Juveniles and migrants as drivers for seasonal epizootics of avian influenza virus

1. Similar to other infectious diseases, the prevalence of low pathogenic avian influenza viruses (LPAIV) has been seen to exhibit marked seasonal variation. However, mechanisms driving this variation in wild birds have yet to be tested. We investigated the validity of three previously suggested drivers for the seasonal dynamics in LPAIV infections in wild birds: (1) host density, (2) immunologically-naïve young, and (3) increased susceptibility in migrants. 2. To address these questions, we sampled a key LPAIV host species, the mallard Anas platyrhynchos, on a small spatial scale, comprehensively throughout a complete annual cycle, measuring both current and past infection (i.e. viral and seroprevalence respectively). 3. We demonstrate a minor peak in LPAIV prevalence in summer, a dominant peak in autumn, during which half of the sampled population was infected, and no infections in spring. Seroprevalence of antibodies to a conserved gene-segment of AIV peaked in winter and again in sp...