Health officials are conducting tests to determine whether there is a link between human cases of swine flu in the United States and a mysterious respiratory illness in Mexico.

Seven human cases of swine flu have been identified in California and Texas, so far.

Meantime, the unidentified illness in Mexico has killed 20 people, according to health officials in Canada.

The National Microbiology Laboratory in Winnipeg is involved in conducting tests on samples from some of the Mexican cases to see if any of them contain the swine flu virus.

Canadians told to be on alert

An advisory sent by the Public Health Agency of Canada to health services across Canada warns that Canadians who have recently vacationed in Mexico should stay on alert for severe flu-like symptoms that could be connected to the respiratory illness that has surfaced in that country.

Anyone who has returned from Mexico within the last two weeks and is suffering the following symptoms should contact a physician:

  • fever
  • cough
  • sore throat
  • shortness of breath
  • muscle and joint pain

The mystery illness has infected hundreds in south and central areas of Mexico, according to Mexican health officials, killing some and leaving others on ventilators. The Mexican health secretary has issued a national alert, following reports of numerous influenza deaths in the first three weeks of April.

At least one Canadian has been treated for severe pneumonia after returning from Mexico, according to the Public Health Agency of Canada.

But the agency said the person's illness was not likely connected to the outbreak in Mexico.

The illness does not appear to have affected the tourist areas on the coast. According to one of the country's Spanish-language newspapers, the regions affected include: the capital region; San Luis Potos�; Oaxaca; and Baja, California.

The flu-like condition has so far affected healthy young adults between the ages 25 and 44. The victims showed influenza-like symptoms that progressed to "severe respiratory distress" within five days. Some health care workers have been affected.

So far, no cases have been confirmed outside of Mexico.

With files from The Canadian press and CTV.ca News Staff