The Molecular Epidemiology of West Nile Virus in North America

West Nile virus (WNV) was introduced into North America in 1999. The original genotype of WNV in North America, NY99, was displaced in 2002 with the current dominant genotype, NA/WN02, which was first identified in 2002. This genotype is characterized by 13 nucleotide changes encoding for one amino acid substitution, E-V159A and has been shown to be transmitted more efficiently in Culex spp. mosquitoes than the NY99 genotype. Studies since 2002 have suggested that WNV has reached genetic homeostasis. The overall objective of this dissertation was to study how WNV has continued to evolve both in Harris County, TX and throughout North America, and to determine if any genetic changes identified have affected the phenotype of the virus. Full-length WNV isolates sequenced from Harris County, Texas from 2002-2011 have shown that WNV continues to evolve with the identification of four new genetic groups since 2005. Furthermore, a new genotype, SW/WN03, was identified which was first detected in 2003 in the southwestern United States (particularly Arizona, Colorado and northern Mexico) and has spread into the Texas Gulf Coast region and other regions throughout the United States. Phenotypic studies on selected isolates from Harris County, TX, representing the different genotypes found in North America, demonstrated relatively few differences in cell culture, neuroinvasiveness in a mouse model, or infectivity and dissemination in mosquitoes. Studies examining the temperature sensitivity of isolates in duck embryo fibroblast cells showed that some isolates exhibited a decreased ability to grow at higher temperatures similar to that seen an isolate belonging to a different lineage 1 clade (KN3829). Finally, studies examining IL-6 induction in A549 cells have shown that an attenuated WNV (Bird1153) induces decreased levels of IL-6, which may be controlled by the NS4B protein. Overall, this dissertation shows that WNV is continuing to evolve in North America with the identification of a new genotype, SW/WN03, and that specific mutations found within some of the isolates modify the phenotype of the virus. Thus, surveillance and study of WNV in North America is important and should be continued, especially as the numbers of cases are rising, as has been seen in 2012.