Officials are trying to determine what kind of food caused at least 70 people to become ill after attending a charity taste-testing event in Sarnia-Lambton, Ont.

They were among 400 people who took part in the Big Sisters Chef's Challenge on May 12.

About a week later, health officials started getting calls from people who had become violently ill with flu-like symptoms, including severe diarrhea, weight loss and fever.

Officials in Lambton County have confirmed seven of the participants were infected with the cyclospora parasite, which is transmitted through food or water contaminated by human feces.

The symptoms of cyclosporiasis usually show up about one week after infection.

Problem could be difficult to trace

Chad Ikert, manager of environmental health for Lambton County, said the parasite is usually carried on fresh produce, but officials are still trying to figure out exactly what food was the culprit.

He's concerned the cyclospora outbreak could spread if the product that was infected is widely distributed.

"There's been an alert to all health units to be on the lookout for increased cases because if it is produce-related that potentially isn't just sent to Lambton we could see this in other areas as well," he said.

Health officials want to question everyone who was at the Chef's Challenge, even if they are not sick, to try to figure out what food was contaminated.

Ikert said it will be difficult to trace the problem because those who became ill were at a taste-testing event, so many people would have eaten a little food from each dish.

Cyclospora infection responds quickly to proper antibiotic treatment and is not considered life-threatening in healthy individuals, according to the Canadian Food Inspection Agency.

If untreated, symptoms last from a few days to six weeks. Sometimes the symptoms can keep recurring for a long time, the agency says.