A hypothesis should be tested to determine if the source has been correctly identified. Investigators use many methods to test their hypotheses. Two main methods are analytic epidemiologic studies and food testing.
Case-control studies or cohort studies are the most common type of analytic study conducted so investigators can analyze information collected from ill persons and comparable well persons to see whether ill persons are more likely than people who did not get sick to have eaten a certain food or to report a particular exposure. Controls for a case-control study may be matched on geography to ensure that cases or ill persons and controls or well persons had the same opportunities for exposure to a contaminated food item. One method to geographically match is to use a reverse digit dialing protocol. A case address is entered into an online directory (such as www.whitepages.com), then a reverse address search is conducted to identify phone numbers for neighbors in that geographic area. Duplicate phone numbers and businesses are excluded.
If eating a particular food is reported more often by sick people than by well people, it may be associated with illness. Using statistical tests, the investigators can determine the strength of the association (i.e., how likely it is to have occurred by chance alone), and whether more than one food might be involved. Investigators look at many factors when interpreting results from these studies:
- 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 can provide useful information and help to support a hypothesis. Finding bacteria with the same DNA fingerprint in an unopened package of food and in the stool samples of people in the outbreak can be convincing evidence of a source of illness. However, relying on food testing can also lead to results that are confusing or unhelpful. This is the case for several reasons:
- Food items with a short shelf life, such as produce, are often no longer available by the time the outbreak is known, so they cannot be tested.
- Even if the actual 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.
Sometimes in testing hypotheses, investigators find no statistical association between the illnesses and any particular food. This is not unusual, even when all the clues clearly point to foodborne transmission. In fact, investigators identify a specific food as the source of illness in about half of the foodborne outbreaks reported to CDC.
Not finding a link between a specific food and illness can happen for several reasons. Public health officials may have learned of the outbreak so long after it occurred that they could not do a full investigation. There may have been competing priorities or not enough staff and other resources to do a full investigation. An initial investigation may not have led to a specific food hypothesis, so no analytic study was done or the initial hypothesis could have been wrong. An analytic study may have been done, but it did not find a specific food exposure because the number of illnesses to analyze was small, because multiple food items were contaminated, or because the food was a "stealth food." Stealth foods are those that people may eat but are unlikely to remember. Examples include garnishes, condiments on sandwiches, and ingredients that are part of a food item (e.g., the filling in a snack cracker). Food testing did not find any pathogen related to the outbreak, or food testing may not have been done at all.
When no statistical association is found, it does not mean that 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.