The H in H5N1 stands for the protein hemagglutinin, which is specially designed to attach to bird cell receptors, and scientists feel that the only way the protein could bind to human cell receptors is if it mutates. An international team led by Yoshirhiro Kawoaka a virologist at the University of Tokyo and the University of Wisconsin, Madison, have been screening viral samples from both birds and humans in search of mutations.
The researchers have pinpointed two amino acid changes on the hemagglutinin molecule that allow the virus to bind to human receptors. According to the team's report in the journal Nature, both mutations were isolated from human infections and were found in 600 bird samples.
Previous studies have found that the upper respiratory tract of humans contains cell receptors that the mutated H5N1 could target, which would make them easily transmittable through coughing or sneezing, Kawaoka said.
"It's an important finding because it shows the possible molecular and structural basis for differences in viral attachment patterns," said Thijs Kuiken, a pathologist at Erasmus University in Rotterdam, the Netherlands, who has also been studying how H5N1 could mutate to a form easily transmittable between humans. Kuiken was quick to point out that just because the virus has a way to cross the species barrier, did not mean that a pandemic was on the horizon. "The fact that the virus attaches to a particular receptor on a cell does not immediately mean that it can replicate in that cell," he said.
According to Kawaoka, additional mutations would probably need to occur befor the virus could become a pandemic.
"The problem is we don't know how many steps away a pandemic strain is," he said.
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