MONDAY, March 20, 2006 (HealthDay News) -- The H5N1 bird flu virus continues to change, with U.S. researchers reporting that two different strains of bird flu are now infecting people in Southeast Asia, representing two distinct genetic subgroups.
Whether these and other changes will increase the likelihood of a human flu pandemic remains unknown, however.
The report, by researchers with the U.S. Centers for Disease Control and Prevention, was presented Monday at the International Conference on Emerging Infectious Diseases, in Atlanta.
"Back in 2003 we only had one genetically distinct population of H5N1 with the potential to cause a human pandemic -- now we have two," lead researcher Rebecca Garten said in a prepared statement.
Since 2003, the H5N1 virus has been found in Asia, Europe, Africa and the Middle East and led to the slaughter of tens of millions of domestic fowl. While infection has primarily been limited to birds, the virus has killed more than 100 people. Scientists worry, however, that the germ could mutate into a form that would make human-to-human transmission far easier, raising the specter of a pandemic that could kill millions of people.
Garten's team analyzed more than 300 H5N1 virus samples taken from both birds and humans from 2003 through the summer of 2005.
In the 2005 samples, Garten's group newly identified a second strain of H5N1, which caused flu in humans in Indonesia. Analysis of that strain found that it belongs to a genetic subgroup of the virus that was not known before to cause human disease.
According to Garten, the pool of H5N1 with the potential to cause a human pandemic is growing more genetically diverse, which makes studying the virus more complex and heightens the need for increased surveillance.
Garten said she expects the virus to continue to mutate.
"Change is the only constant," she said. "Only time will tell whether the virus evolves or mutates in such a way that it can be transmitted from human to human efficiently."
One expert thinks these mutations could have a troubling impact on efforts to develop effective vaccines.
"This complicates vaccine strategy," said James C. Paulson, a professor of molecular biology at the Scripps Research Institute, in La Jolla, Calif. "If the virus keeps changing, we can't just pick one strain and immunize everybody and be done with it."
But Paulson added that just because the virus is mutating, that doesn't mean it will necessarily develop into one that is easily transmitted between people. "These genetic changes are important," he said. "But they don't shed light on whether they will become a pandemic in humans."
Another expert thinks the new study highlights the need to control the disease in birds.
"This presents a greater mandate to control the disease in birds," said Dr. Marc Siegel, a clinical associate professor of medicine at New York University School of Medicine and author of Bird Flu: Everything You Need to Know About the Next Pandemic.
Controlling the disease in birds is the best way to prevent it from triggering a human pandemic, Siegel said.
Based on these new findings, Siegel also doesn't think one can conclude that the virus will become pandemic.
"Because this virus changes a lot, because it rarely affected humans before 1997, you cannot assume that genetic change is responsible for human infection," he said. "It may have to do with the overall amount of the virus. I don't automatically assume that it's a structural change that causes it to infect humans."
Researchers haven't shown that the changes in the virus have made humans more susceptible to it, Siegel said.
In other bird flu news, the U.S. Food and Drug Administration announced Monday a proposed final rule to prohibit the "extra-label use" of two classes of approved human antiviral drugs to combat influenza in poultry. Extra-label use is administering a drug in an animal in a manner not in accordance with approved labeling.
The FDA said it was doing this to help ensure the effectiveness of these drugs for treating or preventing influenza infections in humans.
Specifically, the order prohibits the extra-label use by veterinarians of adamantanes (amantadine and rimantadine) and neuraminidase inhibitors (oseltamivir and zanamivir -- brand named Tamiflu and Relenza) in chickens, turkeys, and ducks.
"Today's action is a preventive measure designed to protect the public health and illustrates FDA's high level of commitment and key role in preparing for a possible influenza pandemic, which is a top priority for our nation," acting FDA Commissioner Dr. Andrew von Eschenbach said in a prepared statement.
The U.S. Centers for Disease Control and Prevention can tell you more about bird flu.