The Link Between Deadly Lettuce Outbreaks and a Warming Planet

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For lettuce growers, even a modest warming of the planet has the potential
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For lettuce growers, even a modest warming of the planet has the potential to accelerate existing risks of foodborne illness in the supply chain — and bring about issues we can’t foresee

In the spring of 2018, dozens of people across the country began falling sick. Infected with E. coli O157:H7, a Shiga toxin-producing E. coli — symptoms of which include severe stomach cramps, bloody diarrhea, vomiting, and potentially life-threatening kidney failure — the majority of patients shared one thing in common: recently eating romaine lettuce.

Federal and state health authorities traced the lettuce’s particular strain of E. coli to a 3.5 mile-long stretch of an irrigation canal supplying water to several farms in Arizona’s Yuma growing region, adjacent to a large cattle feedlot, which is often a source of such pathogens. How exactly that particular bacteria found its way into the water and then the lettuce couldn’t be determined, but growers and investigators had a hypothesis, at least: an unusual late-February freeze had damaged the surfaces of leaves, making them vulnerable to the intrusion of pathogenic E. coli, which could have reached the plants via the spraying of canal water or pesticides diluted with the water, or maybe from contaminated dust blown by wind.

By the end of the outbreak, there had been 210 reported illnesses across 36 states. Ninety-six people were hospitalized; five died. It was the largest multistate E. coli outbreak in the United States since 2006, when a Shiga toxin-producing E. coli outbreak linked to spinach and to cattle manure in a ranch in California’s San Benito County sickened nearly 200 and caused three deaths.

Roughly 48 million people in the U.S. get sick, 128,000 are hospitalized, and 3,000 die from foodborne illnesses each year, the Centers for Disease Control and Prevention (CDC) estimates. It’s not perfect, but statistically speaking, “the reality is that we have a pretty safe food supply,” says Lee-Ann Jaykus, a professor in the department of food, bioprocessing and nutrition sciences at North Carolina State University.

Whether it will remain that way, however, is a different matter, considering the big, flaming, catastrophe-sized elephant in the room: Climate change, food production, and food security are linked, and if we continue down the current trajectory of incremental global warming and everything that entails, the foodborne illness problem is likely going to get worse.

There’s still a lot that we don’t know yet, admittedly. In 2010, a review of climate change and food safety concluded that climate change “may have an impact on the occurrence of food safety hazards.” Jaykus, one of the co-authors of that paper, says that the keyword “may” is still a good working term. More evidence has emerged in the last decade that points to the potential for more people to get sick from eating food contaminated with harmful microbes as a consequence — both direct and indirect — of climate change.

These impacts are likely to be felt across the food supply. But among fresh produce, leafy vegetables may be especially worrisome. This commodity grouping was ranked the “highest priority” concern in a 2007 fresh produce microbiological hazard risk assessment meeting by World Health Organization and Food and Agriculture Organization (FAO) experts, who cited the raw consumption of leafy vegetables, as well as the hazards that animals, climate, the weather, flooding, water, and more pose to their specific growing conditions.

Virtually every one of those hazards — although not solely unique to leafy greens — is vulnerable to the effects of climate change, albeit in complex, interconnected, sometimes uncertain ways. As Jaykus puts it, “It’s like: climate change impacted this factor, which impacted this factor, which impacted this factor.” In other words, think of climate change as both an amplification of existing hazards, as well as a potential trigger for things we can’t foresee.

Climate change is both an amplification of existing hazards, as well as a potential trigger for things we can’t foresee.

To consider the implications of climate change on growing conditions, we have to start from the beginning. E. coli and salmonella, the two pathogens most commonly associated with leafy green-linked foodborne illnesses, live in the intestinal tracts of animals and humans. Their feces can contaminate sources like soil, water, manure, and sewage, which can then reach leafy vegetables via pathways that include irrigation, splash, runoff, direct contact, and more, as summarized in a 2013 International Journal of Food Microbiology paper by researchers in the Netherlands.

As temperatures and humidity rise, certain pathogens may more readily survive or even proliferate. “Some pathogens exposed to increased temperatures experience faster replication,” says Justin Remais, professor of environmental health sciences at the University of California, Berkeley. Others may see increased virulence, meaning a greater ability to cause damage to the host.

Rising temperatures have also been associated with increased rates of antimicrobial resistance in pathogenic bacteria, according to a report by the FAO. That means that, as climate change accelerates, it’s possible that there may be more incidences of foodborne pathogens in feed animals not responding to regular amounts of clinical drugs, leading to the use of more antibiotics, which could further compound the problems of antimicrobial resistance.

More extreme temperatures can also cause cattle — the primary source of E. coli O157:H7 illnesses — to shed higher levels of pathogens into the environment due to stress, says Erin DiCaprio, an assistant cooperative extension specialist in community food safety at the University of California, Davis. Higher temperatures could also result in livestock spending more time outside in warm weather, which may increase pathogen exposure. Climate-related changes will likely result in changes to diet, both to humans and animals, which in turn can change exposure to pathogens, Jaykus posits.

Once pathogens leave the gut, they’re in a hostile environment and will eventually die off. But some, like, salmonella and E. coli, can actually thrive for a short period of time, up to a few days. “Once these organisms leave the gut, in the feces … they’re trying, evolutionarily, to survive and spread themselves to some other organism or host,” says Jay Graham, an assistant professor in residence of environmental health sciences at Berkeley. Often, they’ll end up in water, where they generally don’t last long, but in certain conditions, they can survive and even grow, like in sterile freshwater with low carbon concentrations and a temperature of 86 degrees Fahrenheit.

