Reduced exposure to malaria vectors following indoor residual spraying of pirimiphos-methyl in a high-burden district of rural Mozambique with high ownership of long-lasting insecticidal nets: entomological surveillance results from a cluster-randomized trial

Multiple insecticide resistance in Anopheles funestus from Mopeia, Central Mozambique

BACKGROUND

The main malaria vector control methods implemented in Mozambique are insecticide-treated nets (ITN's) and indoor residual spraying (IRS). These insecticide-based interventions are currently threatened by the rapidly spreading insecticide resistance in several major malaria vectors. Monitoring of insecticide resistance is necessary to inform the selection of insecticides by control programmes. This study describes the insecticide resistance profiles of the main malaria vector, Anopheles funestus sensu lato. in Mopeia district, a malaria holoendemic area of the Zambezia province of Mozambique.

METHODS

Anopheles adults and larvae were collected from 15 sentinel sites across the district between October 2021 and September 2022. Wild-caught, unfed female adults were collected using CDC-light traps and pooled over three days before exposure to the test insecticide. For mosquitoes collected as larvae, F0 adults aged 3-5 days post-emergence were used for insecticide susceptibility testing. Resistance to bendiocarb, DDT, deltamethrin and pirimiphos-methyl was evaluated using the standard WHO tube bioassay. The mechanism of resistance was probed using the PBO (piperonyl butoxide) synergistic bioassay. The presence and frequency of different genetic mutations associated with insecticide resistance was assessed using polymerase chain reaction, including A296S-Rdl, L119F-GSTe2 and 6.5 kb SV (structural variation) insertion.

RESULTS

A total of 1349 female Anopheles mosquitoes (controls included) were used for susceptibility tests with discriminating insecticide concentrations. Phenotypic resistance to bendiocarb, DDT, deltamethrin and pirimiphos-methyl was observed, with 37%, 79%, 14% and 67% mortality rate respectively. Pre-exposure to PBO partially restored susceptibility to deltamethrin to a mortality rate of 80%. The frequency of the insecticide resistance mutations was 0.49, 0.05 and 0.92, for A296S-Rdl, L119F-GSTe2 and 6.5 kb SV insertion, respectively.

CONCLUSION

Malaria vectors in Mopeia exhibit resistance to all four major public health insecticide classes: pyrethroids, organophosphates, organochlorides and carbamates. This highlights the urgent need to adopt new insecticide classes for vector control interventions. The partial restoration of susceptibility by PBO suggests resistance is being driven by various mechanisms including the involvement of metabolic resistance through cytochrome P450 monooxygenase enzymes and glutathione S-transferases.

Why does malaria transmission continue at high levels despite universal vector control? Quantifying persistent malaria transmission by Anopheles funestus in Western Province, Zambia

BACKGROUND

Some settings continue to experience a high malaria burden despite scale-up of malaria vector control to high levels of coverage. Characterisation of persistent malaria transmission in the presence of standard control measures, also termed residual malaria transmission, to understand where and when individuals are exposed to vector biting is critical to inform refinement of prevention and control strategies.

METHODS

Secondary analysis was performed using data collected during a phase III cluster randomized trial of attractive targeted sugar bait stations in Western Province, Zambia. Two seasonal cohorts of children aged 1-14 years were recruited and monitored monthly during the malaria transmission season, concurrent with entomological surveillance using a combination of human landing catch (HLC) and Centres for Disease Control (CDC) light traps at randomly selected households in study clusters. Behavioural data from cohort participants were combined with measured Anopheles funestus landing rates and sporozoite positivity to estimate the human behaviour-adjusted entomological inoculation rate (EIR).

RESULTS

Behavioural data from 1237 children over 5456 child-visits in 20 entomology surveillance clusters were linked with hourly landing rates from 8131 female An. funestus trapped by HLC. Among all An. funestus tested by enzyme-linked immunosorbent assay (ELISA), 3.3% were sporozoite-positive. Mean EIR directly measured from HLC was 0.07 infectious bites per person per night (ib/p/n). When accounting for child locations over the evening and night, the mean behaviour-adjusted EIR was 0.02 ib/p/n. Children not sleeping under insecticide-treated nets (ITNs) experienced 13.6 infectious bites per person per 6 month season, 8% of which occurred outdoors, while ITN users received 1.3 infectious bites per person per 6 month season, 86% of which were received outdoors. Sleeping under an ITN can prevent approximately 90% of potential An. funestus bites among children.

