Influenza in migratory birds and evidence of limited intercontinental virus exchange

Abstract

Migratory waterfowl of the world are the natural reservoirs of influenza viruses of all known subtypes. However, it is unknown whether these waterfowl perpetuate highly pathogenic (HP) H5 and H7 avian influenza viruses. Here we report influenza virus surveillance from 2001 to 2006 in wild ducks in Alberta, Canada, and in shorebirds and gulls at Delaware Bay (New Jersey), United States, and examine the frequency of exchange of influenza viruses between the Eurasian and American virus clades, or superfamilies. Influenza viruses belonging to each of the subtypes H1 through H13 and N1 through N9 were detected in these waterfowl, but H14 and H15 were not found. Viruses of the HP Asian H5N1 subtypes were not detected, and serologic studies in adult mallard ducks provided no evidence of their circulation. The recently described H16 subtype of influenza viruses was detected in American shorebirds and gulls but not in ducks. We also found an unusual cluster of H7N3 influenza viruses in shorebirds and gulls that was able to replicate well in chickens and kill chicken embryos. Genetic analysis of 6,767 avian influenza gene segments and 248 complete avian influenza viruses supported the notion that the exchange of entire influenza viruses between the Eurasian and American clades does not occur frequently. Overall, the available evidence does not support the perpetuation of HP H5N1 influenza in migratory birds and suggests that the introduction of HP Asian H5N1 to the Americas by migratory birds is likely to be a rare event.

Figure 1. Comparison of the Frequencies of Influenza A Virus HA and NA Subtypes from Wild Ducks in Alberta, Canada, and Shorebirds in Delaware Bay (New Jersey), United States

Frequency was determined for each group (i.e., wild ducks or shorebirds) as a percentage of the total number of positive isolates of a subtype per total number of positive samples for that group.

Figure 2. Phylogenetic Tree for the HA Gene (Nucleotides 35–1729) of H13 and H16 Influenza A Viruses

The nucleotide sequences were analyzed by PHYLIP 3.66 software [41] using the neighbor-joining method with 100 bootstraps. The tree was rooted with the A/Duck/Memphis/546/74 (H11) HA sequence.

Figure 3. Phylogenetic Tree for the HA-1 Gene (Nucleotides 76–1026) of H7 Influenza A Viruses

The nucleotide sequences were analyzed by PHYLIP software, version 3.66 [41], using the neighbor-joining method with 100 bootstraps. The tree was rooted with the A/Equine/London/1416/73 HA sequence.