Study: Bird-flu virus takes hold in lungs

NEW YORK — Why doesn't bird flu spread easily between people? Two teams of researchers think they've found a reason.

The virus has a hard time attaching to cells in the nose, throat and upper airways. However, it readily attaches to cells deep in the lungs. That suggests it's not easily coughed or sneezed out into the air, says new research from University of Wisconsin, Madison, virologist Yoshihiro Kawaoka with colleagues in Japan.

It also suggests that people need close and heavy exposure to the H5N1 virus for it to get into the lungs, where it can take hold.

However, once in the lungs, the virus causes extensive damage to the machinery of respiration: the cells and air spaces where oxygen is exchanged for carbon dioxide, according to the second team of researchers, from the Erasmus Medical Center in Rotterdam, the Netherlands.

That scenario closely mimics the clinical experience of many of the 184 human cases of bird flu that have been officially diagnosed and recorded since late 2003. More than half of those have been fatal.

The ease with which the virus spreads could change if it mutates. Experts say the new research doesn't indicate how likely the virus is to change genetically and unleash a worldwide outbreak of lethal flu.

Scientists already knew that bird flu viruses use a specific kind of docking site to enter cells they infect, while human flu viruses use a different one. Kawaoka's group found the bird virus docking site appears mostly on lung cells, while being rare on cells found in higher areas such as the nose and windpipe. Those higher areas were dominated instead by the human-type docking site.

Kawaoka said that for H5N1 to become a pandemic virus, it would have to mutate in a way that lets it attach to the same docking site human viruses use. Other mutations would be needed as well, he said in a statement.

The research was reported in today's issue of the journal Nature and the online journal Science.