CONCLUSIONS

In this setting ITNs have a high personal protective efficacy owing to peak An. funestus biting occurring indoors while most individuals are asleep. However, despite high household possession of ITNs (>90%) and high individual use (>70%), children in this setting experience more than one infectious bite per person per 6 month transmission season, sufficient to maintain high malaria transmission and burden. New tools and strategies are required to reduce the malaria burden in such settings.

Multiple Anopheles species complicate downstream analysis and decision-making in a malaria pre-elimination area in southern Mozambique

BACKGROUND

Different anopheline species (even within a species group/complex) can differ in their feeding and resting behaviours, which impact both malaria transmission patterns as well as the efficacy of vector control interventions. While morphological identification of sampled specimens is an important first step towards understanding species diversity and abundance, misidentification can result in the implementation of less effective vector control measures, and consequently smaller reductions in the number of local malaria cases. Focusing on southern Mozambique, a malaria pre-elimination area where malaria remains persistent, the aims of this preliminary study were to use molecular identification (CO1 and ITS2 barcoding) to (1) validate the results from the morphological identification (with a particular focus on Anopheles pharoensis and Anopheles squamosus), and (2) have a closer look at the Anopheles coustani group (which includes Anopheles tenebrosus and Anopheles ziemanni).

METHODS

Female anopheline mosquitoes (n = 81) were identified morphologically and subsequently sequenced at the ribosomal DNA internal transcribed spacer region 2 (ITS2) and/or cytochrome oxidase subunit 1 (CO1) loci towards species determination.

RESULTS

Out of the 62 specimens that were identified morphologically to species, 4 (6.5%) were misidentified. Regarding the An. coustani group, morphological identification showed that several members are present in southern Mozambique, including An. coustani sensu lato (s.l.), An. ziemanni and An. tenebrosus. However, based on both ITS2 and CO1 sequences, the exact species remains unknown for the latter two members until voucher sequences are available for comparison.

CONCLUSION

The reason(s) for morphological misidentification of anopheline mosquitoes need to be mitigated. This is usually related to both the capacity (i.e. training) of the microscopist to identify anopheline species, and the information provided in the dichotomous identification key. As the An. coustani complex contributes to (residual) malaria transmission in sub-Saharan Africa, it may play a role in the observed persistent malaria in southern Mozambique. A better baseline characterizing of the local anophelines species diversity and behaviours will allow us to improve entomological surveillance strategies, better understand the impact of vector control on each local vector species, and identify new approaches to target those vector species.

BOHEMIA a cluster randomized trial to assess the impact of an endectocide-based one health approach to malaria in Mozambique: baseline demographics and key malaria indicators

BACKGROUND

Many geographical areas of sub-Saharan Africa, especially in rural settings, lack complete and up-to-date demographic data, posing a challenge for implementation and evaluation of public health interventions and carrying out large-scale health research. A demographic survey was completed in Mopeia district, located in the Zambezia province in Mozambique, to inform the Broad One Health Endectocide-based Malaria Intervention in Africa (BOHEMIA) cluster randomized clinical trial, which tested ivermectin mass drug administration to humans and/or livestock as a potential novel strategy to decrease malaria transmission.

METHODS

The demographic survey was a prospective descriptive study, which collected data of all the households in the district that accepted to participate. Households were mapped through geolocation and identified with a unique identification number. Basic demographic data of the household members was collected and each person received a permanent identification number for the study.

RESULTS

25,550 households were mapped and underwent the demographic survey, and 131,818 individuals were registered in the district. The average household size was 5 members and 76.9% of households identified a male household head. Housing conditions are often substandard with low access to improved water systems and electricity. The reported coverage of malaria interventions was 71.1% for indoor residual spraying and 54.1% for universal coverage of long-lasting insecticidal nets. The median age of the population was 15 years old. There were 910 deaths in the previous 12 months reported, and 43.9% were of children less than 5 years of age.

CONCLUSIONS

The study showed that the district had good coverage of vector control tools against malaria but sub-optimal living conditions and poor access to basic services. The majority of households are led by males and Mopeia Sede/Cuacua is the most populated locality in the district. The population of Mopeia is young (< 15 years) and there is a high childhood mortality. The results of this survey were crucial as they provided the household and population profiles and allowed the design and implementation of the cluster randomized clinical trial. Trial registration NCT04966702.

