FRIDAY, April 24, 2009 (HealthDay News) -- Mexican officials took extraordinary steps Friday to try to contain a swine flu outbreak that has killed as many as 20 people, and possibly dozens more, and sickened more than 1,000 other people in recent weeks.
World Health Organization officials worried that it could mark the start of a flu pandemic, according to published reports, although several infectious disease experts in the United States said that was unlikely.
Officials from the U.S. Centers for Disease Control and Prevention said Friday that tests showed some of the Mexico victims died from the same new strain of swine flu that sickened eight people in Texas and California. It's a worrisome new strain that combines genetic material from pigs, birds and humans. All eight U.S. patients have recovered.
The World Health Organization said that 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 Associated Press reported.
"The United States government is working with the World Health Organization and other international partners to assure early detection and warning and to respond as rapidly as possible to this threat," Dr. Richard Besser, acting director of the CDC, said during a Friday afternoon press briefing.
"We do not know if this swine flu virus or some other influenza virus will lead to the next pandemic," he added. "However, scientists around the world continue to monitor the virus and take its threat seriously."
Thomas Abraham, a spokesman for the World Health Organization, said, "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.
Besser said the CDC has issued an outbreak notice for travelers to central Mexico and Mexico City, alerting people to the flu outbreak. "At this time there are no recommendations for U.S. travelers to change, restrict or alter their travel plans to Texas, California or Mexico," he said.
In response to the outbreak, Mexico City closed schools -- from kindergartens to the university level -- as well as museums, libraries, and state-run theaters across the metropolis of 20 million people on Friday, and urged people with flu symptoms to stay home from work, according to published reports.
"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 New York Times reported.
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 had announced that seven people in California and Texas had been diagnosed with a unique form of swine flu.
On Friday, another case of swine flu had been confirmed in a child in San Diego, bringing the total number of U.S. cases to eight, Besser said. The child has recovered from the illness, he added.
All of the U.S. patients 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 a Thursday afternoon teleconference. "So far this is not looking like a very severe influenza," she said.
The seven patients originally reported to the CDC ranged in age from 9 to 54, Schuchat said on Thursday. The first two cases were reported Tuesday in California, she said.
"At this point we don't know the extent of the spread of this strain of human influenza derived from swine," Schuchat 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 found in the U.S. 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.
CDC investigators were working with health officials in California and Texas to identify the source of the infection and to see if any other people have contracted it, Schuchat said.
The CDC is also asking doctors to be on the lookout for cases of flu that are hard to identify, and send samples of the virus to their state health department. In addition, the CDC is working with the virus to prepare a vaccine should there be a need to produce one against the virus, officials said.
Dr. Pascal James Imperato is dean and distinguished service professor of the Graduate Program in Public Health at SUNY Downstate Medical Center Brooklyn, and was director of the Swine Influenza Immunization Program for New York City during the outbreak in 1976. "The virus identified in the recently reported human cases in the U.S. contained genetic elements from several influenza viruses," he said. "While the array of genetic elements seems unusual, it is not unusual for swine flu viruses to contain multiple genetic elements. Also, it is difficult to know how long this virus has been circulating because it was not a special focus of the international influenza surveillance system."
Dr. Marc Siegel, associate professor of medicine at New York University School of Medicine, said the current outbreak was unlikely to become a pandemic. "Swine flu could cause the next pandemic, but it is not likely that this thing is going to erupt and take over the world," he said. Even though the virus is being transmitted human-to-human, "that's a far cry from becoming a pandemic," he added.
Dr. Martin J. Blaser, chairman of the Department of Medicine at New York University Langone Medical Center in New York City, also believes it's unlikely the outbreak will trigger a pandemic.
"The CDC has been doing more surveillance for flu," Blaser said. "So it could be that these cases have been happening all the time, but we just never saw them. Or it is possible that it is a new strain of influenza that is emerging or it's a dangerous new combination. That's why we have to watch it closely."
For more on swine flu, visit the U.S. Centers for Disease Control and Prevention.