As federal and state health authorities continue to trace and track illnesses from a foodborne parasite, infectious disease specialists said clinicians should test for Cyclospora cayetanensis in patients who present with symptoms such as frequent watery diarrhea, bloating, nausea, and weight loss.

Potentially many thousands of Americans have already been sickened this summer, with the CDC reporting 1645 lab-confirmed cases with 141 hospitalizations and no deaths since May 1. The agency said in a Health Alert Network notice that the number is “substantially higher” than the 249 cases reported in the same period in 2025. Meanwhile, Michigan, an epicenter, reported 6148 cases and 102 hospitalizations — far greater than the 40-50 cases it records in a typical year.

“Even in Maryland, we normally have about 30 cases, and we’re up into the 70s,” said Cynthia L. Sears, MD, professor of medicine at the Johns Hopkins University School of Medicine in Baltimore and expert in foodborne and intestinal infections.

Shifting Outbreak Scope

In a July 20 briefing with reporters, FDA Acting Commissioner Kyle Diamantis said that the agency, along with the CDC and state health departments had investigated “five different subclusters” of cyclosporiasis and that three inquiries had been concluded. One of the larger clusters, involving “1600 cases across multiple states,” is still under investigation, with illnesses being reported as recently as the week of July 13, said Diamantis.

He added that cyclosporiasis is a “seasonal hazard,” with cases usually concentrated from May through August. Diamantis said that more than 4500 cases were reported in 2023 and 3500 in 2024.

“Multiple jurisdictions have reported an increase of cases compared to the same period in 2025,” reported the CDC on its cyclosporiasis surveillance page.

All evidence points to the parasite having been transmitted via shredded iceberg lettuce that came from Taylor Farms’ operations in Central Mexico that was served at Taco Bell locations in Indiana, Kentucky, Michigan, Ohio, and West Virginia, said Diamantis.

Ohio reported some 1246 cases, with major spikes in early June and early July, whereas West Virginia has tallied 139 cases, and Indiana said it has had 427 cases since May 1.

However, Taylor has recalled all shredded iceberg lettuce produced in central Mexico and distributed from June 29 to July 16 to 27 states: Alabama, Arkansas, Connecticut, Florida, Georgia, Iowa, Illinois, Indiana, Kansas, Kentucky, Louisiana, Massachusetts, Maryland, Michigan, Missouri, Mississippi, North Carolina, New Hampshire, New Jersey, Ohio, Oklahoma, Pennsylvania, South Carolina, Tennessee, Texas, Virginia, and Wisconsin.

That widespread recall, plus the long incubation period from infection to illness — a week up to a month — means the number of cases will likely continue to rise, said experts.

Awareness of the outbreak’s severity is important because it can help clinicians shift their diagnostic priorities, Sears said. “Cyclospora on an average day would not have been the first thing to leap to mind,” she said.

Even in states that have not declared an outbreak, if someone is presenting with frequent watery diarrhea, the suspicion for cyclosporiasis “should be high, just given the scope of how big this outbreak is,” and the size of operations of the implicated producer, Nahid Bhadelia, MD, founding director of Boston University’s Center on Emerging Infectious Diseases in Boston, said.

“I’ve been a little concerned that there was possibly more than one source for this outbreak,” said Sears. She also said there has been an excess of cases in up to 34 states.

The investigations are also incomplete — in part “because the CDC can’t keep up with this volume,” said Sears. The outbreak potentially could have been detected sooner, she said. The nation’s foodborne “early warning system is impaired,” said Sears, noting that in 2025, the administration directed the Foodborne Diseases Active Surveillance Network to track just two pathogens, down from eight, including C cayetanensis.

Diagnostics, Treatment, Special Populations

There are many open questions about C cayetanensis, but the pathogen is known to be transmitted when feces from an infected individual contaminates food or water.

The parasite’s oocytes are difficult to detect in soil or on produce but can be more readily detected in stool, which has led to an increase in diagnoses. Even so, cyclosporiasis can be tricky because symptoms can come and go, which is different than typical food poisoning, said Bhadelia. And because it is a parasite, it can’t be cultured.

Clinicians should specifically request that stool samples be tested for the parasite. That could involve acid-fast staining of the sample, which is then viewed under a microscope to detect oocytes, or polymerase chain reaction -based diagnostics. There are five commercially available multiplex assays for gastrointestinal pathogens, but the only FDA-approved test is the BIOFIRE FILMARRAY Gastrointestinal Panel. The panel assesses for 22 pathogens, including Salmonella, Campylobacter, Escherichia coli, Cryptosporidium, Giardia, and C cayetanensis.

Hospitals and clinics typically do not have the panel in-house. Even Johns Hopkins Hospital sends samples out, but that may change if cases continue to multiply, said Sears.

Fluid loss is a concern for older patients and children and often leads to hospitalization. Immunocompromised and pregnant individuals are also more vulnerable to the vomiting, diarrhea, and weight loss from the infection.

Cyclosporiasis is readily treatable with a 7- to 10-day course of the antimicrobial trimethoprim-sulfamethoxazole, said Sears. But neither that antibiotic nor the backup choice, ciprofloxacin, are safe during pregnancy.

“I would be talking to my ob/gyn colleagues to try to figure out the path forward,” she said.

The illness may resolve on its own in healthy individuals, but even that is not a given, said Sears. “We don’t have good data” on who gets better and when, she said.

Bhadelia and Sears had no disclosures.

Alicia Ault is a St. Petersburg, Florida-based freelance journalist whose work has appeared in many health and science publications, including Smithsonian.com. You can find her on X @aliciaault and on Bluesky @aliciaault.bsky.social.