A single-letter change in the RNA sequence of an avian influenza virus called H7N9 could explain its continuing ability to infect humans as well as birds.
H7N9 has caused illness in more than 1,000 people since early 2013, and proved fatal in about 40% of cases. Honglin Chen at the University of Hong Kong and his colleagues compared the genome sequence of the 2013 H7N9 strain, which infects people, with all other available flu sequences. They found a single-letter substitution in the RNA for one of the viral proteins in the 2013 strain. This mutation boosts the virus's ability to infect human and mouse cells, without disrupting its ability to replicate in bird cells.
This single-letter change came from another avian flu virus, H9N2, and emerged in 2000. Surveillance programmes that monitor for this sequence in avian viruses may identify those that could potentially jump into humans.
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What makes bird flu jump species?. Nature 543, 593 (2017). https://doi.org/10.1038/543593e