West Nile virus (WNV) has become the leading cause of arboviral encephalitis in the United States. The most notable epidemiological factors associated with the rapid emergence of WNV have been the development of high viral titers within birds and the wide range of mosquito vectors implicated with transmission. Despite the use of bird mortality as a sentinel for WNV activity, the underlying viral genetic basis for WNV pathogenicity in birds and the role of increased vector competence in the transmission of WNV are poorly understood. Generation of fundamental data on the basis of replication within the WNV avian reservoir and insect vector will fill critical gaps in our knowledge of WNV transmission and aid in the improvement of existing surveillance strategies and predictive emergence models for the prevention of human and veterinary disease. The fact that a single gene alteration can impact on the transmission potential of WNV indicates the strong need for genotypic monitoring and virulence characterization of WNV as it moves into new ecological niches that may select for additional genetic change. Our overall hypothesis that that an invading virus must evolve and alter its virulence to adapt to new ecological niches will be addressed by the following three specific aims: (1) Identify WNV fitness differences between the introduced NY99 genotype and strains isolated from North American birds and mosquitoes since 1999 as well as other WNV strains from Australia, Kenya and South Africa. Highly sensitive viral fitness assays will be employed as a marker for the detection of phenotypic differences in standardized high, medium and low susceptible avian and mosquito hosts using in vivo assays (2) Characterize the specific role of genetic substitutions identified for host adaptation by engineering the genetic changes into an infectious cDNA made from an early WNV isolate following North American introduction (3) Determine if long term persistence in avian or mosquito hosts during the overwintering period results in consistent genetic change that alter viremogenic potential after vernal termination. These data could help explain why WNV epidemics seem most extensive the year following introduction into new geographical areas. Virus isolated from chronically infected birds or passed vertically from infected mosquitoes will be examined for genetic changes associated with the avoidance of the host immune systems that allow for persistence. Completion of these specific aims will provide new insight into the selective mechanisms that modulate viral emergence and specifically delineate Iviral genetic changes associated with avian/mosquito infectivity that have facilitated the transcontinental epidemic of West Nile virus in North America.
