[Distribution and circulation of influenza viruses in nature]

All of the known subtypes of influenza A viruses are circulating in ducks, providing the reservoir for the emergence of influenza viruses in all other species including humans. Each of the pandemic influenza virus strains that emerged in 20th century is a reassortant generated in pig co-infected with avian viruses and the preceding human strains. None of the 15 HA and 9 NA subtypes can be ruled out as potential candidates for future pandemics since avian viruses of any subtype can contribute genes in the generation of reassortants in pigs. To expand surveillance for influenza viruses in migratory and domestic ducks and geese, chickens, quail, and pigs of the world and to be informed about what influenza viruses are dominant in the animal reservoirs, we have started 'Program of Excellence for the Control of Influenza'. One of the aims of the program is to establish a library of influenza virus strains and their genes, where influenza virus strains isolated from animal species in the surveillance are thoroughly characterized, stored, and provided for the use of diagnosis and vaccine preparation for the control of future pandemics.