Attacking the mosquito on multiple fronts: insights from the vector control optimization model (VCOM) for malaria elimination

Despite great achievements by insecticide-treated nets (ITNs) and indoor residual spraying (IRS) in reducing malaria transmission, it is unlikely these tools will be sufficient to eliminate malaria transmission on their own in many settings today. Fortunately, field experiments indicate that there are many promising vector control interventions that can be used to complement ITNs and/or IRS by targeting a wide range of biological and environmental mosquito resources. The majority of these experiments were performed to test a single vector control intervention in isolation; however, there is growing evidence and consensus that effective vector control with the goal of malaria elimination will require a combination of interventions.; We have developed a model of mosquito population dynamic to describe the mosquito life and feeding cycles and to optimize the impact of vector control intervention combinations at suppressing mosquito populations. The model simulations were performed for the main three malaria vectors in sub-Saharan Africa, Anopheles gambiae s.s, An. arabiensis and An. funestus. We considered areas having low, moderate and high malaria transmission, corresponding to entomological inoculation rates of 10, 50 and 100 infective bites per person per year, respectively. In all settings, we considered baseline ITN coverage of 50% or 80% in addition to a range of other vector control tools to interrupt malaria transmission. The model was used to sweep through parameters space to select the best optimal intervention packages. Sample model simulations indicate that, starting with ITNs at a coverage of 50% (An. gambiae s.s. and An. funestus) or 80% (An. arabiensis) and adding interventions that do not require human participation (e.g. larviciding at 80% coverage, endectocide treated cattle at 50% coverage and attractive toxic sugar baits at 50% coverage) may be sufficient to suppress all the three species to an extent required to achieve local malaria elimination.; The Vector Control Optimization Model (VCOM) is a computational tool to predict the impact of combined vector control interventions at the mosquito population level in a range of eco-epidemiological settings. The model predicts specific combinations of vector control tools to achieve local malaria elimination in a range of eco-epidemiological settings and can assist researchers and program decision-makers on the design of experimental or operational research to test vector control interventions. A corresponding graphical user interface is available for national malaria control programs and other end users

Structure, syntax and “small-world” organization in the complex songs of California Thrashers (<i>Toxostoma redivivum</i>)

<div><p></p><p>We describe songs of the California Thrasher (<i>Toxostoma redivivum)</i>, a territorial, monogamous species whose complex songs are composed of extended sequences of phonetically diverse phrases. We take a network approach, so that network nodes represent specific phrases, and links or transitions between nodes describe a subgroup structure that reveals the syntax of phrases within the songs. We found that individual birds have large and largely distinct repertoires, with limited phrase sharing between neighbours and repertoire similarity decaying between individuals with distance apart, decaying also over time within individuals. During song sequences, only a limited number of phrases (ca. 15–20) were found to be actually “in play” at any given time; these phrases can be grouped into themes within which transitions are much more common than among them, a feature contributing to a small-world structure. It appears that such “small-world themes” arise abruptly, while old themes are abandoned more gradually during extended song sequences; most individual thrashers switch among 3–4 themes over the course of several successive songs, and some small-world themes appear to have specific roles in starting or ending thrasher songs.</p></div

Attacking the mosquito on multiple fronts: Insights from the Vector Control Optimization Model (VCOM) for malaria elimination.

BackgroundDespite great achievements by insecticide-treated nets (ITNs) and indoor residual spraying (IRS) in reducing malaria transmission, it is unlikely these tools will be sufficient to eliminate malaria transmission on their own in many settings today. Fortunately, field experiments indicate that there are many promising vector control interventions that can be used to complement ITNs and/or IRS by targeting a wide range of biological and environmental mosquito resources. The majority of these experiments were performed to test a single vector control intervention in isolation; however, there is growing evidence and consensus that effective vector control with the goal of malaria elimination will require a combination of interventions.Method and findingsWe have developed a model of mosquito population dynamic to describe the mosquito life and feeding cycles and to optimize the impact of vector control intervention combinations at suppressing mosquito populations. The model simulations were performed for the main three malaria vectors in sub-Saharan Africa, Anopheles gambiae s.s, An. arabiensis and An. funestus. We considered areas having low, moderate and high malaria transmission, corresponding to entomological inoculation rates of 10, 50 and 100 infective bites per person per year, respectively. In all settings, we considered baseline ITN coverage of 50% or 80% in addition to a range of other vector control tools to interrupt malaria transmission. The model was used to sweep through parameters space to select the best optimal intervention packages. Sample model simulations indicate that, starting with ITNs at a coverage of 50% (An. gambiae s.s. and An. funestus) or 80% (An. arabiensis) and adding interventions that do not require human participation (e.g. larviciding at 80% coverage, endectocide treated cattle at 50% coverage and attractive toxic sugar baits at 50% coverage) may be sufficient to suppress all the three species to an extent required to achieve local malaria elimination.ConclusionThe Vector Control Optimization Model (VCOM) is a computational tool to predict the impact of combined vector control interventions at the mosquito population level in a range of eco-epidemiological settings. The model predicts specific combinations of vector control tools to achieve local malaria elimination in a range of eco-epidemiological settings and can assist researchers and program decision-makers on the design of experimental or operational research to test vector control interventions. A corresponding graphical user interface is available for national malaria control programs and other end users

