As cases of an illness stemming from a parasite that causes symptoms like explosive diarrhea rise across the United States, including cases in California, the hunt for the source has so far seen investigators turn up empty-handed.
For years, cyclosporiasis, a stomach-churning illness caused by the cyclospora parasite, often found on produce, has largely been treated as a food safety problem. But as health officials struggle to identify the source of a growing outbreak, some experts believe the focus on food may be obscuring the bigger picture.
Cases of the largely seasonal sickness reached at least 843 people across 31 states as of July 9, according to CDC numbers, and over 1,000 when separate state-reported data is counted.
In California, health officials have recorded 41 provisional Cyclospora cases this year. Most were linked to international travel, and the state has not identified a California-based outbreak or tied infections to a specific food source
“It’s important to understand that California experiences cases of cyclosporiasis every year, with a majority of cases acquired internationally. Cases for 2026 are well within expected ranges for California, and we are not currently one of the states experiencing increased cases or an outbreak,” said Dr. Erica Pan, California Department of Public Health (CDPH) Director and State Public Health Officer. “We are aware of the outbreaks affecting other states, however, and are closely monitoring for any signs of local acquired infection. We encourage Californians to protect themselves and their families by practicing safe food handling and thoroughly washing produce.”
Information from past outbreaks has state and federal health officials issuing warnings about known culprits like fresh herbs and lettuce, but former Department of Agriculture and Food and Drug Administration officials told USA TODAY that the key to the mystery could be in something much more ubiquitous than produce — water.
What Californians should know
California residents are not being advised to avoid public drinking water systems, and health officials have not identified a specific source tied to the current outbreak.
Still, while California’s case numbers remain stable and fall within expected ranges, CDPH urges everyone to follow these prevention steps:
Health officials face challenges tracing a food link
As previously reported by USA TODAY, the source of a cyclosprasis outbreak is more difficult to track than foodborne illnesses like E.coli and listeria, as it has a long incubation period, is hard to detect in food and environmental samples, and does not lend itself to the same genetic tracking tools used for other pathogens, leaving investigators with a lot more legwork and uncertainty.
An additional challenge is what Kalmia Kniel, a professor of microbial food safety at the University of Delaware, described as a “reduced surveillance structure” at the federal level.
Last year, the CDC scaled back a decades-long federal-state partnership that monitors foodborne illnesses, including those caused by cyclospora. The move no longer requires authorities to report cyclospora, along with six other pathogens, to the Foodborne Diseases Active Surveillance Network, known as FoodNet.
The loss of such data could hinder health officials’ ability to track cases across the country and identify trends tying them together, such as a particular kind of produce or food producer, Kniel said. This could risk delaying authorities’ ability to identify the source of an outbreak and stop further transmission.
“We have a lot less information to work with,” Kniel said. “I think we’re seeing some of the effects of that now.”
When contacted by USA TODAY, the FDA said that, “Under the leadership of Secretary Kennedy, FDA is currently investigating cyclospora outbreaks using established epidemiologic, laboratory, and traceback tools in close coordination with CDC and state and local partners.
“Protecting the nation’s food supply is a core FDA responsibility, and the agency has the expertise, personnel, and resources necessary to detect, investigate, and respond to foodborne illness outbreaks and take regulatory action when warranted,” it continued. The CDC did not respond to USA TODAY’s request for comment July 10.
Food may not be parasite’s true origin at all
With all the focus on food, however, some experts say investigators are missing a big piece of the traceback puzzle.
“The role of water in the transmission of cyclospora to humans is probably underestimated or underrepresented and less understood,” former FDA Deputy Commissioner for Food Policy and Response Frank Yiannas told USA TODAY. “There have been some big outbreaks in the United States that were produce-related, but it was likely, and in some cases proven, that the water served as the conduit for the produce contamination.”
Water can serve as a delivery system of sorts for pathogens to reach our produce. This is often where contamination that causes foodborne illness begins: irrigation systems, soil and the environment in which the food is grown.
Waste, human and otherwise, ends up in waterways where it shouldn’t be through things like sewage leaks, wastewater overflow, failing septic systems, flooding and surface water (rivers, canals, reservoirs and streams). When that feces-contaminated water ends up in the irrigation systems used to grow food, cyclospora remains on the produce, which, when eaten raw, then infects people.
