Avian influenza viruses in Hong Kong: zoonotic considerations

Since the mid-1970s, Hong Kong has functioned as an influenza sentinel post for southern China, a region identified as a hypothetical epicentre for the emergence of pandemic influenza viruses. Nineteen ninety-seven marked the coming-of-age of animal-influenza studies with the recognition in Hong Kong of an incipient pandemic situation brought about by the infection of chicken and humans with an avian influenza H5N1 (H5N1/97) virus. Slaughter of all poultry across the Hong Kong SAR possibly averted a pandemic. Tracking down the source of the H5N1/97 virus to geese and quail and precursor avian H5N1, H9N2 and H6N1 viruses revealed that it was a triple reassortant. This provided a framework for influenza-pandemic preparedness at the baseline avian level, H5N1-like viruses being recognized in chicken in 2001 and twice in 2002; at the human level, H9N2 and H5N1-like isolations were made in 1999 and 2003, respectively. In contrast to Europe and elsewhere, where outbreaks of disease in chicken (by H5 and H7 subtypes) often follow migratory bird activity in an area and the subsequent detection of low-pathogenic avian influenza (LPAI) and highly pathogenic avian influenza (HPAI) virus forms, in southern China, chicken and other poultry are raised in a permanent, year-round avian-influenza milieu as a consequence of duck-raising practices. Given this long-established milieu, the question is raised whether EastAsian avian influenza viruses comprise a group with a greater propensity for interspecies transmission and potential for pandemicity. The intensification of the poultry industry worldwide in recent years may influence the behaviour of these viruses in the milieu and elsewhere. Clearly, there are scientific and veterinary health needs to redefine the terms LPAI and HPAI. The extent of their applicability in southern China where there is now evidence of H5 and H9 subtype viruses exhibiting swings in avian host range perhaps with ‘smouldering virulence’ remains to be seen. With animal influenza now part of the World Health Organization’s “Global Agenda on Influenza Surveillance and Control” the time is now opportune for it and its sister organization dealing with animal health, Office International des Epizooties, to strengthen their links to combat collaboratively the serious threat of avian influenza viruses for humans and animals.