Anopheles rufipes implicated in malaria transmission both indoors and outdoors alongside Anopheles funestus and Anopheles arabiensis in rural south-east Zambia

BACKGROUND

The primary malaria vector-control interventions, indoor residual spraying and long-lasting insecticidal nets, are effective against indoor biting and resting mosquito species. Consequently, outdoor biting and resting malaria vectors might elude the primary interventions and sustain malaria transmission. Varied vector biting and resting behaviour calls for robust entomological surveillance. This study investigated the bionomics of malaria vectors in rural south-east Zambia, focusing on species composition, their resting and host-seeking behaviour and sporozoite infection rates.

METHODS

The study was conducted in Nyimba District, Zambia. Randomly selected households served as sentinel houses for monthly collection of mosquitoes indoors using CDC-light traps (CDC-LTs) and pyrethrum spray catches (PSC), and outdoors using only CDC-LTs for 12 months. Mosquitoes were identified using morphological taxonomic keys. Specimens belonging to the Anopheles gambiae complex and Anopheles funestus group were further identified using molecular techniques. Plasmodium falciparum sporozoite infection was determined using sandwich enzyme-linked immunosorbent assays.

RESULTS

From 304 indoor and 257 outdoor light trap-nights and 420 resting collection, 1409 female Anopheles species mosquitoes were collected and identified morphologically; An. funestus (n = 613; 43.5%), An. gambiae sensu lato (s.l.)(n = 293; 20.8%), Anopheles pretoriensis (n = 282; 20.0%), Anopheles maculipalpis (n = 130; 9.2%), Anopheles rufipes (n = 55; 3.9%), Anopheles coustani s.l. (n = 33; 2.3%), and Anopheles squamosus (n = 3, 0.2%). Anopheles funestus sensu stricto (s.s.) (n = 144; 91.1%) and Anopheles arabiensis (n = 77; 77.0%) were the dominant species within the An. funestus group and An. gambiae complex, respectively. Overall, outdoor CDC-LTs captured more Anopheles mosquitoes (mean = 2.25, 95% CI 1.22-3,28) than indoor CDC-LTs (mean = 2.13, 95% CI 1.54-2.73). Fewer resting mosquitoes were collected with PSC (mean = 0.44, 95% CI 0.24-0.63). Sporozoite infectivity rates for An. funestus, An. arabiensis and An. rufipes were 2.5%, 0.57% and 9.1%, respectively. Indoor entomological inoculation rates (EIRs) for An. funestus s.s, An. arabiensis and An. rufipes were estimated at 4.44, 1.15 and 1.20 infectious bites/person/year respectively. Outdoor EIRs for An. funestus s.s. and An. rufipes at 7.19 and 4.31 infectious bites/person/year, respectively.

CONCLUSION

The findings of this study suggest that An. rufipes may play an important role in malaria transmission alongside An. funestus s.s. and An. arabiensis in the study location.

Species Composition, Seasonal Abundance, and Biting Behavior of Malaria Vectors in Rural Conhane Village, Southern Mozambique

Malaria vector surveillance provides important data to inform the effective planning of vector control interventions at a local level. The aim of this study was to determine the species diversity and abundance, biting activity, and Plasmodium infectivity of Anopheles mosquitoes from a rural village in southern Mozambique. Human landing catches were performed monthly between December 2020 and August 2021. All collected Anopheles were identified to the species level and tested for the presence of malaria parasites. Eight Anopheles species were identified among the 1802 collected anophelines. Anopheles gambiae sensu lato (s.l.) were the most abundant (51.9%) and were represented by Anopheles quadriannulatus and Anopheles arabiensis. Anopheles funestus s.l. represented 4.5%. The biting activity of An. arabiensis was more pronounced early in the evening and outdoors, whereas that of An. funestus sensu stricto (s.s.) was more intense late in the night, with no significant differences in location. One An. funestus s.s. and one An. arabiensis, both collected outdoors, were infected with Plasmodium falciparum. The overall entomologic inoculation rate was estimated at 0.015 infective bites per person per night. The significant outdoor and early evening biting activity of An. arabiensis and An. funestus found in this village may negatively impact the effectiveness of current vector control interventions. Additional vector control tools that can target these mosquitoes are needed.

VECTRON™ T500, a new broflanilide insecticide for indoor residual spraying, provides prolonged control of pyrethroid-resistant malaria vectors

Background

Broflanilide is a newly discovered insecticide with a novel mode of action targeting insect γ-aminobutyric acid receptors. The efficacy of VECTRON™ T500, a wettable powder formulation of broflanilide, was assessed for IRS against wild pyrethroid-resistant malaria vectors in experimental huts in Benin.

