How is a foodborne disease outbreak investigated?
Once an outbreak is strongly suspected, an investigation begins. A search is made for more cases among persons who may have been exposed. The symptoms and time of onset, and location of possible cases is determined, and a “case definition” is developed that describes these typical cases. The outbreak is systematically described by time, place, and person. A graph is drawn of the number of people who fell ill on each successive day to show pictorially when it occurred. A map of where the ill people live, work, or eat may be helpful to show where it occurred. Calculating the distribution of cases by age and sex shows who is affected. If the causative microbe is not known, samples of stool or blood are collected from ill people and sent to the public health laboratory to make the diagnosis. To identify the food or other source of the outbreak, the investigators first interview a few persons with the most typical cases about exposures they may have had in the few days before they got sick.
