Thank goodness for Science: Risk assessment of E. coli O157:H7 along the farm-to-fork fresh-cut romaine lettuce supply chain

It was a beautiful day today here in the Pacific Northwest, but instead of enjoying the weather, I was enjoying scientists trying hard to prevent the next romaine lettuce E. coli outbreak.

Abstract

The reality:

• Outbreaks of Escherichia coli O157:H7 (ECO157) in romaine lettuce remain an ongoing public health concern.
• The importance of managing irrigation-related contamination at preharvest to control illness cases. 
• Wildlife intrusions pose lower health risk, followed by runoffs and biological soil amendments of animal origin. 
• When preharvest contamination persists and combines with time–temperature abuses at postharvest, the predicted ECO157 illness cases rise considerably. 

Bulut, E., Murphy, S.I., Strawn, L.K. et al. Risk assessment of Escherichia coli O157:H7 along the farm-to-fork fresh-cut romaine lettuce supply chain. Sci Rep 15, 17421 (2025). https://doi.org/10.1038/s41598-025-01585-z

Introduction

The number of detected foodborne illness outbreaks linked to leafy greens has increased in the United States (USA) between 1996 and 2016, with Escherichia coli O157:H7 (ECO157)-contaminated romaine lettuce emerging as the leading source, followed by other common serotypes such as ECO26 and ECO111. Between 2015 and 2021, ECO157-contaminated romaine has been implicated in 7 outbreaks in the USA, 6 of which were multistate incidents. These outbreaks resulted in 4,274 laboratory-confirmed illnesses, leading to 766 hospitalizations and 11 deaths.

Contamination of leafy greens, including romaine, can occur at any point during preharvest, harvest and postharvest, involving various risk factors. Preharvest risk factors may involve domestic animals in the fields, wild animals, biological soil amendments of animal origin (BSAAO) amended soil, irrigation water and runoff from animal operations. Harvest and postharvest risk factors may include human behavior, harvesting/production equipment and production water. In addition, the lack of processing steps that ensure effective removal or inactivation of ECO157 before consumption,coupled with the recent increase in romaine consumption, further elevates the risk.

Discussion

Irrigation emerged as the most significant source of ECO157 counts in fresh-cut romaine, particularly when considering the impact of other contamination sources: inadequately treated BSAAO, runoff, and wildlife intrusion. However, the public health impact could be mitigated through effective interventions. Our predictions also indicated a high number of illness cases due to the combined effects of preharvest and postharvest factors. Additionally, we identified key knowledge gaps in assessing human ECO157 exposure through contaminated leafy greens.

For the romaine outbreak in 2018, ECO157 introduction into the irrigation water through aerial and land-based spray applications of crop protection chemicals was proposed as a plausible route of introduction. On the other hand, the spinach and iceberg lettuce outbreaks appeared to have been facilitated by cross-contamination, where contaminated surface water and wastewater flowed into groundwater or surface water used for irrigating the lettuce plants.

Conclusion

The analysis suggests that illnesses occur mainly (52%) due to ECO157 contamination originating from untreated irrigation water applied through overhead spray irrigation. Data further suggests that preharvest ECO157 risk, including that from irrigation, can be reduced either through water treatments or by switching to furrow or drip irrigation. In postharvest, maintaining the cold chain is crucial to prevent high numbers of illness cases. While data indicates that an effective postharvest wash can help mitigate the public heath impact of both preharvest and postharvest contamination, future work on the impact of postharvest wash variability on public health risk is important.

Thank you.

Republished with permission from Bill Marler and Marler Clark. Copyright (c) Marler Clark LLP, PS. All rights reserved.

Source: https://www.marlerblog.com/case-news/thank-goodness-for-science-risk-assessment-of-escherichia-coli-o157h7-along-the-farm-to-fork-fresh-cut-romaine-lettuce-supply-chain/