Michigan Sees Exploding Diarrhea Outbreak With No Clear Source

Cases of a diarrhea-causing intestinal parasite have exploded in Michigan over the last two weeks in an outbreak that still has no clear source.

As of July 6, the state has received reports of over 700 cases since June 22, along with 36 hospitalizations, the Michigan Department of Health and Human Services (MDHSS) told Ars Technica on Tuesday.

On June 30, the health department reported 170 cases, which rose to 572 on July 4.

The microscopic unicellular parasite Cyclospora cayetanensis is behind the cases, causing a disease called cyclosporiasis. Although the infection is generally not life-threatening, it usually causes “watery diarrhea with frequent and sometimes explosive bowel movements,” according to the Centers for Disease Control and Prevention. These intestinal eruptions usually last about a week, but can go on for two or more weeks, the CDC says, with dehydration being one of the most significant concerns.

The parasite is shed in feces and spreads by getting on produce and into water. Thoroughly washing and cleaning produce before eating, along with diligent hand hygiene, are key prevention methods.

State and national trends

Previous outbreaks have been linked to bagged salad mixes/kits, cilantro, basil, raspberries, snow peas, and green onions, MDHSS has noted.

“At this time, no specific produce grower, supplier or type of produce has been identified as the source,” an MDHSS spokesperson told Ars. But the state is coordinating a large investigation to identify a source or sources, working with local health departments in the southeastern region of the state, where most of the cases have been reported. Officials are interviewing cases, sharing information, and ultimately trying to identify common exposures. The spokesperson said MDHSS is also coordinating with the CDC and has shared the state’s case data with the agency.

The CDC has a website for cyclosporiasis surveillance. But it only has case reporting up to June 16 and doesn’t include any cases from Michigan. Ars reached out to the US Department of Health and Human Services about why the CDC’s surveillance data has not been updated, but there was no response. HHS also did not answer questions about its work with Michigan or trends in cyclosporiasis cases, which historically peak in June and July each year nationally.

According to the CDC’s outdated numbers, 17 states besides Michigan have reported cases this year, with a total of 145 cases nationally. With Michigan’s cases, the total would be over 845 cases.

Michigan’s case tally this year is far out of line with the state’s trends for cyclosporiasis cases. In the past several years, the state has mostly reported totals of around 50 cases a year, with a high of 97 in 2023, according to CDC data.

While it may be a record year for Michigan, the US does not seem to be out of line with past years so far. Since 2018, the US has been recording between 2,000 and 5,000 cases each year. In 2023, the US reported 4,463 cases, with 3,091 in 2022, 2,424 in 2021, 2,689 in 2020, 4,703 in 2019, and 3,519 in 2018, according to CDC’s data.