204 research outputs found
PMI Activity TZ-1,2: IRS and LLIN: Integration of Methods and Insecticide Mode of Actions for Control of African Malaria Vector Mosquitoes
Long lasting Insecticidal nets (LLINs) and indoor residual spraying (IRS) are the preferred techniques for malaria vector control in Africa, where their application has a proven contribution to the recent significant reductions in the burden of the disease. Even though both methods are commonly used together in the same households, evidence of improved malaria control due to the use of combinations as opposed to use of either method alone has been minimal and inconclusive.To measure the mode of action of three classes of insecticides used for IRS at the WHO recommended dose: the organochlorine DDT 70 wettable powder (AVIMA, South Africa) at 2g/m2; the pyrethroid lambda-cyhalothrin capsule suspension ICON CS, (Syngenta, Switzerland), at 0.03g/m2; and the organophosphate pirimiphos-methyl (PM) emulsified concentrate, also known as actellic (Syngenta, Switzerland), at 2g/m2 used alone or in combination with three leading LLIN brands: PermaNet 2.0® nets (Vastergaard, Switzerland), Olyset® nets (manufactured by A-Z, Tanzania), and Icon Life® nets (Bestnet Europe ltd, Denmark). All LLINS were used intact and were not subjected to repeated washing to reflect their optimum performance. The control was untreated polyester net. Data were collected from experimental huts developed during the project to measure both behavioral and toxic modes of actions of insecticides in Southern Tanzania. The primary malaria vector is Anopheles arabiensis with >90% susceptibility to insecticides of all classes at diagnostic doses in WHO susceptibility assays. Two rounds of data collection were performed: 1) 4 months during the dry season 2) six months during the wet season. Data generated from the experimental hut studies were analysed with Poisson-lognormal generalized linear mixed effects models (GLMM). Data was also simulated using deterministic mathematical model to measure potential impacts of each IRS, LLIN and combination thereof on malaria at a community level. Bite prevention (feeding inhibition): During both rounds, all the IRS treatments, LLINs and the controls (which consisted of intact untreated mosquito nets), provided greater than 99% protection from potentially infectious bites by the malaria vector, An. arabiensis, for the entire duration of the study. Most of the mosquitoes were caught inside the exit traps as opposed to inside the experimental huts, regardless of whether the huts were had LLINs, IRS or non-insecticidal nets. More than 95% of An. arabiensis, Culex pipiens quinquefasciatus and Mansonia africana / uniformis mosquitoes were caught inside the exit traps while exiting the huts. Toxicity: All IRS treatments, all the LLINs and the majority of LLIN/IRS combinations significantly increased proportions of dead An. arabiensis mosquitoes, relative to the control huts. The most toxic IRS relative to the controls was PM (RR = 2.21 (1.82 – 2.68), P < 0.001), followed by ICON CS (RR = 1.55 (1.27 – 1.89), P < 0.001) and then DDT (RR = 1.44 (1.18 – 1.77), P < 0.001). The most toxic LLIN relative to the controls was PermaNet 2.0® nets (RR = 1.65 (1.58 – 1.74), P < 0.001), followed by Icon Life® nets (RR = 1.55 (1.42 – 1.69), P < 0.001) and then Olyset® nets (RR = 1.33 (1.12 – 1.47), P < 0.001). Combinations of IRS and LLINs relative to LLINs alone: In most cases, there was no significant increase in An. arabiensis mortality in huts combining LLINs plus IRS, relative to huts having LLINs only, except in cases where the specific IRS treatment was PM. Addition of PM significantly increased proportional mortality of An. arabiensis when combined with Olyset® nets (RR = 1.38 (1.14 – 1.65), P = 0.001), PermaNet 2.0® nets (RR = 1.42 (1.18 – 1.71), P <0.001) and Icon Life® (RR = 1.24 (1.03 – 1.49), P = 0.023). Combinations of LLINs and DDT or lambda cyhalothrin resulted in marginal increases in An. arabiensis mortality relative to huts with LLINs alone although none of these combinations resulted in a statistically significant increase. Combinations of IRS and LLINs relative to IRS alone: There was a trend of significant increases in An. arabiensis mortality in huts having IRS plus LLINs, relative to huts having just the IRS alone, except for the combinations of 1) Olyset® with ICON CS, 2) DDT with Olyset® or 3) DDT with Icon Life® nets. In the huts that had been sprayed with PM, there was a significant increase in An. arabiensis mortality whenever Icon Life® nets (RR = 1.39 (1.18 – 1.63), P < 0.001), Olyset® nets (RR = 1.32 (1.13 – 1.55), P = 0.001) or PermaNet 2.0® nets (RR = 1.26 (1.08 – 1.48), P = 0.004) were added, relative to the huts where PM IRS was used alone. Similarly, in the huts that had been sprayed with ICON CS, there was a significant increase in An. arabiensis mortality in combination with Icon Life® nets (RR = 1.43 (1.19 – 1.73), P < 0.001) or PermaNet 2.0® nets (RR = 1.70 (1.35 – 2.13), P < 0.001), but not Olyset® nets (RR = 1.16 (0.92 – 1.45), P = 0.210), relative to the IRS alone. In huts sprayed with DDT, none of the LLINs significantly improved proportional mortality of the An. Arabiensis mosquitoes, except PermaNet 2.0® nets (RR = 1.18 (1.06 – 1.32), P = 0.003). Residual efficacy bioassays of IRS: All IRS formulations were highly effective during the first month after spraying and rapidly decayed losing most activity within 1-3 months. In month 1, all An. arabiensis exposed to palm ceilings sprayed with either PM or ICON CS died, and 85% were killed by DDT (despite full susceptibility most likely because it flaked away). On mud walls sprayed with the same chemicals, 100%, 90.0% and 97.5% mortality was observed, respectively, during the first month. Activity of the IRS declined significantly so that by the third month, PM on palm and mud killed 42.5% and 55.0% of exposed An. arabiensis, respectively. ICON CS killed only 46.3% on palm and 52.5% on mud walls. By month 6, PM had nearly entirely decayed, killing only 7.5% of An. arabiensis exposed to sprayed palm ceilings and 27.5% of those exposed to sprayed mud walls; ICON CS killed 30.0% on ceilings and 27.5% on walls. DDT had a longer residual action, killing 42.5% of An. arabiensis exposed to sprayed ceilings, and 36.3% of those exposed to sprayed walls after 6 months. Residual efficacy bioassays of LLINs: While all the LLINs generally performed better (i.e. killed more mosquitoes) on wire frame assays than on the cone assays, their activity rapidly deteriorated by the second month of use relative to new nets. Only PermaNet® nets retained mosquitocidal efficacy of >80% by the sixth month of net use (killing 92.7% on wire ball tests and 84% on cone assays). All the LLINs however retained very high knock-down rates (> 90% in wire ball tests and >80% in cone tests) on the exposed mosquitoes, except Olyset® nets whose knock-down activity reduced to 72.7% on wire ball tests and 62% on cone tests by the sixth month. Both the field studies and the model simulations showed that any synergies or redundancies resulting from LLIN/IRS combinations are primarily a function of modes of action of active ingredients used in the two interventions. None of the IRS or LLINs tested was deterrent so they do not protect by keeping mosquitoes from houses in this setting. Very few mosquitoes were able to obtain a blood meal due to the use of intact LLINs and untreated control nets. Therefore, where households are correctly using and maintaining LLINs there is no added value in the additional application of IRS unless the IRS chemical is highly toxic and non-irritant, as is PM. This compound consistently increased mosquito mortality in combination with any LLIN even though mosquitoes did not rest indoors as they were unable to obtain a blood meal. The average duration of effect of insecticides in this setting was 3 months, far lower than that stated by the manufacturers, so IRS should be carefully timed. Where IRS is the pre-existing intervention, providing households with additional LLINs confers additional protection. Therefore, IRS households should always be supplemented with nets, preferably LLINs, which not only protect house occupants against mosquito bites, but also kill additional mosquitoes. Finally, where resources are limited, priority should be given to providing everybody with LLINs and ensuring that these nets are consistently and appropriately used, rather than trying to implement both LLINs and IRS in the same community at the same time.\ud
\u
Larvicidal effects of a neem (Azadirachta indica) oil formulation on the malaria vector Anopheles gambiae
