Why Does Lettuce Cause So Many Foodborne Illness Outbreaks?

Why Does Lettuce Cause So Many Foodborne Illness Outbreaks?

Why Does Lettuce Cause So Many Foodborne Illness Outbreaks?

By Robert Carlson, Grow Space Vertical Farms | March 2026

Photo by ClickerHappy

Lettuce is one of the most recalled foods in the United States. Between 2009 and 2018 alone, researchers identified 40 outbreaks of Shiga toxin-producing E. coli linked to leafy greens in the US and Canada. Those outbreaks resulted in 1,212 reported illnesses, 420 hospitalizations, 77 cases of kidney failure, and 8 deaths.

And those numbers only reflect the cases that were reported. The CDC estimates that for every confirmed case of E. coli, many more go undiagnosed.

Leafy greens account for roughly 9% of all foodborne illnesses tied to identified pathogens in the US. When you include illnesses from unknown sources, researchers estimate that leafy greens may be responsible for over 2 million illnesses per year, with an annual economic cost of $5.28 billion. Within that category, lettuce — romaine, iceberg, and other varieties — makes up more than 75% of leafy green-related foodborne illness.

As someone who grows lettuce for a living, these numbers are personal. I've spent the last few months digging into the research behind these outbreaks, and what I've found has shaped how I think about the systems we've built at Grow Space.

The Two Main Culprits: E. coli and Listeria

Almost every major lettuce-related outbreak traces back to one of two pathogens: E. coli (specifically Shiga toxin-producing strains like O157:H7) and Listeria monocytogenes. These aren't random. The conditions that allow them to contaminate lettuce are built into the way most lettuce is grown in the United States.

How E. coli Gets on Lettuce

E. coli O157:H7 is an animal-based pathogen. It lives in the intestines of cattle and other ruminants without making the animals sick. The bacteria is shed through their feces — and that's where the problem begins.

Over 90% of America's romaine lettuce is grown in just two regions: Yuma, Arizona (winter) and the Salinas Valley in California (summer and fall). Both regions have large-scale cattle operations located near lettuce fields. The contamination pathway has been well-documented by the FDA and independent researchers:

  • Contaminated irrigation water. Irrigation canals that supply lettuce fields can run directly through or near cattle feeding operations. Cattle feces washes into those canals through runoff, and the contaminated water is then used to irrigate the crops. The FDA found the exact outbreak strain of E. coli in an irrigation canal near a large cattle feedlot in Yuma County during the 2018 investigation.
  • Airborne dust. Particulate matter from feedlot manure can drift onto nearby fields and settle directly on crops. Dust can carry bacteria significant distances.
  • Overhead spray irrigation. Research from Cornell University found that roughly 52% of E. coli O157 illnesses from romaine lettuce are attributable to overhead spray irrigation, which maximizes contact between contaminated water and the edible plant surfaces.

What makes this particularly dangerous is that E. coli can actually enter the lettuce plant through the root system and travel into the internal tissue of the leaves. Once inside, no amount of washing or chlorine rinsing can remove it. Researchers have demonstrated that standard industry chlorine washes (100–200 ppm) are only marginally effective at reducing E. coli on lettuce surfaces — and completely ineffective against bacteria that has been internalized.

How Listeria Gets on Lettuce

Listeria monocytogenes follows a different path. Unlike E. coli, Listeria contamination typically doesn't happen in the field. It happens during processing and packaging.

Listeria thrives in cool, dark, moist environments — the same conditions that are ideal for storing and preserving leafy greens. Packing facilities, cold storage areas, conveyor belts, and cutting equipment all create environments where Listeria can colonize and persist. The bacteria forms biofilms on surfaces, making it difficult to eliminate through routine cleaning.

Because most lettuce passes through multiple human touch points between harvest and the consumer's plate — cutting, washing, sorting, packaging, shipping, stocking — each step introduces another opportunity for contamination. Leafy greens become, as food safety researchers have described it, a vehicle for spreading the infection.

A Pattern That Keeps Repeating

This isn't ancient history. Major lettuce-related E. coli outbreaks have occurred with alarming regularity:

  • Spring 2018 (Yuma, AZ): 210 people sickened across 36 states, 96 hospitalized, 27 cases of kidney failure, 5 deaths. Linked to romaine lettuce from multiple farms in Yuma. The FDA traced contamination to irrigation water near a cattle feeding operation. This was the largest multistate E. coli outbreak linked to leafy greens in decades.
  • Fall 2018 (Santa Maria, CA): 62 people sickened, 25 hospitalized, 2 cases of kidney failure. Linked to romaine from the Central Coast of California.
  • Fall 2019 (Salinas, CA): 167 people sickened across 27 states, 85 hospitalized, 15 cases of kidney failure. Linked to romaine from Salinas Valley. Cattle feces found on public land upslope from the implicated fields.
  • Fall 2020: 40 people sickened across 19 states, linked to leafy greens including romaine. The same recurring E. coli strain from previous years was identified.
  • Late 2024: 89 people sickened across 15 states, 36 hospitalized, 7 cases of kidney failure, 1 death. Linked to romaine lettuce from a major supplier. The CDC and FDA closed the investigation without publicly identifying the source.

