Evolution of Highly Pathogenic H5N1 Avian Influenza Viruses in Vietnam between 2001 and 2007

Abstract

Highly pathogenic avian influenza (HPAI) H5N1 viruses have caused dramatic economic losses to the poultry industry of Vietnam and continue to pose a serious threat to public health. As of June 2008, Vietnam had reported nearly one third of worldwide laboratory confirmed human H5N1 infections. To better understand the emergence, spread and evolution of H5N1 in Vietnam we studied over 300 H5N1 avian influenza viruses isolated from Vietnam since their first detection in 2001. Our phylogenetic analyses indicated that six genetically distinct H5N1 viruses were introduced into Vietnam during the past seven years. The H5N1 lineage that evolved following the introduction in 2003 of the A/duck/Hong Kong/821/2002-like viruses, with clade 1 hemagglutinin (HA), continued to predominate in southern Vietnam as of May 2007. A virus with a clade 2.3.4 HA newly introduced into northern Vietnam in 2007, reassorted with pre-existing clade 1 viruses, resulting in the emergence of novel genotypes with neuraminidase (NA) and/or internal gene segments from clade 1 viruses. A total of nine distinct genotypes have been present in Vietnam since 2001, including five that were circulating in 2007. At least four of these genotypes appear to have originated in Vietnam and represent novel H5N1 viruses not reported elsewhere. Geographic and temporal analyses of H5N1 infection dynamics in poultry suggest that the majority of viruses containing new genes were first detected in northern Vietnam and subsequently spread to southern Vietnam after reassorting with pre-existing local viruses in northern Vietnam. Although the routes of entry and spread of H5N1 in Vietnam remain speculative, enhanced poultry import controls and virologic surveillance efforts may help curb the entry and spread of new HPAI viral genes.

Molecular epidemiology of avian influenza viruses circulating among healthy poultry flocks in farms in northern Vietnam

“…3). The H3N8 viruses isolated from chickens in the study, represented by Previous reports indicated that the distribution of H5N1 strains were different in sublineages in northern and southern Vietnam Wan et al, 2008). Fig.…”

Increased substitution rate in H5N1 avian influenza viruses during mass vaccination of poultry

“…Circulation of H5N1 HPAI virus in Vietnam was firstly identified in 2001, and was almost confirmed in each of the subsequent years [14,15]. The sequence data have revealed several times of introduction of exotic H5N1 viruses into Vietnam [14][15][16].…”

“…The sequence data have revealed several times of introduction of exotic H5N1 viruses into Vietnam [14][15][16]. A vaccination campaign targeting flocks and regions of a higher risk to be infected with the deadly viruses started at the end of 2005 in Vietnam, and resulted in decline of poultry and human cases [17].…”

“…We selected these four periods because mass vaccination was not implemented in China until 2005, and HPAI vaccination has never been approved in Thailand, but was implemented in Indonesia in [2003][2004][2005][2006][2007][2008][2009]. We did not analyze the data of Egypt and Vietnam, because the vaccination strategy were not well implemented in Egypt, and some exotic H5N1 viruses have been introduced into Vietnam in the past several years for multiple times [14][15][16]19]. The substitution rate can be overestimated if these exotic viruses were not well identified and excluded from the calculation.…”

Phylogenetic Relationship of the H5N1 Viruses Isolated in Vietnam, 2012 Based on Sequencing Analyses of HA and NA Gene Segments

“…The outbreaks of HPAI H5N1 virus was first identified in Vietnam in 2001 [7], and outbreaks in poultry have been reported in more than 59 of the 64 Vietnamese provinces since December, 2003(OIE, 2010 [8].…”