Phylogenetic analysis of the hemagglutinin gene of low pathogenic avian influenza virus H7N7 strains in mallards in Northern Europe.

The H7 subtype of avian influenza (AI) has the capability to evolve into a highly pathogenic AI (HPAI) virus. In this study, we have characterized the hemagglutinin (HA) genes of three avian H7N7 influenza A viruses isolated from healthy migratory mallards in Northern Europe in three different years to study the natural variation of these viruses in the natural reservoir. Phylogenetic analysis demonstrated that the H7 HA genes were all closely related to recent H7 isolates responsible for influenza outbreaks in poultry in Europe. The A/mallard/Sweden/S90735/2003 isolate clustered together with the HA gene of A/mallard/Netherlands/12/00/H7N3 (AY338460), which has been shown to be closely related to the H7N7 responsible for HPAI outbreaks in The Netherlands and Germany in 2003. In contrast, the HA gene of the two mallard strains A/mallard/Sweden/ S90597/2005 and A/mallard/Sweden/100993/2008 were more related to the chicken strain isolated in domestic poultry in England in 2006, A/Ch/England/4054/2006/H7N3 (EF467826), and 2008, A/Ch/England/2008/H7N7 (214011964). Analysis of the deduced HA amino acid sequence shows two different HA cleavage sites in these isolates. Although these HA cleavage sites are consistent with a low pathogenicity AI, the cleavage sites appear to posses an increasing numbers of basic amino acids over time (PEIPKGRGLF in 2003 and 2005 and PEIPKKRGLF in 2008). The conclusion from this study is that H7 subtypes isolated from healthy mallards are closely related to the H7 subtypes that have caused recent influenza outbreaks in poultry in Europe.

Phylogenetic analysis of the non-structural (NS) gene of influenza A viruses isolated from mallards in Northern Europe in 2005.

BACKGROUND: Although the important role of the non-structural 1 (NS) gene of influenza A in virulence of the virus is well established, our knowledge about the extent of variation in the NS gene pool of influenza A viruses in their natural reservoirs in Europe is incomplete. In this study we determined the subtypes and prevalence of influenza A viruses present in mallards in Northern Europe and further analysed the NS gene of these isolates in order to obtain a more detailed knowledge about the genetic variation of NS gene of influenza A virus in their natural hosts. RESULTS: A total number of 45 influenza A viruses of different subtypes were studied. Eleven haemagglutinin- and nine neuraminidase subtypes in twelve combinations were found among the isolated viruses. Each NS gene reported here consisted of 890 nucleotides; there were no deletions or insertions. Phylogenetic analysis clearly shows that two distinct gene pools, corresponding to both NS allele A and B, were present at the same time in the same geographic location in the mallard populations in Northern Europe. A comparison of nucleotide sequences of isolated viruses revealed a substantial number of silent mutations, which results in high degree of homology in amino acid sequences. The degree of variation within the alleles is very low. In our study allele A viruses displays a maximum of 5% amino acid divergence while allele B viruses display only 2% amino acid divergence. All the viruses isolated from mallards in Northern Europe possessed the typical avian ESEV amino acid sequence at the C-terminal end of the NS1 protein. CONCLUSION: Our finding indicates the existence of a large reservoir of different influenza A viruses in mallards population in Northern Europe. Although our phylogenetic analysis clearly shows that two distinct gene pools, corresponding to both NS allele A and B, were present in the mallards populations in Northern Europe, allele B viruses appear to be less common in natural host species than allele A, comprising only about 13% of the isolates sequenced in this study.