Methods

VECTRON™ T500 was evaluated at 100 mg/m2 in mud and cement-walled experimental huts against wild pyrethroid-resistant Anopheles gambiae sensu lato (s.l.) in Covè, southern Benin, over 18 months. A direct comparison was made with Actellic® 300CS, a WHO-recommended micro-encapsulated formulation of pirimiphos-methyl, applied at 1000 mg/m2. The vector population at Covè was investigated for susceptibility to broflanilide and other classes of insecticides used for vector control. Monthly wall cone bioassays were performed to assess the residual efficacy of VECTRON™ T500 using insecticide susceptible An. gambiae Kisumu and pyrethroid-resistant An. gambiae s.l. Covè strains. The study complied with OECD principles of good laboratory practice.

Results

The vector population at Covè was resistant to pyrethroids and organochlorines but susceptible to broflanilide and pirimiphos-methyl. A total of 23,171 free-flying wild pyrethroid-resistant female An. gambiae s.l. were collected in the experimental huts over 12 months. VECTRON™ T500 induced 56%-60% mortality in wild vector mosquitoes in both cement and mud-walled huts. Mortality with VECTRON™ T500 was 62%-73% in the first three months and remained > 50% for 9 months on both substrate-types. By comparison, mortality with Actellic® 300CS was very high in the first three months (72%-95%) but declined sharply to < 40% after 4 months. Using a non-inferiority margin defined by the World Health Organization, overall mortality achieved with VECTRON™ T500 was non-inferior to that observed in huts treated with Actellic® 300CS with both cement and mud wall substrates. Monthly in situ wall cone bioassay mortality with VECTRON™ T500 also remained over 80% for 18 months but dropped below 80% with Actellic® 300CS at 6–7 months post spraying.

Conclusion

VECTRON™ T500 shows potential to provide substantial and prolonged control of malaria transmitted by pyrethroid-resistant mosquito vectors when applied for IRS. Its addition to the current list of WHO-approved IRS insecticides will provide a suitable option to facilitate rotation of IRS products with different modes of action.

Microsporidia: a promising vector control tool for residual malaria transmission

Long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) have resulted in a major decrease in malaria transmission. However, it has become apparent that malaria can be effectively transmitted despite high coverage of LLINs/IRS. Residual transmission can occur due to Plasmodium-carrying Anopheles mosquitoes that are insecticide resistant and have feeding and resting behavior that reduces their chance of encountering the currently deployed indoor malaria control tools. Residual malaria transmission is likely to be the most significant hurdle to achieving the goal of malaria eradication and research and development towards new tools and strategies that can control residual malaria transmission is therefore critical. One of the most promising strategies involves biological agents that are part of the mosquito microbiome and influence the ability of Anopheles to transmit Plasmodium. These differ from biological agents previously used for vector control in that their primary effect is on vectoral capacity rather than the longevity and fitness of Anopheles (which may or may not be affected). An example of this type of biological agent is Microsporidia MB, which was identified in field collected Anopheles arabiensis and caused complete inhibition of Plasmodium falciparum transmission without effecting the longevity and fitness of the host. Microsporidia MB belongs to a unique group of rapidly adapting and evolving intracellular parasites and symbionts called microsporidia. In this review we discuss the general biology of microsporidians and the inherent characteristics that make some of them particularly suitable for malaria control. We then discuss the research priorities for developing a transmission blocking strategy for the currently leading microsporidian candidate Microsporidia MB for malaria control.

To spray or target mosquitoes another way: focused entomological intelligence guides the implementation of indoor residual spraying in southern Mozambique

BACKGROUND

To eliminate malaria in southern Mozambique, the National Malaria Control Programme and its partners are scaling up indoor residual spraying (IRS) activities in two provinces, Gaza and Inhambane. An entomological surveillance planning tool (ESPT) was used to answer the programmatic question of whether IRS would be effective in target geographies, given limited information on local vector bionomics.

METHODS

Entomological intelligence was collected in six sentinel sites at the end of the rainy season (April-May 2018) and the beginning of the dry season (June-July 2018). The primary objective was to provide an 'entomological snapshot' by collecting question-based, timely and high-quality data within one single week in each location. Host-seeking behaviour (both indoors and outdoors) was monitored by human-baited tent traps. Indoor resting behaviour was quantified by pyrethrum spray catches and window exit traps.

