MEXICO CITY — Mexico said a swine flu epidemic appears to be easing, but urged citizens Saturday not to let their guard down against a virus that has killed 17 people and is spreading across Asia and Europe. Experts warned the virus could mutate and come back with a vengeance.

With no suspected swine flu deaths since Wednesday and fewer people turning up at hospitals with virus symptoms, Mexican officials were guardedly optimistic that the worst was over in the outbreak's epicenter.

Cases outside Mexico suggested the new swine flu strain is weaker than feared. But governments moved quickly anyway to ban flights and prepare quarantine plans.

In the first known reported case of the new, mutated virus infecting another species, pigs in the province of Alberta, Canada, have become infected and are under quarantine. They apparently got the virus from a Canadian farm worker who recently visited Mexico and got sick with swine flu, Canadian officials said Saturday.

They told a press conference in Ottawa that the pigs do not pose a food safety risk, adding that the traveler recovered from the swine flu and the pigs are "well on their way to recovery." The outbreak occurred on a single farm, where about 10 percent of 2,200 pigs showed a fever and loss of appetite. No pigs have died from the virus, officials said.

The U.S. Centers for Disease Control and Prevention said it's too early to declare victory.

The World Health Organization also decided against a full pandemic alert, but that doesn't mean people can relax, said Dr. Mike Ryan, WHO's global alert and response director.

"These viruses mutate, these viruses change, these viruses can further reassort with other genetic material, with other viruses," he said. "So it would be imprudent at this point to take too much reassurance" from the small number of deaths.

"We have seen times where things appear to be getting better and then get worse again," said Dr. Anne Schuchat, the U.S. centers' interim science and public health deputy director. "I think in Mexico we may be holding our breath for some time."

The global caseload was 763 and growing – the vast majority in Mexico, the United States and Canada. Costa Rica reported its first confirmed swine flu case – the first in Latin America outside Mexico.

Swine flu cases have been confirmed in 18 countries so far – including Europe, the Middle East and the Asia-Pacific region – and experts believe the actual spread is much wider than the numbers suggest.

U.S. President Barack Obama urged caution Saturday.

"This is a new strain of the flu virus, and because we haven't developed an immunity to it, it has more potential to cause us harm," Obama said. Later, he spoke with Mexican President Felipe Calderon for about 20 minutes to share information.

What started as a swine flu outbreak more than a week ago in Mexico quickly ballooned to a global health threat, with the WHO declaring a pandemic was imminent. Now public health officials are having to carefully calibrate their statements. Push the message too far, and they could lose credibility if the virus fizzles out. But if they back off and it suddenly surges, the consequences could be much more dire.

Health Secretary Jose Angel Cordova said hospitals are now handling fewer patients with swine flu symptoms, a sign that the disease is not very contagious. Mexican investigators who visited 280 relatives of victims found only four had the virus.

But experts said there is much they don't know about the outbreak in Mexico, where tests confirmed 16 deaths and nearly 450 people sickened. A multinational team of virus sleuths is trying to piece together the epidemiological puzzle.

Pablo Kuri, an epidemiologist and adviser to Cordova, said three of the dead were children: a 9-year-old girl, a 12-year-old girl and a 13-year-old boy. Four were older than 60.

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