Extreme climate whiplash events drive divergent responses of mosquito-borne disease.

There is mounting concern surrounding climate change effects on human health. Vector-borne diseases-transmitted by ectotherms like mosquitos-are sensitive to abiotic conditions, and there is a significant interest in modeling their response to future climate change. However, changing climate also contributes to increasing variability and frequency of extreme weather, including "climate whiplash" events, when weather conditions abruptly shift between extremes. These events may have more immediate effects on vector-borne diseases, though they have received less attention. Here, we use the series of extreme atmospheric rivers of 2022/2023 in California, following years of extreme drought, as a natural experiment to assess the effect of climate whiplash on vector-borne disease risk. Using high spatiotemporal resolution standing water remote sensing, mosquito and viral surveillance, and community science observations of key reservoir hosts, we estimate mosquito species- and virus-specific responses to flooding following atmospheric rivers using panel regression models. We find significant positive effects of flooding on abundance of the rural West Nile virus (WNV) vector in California's Central Valley, with the largest effects at landscape scales. We find no significant effects for the urban WNV vector, or globally invasive yellow fever mosquito. Finally, we find similarly divergent effects on WNV and Saint Louis Encephalitis virus (SLEV) infection: WNV rates decline significantly in the urban vector and SLEV increases significantly in the rural vector. These results reveal species-specific responses to climate whiplash that are predictable by mosquito ecology, relevant to globally important mosquito vectors and diseases, and inform public health response to future extreme events.