Researchers have discovered numerous changes a flu virus likely would have to make in order to spread as a human pandemic, but so far it seems the avian-flu virus has made just a few of those changes.
"It's reassuring," said David Finkelstein, who led a team of researchers from St. Jude Children's Research Hospital in Memphis, Tenn. Their findings were recently published in the Journal of Virology.
The scientists identified 32 "markers," or clear differences between influenza viruses found in birds and in those that infect humans.
Scientists hope that by learning these markers they will be able to monitor whether the bird flu, referred to as H5N1, is becoming more of a "human" virus and thus a global threat, Mr. Finkelstein said.
"While we can't directly estimate how long it would take an avian virus such as H5N1 to acquire these traits, we can use these markers to roughly measure the distance between an avian influenza and a pandemic," said Clayton Naeve, St. Jude Hartwell Center's director and senior author of the paper.
The good news is that so far, bird-flu viruses in humans have only occasionally shown any of these markers. Current bird-flu viruses "are no more adapted to humans today than they were in the past," the researchers say.
The team compared the genetic makeup of thousands of avian viruses to thousands of viruses from human patients and found 32 amino acids that differentiated the two groups. This indicates that these 32 are "markers on a road" toward a pandemic, said Mr. Finkelstein, a research associate at St. Jude Hartwell Center.
Thirteen of the markers were present in the flu viruses that caused deadly pandemics in 1918, 1957 and 1968, the researchers found.
"The 13 are sort of the minimum dangerous set, as far as we can tell," Mr. Finkelstein said.
But so far, no single human infection of bird flu that was examined showed more than two of the markers, which means bird flu is probably "more than a couple of small steps away" from becoming a human pandemic, he said.
Mr. Finkelstein cautioned, however, that the team's research was a statistical analysis and must be verified in a lab. And he stressed that researchers don't know definitively how many steps or how fast it would take for a virus to become a pandemic.
There have been 321 cases of human infection with bird flu and 194 deaths since 2003, mostly in Asia, according to the World Health Organization.
But it hasn't spread rapidly, like the 1918 flu outbreak that killed millions. Scientists want to know why this hasn't happened yet and what signs to look for, Mr. Finkelstein said.
According to a factsheet from the Centers for Disease Control and Prevention, the unknowns about bird flu give health authorities cause for concern.
"Most cases of avian influenza infection in humans are thought to have resulted from direct contact with infected poultry or contaminated surfaces. However, there is still a lot to learn about how different subtypes and strains of avian influenza virus might affect humans," it states.
Mr. Finkelstein said he hopes their research is one more "nugget of information" in that effort.