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When a foodborne illness outbreak hits, finding the source isn't easy

People shop for raspberries and cherries at the Granville Island Public Market in Vancouver on July 26, 2010. Some fresh produce has been making some Canadians sick this summer. But with grocery stores a cornucopia of produce at this time of year, how does public health figure out what's to blame?
Image Credit: THE CANADIAN PRESS/Darryl Dyck
August 14, 2015 - 7:00 AM

TORONTO - Some sort of fresh produce has been making a number of Canadians sick this summer. But with grocery stores a veritable cornucopia of fruits and vegetables, how do public health experts figure out what is to blame?

It ain't easy.

In fact, it takes a lot of dogged work to figure out what is the cause of something like the Cyclospora outbreak that has sickened more than 80 people in Ontario, British Columbia, Alberta and Quebec this summer.

And given the fact that it's actually been several weeks since the most recent person to be diagnosed became ill, chances are decent the source of this outbreak is going to remain a mystery.

"At this point in time since nothing's really coming up strongly I suspect that there is diminishing likelihood that we will detect the cause or causes," says Dr. Doug Sider, medical director of communicable disease prevention and control at Public Health Ontario.

Sider says it appeared for a time that the culprit might have been contaminated blackberries, but some of the people who were ill did not eat that type of berry.

Ontario has reported the lion's share of the cases — 74 — in this outbreak, which is caused by a parasite called Cyclospora. The bug only infects people, triggering a gastrointestinal illness called cyclosporiasis.

Its presence suggests some human feces came in contact with the produce carrying it. And that suggests what public health folks call poor toileting practices — farm workers defecating in fields, or not washing their hands before returning to picking after having a bowel movement, for instance.

These parasites aren't found here, Sider says. So if an infected person hasn't recently travelled outside the country, he or she picked up the parasite by eating an imported contaminated food.

While some bacteria that cause foodborne illnesses can be transported via produce, eggs and meat — or even contact with reptiles or baby chicks — Cyclospora are only known to be transmitted on produce.

And yet, even with that narrowing down of the possible sources, these outbreaks are devilishly difficult to solve, suggests Dr. Eleni Galanis, a physician-epidemiologist with the British Columbia Centre for Disease Control.

"Yes, the number of possible sources for a Cyclospora outbreak occurring in Canada are limited — in fact, very limited. Likely just herbs, greens and berries," she says.

"But for every outbreak that we've solved with Cyclospora there's likely five to 10 that we haven't solved."

So how are the cases cracked? Detective work.

Investigators start by trying to establish what the people who became sick ate, comparing their diets in the week or two before they became ill to people who weren't sick. If all the sick people ate cilantro and none of the healthy people did, that would plant a red flag.

But it's never that easy. People have a hard time remembering exactly what they ate, especially if the questions are being asked three or four weeks later.

And the kinds of foods Cyclospora travel on are highly perishable. Samples of contaminated cheese or deli meats might still be in a fridge and available for testing if you are looking for a source of a Salmonella outbreak. But berries and greens will be long gone. Plus, Sider says testing foods for Cyclospora is difficult and not yet terribly useful.

Another way investigators search for clues is by looking at the ages and demographics of the people who got sick, Galanis says.

If it's mainly adult women, public health will always look to see if a restaurant chain's salad bar might have been the culprit. If it's young kids, investigators will ask about petting zoos or kid-friendly foods.

Given the amount of work it takes and the poor likelihood of finding the source, why does public health bother? Galanis says these kinds of investigations are useful for trying to prevent future outbreaks.

She points to the fact that for several years, B.C. had Cyclospora outbreaks every spring. The problem was finally discovered: basil imported from Mexico. The company that had been importing the basil found another producer and the outbreaks stopped.

News from © The Canadian Press, 2015
The Canadian Press

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