161 - A Spatially Resolved and Environmentally Informed Forecast Model of St. Louis Encephalitis Virus in Coachella Valley, California

Abstract: St. Louis encephalitis virus (SLE) is endemic to California and was detected annually until 2003 when West Nile virus (WNV) was first identified in California. SLE was not detected again in California until 2015, but has since reestablished itself as an endemic arbovirus. WNV and SLE exist in a complex transmission cycle that uses the same amplifying avian and vector mosquito hosts, but the interplay between these two flaviviruses remains unclear. Preliminary analyses suggest that since the reintroduction of SLE to California in 2015, WNV and SLE transmission have occurred simultaneously. Using mosquito surveillance data from 2015 to 2025, we developed a spatially resolved multi-model inference system of SLE in Coachella, which aims to understand the combination of environmental conditions that have the greatest association with mosquito infection rates. Given the land use characteristics in Coachella and their associations with annual infection rates, monthly temperature and evapotranspiration from October to July were used as environmental predictors. The resulting ensemble model was associated with a combination of environmental drivers yielding significantly higher infection rates relative to an average year. This process was a cooler wetter fall, followed by a drier, warmer winter, leading into a cooler, wetter spring and a cooler summer, which had the highest rates. A wetter and cooler spring followed by a cooler summer works in concert to provide more available water, while cooler than usual desert temperatures reduce mosquito mortality associated with heat stress, giving more time to amplify the virus. Accurate early-season predictions of arboviral risk have the potential to allow local abatement districts and public health entities to implement early-season interventions such as targeted adulticiding and public health messaging before mosquito amplification builds to where human transmission occurs.