In the late summer of 2022, the United Kingdom faced a bizarre health threat: a large outbreak of Shiga-toxin-producing Escherichia coli (STEC) O157. The outbreak affected 259 people across the country, hospitalizing dozens. Given that the country typically gets only a few cases per month, authorities declared a national outbreak and created a task force to manage it.

The outbreak eventually subsided and the culprit was found to be contaminated lettuce. But how did this pathogen, typically carried by animals like cattle and sheep, find its way to the food supply through lettuce? According to a new investigation, a series of cascading climate-driven events created the perfect conditions for the outbreak.

Finding the culprit

E. coli is a common infection. However, STEC O157 is a potent strain of E. coli that can cause severe gastrointestinal illness in humans. Symptoms range from mild diarrhea to life-threatening complications such as hemolytic uremic syndrome (HUS), which affects the kidneys. It is one of the most virulent foodborne pathogens, commonly linked to contaminated meat, dairy, and fresh produce. This bacterial strain is special because it produces Shiga toxin, which can be devastating.

Many of those affected had recently consumed salad items, particularly lettuce, leading investigators to suspect fresh produce as the vehicle for infection. This hypothesis was supported by food chain analysis, which traced the lettuce back to a single grower referred to in the report as Grower X.

But that doesn’t explain how the lettuce got tainted. It took two years for the investigation to come out.

Heavy rainfall followed by flooding likely transported fecal matter from livestock fields into nearby agricultural areas. Lettuce, grown in these fields, was exposed to contaminated water, setting the stage for the STEC bacteria to make its way onto the plates of unsuspecting consumers.

How climate change plays into this

The summer of 2022 saw extreme weather conditions in the UK, with long periods of drought followed by intense rainfall. The UK Met Office recorded over 100 mm of rain in a single 24-hour period in August. And some regions experienced even higher totals.

This sudden influx of water caused localized flooding, particularly in agricultural areas where livestock were grazing. The investigation revealed that lettuce fields were exposed to standing water after these floods, which likely carried fecal contaminants from nearby sheep farms.

Sheep are known carriers of STEC O157, and their proximity to lettuce fields created a high-risk scenario for contamination. Lettuce itself is not inherently a high-risk food. But the circumstances of lettuce cultivation in this case — growing in fields exposed to floodwaters carrying animal feces — made it a dangerous vehicle for disease transmission.

So, in summary, a series of events created this outbreak, with climate change catalyzing it.

“Investigations determined how a series of adverse weather events in July and August 2022 likely led to crop contamination. The prolonged periods of extremely dry weather and heavy rainfall that followed likely contributed to the amplification and spread of bacteria through the environment, with standing water and flooding ultimately leading to crop contamination,” states the U.K. Health Security Agency.

The many effects of climate change

We often hear about the direct effects of climate change (like extreme weather), but this global phenomenon affects us in far more ways than we imagine.

This outbreak of STEC O157 is part of a growing trend of foodborne illnesses associated with fresh produce. The complex supply chains involved in modern agriculture, combined with the increasing frequency of extreme weather events due to climate change, are creating new risks for food safety. The UK’s experience with this outbreak is a good example.

The World Health Organization (WHO) has warned that rising temperatures and shifting weather patterns are likely to increase the frequency and severity of foodborne disease outbreaks worldwide. In particular, heavy rainfall and flooding are known to elevate the risk of contamination. Pathogens from animal waste flow with rainwater into water supplies and agricultural fields.

Yet this research also shows that we’re not defenseless against such outbreaks.

In this case, geospatial analysis proved to be a valuable tool in identifying the source of the outbreak. The UKHSA used land-use data, flood risk maps, and animal density information to enable investigators to pinpoint Grower X as the likely culprit. And they achieved this even in the absence of direct microbiological evidence from the lettuce itself. This approach could serve as a model for future investigations, as climate-related risks continue to escalate.

Ideally, though, we’d try to prevent such outbreaks rather than trace them.

With climate change causing more and more challenges, the food industry will need to adapt to this new reality. Farmers, in particular, will need to implement more robust risk management strategies to protect their crops from contamination. This could include measures such as testing irrigation water for pathogens, improving drainage systems to prevent flooding, and developing contingency plans for periods when fields are exposed to standing water.

The study was published in the journal Eurosurveillance.