The paradoxical impact of drought on West Nile virus risk: Insights from long-term ecological data

Mosquito-borne diseases are deeply embedded within ecological communities,
with environmental changes – particularly climate change – shaping their
dynamics. Increasingly intense droughts across the globe have profound
implications for the transmission of these diseases, as drought conditions
can alter mosquito breeding habitats, host-seeking behaviors, and
mosquito-host contact rates. To quantify the effect of drought on disease
transmission, we use West Nile virus (WNV) as a model system and leverage
a robust mosquito and virus dataset consisting of over 500,000 trap nights
collected from 2010-2023, spanning a historic drought period followed by
atmospheric rivers. We pair this surveillance dataset with a novel
modeling approach that incorporates monthly changes in bird host community
competence, along with drought conditions, to estimate the effect of
drought severity on WNV risk using panel regression models. Our results
show that while drought decreases mosquito abundances, it paradoxically
increases WNV infection rates. This counterintuitive pattern likely stems
from reduced water availability, which concentrates mosquitoes and
pathogen-amplifying bird hosts around limited water sources, thereby
increasing disease transmission risk. However, the magnitude of the effect
depends critically on mosquito species, suggesting species-specific
behavioral traits are key to understanding the effect of drought on
mosquito-borne disease risk across real landscapes.