AlaskaDenise
In Memoriam
Global Patterns of Influenza A Virus in Wild Birds
Bj?rn Olsen,<SUP>1</SUP><SUP>,2</SUP> <NOBR>Vincent J. Munster,<SUP>3</SUP></NOBR> <NOBR>Anders Wallensten,<SUP>4</SUP><SUP>,5</SUP></NOBR> <NOBR>Jonas Waldenstr?m,<SUP>6</SUP></NOBR> <NOBR>Albert D. M. E. Osterhaus,<SUP>3</SUP></NOBR> <NOBR>Ron A. M. Fouchier<SUP>3</SUP><SUP>*</SUP></NOBR>
The outbreak of highly pathogenic avian influenza of the H5N1<SUP> </SUP>subtype in Asia, which has subsequently spread to Russia, the<SUP> </SUP>Middle East, Europe, and Africa, has put increased focus on<SUP> </SUP>the role of wild birds in the persistence of influenza viruses.<SUP> </SUP>The ecology, epidemiology, genetics, and evolution of pathogens<SUP> </SUP>cannot be fully understood without taking into account the ecology<SUP> </SUP>of their hosts. Here, we review our current knowledge on global<SUP> </SUP>patterns of influenza virus infections in wild birds, discuss<SUP> </SUP>these patterns in the context of host ecology and in particular<SUP> </SUP>birds' behavior, and identify some important gaps in our current<SUP> </SUP>knowledge.
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<SUP>Genetic Variation of Influenza Viruses in Wild Birds </SUP>
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Evolution of avian influenza viruses in their natural hosts is slow, but not negligible. Avian influenza viruses can be divided into two lineages, Eurasian and American (Fig. 2), probably as a result of long-term ecological and geographical separation of hosts. However, the avifauna of North America and Eurasia are not completely separated; some ducks and shorebirds cross the Bering Strait during migration or have breeding ranges that include both the Russian Far East and northwestern North America (6). The majority of tundra shorebirds from the Russian Far East winter in Southeast Asia and Australia, but some species winter along the west coast of the Americas (22). The overlap in distribution of ducks is not as profound as that of shorebirds, but a few species (e.g., Northern Pintail, Anas acuta) are common in both North America and Eurasia (6) and could also provide an intercontinental bridge for influenza virus. Indeed, influenza viruses carrying a mix of genes from the American and Eurasian lineages have been isolated, indicating that allopatric speciation is only partial (23?25). The partial ecological isolation of influenza virus hosts seems sufficient to facilitate divergent evolution of separate gene pools, but allows occasional spillover of gene segments from one gene pool to the other.
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<CENTER><TABLE cellSpacing=0 cellPadding=0 width="95%"><TBODY><TR bgColor=#e1e1e1><TD><TABLE cellSpacing=2 cellPadding=2><TBODY><TR bgColor=#e1e1e1><TD vAlign=top align=middle bgColor=#ffffff>
</TD><TD vAlign=top align=left>Fig. 2. Phylogenetic tree for the matrix gene of influenza A viruses from a variety of hosts. Nucleotide sequences were selected from public databases and aligned, after which a maximum likelihood tree was generated using influenza virus A/Equine/Prague/57 (H7N7) as outgroup. Sequences were selected from each host to reflect the longest possible time frame and variation in locations of virus isolation. The avian influenza viruses are divided in an American lineage (pink) and a Eurasian lineage (yellow), and there are no clear patterns of host, temporal, or spatial correlation within these lineages. In contrast, the human influenza A virus lineage (light blue), the Eurasian swine lineage (purple), and the HPAI H5N1 lineage (orange) display clear temporal patterns of virus evolution. <NOBR>[View Larger Version of this Image (21K GIF file)]</NOBR> </TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE></CENTER>
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Within each genetic lineage, multiple sublineages of viral genes<SUP> </SUP>cocirculate, but there appear to be no consistent temporal or<SUP> </SUP>spatial correlations. Moreover, genetic data from duck and shorebird<SUP> </SUP>influenza virus isolates from the Americas suggest an active<SUP> </SUP>interplay between these host species (20, 21). Although certain<SUP> </SUP>HA subtypes are reported to be more prevalent in either shorebirds<SUP> </SUP>or ducks in North America, this also does not seem to have resulted<SUP> </SUP>in differences in the genetic composition of influenza viruses<SUP> </SUP>obtained from these two reservoirs (19, 26).
