Key Differences Between Bird Flu And Human Flu

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Scientists have found key features that distinguish influenza viruses found in birds from those that infect humans.

The St. Jude team used a mathematical technique to identify specific amino acid building blocks that are statistically more likely to appear in avian influenza virus proteins and those that are more likely to be in human influenza virus proteins. The differences in these amino acids can be used as markers to track changes in H5N1 avian influenza strains that threaten humans. "Influenza mutates rapidly, so that any marker that is not the same in bird flu but remains stable in human flu is likely to be important," said David Finkelstein, Ph.D., research associate at the St. Jude Hartwell Center for Bioinformatics and Biotechnology. "If human specific markers start accumulating in bird flu viruses that infect humans, that suggests that the bird flu may be adapting to humans and could spread."

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The researchers also found that various strains of H5N1 that have infected humans are more likely to contain human markers than are H5N1 strains that have not infected humans. Only occasionally have H5N1 samples obtained from human patients shown any of these markers, and no H5N1 strain has permanently acquired any of them.

A report on this work appears in the advanced online edition of "Journal of Virology."

The investigators cautioned that there is no proof yet that the human markers in H5N1 and other bird flu viruses directly contribute to the ability of these viruses to cause pandemics among humans; and H5N1 is not any more adapted to humans today than in the past. However, the fact that the bird viruses accumulate and retain these markers after infecting humans suggests that these changes are important. Therefore, scientists should monitor avian influenza viruses to see if they are acquiring human markers.

"While we can't directly estimate how long it would take an avian virus such as H5N1 to acquire these traits, we can use these markers to roughly measure the distance between an avian influenza and a pandemic," said Clayton Naeve, Ph.D., St. Jude Hartwell Center director and the paper's senior author.

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