A Closer Look at the CDC’s Report on Foodborne Illness in Leafy Greens

This article originally appeared on the Leafy Greens website.

In the March 2013 issue of Emerging Infectious Diseases, the United States Centers for Disease Control and Prevention (CDC) published a report attributing incidents of foodborne illness to specific food commodities using outbreak data from 1998-2008. The CDC organized all food products into 17 mutually-exclusive commodities to determine the prevalence of food-related illnesses for each commodity, based on an estimated 9.6 million illnesses annually. The 17 commodities were grouped into three larger categories: aquatic animals, land animals, and plants.

The CDC concluded that slightly over half (51%) of outbreak-related illnesses were attributable to plant commodities, with 42% and 6% being assigned to land animal and aquatic animal commodities, respectively. Produce (fruits-nuts and 5 vegetable commodities) accounted for 46% of illnesses, and meat-poultry products accounted for 22%. Importantly, the CDC concluded that “[a]mong the 17 commodities, more illnesses were associated with leafy vegetables (2.2 million [22%]) than any other commodity.” On the other hand, meat – particularly poultry – was responsible for the most deaths, with 43% of all deaths estimated to have come from land animals, and 25% from plant products.

On February 4, The New York Times featured a short recap of the study, entitled “Most Food Illnesses Come From Greens.” The article was coupled with an image of a spinach field, and in its first sentence proclaimed that “the most common cause of food-borne illness is leafy vegetables.” The CDC’s seemingly dire conclusions with respect to leafy greens, however, must be read in context: a closer examination of the CDC’s methodology reveals several assumptions that may have exaggerated the prevalence of foodborne illness in leafy greens.

First, the “plants” group included a wide range of food products, constituting 8 of the 17 mutually-exclusive commodities: grains-beans, oils-sugars, fruits-nuts, and 5 vegetable groups (fungi, leafy, root, sprout, and vine-stalk). The “produce” sub-group included the latter 6 commodities. Thus, although leafy greens accounted for 22% of all foodborne illnesses, they constituted less than half of both the plant- and produce-related illnesses (51% and 46% of all illnesses, respectively). The CDC also noted that the large number of norovirus illnesses was a “major driver” of the high percentages for produce products.

Second, the CDC’s report states that leafy vegetables frequently were found in complex foods, i.e., those food vehicles containing ingredients from one or more commodity. The CDC attempted to partition illnesses among the commodities in complex-food outbreaks based on each ingredient’s prevalence in single-commodity outbreaks. However, this methodology does not eliminate the possibility that some or even most of the complex food outbreaks could have been influenced (or caused) by cross-contamination from non-leafy-green ingredients.

Finally, the CDC’s report was based only on published data, and it is not clear how other information would have affected the results. For example, the CDC attempted to maximize the amount of outbreak data it considered but noted that “[t]o improve the quality and accuracy of outbreak attribution, models can be developed that include other types of data (e.g., studies of sporadic cases, isolates from foods and animals, agent subtypes).”

In sum, the CDC’s recent report perhaps over-emphasizes the prevalence of foodborne illness from leafy greens in several respects. These shortcomings, however, should not downplay the importance of proper food safety practices, from the farm to the dinner table. That is, with leafy greens in the proverbial spotlight, it becomes increasingly important for the industry not only to proactively implement proper food safety practices, but also to reach out to lawmakers, the media, and the public at large. Such efforts go a long way in dispelling common misconceptions of the leafy greens industry and educating consumers of important food handling practices that can, and should, be implemented in homes and restaurants. 

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