An ongoing outbreak of cyclosporiasis, a parasitic infection causing severe diarrhea, has reached over 1,000 cases, primarily in Michigan and Ohio. Michigan alone has reported 992 cases, including approximately 40 hospitalizations, marking its largest such outbreak in history. Neighboring northwest Ohio has seen over 500 cases, with 306 in Lucas County. Investigations are also underway in 28 other states. Despite the growing numbers, no deaths have been reported.
Health officials face significant challenges in identifying the source of the cyclospora outbreak. Unlike bacterial foodborne illnesses, the cyclospora parasite is difficult to culture in lab settings, hindering traditional traceback methods. The incubation period for cyclosporiasis is also relatively long, averaging one week but ranging from two days to over two weeks, making it difficult for sick individuals to recall specific food exposures. Past outbreaks have shown that investigations can take months and sometimes never pinpoint a clear source.
Previous major outbreaks include a 1997 incident linked to Guatemalan raspberries, affecting over 1,000 people in the U.S. and Canada, and a 2019 outbreak tied to Mexican basil, sickening more than 2,400. Common sources of cyclospora are often unwashed produce like cilantro, basil, parsley, raspberries, snow peas, and green onions, which can be contaminated by fecal matter during growing or processing. The CDC defines the cyclospora "season" from May 1 to August 31, and while this surge falls within that period, its scale has prompted intensified efforts by the CDC and FDA to determine its origins.
Michigan officials recommend consumers wash produce thoroughly, cook vegetables when possible, and for lettuce, remove the outer two to three leaves from whole heads before washing. The difficulty in tracing the parasite, coupled with challenges in obtaining enough pure DNA for Whole Genome Sequencing (WGS), further complicates the investigation. A 2025 outbreak, which sickened nearly 1,000 people across 37 states, ultimately implicated parsley as the likely source, but no specific brand or grower was ever publicly identified.
