108 - DEVELOPMENT OF WNV AREA-WIDE CONTROL TRIGGERS FOR GRIDED SURVEILLANCE SYSTEM IN ORANGE COUNTY, CA

Abstract: The Orange County Mosquito and Vector Control District (OCMVCD) modified its long-standing West Nile virus (WNV) surveillance program in 2019 with the aim of evolving a more equitable, transparent, and evidence-based disease risk response program. Using spatial analysis tools, we examined the distribution of historical WNV positive humans and dead birds to delineate three risk areas in Orange County, CA, designated high, medium, and low. In each risk area, blocks were formed from nine grid cells and a routine trap site was established in the center, resulting in an equidistant trap distribution. Upon detection of WNV, additional traps were deployed around the centralized site, for a fine-scale evaluation. This discussion details the development of WNV control triggers used to manage area-wide truck-mounted ultra-low volume (ULV) space spray applications within the grided system. The triggers were developed using a geospatial analysis of historical WNV human cases and associated vector indices (VI) on a relatively fine spatial scale. VIs from 2004 to 2018 were cumulated by disease week for outbreak and non-outbreak years separately and compared graphically. The lowest VI threshold that was associated with elevated human cases in outbreak years (≥40 cases/yr) was selected as a treatment trigger, a VI of 800. A brief overview of results from a controlled urban treatment study, while operating under the new program, is presented. In conclusion, OCMVCD’s surveillance grid and evidence-based trigger response plan has worked to detect early season hotspots and successfully reduced WNV mosquito infection rates.