Modeling Aedes albopictus response to control methods based on sterilized males release
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The tiger mosquito, Ae. albopictus, is emerging throughout the world as a public health hazard through the transmission of many human pathogens for which no effective antiviral agent or vaccine are available. Chemical insecticides remain the main tools to control the tiger mosquito populations, but the development of resistances is threatening their effectiveness. In this context, it is necessary to develop alternative control methods, one of which is the Sterile Insect Technique (SIT) consisting in releasing males that have been sterilized using ionizing radiation. Those males reduce the reproductive success of the encountered wild females, hence causing the target population to decline. Another method is the boosted SIT, a SIT improvement, where sterile males are also vectors of a biocide transmitted to the females through mating. The biocides (e.g. growth regulators affecting larval development) can be then specifically dispersed to the breeding sites by the females. Yet, experimenting those practices across the relevant space and time scales is costly, as well potentially hazardous. In this context, mathematical models can provide useful tools to test and optimize such control strategies. We will present an age-structured population dynamics model of the vector Ae. albopictus, designed to explore the demographic effects of SIT control strategies. The model accounts for the spatiotemporal heterogeneity of environmental and meteorological conditions across La Reunion Island (Indian Ocean, France). The control strategies and their optimization will be analyzed and discussed.Keywords:
Sterile Insect Technique
Aedes albopictus
Biocide
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The Aedes aegypti mosquito is the vector for four infectious diseases of global concern – Yellow Fever, Dengue, Chikungunya, and Zikavirus. Previous attempts to model the expansion of the vector habitat due to global climate change have rarely included characteristics related to the human populations on which this mosquito is dependent. The purpose of this research was to determine whether the inclusion of human population density improves model performance while creating risk maps that can be used to determine where humans are most likely to be exposed to the vector in the future. The resulting model demonstrated that the inclusion of human population density improves the predictive power for A. aegypti and should be considered during model development. Maps produced by the model were also suitable for identifying regions where human populations are most likely to experience increased risk. Finally, two areas at risk of expansion were highlighted as a case study in pairing risk mapping with evidence-based intervention strategies to identify sites that would benefit from mosquito-control efforts. In this case, a low-cost program of insecticide-treated covers for water storage containers would likely work well in both Minas Gerais, Brazil and Northwestern Province, Zambia to mitigate mosquito risk. This research demonstrates that human population characteristic not only improve model fit but also increase the extent to which risk maps are actionable by aiding in targeting interventions.
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In the last decades, the development of sustainable insect control methods, like sterile insect technique (SIT), has become one of the most challenging issue to reduce the risk of human vector-borne diseases, like malaria, dengue, chikungunya or crop pests, like fruit flies. control generally consists of massive releases of sterile insects in the targeted area with the aim to reach elimination or to lower the pest population under a certain threshold. Practically, due to e.g. manufacturing limitations/constraints and the (economic) cost of such operations, massive releases of sterile males are only possible for a short period of time. Despite that restriction, the main issue is to quantify the size and the duration of massive releases, before, eventually, shift to a low level and more sustainable releases of sterile insects, in order to reach elimination or maintain wild insects below an epidemiological risk threshold. Mathematical modelling can be a powerful tool to provide insights in the long-term dynamics of complex systems, like wild insects population experiencing control. In this poster, we present minimalistic entomo-mathematical models for wild insect population when is taken into account. Using Mathematical analysis and simulations, we show that different strategies can be developed, like, for instance a strategy that maintains the wild population under a certain threshold, for a permanent and sustainable low level of control and ensuing their elimination in the long-term dynamics. Taking into account the spatial component in the previous strategy, we also show that it can be used to stop a pest/vector invasion and eventually push them back. This work is partly supported by the SIT feasibility project against Aedes albopictus in Reunion Island phase 2B, jointly funded by the French Ministry of Health and the European Regional Development Fund (ERDF). The authors were supported by the DST/NRF SARChI Chair M2B3, in Mathematical Models and Methods in Biosciences and Bioengineering, at the University of Pretoria (grant 82770).
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Economic threshold
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Eliminating malaria requires vector control interventions that dramatically reduce adult mosquito population densities and survival rates. Indoor applications of insecticidal nets and sprays are effective against an important minority of mosquito species that rely heavily upon human blood and habitations for survival. However, complementary approaches are needed to tackle a broader diversity of less human-specialized vectors by killing them at other resource targets. Impacts of strategies that target insecticides to humans or animals can be rationalized in terms of biological coverage of blood resources, quantified as proportional coverage of all blood resources mosquito vectors utilize. Here, this concept is adapted to enable impact prediction for diverse vector control strategies based on measurements of utilization rate s for any definable, targetable resource subset, even if that overall resource is not quantifiable. The usefulness of this approach is illustrated by deriving utilization rate estimates for various blood, resting site, and sugar resource subsets from existing entomological survey data. Reported impacts of insecticidal nets upon human-feeding vectors, and insecticide-treated livestock upon animal-feeding vectors, are approximately consistent with model predictions based on measured utilization rates for those human and animal blood resource subsets. Utilization rates for artificial sugar baits compare well with blood resources, and are consistent with observed impact when insecticide is added. While existing data was used to indirectly measure utilization rates for a variety of resting site subsets, by comparison with measured rates of blood resource utilization in the same settings, current techniques for capturing resting mosquitoes underestimate this quantity, and reliance upon complex models with numerous input parameters may limit the applicability of this approach. While blood and sugar consumption can be readily quantified using existing methods for detecting natural markers or artificial tracers, improved techniques for labelling mosquitoes, or other arthropod pathogen vectors, will be required to assess vector control measures which target them when they utilize non-nutritional resources such as resting, oviposition, and mating sites.
