Despite the importance of migratory birds in the ecology and evolution of avian influenza virus (AIV), there is a lack of information on the patterns of AIV spread at the intra-continental scale. We applied a variety of statistical phylogeographic techniques to a plethora of viral genome sequence data to determine the strength, pattern, and determinants of gene flow in AIV sampled from wild birds in North America. These analyses revealed a clear isolation-by-distance of AIV among sampling localities. In addition, we show that phylogeographic models incorporating information on the avian flyway of sampling proved a better fit to the observed sequence data than those specifying homogeneous or random rates of gene flow among localities. In sum, these data strongly suggest that the intra-continental spread of AIV by migratory birds is subject to major ecological barriers, including spatial distance and avian flyway.

SUMMARY. In 2006 the U.S. Department of Agriculture, U.S. Department of Interior, and cooperating state fish and wildlife agencies began surveillance for high-pathogenicity avian influenza (HPAI) H5N1 virus in wild birds in the Pacific Flyway of the United States. This surveillance effort was highly integrated in California, Oregon, Washington, Idaho, Nevada, Arizona, Utah, and western Montana, with collection of samples coordinated with state agencies. Sampling focused on live wild birds, hunterkilled waterfowl during state hunting seasons, and wild bird mortality events. Of 20,888 samples collected, 18,139 were from order Anseriformes (waterfowl) and 2010 were from order Charadriiformes (shorebirds), representing the two groups of birds regarded to be the primary reservoirs of avian influenza viruses. Although 83 birds were positive by H5 real-time reverse transcription polymerase chain reaction (rRT-PCR), no HPAI H5N1 virus was found. Thirty-two virus isolates were obtained from the H5-positive samples, including low-pathogenicity H5 viruses identified as H5N2, H5N3, and H5N9.RESUMEN. Muestreo para detectar influenza aviar de alta patogenicidad en aves silvestres de la ruta migratoria del Pacífico de los Estados Unidos, 2006Unidos, -2007 En el año 2006 el Departamento de Agricultura y el Departamento del Interior de los Estados Unidos, con la cooperación de agencias estatales de pesca y vida silvestre iniciaron un muestreo para detectar la influenza aviar de alta patogenicidad subtipo H5N1 en aves silvestres de la ruta migratoria del Pacífico de los Estados Unidos. Este proyecto estuvo integrado principalmente en California, Oregón, Washington, Idaho, Nevada, Arizona, Utah, y la parte oeste de Montana, con la recolección de muestras coordinada por las agencias estatales. El muestreo estuvo enfocado en aves silvestres vivas, aves acuáticas muertas por cazadores durante las estaciones de caza y en eventos de mortalidad de aves silvestres. De las 20,888 muestras recolectadas, 18,139 muestras fueron de aves del orden Anseriformes (aves acuáticas) y 2010 fueron de aves del orden Caradriformes (aves playeras), que representan los dos grupos de aves considerados como los principales reservorios de los virus de influenza aviar. Aunque 83 aves fueron positivas para la hemaglutinina 5 (H5) por la prueba de transcripción reversa y reacción en cadena de la polimerasa en tiempo real, no se encontró la presencia del virus de alta patogenicidad subtipo H5N1. Treinta y dos aislamientos fueron obtenidos de las muestras positivas a H5, incluyendo virus de baja patogenicidad identificados como H5N2, H5N3 y H5N9.

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