LANSING, Mich. – Michigan’s summer outbreak of cyclosporiasis has produced more than 300 confirmed cases in under two weeks, spreading across 21 counties and reaching a total that, in a normal year, the state would not accumulate until December.
The Michigan Department of Health and Human Services confirmed the case count Thursday, describing the spread as “large and growing.” The agency began investigating at the end of June after detecting what it called a “sudden and large increase” of Cyclospora cayetanensis infections in the Lower Peninsula. Within days, what looked like a cluster in seven counties – Jackson, Lenawee, Livingston, Monroe, Shiawassee, Washtenaw, and Wayne – had more than doubled in geographic reach, stretching to 21 counties by Wednesday.
Michigan typically records about 50 cyclosporiasis cases in an entire year. The state now has that many cases in Wayne County alone, which is tracking at least 27 potential infections.
Cyclosporiasis is caused by a microscopic parasite – not a bacterium, not a virus – that spreads through food or water contaminated with fecal matter. The parasite cannot be passed directly from person to person because Cyclospora cayetanensis requires one to two weeks in the environment before it becomes infectious. That means every Michigan resident who is currently symptomatic consumed the parasite in contaminated food or water sometime in late June, probably from the same source, without knowing it.
That source has not been found.
Michigan’s outbreak is occurring inside a broader national surge. Seventeen states had documented domestically acquired cyclosporiasis cases in 2026, with 145 confirmed infections and 20 hospitalizations as of mid-June, according to Fox News and CDC surveillance data – figures that predate Michigan’s explosion by weeks. The CDC, the Food and Drug Administration, and state officials are investigating multiple clusters but have not linked all cases to a single food item. Michigan’s 300-case spike, if it is driven by one source, would constitute a separate and far more concentrated outbreak than what national surveillance had tracked through June.
Past U.S. outbreaks have most often been traced to fresh produce eaten raw. Raspberries, mesclun salad greens, cilantro, and basil have all been implicated in prior Cyclospora outbreaks over the past three decades. The summer months align with peak consumption of fresh herbs and garden salads, and the seasonal window for cyclosporiasis – roughly May through August – coincides with the height of the fresh produce season. What, specifically, Michigan residents ate in common around the third week of June is the question state and federal investigators are trying to answer.
The illness typically begins about a week after exposure, though symptoms can appear as soon as two days after infection or as late as two weeks. What follows is often debilitating: watery diarrhea that can be frequent and explosive, along with severe abdominal cramping, bloating, nausea, appetite loss, and fatigue. Without treatment, the illness can persist for several days to more than a month, with possible relapses. For most otherwise healthy adults, the recommended treatment is a course of trimethoprim-sulfamethoxazole – the antibiotic sold under the brand names Bactrim and Septra – along with adequate hydration. State health officials are urging anyone with those symptoms to contact a health care provider and mention possible parasite exposure.
The case count will likely continue to climb. Diagnosis of cyclosporiasis requires laboratory testing of stool samples, and clinicians sometimes need multiple specimens because the parasite is difficult to detect. Some residents who recovered from a gastrointestinal illness in late June or early July may not realize they were infected, and their cases may not have been reported to the state.
Michigan’s cyclosporiasis spike arrives amid an already difficult summer for infectious disease in the United States. Tick bites are sending Americans to emergency rooms at their highest rate in nearly a decade, as warming winters push blacklegged and lone star ticks into states that rarely had to contend with them before. Measles has neared its full-year case record in just six months, with 2,134 cases confirmed through the first half of 2026. Each of those situations has its own cause and its own trajectory, arriving at the same moment in the same summer.
What makes cyclosporiasis unusual among this season’s threats is that the source is likely something people have already stopped consuming, or may still be consuming without knowing it. The two-week environmental delay before Cyclospora becomes infectious means the contaminated food was eaten sometime around the third week of June. If it was a single item – a package of herbs, a bag of salad greens, a batch of fresh berries – it may already be out of circulation. Or it may still be on store shelves.
The state health department is working with the CDC and FDA to identify any common food purchases, suppliers, or dining establishments among confirmed case patients. Until that link is established, officials recommend washing all produce thoroughly under running water, scrubbing firm fruits and vegetables with a brush, removing and discarding damaged portions, and refrigerating cut produce within two hours. Standard washing does not eliminate Cyclospora entirely – the parasite is difficult to remove from leafy produce – but it reduces the risk of ingesting a sufficient quantity to cause illness.
What Michigan’s outbreak makes clear is that when Cyclospora appears in significant numbers, it does so fast. Three hundred cases in under two weeks is not a background risk rising gradually. It is a concentrated event, suggesting a specific exposure moment that a broad population shared. Whether investigators can trace it back to a specific food item, grower, or distributor before more cases accumulate is the central question – and it is one the state cannot yet answer.
