Identification of precursors of Indonesia and Vietnam avian influenza A (H5N1) viruses from southern China

Abstract

The transmission of highly pathogenic avian influenza (HPAI) H5N1 virus to Southeast Asian countries triggered the first outbreak wave of this virus in late 2003. Subsequently H5N1 influenza virus has become endemic in poultry in this region, which has lead to the sustained transmission of those viruses and repeated outbreaks in poultry and human infection cases. This situation has raised global concern of a coming influenza pandemic sometime in the near future. Although surveillance work in market poultry had been strengthened following this initial outbreak in Southeast Asian countries, the lack of influenza surveillance prior to the outbreaks made it difficult to identify the precursors and transmission pathways of those H5N1 viruses. To determine the possible source of those H5N1 viruses responsible for this first transmission wave we recently conducted further sequencing of samples collected in live-poultry markets from Guangdong, Hunan and Yunnan Provinces in southern China from 2001 to 2003. Phylogenetic analysis of the HA gene of 50 H5N1 isolates from this period indicated that eight viruses, exclusively from Yunnan, fell as the direct progenitor to viruses isolated from Vietnam. A further two viruses isolated from Hunan were the direct precursor to those viruses from Indonesia. In general, phylogenetic analysis revealed similar relationships for the NA gene and each of the 6 internal genes, indicating that these viruses also belonged to the same H5N1 genotype Z that is predominant throughout Southeast Asia. These results clearly show a transmission of H5N1 viruses from Yunnan to Vietnam and from Hunan to Indonesia. Trade of poultry may be the major route of virus transmission between Yunnan and Vietnam, while the transmission route from Hunan to Indonesia remains unclear and could be either via migratory birds or poultry movement.