Larviciding is a key strategy used in many vector control programmes around the world. Costs could be reduced if larvicides could be manufactured locally. The potential of natural products as larvicides against the main African malaria vector, Anopheles gambiae s.s was evaluated. To assess the larvicidal efficacy of a neem (Azadirachta indica) oil formulation (azadirachtin content of 0.03% w/v) on An. gambiae s.s., larvae were exposed as third and fourth instars to a normal diet supplemented with the neem oil formulations in different concentrations. A control group of larvae was exposed to a corn oil formulation in similar concentrations. Neem oil had an LC50 value of 11 ppm after 8 days, which was nearly five times more toxic than the corn oil formulation. Adult emergence was inhibited by 50% at a concentration of 6 ppm. Significant reductions on growth indices and pupation, besides prolonged larval periods, were observed at neem oil concentrations above 8 ppm. The corn oil formulation, in contrast, produced no growth disruption within the tested range of concentrations. Neem oil has good larvicidal properties for An. gambiae s.s. and suppresses successful adult emergence at very low concentrations. Considering the wide distribution and availability of this tree and its products along the East African coast, this may prove a readily available and cheap alternative to conventional larvicides
Using a New Odour-Baited Device to Explore Options for Luring and Killing Outdoor-biting Malaria Vectors: Design and Field Evaluation of the Mosquito Landing Box
A Geographical Location Model for Targeted Implementation of Lure-and-Kill Strategies Against Disease-Transmitting Mosquitoes in Rural Areas
Outdoor devices for luring and killing disease-transmitting mosquitoes have been proposed as potential com- plementary interventions alongside existing intra-domiciliary methods namely insecticide treated nets and house spraying with residual insecticides. To enhance effectiveness of such outdoor interventions, it is essential to optimally locate them in such a way that they target most of the outdoor mosquitoes. Using odour-baited lure and kill stations (OBS) as an example, we describe a map model derived from: 1) com-munity participatory mapping conducted to identify mosquito breeding habitats, 2) entomological field studies conducted to estimate outdoor mosquito densities and to determine safe distances of the OBS from human dwellings, and 3) field surveys conducted to map households, roads, outdoor human aggregations and landmarks. The resulting data were combined in a Ge- ographical Information Systems (GIS) environment and analysed to determine optimal locations for the OBS. Separately, a GIS-interpolated map produced by asking community members to rank different zones of the study area and show where they expected to find most mosquitoes, was visually compared to another map interpolated from the entomological survey of outdoor mosquito densities. An easy-to-interpret suitability map showing optimal sites for placing OBS was produced, which clearly depicted areas least suitable and areas most suitable for locating the devices. Comparative visual interpretation of maps derived from interpolating the community knowledge and entomological data revealed major similarities between the two maps. Using distribution patterns of human and mosquito populations as well as characteristics of candidate outdoor interventions, it is possible to readily determine suitable areas for targeted positioning of the interventions, thus improve effectiveness. This study also highlights possibilities of relying on community knowledge to approximate areas where mosquitoes are most abundant and where to locate outdoor complementary interventions such as odour-baited lure and kill stations for controlling disease-transmitting mosquitoes.\u
Insecticide-Treated Nets Can Reduce Malaria Transmission by Mosquitoes Which Feed Outdoors
Insecticide treated nets (ITNs) represent a powerful means for controlling malaria in Africa because the mosquito vectors feed primarily indoors at night. The proportion of human exposure that occurs indoors, when people are asleep and can conveniently use ITNs, is therefore very high. Recent evidence suggests behavioral changes by malaria mosquito populations to avoid contact with ITNs by feeding outdoors in the early evening. We adapt an established mathematical model of mosquito behavior and malaria transmission to illustrate how ITNs can achieve communal suppression of malaria transmission exposure, even where mosquito evade them and personal protection is modest. We also review recent reports from Tanzania to show that conventional mosquito behavior measures can underestimate the potential of ITNs because they ignore the importance of human movements
Most outdoor malaria transmission by behaviourally-resistant Anopheles arabiensis is mediated by mosquitoes that have previously been inside houses
Background
Anopheles arabiensis is stereotypical of diverse vectors that mediate residual malaria transmission globally, because it can feed outdoors upon humans or cattle, or enter but then rapidly exit houses without fatal exposure to insecticidal nets or sprays.
