Ecology and Epidemiology of Influenza A virus in Mallards Anas platyrhynchos

Abstract: The influenza A virus (IAV) is a single-stranded RNA-virus with a segmented genome. It occurs in numerous genetic lineages and can infect a broad range of animal hosts, including humans, pigs, horses, mustelids and birds. Birds of the orders Anseriformes and Charadriiformes are believed to be the natural hosts of most IAV strains. The aim of this thesis was to increase the understanding of naturally circulating IAV in wild birds. My work focused on low pathogenic avian influenza virus (LPAI) in Mallards and their function as hosts.Migratory Mallards were trapped at a stopover site on the island of Öland, Southern Sweden. Over the course of 9 years (2002-2010) a total of 26586 samples from 8529 Mallards were screened for IAV. The prevalence showed repeatable temporal patterns with low virus occurrence in spring and summer, and a marked increase in autumn coinciding with migration. Infections were mainly observed in the gastrointestinal tract, and respiratory infections were rarer. Subtype diversity of the membrane proteins hemagglutinin (HA) and neuraminidase (NA) was very high, with 74 detected subtype combinations. H5 and H7 viruses were common in the duck population and represent a risk of transmission to poultry and thereby potential genesis of highly-pathogenic virus forms.Infected Mallards had a lower body mass than uninfected individuals, which was interpreted as a cost of infection. Thus, transient consequences of infection occurred, but no further effects on migration or survival were found. A first infection with IAV may, comparable to vaccination, induce immunity against future infections with the same virus. By analyzing individual infection histories, we could show that Mallards exhibited both homosubtypic and partial heterosubtypic immunity. The latter was largely explained by HA phylogenetic relatedness at the clade level. The HA and NA are important targets for host immune responses. Hence, in an evolutionary context, changes in host immune pressures will drive changes in the virus, which may result in the diversification of viral HA and NA subtypes to avoid cross-immunity. My studies suggest that heterosubtypic immunity is an important determinant of the evolution of IAV diversity in the Mallard system.