Aedes aegypti is the primary vector of several arboviruses (such as dengue, chikungunya, and Zika) that cause frequent human disease outbreaks in tropical and subtropical regions. Management of these outbreaks relies on vector control, often in the form of insecticide sprays targeting adult female mosquitoes. However, the spatial coverage and frequency of spraying required for optimal effectiveness are unclear. In this study, we describe the impact of indoor spraying with ultra-low volume (ULV) pyrethroid insecticides on household Aedes aegypti mosquito populations.
Our results show that within-household declines in Aedes aegypti are primarily due to spraying occurring within the same household, with no additional effect from spraying in neighbouring households. Spray effectiveness should be measured in terms of time since the last spray, as we found no cumulative effect from successive sprays. Based on our model, we estimate that spray effectiveness declines by 50% approximately 28 days after spraying.
The decline in Aedes aegypti abundance within a household was primarily determined by the number of days since the last spraying in that household, highlighting the importance of spraying coverage in high-risk areas, with spraying frequency dependent on local virus transmission dynamics.
In this study, we used data from two large field trials of repeated ultra-low volume indoor pyrethroid spraying in the city of Iquitos, in the Peruvian Amazon region to estimate the impact of ultra-low volume spraying on each individual aedes aegypti mosquito population within a household, extending beyond the boundaries of a single household. Previous research has estimated the effect of ultra-low volume treatments based on whether households were inside or outside a larger intervention area. In this study, we aim to disaggregate treatment effects at a finer level of individual households to understand the relative contribution of within-household treatments compared to treatments in neighbouring households. Over time, we estimated the cumulative effect of repeat spraying compared to the most recent spraying on Aedes aegypti reduction in poultry houses to understand the frequency of spraying needed and to assess the decline in spray effectiveness over time. This analysis may assist in the development of vector control strategies and provide information for the parameterization of models to predict their effectiveness.
The outcome of interest is defined as the total number of adult Aedes aegypti collected per household i and time t , which is modeled in a multilevel Bayesian framework using a negative binomial distribution to account for overdispersion, especially since a large number of null Aedes aegypti adults were collected . Given the differences in location and experimental designs between the two studies, all candidate models were fitted to the S-2013 and L-2014 datasets, respectively. Candidate models are developed according to the general form:
a represents any one of a set of candidate variables measuring the impact of spraying on household i at time t, as described below .
b represents any one of a set of candidate variables measuring the impact of spraying on neighbours around household i at time t, as described below .
We tested a simple b-statistic by calculating the proportion of households within a ring at a given distance from household i that were sprayed in the week before t .
where h is the number of households in ring r, and r is the distance between the ring and household i. The distance between rings is assigned based on the following factors:
Relative model fit for time-weighted within-household spray exposure functions. The thicker red line represents the best-fitting model, with the thickest line representing the best-fitting model and the other thick lines representing models whose WAIC is not significantly different from the best-fitting model’s WAIC. B A decay function is applied to the number of days since last spraying that are in the top five best-fitting models based on the average WAIC ranking across the two experiments.
The model estimated that spray effectiveness declined by 50% approximately 28 days after spraying, while Aedes aegypti populations were almost fully recovered approximately 50-60 days after spraying.
In this study, we describe the impact of indoor ultra-low volume pyrethrin spraying on indoor Aedes aegypti populations in relation to spraying events that occur temporally and spatially close to the home. A better understanding of the duration and spatial extent of spraying impacts on Aedes aegypti populations will help to identify optimal targets for the spatial coverage and frequency of spraying required during vector control interventions, and will provide a basis for comparing different potential vector control strategies. information. Our results show that intra-household Aedes aegypti population reductions are due to spraying within a single household, with no additional effect from spraying by households in neighbouring areas. The impact of spraying on indoor Aedes aegypti populations depends primarily on the time since the last spraying and gradually declines over 60 days. No further reduction in Aedes aegypti populations was observed due to the cumulative effect of multiple intra-household spraying events. Overall, the Aedes aegypti population has decreased. The number of Aedes aegypti mosquitoes in a household depends mainly on the time that has passed since the last spraying in that household.
An important limitation of our study is that we did not control for the age of the adult Aedes aegypti mosquitoes collected. Previous analyses of these experiments [14] showed that the age distribution of adult females tended to be younger (increased proportion of nulliparous females) in the L-2014 spraying zone compared to the buffer zone. Thus, although we did not find an additional explanatory role of spraying events in surrounding households on Aedes aegypti abundance in a given household, we cannot be certain that there are no regional effects on Aedes aegypti population dynamics in areas where spraying events occur frequently.
Other limitations of our study include the inability to account for the emergency spraying by the Ministry of Health, which occurred approximately 2 months before the experimental spraying of L-2014, due to the lack of detailed information on its location and timing. Previous analyses have shown that these sprays had a similar effect throughout the study area, forming a common baseline level of Aedes aegypti density; in fact, by the time the experimental spraying began, Aedes aegypti populations had begun to recover . Furthermore, the difference in results between the two experimental periods may be due to differences in study design and different susceptibility of Aedes aegypti to cypermethrin, with S-2013 being more sensitive than L-2014 .
Finally, our results show that the effects of indoor spraying were limited to the household where spraying occurred, and that spraying in neighbouring households did not further reduce Aedes aegypti populations. Adult Aedes aegypti mosquitoes can remain close to or inside houses, congregating within 10 m and travelling an average distance of 106 m . Thus, spraying the area around a house may not have a large impact on the Aedes aegypti population in that house. This supports previous findings that spraying outside or around the house has no effect . However, as mentioned above, there may be regional influences on Aedes aegypti population dynamics, and our model was not designed to detect such effects.
Taken together, our results highlight the importance of reaching every household at higher risk of transmission during an outbreak, as households that have not been recently sprayed cannot rely on nearby interventions or even multiple past interventions to reduce current mosquito populations. Because some houses were inaccessible, initial spraying efforts always resulted in partial coverage. Repeated visits to missed households can increase coverage, but the returns diminish with each round of attempts and the cost per household increases. Vector control programmes therefore need to be improved by targeting areas where the risk of dengue transmission is higher. Dengue transmission is heterogeneous in space and time, and local assessment of high-risk areas, including demographic, environmental and social conditions, should guide targeted vector control efforts . Other targeted strategies, such as combining indoor residual spraying with contact tracing, have been effective in the past and may be successful in some settings . Mathematical models can also help select optimal vector control strategies to reduce transmission in each local setting without the need for expensive and logistically complex field trials . Our results provide a detailed parameterization of the spatial and temporal effects of ultra-low volume indoor spraying, which may inform future mechanistic modeling efforts.
Post time: Jan-13-2025