RESULTS

Five different species or species groups were identified: Anopheles funestus sensu lato (s.l.) (66.0%), Anopheles gambiae s.l. (14.0%), Anopheles pharoensis (1.4%), Anopheles tenebrosus (14.1%) and Anopheles ziemanni (4.5%). Anopheles funestus sensu stricto (s.s.) was the major vector among its sibling species, and 1.9% were positive for Plasmodium falciparum infections. Anopheles arabiensis was the most abundant vector species within the An. gambiae complex, but none tested positive for P. falciparum infections. Some An. tenebrosus were positive for P. falciparum (1.3%). When evaluating behaviours that impact IRS efficacy, i.e. endophily, the known primary vector An. funestus s.s., was found to rest indoors-demonstrating at least part of its population will be impacted by the intervention if insecticides are selected to which this vector is susceptible. However, other vector species, including An. gambiae s.l., An. tenebrosus, An. pharoensis and An. ziemanni, showed exophilic and exophagic behaviours in several of the districts surveilled.

CONCLUSION

The targeted approach to entomological surveillance was successful in collecting question-based entomological intelligence to inform decision-making about the use of IRS in specific districts. Endophilic An. funestus s.s. was documented as being the most prevalent and primary malaria vector suggesting that IRS can reduce malaria transmission, but the presence of other vector species both indoors and outdoors suggests that alternative vector control interventions that target these gaps in protection may increase the impact of vector control in southern Mozambique.

Pyrethroid-piperonyl butoxide (PBO) nets reduce the efficacy of indoor residual spraying with pirimiphos-methyl against pyrethroid-resistant malaria vectors

Pirimiphos-methyl is a pro-insecticide requiring activation by mosquito cytochrome P450 enzymes to induce toxicity while PBO blocks activation of these enzymes in pyrethroid-resistant vector mosquitoes. PBO may thus antagonise the toxicity of pirimiphos-methyl IRS when combined with pyrethroid-PBO ITNs. The impact of combining Olyset Plus and PermaNet 3.0 with Actellic 300CS IRS was evaluated against pyrethroid-resistant Anopheles gambiae s.l. in two parallel experimental hut trials in southern Benin. The vector population was resistant to pyrethroids and PBO pre-exposure partially restored deltamethrin toxicity but not permethrin. Mosquito mortality in experimental huts was significantly improved in the combinations of bendiocarb IRS with pyrethroid-PBO ITNs (33–38%) compared to bendiocarb IRS alone (14–16%, p < 0.001), demonstrating an additive effect. Conversely, mortality was significantly reduced in the combinations of pirimiphos-methyl IRS with pyrethroid-PBO ITNs (55–59%) compared to pirimiphos-methyl IRS alone (77–78%, p < 0.001), demonstrating evidence of an antagonistic effect when both interventions are applied in the same household. Mosquito mortality in the combination was significantly higher compared to the pyrethroid-PBO ITNs alone (55–59% vs. 22–26% p < 0.001) showing potential of pirimiphos-methyl IRS to enhance vector control when deployed to complement pyrethroid-PBO ITNs in an area where PBO fails to fully restore susceptibility to pyrethroids.

Pirikool® 300 CS, a new long-lasting capsule suspension formulation of the organophosphate insecticide pirimiphos-methyl for indoor residual spraying against pyrethroid-resistant malaria vectors

Background

Indoor residual spraying (IRS) using a capsule suspension formulation of the organophosphate insecticide, pirimiphos-methyl, has provided substantial malaria control in many communities in Africa. However, only one brand of this product has been recommended by the World Health Organisation for IRS. To help increase the diversity of the portfolio of IRS insecticides and offer suitable options to procurers and malaria vector control programmes, additional product brands of this highly effective and long-lasting insecticide formulation for IRS will be needed.

Methods

We evaluated the efficacy of Pirikool® 300CS, a new capsule suspension formulation of pirimiphos-methyl developed by Tianjin Yorkool, International Trading, Co., Ltd in standard WHO laboratory bioassays and experimental hut studies. The efficacy of the insecticide applied at 1000mg/m² was assessed in laboratory bioassays for 6 months on cement, plywood and mud block substrates and for 12 months in cement and mud-walled experimental huts against wild free-flying pyrethroid-resistant Anopheles gambiae sensu lato in Covè, Benin. Actellic® 300CS, a WHO-recommended capsule suspension formulation of pirimiphos-methyl was also tested. WHO cylinder tests were performed to determine the frequency of insecticide resistance in the wild vector population during the hut trial.

