P04.3 Spatial and temporal analysis on the impact of ultra-low volume indoor insecticide spraying on Aedes aegypti household density

Abstract

Aedes aegypti is the primary vector for several arboviruses that can cause widespread epidemics, such as Dengue and Zika. This mosquito feeds primarily and frequently on humans, and is well adapted to the urban environment, where it has successfully established itself in many areas throughout the tropics and subtropics. Ultra-low volume (ULV) indoor insecticide spraying has been shown to be effective at reducing Ae. aegypti density for short periods of time (Gunning et al., 2018), however the spatial extent of the vector control effect along the margins of a treated area is unclear, as is the potentially reduced effect of incomplete spray coverage within the treated area. In this study, we characterize the impact of pyrethroid ULV indoor spraying on the Ae. aegypti population within households in relation to the spray events occurring in the proximity of that household in time and space. Improved understanding of the duration and the distance of the effect of a spray intervention on Ae. aegypti populations can better inform planning the coverage and frequency of sprays needed during vector control interventions, as well as modeling efforts that contrast different vector control strategies. 

This project uses data from two large-scale experiments that involved six cycles of indoor pyrethroid spray applications in the Amazonian city of Iquitos, Peru (Gunning et al. 2018). We developed spatial multi-level models to disentangle the reduction in Ae. aegypti numbers that resulted from (1) recent ULV treatment of a specific household and (2) ULV treatment of households in the neighboring area. We compared the fit of models including different weighting schemes for the spray effect, based on different temporal and spatial decay functions to understand lagged ULV effects. 

Our results suggest that the effect of a spray event in a house wanes over time following a gaussian decay, with a spray that occurred between 9-30 days ago having half the effect of a spray that occurred the day before. In addition, we found that the reduction of Ae. aegypti density is mainly determined by time since the most recent spray event, with no significant explanatory value added by the cumulative effect of multiple past sprays. Our results also indicate that the reduction of the Ae. aegypti in a house is due to spray events occurring in that same house, with no significant effect of sprays occurring in adjacent houses or the neighboring area. In summary, the reduction of Ae. aegypti in a house is determined mainly by the time since the last spray intervention in that same house. Together, our results suggest that increased coverage rather than increased frequency of sprays should be the priority of indoor vector control programs.