Limited evidence of trans-hemispheric movement of avian influenza viruses among contemporary North American shorebird isolates.

Migratory routes of gulls, terns, and shorebirds (Charadriiformes) are known to cross hemispheric boundaries and intersect with outbreak areas of highly pathogenic avian influenza (HPAI). Prior assessments of low pathogenic avian influenza (LPAI) among species of this taxonomic order found some evidence for trans-hemispheric movement of virus genes. To specifically clarify the role of shorebird species in the trans-hemispheric movement of influenza viruses, assess the temporal variation of Eurasian lineages observed previously among North American shorebirds, and evaluate the necessity for continued sampling of these birds for HPAI in North America, we conducted a phylogenetic analysis of >700 contemporary sequences isolated between 2000 and 2008. Evidence for trans-hemispheric reassortment among North American shorebird LPAI gene segments was lower (0.88%) than previous assessments and occurred only among eastern North American isolates. Furthermore, half of the reassortment events occurred in just two isolates. Unique phylogenetic placement of these samples suggests secondary infection and or involvement of other migratory species, such as gulls. Eurasian lineages observed in North American shorebirds before 2000 were not detected among contemporary samples, suggesting temporal variation of LPAI lineages. Results suggest that additional bird migration ecology and virus phylogenetics research is needed to determine the exact mechanisms by which shorebirds in eastern North America become infected with LPAI that contain Eurasian lineage genes. Because of the low prevalence of avian influenza in non-eastern North America sites, thousands more shorebirds will need to be sampled to sufficiently examine genetic diversity and trans-hemispheric exchange of LPAI viruses in these areas. Alternatively, other avian taxa with higher virus prevalence could serve as surrogates to shorebirds for optimizing regional surveillance programs for HPAI through the LPAI phylogenetic approach.