Abstract Background Following agricultural use and large-scale distribution of insecticide treated nets (ITNs), malaria vector resistance to pyrethroids is widespread in sub-Saharan Africa. Interceptor® G2 is a new dual active ingredient (AI) ITN treated with alpha-cypermethrin and chlorfenapyr for the control of pyrethroid-resistant malaria vectors. In anticipation of these new nets being more widely distributed, testing was conducted to develop a chlorfenapyr susceptibility bioassay protocol and gather susceptibility information. Methods Bottle bioassay tests were conducted using five concentrations of chlorfenapyr at 12.5, 25, 50, 100 and 200µg AI/bottle in ten countries in sub-Saharan Africa using 13,639 wild collected An. gambiae s.l. (56 vector populations per dose) and 4,494 pyrethroid susceptible insectary mosquitoes from 8 colonized strains. In parallel, susceptibility tests were conducted using a provisional discriminating concentration of 100µg AI/bottle in 16 countries using 23,422 wild collected pyrethroid resistant An. gambiae s.l . (259 vector populations). Exposure time was 60 minutes, with mortality recorded at 24, 48 and 72 hours after exposure. Results Median mortality rates (up to 72h after exposure) of insectary colony mosquitoes was 100% at all five concentrations tested, but the lowest dose to kill all mosquitoes tested was 50µg AI/bottle. The median 72h mortality of wild An. gambiae s.l. in 10 countries was 71.5%, 90.5%, 96.5%, 100% and 100% at concentrations of 12.5, 25, 50, 100 and 200µg AI/bottle, respectively. Log-probit analysis of the five concentrations tested determined that the LC 95 of wild An. gambiae s.l. was 67.9µg AI/bottle (95% CI: 48.8-119.5). The discriminating concentration of 203.8µg AI/bottle (95% CI: 146-359) was calculated by multiplying the LC 95 by three. However, the difference in mortality between 100 and 200µg AI/bottle was minimal and large-scale testing using 100µg AI/bottle with wild An. gambiae s.l. in 16 countries showed that this concentration was generally suitable, with a median mortality rate of 100% at 72h. Conclusions This study determined that 200µg AI/bottle chlorfenapyr in bottle bioassays is the most suitable discriminating concentration for monitoring susceptibility of wild An. gambiae s.l., using mortality recorded up to 72h. Testing in 16 countries in sub-Saharan Africa demonstrated vector susceptibility to chlorfenapyr, including mosquitoes with multiple resistance mechanisms to pyrethroids.
Ficam (bendiocarb) was tested for its residual efficacy and irritation in malaria vector control by using a laboratory bred colony of Anopheles arabiensis. In the study area, the insecticide remained active for up to 8 wk (96% mortality) on thatch. In similar, especially constructed huts, 74% mortality was achieved up to 20 wk on mud compared with up to 100% on thatch. In the special huts, release/capture studies indicated that the lethal effect of Ficam on the insects was more pronounced than its irritant effect. This was shown by the low recapture numbers in exit traps as compared with the hut-floor mortalities. The implications of these findings in relation to studies elsewhere and the potential of Ficam use in malaria control are discussed.
Following agricultural use and large-scale distribution of insecticide-treated nets (ITNs), malaria vector resistance to pyrethroids is widespread in sub-Saharan Africa. Interceptor® G2 is a new dual active ingredient (AI) ITN treated with alpha-cypermethrin and chlorfenapyr for the control of pyrethroid-resistant malaria vectors. In anticipation of these new nets being more widely distributed, testing was conducted to develop a chlorfenapyr susceptibility bioassay protocol and gather susceptibility information.Bottle bioassay tests were conducted using five concentrations of chlorfenapyr at 12.5, 25, 50, 100, and 200 µg AI/bottle in 10 countries in sub-Saharan Africa using 13,639 wild-collected Anopheles gambiae sensu lato (s.l.) (56 vector populations per dose) and 4,494 pyrethroid-susceptible insectary mosquitoes from 8 colonized strains. In parallel, susceptibility tests were conducted using a provisional discriminating concentration of 100 µg AI/bottle in 16 countries using 23,422 wild-collected, pyrethroid-resistant An. gambiae s.l. (259 vector populations). Exposure time was 60 min, with mortality recorded at 24, 48 and 72 h after exposure.Median mortality rates (up to 72 h after exposure) of insectary colony mosquitoes was 100% at all five concentrations tested, but the lowest dose to kill all mosquitoes tested was 50 µg AI/bottle. The median 72-h mortality of wild An. gambiae s.l. in 10 countries was 71.5, 90.5, 96.5, 100, and 100% at concentrations of 12.5, 25, 50, 100, and 200 µg AI/bottle, respectively. Log-probit analysis of the five concentrations tested determined that the LC95 of wild An. gambiae s.l. was 67.9 µg AI/bottle (95% CI: 48.8-119.5). The discriminating concentration of 203.8 µg AI/bottle (95% CI: 146-359) was calculated by multiplying the LC95 by three. However, the difference in mortality between 100 and 200 µg AI/bottle was minimal and large-scale testing using 100 µg AI/bottle with wild An. gambiae s.l. in 16 countries showed that this concentration was generally suitable, with a median mortality rate of 100% at 72 h.This study determined that 100 or 200 µg AI/bottle chlorfenapyr in bottle bioassays are suitable discriminating concentrations for monitoring susceptibility of wild An. gambiae s.l., using mortality recorded up to 72 h. Testing in 16 countries in sub-Saharan Africa demonstrated vector susceptibility to chlorfenapyr, including mosquitoes with multiple resistance mechanisms to pyrethroids.