Methods
Life histories of a well-characterized An. arabiensis population were simulated with a simple but process-explicit deterministic model and relevance to other vectors examined through sensitivity analysis.
Results
Where most humans use bed nets, two thirds of An. arabiensis blood feeds and half of malaria transmission events were estimated to occur outdoors. However, it was also estimated that most successful feeds and almost all (>98 %) transmission events are preceded by unsuccessful attempts to attack humans indoors. The estimated proportion of vector blood meals ultimately obtained from humans indoors is dramatically attenuated by availability of alternative hosts, or partial ability to attack humans outdoors. However, the estimated proportion of mosquitoes old enough to transmit malaria, and which have previously entered a house at least once, is far less sensitive to both variables. For vectors with similarly modest preference for cattle over humans and similar ability to evade fatal indoor insecticide exposure once indoors, >80 % of predicted feeding events by mosquitoes old enough to transmit malaria are preceded by at least one house entry event, so long as ≥40 % of attempts to attack humans occur indoors and humans outnumber cattle ≥4-fold.
Conclusions
While the exact numerical results predicted by such a simple deterministic model should be considered only approximate and illustrative, the derived conclusions are remarkably insensitive to substantive deviations from the input parameter values measured for this particular An. arabiensis population. This life-history analysis, therefore, identifies a clear, broadly-important opportunity for more effective suppression of residual malaria transmission by An. arabiensis in Africa and other important vectors of residual transmission across the tropics. Improved control of predominantly outdoor residual transmission by An. arabiensis, and other modestly zoophagic vectors like Anopheles darlingi, which frequently enter but then rapidly exit from houses, may be readily achieved by improving existing technology for killing mosquitoes indoors
Interventions that effectively target Anopheles funestus mosquitoes could significantly improve control of persistent malaria transmission in south–eastern Tanzania
Malaria is transmitted by many Anopheles species whose proportionate contributions vary across settings. We re-assessed the roles of Anopheles arabiensis and Anopheles funestus, and examined potential benefits of species-specific interventions in an area in south-eastern Tanzania, where malaria transmission persists, four years after mass distribution of long-lasting insecticide-treated nets (LLINs). Monthly mosquito sampling was done in randomly selected households in three villages using CDC light traps and back-pack aspirators, between January-2015 and January-2016, four years after the last mass distribution of LLINs in 2011. Multiplex polymerase chain reaction (PCR) was used to identify members of An. funestus and Anopheles gambiae complexes. Enzyme-linked immunosorbent assay (ELISA) was used to detect Plasmodium sporozoites in mosquito salivary glands, and to identify sources of mosquito blood meals. WHO susceptibility assays were done on wild caught female An. funestus s.l, and physiological ages approximated by examining mosquito ovaries for parity. A total of 20,135 An. arabiensis and 4,759 An. funestus were collected. The An. funestus group consisted of 76.6% An. funestus s.s, 2.9% An. rivulorum, 7.1% An. leesoni, and 13.4% unamplified samples. Of all mosquitoes positive for Plasmodium, 82.6% were An. funestus s.s, 14.0% were An. arabiensis and 3.4% were An. rivulorum. An. funestus and An. arabiensis contributed 86.21% and 13.79% respectively, of annual entomological inoculation rate (EIR). An. arabiensis fed on humans (73.4%), cattle (22.0%), dogs (3.1%) and chicken (1.5%), but An. funestus fed exclusively on humans. The An. funestus populations were 100% susceptible to organophosphates, pirimiphos methyl and malathion, but resistant to permethrin (10.5% mortality), deltamethrin (18.7%), lambda-cyhalothrin (18.7%) and DDT (26.2%), and had reduced susceptibility to bendiocarb (95%) and propoxur (90.1%). Parity rate was higher in An. funestus (65.8%) than An. arabiensis (44.1%). Though An. arabiensis is still the most abundant vector species here, the remaining malaria transmission is predominantly mediated by An. funestus, possibly due to high insecticide resistance and high survival probabilities. Interventions that effectively target An. funestus mosquitoes could therefore significantly improve control of persistent malaria transmission in south–eastern Tanzania
