Mexico said a swine flu epidemic appears to be easing, but today urged citizens not to let their guard down against a virus that has killed 19 people and is spreading across Asia and Europe.
Experts warned the virus could mutate and come back with a vengeance.
The Mexican health secretary this morning said the confirmed swine flu death toll there had risen by three to 19.
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 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 last night.
They told a press conference in Ottawa that the pigs do not pose a food safety risk, adding that the traveller 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 per cent of 2,200 pigs showed a fever and loss of appetite. No pigs have died from the virus, officials said.
The US Centres for Disease Control and Prevention said it is too early to declare victory.
The World Health Organisation also decided against a full pandemic alert, but that does not 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 US 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 US 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.
US President Barack 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," Mr 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.
Some Mexicans have criticised their government for reacting too slowly to the outbreak at first, and now for overreacting in ordering a five-day, nationwide shutdown of all non-essential government and private business.
Responding to the attacks, Health Secretary Jose Angel Cordova said: "It's absurd to think that Mexico was putting on a show. I think it's preferable, at a certain moment, to take advanced measures and succeed in containing the problem than to not take them and ask, 'Why didn't we take them?"'
Mexico's last confirmed swine flu death occurred Tuesday, and the last suspected death came on Wednesday, said Pablo Kuri, an epidemiologist and adviser to Cordova.
Mr Cordova said hospitals are now handling fewer patients with swine flu symptoms, a sign that the disease is presently not very contagious. Mexican investigators who visited 280 relatives of victims found only four had the virus.
Many of the sick around the world were people who had visited Mexico, including 13 of Britain's 15 cases.
AP