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  1. Sampling to Protect the Food Supply

Microbiological Surveillance Sampling: FY19 Romaine Lettuce at Commercial Coolers in Yuma, Arizona

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The U.S. Food and Drug Administration conducted an assignment to collect romaine lettuce samples and test them for Shiga toxin-producing Escherichia coli (STEC) and Salmonella spp. following the spring 2018 multistate E. coli O157:H7 outbreak linked to romaine lettuce from the Yuma, Arizona growing region. The agency implemented this sampling assignment to help determine whether romaine lettuce introduced into commerce from the Yuma region continued to pose a public health concern.

The FDA began its sampling of romaine lettuce on December 17, 2018, shortly after the start of the winter harvest season in Yuma. The FDA, Centers for Disease Control and Prevention and state officials identified romaine lettuce from Yuma as the food vehicle involved in the multistate STEC outbreak based on epidemiologic, laboratory and traceback evidence. The outbreak began at the end of the prior winter harvest season in Yuma (in March); it involved 210 people in 36 states, and resulted in 96 hospitalizations, 27 cases of hemolytic uremic syndrome (a type of kidney failure), and five deaths. The outbreak appeared to be over as of June 28, 2018, as reported by the CDC.

The objectives of the FDA’s assignment were to determine if the target pathogens were present in the romaine lettuce sampled, and to respond swiftly in the event of detections to keep contaminated product from reaching consumers.

The FDA concentrated its sampling in the Yuma area for two reasons. An environmental assessment conducted by the FDA and CDC concluded that water from an irrigation canal in Yuma County most likely led to contamination of romaine lettuce eaten during the spring 2018 E. coli outbreak. The investigations and traceback by the federal agencies and state partners at the time implicated one farm but also indicated that other farms in Yuma may have contributed to the outbreak as well. Additionally, there have been five suspected or confirmed multistate outbreaks of foodborne illness linked to produce harvested in the Yuma growing region since 2012.[1]

The FDA’s assignment focused on commercial cooler and cold storage facilities where field heat is removed from harvested romaine and product is cold stored before shipment. The focus on these sites enabled the FDA to efficiently collect samples from multiple farms at centralized locations. This approach also would facilitate traceback because the FDA would know the identity of the farm associated with each sample. All samples were collected prior to handling by workers at the cooling and cold storage facilities, with many of the facilities opting to hold the product pending notification of test results. In all, agency field staff visited 26 cooling and cold storage facilities in Yuma, with the assistance of the Arizona Department of Agriculture, and the Arizona Department of Health Services. The 26 sites included most of the commercial cooler and cold storage facilities in Yuma registered with or known to the FDA.

The FDA collected and tested 118 samples for each pathogen, with STEC and Salmonella spp. selected because they are the two pathogens most closely associated with leafy vegetables.[2], [3] Each sample consisted of 10 subsamples, and each subsample consisted of one head or two hearts (i.e., the center leaves) of romaine. The FDA did not detect any STEC of concern to human health, though it did find one non-pathogenic STEC strain. The FDA did not detect Salmonella spp. in any of the samples.

Following last spring’s outbreak, and prior to conducting this assignment, the FDA sent a letter to the Arizona and California departments of agriculture, and to the leafy greens industry in those states, calling on all stakeholders to strengthen their efforts, each in keeping with its role, to help ensure the safety of romaine.

The FDA’s letter detailed a series of recommendations to industry. The agency urged all segments of the leafy green industry to review their operations, policies, and practices in light of the findings of the environmental assessment and to make modifications as warranted. Specific recommendations included the use of agricultural water that is safe and adequate for its intended purpose, the mitigation of risks related to nearby land use, and the adoption of traceability best practices to facilitate rapid response in the event of an outbreak or recall. The full list of recommendations is in the letter.

The findings of this assignment suggest that there was no widespread Salmonella or STEC contamination of romaine lettuce from the Yuma growing region during the period when sampling occurred. As a next step, the FDA is working with leafy green stakeholders in the Yuma region to consider a longer-term environmental study to identify and control risks that will prevent future outbreaks, with the ultimate goal of protecting consumers.


[1] Food and Drug Administration. (2019). Outbreaks of Foodborne Illness Suspected or Confirmed to be Linked to FDA-Regulated Produce Harvested in the Yuma Growing Region, 2012 to Present (i.e., June 6, 2019). Reference Type: Query Result of CORE Outbreak Database.

[2] Turner, K., Moua, C.N., Hajmeer, M., Barnes, A., & Needham, M. (2019). Overview of Leafy Greens–Related Food Safety Incidents with a California Link: 1996 to 2016. Journal of Food Protection, Vol. 82, No. 3, pages 405 to 414.

[3] Herman, K.M., Hall, A.J., & Gould, L.H. (2015). Outbreaks attributed to fresh leafy vegetables, United States, 1973–2012. Epidemiology & Infection, Vol. 143, No. 14, pages 3,011 to 3,021.