A single persistent strain of E. coli O157:H7 (known as REPEXH01 in surveillance databases) has been linked to multiple outbreaks since 2016. As of early 2024, over 760 people across 46 states had been infected with this one strain alone.

Why This Keeps Happening

The uncomfortable truth is that the fundamental conditions causing these outbreaks haven't changed. Lettuce is still grown in open fields. Those fields are still located near cattle operations. The same irrigation infrastructure is still in use. And lettuce is still consumed raw, meaning there's no cooking step to kill bacteria before it reaches your plate.

The industry has made some improvements. California and Arizona established leafy greens marketing agreements with updated food safety recommendations. The FDA's Food Safety Modernization Act created new produce safety standards. Some growers have shifted from overhead spray to drip irrigation.

But the core structural problem remains: when you grow food outdoors, in soil, near animals, using open-air irrigation, you cannot fully control what ends up on your produce. You can reduce risk. You can test more frequently. But you cannot eliminate the exposure.

The Vertical Farming Difference

This is exactly why we built Grow Space the way we did.

Indoor vertical farms operate in a fundamentally different environment than outdoor field agriculture. At Grow Space, our lettuce never touches soil. It's never exposed to animal waste, contaminated irrigation water, or airborne dust from nearby feedlots. We don't use pesticides, because we don't have pest pressure. Our water is clean, recirculated, and monitored.

The track record reflects this. While outdoor field-grown lettuce has been linked to dozens of outbreaks and thousands of illnesses over the past 15 years, the indoor vertical farming industry has an overwhelmingly clean food safety record. The controlled environment eliminates the primary contamination pathways — animal proximity, contaminated water, and environmental exposure — that drive nearly every major lettuce outbreak.

That said, no food system is completely immune to risk. In 2021, an indoor greenhouse grower experienced a Salmonella recall traced to environmental contamination at their facility. The incident underscored that indoor growing is not automatically safer — it has to be designed and operated correctly. As one industry expert put it, vertical farms can be much safer than conventional farms, but only if food safety is built into the system from the ground up.

At Grow Space, food safety isn't an afterthought. It's the reason we exist. Our NFT hydroponic systems are designed to minimize contamination vectors. We grow locally, which means our lettuce travels miles instead of thousands of miles before it reaches you. Fewer hands touch it. Fewer days pass between harvest and your plate. And we operate with full transparency — you can see exactly where and how your food is grown.

What This Means for You

You shouldn't have to worry about whether your salad is going to make you sick. But the reality is that the conventional lettuce supply chain has a structural problem that band-aid solutions haven't fixed.

The good news is that you have a choice. Buying from local indoor growers — whether that's Grow Space or another controlled-environment farm in your area — significantly reduces your exposure to the contamination pathways that cause these outbreaks.

We're not here to scare you away from eating lettuce. Lettuce is one of the most nutrient-dense, fiber-rich, and accessible vegetables you can eat. We're here to make sure you can eat it with confidence.

Sources

  1. Marshall KE, et al. "Lessons Learned from a Decade of Investigations of Shiga Toxin–Producing Escherichia coli Outbreaks Linked to Leafy Greens, United States and Canada." Emerging Infectious Diseases, 2020;26(10):2319–28. PMC7510726

  2. Yang X, Scharff RL. "Foodborne Illnesses from Leafy Greens in the United States: Attribution, Burden, and Cost." Journal of Food Protection, 2024. ScienceDirect

  3. CDC. "Summary of Possible Multistate Enteric Disease Outbreaks, 2017–2020." Centers for Disease Control and Prevention. CDC.gov

  4. Wirth JS, et al. "Genomic Characterization of Escherichia coli O157:H7 Associated with Multiple Sources, United States." Emerging Infectious Diseases, 2025;31(13). CDC EID Journal

  5. Solomon EB, et al. "Transmission of Escherichia coli O157:H7 from Contaminated Manure and Irrigation Water to Lettuce Plant Tissue and Its Subsequent Internalization." Applied and Environmental Microbiology, 2002;68(1):397–400. PMC126537

  6. Environmental Working Group. "E. coli from Factory Farms Threatens America's Leafy Greens." January 2024. EWG.org

  7. FDA. "Outbreak Investigation of Salmonella Typhimurium: BrightFarms Packaged Salad Greens (July 2021)." U.S. Food and Drug Administration. FDA.gov

  8. Bottichio L, et al. "Shiga Toxin–Producing Escherichia coli Infections Associated with Romaine Lettuce — United States, 2018." Clinical Infectious Diseases, 2020;71:e323–30.

  9. FDA. "Investigation Report: Factors Potentially Contributing to the Contamination of Romaine Lettuce from the Salinas Valley Growing Region." 2020. FDA.gov

  10. Agritecture. "Are Vertical Farms Truly the Answer to Food Safety Concerns?" 2022. Agritecture.com

  11. Food Safety News. "Safety Aspects of Indoor Farming Signal a Change in Agriculture." February 2020. FoodSafetyNews.com

  12. Bulut E, et al. "Quantitative microbial risk assessment for Escherichia coli O157 contamination of romaine lettuce." Cornell University / Food Safety News, 2025. FoodSafetyNews.com

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