(AP) -- A new strain of swine flu has this metropolis of 20 million people increasingly fearful as suspected flu deaths grow, and world health officials warn that Mexico City could be at the epicenter of a global epidemic.
Everything from concerts to sports matches and church services were canceled Sunday to keep people from congregating and spreading the virus in large crowds.
President Felipe Calderon assumed new powers to isolate people infected with a deadly swine flu strain that Mexico's health minister says has killed up to 81 people and likely sickened 1,324 since April 13.
Mexican soldiers and health workers patrolled airports and bus stations, looking for people showing symptoms, which include a fever of more than 100 degrees, body aches, coughing, a sore throat, respiratory congestion and, in some cases, vomiting and diarrhea.
Markets and restaurants were nearly empty. And throngs of Mexicans - some with just a fever - rushed to hospitals.
Mexico appears to have lost valuable days or weeks in detecting the new flu strain, a combination of pig, bird and human viruses that humans may have no natural immunity to. Health officials have found cases in 16 Mexican states. Two dozen new suspected cases were reported in the capital on Saturday alone.
Eleven cases of swine flu were confirmed in California, Texas and Kansas, with more suspected in New York City.
The World Health Organization on Saturday asked all countries to step up reporting and surveillance of the disease, as airports around the world were screening travelers from Mexico for flu symptoms.
WHO Director-General Margaret Chan said the outbreak of the never-before-seen virus has "pandemic potential." But she said it is still too early to tell if it would become a pandemic.
WHO guidance calls for isolating the sick and blanketing everyone around them with anti-viral drugs such as Tamiflu. Too many patients have been identified in Mexico's teeming capital for such a solution now. But some pandemic flu experts say it's also too late to contain the disease to Mexico and the United States.
"Anything that would be about containing it right now would purely be a political move," said Michael Osterholm at the University of Minnesota.
Mexican authorities ordered schools closed in the capital and the states of Mexico and San Luis Potosi until May 6, and the Roman Catholic Church announced the cancellation of Sunday masses in the capital.
A team from the Centers for Disease Control had arrived in Mexico to help set up detection testing for the swine flu strain, something Mexico previously lacked.
Health authorities started noticing a threefold spike in flu cases in late March and early April, but they thought it was a late rebound in the December-February flu season.
Testing at domestic labs did not alert doctors to the new strain. Health Secretary Jose Cordova acknowledged Mexican labs lacked the necessary profiling data to detect the previously unknown strain.
The first death occurred in southern Oaxaca state on April 13, but Mexico didn't send the first of 14 mucous samples to the CDC until April 18, around the same time it dispatched health teams to hospitals looking for patients with severe flu or pnuemonia-like symptoms.
Those teams noticed something strange: The flu was killing people aged 20 to 40. Flu victims are usually either infants or the elderly. The Spanish flu pandemic, which killed at least 40 million people worldwide in 1918-19, also first struck otherwise healthy young adults.
Even though U.S. labs detected the swine flu in California and Texas before last weekend, Mexican authorities as recently as Wednesday were referring to it as a late-season flu.
But mid-afternoon Thursday, Mexico City Health Secretary Dr. Armando Ahued said, officials got a call "from the United States and Canada, the most important laboratories in the field, telling us this was a new virus."
Asked why there were so many deaths in Mexico, and none so far among the U.S. cases, Cordova noted that the U.S. cases involved children - who haven't been among the fatal cases in Mexico, either.
"There are immune factors that are giving children some sort of defense, that is the only explanation we have," he said.
Another factor may be that some Mexican patients may have delayed seeking medical help too long, Cordova said.
Scientists have warned for years about the potential for a pandemic from viruses that mix genetic material from humans and animals.
A "seed stock" genetically matched to the new swine flu virus has been created by the CDC, said Dr. Richard Besser, the agency's acting director. If the government decides vaccine production is necessary, manufacturers would need that stock to get started.
Associated Press Writers David Koop in Mexico City; Frank Jordans in Geneva; Mike Stobbe in Atlanta; Malcolm Ritter in New York; and Maria Cheng in London contributed to this report.
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