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Bj?rn Olsen,<SUP>1</SUP><SUP>,2</SUP> <NOBR>Vincent J. Munster,<SUP>3</SUP></NOBR> <NOBR>Anders Wallensten,<SUP>4</SUP><SUP>,5</SUP></NOBR> <NOBR>Jonas Waldenstr?m,<SUP>6</SUP></NOBR> <NOBR>Albert D. M. E. Osterhaus,<SUP>3</SUP></NOBR> <NOBR>Ron A. M. Fouchier<SUP>3</SUP><SUP>*</SUP></NOBR>
The outbreak of highly pathogenic avian influenza of the H5N1<SUP> </SUP>subtype in Asia, which has subsequently spread to Russia, the<SUP> </SUP>Middle East, Europe, and Africa, has put increased focus on<SUP> </SUP>the role of wild birds in the persistence of influenza viruses.<SUP> </SUP>The ecology, epidemiology, genetics, and evolution of pathogens<SUP> </SUP>cannot be fully understood without taking into account the ecology<SUP> </SUP>of their hosts. Here, we review our current knowledge on global<SUP> </SUP>patterns of influenza virus infections in wild birds, discuss<SUP> </SUP>these patterns in the context of host ecology and in particular<SUP> </SUP>birds' behavior, and identify some important gaps in our current<SUP> </SUP>knowledge.
.
.
.
.
<SUP>Genetic Variation of Influenza Viruses in Wild Birds </SUP>
<SUP>
Evolution of avian influenza viruses in their natural hosts is slow, but not negligible. Avian influenza viruses can be divided into two lineages, Eurasian and American (Fig. 2), probably as a result of long-term ecological and geographical separation of hosts. However, the avifauna of North America and Eurasia are not completely separated; some ducks and shorebirds cross the Bering Strait during migration or have breeding ranges that include both the Russian Far East and northwestern North America (6). The majority of tundra shorebirds from the Russian Far East winter in Southeast Asia and Australia, but some species winter along the west coast of the Americas (22). The overlap in distribution of ducks is not as profound as that of shorebirds, but a few species (e.g., Northern Pintail, Anas acuta) are common in both North America and Eurasia (6) and could also provide an intercontinental bridge for influenza virus. Indeed, influenza viruses carrying a mix of genes from the American and Eurasian lineages have been isolated, indicating that allopatric speciation is only partial (23?25). The partial ecological isolation of influenza virus hosts seems sufficient to facilitate divergent evolution of separate gene pools, but allows occasional spillover of gene segments from one gene pool to the other.
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<CENTER><TABLE cellSpacing=0 cellPadding=0 width="95%"><TBODY><TR bgColor=#e1e1e1><TD><TABLE cellSpacing=2 cellPadding=2><TBODY><TR bgColor=#e1e1e1><TD vAlign=top align=middle bgColor=#ffffff>
</TD><TD vAlign=top align=left>Fig. 2. Phylogenetic tree for the matrix gene of influenza A viruses from a variety of hosts. Nucleotide sequences were selected from public databases and aligned, after which a maximum likelihood tree was generated using influenza virus A/Equine/Prague/57 (H7N7) as outgroup. Sequences were selected from each host to reflect the longest possible time frame and variation in locations of virus isolation. The avian influenza viruses are divided in an American lineage (pink) and a Eurasian lineage (yellow), and there are no clear patterns of host, temporal, or spatial correlation within these lineages. In contrast, the human influenza A virus lineage (light blue), the Eurasian swine lineage (purple), and the HPAI H5N1 lineage (orange) display clear temporal patterns of virus evolution. <NOBR>[View Larger Version of this Image (21K GIF file)]</NOBR> </TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE></CENTER><SUP></SUP>
Within each genetic lineage, multiple sublineages of viral genes<SUP> </SUP>cocirculate, but there appear to be no consistent temporal or<SUP> </SUP>spatial correlations. Moreover, genetic data from duck and shorebird<SUP> </SUP>influenza virus isolates from the Americas suggest an active<SUP> </SUP>interplay between these host species (20, 21). Although certain<SUP> </SUP>HA subtypes are reported to be more prevalent in either shorebirds<SUP> </SUP>or ducks in North America, this also does not seem to have resulted<SUP> </SUP>in differences in the genetic composition of influenza viruses<SUP> </SUP>obtained from these two reservoirs (19, 26).
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..
<SUP></SUP>
</SUP><SUP></SUP>
<SUP></SUP>
<SUP></SUP>
<SUP></SUP>