The diarrhea-inducing outbreak from infected Mexican lettuce is mercifully over. But some food safety watchers fear this parasite is here to stay.
Somewhere in the middle of this summer’s unprecedented Cyclospora outbreak, food safety lawyer Bill Marler paused his upward-ticking tally of infections to ponder a disturbing thought. With so many people contracting explosive diarrhea from this parasite clinging to lettuce leaves, where were all the egg-like structures called oocysts, one of Cyclospora’s lifecycle stages, ending up? Oocysts were being excreted from lettuce that was grown for Taylor Farms in Mexico by the billions or trillions into our sewage systems. Was it possible some of them were evading the wastewater treatment process to settle in and mature in our fields and waterways, too?
The conventional wisdom about Cyclospora is that it’s endemic—that is, native to—countries in the Global South where warm conditions are favorable to its thriving, and that it mostly gets too cold for it to linger full-time in environments in the U.S. But as the outbreak of 2026 raged on, “I was just thinking about, how does a parasite become endemic?” Marler recalled. By his accounting, 32,500 people were infected over four months, although those numbers were likely much, much higher; epidemiologists multiply known Cyclospora infections by 83.1 to account for cases that go untested.
“You look at hepatitis A, which is endemic around the world except in the U.S. and Europe and Japan,” Marler continued. “Why is that? The U.S., Europe, and Japan, generally speaking, have better sewage treatment systems. So how could [endemic Cyclospora] happen in the U.S.? And then it just dawned on me: If you have tens of thousands of people with explosive diarrhea, and you have a pathogen that isn’t killed by sewage treatment plants, can’t you, by sheer volume, start to populate the United States’ water supplies?”
That question had also been on the mind of Joan Rose, a water microbiologist at Michigan State University and director of its Water Alliance. “If the techniques they use in wastewater treatment are not sufficient to remove the oocyst,” like chlorination, “then the oocyst will be discharged with the effluent,” she said.
Alternatively, about 20 percent of American homes are on septic rather than municipal sewage systems, in which case those excreted oocysts linger in septic tanks. “One of the things we don’t know is how long the oocysts survive before they mature” and become infectious, Rose said. “Could they survive a long time, like years? I don’t know. And then when you pump the septage out … it may be land-applied.” Meaning, any remaining oocysts would be land-applied, too, with the potential for them to mature there and go on to infect the next cohort of humans, starting the illness cycle all over again.
Tracking Cyclospora to understand the likelihood of this, however, remains challenging. Scientific studies started finding the parasite bouncing around the U.S. in the late 1980s. Every year around May when temperatures rise, through August or early September when they begin to fall, CDC reports a few thousand cases of cyclosporiasis, the infection caused by Cyclospora ingestion which, in addition to diarrhea, can cause weeks’ worth of cramping, nausea, and fatigue.
“If you have a pathogen that isn’t killed by sewage treatment plants, can’t you, by sheer volume, start to populate the United States’ water supplies?”
The educated assumption among public health professionals is that once that seasonal cycle ends, the parasite vanishes, then reappears the following spring on produce imported from Latin America. But as drought conditions increase around the U.S., Rose has concerns about what happens when farms here turn to recycled water to irrigate crop fields, and if that could be a pathway to endemic Cyclopora.
Little covered in the media (if at all) are the findings of a research organization called the Center for Produce Safety, funded partly by industry players including Taylor Farms. Their website states that there have been Cyclospora outbreaks linked to U.S.-grown produce since 2018 (also reported in Food Safety News), which CPS called representative of “an increased threat to U.S. consumers.” Additionally, based on its own surveillance of the wastewater treatment process starting in 2014, the organization believes the parasite is actually already endemic here—a finding that has not been confirmed by outside researchers. (Offrange reached out to CPS and to an affiliated researcher for clarification on their findings but had not heard back by press time.)
Rose is quick to point out that there’s a lot we still do not understand about Cyclospora that makes tracking and studying it difficult. One thing we do know is that “It takes very, very few [mature oocysts] to infect a person, possibly even as low as one,” said Joel Barratt. He’s a parasitologist at Emory University who worked at CDC during the 2018 outbreak and believes CPS’s findings about it are likely faulty—possibly confusing Cyclospora with a related parasite common in poultry called Eimeria.
An oocyst matures in the environment for seven to 14 days at temperatures of 23-32 degrees Celsius/73.4-89.6 degrees Fahrenheit. You eat a piece of lettuce contaminated with some of those then an unknown trigger in our bodies (Warmth? Moisture? Stomach acid?) causes what Barratt called two “banana-shaped wiggly things”—sporozoites—to pop out and penetrate our gastrointestinal cells. The sporozoites replicate in there, producing male and female gametes that combine to create a new batch of un-mature non-infectious oocysts that get pooped out and wend their way into our wastewater or septic systems.
