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One of the fundamental unknowns in the field of influenza biology is a panoramic understanding of the role wild birds play in the global maintenance and spread of influenza A viruses. Wild aquatic birds are considered a reservoir host for all lowly pathogenic avian influenza A viruses (AIV) and thus serve as a potential source of zoonotic AIV, such as Australasian-origin H5N1 responsible for morbidity and mortality in both poultry and humans, as well as genes that may contribute to the emergence of pandemic viruses. Years of broad, in-depth wild bird AIV surveillance have helped to decipher key observations and ideas regarding AIV evolution and viral ecology including the trending of viral lineages, patterns of gene flow within and between migratory flyways and the role of geographic boundaries in shaping viral evolution (Bahl et al. 2009; Lam et al. 2012). While these generally ‘virus-centric’ studies have ultimately advanced our broader understanding of AIV dynamics, recent studies have been more host-focused, directed at determining the potential impact of host behaviour on AIV, specifically, the influence of bird migration upon AIV maintenance and transmission. A large number of surveillance studies have taken place in Alaska, United States—a region where several global flyways overlap—with the aim of detecting the introduction of novel, Australasian-origin highly pathogenic H5N1 AIV into North America. By targeting bird species with known migration habits, long-distance migrators were determined to be involved in the intercontinental movement of individual AIV gene segments, but not entire viruses, between the Australasian and North American flyways (Koehler et al. 2008; Pearce et al. 2010). Yet, bird movement is not solely limited to long-distance migration, and the relationship of resident or nonmigratory and intermediate-distance migrant populations with AIV ecology has only recently been explored by Hill et al. (2012) in this issue of Molecular Ecology. Applying a uniquely refined, multidimensional approach, Hill et al. validate the innovative use of stable isotope assays for qualifying migration status of wild mallards within the Pacific flyway. The authors reveal that AIV prevalence and diversity did not differ in wintering mallard ducks with different migration strategies, and while migrant mallards do indeed introduce AIV, these viruses do not circulate as the predominant viruses in resident birds. On the other hand, resident mallards from more temperate regions act as reservoirs, possibly contributing to the unseasonal circulation and extended transmission period of AIV. This study highlights the impact of animal behaviour on shaping viral evolution, and the unique observations made will help inform prospective AIV surveillance efforts in wild birds.
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.
Figure 1. Graphical 3D representation of a generic influenza A virus. Surface proteins hemagglutinin (HA) and neuraminidase (NA) are represented by purple and yellow projections from the blue outer lipid envelope of the virus. Illustration by Jason Galeon, Esq.
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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.
Figure 2. Three pairs of mallard ducks, Anas platyrhynchos, exhibiting winter plumage at one of the study sites. Sacramento National Wildlife Refuge, California, US. Photo credit: Dr. Nichola J. Hill, Massachusetts Institute of Technology.
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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.