Mexico's president assumed new powers Saturday to isolate people infected with a deadly swine flu strain as authorities struggled to contain an outbreak that world health officials warned could become a global epidemic.
New cases of swine flu were confirmed in Kansas and California and suspected in New York City. But officials said they didn't know whether the New York cases were the strain that now has killed up to 81 people in Mexico and likely sickened 1,324 since April 13, according to figures updated late Saturday by Mexico's health secretary.
Tests have confirmed swine flu as the cause of death in 20 of the cases.
Mexican soldiers and health workers patrolled airports and bus stations as they tried to corral people who may be infected with the swine flu, as it became clearer that the government may have been slow to respond to the outbreak in March and early April.
Now, even detaining the ill may not keep the strain — a combination of swine, bird and human influenza that people may have no natural immunity to — from spreading, epidemiologists say.
The World Health Organization on Saturday asked countries around the world to step up reporting and surveillance of the disease and implement a coordinated response to contain it.
Two dozen new suspected cases were reported in Mexico City alone, where authorities suspended schools and all public events until further notice. More than 500 events, including concerts and sports games, were canceled in the metropolis of 20 million.
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.
The Mexican government issued a decree authorizing President Felipe Calderon to invoke special powers letting the Health Department isolate patients and inspect homes, incoming travelers and baggage. But officials said it was designed to free health workers from possible legal reprisals and to speed disease control efforts.
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.
The U.S. Embassy said the U.S. has not imposed travel constraints to and from Mexico but is suspending the processing of visas and other services through Wednesday to avoid creating crowds.
It issued an earlier message advising U.S. citizens to avoid large crowds, shaking hands, greeting people with a kiss or using the subway.
While suspected swine flu cases have been reported in about 16 Mexican states, Health Secretary Jose Cordova said "it has not spread to the entire country."
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 lays out three criteria necessary for a global epidemic: The virus is able to infect people, can readily spread person-to-person and the global population has no immunity to it.
Early detection and treatment are key to stopping any outbreak. WHO guidance calls for isolating the sick and blanketing everyone around them with anti-viral drugs such as Tamiflu.
Now, with patients showing up all across Mexico and its teeming capital, simple math suggests that kind of response is impossible.
Mexico appears to have lost valuable days or weeks in detecting the new virus.
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, and 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."
"That was what led us to realize it wasn't a seasonal virus ... and take more serious preventative measures," Cordova said.
Asked why there were so many deaths in Mexico, and none so far among the 11 cases in the United States, 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.
Some Mexicans suspected the government had been less than forthcoming. "They always make a big deal about good things that happen, but they really try to hide anything bad," Mexico City paralegal Gilberto Martinez said.
Airports around the world were screening travelers from Mexico for flu symptoms. But containing the disease may not be an option.
"Anything that would be about containing it right now would purely be a political move," said Michael Osterholm, a University of Minnesota pandemic expert.
Scientists have warned for years about the potential for a pandemic from viruses that mix genetic material from humans and animals.
This swine flu and regular flu can have similar symptoms — mostly fever, cough and sore throat, though some of the U.S. victims who recovered also experienced vomiting and diarrhea. But unlike with regular flu, humans don't have natural immunity to a virus that includes animal genes — and new vaccines can take months to bring into use.
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.
Mexican authorities did lay to rest one persistent doubt, after Mexican museum director Felipe Solis died this week, just days after accompanying U.S. President Barack Obama on a tour of National Anthropology Museum on April 16. Cordova said Solis had a pre-existing illness and died of pneumonia unrelated to influenza.