Rose thinks it’s unlikely that oocysts mature inside sewage treatment plants and she surmises that 90 to 99 percent of them wind up in the solids—the sewage sludge—that’s separated from treated wastewater. Unless the solids are sent to a digester that operates at temperatures of at least 400 degrees C, she believes some oocysts could mature to their infectious stage inside the sludge; this is a problematic substance of recent renown for its high PFAS “forever chemical” content, which frequently gets spread on farm fields as a nutrient-rich amendment.
“It takes very, very few [mature oocysts] to infect a person, possibly even as low as one.”
As for the one to 10 percent of oocysts that might remain in the water, chlorination won’t kill them. But Rose said ultraviolet treatment, which is used in 22 percent of U.S. public wastewater treatment plants, according to one independent analysis, probably will at certain wavelengths (this requires confirmation). Rose believes some oocysts were in fact discharged through wastewater effluents during this most recent outbreak. But much better tracking is needed to understand where they are, where they’re going, and whether they’re remaining viable in the environment.
“Given that the United States has adequate health and sanitation, I would say the risk is probably low for having more and more people become infected via the oocysts being excreted during this unprecedented 2026 cyclospora season,” Barratt said. What few studies about Cyclospora exist suggest to him that oocysts that do make it into the environment in places like Michigan will get killed off by its cold winters. Viability, he said, also “appears to drop dramatically with time.”
Although, “Never say never. I mean, some people are on septic, those tanks overflow, and there also tend to be people on septic living rurally close to where food is grown.” Marler countered that some food-growing places in the U.S. aren’t actually susceptible to winter freezes. “We don’t really know for sure whether or not we’ve got Cyclospora endemic in certain areas that we grow leafy greens in, like the Salinas Valley, Yuma, anywhere in the South, or in warmer climates,” Marler said. “And it doesn’t seem like the FDA or anybody’s researching that in any great depth.”
A huge bone of contention for public health professionals is the fact that the Trump administration cut the number of food pathogens it tracks from eight to two, although Barratt said this does not actually impact CDC’s ability to track current outbreaks, just its ability to monitor foodborne outbreak trends over time. The administration also cut the team at CDC’s domestic parasite lab, where Barratt used to work, from 11 to three; that meant people less experienced with parasite protocols were brought in to help out during the outbreak—what Barratt called “running this thing off the smell of an oily rag.”
Marler is irritated that this administration has delayed a food traceability rule that would have helped the FDA better protect our food system from outbreaks. But he’s also incredulous that American leafy-greens growers haven’t implemented their own traceability metrics. With consumers afraid to purchase lettuce due to the outbreak, the industry is “plowing fields under, they’re not hiring employees, they’re losing a shitpot of money. That just ripples through the supply chain,” he said. One industry member he spoke with at a recent food safety conference explained it to him thusly: Tracing potential pathogens would be “‘a cost up front and you never know for sure whether that cost is going to save you money’ so … they’ve got no interest because it’s going to cost them something.
He’s incredulous that American leafy-greens growers haven’t implemented their own traceability metrics.
When it comes to recycled water, Rose said she’s a big supporter and, optimistically, “The wastewater reuse community that uses treated water for irrigation of food crops has more stringent rules, and has done studies to be sure that they’ve got filtration on, they’ve got extra treatment to take out the oocysts.” However, she emphasized that we’re also not monitoring recycled water as robustly as we could. “We should be able to show the public that wastewater treatment is taking care of these pathogens” and removing all oocysts present, Rose said.
Barratt doesn’t believe Cyclospora is endemic in the U.S. or that it will become so anytime soon. Although, if there were a chance of that happening, the warm (and warming) Southwest and maybe the Southeast, too, might be the likeliest points of takeoff. Unfortunately, further research into this is stymied by the fact that there are so many things we still don’t understand about Cyclospora’s behavior in the environment and its vulnerabilities.
We can’t even culture it to study it better, because scientists haven’t been able to replicate the environment it needs in order to grow—i.e., the inside of human gastrointestinal cells. Cyclospora “does not seem to grow readily in artificially cultured human cells for some reason,” Barratt said. Instead, researchers have to study Eimeria and other close relatives, like Cryptosporidium, and try to extrapolate useful data from those.
“If we don’t want [me] sitting here going, ‘I don’t really know,’ you have to throw some money at the problem,” Barratt said. “It really hasn’t been sufficiently funded for a very, very, very long time.”
