An international team of researchers has identified a combination of 5 mutations that allows for airborne transmission of H5N1 avian influenza virus between ferrets (Herfst S et al. Science. 2012;336:1534-1541).
Researchers infected ferrets with H5N1 strains containing mutations that would be expected to increase the virus' affinity for mammalian hosts. They then swabbed the animals' noses and inoculated more ferrets, repeating the process several times. Novel mutations appeared, ultimately allowing the virus to become transmissible through the air, as the researchers found when they placed healthy and infected ferrets in adjacent cages but not in direct contact. Four mutations in the gene for hemagglutinin, a protein that helps the virus enter host cells, and 1 in a protein that helps the virus replicate its genome, were consistently present in airborne-transmitted viruses.
Hampton T. Airborne Transmission of H5N1. JAMA. 2012;308(5):444. doi:10.1001/jama.2012.9461