This year's growing season has been dominated by news coverage of the salmonella food poisoning outbreak linked to tomato consumption. An announcement of the outbreak was first made by the Food and Drug Administration (FDA) in early June. The search has now expanded to include produce that is commonly served with tomatoes. As details of the outbreak have unfolded in the news, many members of the general public have asked questions about the contamination. Although the outbreak investigation by the FDA and the Centers for Disease Control and Prevention (CDC) is still ongoing, some comments can be made.
The answer to this question is not known at present. However, based on what we know about food safety in general and produce-borne food poisoning in particular, we can suggest some possibilities. Because a large number of people in a large number of states have been affected, it's likely that the contamination occurred in the field or in the packinghouse (rather than later in the process). Possible sources of contamination in the field or packinghouse could include use of contaminated irrigation or wash water (from a bacterially contaminated well or pond), use of improperly composted manure in the field, or handling of the produce by sick field or packinghouse workers. Scientific research has shown that when produce is contaminated, washing with water or treatment with sanitizers may not remove all the contamination.
A variety of factors come into play that can frustrate FDA and CDC efforts to investigate any outbreak. First, the nature of foodborne disease is that people may not experience their first symptoms until several days after consuming the implicated food. Once sick, people may not seek medical attention for several days. After medical treatment, identifying the microbiological cause from culturing fecal samples may also take several days or more. This means that a bacterial cause may not be known until a week or more after the food is eaten.
Once a common cause for two or more cases is known, it takes even more time for public health officials to interview all the sick individuals and try to determine which food or foods are the likely culprit. Once a food is implicated, a complex process of"traceback" begins, where the history of the implicated food is traced from the point of consumption back through the channels of distribution to its point of origin. Traceback is especially complex for produce items because these foods may pass from a farm to a packinghouse through several distributors and before reaching a final point of consumption. Add on top of that the fact that similar produce items from different farms or distributors may be blended at each step, or items from one distributor may pass to several customers, and this long chain grows into a complex web. One must also factor in that each stop along the way may have its own computer- or paper-based recordkeeping system, none of which are compatible, making it a wonder tracebacks ever happen at all! Because produce growing is highly seasonal, if a traceback does eventually implicate a particular farm, that farm is likely to have finished harvesting all of the implicated items, and may even be closed for the season, when investigators arrive.
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