To achieve these results, Hill et al. honed in on mallard populations within the Pacific flyway to discriminate migration status by way of stable isotope assays (SIA) (δ2H) of primary flight feathers. Multiple sites were surveyed including the Central Valley of California that represents a unique ecosystem due to large populations of over-wintering, migrating, resident and locally bred mallards that coexist within a single migration season. As mentioned earlier, avian influenza A viruses (AIV) have previously been detected in mallards and other bird species sampled within this flyway, including viruses containing Australasian-origin gene segments. Taken together, this collective sampling region spanning Alaska and California is well founded for studying AIV infection dynamics in mallard populations using stable isotope analyses as a tool.
The SIA technique has successfully determined the origin of birds in various ecological studies (Hobson 1999, 2005); however, its application in the context of AIV epidemiology and animal movement is transformative. Focusing on a single migratory season, Hill et al. collected samples from wintering migrant mallards and summer breeding mallards, the latter being used as a subset population to substantiate SIA and assign migration strategies to the population as a whole. Three types of migrant mallards were identified: resident mallards, intermediate migrators and long-distance migrators. A fourth category—breeding mallards—was assigned based on the time of sampling (summer). By extracting more precise migratory data, SIA creates a fine-scale measurement of migratory status that, when combined with general migratory knowledge, can create a more thorough assessment of AIV infection dynamics.
The notion that long-distance bird migration is responsible for moving and increasing the transmission of potentially zoonotic emerging diseases and other avian pathogens has been illustrated for AIV in Asia and on a continental scale for AIV in North America (Lam et al. 2012; Newman et al. 2012); however, this idea was not supported for the particular geographic area and avian species studied by Hill et al. AIV prevalence and viral diversity were similar across the three types of migratory mallards defined in the Pacific flyway, and migration status was not significantly linked to AIV infection within this single migratory cycle. In fact, the only factors linked with AIV prevalence were mallard age (young, hatch year mallards more likely to be infected), strongly documented in past studies and, more importantly, the time of sample collection. These findings further substantiate the importance of mindful, carefully planned surveillance strategies.
As additional support, Hill et al. used phylogenetic tools to analyse the hemagglutinin (HA) and neuraminidase (NA) gene segments of AIV isolates collected in the study within the context of AIV sequences from wild birds in the Pacific flyway (Fig 1). A general movement of AIV in mallards was revealed, occurring in a general southward direction from Alaska to California, which mirrors long- and intermediate-migrant bird patterns. Nevertheless, AIV introduced by long-distance migrators were not predominant in resident or breeding mallard populations, leading the authors to conclude that long-distance migratory mallards play a limited role in moving AIV subtypes within the Pacific flyway.
More broadly, Hill et al. provide an alternate view to the central dogma of AIV transmission dynamics in wild birds by devising crucial observations within this flyway-specific, temporal study of AIV in mallards. The generally accepted paradigm is that AIV thrives at colder temperatures (i.e. high latitudes and/or winter seasons), with long-distance migrants held responsible for the movement of viruses. Interestingly, within the lower latitudes of California, the authors discovered that wintering resident birds served as a likely source population for summer breeding mallards (Fig 2). AIV detected in wintering resident mallards became the predominant circulating virus of summer breeding mallards, lending credence to the idea that nonmigratory mallards play a larger role in AIV transmission than previously thought and more importantly, substantiating their role as a valid reservoir host. Furthermore, resident mallards can facilitate AIV transmission at lower latitudes (i.e. warmer temperatures) causing dogmatic atypical AIV circulation that may increase AIV transmission within the Pacific flyway.
In summary, Hill et al. have provided one of the first extensive, targeted surveillance studies investigating the role of wild bird migration on AIV ecology. While these trends potentially represent a narrow view of a geographically restricted, single migration period of mallards, the multidisciplinary approach employed sheds new light on the role of bird migratory behaviour in the natural history of AIV that eventually donate gene segments to mammalian viruses thus creating novel strains with zoonotic potential. Despite the substantial financial and scientific investments made in influenza surveillance, efforts have been criticized as insufficient (Butler 2012), underscoring the clear need for continued efforts involving advantageous tools, such as SIA, for investigating the complexity of AIV movement and transmission in wild birds.