Water is a major area of concern to many experts. “You can’t have a safe and secure food supply if you don’t have a safe and secure water supply,” says Jaykus. Heavy rain or flood events, especially after periods of drought or in arid conditions, can contaminate irrigation water with human and animal waste. There’s a clear link between these weather patterns, which are becoming more prevalent and more extreme due to climate change, and upticks in gastrointestinal infections, according to Graham.

“Think about all the septic tanks in the United States, all the leaky sewers and all of the livestock waste spread on land,” he says. “Then you have this dry spell, so there can be an accumulation of fecal waste on the land … That rain event basically spreads all that feces out over the environment and contaminates waterways that are then used to irrigate all of our crops.”

The scale of the problem becomes more serious once you consider how much feces is within the vicinity of the fields where fresh produce is grown. “The way we get rid of animal waste is to apply it to agricultural land,” says Lance Price, a professor of environmental and occupational health at George Washington University. And even when these kinds of soil amendments aren’t used, there may still be fecal matter nearby; much of America’s leafy vegetable production takes place in Yuma and the Salinas Valley, where there are also concentrated animal feeding operations (CAFO) that can have upward of 100,000 head of cattle or other livestock.

“These are animals that are in very close proximity to one another,” is how Jaykus describes CAFOs. “When you have animals very close to each other, you also have a lot of fecal material, and fecal material is close to irrigation in that region.”

You can imagine where some of that fecal material ends up after heavy downpours. “What you get are little streams of cow and pig shit flowing downstream,” says Price. “They’re just like big fecal wastelands. There’s nothing to really stop the flow of shit and bacteria when there are these heavy rain events.” In some cases, entire fields may be flooded, potentially exposing the edible portions of plants to waste, agrochemicals, and heavy metals, according to FAO food safety officer Vittorio Fattori. Leafy greens, which are grown close to the ground, are more likely to be contaminated compared to other produce under these conditions.

“They’re big fecal wastelands. There’s nothing to really stop the flow of shit and bacteria when there are these heavy rain events.”

It doesn’t even have to be flooding. Contaminated runoff could flow into irrigation canals, which are then used to water crops. Dust storms as a result of droughts and dry spells can lead to contaminated dust particles settling on leafy greens. Those vegetables, delicate as they are, could suffer surface damage from unexpected freezes or other unpredictable weather events, creating the potential for bacteria to work its way into the plant.

Wildlife is another contamination source that could be exacerbated by climate change. “If there are more precipitation events and higher temperatures, you could see this explosion of vegetation and some of these wildlife populations,” says DiCaprio. If the number of pathogen-carrying animals and insects increases, there’s a higher probability that they’ll infiltrate irrigation water, soil sources, or growing fields themselves, potentially leading to more foodborne illnesses.

Not all of these hazards — just a few among many possibilities, should climate change continue accelerating — are going unchecked. “There have been huge improvements related to irrigation practices, like doing site assessments to make sure there is not a concentrated animal feeding operation in close proximity to a field that’s being used to produce leafy greens,” says DiCaprio, also highlighting irrigation water-quality requirements, soil-amendment regulation, pre-harvest checks for wildlife intrusion, and even more stringent requirements for leafy greens in particular.

Still, DiCaprio acknowledges, “even with all those tight controls, you can’t control everything.” That’s especially true when you add climate change and all its variables into the mix, complicating the already difficult task of managing food safety.

“I do think we are going to see more of these events. There are a lot of drivers involved, but it is going to result in more of these kinds of outbreaks,” says Graham.

While the U.S.’s fairly safe food system may be able to shoulder that burden, the same is not true everywhere. That’s what Graham is primarily concerned with: this “perfect storm” of climate change-related impacts in countries with fewer resources and less developed food safety systems.

“We’re seeing significant increases in industrial ag in places that don’t even have good human sanitation, so there are very few safeguards against animal waste going into the environment,” he says. “The U.S. does a poor job in managing waste from livestock and poultry, but when I go to these other countries, there’s not the oversight or even local regulations and policies to prevent the spread of any organisms from these farms.”

Some of those farms with fewer hygiene controls grow fruits and vegetables for export, which poses its own risks for consumers in the U.S. Food imported into this country has steadily increased over the past decades, thanks to higher demand for a wider selection of foods and for produce items year round, according to a 2017 study published in the CDC journal Emerging Infectious Diseases. At the same time, there has been “a small but increasing number of foodborne disease outbreaks associated with imported foods, most commonly fish and produce,” the researchers found. The globalized food supply means that we can’t just focus on our own well-being; our problems are shared across oceans, across borders.

With food safety, as with climate change, there’s no magic bullet that can fix everything. “Our world is so complex now, and the problems we face are so fundamental and big, that we can’t use a reductionist approach,” says Max Teplitski, chief science officer of the Produce Marketing Association. Addressing all the components — sustainability, labor, hygiene and health, soil quality, environmental protection, wildlife management — will take a systems approach rather than a siloed one. That means getting all the right people to the table: farmers, the scientific community, and the government, which Teplitski finally feels hopeful about, thanks to the new administration. Collaboration needs to happen on local, national, and global levels.

“A few months ago, we had this conversation about how essential food industry workers are, and I think we also realized that the food industry itself is essential,” he says. “If we say that food production is essential for life on this planet, then we must treat this industry as essential.” To him, that means investing in agricultural research and practices that foster sustainability and produce safety, whether that’s incentivizing carbon soil sequestration or the protection of soil health.

“The health of people, animals, plants, and the environment are inextricably linked” under the umbrella of the One Health paradigm, says Fattori. That’s a lesson we should have learned from the coronavirus pandemic: fundamentally, public health is collective. The only way we can protect ourselves — and the planet — is to put in the work to protect each other.

Yadi Liu is an award-winning visual artist who is passionate about finding the optimum balance between illustration and modern art.
Fact checked by Andrea López Cruzado
Copy edited by Rachel P. Kreiter

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