UC Davis

West Nile virus (WNV) can cause a potentially fatal neuroinvasive mosquito-borne disease. The virus is maintained in an enzootic cycle between birds and Culex mosquitoes but can spillover to cause infections in horses and humans. Approximately 80% of human infections are asymptomatic, but 20% develop a febrile illness and <1% result in a neuroinvasive disease which can result in long-term physical and mental disabilities. During periods of high risk for human infections, insecticides are used to rapidly reduce the abundance of infectious mosquitoes in proximity to humans, thereby reducing zoonotic transmission risk. However, the degree to which mosquito populations are reduced following insecticide applications is highly variable and difficult to measure in operational settings. New vector control strategies, like ivermectin (IVM), a drug that increases mosquito mortality when ingested, are under investigation to improve the specificity of control strategies. This dissertation assesses the spatio-temporal impacts of aerial applications of insecticides and bird-delivered ivermectin on Culex mosquito populations and WNV transmission dynamics.In Chapter 1, we developed generalized additive models to estimate the duration and magnitude of reduction in abundance of Culex (Cx.) tarsalis and Cx. pipiens, the primary WNV vectors in California, following application of aerial adulticides in Sacramento and Yolo counties from 2006-2017. Aerial applications are utilized during periods of epidemic risk to rapidly reduce the abundance of infectious mosquitoes in proximity to humans. The efficacy of these applications for reducing Culex mosquito abundance is difficult to assess for single events due to stochastic variation in trapping success as well as natural variation in mosquito abundance due to season, land use, and weather. Peridomestic Cx. pipiens populations were reduced to a greater extent compared with Cx. tarsalis after both single and repeated spray events, likely due to the species’ focal distribution within urbanized areas. In contrast, impacts of aerial sprays on Cx. tarsalis populations are likely diluted by the species’ large dispersal ability and the broad distributions of productive larval habitat present in the study area. In Chapter 2, we conducted a randomized field trial to investigate the impacts of a novel WNV control strategy, IVM-treated backyard chickens, on mosquito population and WNV transmission dynamics. We placed eight flocks (4 treated and 4 untreated) of six chickens each in backyards across Davis, California for the WNV season (Jun-Sep 2019). We detected a reduction in WNV seroconversions in treated flocks, paired with increased mortality in wild Cx. tarsalis feeding on treated chickens and a reduction in parity in female mosquitoes near treated flocks, suggest that there was a reduction in WNV transmission around treated vs. untreated flocks resulting from ivermectin-administration. We did not detect a difference in abundance or WNV infection rates in mosquitoes. In Chapter 3, we developed a spatially implicit dynamical model of WNV transmission in the presence of IVM-treated birdfeeders to assess the feasibility and efficacy of treating backyard birds with IVM to reduce local WNV transmission. Using field-collected data on birdfeeder usage and nocturnal roosting habits of common backyard birds to parameterize the model, we estimated that reductions up to 83.9% in infectious mosquito-days and 61.3% in infections in competent birds could be obtained under ideal conditions. Both the probability of IVM-induced mosquito mortality and the number of treated lots strongly affect the magnitude of reduction while the spatial distribution of treated lots within a neighborhood did not. Increasing the total number of treated birds in a neighborhood, irrespective of WNV competency, reduced WNV transmission intensity, indicating that IVM deployment should target a wide variety of common backyard bird species. Taken together, the results of this dissertation provide new insights into the efficacy of existing and novel methods of vector control for reducing mosquito abundance and WNV transmission. An improved understanding of these processes will inform future vector control strategies, providing more efficient and targeted approaches to prevent arboviral transmission.