Highly pathogenic avian H5N1 influenza A viruses occasionally infect humans, but currently do not transmit efficiently among humans. The viral haemagglutinin (HA) protein is a known host range determinant since it mediates virus binding to host-specific cellular receptors1–3. Here, we therefore assessed the molecular changes in HA that would allow an H5 HA-possessing virus to transmit among mammals. We identified a reassortant virus with H5 HA possessing four mutations in a 2009 pandemic H1N1 virus backbone capable of droplet transmission in a ferret model. The transmissible H5 reassortant virus preferentially recognized human-type receptors, replicated efficiently in ferrets, caused lung lesions and weight loss, but it was not highly pathogenic and did not cause mortality. These results suggest that H5 HA can convert to an HA that supports efficient viral transmission in mammals. However, we do not know whether the four mutations in the H5 HA identified in this study would render a wholly avian H5N1 virus transmissible. The genetic origin of the remaining seven viral genes may also critically contribute to transmissibility in mammals. Nevertheless, as H5N1 viruses continue to evolve and infect humans, receptor-binding variants of H5N1 viruses with pandemic potential, including avian-human reassortant viruses as tested here, may emerge. Our findings emphasize the need for pandemic preparedness for H5 HA-possessing viruses and will help individuals conducting surveillance in regions with circulating H5N1 viruses to recognize key residues that predict the pandemic potential of isolates, which will inform the development, production, and distribution of effective countermeasures.

H5N1 influenza A viruses have spread to numerous countries in Asia, Europe and Africa, infecting not only large numbers of poultry, but also an increasing number of humans, often with lethal effects. Human and avian influenza A viruses differ in their recognition of host cell receptors: the former preferentially recognize receptors with saccharides terminating in sialic acid-alpha2,6-galactose (SAalpha2,6Gal), whereas the latter prefer those ending in SAalpha2,3Gal (refs 3-6). A conversion from SAalpha2,3Gal to SAalpha2,6Gal recognition is thought to be one of the changes that must occur before avian influenza viruses can replicate efficiently in humans and acquire the potential to cause a pandemic. By identifying mutations in the receptor-binding haemagglutinin (HA) molecule that would enable avian H5N1 viruses to recognize human-type host cell receptors, it may be possible to predict (and thus to increase preparedness for) the emergence of pandemic viruses. Here we show that some H5N1 viruses isolated from humans can bind to both human and avian receptors, in contrast to those isolated from chickens and ducks, which recognize the avian receptors exclusively. Mutations at positions 182 and 192 independently convert the HAs of H5N1 viruses known to recognize the avian receptor to ones that recognize the human receptor. Analysis of the crystal structure of the HA from an H5N1 virus used in our genetic experiments shows that the locations of these amino acids in the HA molecule are compatible with an effect on receptor binding. The amino acid changes that we identify might serve as molecular markers for assessing the pandemic potential of H5N1 field isolates.

The persistence of H5N1 avian influenza viruses in many Asian countries and their ability to cause fatal infections in humans have raised serious concerns about a global flu pandemic. Here we report the isolation of an H5N1 virus from a Vietnamese girl that is resistant to the drug oseltamivir, which is an inhibitor of the viral enzyme neuraminidase and is currently used for protection against and treatment of influenza. Further investigation is necessary to determine the prevalence of oseltamivir-resistant H5N1 viruses among patients treated with this drug.

The distribution of sialic acid (SA) species varies among animal species, but the biological role of this variation is largely unknown. Influenza viruses differ in their ability to recognize SA-galactose (Gal) linkages, depending on the animal hosts from which they are isolated. For example, human viruses preferentially recognize SA linked to Gal by the ␣2,6(SA␣2,6Gal) linkage, while equine viruses favor SA␣2,3Gal. However, whether a difference in relative abundance of specific SA species (N-acetylneuraminic acid [NeuAc] and N-glycolylneuraminic acid [NeuGc]) among different animals affects the replicative potential of influenza viruses is uncertain. We therefore examined the requirement for the hemagglutinin (HA) for support of viral replication in horses, using viruses whose HAs differ in receptor specificity. A virus with an HA recognizing NeuAc␣2,6Gal but not NeuAc␣2,3Gal or NeuGc␣2,3Gal failed to replicate in horses, while one with an HA recognizing the NeuGc␣2,3Gal moiety replicated in horses. Furthermore, biochemical and immunohistochemical analyses and a lectin-binding assay demonstrated the abundance of the NeuGc␣2,3Gal moiety in epithelial cells of horse trachea, indicating that recognition of this moiety is critical for viral replication in horses. Thus, these results provide evidence of a biological effect of different SA species in different animals.

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