Parasites found in pre-washed packages of lettuce: Health Canada

In the first-ever large-scale study of its kind, Canadian researchers have tested how clean pre-washed packages of leafy greens really are, and found parasites in dozens of samples purchased in Ontario.

Looking at 544 samples of store-bought, pre-washed salads, researchers from Health Canada and the Public Health Agency of Canada found nearly one-tenth of the samples were contaminated with either cyclospora, cryptosporidium or giardia -- parasites that can cause intestinal illness, such as diarrhea.

"In the present study, a relatively high prevalence of all three parasites was found in packaged, ready-to-eat leafy greens," said the study, published recently in the Journal of Food Protection.

"This, along with the fact that all isolates tested represented species and genotypes commonly reported in humans, suggests that there is a potential for transmission to consumers, particularly since these leafy greens are typically consumed raw."

None of the products that tested positive are believed to have been associated with any reported illness outbreaks.

While cryptosporidium has been reported in such food products as apple cider, apples and locally-grown spinach, the study is the first to positively identify cyclospora and giardia in North American produce samples.

Giardia and cryptosporidium are water-borne microscopic parasites that can cause intestinal illnesses such as diarrhea. Cyclospora is a human-borneparasite, spread when food or water is contaminated with human feces.

"The relatively high prevalence of these parasites in packaged salads and leafy greens establishes a baseline for further studies and suggests a need for additional research with respect to the possible sources of contamination of these foods," the study said.

To conduct the study, the research team purchased a total of 544 prewashed salad samples between April 2009 and March 2010 -- all in the Waterloo, Ont. area. After testing the samples, the team found:

*Two of the samples were contaminated with two of the parasites

In total, 507 of the samples were grown in the U.S., with 23 coming from Canada and seven from Mexico. Two were labelled as the products of two countries. Of the contaminated samples, 46 were grown in the U.S. and three were grown in Canada. None of the three Mexican samples tested positive for the parasites.

Brent Dixon, a parasite scientist with Health Canada and one of the report’s authors, said the study breaks new ground for scientists studying the North American food supply chain -- and serves as a wake-up call.

"The fact (the parasites) are there at all is of some concern to us," he told CTV News, adding all the samples tested were labelled as pre-washed and, in some cases, triple-washed.

"Consumers that are concerned can do additional washing, but from what we know it does not remove 100 per cent of pathogens from produce."

Still, Dixon doesn't want Canadians turning away from salad; he noted that the health benefits of eating raw greens will likely outweigh the risks of possibly developing diarrhea from eating them.

The study didn't specify how the samples became contaminated. It is possible that either the food itself or equipment used to process it could have been contaminated during harvest, packaging or transport -- or directly from the hands of food handlers who are infected or have poor sanitary practices. Crops can also become contaminated through the use of contaminated water used to mix pesticides or wash produce.

Rick Holley, a microbiology and food safety professor with the University of Manitoba, said the study points to the need for greater scrutiny of Canada’s food-processing systems.

"We really should be at the lower levels, down around less than a one-per-cent (contamination rate)," he said. "These products are eaten raw: we are not cooking these and they have organisms on them that can cause mild to severe health effects in humans."

Keith Warriner, a food scientist with Guelph University, said the study underlines the fact there are food safety risks associated with leafy greens.

"The issue is that the leafy greens are harvested (and) they are washed, but washing doesn't do that much … What's acquired in the field is usually taken straight onto the plate."

Warriner said consumers wishing to reduce the risk of illness can keep their leafy greens refrigerated, which stops the growth of pathogens.

And for those who are susceptible to a parasitic illness -- including the ill, elderly or pregnant -- Warriner recommends purchasing intact lettuce as opposed to pre-cut products.

With a report from CTV medical specialist Avis Favaro and producer Elizabeth St. Philip