Swine flu cases rise in Mexico as experts warn of mutations

MEXICO CITY - Mexico's health secretary says the number of confirmed swine flu cases in his country has risen to 473, including 19 people who died.

The previous confirmed death toll was 16.

Jose Angel Cordova urged citizens not to let their guard down against a virus that is spreading across Asia and Europe. Experts warned the virus could mutate and come back with a vengeance.

Cordova spoke at a news conference yesterday.

Cases outside Mexico suggest the new swine flu strain is weaker than feared. But governments moved quickly anyway to ban flights and prepare quarantine plans. A total of 20 people are confirmed dead worldwide.

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

They told a news conference in Ottawa that the pigs did not pose a food-safety risk, adding that the traveler recovered from the swine flu and that 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 was too early to declare victory.

The World Health Organization decided against a full pandemic alert, but that doesn't mean people can relax, said 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 Anne Schuchat, the U.S. agency's 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.

President Obama urged caution yesterday.

"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 calibrating 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 dire.

Some Mexicans have criticized their government for reacting too slowly to the outbreak at first, and now for overreacting in ordering a five-day, nationwide shutdown of all nonessential government and private business.

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

But experts said there was much they did not know about the outbreak in Mexico. A multinational team of virus sleuths is trying to piece together the puzzle.

Many of the sick around the world were people who had visited Mexico, including 13 of Britain's 15 cases.