153 - The Persistence of West Nile Virus in the Southwestern United States

Abstract: Since the first detection of West Nile virus (WNV) in the US in 1999, nearly 55,000 people have tested positive and more than 2,700 have died. Over the past decade, Arizona has ranked 3rd for highest total and neuroinvasive disease cases, with the majority occurring in Maricopa County. While 2021 was an average year for WNV cases across much of the US, AZ faced its highest number of cases ever, with Maricopa County residents comprising ~86% of those cases. At the onset of this study in 2016, we hypothesized that the county likely experienced repeated introductions year after year, and that given the distance between several southwestern counties that frequently report human cases (eg. Riverside, Clark, Washington, and Yuma counties), outbreaks were geographically distinct. To address our hypotheses, we’ve partnered with vector control agencies throughout the southwest, who have provided more than 800 WNV positive samples that we sequenced using a novel tiled amplicon sequencing approach. Our study reveals novel insights regarding West Nile virus in Maricopa County and the greater southwest: 1) WNV is endemic in Maricopa County, overwintering and reemerging annually, with a limited number of new and short-lived importations, 2) This endemic WNV population is the longest known in any US county, persisting over the past decade, 3) Preliminarily, that genomically-derived effective population size estimates are strong predictors of spillover risk, and 4) WNV in Maricopa County repeatedly spills over into other southwestern counties, indicating that this viral population is not only important for the public health of Maricopa County residents, but the rest of the region as well. Genomic results have been made publicly available, to provide situational awareness to our partners and the public: https://nextstrain.org/community/HeppLab/WestNileVirus@main/NorthAmerica?c=county