Ecology and evolution of influenza A (H5N1) virus in Asia: Evidence from systematic influenza surveillance

Abstract

The development of highly pathogenic avian influenza A (H5N1) viruses in poultry in Eurasia accompanied with the increase in human infection in 2006 and the recent reemergence of poultry outbreaks throughout the region, suggests that the virus has not been effectively contained and that the pandemic threat persists. Virological and epidemiological findings from our surveillance in live-poultry markets conducted over the last seven years in southern China has provided a comprehensive view of the ecology and evolution of H5N1 influenza viruses and revealed that they are endemic in different types of market poultry. Genetic and antigenic analyses have also demonstrated the dynamic evolution of these viruses in the region, with repeated introductions from southern China to neighboring regions, including Cambodia, Indonesia, Malaysia, Thailand, and Vietnam. A further dramatic spread of the virus westwards throughout Central and Southern Asia, Europe and Africa was also seen after detection of an initial outbreak in wild birds at Qinghai Lake in Central China, in line with our predictions. Genetic analyses revealed that after the H5N1 influenza viruses had established and been endemic several years, they developed into regionally distinct sublineages that allowed us to further trace their transmission pathways. However, recent surveillance data has revealed the emergence and predominance of a single H5N1 virus sublineage in poultry since late 2005 that has gradually replaced those previously detected multiple regional distinct sublineages in China. These viruses have already transmitted to neighboring regions thereby resulting in a new transmission and outbreak wave in Southeast Asia. The persistence of H5N1 virus since it first caused human disease in Hong Kong 11 years ago, as demonstrated by its endemicity over a large geographical region, along with repeated disease outbreaks in both poultry and humans indicates that it will be a long-term and difficult task to bring this virus under control in the absence of well developed disease control systems.