168 - Enhanced Mosquito Surveillance Project Aimed at Stabilizing the Vector Index to Determine an Adulticide Application Threshold, Larimer County, Colorado – 2024

Abstract: In 2023, Colorado experienced a historic West Nile virus (WNV) season with the most human cases (n=634) reported annually in the state since the epidemic of 2003 (n=2,948).¹



During the summer of 2023, the Town of Berthoud, located in Larimer County, Colorado, began reporting Vector Index (VI) values exceeding 1.0. Berthoud’s policy required that VI of 0.75 or higher from the town’s five light traps result in a discussion regarding adulticide use. Despite VI values exceeding 3.0 in the Town throughout the summer of 2023, no adulticide applications were conducted. The week-to-week fluctuations in the VI, along with no reported human cases among residents, led the town’s decision-makers to lose confidence in the VI as an accurate measure of the risk of WNV to humans.



To address Berthoud’s concerns, the Larimer County Department of Health and Environment partnered with the Centers for Disease Control and Prevention (CDC)’s Division of Vector-Borne Diseases, Colorado State University, and Vector Disease Control International to conduct a pilot study to increase the number of mosquito traps located within the town from five to ten traps to stabilize the week-to-week variation in the VI and provide more data to the town’s decision-makers to determine the need for adulticide applications. The pilot study was launched in Week 24 (June 2024) and concluded in Week 34. All data from the new traps was integrated with existing trap data to calculate a cumulative weekly VI.



Findings from the project included detection of WNV in mosquitos from the supplemental traps, which would have been undetected without the additional traps in the newly surveilled locations. Additionally, the statistical power added by the supplemental traps resulted in less variation and smaller confidence intervals in the weekly VI calculation, which provided the town with better data with which base policy decisions on.