Virginia health officials are investigating an increase in cases of a foodborne intestinal illness and dozens of suspected cases.

The Virginia Department of Health reported Tuesday on a recent increase of cyclosporiasis, an intestinal illness caused by the foodborne parasite cyclospora, since mid-June in the state.

The source of the outbreak has not been identified, but officials said three workplace cafeterias were part of the investigation: the Capital One building at 1600 Capital One Drive in McLean; the Valo Park building at 7950 Jones Branch Drive in McLean; and CarMax at 12800 Tuckahoe Creek Parkway in Richmond.

There have been 39 confirmed cases of the illness in Virginia since May, an increase in a five-year average of nine cases for the same time from 2014 to 2018.

Katherine G. McCombs, foodborne disease epidemiology program coordinator for the state health department, said it was hard for officials to determine the source of the parasite because those who become infected don’t immediately feel sick.

“One of the hard things about the investigation is you might not get sick for one or two weeks,” she said. “We have to try to get a hold of that person and figure out what they ate. That takes time.”

Northern Virginia health districts involved in the investigation include Arlington, Fairfax, Loudoun and Prince William counties, as well as the city of Alexandria.

Elsewhere in the region, Maryland Department of Health officials have confirmed 42 cases of cyclosporiasis through Saturday, with 37 of those reported over the past two weeks. D.C. health officials also have seen a recent increase, with one case tied to the Virginia outbreak.

Cyclosporiasis infections can occur after people consume food or water contaminated with feces that contains cyclospora, Virginia health officials said. Past outbreaks have been linked to imported fresh produce such as raspberries, basil, arugula, snow peas, mesclun and cilantro.

Symptoms include diarrhea, weight loss, loss of appetite, abdominal cramping, nausea, prolonged fatigue and other flulike symptoms. The illness can last from a few days to more than a month, according to the Centers for Disease Control and Prevention.

To prevent the spread of cyclosporiasis, residents are asked to wash produce thoroughly and wash their hands before and after preparing it. Cut, peeled or cooked fruits and vegetables should be refrigerated within two hours, officials said. Anyone who suspects being infected is being asked to visit a health-care provider, who might prescribe antibiotics.

Officials elsewhere in the country have previously warned the public about cyclosporiasis cases. McDonald’s pulled salads from 3,000 locations across the country last year after an outbreak.