The discovery of the new swine flu in pigs on an Alberta farm raises a spectre that worries influenza experts: the possibility of the virus moving back and forth between humans and pigs, giving it more chances to mutate along the way.
About 220 pigs in a herd of 2,200 began showing signs of the flu April 24, Canadian officials revealed over the weekend. A farmhand who travelled to Mexico and fell ill upon his return is believed to have infected the pigs with the H1N1 influenza virus.
While the development did not come as a surprise to the World Health Organization or other experts, they expressed concern.
"We expected that at some point since this virus has swine virus elements that we would find possibly the virus in swine pigs in the region where the virus is circulating," Dr. Peter Ben Embarek, a WHO food safety scientist, said Sunday from Geneva.
Measures should be taken to prevent further human exposure to sick animals because of a risk people around the pigs could become infected, Embarek said.
"It has happened in the past with classical swine influenza," he said.
Dr. Ruben Donis, head of the molecular genetics branch of the influenza division at the US Centers for Disease Control, said the movement of a virus from one species to another creates more opportunities for mutations.
While it isn't a given that any changes in the virus would mean it becomes more virulent - causes more severe disease - that cannot be ruled out, he said.
"It's possible," Donis said in an interview from Atlanta. "We have to consider all options."
Donis was especially concerned about the virus getting seeded in pig populations on small farms that don't have the same level of biosecurity as larger operations.
Another worker on the Alberta farm subsequently fell ill, but it's not yet known if that person caught the swine flu.
The herd in central Alberta has been quarantined, and all of the pigs are recovering or have recovered. The farm worker has also recovered.
Meanwhile, Mexico's health secretary declared the swine flu outbreak to be declining in his country, though health officials warned against complacency in combatting the spread of the disease.
There are no recommendations at this point from various agencies to cull pigs in Alberta or anywhere in the world, Embarek said.
He reassured the public that the virus is not a food-borne disease, saying there is no reason to be afraid of consuming pork products.
In Egypt police and armoured cars charged into a crowd of a 1,000 irate pig farmers armed with stones and bottles Sunday.
Twelve people were injured as residents of a Cairo slum resisted government efforts to slaughter the nation's pigs to guard against swine flu.
Dr. Christopher Olsen, a swine flu expert at the University of Wisconsin-Madison, said having this H1N1 influenza A virus go back into swine creates opportunities for it to pick up genetic mutations or swap genes with other flu viruses. The latter process is called reassortment.
"Putting it back into pigs creates more potential for genetic reassortment than in people alone," he said from Madison, Wis.
Canada's swine flu caseload swelled Sunday to 101 after health officials in British Columbia, Alberta, Manitoba Quebec, Ontario and Nova Scotia reported new confirmed cases. Worldwide the WHO confirmed 787 cases in 17 countries.
But even as the tally of people infected with swine flu continued to rise Sunday - at least six other countries reported new cases - Mexico's health secretary said the swine flu epidemic in his country "is now in its declining phase."
Jose Angel Cordova said data suggest the epidemic peaked sometime between April 23 and April 28, and that drastic measures - closing the nation's schools, shuttering most of its businesses and banning mass public gatherings - apparently have helped curb the flu's spread.
But Gregory Hartl, the WHO spokesman for epidemic and pandemic diseases, cautioned against any premature declarations.
"That might be certainly what the current epidemiology is showing," he said from Geneva in response to Cordova's comments.
"I also would like to remind people that in 1918 the Spanish flu showed a surge in the spring and then disappeared in the summer months, only to return in the autumn of 1918 with a vengeance."
It is estimated that upwards of 50 million people died in that pandemic.
Hartl said there is a "high possibility" this H1N1 influenza A virus will come back in colder periods.
"Maybe this current round of activity has peaked, but really, we are only 10 days into the outbreak so we must wait and see," he said.
The death toll in Mexico remains at 19, and the number of confirmed cases has increased slightly, from 473 to 506.
Cordova said 12 of the dead were between 21 and 40 - unusual ages for people to die of the flu because they tend to have stronger immune systems.
Three of the dead were children: a nine-year-old girl, a 12-year-old girl and a 13-year-old boy, said Pablo Kuri, a Mexican epidemiologist and adviser to Cordova. Four were older than 60.
Although most of the dead were from the Mexico City area, they came from different neighbourhoods in the metropolis of 20 million, and there were no similarities linking their medical backgrounds.
One theory for the deaths is that perhaps they sought treatment too late - falling sick an average of seven days before seeing a doctor. Many of the sick around the world were people who had recently visited Mexico.
In China more than 70 Mexican travellers were quarantined in hospitals and hotels as part of that country's sweeping anti-swine flu measures.
Mexicans were being asked to identify themselves on arriving flights and isolated from other travellers after landing, Jorge Guajardo, the country's ambassador to Beijing said Sunday.
Not even the country's diplomats have been immune. The Mexican consul general in the southern city of Guangzhou was briefly held for checks after returning from a Cambodian vacation last week, Guajardo said.
- With files from The Associated Press
