At left, oocysts of C. cayetanensis stained with modified acid-fast stain; at right, oocyst of C. cayetanensis in an unstained wet mount of stool. Image taken at 1000x magnification.
At left, oocysts of C. cayetanensis stained with modified acid-fast stain; at right, oocyst of C. cayetanensis in an unstained wet mount of stool. Image taken at 1000x magnification.Centers for Disease Control and Prevention
A nightmare parasitic disease outbreak that has sickened thousands in the U.S., but somehow spared California so far, continues to baffle public health experts as they search for a source of the contamination.
On Tuesday, the U.S. Centers for Disease Control and Prevention reported that at least 1,645 confirmed cases of the parasitic disease cyclosporiasis have been detected since May 1. Another 5,100 cases are undergoing further analysis to determine whether they were acquired within the U.S.
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While cyclosporiasis cases are detected every year, these numbers are a major increase over average years. At the same time last year, the CDC documented just 249 cases.
The highest number of annual cases reported in the U.S. was about 4,700 in 2019. So far, cases have been reported in 34 states in 2026, and CDC officials said they believe a cluster of cases in Michigan, Kentucky, West Virginia and Ohio are linked.
But California is showing a very different trend with a significant drop in cyclosporiasis cases since the start of the year. Currently, just 41 people have tested positive for the disease in 2026, compared to 80 at the same time last year in California.
The California Department of Public Health released a statement earlier this week emphasizing that no cases in the state have been linked to any of the ongoing outbreaks seen nationwide.
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“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,” California Department of Public Health Director Dr. Erica Pan said in a statement.
Health experts are still unraveling why there’s been a surge in cases nationwide and why Californians appear to have been spared, at least so far.
Cyclosporiasis is an intestinal disease caused by a microscopic parasite called Cyclospora cayetanensis. It spreads when people consume food or drink contaminated by the parasite via fecal material. It can result in weeks of uncomfortable and debilitating symptoms, mainly watery diarrhea. It can take up to two weeks for symptoms to develop after exposure, and many people diagnosed in the U.S. contract the disease while traveling internationally.
In California, 31 of the cases reported this year were among people who traveled internationally. Seven were among people who did not travel outside the state and three were in people who traveled domestically outside California.
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Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center, said produce is always a top focus for public health investigators during cyclosporiasis outbreaks.
“Produce can be contaminated in the field through either animals depositing feces or because the irrigation water that’s used is not purified,” he told SFGATE. “So this parasite can be transmitted to you when you eat uncooked, unpeeled or otherwise raw produce.”
Schaffner said the sheer size and wide number of states reporting high cases this year is unusual and said past outbreaks have had “a more local source or a time-limited source.”
FILE: Green vegetables growing in a sunny farm field in California.
FILE: Green vegetables growing in a sunny farm field in California.Getty Images
Dr. Matt Willis, an epidemiologist and former Public Health Officer for Marin County, said the current pattern of cases nationwide may give another clue about the outbreak source by highlighting a potential delivery network for the product.
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“It’s a Midwestern food distribution network of a contaminated product or products that California seems to be outside of,” Willis theorized.
By far the most cases have been reported in Michigan, with over 3,300 cases reported by the state’s Department of Public Health. Early interviews point to lettuce as a possible culprit for the outbreak, according to the department’s chief medical executive.
Willis said if lettuce or other produce items grown in the Midwest are the cause of the outbreak, California’s robust agricultural production may have helped protect state residents.
“California is a food-producing state,” he said. “We are less dependent on produce coming from other areas.”
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While the cause of the outbreak remains unknown, the treatment is not. Willis said the condition can be treated with a common antibiotic. He cautioned that the disease can be serious for people at risk for dehydration, including those with certain health conditions or the very young and old.
Willis also advised that California may still see an uptick in cases in the coming weeks, even if no major outbreak occurs.
“If we do see little blips in cases, it could be attributable to just the fact that we’re more vigilant now about testing for it in people who have the very common problem of diarrhea,” he said.
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