BackgroundMosquito nets containing synergists designed to overcome metabolic resistance mechanisms in vectors have been developed. These may enhance excitability in the mosquitoes and affect how they respond to CDC light-traps. Investigating the behaviour of vectors of disease in relation to novel mosquito nets is, therefore, essential for the design of sampling and surveillance systems.MethodsIn an initial experiment in Muleba, Tanzania, nine bedrooms from three housing clusters were sampled. CDC light-traps were operated indoors next to occupied untreated nets (UTN), Olyset® long lasting insecticidal net (LLIN) and Olyset Plus® LLIN containing piperonyl butoxide (PBO) synergist. Nets were rotated daily between the nine rooms over nine nights. A further series of experiments using the nets on alternate nights in a single room was undertaken during the short rains. Anopheles gambiae s.l. were collected in CDC light-traps, a window-trap and Furvela tent-trap. Anopheles gambiae s.l. were identified to species by polymerase chain reaction (PCR).ResultsIn the initial experiment 97.7% of the 310 An. gambiae s.l. were An. gambiae s.s., the remainder being Anopheles arabiensis. The number of mosquitoes collected from 81 light-trap collections was greater in the presence of an Olyset [density rate ratio 1.81, 95% CI (1.22–2.67), p = 0.003] relative to an UTN. In a second experiment, in the wet season 84% of the 180 An. gambiae s.l. identified were An. arabiensis. The number of An. gambiae s.l. collected from a light-trap compared to a tent-trap was significantly higher when an Olyset Plus net was used compared to an UTN. Survival of the mosquitoes in the window trap was not reduced by the use of an Olyset Plus net in the bedroom relative to an Olyset net.ConclusionMosquitoes entering bedrooms, even those susceptible to pyrethroids, were not killed by contact with an Olyset Plus LLIN. The enhanced numbers of An. gambiae or An. arabiensis collected in light-traps when a treated net is used requires further experimentation and may be because of a heightened escape reaction on the part of the mosquito.
The relative efficacy of a mechanical (Prokopack) collection method vs. manual aspiration in the collection of resting mosquitoes was evaluated in northern Tanzania before and after an intervention using indoor residual spraying and longlasting insecticide-treated nets. In smoke-free houses mosquitoes were collected from the roof and walls, but in smoky houses mosquitoes were found predominantly on the walls. Anopheles gambiae (Diptera: Culicidae) constituted 97.7% of the 312 An. gambiae complex specimens identified before but only 19.3% of the 183 identified after the intervention. A single sampling with the Prokopack collected a third of the available insects. Anopheles gambiae completed its gonotrophic development indoors, whereas Anopheles arabiensis did so outdoors. In both species gonotrophic development took 2 days. Most unfed resting An. arabiensis collected outdoors were virgins, whereas the majority of engorged insects were parous (with well-contracted sacs). Daily survival was estimated to be 80.0%. Only 9.4% of the engorged An. arabiensis collected outdoors and 47.1% of those collected indoors had fed on humans. Using the Prokopack sampler is more efficient than manual methods for the collection of resting mosquitoes. Malaria transmission may have been affected by a change in vector composition resulting from a change in feeding, rather than reduced survival. Monitoring the proportions of members of the An. gambiae complex may provide signals of an impending breakdown in control.
After decades of success in reducing malaria through the scale-up of pyrethroid long-lasting insecticidal nets (LLINs), malaria decline has stalled, coinciding with the rapid spread of pyrethroid resistance. A new class of net, treated with a mixture of a pyrethroid and a synergist, piperonyl butoxide (PBO), demonstrated superior efficacy compared to standard (std) pyrethroid LLINs against malaria in an area of intense pyrethroid resistance, reducing malaria prevalence by 44% over 2 years in the present trial. However, an important question is left unanswered regarding the performance of this PBO-LLIN over the World Health Organization recommended lifespan of 3 years for LLINs. We conducted a four-arm randomized controlled trial using a two-by-two factorial design that evaluated the effectiveness of PBO-LLIN arms (12 clusters PBO-LLIN alone and 12 clusters PBO-LLIN + Indoor Residual Spraying; IRS) compared to std-LLIN (12 clusters std-LLIN alone and 12 clusters std-LLIN + IRS) and IRS arms versus no IRS arms from January 2014 to December 2017 in Muleba, Tanzania. Malaria infection prevalence in 80 children, 6 months to 14 years, per cluster was measured twice a year and analysed in an intention to treat (ITT) and per protocol (PP) approach. Density of malaria mosquito vectors and entomological inoculation rate (EIR) were assessed monthly in 7 houses per cluster. Logistic regression allowing for within cluster correlation of responses was used to compare malaria prevalence between PBO-LLIN groups vs std-LLIN groups and IRS groups vs no IRS groups during the third-year follow-up at 28- and 33-months post-intervention. No further IRS was conducted after the first spray round in 2015; as yearly IRS is recommended by WHO, results need to be interpreted in light of this limitation. Vector density and EIR were analysed using negative binomial regression. Malaria results were available for 7471 children. At 28 months, malaria infection prevalence was lower in the PBO-LLIN groups (69.3%) compared to the std-LLIN groups (80.9%, Odds Ratio: 0.45, 95% Confidence Interval: 0.21-0.95, p value: 0.0364). The effect was weaker at 33 months post-intervention (OR: 0.60, 95%CI:0.32-1.13, p value: 0.1131), in the ITT analysis but still evident in the PP analysis (OR: 0.34, 95%CI: 0.16-0.71, p value: 0.0051). At this time point, net usage in household participants was 31% and PBO concentration in PBO-LLINs was reduced by 96% compared to those of new nets. A total of 17,451 Anopheles mosquitoes were collected during the 3150 collection nights done in the third year. There was no reduction in EIR (DR: 0.63, 95%CI: 0.25-1.61, p value: 0.3296) between the PBO groups and std-LLIN groups or between IRS and no IRS groups (DR: 0.7, 95%CI: 0.41-2.28, p value: 0.9426). PBO-LLINs no longer provided community protection from malaria infection, compared to std-LLINs by the third year of use due to low net usage. Children still sleeping under PBO-LLINs had lower odds of infection than those sleeping under a std-LLIN, however prevalence remained unacceptably high. It is urgent that net distribution frequencies and effective lifespan of this class of LLIN are aligned for maximum impact.
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