In one investigation Yiannas conducted while at the FDA, the agency traced an outbreak back to red cabbage grown in South Florida, where two years of testing found that Cylospora was “readily prevalent” in canals used by local farmers to irrigate crops.
Cyclospora is also fairly resilient and not killed by chlorine, the primary disinfectant used in municipal water supplies, Dr. Robert Mandrell, microbiology researcher and former USDA official, told USA TODAY.
“It’s very resistant,” he said, noting that the parasite’s tough outer shell allows it to survive conditions that kill many other pathogens. “One expects that when you do treatment of water in a wastewater facility, that it can withstand that chlorine.”
Flooding, sewage and wastewater could be key
Exposure doesn’t have to come only from eating produce grown with contaminated water, Mandrell said.
When heavy rainfall overwhelms sewage systems or septic tanks, raw or partially treated sewage mixes with floodwater, which then spreads to neighborhoods, fields, waterways, and low-lying areas, and can eventually filter into recreational water, wells, or reservoirs. This could result in more direct exposure than the water-to-food route.
“It may not be the drinking water that’s contaminated, but we don’t know that. In some cases, it may be, but also just exposure to flood water that has some level of sewage in it,” Mandrell said. We don’t know many of these things for sure, however, because surveillance of Cyclospora in water sources is far from routine or robust in the United States.
“You need to do a major study, not just of drinking water, but of the rivers and streams that get the outflow from wastewater in our country,” he said.
Isolated incidences have proven this is possible, said Yiannas. In one case, cyclosporiasis infections were traced back to tap water in a residential dormitory. In another case, a child contracted it while swimming in Lake Michigan and in yet another, a man was infected while cleaning up sewage.
Still, both Yiannas and Mandrell said there is a lot scientists and health officials don’t know, as the parasite’s nature makes it more difficult to study than other foodborne pathogens.
Why a parasite causing explosive diarrhea remains a mystery
Scientists understand how Cyclospora makes people sick, but they still do not fully understand how it moves through the environment before it reaches people, or what environmental conditions transform it into an infectious organism, or by what process, explained Mandrell. It is notoriously difficult to find in environmental samples, has unique reproductive properties, and a lifecycle that is nearly impossible to replicate in a lab.
In the face of outbreaks like this one, both Mandrell and Yiannas said U.S. public health agencies need to invest significantly more energy and resources into studying Cyclospora and its potential for water-based transmission. In fact, said Yiannas, the questionnaire the FDA uses to trace common threads between sick people for a potential outbreak source still did not include questions about water the week of July 6.
“When we’re investigating these outbreaks, we’re not asking enough questions about it,” he said. “We don’t have a lot of data, and we should.”
The FDA and CDC did not respond to USA TODAY’s questions about water tracing.
And while cyclospora was previously considered an imported food issue, which is still at least somewhat the case, said Mandrell, who has extensively studied the parasite in Mexico, it is now likely also a domestic one – even if this outbreak traces back to something imported.
“We say it’s not endemic, but I’m not so sure,” he said. “When you think about the amount of waste, as I said, that’s coming out now during this outbreak, where’s it going?”
Yiannas agreed, saying, “I do think the parasite is endemic in the United States now and I think it would be foolish to just assume … that it’s a food product from other countries.”
“If we can’t solve this outbreak, then I think that really questions the effectiveness of the state of our public health system,” he said.
In the meantime, Mandrell said, people in areas with a high degree of flooding can swap to bottled water until things dry up if they are concerned about their water supply.
“If you want to be proactive, you might just stay away from your water if you’re worried about it, especially if you can see outside your windows that this is definitely contaminating something,” he said.
(A new video and information have been added to this story)
Christopher Cann, USA TODAY, contributing
This article originally appeared on Palm Springs Desert Sun: California tracks 41 provisional Cyclospora cases statewide
Reporting by Mary Walrath-Holdridge, Chad Murphy and James Ward, USA TODAY NETWORK / Palm Springs Desert Sun
USA TODAY Network via Reuters Connect
By Mary Walrath-Holdridge, Chad Murphy and James Ward, USA TODAY NETWORK | USA TODAY Network
