Researchers find that overhead irrigation is most likely cause of E. coli contamination of romaine

Foodborne illness outbreaks of E. coli O157 infections traced to romaine lettuce continue to be an ongoing public health concern. 

The most recent such outbreak in November and December 2024 saw 89 people across 15 states sickened. The outbreak hospitalized 36 people and killed one.

To better understand the problem, a group of scientists took a look at several factors, including irrigation water used in the production of romaine lettuce. The researchers’ review cited a large number of outbreaks, including seven between 2015 and 2021. These outbreaks resulted in 4,274 laboratory-confirmed illnesses, leading to 766 hospitalizations and 11 deaths.

The scientists considered a wide variety of sources of E. coli O157 contamination of romaine lettuce, but kept coming back to overhead irrigation as a problem. Other factors included runoff from feedlots and the lack of treatment for irrigation water.

“Harvest and postharvest risk factors may include human behavior, harvesting/production equipment, and production water,” according to the research report. “In addition, the lack of processing steps that ensure effective removal or inactivation of E. coli O157 before consumption, coupled with the recent increase in romaine consumption, further elevates the risk.”

The researchers focused their investigation on romaine lettuce grown in California and Arizona because 90 percent of the country’s romaine is grown in those states. The research model included three different irrigation systems: overhead spray, furrow and drip.

Nine risk management strategies for evaluation in the scenario analysis of the study were identified. These scenarios were vaccination of cattle in nearby feedlots; four scenarios involving treatment of surface water used for overhead spray irrigation, by either chlorine, peracetic acid or ultraviolet radiation; transition from overhead spray to furrow irrigation system; transition from overhead spray and furrow to drip irrigation; chlorine wash alternatives during postharvest processing; temperature reduction during retail; and consumer wash habits.

The highest E. coli O157 counts in romaine batches were predicted when the irrigation water was applied via an overhead spray system. 

“On the other hand, irrigation from furrow or drip systems led to substantially lower E. coli O157 counts in romaine batches,” according to the researchers.

“. . . Surface water treatments and transitioning to furrow or drip irrigation resulted in slightly lower but highly effective reductions in E. oli O157 counts at the preharvest stage, achieving between 90.5 percent and 96.8 percent reductions in median illness cases.

“. . . Irrigation emerged as the most significant source of E. coli O157 counts in fresh-cut romaine, particularly when considering the impact of other contamination sources. . .”

The researchers repeatedly found that contaminated irrigation water — particularly from overhead spray methods — was a key route for E. coli O157 contamination of romaine lettuce. They say that treatment of irrigation water from surface sources — such as open canals like those often used in much of the California and Arizona growing regions — is a key factor in controlling E. coli O157 contamination of romaine lettuce.

The data from the study suggested that transitioning from overhead spray irrigation to furrow or, ideally, drip irrigation systems could provide a viable solution for reducing the number of E. coli O157 illnesses from romaine lettuce. 

“Despite introducing numerous parameters specific to each irrigation system that reasonably differentiate between the three irrigation systems, our model suggested that the primary driver behind the higher number of illness cases associated with overhead spray irrigation was its capacity for increased direct contact between irrigation water and romaine,” according to the research report. 

“. . . In our study, transitioning fully from overhead spray and furrow irrigation to drip irrigation was predicted to be more effective than chlorination and UV treatments of surface irrigation water. . .”

The research study found that 52 percent of E. coli O157 illnesses caused by romaine lettuce are because of contamination through overhead spray irrigation. The scientists said that switching to drip or furrow irrigation systems would greatly reduce the contamination.

Research for the study was supported by a grant from the USDA’s National Institute of Food and Agriculture. 

Authors and Affiliations

• Ece Bulut, Sarah I. Murphy & Renata Ivanek from the Department of Population Medicine and Diagnostic Sciences, Cornell University
• Laura K. Strawn  from the Department of Food Science and Technology, Virginia Tech
• Michelle D. Danyluk from the Food Science and Human Nutrition Department, University of Florida
• Martin Wiedmann from the Department of Food Science, Cornell University

To read the full research report, click here.

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Source: https://www.foodsafetynews.com/2025/07/researchers-find-that-overhead-irrigation-is-most-likely-cause-of-e-coli-contamination-of-romaine/