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Nearly 1,000 people in Michigan have been diagnosed with a parasitic infection that can cause weeks of watery diarrhea, making it the largest such outbreak in state history and one of the nation’s largest in years.

No deaths have been reported and the source of the cyclospora infections hasn't been identified. Meanwhile, investigations into similar illnesses have been going on in 28 other states, including in Ohio, where people just across the Michigan border are also becoming sick.

Michigan officials first announced the outbreak last week, when they were aware of more than 170 cases — all in the southeastern corner of the state — since June 22. Michigan usually identifies only about 50 cases each year.

 had grown to 992, including about 40 hospitalizations. Just across the state line, Lucas County, Ohio, reported 306 cases as of Wednesday. Northwest Ohio has seen more than 500 cases.

So far the outbreak does not include Minnesota. A spokesperson at the Minnesota Department of Health said Wednesday that the state hasn’t seen any increased cases associated with the current wave of cyclosporiasis.

Cyclospora surges can be tricky to investigate, and food poisoning sources can be hard to establish. But “there is clearly a linked outbreak happening right now,” Dr. Natasha Bagdasarian, Michigan’s chief medical executive, told The Associated Press on Wednesday.

Here's what to know about the current situation:

Cyclospora is a microscopic, spherical parasite that commonly causes watery diarrhea “with frequent and sometimes explosive bowel movements,” according to the U.S. Centers for Disease Control and Prevention. The illness, called cyclosporiasis, is not usually life threatening and is typically treated with antibiotics. 

 tend to occur most often in the late spring and summer.

The heat-loving parasite infects the bowels and spreads through feces. In the past, people have been infected by consuming fruits or 

 that were exposed to feces-contaminated irrigation water.

It’s less common than a number of other kinds of foodborne illnesses, including salmonella and E. coli. For years, few U.S. cyclospora outbreaks were reported each year. But the number started rising about a decade ago, with a particularly notable spike in 2018 and 2019. Experts attribute the increases to climate change and better detection.

How does this outbreak compare to previous ones in the U.S.?

Comprehensive data on cyclospora outbreaks is lacking. But available information shows only a small number of documented outbreaks in the last 20 years have surpassed 1,000 cases. That short list includes a 1997 outbreak tied to Guatemalan raspberries that sickened more than 1,000 in the U.S. and Canada, and a 2019 outbreak linked to Mexican basil that sickened more than 2,400.

There are several reasons it's challenging to know the exact toll, said Melanie Firestone, a University of Minnesota foodborne illness researcher. Some tests used to check for types of food poisoning are not geared to detect cyclospora, “so there is a lot of underreporting when it comes to this,” she said.

Other challenges: Technicians aren't able to grow the parasite in labs, making it hard to draw evidence from contaminated produce. And it can be hard to figure out what food sick people had in common, because sometimes it’s a single ingredient that might be common in multiple recipes — like basil or cilantro.

Also, it's possible that food distributors may channel contaminated foods to both grocery stores and restaurants, making it hard to discern where tainted food came from. Investigations can take months and sometimes never find a clear source.

Cases seem to be surging in and around southeastern Michigan. But it's not considered a national health emergency.

There's no evidence that the parasite has evolved to become more infectious, said Dianna Blau, the CDC's acting parasitic diseases branch chief.

Thousands of cyclospora illnesses are reported in the U.S. each year and it's not yet clear how unusual this year will be, she added. That said, the case total so far is four times higher than at the same point last year, according to current CDC national data, which lags dramatically from what's being reported by the states.

Michigan appears to be suffering the worst of it, but the state's aggressiveness in investigating and reporting cases may be “part of the reason why this looks like a Michigan problem,” Bagdasarian said.

How can you protect yourself from cyclospora?

People who have diarrhea that hasn’t gone away on its own within a few days should see a health provider and discuss the possibility of cyclospora, officials say.

The best way to prevent infection with a parasite is to avoid food or water that may have been contaminated.

Fresh produce should be thoroughly washed before being eaten. But be aware that cyclospora can really stick to some foods, so washing may not eliminate the risk of infection.

As Michigan officials investigate the potential source, they recommend consumers purchase whole heads of lettuce rather than prewashed, bagged lettuce or salad mixes, and to remove the outer two to three leaves before washing the remaining leaves under running water.

They also say to cook vegetables when possible.

The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education and the Robert Wood Johnson Foundation. The AP is solely responsible for all content.

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Michigan is experiencing its largest outbreak of a parasitic infection that causes severe diarrhea. Nearly 1,000 people have been diagnosed with cyclospora as of Wednesday, though no deaths have been reported. So far the outbreak does not include Minnesota. 

Outbreak of diarrhea-causing parasite grows to more than 1,000 cases

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