Results of a testing program in February and March are now available and show that a sample of romaine lettuce from the Yuma, AZ, growing area was positive for E. coli O130:H11, which can cause serious infections in people.

The Food and Drug Administration conducted the two-month sampling program in response to a string of foodborne illness outbreaks linked to leafy greens, some specifically involving romaine lettuce, in recent years.

“Since 2009, the FDA and Centers for Disease Control and Prevention (CDC) identified 40 foodborne outbreaks of STEC (Shiga toxin-producing E. coli) infections in the U.S. with a confirmed or suspected link to leafy greens. Most recently, in 2018 and 2020 multistate outbreak investigations of E. coli O157:H7 infections were associated with or potentially associated with the consumption of romaine lettuce from the Yuma agricultural region,” according to the FDA’s report released today.

The testing program in February and March checked 504 samples of romaine lettuce collected from centrally located cooling facilities used to remove “field heat” from fresh produce before sending it for processing and packaging. The 504 samples represented more than tens of thousands of acres of romaine.

Investigators also tested the samples for Salmonella spp., but there were not any positive tests results for that pathogen. Contamination of romaine with Salmonella is also of concern because the United States experienced a multistate outbreak of Salmonella Newport infections linked to romaine lettuce consumption in 2013, according to the FDA report. 

The report did not discuss the proximity of the growing fields to large feedlots, which the FDA has previously reported could have contaminated romaine with E. Coli via dust and contaminated irrigation water in open trenches.

The goal of the testing program was to help determine whether E. Coli and Salmonella are present and what to do to control them. The program is part of the FDA’s Leafy Greens STEC Action Plan. The samples were collected at cooling operations in Yuma County, AZ, during the second half of the harvest season there. The focus on commercial coolers for sample collection enabled the agency to collect samples from multiple farms while minimizing travel.

“Romaine lettuce is grown low to the ground making it susceptible to contamination from irrigation water splashing off the soil, run-off from adjacent land, animal intrusion, and windblown dust, among other routes. In addition, this food is typically eaten without having undergone a ‘kill step,’ such as cooking, to reduce or eliminate bacteria,” the report states.

Only harvested romaine was sampled.

The sampling project involved special precautions because of the COVID-19 pandemic and included notifying growers before inspectors were on site. Also, the FDA used a nearby independent laboratory “to eliminate the need to ship samples to FDA’s own laboratories, reducing the time between sample collection and results reporting, and facilitating swift action by the agency in the event that a sample was to yield a pathogen.”

The use of the independent laboratory was a pilot project by the FDA, the objective of which was to quickly notify firms of negatives and “cannot rule out” (CRO) initial test results, generally within a benchmark of 24 hours of the laboratory’s receipt of the samples.

Findings and followup
Using whole-genome sequencing, investigators checked the strain of E. Coli O130:H11 that was found on the romaine sample and discovered it was not a match for any in the database of known human illnesses. However, the grower was notified and chose not to sell any romaine from the field where a positive sample was collected.

“Most of the romaine lettuce associated with the positive had been voluntarily held at the cooling operation. A number of pallets of the lettuce were released but were called back, and the product never reached consumers,” according to the report.

After finding the positive test result for the pathogenic E. coli, the FDA opened an investigation at the farm in question to identify possible sources and routes of contamination. At the time of the investigation, romaine lettuce remained in the field, which allowed the agency to collect romaine from various locations within the field. The FDA also collected multiple samples of soil, water, sediment, and animal fecal material, and assessed surface contamination using drag swabs. 

One sample collected during the follow-up investigation, which came from the outer leaves of romaine lettuce, yielded STEC (specifically, E. coli O116:H-). The strain was characterized as low risk to human health and was not associated with any clinical illnesses upon WGS analysis.

“The findings of this assignment may suggest that STEC and Salmonella spp. were not widespread in romaine lettuce grown in the Yuma agricultural region during this time period, even though such contamination continues to pose a public health concern given the multistate outbreaks of foodborne illness that have been linked to or potentially linked to romaine lettuce in recent years. Importantly, even a low level of either pathogen can result in an outbreak of foodborne illness,” the FDA investigators reported. 

As part of the action plan, the FDA has launched two multi-year research studies that seek to shed light on the cause(s) of foodborne illness outbreaks linked to romaine lettuce produced in California and Arizona. The studies are designed to increase understanding of how human pathogens survive, move, and possibly contaminate produce prior to harvest.  

Research teams collect and examine environmental samples, including from adjacent land, well and surface waters, and soil inputs that may contain compost, dust, and animal fecal material. Research teams also collect scat samples to assess the impact that animal intrusion and native wildlife may have on the growing environment. The studies are being conducted in partnership with state and local governments and universities. 

“We anticipate that study results will lead to improved practices to prevent or mitigate food safety risks, and ultimately enhance the safety of produce grown in the region,” FDA officials said in today’s report.

Yuma is referred to as the nation’s “winter salad bowl” because most of the domestically grown leafy greens and other vegetables eaten by U.S. consumers in the winter and early spring, including romaine lettuce, are grown in the region. California’s Imperial Valley growing region also produces a large portion of the leafy greens eaten in the U.S. during the same period.

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