This review outlines and discusses the new challenges in malaria control and prospects for its elimination in Mutare and Mutasa Districts, Zimbabwe. The burden of malaria has declined significantly over the past 5 years in most regions in Zimbabwe, including Mutare and Mutasa Districts. The nationwide malaria reduction has been primarily linked to scaled-up vector control interventions and early diagnosis and treatment with effective anti-malarial medicines. The successes recorded have prompted Zimbabwe's National Malaria Control Programme to commit to a global health agenda of eliminating malaria in all districts in the country. However, despite the decline in malaria burden in Mutare and Mutasa Districts, there is clear evidence of new challenges, including changes in vector behaviour, resistance to insecticides and anti-malarial medicines, invasion of new areas by vectors, vectors in various combination of sympatry, changes in vector proportions, outdoor malaria transmission, climate change and lack of meticulousness of spray operators. These new challenges are likely to retard the shift from malaria control to elimination in Mutare and Mutasa Districts.
Questions have been raised about the quality of indoor residual spraying for malaria vector control after the decentralization of the national malaria control program in Zimbabwe. Given the critical role this control method plays, we conducted an exercise to determine the amount of insecticide (mg active ingredient/m2 of lambda-cyhalothrin) applied during routine house spraying. Severe insecticide underdosing was detected. Spraying efficiency ranged between 63.4 and 76.1% on walls, and 52.7 and 63.2% on roofs. Differences between 2 districts suggested the problem originates from deficient training and lack of pump calibration. Underdosing can undermine effective residual insecticide activity and the expected reduction in disease transmission.
Abstract Background Insecticide resistance can present a major obstacle to malaria control programmes. Following the recent detection of DDT resistance in Anopheles arabiensis in Gokwe, Zimbabwe, the underlying resistance mechanisms in this population were studied. Methods Standard WHO bioassays, using 0.75% permethrin, 4% DDT, 5% malathion, 0.1% bendiocarb and 4% dieldrin were performed on wild-collected adult anopheline mosquitoes and F 1 progeny of An. arabiensis reared from wild-caught females. Molecular techniques were used for species identification as well as to identify knockdown resistance ( kdr ) and ace-1 mutations in individual mosquitoes. Biochemical assays were used to determine the relative levels of detoxifying enzyme systems including non-specific esterases, monooxygenases and glutathione-S-transferases as well as to detect the presence of an altered acetylcholine esterase (AChE). Results Anopheles arabiensis was the predominant member of the Anopheles gambiae complex. Of the 436 An. arabiensis females, 0.5% were positive for Plasmodium falciparum infection. WHO diagnostic tests on wild populations showed resistance to the pyrethroid insecticide permethrin at a mean mortality of 47% during February 2006 and a mean mortality of 68.2% in January 2008. DDT resistance (68.4% mean mortality) was present in February 2006; however, two years later the mean mortality was 96%. Insecticide susceptibility tests on F 1 An. arabiensis families reared from material from two separate collections showed an average mean mortality of 87% (n = 758) after exposure to 4% DDT and 65% (n = 587) after exposure to 0.75% permethrin. Eight families were resistant to both DDT and permethrin. Biochemical analysis of F 1 families reared from collections done in 2006 revealed high activity levels of monooxygenase (48.5% of families tested, n = 33, p < 0.05), glutathione S-transferase (25.8% of families tested, n = 31, p < 0.05) and general esterase activity compared to a reference susceptible An. arabiensis colony. Knockdown resistance ( kdr ) and ace-I R mutations were not detected. Conclusion This study confirmed the presence of permethrin resistance in An. arabiensis populations from Gwave and emphasizes the importance of periodic and ongoing insecticide susceptibility testing of malaria vector populations whose responses to insecticide exposure may undergo rapid change over time.
