Finding the Point of Contamination and Source of the Food

If a likely source is identified, investigators may also do an environmental assessment or evaluation to find out how the food was contaminated. The assessment could involve one food facility or several. If the people who got sick ate food prepared in only one kitchen, it is likely the contamination occurred in that kitchen. Investigators interview the people who prepared the food to find out the ingredients used, the steps followed in preparing the food, and the temperatures used to prepare and hold the food. They look at the health practices and training of the workers and at the cleanliness of the kitchen. They also check the health status of the workers at the time the exposures took place. In a commercial or institutional kitchen, they look at past inspection reports to see if there has been a history of problems.

If an outbreak is linked to a food prepared in a number of different kitchens (like hamburgers from many stores of the same chain) or to a food that was bought from many stores and eaten without further preparation (like peanut butter), it is likely that contamination happened somewhere in the food production chain before the final kitchen. In that case, investigators do a “source traceback” to find out where contamination occurred.

Tracebacks typically start from several ill persons or restaurants to see whether and where the food production chain comes to a common point. Finding this point helps to define where contamination occurred and can help to confirm the hypothesis. Investigators ask about suppliers of the suspect food item for stores, restaurants, or cafeterias where they believe the suspect food was bought or eaten. They then ask food suppliers where they received the suspect food item from, and so on. They study purchase and shipment information to find food items that are most closely associated with the illnesses.

These steps usually involve local or state environmental health specialists. For widespread or severe outbreaks, they often involve state environmental health specialists or other state public health officials and investigators from the Food and Drug Administration (FDA), U.S. Department of Agriculture (USDA), and CDC. Information from the environmental assessment and source traceback suggests ways to control the outbreak and prevent similar outbreaks from happening in the future.