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.
See detailed information on what to expect when the FDA collects samples.
We will be pre-announcing visits to cooling operations in light of the COVID-19 pandemic to help assure the safety of cooling operation employees and FDA investigators, and to improve the efficiency of sample collection. Additionally, FDA field staff will be equipped with personal protective equipment (PPE) and will adhere to local, state, and applicable CDC guidance to help protect their health and the health of firm employees. The FDA offered our investigators the opportunity to get the COVID-19 vaccination, on a voluntary basis, and is providing access to COVID-19 testing.
For the purposes of this assignment, the FDA considers a “lot” to be product from a day’s harvest, from the same land/field, and same harvest crew. While the agency recognizes that this definition may differ from industry’s definition, the FDA will use its own definition of a lot for consistency, and to support recall actions and traceback investigations, during the assignment.
We considered the option of using an independent third-party laboratory for this assignment. However, we decided to use FDA laboratories to conduct the testing primarily for two reasons.
First, it would be premature for the agency to make use of an independent laboratory for our Salinas Valley assignment while we are assessing what circumstances are most appropriate for using this kind of testing in assignments. Between February-March 2021, FDA utilized an independent laboratory as part of a pilot assignment in Yuma, AZ to explore how and when we might use this model in the future.
Second, we lacked an existing contract with an independent laboratory in the Central Coast of California region. The FDA is bound by the requirements of the Federal Acquisition Regulations. Arranging for a new contract at this time would have delayed the assignment beyond the initial months of the growing season.
The FDA recognizes that lettuce is perishable and therefore notifies the entity that is in possession of the product so it can take action, as appropriate, as soon as possible. In the case of this sampling assignment, cooling operation management is generally holding the product. The FDA understands that some firms may opt to hold product pending notification of test results.
We will notify the primary contact, as provided by the cooling operation management, of negatives and “cannot rule out” (CRO) samples within three days of sample collection, as we expect our initial testing and outreach to firms to be completed within 48 hours of the lab’s receipt of the samples. More specifically, if we collect a sample on a Monday, we expect to communicate the initial test results by Wednesday. Similarly, if we collect a sample on a Friday, we expect to communicate the initial test results by Sunday afternoon, or early evening.
With respect to the CRO samples – or, more precisely, to the time we need to determine whether they are positive for a target pathogen – generally it will take us an additional week to notify the firms of the final test results as we must microbiologically confirm the organism’s ability to cause disease. WGS analysis generally will take two weeks following the lab’s receipt of the sample(s).
If the FDA detects Salmonella spp. or pathogenic E. coli in a sample, the agency will notify the cooling operation management, the grower and the owner of the product by phone and email.
Initial follow-up activities will center on the removal of potentially contaminated food from the market, if necessary. Additionally, the FDA may conduct a follow-up investigation at the farm where the lettuce was grown, particularly if the detected bacteria is linked to an outbreak strain.
The FDA plans to publish the test results in the form of a summary report after sampling concludes and the final data are analyzed.
The FDA is sensitive to the concerns of industry and is working on multiple fronts to minimize disruption to businesses during this assignment, while still protecting public health:
- The FDA laboratories that test the samples are operating seven days a week.
- To assist with prompt notification, we ask that cooling operations management provide the FDA investigators up-to-date contact information (i.e., phone number and email address) for the primary contact and an alternate contact at the time of sample collection. We are also encouraging the product owners to ensure that the cooling operation management has the correct contact information to provide. The FDA notifies firms of all test results as soon as possible. Two-way communication between the agency and industry will help minimize any disruption to business associated with this sampling activity.
- To reduce the number of samples requiring lengthier analysis to determine public health significance we have revised the assignment to only test for E.coli O157:H7 and Salmonella spp.
- This change was made as we have been closely monitoring the sampling assignment and are aware that about 9% of samples have needed to be further analyzed for both STEC and Salmonella spp. to determine the public health significance.
- 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 Shiga toxin-producing E.coli are of public health significance.
- We recognize that common industry practice is to destroy product from lots undergoing further analysis, even if later they are confirmed negative once testing is completed, because the product is outside of its useful shelf life.
- Accordingly, we decided that, for now, we can address the risks of greatest concern by revising the assignment to test for E. coli O157:H7 and Salmonella spp, rather than the fuller range of STEC.
- This portion of the analysis will be completed within the same timeframe as previously reported, or within three days of sample collection, but will reduce the number of samples requiring a lengthier analysis to determine the public health significance of the STEC.
- The changes to the assignment were made and implemented during the week of July 26, 2021.
- The FDA may expand testing or consider other follow-up activities as appropriate to protect public health
- To improve the efficiency of sample collection we intend to provide advance notification to industry prior to visiting.
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.