The U.S. Centers for Disease Control and Prevention expects cases of severe diarrhea caused by the Cyclospora parasite to keep rising, possibly until the end of August, as it searches for the source of the outbreak that’s already sickened thousands.

At least 1,645 cases across 34 U.S. states have been confirmed by lab tests, the CDC said, while the agency is tracking another 5,100 reports that require further testing. The outbreak is especially bad in at least four U.S. states, with the largest number of cases in Michigan.

Bringing the outbreak to an end will prove challenging. Health authorities don’t know how people are coming into contact with the parasite, though authorities in Michigan said it may be tied to contaminated lettuce or salad greens.

The disease isn’t spread person to person, which can make it more difficult to track. And it can take up to two weeks between the time a person ingests infected food or water and when symptoms appear, making it hard to pinpoint the cause of the illness. The type of genotype testing the CDC is doing to identify the source is slower and less accurate than it would be if it were a bacterial outbreak, also creating delays.

Health authorities didn’t give a timeline for when they’d have a definitive answer.

“Our outbreak investigation team is hard at work to follow all the leads we have from CDC on potential food sources for all the various outbreak clusters we are tracking at the moment,” Donald Prater, the U.S. Food and Drug Administration’s acting deputy commissioner for food, said.

Restaurants have been pulling lettuce and greens from menus as a precaution while they wait for more information about the cause. Taco Bell removed some ingredients at select restaurants as a precaution while authorities investigate the outbreak, according to a statement Tuesday. Public health officials haven’t confirmed a link to Taco Bell, nor to any specific ingredient or supplier.

There were 3,309 cases of cyclosporiasis reported in Michigan as of Tuesday, a 25% jump from a day earlier and well beyond the 40 to 50 cases the state normally sees every year, according to the Michigan Department of Health and Human Services.

CDC figures tend to trail state figures. Because many people have mild symptoms and don’t go to a doctor’s office where they would be tested, the true case count is probably a lot higher, the CDC said.

Michigan health authorities are continuing to investigate the cause and said other sources besides salad greens can’t be ruled out. The CDC suspects that cases across Midwestern states are connected to a common source.

The illness stems from consuming the microscopic Cyclospora parasite, which takes hold in the intestines and can lead to nausea, stomach cramps and potentially explosive diarrhea that can last for weeks.

Cyclosporiasis outbreaks tend to be seasonal, peaking between May and August. The number of infections this year were much higher than during the previous two years, said Gwen Biggerstaff, a CDC official who tracks foodborne illnesses.

Public health officials recommend buying whole heads of lettuce rather than pre-washed or bagged lettuce, discarding the outer layers and thoroughly washing the inner leaves. For any greens that can be cooked, the safest option is heating to at least 158F (70C), since washing and chemical disinfection don’t always get rid of the parasite.

Outbreaks have historically been linked to produce like basil, cilantro, lettuce and raspberries, or stem from infections with origins outside the U.S.

“This is a notoriously difficult pathogen to track down,” given the length of time it can take some people to have symptoms, said Vanessa Coffman, director of the Alliance to Stop Foodborne Illness, which includes consumer advocates and companies.

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With assistance from Jessica Nix, Daniela Sirtori, Rachel Cohrs Zhang, Kristina Peterson and Martin Keohan.

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