Further evidence emerged Friday that there is person-to-person spread of a new flu virus to which many people may have little or no immunity -- key ingredients for the development of a flu pandemic.
But international authorities, including those at the World Health Organization, were not ready to say that is what the world is watching.
Still, the WHO signalled it will convene a meeting of experts -- likely by teleconference -- who will advise Director General Margaret Chan on whether to raise the pandemic alert level from the current phase 3 to 4 or beyond. Phase 6 is a pandemic. There is little doubt among influenza experts the alert phase will rise as a result of that meeting.
Laboratories in Canada and the United States confirmed Friday that large outbreaks of unusual respiratory illnesses in Mexico are caused by swine flu viruses closely related to those that infected eight people in California and Texas.
The WHO said Friday that so far, no cases have been reported from other countries. In Canada, authorities said while public health officials continue to investigate cases of illness in people returning from Mexico, nothing has yet turned up.
"There's no confirmed cases in Canada," said Dr. David Butler-Jones, Canada's chief public health officer, adding that at this time there is no reason to defer travel to Mexico.
U.S. officials also said they would not tell Americans to avoid travel to Mexico at this point.
Canada's National Microbiology Laboratory in Winnipeg tested a variety of samples sent by Mexican authorities earlier this week because the Mexicans were having problems diagnosing what was behind the large spike of illness.
"We're working very closely with the Mexicans and the Americans and continuing to provide technical support," Butler-Jones said.
Mexican officials said 68 people have died of flu and the new swine flu strain had been confirmed in 20 of those deaths. More than 1,000 people nationwide were sick from the suspected flu. In Mexico City, authorities closed schools, museums, libraries and movie theatres Friday to try to limit spread.
In contrast, only one of the American cases was hospitalized and that person had other medical conditions.
The Winnipeg lab found 16 samples containing the unusual influenza A H1N1 virus that American authorities discovered last week in California and again this week in Texas.
Butler-Jones said Canada was on the alert after the Americans notified the World Health Organization of the swine flu infections on April 17. When Mexico then sought help diagnosing an unusual respiratory illness, he said it was hard not to think there might be a link.
"The spidey senses did go off," Butler-Jones said in an interview.
The acting head of the U.S. Centers for Disease Control, Dr. Richard Besser, said the CDC's labs also confirmed seven cases in 14 samples sent from Mexico, and that early analysis of the viruses suggested they are very similar to those responsible for the American infections.
Besser revealed the U.S. case count has risen to eight, with discovery of an additional infection in a child living in San Diego. The child has recovered.
Besser said the CDC is hearing from the public that they are concerned, adding: "We are worried as well."
"Our concern has grown since yesterday in light of what we've learned since then," he said during a news teleconference.
Heightened anxiety was also apparent among WHO officials in Geneva. The organization activated its 24-hour emergency operations centre Friday. Both Canada and the U.S. have activated theirs as well.
It wasn't clear when a decision would be taken on raising the pandemic alert level, though experts expect events to move quickly in the coming days.
"We're very concerned. And this is why we're running our operations centre 24 hours a day and why the DG (director general) is moving to call the emergency committee.... It all indicates the very high level of our concern," said Gregory Hartl, a spokesperson for the organization.
The H1N1 viruses responsible for the cases are made up predominantly of swine flu genes. Though human H1N1 viruses have been circulating for decades, it is not clear how much protection previous infection with them would confer against a swine flu virus and in particular this swine flu virus, which looks unlike any experts have seen before.
Flu viruses from an animal source -- whether pigs, birds or other mammals -- have the potential to cause pandemics because they are generally so different from human viruses that people have little or no immunity to them.
Widespread illness would be expected to occur if the viruses take off and become a pandemic strain. But the severity of the illness caused depends on the virus, and at present these viruses seem to be causing symptoms similar to regular seasonal flu, said Dr. Allison McGeer, an infectious diseases expert at Toronto's Mount Sinai Hospital.
"What I'm feeling best about is that it does not seem to be a terrible illness. And if this is going to go (pandemic), then it's going to be at least to some degree manageable," she said, noting that even though Mexico is reporting deaths, there are no reports yet that hospitals there are being overwhelmed.
Testing has shown the viruses are resistant to two old flu drugs, amantadine and rimantadine, but are susceptible to the flu antivirals, Tamiflu and Relenza, which are in many national emergency stockpiles, including Canada's.
As would be expected in the early hours of a major disease outbreak, there was a lot of confusion about the scope of the problem and the number of confirmed and suspected cases.
Of the cases found in California and Texas, only one travelled recently to Mexico and most have had no contact with each other. None have reported exposures to pigs.
Besser said based on the pattern so far, it seems that there may have been a number of generations of person-to-person spread involved. He said that and the multiple locations of the outbreak make it unlikely that an effort to snuff out the virus could be mounted.
The WHO has planned for some time to try to extinguish an emerging pandemic virus at source, something modelling exercises suggest might be workable under the right circumstances. Hartl said late Friday the option of trying to contain the outbreak before it could trigger a pandemic is still on the table at the WHO.
Both the WHO and the CDC are sending staff to Mexico to help authorities there get a better handle on the scope of the problem.
Mexican authorities were reporting three separate events in different parts of the country. The earliest report of an uptick of influenza-like illness dates to March 18, the WHO said in a release posted on its website.
In the U.S., the ages of cases range from children to 54 years. It has been reported that in Mexico the majority of cases are young, previously healthy adults in their mid 20s to mid 40s.