Flow diagram for the mosquito ecological model, mosquito SEI model, and human SI model.

<p><i>E</i>, Early Instar; <i>L</i>, Late Instar; <i>P</i>, Pupae; <i>S</i>_<i>v</i>, Susceptible Vectors; <i>E</i>_<i>v</i>, Exposed Vectors, <i>I</i>_<i>v</i>, Infected Vectors; <i>S</i>_<i>H</i>, Susceptible Humans; and <i>I</i>_<i>H</i>, Infected Humans.</p

Targeting the mosquito on multiple fronts.

<p>The schematic highlights opportunities for existing and novel vector control tools that can be used to target mosquitoes both indoors and outdoors and at all stages of the mosquito life and feeding cycles. Synergy and layering between interventions follows from this schematic diagram (Fig 2) and how all the interventions are encoded from it.</p

Sensitivity analysis for vector control optimization model.

<p>Sensitivity analysis is performed using Latin Hypercube Sampling/Partial Rank Correlation Coefficient (LHS/PRCC) sensitivity analysis approach based on the impact of selected vector control tools at 50% coverage in reducing entomological inoculation rate for <i>An</i>. <i>gambiae s</i>.<i>s</i>. (<b>A</b>) and <i>An</i>. <i>arabiensis</i> (<b>B</b>). The names for parameters are given in the second column titled “Key” of Table C in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0187680#pone.0187680.s011" target="_blank">S2 Appendix</a>.</p

Evaluating the impact of combining ITNs at 80% coverage with other interventions.

<p>This figure shows equilibrium values of EIR simulated based on selected tools added to high transmission areas against <i>An</i>. <i>arabiensis</i> (<b>A</b>), <i>An</i>. <i>gambiae</i> (<b>B</b>), and <i>An</i>. <i>funestus</i> (<b>C</b>). The box plot shows the median, the interquartile ranges and the 95% confidence ranges of the EIR for different parameter values shown on <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0187680#pone.0187680.g003" target="_blank">Fig 3</a> corresponding to either specific mosquito parameters and/or vector control tools used to interrupt transmission. Adding larviciding at 80% to ITNs at 50% might not be sufficient to interrupt transmission (<b>A</b>, <b>B</b>, and <b>C</b>); but adding another intervention (e.g. ATSB at 50%) reduces transmission dramatically and adding the fourth intervention (e.g. endectocide-treated cattle at 80%) is sufficient to interrupt transmission with baseline EIR of 100.</p

Evaluating the impact of combining ITNs at 50% and 80% coverage with additional tools against <i>An</i>. <i>gambiae s</i>.<i>s</i>.

<p>The tools selected in this example are attractive toxic sugar baits (ATSB), topical (ECT) endectocide-treated cattle, mosquito-proofed housing (HOU), larviciding (LAR) and personal protection measure (e.g. insecticide-treated clothing) (PPM). Adding one or two tools to ITNs at 50% coverage might be sufficient to interrupt transmission in low transmission settings (<b>A</b>); but in most cases not sufficient in moderate (<b>C</b>) and high transmission (<b>E</b>) settings. Scaling up ITNs to 80% coverage and adding another tool with 50% coverage might be sufficient to interrupt transmission in low (<b>B</b>) transmission but not necessarily in moderate (<b>D</b>) and high (<b>F</b>) transmission settings unless the tool added is mosquito proofed housing.</p

Evaluating the impact of combining ITNs at 80% coverage with additional tools against <i>An</i>. <i>arabiensis</i>.

<p>Similar tools presented in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0187680#pone.0187680.g004" target="_blank">Fig 4</a> are presented here showing EIR values at equilibrium but with ITN coverage set to 80% and coverage with additional vector control tools set to 50% in panels A, C and E and 80% panels B, D and F. Adding a tool or two is insufficient to interrupt transmission in most cases for <i>An</i>. <i>arabiensis</i> in moderate and high transmission settings, except when larviciding or endocticide-treated cattle is added (either alone or in combination).</p