( em (cell) receptors for avian viruses...)Cell barrier shows why bird flu not so easily spread among humans

em, (cell) receptors for avian viruses, including avian H5N1 viruses, are present," explains Kawaoka, who also holds an appointment at the University of Tokyo. "But these receptors are rare in the upper portion of the respiratory system. For the viruses to be transmitted efficiently, they have to multiply in the upper portion of the respiratory system so that they can be transmitted by coughing and sneezing."

The upshot of the new finding, says Kawaoka, a professor of pathobiological sciences at the UW-Madison School of Veterinary

Medicine, is that existing strains of bird flu must undergo key genetic changes to become the type of flu pathogen most feared by biomedical scientists.

"No one knows whether the virus will evolve into a pandemic strain, but flu viruses constantly change," Kawaoka says. "Certainly, multiple mutations need to be accumulated for the H5N1 virus to become a pandemic strain."

The finding suggests that scientists and public health agencies worldwide may have more time to prepare for an eventual pandemic of avian influenza. Periodically, animal forms of influenza such as bird flu evolve to become highly contagious human pathogens.

Most scientists agree a pandemic of avian influenza will occur at some time. The worst-case scenario would be a form of influenza similar to the strain of 1918 that killed between 30 million and 50 million people globally.The new work may also help scientists keep track of evolving strains of influenza and provide earlier warning of potential pandemics. For the H5N1 strain of flu virus to evolve to a pathogen easily transmissible from one human to another, changes need to occur in the virus' hemagglutinin surface protein -- a molecule embedded in the virus membrane -- to recognize human receptors, Kawaoka says.

"Mutations in the hemagglutinin for avian H5N1 viruses to recognize human receptors are needed for the virus to become a pandemic strain," Kawaoka explains.

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Source:University of Wisconsin-Madison


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