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The invasive temperate mosquito Aedes japonicus japonicus is a potential vector for various infectious diseases and therefore a target of vector control measures. Even though established in Germany, it is unclear whether the species has already reached its full distribution potential. The possible range of the species, its annual population dynamics, the success of vector control measures and future expansions due to climate change still remain poorly understood. While numerous studies on occurrence have been conducted, they used mainly presence data from relatively few locations. In contrast, we used experimental life history data to model the dynamics of a continuous stage-structured population to infer potential seasonal densities and ask whether stable populations are likely to establish over a period of more than one year. In addition, we used climate change models to infer future ranges. Finally, we evaluated the effectiveness of various stage-specific vector control measures. Aedes j. japonicus has already established stable populations in the southwest and west of Germany. Our models predict a spread of Ae. j. japonicus beyond the currently observed range, but likely not much further eastwards under current climatic conditions. Climate change models, however, will expand this range substantially and higher annual densities can be expected. Applying vector control measures to oviposition, survival of eggs, larvae or adults showed that application of adulticides for 30 days between late spring and early autumn, while ambient temperatures are above 9 °C, can reduce population density by 75%. Continuous application of larvicide showed similar results in population reduction. Most importantly, we showed that with the consequent application of a mixed strategy, it should be possible to significantly reduce or even extinguish existing populations with reasonable effort. Our study provides valuable insights into the mechanisms concerning the establishment of stable populations in invasive species. In order to minimise the hazard to public health, we recommend vector control measures to be applied in 'high risk areas' which are predicted to allow establishment of stable populations to establish.
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Two case studies, one from Hawaii involving pest populations of mosquitoes and one from Trinidad looking at dengue, are presented to demonstrate the usefulness of spatial statistics in the study and control of vector-borne disease; we focus on monitoring methodologies, risk assessment, and mitigation. In Hawaii, the spatial distribution of Aedes albopictus (Skuse) oviposition within a small military facility located on a forest preserve indicated that the source of adult, biting mosquitoes was the untreatable sylvan areas surrounding the base where breeding occurred. A simple spatial analysis of oviposition during weeks with and without insecticide aerosol applications indicated that treatments were ineffective. From these analyses we concluded that if source reduction was not a viable alternative, then, without a tangible threat of A. albopictus -borne illness, no insecticide control measures should be attempted on the base; alternatively, the spatial analysis would be beneficial in directing the extent of a source reduction (e.g., deforestation) effort designed to eliminate migration into the base. In the Trinidad study, spatial visualization demonstrated dramatically that the traditional Stegomyia indices no longer should be considered viable surrogates of dengue transmission risk because they did not correlate spatially within themselves nor with the absolute densities of the vector Aedes aegypti (L.). Concerning risk assessment, spatial visualization of the number of pupae per person indicated that the threat of dengue was not distributed uniformly throughout the island. Regarding mitigation, spatial analysis of site differences in the types, frequencies, and productivities of water-holding domestic and trash containers indicated that targeted source reduction programs would have outcomes that varied spatially, perhaps justifying tailoring control efforts on a site-by-site basis. These studies also demonstrated the utility of combining spatial analysis with computer models in vector-borne disease systems. Finally, this report presents initial estimates of dengue transmission thresholds in terms of pupae per person as a function of herd immunity in the human population for the average annual ambient air temperature in Trinidad.
Aedes albopictus
Spatial heterogeneity
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This paper focuses on the design and analysis of short-term control intervention measures seeking to suppress local populations of Aedes aegypti mosquitoes, the major transmitters of dengue and other vector-borne infections. Besides traditional measures involving the spraying of larvicides and/or insecticides, we include biological control based on the deliberate introduction of predacious species feeding on the aquatic stages of mosquitoes. From the methodological standpoint, our study relies on application of the optimal control modeling framework in combination with the cost-effectiveness analysis. This approach not only enables the design of optimal strategies for external control intervention but also allows for assessment of their performance in terms of the cost-benefit relationship. By examining numerous scenarios derived from combinations of chemical and biological control measures, we try to find out whether the presence of predacious species at the mosquito breeding sites may (partially) replace the common practices of larvicide/insecticide spraying and thus reduce their negative impact on non-target organisms. As a result, we identify two strategies exhibiting the best metrics of cost-effectiveness and provide some useful insights for their possible implementation in practical settings.
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Abstract BACKGROUND: Microbial and insect‐growth‐regulator larvicides dominate current vector control programmes because they reduce larval abundance and are relatively environmentally benign. However, their short persistence makes them expensive, and environmental manipulation of larval habitat might be an alternative control measure. Aedes vigilax is a major vector species in northern Australia. A field experiment was implemented in Darwin, Australia, to test the hypotheses that (1) aerial microbial larvicide application effectively decreases Ae. vigilax larval presence, and therefore adult emergence, and (2) environmental manipulation is an effective alternative control measure. Generalised linear and mixed‐effects modelling and information‐theoretic comparisons were used to test these hypotheses. RESULTS: It is shown that the current aerial larvicide application campaign is effective at suppressing the emergence of Ae. vigilax , whereas vegetation removal is not as effective in this context. In addition, the results indicate that current larval sampling procedures are inadequate for quantifying larval abundance or adult emergence. CONCLUSIONS: This field‐based comparison has shown that the existing larviciding campaign is more effective than a simple environmental management strategy for mosquito control. It has also identified an important knowledge gap in the use of larval sampling to evaluate the effectiveness of vector control strategies. Copyright © 2011 Society of Chemical Industry
Larvicide
Mosquito control
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