The Asia-to-America Influx of Avian Influenza Wild Bird Hosts Is Large

Recent literature has underestimated the number and taxonomic diversity of wild birds moving between Asia and North America. Our analyses of the major avian influenza (AI) host groups show that fully 33 species of waterfowl (Anatidae), 46 species of shorebirds (Charadriidae and Scolopacidae), and 15 species of gulls and terns (Laridae) are involved in movements from Asia to Alaska across northern oceans (Table 1). Our data suggest that about 1.5–2.9 million individuals in these important host groups move from Asia to Alaska annually. Among all of the host groups we consider most relevant for AI virus movement models in this region (waterfowl, shorebirds, and gulls and terns), it seems likely that thousands of AI-infectious birds may be involved in annual Asia-to-America migrations. Importantly, host availability in Alaska once these vectors arrive is also very high, representing at least 5–10 times more birds and infectious birds than the host populations moving from Asia to North America. Incorporating our data into a recent model of the global spread of the highly pathogenic H5N1 suggests that wild birds are a more likely source of this strain being brought into the United States than trade in domestic birds, although the latter remain a numerically more probable source of introduction into the New World. Our results should help in defining the key taxonomic, geographic, and seasonal factors involved in this complex intercontinental association of wild bird AI hosts. The next steps are to determine infection rates of low pathogenicity and highly pathogenic viruses among these hosts and to incorporate these into dynamic models.