The World Health Organization has been warning for years that the world is overdue for a flu pandemic – and that the avian flu virus circulating in Asia and Europe is the most likely culprit. Those concerns have grown with the impending arrival of bird flu in the United States this year. But, so far, the virus (H5N1), which is deadly to birds, has killed relatively few people.

In order to become a pandemic human virus, the H5N1 strain would need to mutate to become easily passed from person to person. Scientists don’t know exactly what changes would allow the virus to make that deadly jump – but they hope to find some clues with new technologies that can assess viral genomes and proteins with greater accuracy than ever before.

Viruses infiltrate host cells by binding to specific sugar molecules coating the cell. Human and bird cells carry different sugars, so viruses that infect birds don’t usually infect humans.

However, viruses can evolve or swap genes – many of the deadliest human flu viruses of the last century have had bird flu genes. The 1918 flu virus, for example, was a bird virus that acquired the ability to infect human cells and killed 30 to 50 million people worldwide. The viruses behind other more recent flu pandemics were human strains that acquired a few bird virus genes, making them more infectious to people.

Scientists hope that by studying the unique characteristics of previous pandemic flu strains, they will be able to predict how the current strain could mutate to become more deadly to humans. They have zeroed in on the hemagglutinin protein (HA), a molecule that sits on the outside of the influenza virus. The protein binds to sugars and determines which cells the virus can infiltrate.

In a Science paper published this month, researchers at The Scripps Research Institute in La Jolla, CA identified two mutations that could make the H5N1 virus more likely to infect human cells. “This a possible route by which the virus could…gain a foothold in the human population,” says Ian Wilson, the Scripps scientist who led the research. “But it’s not a doom or gloom story…I find it encouraging. Here’s a possible avenue by which the virus can make the switch, and we should look out for it.”

In previous research, the Scripps team tested how well the proteins from different viruses bind to human sugars, by using a new technology called a glycan microarray – a glass slide coated with the different kinds of sugars found in bird and human cells. They discovered that two small mutations in the protein from the 1918 flu strain transformed that virus from one that binds only to bird cells to one that binds only to human cells. Two other small changes in the strain from the 1968 pandemic had a similar effect.

In the new study, researchers copied mutations from the 1918 and 1968 strains into the HA protein of the current H5N1 virus. They found that the same mutations that transformed the 1968 strain could also make the H5N1 HA protein better able to bind to human sugars. (Inducing the changes from the 1918 virus did not have this effect.)

Scientists were relieved to discover that the two mutations only partly transform the H5N1 protein – it can bind to human receptors, but not as well as proteins from the 1918 or 1968 strains. “The ultimate conclusion, at least for mutations we know about, is that it’s a lot harder to tailor H5 types of hemagglutinin to bind to human forms of receptors than it is for [other varieties] of influenza,” says Jeffery Taubenberger, a scientist at the Armed Forces Institute of Pathology in Rockville, MD, who led the effort to sequence the 1918 virus.

Wilson adds that these changes have not been observed in the circulating H5N1 virus, and it’s unclear what kind of impact they would have. “We’re not saying it only takes those two changes to cross the barrier,” he says. “You likely need a lot of other changes to get human virus.”

Some experts are skeptical that the H5N1 strain will ever morph into a human virus. But they emphasize that, if it isn’t H5N1, another deadly strain is likely to emerge, and better surveillance and analysis technologies are crucial for making the world better prepared for a pandemic flu, wherever it emerges. “As a screening tool, the [microarray] will become a really valuable test,” says Taubenberger.

Government organizations are now considering using the technology to monitor the H5N1 virus as it spreads throughout the globe. So the technology could come in handy immediately.

The Scripps researchers are already using the technology to test new variants. An H5N1 strain isolated from people in Turkey was found to have some mutations in the HA receptor binding region. But, according to analysis with the glycan array, those changes did not boost the virus’s ability to bind to human sugars.

Another new variant of H5N1, which began infecting people in Indonesia in 2005, was announced last week at the International Conference on Emerging Infectious Diseases in Atlanta, GA. According to scientists at the Centers for Disease Control, who led the research, the new variant does not appear to be more transmissible between humans, but it does have mutations in the crucial HA protein.

Says Anthony Fauci, director of the National Institute of Allergy and Infectious Disease in Bethesda, MD, “Understanding the molecular basis of the [virus’s] ability to bind to various receptors gets us closer to understanding how viruses adapt to different species and has implications for understanding the response you want in a vaccine.”