Results

The vector population at the hut station was resistant to pyrethroids but susceptible to pirimiphos-methyl. Overall mortality rates of wild free-flying pyrethroid-resistant An. gambiae (s.l.) entering Pirikool®300CS treated experimental huts during the 12-month trial were 86.7% in cement-walled huts and 88% in mud-walled huts. Mortality of susceptible An. gambiae (Kisumu) and pyrethroid-resistant An. gambiae s.l. (Covè) mosquitoes in monthly wall cone bioassays on Pirikool® 300CS treated hut walls remained over 80% for 10–12 months. The laboratory bioassays corroborated the hut findings with Pirikool® 300CS on mud and wood block substrates but not on cement block substrates.

Conclusion

Indoor residual spraying with Pirikool® 300CS induced high and prolonged mortality of wild pyrethroid-resistant malaria vectors for 10–12 months. Addition of Pirikool®300CS to the current portfolio of IRS insecticides will provide an extra choice of microencapsulated pirimiphos-methyl for IRS.

Malaria risk and receptivity: Continuing development of insecticide resistance in the major malaria vector Anopheles arabiensis in northern KwaZulu-Natal, South Africa

Malaria incidence in South Africa is highest in the three endemic provinces: KwaZulu-Natal, Mpumalanga and Limpopo. The contribution to malaria transmission by several mosquito species, variation in their resting behaviours and low levels of insecticide resistance makes it necessary to periodically monitor Anopheles species assemblages and resistance phenotypes in vector populations. The aim of this study was therefore to assess Anopheles species assemblage in northern KwaZulu-Natal and to collect insecticide susceptibility data for An. arabiensis, the primary vector of malaria in that province. Anopheles specimens were collected from Mamfene, Jozini, northern KwaZulu-Natal from November 2019 to April 2021. Progeny of wild-collected An. arabiensis females were used for standard insecticide susceptibility tests and synergist bioassays. Anopheles arabiensis contributed 85.6% (n=11 062) of the total catches. Samples for subsequent insecticide susceptibility bioassays were selected from 212 An. arabiensis families. These showed low-level resistance to DDT, permethrin, deltamethrin, and bendiocarb, as well as full susceptibility to pirimiphos-methyl. Synergist bioassays using piperonyl butoxide and triphenyl phosphate suggest oxygenase-based pyrethroid and esterase-mediated sequestration of bendiocarb. These low levels of resistance are unlikely to be operationally significant at present. It is concluded that northern KwaZulu-Natal Province remains receptive to malaria transmission despite ongoing control and elimination interventions. This is due to the perennial presence of the major vector An. arabiensis and other secondary vector species. The continued detection of low-frequency insecticide resistance phenotypes in An. arabiensis is cause for concern and requires periodic monitoring for changes in resistance frequency and intensity.

Indoor residual spraying for preventing malaria in communities using insecticide-treated nets

BACKGROUND

Insecticide-treated nets (ITNs) and indoor residual spraying (IRS) are used to prevent malaria transmission. Both interventions use insecticides to kill mosquitoes that bite and rest indoors. Adding IRS to ITNs may improve malaria control simply because two interventions can be better than one. Furthermore, IRS may improve malaria control where ITNs are failing due to insecticide resistance. Pyrethroid insecticides are the predominant class of insecticide used for ITNs, as they are more safe than other insecticide classes when in prolonged contact with human skin. While many mosquito populations have developed some resistance to pyrethroid insecticides, a wider range of insecticides can be used for IRS. This review is an update of the previous Cochrane 2019 edition.

OBJECTIVES

To summarize the effect on malaria of additionally implementing IRS, using non-pyrethroid-like or pyrethroid-like insecticides, in communities currently using ITNs.

SEARCH METHODS

We searched the Cochrane Infectious Diseases Group Specialized Register; CENTRAL; MEDLINE; and five other databases for records from 1 January 2000 to 8 November 2021, on the basis that ITN programmes did not begin to be implemented as policy before the year 2000.

SELECTION CRITERIA

We included cluster-randomized controlled trials (cRCTs), interrupted time series (ITS), or controlled before-after studies (CBAs) comparing IRS plus ITNs with ITNs alone. We included studies with at least 50% ITN ownership (defined as the proportion of households owning one or more ITN) in both study arms.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed studies for eligibility, analyzed risk of bias, and extracted data. We used risk ratio (RR) and 95% confidence intervals (CI). We stratified by type of insecticide, 'pyrethroid-like' and 'non-pyrethroid-like'; the latter could improve malaria control better than adding IRS insecticides that have the same way of working as the insecticide on ITNs ('pyrethroid-like'). We used subgroup analysis of ITN usage in the studies to explore heterogeneity. We assessed the certainty of evidence using the GRADE approach.

