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More than 250 people have been sickened, mostly in the Midwest, by a rare parasite that may have contaminated fresh produce shipped across state lines, said federal health officials, who’ve stepped in to help coordinate the growing outbreak.

At least 118 cases of cyclospora infection have been reported in Iowa, another 65 in Texas and 68 in Nebraska, state officials said. Four more cases also have been reported in Wisconsin and one each in Illinois and Kansas, although the Illinois case may have originated in Iowa.

At least eight people have been hospitalized, the Centers for Disease Control and Prevention reported Monday.

So far, there’s no clear source for the illnesses, which were reported from mid-June through July, said Dr. Barbara Herwaldt, a medical epidemiologist with the CDC’s division of parasitic diseases and malaria centers.

“Nothing has been implicated yet in a formal sense,” Herwaldt said. “No food item has been identified as the source of the outbreak.”

But officials in Nebraska and other states suggest that fresh vegetables may be the source, based on interviews with people who got sick. Tainted produce could have been shipped across state lines, accounting for the illnesses in multiple states, Herwaldt said. More than one food source could be behind the outbreak and contaminated water used in growing practices could be a culprit.

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Cyclospora is a microscopic protozoan parasite excreted in human stool. Protozoa are tiny, one-celled animals that breathe, move and reproduce.

Symptoms of infection can include weeks or months of watery diarrhea -- but the infection is treatable with common antibiotics, Herwaldt said. She encouraged people who have unexplained stomach troubles to seek medical help and to ask whether a test for cyclospora infection might be necessary. Such tests are not routinely performed, and have to be requested.

Other symptoms can include fatigue, loss of appetite, weight loss, bloating, intestinal gas, stomach cramps, nausea, vomiting, muscle aches and low-grade fever.

There’s no known natural source in the U.S. of cyclospora. That raises the possibility that imported produce could be behind the outbreak, Herwaldt added.

Indeed, raspberries imported from Guatemala were responsible for a 1996 outbreak that sickened 1,465 people in the U.S. and Canada, and also for a 1997 outbreak that made more than 1,000 people ill, CDC records show. Other foods considered potential culprits in other outbreaks include fresh herbs and lettuce.

Cyclospora infections must be reported in 39 states, plus New York City and Washington, D.C., Herwaldt said. CDC encourages other states to report infections as well, both in order to help treat individual people and to help stop the future spread of the parasite.

Consumers should wash their hands well when handling fresh produce and scrub it well, if possible. Refrigeration seems to slow the parasite’s ability to infect people, Herwaldt said.

It’s possible that the contaminated produce has made its way through the food supply in the past month, but it’s still too soon to tell, she said.

“What we don’t know yet is whether the transmission or spread of the parasite is ongoing,” she said.

JoNel Aleccia is a senior health writer with NBC News. You can reach her on Twitter at @JoNel_Aleccia or send her an email.