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Lettuce harvested in Watsonville Tim Hearden
Lettuce is harvested on California’s Central Coast in August 2019. A pair of E. coli outbreaks linked to romaine lettuce in 2018 sickened hundreds.

Researchers track causes of 2018 E. coli outbreak

Food detectives looking for origin, distribution channels of the strain that contaminated romaine lettuce in the Yuma area

E. coli bacteria are kind of like those family relatives you really don’t like.  You can’t get rid of them because they’re found living, innocuously enough, in people’s intestines where they are generally harmless.

But found in leafy greens that were contaminated in the field, they can cause all kinds of health havoc --- witness the outbreak in Yuma, Ariz., romaine lettuce that made Shiga toxin-producing Escherichia coli O157:H7 a bad actor in 2018.

Before all was said and done, the outbreak resulted in over 200 reported illnesses in 36 states along with nearly 100 hospitalizations and five fatalities. Across that sector of the food industry, from growers to servers, millions of dollars in revenue were lost as Romaine prices fell by more than 50 percent and farmers plowed under their fields or left them to rot.

Another E. coli outbreak linked to romaine occurred just before Thanksgiving in 2018, sickening 62 people in 16 states and prompting stores and restaurants across the country to agree to temporarily stop selling the lettuce. That outbreak was linked to farms on California’s Central Coast.

Based on the Yuma outbreak and the resultant U.S. Food and Drug Administration/Arizona Department of Agriculture environmental assessment that determined the scope of the flare-up, food detectives are now on the case to further uncover the specific origin, environmental distribution, and potential reservoirs of the outbreak strain that remain unknown.


FDA, the Arizona Department of Agriculture, University of Arizona Cooperative Extension scientists, growers in Yuma and the Wellton Mohawk Irrigation and Drainage District are involved in a multi-year study to look at the environment and growing practices in the Yuma produce growing region.

That so-called Winter Leafy Greens Capitol of the World encompasses Yuma County in Arizona, the adjacent cross border area in Mexico, and the Imperial Valley in California.

Although the last year’s environmental assessment provided useful information, the source of the E. coli and how widely it was distributed remain unknown and as another growing season approaches, solving that mystery is still a priority.

Microbial contamination remains a prime suspect. 

“We’ll be collecting surface water, canal sediment, dust, and scat samples from animal intrusion to determine their impact on the growing environment,” said Dr. Channah Rock, University of Arizona Cooperative Extension Specialist in describing the 2-4 year research project funded with several hundred thousand FDA grant dollars.

“While the production of food safe for human consumption is paramount to growers in the Yuma Area Leafy Greens Industry, outbreaks still occur.  We’re working side-by-side to find solutions and enhance food safety and protect public health.  Our findings will be used to provide recommendations to Yuma growers toward best management practices to enhance their regional food safety efforts.”


Lettuce is among the most popular produce items in supermarkets, and because it’s typically eaten without cooking, E. coli contamination can occur at any point from farm to table, food safety experts say.

Leafy greens can become contaminated in the field by a variety of means, through soil, contaminated water, improperly composted manure, or animal feces.  Lettuce can also be contaminated by bacteria during and after harvest via handling, storing, and transporting the produce, the experts say.

“The work we’re doing will specifically evaluate the Yuma growing area, but we’re hopeful some of our findings will be applicable to other regions where soil composition and water input might be somewhat similar.  There could be lessons learned here that would be applicable,” said Rock.

Field personnel will be collecting air and soil samples, irrigation water, and sediment in the initial phase while other researchers survey wildlife in the growing region, collecting scat samples as part of the investigation

“What we’re hoping to learn from this study will paint a broad brush, a large umbrella that will cover multiple potential avenues of how contamination could occur,” Rock said. “We’re diving deeper than specific outbreak investigations, obtaining sufficient samples needed to statistically say that something might be valid or not.  Our efforts will allow us to hone in and answer multiple questions based on a greater understanding of the field environment translated into best management practices for growers.”


The job of learning about agricultural field production in relation to food safety is ongoing, paving the way for future investigations. 

“Folks in the governmental realm, the FDA and the CDC, learn from the experiences of folks on the ground and the more they learn from each other about the realities of what does and does not happen, that benefits food safety efforts in the long run,” Rock says.

Although not a plant physiologist, she noted that lettuce and leafy greens were often the bad guys in E. coli outbreaks because number one, they are the most popular produce items, and number two, they are generally eaten without being cooked.

“In most cases, this produce is grown in the open environment and while you can control for some things, you can’t control for everything.  That’s the challenge from a grower’s perspective is to manage the things they can and reduce the risks where it makes the most sense, trying to control as best they can.

“This environmental monitoring work is a challenging matrix to look at.  Oftentimes when you’re looking at a specific outbreak, it could be a perfect storm that you can’t replicate again.   We’re not conducting needle-in-a-haystack work, we’re looking at trends over time and evaluating prevailing practices.

“We’re hopeful our research can shed further light on what occurred previously and at a minimum, we anticipate our findings should be able to shed light on ways to improve safety moving forward.”

One of the project partners is the Wellton Mohawk Irrigation District where General Manager Elston Grubaugh says:  “We operate a 68-mile-long open canal system that is what it is, but our goal is to support our growers and protect our food supply, so we’d like to learn if there are additional steps we can take to protect our system and promote a safer growing environment.”

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