Semi-field assessment of the BG-Malaria trap for monitoring the African malaria vector, Anopheles arabiensis
Odour-baited technologies are increasingly considered for effective monitoring of mosquito populations and for the evaluation of vector control interventions. The BG-Malaria trap (BGM), which is an upside-down variant of the widely used BG-Sentinel trap (BGS), has been demonstrated to be effective to sample the Brazilian malaria vector, Anopheles darlingi. We evaluated the BGM as an improved method for sampling the African malaria vectors, Anopheles arabiensis. Experiments were conducted inside a large semi-field cage to compare trapping efficiencies of BGM and BGS traps, both baited with the synthetic attractant, Ifakara blend, supplemented with CO2. We then compared BGMs baited with either of four synthetic mosquito lures, Ifakara blend, Mbita blend, BG-lure or CO2, and an unbaited BGM. Lastly, we compared BGMs baited with the Ifakara blend dispensed via either nylon strips, BG cartridges (attractant-infused microcapsules encased in cylindrical plastic cartridge) or BG sachets (attractant-infused microcapsules encased in plastic sachets). All tests were conducted between 6P.M. and 7A.M., with 200–600 laboratory-reared An. arabiensis released nightly in the test chamber. The median number of An. arabiensis caught by the BGM per night was 83, IQR:(73.5–97.75), demonstrating clear superiority over BGS (median catch = 32.5 (25.25–37.5)). Compared to unbaited controls, BGMs baited with Mbita blend caught most mosquitoes (45 (29.5–70.25)), followed by BGMs baited with CO2 (42.5 (27.5–64)), Ifakara blend (31 (9.25–41.25)) and BG lure (16 (4–22)). BGM caught 51 (29.5–72.25) mosquitoes/night, when the attractants were dispensed using BG-Cartridges, compared to BG-Sachet (29.5 (24.75–40.5)), and nylon strips (27 (19.25–38.25)), in all cases being significantly superior to unbaited controls (p < 000.1). The findings demonstrate potential of the BGM as a sampling tool for African malaria vectors over the standard BGS trap. Its efficacy can be optimized by selecting appropriate odour baits and odour-dispensing systems
Correlations Between Household Occupancy and Malaria Vector Biting Risk in Rural Tanzanian Villages: Implications for High-resolution Spatial Targeting of Control Interventions.
Fine-scale targeting of interventions is increasingly important where epidemiological disease profiles depict high geographical stratifications. This study verified correlations between household biomass and mosquito house-entry using experimental hut studies, and then demonstrated how geographical foci of mosquito biting risk can be readily identified based on spatial distributions of household occupancies in villages. A controlled 4 × 4 Latin square experiment was conducted in rural Tanzania, in which no, one, three or six adult male volunteers slept under intact bed nets, in experimental huts. Mosquitoes entering the huts were caught using exit interception traps on eaves and windows. Separately, monthly mosquito collections were conducted in 96 randomly selected households in three villages using CDC light traps between March-2012 and November-2013. The number of people sleeping in the houses and other household and environmental characteristics were recorded. ArcGIS 10 (ESRI-USA) spatial analyst tool, Gi* Ord Statistic was used to analyse clustering of vector densities and household occupancy. The densities of all mosquito genera increased in huts with one, three or six volunteers, relative to huts with no volunteers, and direct linear correlations within tested ranges (P < 0.001). Significant geographical clustering of indoor densities of malaria vectors, Anopheles arabiensis and Anopheles funestus, but not Culex or Mansonia species occurred in locations where households with highest occupancy were also most clustered (Gi* P ≤ 0.05, and Gi* Z-score ≥1.96). This study demonstrates strong correlations between household occupancy and malaria vector densities in households, but also spatial correlations of these variables within and between villages in rural southeastern Tanzania. Fine-scale clustering of indoor densities of vectors within and between villages occurs in locations where houses with highest occupancy are also clustered. The study indicates potential for using household census data to preliminarily identify households with greatest Anopheles mosquito biting risk
Implications of bio-efficacy and persistence of insecticides when indoor residual spraying and longlasting insecticide nets are combined for malaria prevention.
