The spread of avian influenza to at least 29 new countries in the last seven weeks -- one of the biggest outbreaks of the virus since it emerged nine years ago -- is prompting a sobering reassessment of the strategy that has guided efforts to contain the disease.
Since February, the virus has cut a wide swath across the globe, felling tens of thousands of birds in Nigeria, Israel, India, Sweden and elsewhere. Health officials in the United States say bird flu is likely to arrive in North America this year, carried by wild birds migrating thousands of miles to their summer breeding grounds.
The speed of its migration, and the vast area it has infected, has forced scientists to concede there is little that can be done to stop its spread across the globe.
"We expected it to move, but not any of us thought it would move quite like this," said Dr. David Nabarro, the United Nations' coordinator on bird flu efforts.
The hope was once that culling millions of chickens and ducks would contain or even eradicate the virus. Now, the strategy has shifted toward managing a disease that will probably be everywhere. Officials are hoping to buy a little more time to produce human vaccines and limit the potential economic damage.
"We cannot contain this thing anymore. Nature is in control," said Robert G. Webster, a virologist at St. Jude Children's Research Hospital in Memphis, Tenn., who has been studying the virus since it emerged in 1997.
Formally known as avian influenza A, or H5N1, the virus is rarely transmitted to humans.
There have been 186 human cases and 105 deaths since 2003, according to the World Health Organization. More than a quarter of the deaths -- 29 -- have occurred this year.
Many fear the virus will mutate into a form that is more easily transmitted among people, introducing a deadly flu strain unfamiliar to the human immune system.
Though the virus also could mutate into a harmless strain, scientists have recently found that it has infected domestic cats and a stone marten in Germany, increasing concerns over its ability to cross into mammals.
"Something generally disturbing is going on at the moment," Nabarro said. "It's certainly in the bird world, and it's pushing up against the human world in a serious way."
For most of its existence, H5N1 stewed in Southeast Asia.