In 2017, more than 5 million house structures were sprayed through the U.S. President's Malaria Initiative, protecting more than 21 million people in sub-Saharan Africa. New IRS formulations, SumiShield™ 50WG and Fludora Fusion™ WP-SB, became World Health Organization (WHO) prequalified vector control products in 2017 and 2018, respectively. Both formulations contain the neonicotinoid active ingredient, clothianidin. The target site of neonicotinoids represents a novel mode of action for vector control, meaning that cross-resistance through existing mechanisms is less likely. In preparation for rollout of clothianidin formulations as part of national IRS rotation strategies, baseline susceptibility testing was conducted in 16 countries in sub-Saharan Africa.While work coordinated by the WHO is ongoing to develop a suitable bottle bioassay procedure, there was no published guidance regarding clothianidin susceptibility procedures or diagnostic concentrations. Therefore, a protocol was developed for impregnating filter papers with 2% w/v SumiShield™ 50WG dissolved in distilled water. Susceptibility tests were conducted using insectary-reared reference Anopheles and wild collected malaria vector species. All tests were conducted within 24 h of treating papers, with mortality recorded daily for 7 days, due to the slow-acting nature of clothianidin against mosquitoes. Anopheles gambiae sensu lato (s.l.) adults from wild collected larvae were tested in 14 countries, with wild collected F0 Anopheles funestus s.l. tested in Mozambique and Zambia.One-hundred percent mortality was reached with all susceptible insectary strains and with wild An. gambiae s.l. from all sites in 11 countries. However, tests in at least one location from 5 countries produced mortality below 98%. While this could potentially be a sign of clothianidin resistance, it is more likely that the diagnostic dose or protocol requires further optimization. Repeat testing in 3 sites in Ghana and Zambia, where possible resistance was detected, subsequently produced 100% mortality. Results showed susceptibility to clothianidin in 38 of the 43 sites in sub-Saharan Africa, including malaria vectors with multiple resistance mechanisms to pyrethroids, carbamates and organophosphates.This study provides an interim diagnostic dose of 2% w/v clothianidin on filter papers which can be utilized by National Malaria Control Programmes and research organizations until the WHO concludes multi-centre studies and provides further guidance.
Regular entomological monitoring is important to determine changes in mosquito species composition and relative densities of malaria vectors in relation to vector control interventions. A study to gain insights into malaria vector species composition and relative abundance was undertaken in Mutare and Mutasa districts, Zimbabwe. Two methods; indoor resting catches and larval sampling were used to collect indoor resting adults and larvae from May 2013 to April 2014. Mosquitoes collected as adults and reared from larvae that were identified morphologically as potential malaria vectors were further processed to sibling species by polymerase chain reaction (PCR). Morphological identification of anopheline mosquitoes showed presence of two complexes: <em>An. funestus</em> and <em>An. gambiae</em>. The total number of female members of the <em>An. funestus</em> group and <em>An. gambiae</em> complex collected by both methods from the two sites was 840 and 31 respectively. Malaria vector species of both complexes were more abundant in Mutare than in Mutasa. The PCR-based assays showed the presence of four sibling species: <em>An. funestus</em> <em>sensu</em> <em>stricto</em> (90.8%, 267/294) and <em>An. leesoni</em> (5.1%, 15/294), of <em>An. funestus</em> group; <em>An. arabiensis</em> (41.9%, 13/31) and <em>An. quadriannulatus</em> (48.4%, 15/31) of the <em>An. gambiae</em> complex. About 4% and 5% of specimens of <em>An. gambiae</em> complex and A<em>n. funestus</em> group respectively did not amplify. Of the two identified malaria vector sibling species, An. funestus sensu stricto was more abundant (95.4%, 267/280) than <em>An. arabiensis</em> (4.6%, 13/280), suggesting the replacement to secondary vector of <em>An. arabiensis</em>, which was previously the predominant vector species. <em>An. funestus</em> <em>sensu</em> <em>stricto</em> and <em>An</em>. <em>arabiensis</em>, the most important vectors of human malaria were identified in this study, but their resting and biting habits as well as insecticide susceptibility are unclear. Further studies on vector behaviour are therefore recommended.
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