Avian influenza viruses may spark next pandemic.(Clinical Rounds)

Avian influenza viruses could spark the next influenza pandemic, if they acquire the ability to pass from human to human, experts say.

The A(H5N1) strain is of most concern, and has been associated with unusually severe human infection since the first outbreak in 1997 in Vietnam. The virus reappeared in 2003 and again this year, affecting domestic poultry in unprecedented numbers in eight countries in Southeast Asia. So far, 36 humans have been infected, presumably from contact with infected birds. The disease is highly lethal: Of the infected patients, 24 died from viral pneumonia.

"Of the viruses around at the moment, H5N1 is certainly at the top of the list as a potential candidate" to cause a pandemic, said Richard J. Webby, Ph.D., of St. Jude Children's Research Hospital, Memphis. "The virus has never been as widespread as it is now. With so many more millions of birds infected, the chance of people getting infected has increased."

The World Health Organization has recently warned just that, saying that the H5N1 virus has the potential to ignite a global influenza pandemic. "In a number of these outbreaks since the beginning of 2004, the virus has jumped from infected chickens or ducks directly to humans. These direct human infections have produced severe and sometimes fatal outcomes."

So far, human disease has been contained by slaughtering millions of domestic birds in an effort to reduce the possibility of transmission and, perhaps, to slow the virus's evolutionary march. But disturbing news arose last month when Chinese researchers discovered that the virus has now appeared in pigs, revealing its ability to jump yet another species barrier.

Swine infection is of great concern, Dr. Webby said, because these animals can carry not only porcine viruses, but human and avian viruses as well.

"Certainly when they start going into pigs it's a ...