U.S. flag An official website of the United States government
  1. Home
  2. Food
  3. Compliance & Enforcement (Food)
  4. Sampling to Protect the Food Supply
  5. Microbiological Surveillance Sampling: FY21 Collection and Testing of Lettuce Grown in Salinas Valley, CA
  1. Sampling to Protect the Food Supply

Microbiological Surveillance Sampling: FY21 Collection and Testing of Lettuce Grown in Salinas Valley, CA

The FDA is conducting a sampling assignment to collect and test lettuce (iceberg, leaf and romaine) grown in Salinas Valley, California from commercial coolers that service the Salinas Valley area, from mid-May to the end of the harvest season in November 2021. All samples are being tested for E. coli O157:H7, a type of Shiga toxin-producing E.coli (STEC) and Salmonella spp. The FDA’s aim in conducting the assignment is to identify potentially contaminated lettuce and if detected, prevent it from entering commerce, when possible. This assignment is also part of the FDA’s Leafy Greens STEC Action Plan, an ongoing preventive effort to ensure the microbiological safety of leafy greens in the U.S.

In November and December 2019, three E. coli O157:H7 foodborne illness outbreaks resulted in 204 illnesses associated with the consumption of romaine lettuce or leafy greens sourced from the Salinas Valley. In response to these outbreaks, the FDA conducted an investigation of E. coli in romaine lettuce in Salinas, CA. Due to the multistate outbreaks of E. coli O157:H7 infections over the past three years that were linked to or potentially linked to the consumption of leafy greens grown in California, the agency designated this assignment as “mission critical.”

Salinas Valley and Lettuce

Annually, California and Arizona produce 98% the nation’s domestically grown leafy greens, including various types of lettuce.[1] Salinas Valley is known as “The Salad Bowl of the World” because it produces roughly 70% of the nation’s lettuce crops.[2] Over the last decade, the consumption of leafy greens, especially lettuce, has become associated with foodborne illness outbreaks caused by Salmonella enterica and STEC. Leafy greens are the second largest cause of food related infections with STEC in humans.[3] This large number of infections is linked to the popularity of lettuce in the American diet and the fact that lettuce is usually eaten without having undergone a ‘kill step,’ such as cooking, to reduce or eliminate bacteria.

Questions and Answers:

The FDA plans to collect 500 samples of iceberg, leaf, and romaine lettuce for this assignment. Each sample will consist of 10 subsamples, and each subsample will be made up of one or two heads of lettuce (trimmed, cored and possibly wrapped), or in the case of romaine lettuce, loose leaves or one package of hearts. The agency does not plan to collect baby leaf lettuce types such as spinach, arugula, radicchio, red oak leaf or other types of lettuce commonly found in spring mix.

The FDA will only collect samples from FDA-regulated commercial coolers that service the lettuce growers of Salinas Valley, CA. Sample collection at commercial coolers helps the FDA to efficiently obtain samples from multiple farms at centralized locations, typically before the distribution of the product, and facilitates prompt traceback and follow-up if contamination is detected.

The FDA plans to collect samples from May to November 2021, unless the season ends early. The agency plans to increase sample collection frequency during peak harvest season (August to October 2021).

The FDA will test all samples for E. coli O157:H7 and Salmonella spp. The agency will further analyze any confirmed STEC or Salmonella spp. isolates using whole genome sequencing (WGS). WGS can provide the FDA with important information, including serotype, pathogenicity, and possible linkage to past or ongoing outbreaks of foodborne illness.

Of note, at the start of the assignment the FDA was testing for other STEC strains in addition to E. coli O157:H7. However, as agency personnel monitored the assignment they became aware that 9% of samples were requiring further analysis for both STEC and Salmonella spp. to determine public health significance. We recognize that common industry practice is to destroy those lots of product, even if later they are confirmed negative once testing is completed, because the product is outside of its useful shelf life.

The FDA has been and continues to carefully monitor all assignment data and information, including on the destruction of product, to assess how we can minimize industry disruption or product loss while  continuing to protect public health.

While we have seen non-O157-related outbreaks in leafy greens, E. coli O157:H7 has been identified as the causative agent in the majority of outbreaks linked to this region in recent years and not all STEC strains are of public health significance.

Accordingly, we decided that, for now, we can address the risks of greatest concern by revising the assignment to test for Salmonella spp. and E. coli O157:H7, rather than the fuller range of STEC.  This change should reduce the number of samples requiring lengthier analysis to determine public health significance.

1. Agricultural Marketing Resource Center. Lettuce. 2018 [updated December 2018; cited 2019 April 2, 2019]; Available online at: https://www.agmrc.org/commodities-products/vegetables/lettuce.

2. USDA NASS (2017). Available online at http://www.nass.usda.gov/Statistics_by_State/California/index.asp.

3. Herman, K.M., A.J. Hall and L.H. Gould. 2015. Outbreaks attributed to fresh leafy vegetables, United States, 1973– 2012. Epidemiol Infect.143(14): 3011–3021.


Back to Top