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Passengers ride a subway in Mexico City as the Mexican capital began to get back to normal on Wednesday after authorities lifted a five-day shutdown of businesses to try to contain the deadly swine flu virus. ((Jorge Dan/Reuters))

Canadian scientists who sequenced Canadian and Mexican samples of the swine flu virus say it is the same strain, even though the virus seems to cause more severe symptoms in Mexico.

Scientists at Health Canada's National Microbiology Laboratory in Winnipeg genetically sequenced and compared samples of the H1N1 flu virus from Nova Scotia, Ontario and Mexico. It's the first time the sequence has been completed on samples from Mexico and Canada, said officials during a news conference in Ottawa Wednesday.

The results have ruled out a mutation to explain why the Mexican cases have been much more severe than elsewhere, said Dr. Frank Plummer, the chief science adviser of the national lab.

"Essentially, what it appears to suggest, is that there is nothing at the genetic level that differentiates this virus that we got from Mexico and those from Nova Scotia and Ontario, that explains apparent differences in disease severity between Mexico and Canada and the United States," said Plummer.

"That's one of the big questions that everybody's been asking, so part of the answer is that it's likely not the virus itself that is explaining the differential and severity of disease between Mexico and the rest of North America."

One possibility being considered is that the Mexican victims may have had underlying medical conditions that made them more susceptible to the bug.

Mexico has reported 42 deaths from swine flu, while two have been reported in the U.S., including one Mexican infant visiting family in Texas.

Canada's cases have all been mild, with the exception of a young Alberta girl who came down with a severe case of the flu.

Plummer said the genetic sequencing results have been submitted to GenBank, an international database where genetic sequences can be studied and compared by scientists around the world.

He also praised scientists for completing the task in less than a week, often working through the night.

The completed sequence will help in the development of a vaccine, said Plummer.

As of Wednesday afternoon, the World Health Organization reported that 1,893 cases of the virus have been confirmed in 23 countries.

The United States has reported 642 confirmed human cases, while Canada has reported 201.

The 36 new cases confirmed in Canada on Wednesday include:

  • 8 new cases in B.C., bringing the provincial total to 54.
  • 4 new cases in Alberta, for a total of 30.
  • 13 new cases in Ontario, for a total of 49.
  • 6 new cases in Quebec, for a total of 10.
  • 5 new cases in Nova Scotia, for a total of 53.

The number of confirmed cases and deaths reported by WHO may not match the numbers reported by countries, because those countries may not yet have reported them to the international health body.

With files from The Canadian Press