Two separate research groups discover the sole trait that keeps avian flu from going airborne between humans.

The mutation of avian flu into a virus communicable between humans remains, for now, only a nightmare, but the fact that it hasn’t happened yet had been a mystery to scientists. This week, two groups of researchers independently identified the trait holding blocking the virus’ spread: an inability to attach to the upper respiratory tract.

“Deep in the respiratory system, [cell] receptors for avian viruses, including avian H5N1 viruses, are present,” said a press statement by Yoshihiro Kawaoka, a virologist at the University of Wisconsin and primary author of a paper published in the March 23rd issue of Nature. “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.”

Avian flu takes hold in an organism’s respiratory tract through proteins that grasp onto the surface receptors of the host’s cells. Previously, scientists had assumed that the virus simply didn’t have the capacity to bind itself very well to host cells in the human respiratory system. But the two research teams, using disparate methods to stain and mark the interactions between the H5N1 virus and tissue from different areas of the human respiratory tract, both overturned this theory.

“Our study shows that that was in fact not the case,” said Thijs Kuiken, a veterinary pathologist at the Erasmus Medical Center in the Netherlands, whose work appears in the March 24th issue of Science. “Although [the virus] had poor affinity for a particular part of the human respiratory tract that had always been examined before—namely the human trachea—if you looked lower down in the human respiratory tract, in fact the virus was able to bind quite well.”

If the virus cannot invade the upper respiratory tract in humans, it becomes much more difficult for human-to-human transmission to take place. Kuiken noted that in the cases where scientists suspect human transmission, very intimate contact—such as that between a mother and her sick child—occurred.

For the avian flu to become the global pandemic that many scientists and public health officials fear, this specific trait blocking the virus’ compatibility in the upper respiratory tract would need to mutate.

“No one knows whether the virus will evolve into a pandemic strain, but flu viruses constantly change,” said Wisconsin’s Kawaoka. “Certainly, multiple mutations need to be accumulated for the H5N1 virus to become a pandemic strain.”

Originally published March 22, 2006


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