More than 30 states have reported infections this year, and current data from them shows the number of infections surpassing the record U.S. mark of about 4,700 set in 2019. The illness is not usually life threatening and is typically treated with antibiotics.

Health officials have not yet definitively identified what is causing the infections. On Tuesday, federal health officials said there may be different infection patterns in different places, although they believe cases in at least four states — Michigan, Ohio, Kentucky and West Virginia — are linked.

Michigan officials blame lettuce

In Michigan — where more than 3,300 cases have been reported — officials say early information points to lettuce or salad greens as a possible culprit.

After conducting more than 1,000 interviews with patients, “early information has shown lettuce as a common product that regularly comes up during the investigation,” said Natasha Bagdasarian, the Michigan health department’s chief medical executive.

Because of that commonality and because produce has been behind some past cyclospora outbreaks, Michigan officials advised consumers to buy whole heads of lettuce, discard the outer layers and thoroughly wash what is left. They also suggested people avoid bagged lettuce and pre-mixed salad kits.

On Tuesday, the Taco Bell restaurant chain issued a statement saying it had “voluntarily and temporarily removed limited ingredients at select restaurants as a precautionary measure. We will continue to closely monitor the situation and follow the guidance of public health authorities.”

In a call with reporters on Tuesday, federal health officials did not directly respond to a question about whether they are looking at Taco Bell or any specific food vendor or distributor.

“FDA certainly is continuing its traceback investigation on multiple produce items, also including locations that are reported by the case patients before they became sick,” said Donald Prater, The U.S. Food and Drug Administration’s Acting Deputy Commissioner for Food, in response.

Cyclospora causes ‘explosive’ diarrhea

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. Outbreaks 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 vegetables that were exposed to feces-contaminated irrigation water.

The illness, called cyclosporiasis, is less common than foodborne illnesses caused by other germs, including salmonella and E. coli. Many cases are never linked to a specific food or other source and, for years, few U.S. cyclospora outbreaks were reported. But the number started rising about a decade ago, with a particularly notable spike in 2018 and 2019.

Experts say it’s likely that cyclospora cases historically were underreported, in part because some common tests used to check for food poisoning have not been geared to detect cyclospora. They attribute the increasing trend in cases to climate change and better detection.

2019 was the worst year for infections

The worst year in the U.S. for infections was 2019, when about 4,700 illnesses were reported, according to federal data on confirmed and suspected cases.

The Centers for Disease Control and Prevention keeps those numbers, but they traditionally lag what is posted by individual city and state health departments dealing with local outbreaks in real time. On Tuesday, CDC officials issued a health alert that said that since May 1, the agency is aware of 1,645 confirmed domestic cases and more than 5,100 that require further analysis to confirm whether the infections were acquired in the U.S. The reports come from 34 states, and no deaths have been reported, the CDC said.

The true number of cases is likely larger than what has been reported so far because some people may have milder illnesses that they don’t seek care for, said Gwen Biggerstaff, deputy director of CDC’s Division of Foodborne, Waterborne, and Environmental Diseases, in the call with reporters.

It can take days to weeks for someone exposed to the parasite to develop symptoms. It’s not clear to what extent people currently are being infected, or whether many of the exposures happened earlier. But Biggerstaff said official expect that case numbers will continue to grow through August.

The CDC also posted information about the investigation of an outbreak affecting the four states.

The federal alert did not detail how many reports came from each state. But Michigan officials have reported more than 3,300 cases, officials in northwest Ohio said they have seen more than 1,100, New York City officials counted more than 400 and Illinois reported more than 200.

Officials do not think all can be tied to a common source. In Illinois, for example, more than half the people with infections said they had traveled outside the United States, and at least some may have been infected elsewhere.

But Michigan officials believe a large share of their total is likely due to a linked domestic outbreak. CDC officials did not go into detail about why they currently think cases in Michigan, Ohio, Kentucky and West Virginia are part of the same outbreak.

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Associated Press writer Dee-Ann Durbin in Detroit contributed to this report.

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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.