The importance of animal influenza for human disease.

Influenza is a zoonotic disease caused by a constantly varying RNA virus resulting in a need for continuous surveillance to update human vaccines. Our knowledge indicates that the intermittent pandemics of influenza originate from influenza viruses or gene segments from influenza viruses in lower animals and birds. These pandemics can be mild to catastrophic. While we have learned a great deal about the ecology and molecular properties of "animal" influenza viruses, we do not have a system for comprehensive international surveillance. The 1918 Spanish influenza pandemic that originated from lower animals and the recent H5N1 bird flu incident in Hong Kong serves to remind us that influenza is an emerging disease. The challenge for the 21st century is to accumulate the necessary epidemiological data on animal influenza viruses so that an international surveillance system can be devised. This epidemiological data may provide clues on how to reduce interspecies transmission of influenza. The separation of aquatic birds from other "land based" domestic poultry in Hong Kong after the H5N1 bird flu incident indicates that animal husbandry practices could influence the interspecies transmission of influenza viruses.