (germ with the same DNA fingerprint as those making people sick) in a food or in a food-growing or production environment can also confirm the source of an outbreak. Public health officials look for opportunities to test suspected foods in outbreaks, such as leftovers in sick people’s homes and suspected foods from restaurants, grocery stores, or in the supply chain. Sometimes foods being tested for other reasons will identify an outbreak strain and help confirm the source of the outbreak.

Food testing is most useful when it is driven by results of the epidemiologic investigation. Identifying the outbreak strain in a food cannot by itself confirm that the food is the source of the outbreak. Information from interviews must also show that sick people consumed the contaminated food.

Challenges confirming the outbreak source

Traceback can be challenging for several reasons. Sometimes records documenting the movement of foods through the supply chain are missing, incomplete, or not linkable across companies. Also, if sick people consumed the same food multiple times before getting sick, it can be difficult to know which exposure to trace back. Finally, investigators often don’t have enough detail about a food exposure to start a traceback. For example, a sick person might not have information on the specific brand or production date of a food they ate.

Food and environmental testing can also be challenging. Investigators need to collect enough epidemiologic information to know which foods or environments to test. Even if they have enough information to test specific foods, it may be difficult to find foods to test if the product has a short shelf life. Sometimes leftover foods in open containers can include a mix of ingredients, which also makes test results challenging to interpret. Finally, only a small fraction of any given food can be tested during an outbreak, so a negative test does not prove that a food was not contaminated.