Hiram Diaz, 8, gives his six-year-old sister Adely Diaz a ride on the pegs of his bicycle while they wear protective masks Saturday near the market where their parents own a store in Mexico City. (Julio Cortez/Associated Press-Houston Chronicle)Calling the swine flu outbreak in Mexico and the United States a "public health emergency of international concern," the World Health Organization asked countries around the world Saturday to step up their reporting and surveillance of influenza.

The move signals that the WHO fears the outbreak could spread to other countries and is calling for a co-ordinated international response to contain it.

CBC News has learned that students in Nova Scotia are among the first suspected Canadian cases of the virus. Nose and throat swabs collected from them have been sent to the national laboratory in Winnipeg for testing.

Elsewhere:

• As many as 81 flu deaths are suspected and 20 are confirmed, with more than 1,000 people ill across Mexico. The Mexican government has given its health department the power to isolate patients and inspect homes in the swine flu outbreak. The mayor of Mexico City cancelled all public events until further notice and classes have been suspended until May 6.
• Eleven cases are confirmed in the U.S. — in California, Texas and Kansas. All are expected to recover. At least eight students at a New York City high school likely have the swine flu but it won't be known whether it is the same strain of virus that killed people in Mexico until testing is completed.

Passengers, wearing surgical masks as a precaution against infection, arrive at the airport in Tijuana, Mexico, on Saturday. (Guillermo Arias/Associated Press)Dr. Margaret Chan, director general of the WHO, said the outbreak involves "an animal strain of the H1N1 virus and it has pandemic potential," though it's too early to announce a full pandemic alert.

The organization uses six phases to categorize the risks of such an outbreak and its pandemic alert level is currently at Phase 3, for very limited human-to-human transmission.

Phase 6 is a full "pandemic"— sustained human-to-human transmission of disease across national borders.

"The situation is evolving quickly," said Chan. "A new disease is by definition poorly understood. We do not yet have a complete picture of the epidemiology or the risk, including possible spread beyond the currently affected areas."

Swine flu is a respiratory disease of pigs caused by type-A flu viruses. Human cases of swine flu are uncommon but can occur in people who are exposed to pigs and can be spread from person to person.

Symptoms include fever of more than 37.8 C (100 degrees F), body aches, coughing, sore throat, respiratory congestion and, in some cases, diarrhea and vomiting.

Health officials are worried because people appear to have no immunity to the virus, a combination of bird, swine and human influenzas.

Human-to-human transmission of swine flu is thought to occur in the same way as seasonal influenza — through coughing and sneezing, or touching contaminated surfaces.

A Toronto infection control specialist said it's important to keep the outbreak in perspective.

"This sounds like a pandemic — while it's not trivial — that is less severe. And less severe is something that we spent a lot of time planning for and a lot of time working on," Dr. Allison McGeer, director of infection control at Mount Sinai Hospital told CBC News.

While the cases in Mexico sound like a large number, "in truth for influenza, that's a very small number," she added. "You need to remember that in Canada alone, which is not that much bigger than Mexico City actually in population terms, 4,000 people die of seasonal flu every year."

With files from The Associated Press