MAIN RESULTS

Eight cRCTs (10 comparisons), one CBA, and one ITS study, all conducted since 2008 in sub-Saharan Africa, met our inclusion criteria. The primary vectors in all sites were mosquitoes belonging to the Anopheles gambiae s.l. complex species; five studies in Benin, Mozambique, Ghana, Sudan, and Tanzania also reported the vector Anopheles funestus. Five cRCTs and both quasi-experimental design studies used insecticides with targets different to pyrethroids (two used bendiocarb, three used pirimiphos-methyl, and one used propoxur. Each of these studies were conducted in areas where the vectors were described as resistant or highly resistant to pyrethroids. Two cRCTs used dichloro-diphenyl-trichlorethane (DDT), an insecticide with the same target as pyrethroids. The remaining cRCT used both types of insecticide (pyrethroid deltamethrin in the first year, switching to bendiocarb for the second year). Indoor residual spraying using 'non-pyrethroid-like' insecticides Six studies were included (four cRCTs, one CBA, and one ITS). Our main analysis for prevalence excluded a study at high risk of bias due to repeated sampling of the same population. This risk did not apply to other outcomes. Overall, the addition of IRS reduced malaria parasite prevalence (RR 0.61, 95% CI 0.42 to 0.88; 4 cRCTs, 16,394 participants; high-certainty evidence). IRS may also reduce malaria incidence on average (rate ratio 0.86, 95% CI 0.61 to 1.23; 4 cRCTs, 323,631 child-years; low-certainty evidence) but the effect was absent in two studies. Subgroup analyses did not explain the qualitative heterogeneity between studies. One cRCT reported no effect on malaria incidence or parasite prevalence in the first year, when a pyrethroid-like insecticide was used for IRS, but showed an effect on both outcomes in the second year, when a non-pyrethroid-like IRS was used. The addition of IRS may also reduce anaemia prevalence (RR 0.71, 95% CI 0.38 to 1.31; 3 cRCTs, 4288 participants; low-certainty evidence). Four cRCTs reported the impact of IRS on entomological inoculation rate (EIR), with variable results; overall, we do not know if IRS had any effect on the EIR in communities using ITNs (very low-certainty evidence). Studies also reported the adult mosquito density and the sporozoite rate, but we could not summarize or pool these entomological outcomes due to differences in the reported data. Three studies measured the prevalence of pyrethroid resistance before and after IRS being introduced: there was no difference detected, but these data are limited. Indoor residual spraying using 'pyrethroid-like' insecticides Adding IRS using a pyrethroid-like insecticide did not appear to markedly alter malaria incidence (rate ratio 1.07, 95% CI 0.80 to 1.43; 2 cRCTs, 15,717 child-years; moderate-certainty evidence), parasite prevalence (RR 1.11, 95% CI 0.86 to 1.44; 3 cRCTs, 10,820 participants; moderate-certainty evidence), or anaemia prevalence (RR 1.12, 95% CI 0.89 to 1.40; 1 cRCT, 4186 participants; low-certainty evidence). Data on EIR were limited so no conclusion was made (very low-certainty evidence).

AUTHORS' CONCLUSIONS

in communities using ITNs, the addition of IRS with 'non-pyrethroid-like' insecticides was associated with reduced malaria prevalence. Malaria incidence may also be reduced on average, but there was unexplained qualitative heterogeneity, and the effect may therefore not be observed in all settings. When using 'pyrethroid-like' insecticides, there was no detectable additional benefit of IRS in communities using ITNs.

Assessing the effectiveness of malaria interventions at the regional level in Ghana using a mathematical modelling application

Supporting malaria control with interfaced applications of mathematical models that enables investigating effectiveness of various interventions as well as their cost implications could be useful. Through their usage for planning, these applications may improve the prospects of attaining various set targets such as those of the National Strategic Plan policies for malaria control in Ghana. A malaria model was adapted and used for simulating the incidence of malaria in various regions of Ghana. The model and its application were developed by the Modelling and Simulation Hub Africa and calibrated using district level data in Ghana from 2012 to 2018. Average monthly rainfall at the zonal level was fitted to trigonometric functions for each ecological zone using least squares approach. These zonal functions were then used as forcing functions. Subsequently, various intervention packages were investigated to observe their impact on averting malaria incidence by 2030. Increased usage of bednets but not only coverage levels, predicted a significant proportion of cases of malaria averted in all regions. Whereas, improvements in the health system by way of health seeking, testing and treatment predicted a decline in incidence largely in all regions. With an increased coverage of SMC, to include higher age groups, a modest proportion of cases could be averted in populations of the Guinea savannah. Indoor residual spraying could also benefit populations of the Transitional forest and Coastal savannah as its impact is significant in averting incidence. Enhancing bednet usage to at least a doubling of the current usage levels and deployed in combination with various interventions across regions predicted significant reductions, in malaria incidence. Regions of the Transitional forest and Coastal savannah could also benefit from a drastic decline in incidence following a gradual introduction of indoor residual spraying on a sustained basis.

