A rapidly growing cyclospora outbreak has sickened more than 2,000 people across at least 21 states, prompting health officials to intensify efforts to identify the contaminated food source behind the illnesses.

Investigators have not yet determined which grower, supplier, or type of fresh produce is responsible, but experts say the unusually high number of cases could help authorities identify the source.

"With the number of cases we're seeing nationwide, I would hope, and I would suspect, that we would have a pretty good idea of what are the food vehicles that are likely to be causing these illnesses," Frank Yiannas, the former Food and Drug Administration deputy commissioner for food policy and response, told The Washington Post.

Cyclospora is a microscopic parasite that spreads through contaminated food or water, most often fresh produce. It is not transmitted from person to person but can cause prolonged bouts of watery diarrhea, abdominal cramping, nausea, and other gastrointestinal symptoms.

Officials say the true scope of the outbreak is likely even larger than reported. The Centers for Disease Control and Prevention's case totals often trail state figures because the agency primarily counts confirmed infections, while some states also report probable cases or wait until investigations are complete before submitting data.

Michigan has emerged as the hardest-hit state, reporting more than 1,200 cases — roughly 25 times its typical annual total. State officials say at least 44 people have been hospitalized, largely because of dehydration.

Ohio has reported 477 cases as of July 8, with at least 28 hospitalizations.

Investigators say cyclospora is one of the most difficult foodborne illnesses to trace because symptoms can take two days to two weeks to appear.

"Compared to other foodborne outbreaks, the symptoms show up anywhere from two days to two weeks, where some of the more common foodborne outbreaks, the duration between exposure and illness is one or two days," Dianna Blau, acting chief of the CDC's parasitic diseases branch, told the Post.

The delayed onset of symptoms and the need for specialized testing make the source especially difficult to identify. Investigators must rely on patients to remember what they ate, where they shopped, and which restaurants they visited as far back as two weeks.

In Michigan, authorities are also reviewing supermarket loyalty card records in an effort to pinpoint the contaminated food behind the outbreak.

Michigan Chief Medical Executive Natasha Bagdasarian said increased testing is likely contributing to the growing case count.

"There's greater awareness, so more people are watching for symptoms, more people are getting tested," she told the Post. "Cyclospora is not something that is routinely tested for when someone has GI symptoms, and as a result, I think we're picking up more cases."

Ohio Health Director Bruce Vanderhoff warned that the illness should not be dismissed.

"This is a serious illness that can cause dehydration and require people to seek emergency medical care, and it should be taken seriously," he said in a statement.

According to the CDC, previous U.S. cyclospora outbreaks have been linked to fresh produce, including basil, cilantro, raspberries, salad mixes, and snow peas.

Until investigators identify the source of the current outbreak, health officials recommend thoroughly washing fruits and vegetables, washing hands before preparing food, and scrubbing firm produce such as cucumbers and melons before eating.