Cross-seasonal patterns of avian influenza virus in breeding and wintering migratory birds: a flyway perspective

Abstract

The spread of avian influenza viruses (AIV) in nature is intrinsically linked with the movements of wild birds. Wild birds are the reservoirs for the virus and their migration may facilitate the circulation of AIV between breeding and wintering areas. This cycle of dispersal has become widely accepted; however, there are few AIV studies that present cross-seasonal information. A flyway perspective is critical for understanding how wild birds contribute to the persistence of AIV over large spatial and temporal scales, with implications for how to focus surveillance efforts and identify risks to public health. This study characterized spatio-temporal infection patterns in 10,389 waterfowl at two important locations within the Pacific Flyway--breeding sites in Interior Alaska and wintering sites in California's Central Valley during 2007-2009. Among the dabbling ducks sampled, the northern shoveler (Anas clypeata) had the highest prevalence of AIV at both breeding (32.2%) and wintering (5.2%) locations. This is in contrast to surveillance studies conducted in other flyways that have identified the mallard (Anas platyrhynchos) and northern pintail (Anas acuta) as hosts with the highest prevalence. A higher diversity of AIV subtypes was apparent at wintering (n=42) compared with breeding sites (n=17), with evidence of mixed infections at both locations. Our study suggests that wintering sites may act as an important mixing bowl for transmission among waterfowl in a flyway, creating opportunities for the reassortment of the virus. Our findings shed light on how the dynamics of AIV infection of wild bird populations can vary between the two ends of a migratory flyway.

FIG. 1.

The Pacific Flyway (highlighted in white) and study sites in Alaska (breeding habitat=green) and in California (wintering habitat=orange) where sampling took place during 2007–2009.

FIG. 2.

The prevalence of avian influenza virus (with standard error bars) in dabbling ducks from Alaska and California according to (a) sampling year and (b) sampling site (significant differences between species at each location are indicated by nonmatching letters). AGWT, American green-winged teal; AMWI, American wigeon; MALL, mallard; NOPI, northern pintail; NSHO, northern shoveler.

FIG. 3.

Distribution of hemagglutinin and neuraminidase subtypes in the five dabbling duck species from Alaska (n=17 virus subtypes) and California (n=42 virus subtypes) during 2007–2009.