What are avian influenza viruses?
• Avian influenza viruses (AIV) are Type A influenza viruses associated with avian species. They have been isolated from more than 100 species of free-living birds worldwide. • Classification of these viruses is based on their hemagglutinin (H) and neuraminidase (N) subtypes. There currently are 16 H and nine N recognized subtypes, all of which are represented in viruses isolated from wild birds. • Wild birds represent the historic source for Type A influenza viruses affecting both domestic bird and mammalian species. • The host adaptation that occurs after the movement of these viruses from wild birds to domestic animals to humans often results in the evolution of “new” viruses, which can become adapted to the new host population. These “new” viruses (which include human Type A influenza viruses) differ from the original viruses detected in wild birds and no longer are associated with wild avian populations. • The movement and adaptation of Type A influenza viruses from wild birds to
