June 21, 2024

Investigation of Leafy Greens E. coli O157:H7 Outbreak in Fall 2020

The FDA has released an investigation report into factors that may have contributed to the leafy greens E. coli O157:H7 outbreak in fall 2020 that sickened at least 40 people in 19 states across the country. The outbreak was linked via whole genome sequencing and geography to outbreaks in 2019 and 2018 that were associated to the California growing region.; the proximity of cattle is a likely contributing factor.

Investigation of Leafy Greens E. coli O157:H7 Outbreak in Fall 2020

The outbreak caused 20 hospitalizations. Four people developed hemolytic uremic syndrome (HUS), a complication of an E. coli infection that is a type of kidney failure. The illness onset dates ranged from August 10 through October 31, 2020. Leafy greens were declared the likely source of this outbreak after interviews with patients.

The E. coli O157:H7 outbreak strain was found in a cattle feces composite sample taken alongside a road about 1.3 miles upslope from a produce farm with multiple fields tied to the outbreaks by traceback. The FDA also found several potential contributing factors to the 2020 leafy greens outbreak.

Isolates within this cluster of illnesses are part of a recurring strain and are associated with outbreaks that have contaminated leafy greens every fall since 2017. Clinical isolates from cases in the 2020 outbreak are closely related to those from the 2019 outbreak. Several more isolates from food and the environment include a fecal-soil composite collected by FDA investigators in February 2020 from the Salinas growing region, and two leafy green samples collected in 2019 that were also tried back to the Salinas region.

The FDA has identified key trends regarding the issues of a reoccurring strain, a reoccurring region, and reoccurring issues around adjacent and nearby land use. One of the problems with this investigation is that it occurred near the end of the growing season, which means that “Due to the timing of the three investigations, each near the end of or after the growing and harvest season had concluded, investigators were largely unable to observe product in the field or harvest conditions and in many cases could not observe any activities. This severely limited the investigators’ ability to observe practices and factors that may have contributed to contamination, necessitating increased reliance on sample results during the investigations.”

The investigations were not able to confirm the direct source of route of contamination, but in all cases, they noticed lands adjacent and upslope to farms and ranches of interest were noted as “potential concerns.” Those lands included “year-round cattle grazing; other produce farming activities; and uncultivated land, which could serve as wildlife habitats during the growing and harvest season; all occurred near many of the fields investigated. Additionally, evidence of animal activity (including cattle, deer, feral pigs, rodents, coyotes, and birds) was noted on the hillsides of adjacent lands during the investigations.”

Visual observations of the growing fields suggested several plausible routes for contamination, including from cattle grazing on adjacent land and from animal intrusion. Cattle are a natural reservoir for E. coli bacteria; FDA’s leading hypothesis is that cattle are the most likely sources of outbreak strains of E. coli O157:H7 associated with the 2019 and 2020 leafy greens outbreaks.

As a result of this investigation of the leafy greens E. coli O157:H7 outbreak  the FDA is encouraging collaboration between the agricultural community and academic and government partners to identify and implement measures to prevent this contamination. Among the long list of recommendations, the government says that this particular strain of E. coli O157:H7 should be considered a reasonably foreseeable hazard, so farms must take all measures “reasonably necessary” to identify and not harvest covered produce that is “reasonably likely” to be contaminated, and to handle harvested product during covered activities in a manner that protects against contamination.

In addition, leafy greens producers must emphasize efforts around prevention, improve traceability, improve industry-led provenance labeling, and conduct a root cause analysis when a foodborne pathogen is identified in the growing environment, in agricultural inputs, in raw commodities, or in fresh-cut ready-to-eat produce.

The FDA is also calling upon livestock owners to consider implementing monitoring programs for human pathogenic E. coli and to share this information with leafy greens growers. The FDA will release an update to the 2020 Leafy Greens STEC Action Plan.

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