Some recent research has shed light on the reasons why the H5N1 virus has spread much more easily among birds than among humans. Apparently this virus has a harder time infecting the nose and throat of humans and needs to penetrate all the way into the lower respiratory tract. This also makes it more dangerous when it does manage to infect a human.
Let's hope that H5N1's sugar linkages do not change to alpha 2,3.
Virologists have known for years that human flu viruses attach through a sugar "linkage" designated alpha 2,6. The vast family of avian flu viruses favors a linkage with a different shape, designated alpha 2,3.
A research team led by Kyoko Shinya at the University of Wisconsin and the University of Tokyo looked at what cells in the human respiratory tract contain which linkage. They found that alpha 2,6 -- the receptor for human flu -- predominated in the nose and down the airways to the microscopic passageways that lead to the air sacs, or alveoli. At that point, cells with the alpha 2,3 linkage -- the receptor for bird viruses -- become common. Human viruses attached to the upper airways, while avian viruses attached to cells deep in the lungs.
The Japanese researchers, who published their findings in Nature, also tested an H5N1 virus taken from a person who died of bird flu in Hong Kong in 2003. That microbe's hemagglutinin recognized both the human and bird linkages, and it attached to cells from nose to lung. Most H5N1 samples isolated from people do not generally follow that pattern. The ability to bind to alpha 2,6 human receptors is one of several features that would have to become dominant for the H5N1 virus to become easily transmitted from human to human.