Three more Canadian cases have been confirmed in the current E. coli outbreak associated with romaine lettuce, the Public Health Agency of Canada said on Friday.

That brings the total number of people who have fallen ill in this country to 22, including four cases in Ontario, 17 in Quebec and one in New Brunswick.

The agency said there is no evidence that other parts of Canada have been affected.

The illnesses occurred between late October and early November, Dr. Howard Njoo from the Public Health Agency of Canada, said in a telephone briefing with reporters on Friday.

There's a lag time of 14 to 29 days between someone becoming ill and the agency confirming that it's an E. coli O157 infection — the strain that's behind this outbreak, he said.

When patients start experiencing symptoms, which can include diarrhea and vomiting, it may take time before they seek medical attention. Once they do, a stool sample has to be sent to a lab to confirm whether or not it's an E. coli infection. If the test is positive, local public health authorities are notified and that information is then conveyed to provincial and federal health officials.

The outbreak has also sickened more than 30 people in the U.S.

The romaine lettuce contaminated with the bacteria likely came from California, the head of the U.S. Food and Drug Administration (FDA) said on Friday.

That conclusion is "based on growing and harvesting patterns," FDA Commissioner Dr. Scott Gottlieb tweeted.

"The goal now is to withdraw the product that's at risk of being contaminated from the market, and then re-stock the market," he said.

The Canadian Food Inspection Agency said on Friday that although it was incorporating the information from the FDA into its investigation, it could not yet confirm whether the romaine lettuce making Canadians sick was grown in California.

UPDATE ON OUTBREAK: The romaine implicated in the current outbreak is likely from California based on growing and harvesting patterns. The goal now is to withdraw the product that’s at risk of being contaminated from the market, and then re-stock the market..... —@SGottliebFDA

Gottlieb said the FDA wants "to help unaffected growers get back into production," noting that romaine lettuce would soon be harvested from other growing regions in the U.S., including Florida and Arizona.

The agency also wants to make more specific product labelling "the new standard" to help trace food back to the source, he said. The FDA was "working with growers and distributors on labelling produce for location and harvest date and possibly other ways of informing consumers."

Labelling has proven to be a challenge in investigating food contamination, Matthew Wise, deputy branch chief for outbreak response at the U.S. Centers for Disease Control and Prevention, told CBC News on Friday.

"One of the things that we don't sort of see on packaging is where a leafy green product was grown," Wise said. "A lot of times those bags are listed with ... the location of the corporate headquarters from the company or something like that.

He said that makes it "almost impossible" for someone looking at a bag of lettuce to know whether, for example, it was grown in Arizona, California, Florida or Texas.

Another challenge in tracing E. coli to a particular product in this outbreak, Wise said, is the fact that lettuce has a short shelf life.

"One of the types of evidence that we rely on ... is to test the foods that we think are causing the outbreak and to see if we can find that specific outbreak strain in the foods," he said. "By the time we know that someone's part of an outbreak and we get back to, say, the restaurant that they ate at ... they're not serving the same lettuce that they were serving three weeks ago."

Even when investigators are able to trace E. coli contamination back to a particular region, it's still difficult to narrow down the precise source of the bacteria. It could come from the water used to irrigate a particular farmer's field or somewhere in transport, at a distribution centre, or in a store.

In an outbreak of the same bacterial strain in leafy greens — including romaine lettuce — last year, neither Canadian nor U.S. officials were able to pinpoint where the problem lay.

"It's a complicated distribution chain. It's a tough product to investigate," Wise said.

What people should do

The Public Health Agency of Canada is continuing to urge people in Ontario, Quebec and New Brunswick not to buy or eat romaine lettuce or salad mixes containing romaine lettuce.

If people in those provinces have romaine at home, they should throw it out and clean and sanitize the container it was in, including bins in the fridge.

The agency also advises anyone experiencing vomiting or diarrhea who ate romaine lettuce to see their health-care provider.

Canadian officials have stopped short of advising people living in provinces and territories where cases haven't appeared to take the same precautions. The U.S. Centers for Disease Control and Prevention has taken a much more aggressive stance, advising all American consumers to avoid romaine lettuce and throw any in their homes away.