Bio-efficacy and residual activity of insecticides used for indoor residual spraying (IRS) and long-lasting insecticide nets (LLINs) were assessed against laboratory-reared and wild populations of the malaria vector, Anopheles arabiensis in south eastern Tanzania. Implications of the findings are examined in the context of potential synergies and redundancies where IRS and LLINs are combined. METHODS: Bioassays were conducted monthly for six months on three LLIN types (Olyset(R) PermaNet 2.0(R),and Icon Life(R)) and three IRS treatments (2 g/m2 pirimiphos-methyl, 2 g/m2 DDT and 0.03 g/m2 lambda-cyhalothrin, sprayed on mud walls and palm ceilings of experimental huts). Tests used susceptible laboratory-reared An. arabiensis exposed in cones (nets and IRS) or wire balls (nets only). Susceptibility of wild populations was assessed using WHO diagnostic concentrations and PCR for knock-down resistance (kdr) genes. IRS treatments killed [greater than or equal to] 85% of mosquitoes exposed on palm ceilings and [greater than or equal to] 90% of those exposed on mud walls, but up to 50% of this toxicity decayed within 1-3 months, except for DDT. By 6th month, only 7.5%, 42.5% and 30.0% of mosquitoes died when exposed to ceilings sprayed with pirimiphos-methyl, DDT or lambda-cyhalothrin respectively, while 12.5%, 36.0% and 27.5% died after exposure to mud walls sprayed with the same insecticides. In wire-ball assays, mortality decreased from 98.1% in 1st month to 92.6% in 6th month in tests on PermaNet 2.0(R), from 100% to 61.1% on Icon Life(R) and from 93.2% to 33.3% on Olyset(R) nets. In cone bioassays, mortality reduced from 92.8% in 1st month to 83.3% in 6th month on PermaNet 2.0(R), from 96.9% to 43.80% on Icon Life(R) and from 85.6% to 14.6% on Olyset(R). Wild An. arabiensis were 100% susceptible to DDT, 95.8% to deltamethrin, 90.2% to lambda cyhalothrin and 95.2% susceptible to permethrin. No kdr gene mutations were detected. CONCLUSIONS: In bioassays where sufficient contact with treated surfaces is assured, LLINs and IRS kill high proportions of susceptible An. arabiensis mosquitoes, though these efficacies decay gradually for LLINs and rapidly for IRS. It is, therefore, important to always add intact nets in sprayed houses, guaranteeing protection even after the IRS decays, and to ensure accurate timing, quality control and regular re-spraying in IRS programmes. By contrast, adding IRS in houses with intact LLINs is unlikely to improve protection relative to LLINs alone, since there is no guarantee that unfed vectors would rest long enough on the sprayed surfaces, and because of the rapid IRS decay. However, there is need to clarify these effects using data from observations of free flying mosquitoes in huts. Physiological susceptibility of An. arabiensis in the area remains 100% against DDT, but is slightly reduced against pyrethroids, necessitating caution over possible spread of resistance. The loss of LLIN toxicity, particularly Olyset(R) nets suggests that protection offered by these nets against An. arabiensis may be primarily due to physical bite prevention rather than insecticidal efficacy
- …