Guo, Xi2019-02-122019-02-122018-02https://hdl.handle.net/11299/201698University of Minnesota Ph.D. dissertation. 2018. Major: Veterinary Medicine. Advisors: Carol Cardona, Randall Singer. 1 computer file (PDF); 323 pages.The fall introductions of Influenza A virus (IAV) to turkey flocks in Minnesota (MN) have occurred annually since 1982 with very few exceptions. These cases were associated with outdoor production of turkeys and their exposure to wild birds during migration. Since 2007, there has been an emerging trend of Spring introductions (SIs) of IAV in MN. The subtypes associated with SIs includes H3, H4, H7, H9, and highly pathogenic avian influenza (HPAI) H5N2 which were all avian-origin strains. Outdoor production practices were stopped in 1998 eliminating direct links between wild birds and domesticated turkeys. The factors that drive the Spring seasonality of IAV introductions has never been studied. The goal of this study is to identify these factors. We hypothesized that SIs of IAVs are driven by two synergistic factors, i.e. the presence of sufficient quantities of IAVs that can infect domesticated turkeys, and farm practices that link the flocks with a virus source. The approaches include to characterize the climate conditions, to investigate the epidemiological factors and to evaluate the virological basis. The findings demonstrate that these two factors work synergistically and were not deterministic to the occurrence of SIs. By detecting the viral persistence under SI related temperatures and by measuring the infectious dose of IAVs in turkeys, we show that IAV has long-term persistence under the temperature of both SI-years and non-SI years. Sufficient quantity of IAVs are necessary to drive SIs. By investigating risks associated with farm practices during 2014 – 2016, we illustrate that on-site mortality disposal and the activities of farm workers are critical factors that drive SIs. Improving the current understanding of SIs requires effective surveillance on the natural hosts of SIs. To control and prevent SIs in the future, on farm practices need to be adjusted for Spring conditions and to address risk factors like on site mortality disposal.enavian speciesecologyepidemiologyinfluenza A virusMinnesotasurveyThesis or Dissertation