Pandemic threat posed by avian influenza A viruses

Influenza pandemics, defined as global outbreaks of the disease due to viru ses with new antigenic subtypes, have exacted high death tolls from human p opulations. The last two pandemics were caused by hybrid viruses, or reasso rtants, that harbored a combination of avian and human viral genes. Avian i nfluenza viruses are therefore key contributors to the emergence of human i nfluenza pandemics. In 1997 an H5N1 influenza virus was directly transmitte d from birds in live poultry markets in Hong Kong to humans. Eighteen peopl e were infected in this outbreak, six of whom died. This avian virus exhibi ted high virulence in both avian and mammalian species, causing systemic in fection in both chickens and mice. Subsequently, another avian virus with t he H9N2 subtype was directly transmitted from birds to humans in Hong Kong. Interestingly, the genes encoding the internal proteins of the H9N2 virus are genetically highly related to those of the H5N1 virus, suggesting a uni que property of these gene products. The identification of avian viruses in humans underscores the potential of these and similar strains to produce d evastating influenza outbreaks in major population centers. Although highly pathogenic avian influenza viruses had been identified before the 1997 out break in Hong Kong, their devastating effects had been confined to poultry. With the Hong Kong outbreak, it became clear that the virulence potential of these viruses extended to humans.