The number of Listeria cases being reported is increasing as technology to detect food contamination improves, a former Canadian Food Inspection Agency inspector says.

Bob Kingston told CTV News Channel on Saturday that the reason for a perceived increase in recalls and contaminated produce is because of better technology.

"Detection is better and there's more testing going on so that's inevitable," he said, adding that the bigger risk is how fast contamination can spread because of big farming operations.

"It spreads quickly because you're shipping it all over the continent in a short time period," he said.

On Saturday, the Canadian Food Inspection Agency warned the public not to eat certain packages of True Leaf Farms brand bagged and chopped romaine lettuce because it may be contaminated with Listeria monocytogenes.

The affected product, distributed in Calgary and Edmonton, was sold in 6 x 2 lb. bags with a best before date of Sept. 29, 2011.

Northern Lights Produce Network Ltd., the Edmonton-based importers, is voluntarily recalling the affected product.

Listeria can cause listeriosis, a serious but rare illness that usually manifests as a mild flu-like illness. Symptoms can include vomiting, nausea, cramps, severe headache, constipation or fever.

Pregnant women, the elderly and people with weakened immune systems are particularly at risk.

Food contaminated with Listeria monocytogenes may not look or smell spoiled.

Kingston said that consumers should treat their food carefully to prevent illness.

"Unless you grow it in your backyard and know exactly what's put on it, including what type of irrigation water, then treat it like it could potentially be a problem," he said.

Kingston said that since Listeria is not a commonly detected contamination on produce, it is not always sought during testing.

"It's not looked for, so when our labs are doing analysis of produce for microbials when they come into the country, they're not looking for Listeria," he told CTV News Channel on Saturday, adding that because of the sheer volume of imports and limited resources, monitoring is usually restricted to trends.

"It's not aimed at identifying a problem with a particular shipment and then stopping it before it reaches your plate," he said. "It's more about monitoring trends after you've already consumed it."

The latest recall comes on the heels of several similar warnings.

On Friday, Ontario's chief medical officer warned that the Strubs brand of smoked salmon may also be contaminated with Listeria.

The salmon, a ready-to-eat product, is distributed in 250 gram vacuum packs and sold in supermarkets, grocery stores or delis.

The Canadian Food Inspection Agency warned Canadians on Wednesday that packs of Compliments brand Sensations Old-Style Smoked Ham may be contaminated with listeria.

The affected product was sold in Ontario, Quebec, Nova Scotia, New Brunswick, Prince Edward Island and Newfoundland and Labrador.

There have been no reported illnesses connected with the affected products.

In the U.S., health officials said that as many as 16 people have died from possible listeria illnesses traced to Colorado cantaloupes that were shipped to 20 states.

The Centers for Disease Control and Prevention said that 72 illnesses have been linked to the tainted fruit from Jensen Farms in Colorado.

"We've identified the source of the bad cantaloupe," Doug Karas, from the Food and Drug Administration, said. "We're recalled the product, but now we might still see illnesses and even fatalities for the next month up into October."

With files from The Canadian Press