There are promising signs that illnesses linked to contaminated lettuce may be slowing in Michigan, the hot spot of the multistate outbreak of cyclosporiasis in the U.S. That doesn’t mean all the affected produce is gone from stores or restaurants, or that more people aren’t still getting sick, experts say.

On Tuesday, the Centers for Disease Control and Prevention reported more than 18,000 confirmed or suspected nationwide cases of cyclosporiasis, the illness caused by a parasite found in human feces. In nine states, most of the illnesses have been traced to shredded iceberg lettuce sourced from Taylor Farms de Mexico. Other states, such as New York and North Carolina, have also seen higher-than-normal clusters, from unknown sources.

The Michigan Department of Health and Human Services said Tuesday that more than 9,500 people have gotten sick from the parasite in the state, with about 160 hospitalizations, in the last two months.

The Food and Drug Administration didn’t immediately respond to questions about its progress in tracing the source or sources of the illness.

It has been over a week since Taylor Farms voluntarily recalled iceberg lettuce from some Taco Bell restaurants, groceries and food service suppliers.

While the outbreak isn’t over in Michigan, cases may have peaked there. Dr. Natasha Bagdasarian, the state’s chief medical executive, said a significant number of recently reported cases, roughly 475 since Friday, were a result of a reporting delay at one particular healthcare system and don’t necessarily reflect new illnesses.

Bagdasarian said another positive indicator they are seeing is that the percentage of visits to emergency departments with diarrhea as a main complaint is slowing.

However, people with milder symptoms may not be getting tested.

“I do have a big caveat,” said Bagdasarian. “We have heard recently from some of our health systems that because of backlogs in their labs, and because of limitations with testing supplies, they have been limiting tests to those who are severely ill, or those who are immunocompromised.”

Because of this limitation, investigators in Michigan are unsure about the projected spread of cyclospora, especially if other, unidentified culprits are still in the food supply.

“I don’t know if a single product recall is related to the slowdown that we’ve seen,” said Bagdasarian.

While the purportedly contaminated lettuce has been recalled, Bagdasarian also credited public health interventions, such as guidance to peel, cook and remove the outer layers of produce, for the slowdown.

Bagdasarian said it’s up to the federal health agencies to trace, test and find the contaminated produce, based on interviews with patients and other data gathering.

“There are a fair number of folks out there who have not reported eating lettuce or who have not reported eating at any particular fast-food restaurant,” she said. “So I don’t know, based on my data, that I can think this is exclusively lettuce or whether there are other things going on, especially knowing that there are multiple other outbreaks happening across the United States.”

Health officials don’t have access to real-time genetic sequencing, which makes it harder to match a contaminated product to a specific case.

Jennifer McEntire, the founder of Food Safety Strategy, a consulting firm, said it’s also difficult to pinpoint a specific source because many products “move or change hands many times, go through numerous distribution centers [and] sometimes brokers.”

Health officials have suspected other food sources in this season’s outbreaks, namely cilantro and parsley, but have not confirmed either one. In New York, state officials investigated an earlier, smaller cluster of cyclosporiasis cases in April and traced the outbreak to cilantro served at a Brooklyn restaurant, though they never definitively confirmed it was the source.

Last week, officials in North Carolina — which has reported 561 cyclospora cases — said their preliminary investigation suggests cilantro and parsley could be behind those cases. Neither state’s outbreaks have been linked to Taylor Farms lettuce.

Dr. Stephen Ostroff, a former acting FDA commissioner, said it’s possible other illnesses could stem from contaminated lettuce that was grown in the same region as Taylor Farms de Mexico and was also served in the U.S.

Frank Yiannas, former deputy commissioner for food policy and response at the FDA, said it’s clear there is more than one product causing illnesses.

“If, in fact, all the lettuce is pulled from the market, consumers should not be surprised that they continue to see states report illnesses, and they identify food vehicles other than lettuce,” he said.

In Michigan, there have been two distinct waves of cyclospora, which tends to be a summer problem in the U.S. This year, the first wave peaked June 21 through June 28, and again July 5 through July 12, Bagdasarian said.

Barbara Kowalcyk, director of the Institute for Food Safety and Nutrition Security at George Washington University, believes that’s because the contaminated product was supplied in two separate batches.

“If you have ongoing contamination of irrigation water or processing water, then you will see the outbreak going on,” she said.

“So the question that raises in my mind is: Is there a larger contamination event in that part of the country?” Kowalcyk said, referring to the Taylor Farms site in central Mexico.

Contaminated water has the potential to spread to different crops and multiple batches of products.

“There’s still a lot that we don’t know,” said Kowalcyk. “This goes back to the need for better laboratory methods in terms of detecting cyclospora in the environment.”

Kowalcyk said that the continued search for the source of the cyclospora outbreak highlights vulnerabilities in the U.S.’ public health system, a result of a lack of funding.

“We still need increased capacity. We aren’t investing enough in science. We’re not investing enough in research,” Kowalcyk said.