By Steven Reinberg
FRIDAY, April 24 (HealthDay News) -- Mexican officials took extraordinary steps Friday to try to contain a swine flu outbreak that has killed at least 16 people, and possibly dozens more, and sickened more than 900 other people in recent weeks.
World health officials worried that it could mark the start of a flu pandemic, the Associated Press reported.
It's not yet clear if the swine flu virus in Mexico is the same as the virus blamed for seven cases of swine flu in California and Texas. All seven patients have recovered.
The World Health Organization said at least 57 people have died in the outbreak in Mexico, but it wasn't yet clear if this larger number of deaths was due to swine flu, the AP reported.
"We are very, very concerned," said Thomas Abraham, a spokesman for the agency. "We have what appears to be a novel virus and it has spread from human to human." If international spread is confirmed, that meets WHO's criteria for raising the pandemic alert level, he added.
In response to the outbreak, Mexico City closed schools -- from kindergartens to the university level -- across the metropolis of 20 million people on Friday, and urged people with flu symptoms to stay home from work, The New York Times reported.
"We're dealing with a new flu virus that constitutes a respiratory epidemic that so far is controllable," Health Minister Jose Angel Cordova told reporters late Thursday, after meeting with President Felipe Calderon and other top officials. He said the virus had mutated from pigs and had at some point been transmitted to humans, the Times said.
While Mexico's flu season is usually over by now, health officials noticed a sizeable uptick in flu cases in recent weeks. The World Health Organization reported about 800 cases of flu-like symptoms in Mexico in recent weeks, most of them among healthy young adults, with 57 deaths in Mexico City and three in central Mexico, the Times said.
That could be worrisome. Seasonal flus usually strike hardest at infants and the elderly, but pandemic flus -- such as the 1918 Spanish flu -- often strike young, healthy people, the newspaper reported.
On Thursday, U.S. health officials announced that seven people in California and Texas had now been diagnosed with a unique form of swine flu.
All seven people have recovered, Dr. Anne Schuchat, director of the U.S. Centers for Disease Control and Prevention's National Center for Immunization and Respiratory Diseases, said during an afternoon teleconference. "So far this is not looking like a very severe influenza," she said.
The patients -- three females and four males -- range in age from 9 to 54, Schuchat said.
The first two cases were reported Tuesday in California. There are now five cases in California, including the father of one of the original patients. The other two cases are near San Antonio, Texas, Schuchat said.
"At this point we don't know the extent of the spread of this strain of human influenza derived from swine," she said. "We don't know exactly how people got the virus. None of the patients have had direct contact with pigs."
People can get the virus without contact with pigs, but that's unusual, Schuchat said. "We believe at this point that human-to-human spread is occurring," she said. "We are likely to find more cases and that will not be surprising."
According to Schuchat, the virus in the United States is influenza A N1H1 mixed with swine influenza viruses. The virus contains genetic pieces from four different flu viruses -- North American swine influenza, North American avian influenza, human influenza viruses and swine influenza viruses found in Asia and Europe, she said.
"That particular genetic combination of swine influenza viruses has not been recognized before in the U.S. or elsewhere," Schuchat said.
The viruses are resistant to two antiviral medications -- amantadine and rimantadine -- but are susceptible to the antivirals oseltamivir (Tamiflu) and zanamivir (Relenza), Schuchat said.
Swine flu is a respiratory disease of pigs caused by type A influenza. Swine flu does not normally infect humans. However, human infections do occur, usually after exposure to pigs. Symptoms resemble those of the regular flu, including sore throat, coughing and fever.