The next influenza pandemic: lessons from Hong Kong, 1997.

Pandemic influenza is a zoonosis. Studies on influenza ecology conducted in Hong Kong since the 1970s in which Hong Kong essentially functioned as an influenza sentinel post indicated that it might be possible, for the first time, to have influenza preparedness at the baseline avian level. This appreciation of influenza ecology facilitated recognition of the H5N1 'bird flu' incident in Hong Kong in 1997 in what was considered to be an incipient pandemic situation, the chicken being the source of virus for humans and, if so, was the first instance where a pandemic may have been averted. The 2001 and 2002 H5N1 incidents demonstrated that it was possible to have an even higher order of baseline preparedness with the recognition in chicken of a range of genotypes of H5N1-like viruses before they had the opportunity to infect humans. Investigations of these incidents revealed a complex ecology involving variously precursor avian H5N1 virus in geese and ducks, and H9N2 and H6N1 viruses in quail, the quail possibly functioning as an avian 'mixing vessel' for key genetic reassortment events for onward transmission of H5N1 viruses highly pathogenic for chicken and humans. These findings highlight the importance of systematic virus surveillance of domestic poultry in recognizing changes in virus occurrence, host range and pathogenicity as signals at the avian level that could presage a pandemic. For example, there is now an increasing prevalence of avian influenza viruses in terrestrial (in contrast to aquatic) poultry. Prior to 1997, no particular virus subtype other than H4N6 would have been considered a candidate for pandemicity and this was based, in the absence of any other data, on its high frequency of occurrence in ducks in southern China. Now,with the isolation of H5N1 and H9N2 viruses from humans supported by genetic, molecular and biological studies on these and other avian isolates, there is credible evidence for the candidacy, in order, of H5N1, H9N2 and H6N1 viruses. These viruses have been made available for the production of diagnostic reagents and exploratory vaccines. The 1997 incident upheld the hypothesis that southern China is an epicentre for the emergence of pandemic influenza viruses. However, the intensification of the poultry (chicken) industry worldwide coupled with the spread of viruses such as the Eurasian lineage of H9N2 suggest that the genesis of a pandemic could take place elsewhere in the world. This re-emphasizes the importance of systematic virus surveillance of poultry globally for international public health and for economic and food concerns. Faced with an incipient pandemic in 1997, Hong Kong brought in international experts to join the investigative effort. Good teamwork at all levels is essential in dealing with the many facets. The threat of a pandemic should not be minimized, nor should governments be lulled into a sense of false security. The media is a powerful channel and has the responsibility and the avenues to convey and influence public perception of events. Close liaison between the media and those on the operational side ensures effective, accurate and timely dissemination of information. This will enhance public confidence in the investigative process and in steps taken for its safety and health.