Evaluating the efficacy, impact, and feasibility of community-based house screening as a complementary malaria control intervention in southern Africa: a study protocol for a household randomized trial

BACKGROUND

Concerted effort to control malaria has had a substantial impact on the transmission of the disease in the past two decades. In areas where reduced malaria transmission is being sustained through insecticide-based vector control interventions, primarily long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS), non-insecticidal complementary tools will likely be needed to push towards malaria elimination. Once interruption in local disease transmission is achieved, insecticide-based measures can be scaled down gradually and eventually phased out, saving on costs of sustaining control programs and mitigating any unintended negative health and environmental impacts posed by insecticides. These non-insecticidal methods could eventually replace insecticidal methods of vector control. House screening, a non-insecticidal method, has a long history in malaria control, but is still not widely adopted in sub-Saharan Africa. This study aims to add to the evidence base for this intervention in low transmission settings by assessing the efficacy, impact, and feasibility of house screening in areas where LLINs are conventionally used for malaria control.

METHODS

A two-armed, household randomized clinical trial will be conducted in Mozambique, Zambia, and Zimbabwe to evaluate whether combined the use of house screens and LLINs affords better protection against clinical malaria in children between 6 months and 13 years compared to the sole use of LLINs. Eight hundred households will be enrolled in each study area, where 400 households will be randomly assigned the intervention, house screening, and LLINs while the control households will be provided with LLINs only. Clinical malaria incidence will be estimated by actively following up one child from each household for 6 months over the malaria transmission season. Cross-sectional parasite prevalence will be estimated by testing all participating children for malaria parasites at the beginning and end of each transmission season using rapid diagnostic tests. CDC light traps and pyrethrum spray catches (PSC) will be used to sample adult mosquitoes and evaluate the impact of house screening on indoor mosquito density, species distribution, and sporozoite rates.

DISCUSSION

This study will contribute epidemiological data on the impact of house screening on malaria transmission and assess the feasibility of its implementation on a programmatic scale.

TRIAL REGISTRATION

ClinicalTrials.gov PACTR202008524310568 . Registered on August 11, 2020.

Cost and cost-effectiveness of indoor residual spraying with pirimiphos-methyl in a high malaria transmission district of Mozambique with high access to standard insecticide-treated nets

BACKGROUND

As malaria cases increase in some of the highest burden countries, more strategic deployment of new and proven interventions must be evaluated to meet global malaria reduction goals.

METHODS

The cost and cost-effectiveness of indoor residual spraying (IRS) with pirimiphos-methyl (Actellic®300 CS) were assessed in a high transmission district (Mopeia) with high access to pyrethroid insecticide-treated nets (ITNs), compared to ITNs alone. The major mosquito vectors in the area were susceptible to primiphos-methyl, but resistant to pyrethoids. A decision analysis approach was followed to conduct deterministic and probabilistic sensitivity analyses in a theoretical cohort of 10,000 children under five years of age (U5) and 10,000 individuals of all ages, separately. Model parameters and distributions were based on prospectively collected cost and epidemiological data from a cluster-randomized control trial and a literature review. The primary analysis used health facility-malaria incidence, while community cohort incidence and cross-sectional prevalence rates were used in sensitivity analyses. Lifetime costs, malaria cases, deaths and disability-adjusted life-years (DALYs) were calculated to determine the incremental costs per DALY averted through IRS.

RESULTS

The average IRS cost per person protected was US$8.26 and 51% of the cost was insecticide. IRS averted 46,609 (95% CI 46,570-46,646) uncomplicated and 242 (95% CI 241-243) severe lifetime cases in a theoretical children U5 cohort, yielding an incremental cost-effectiveness ratio (ICER) of US$400 (95% CI 399-402) per DALY averted. In the all-age cohort, the ICER was higher: US$1,860 (95% CI 1,852-1,868) per DALY averted. Deterministic and probabilistic results were consistent. When adding the community protective effect of IRS, the cost per person protected decreased (US$7.06) and IRS was highly cost-effective in children U5 (ICER = US$312) and cost-effective in individuals of all ages (ICER = US$1,431), compared to ITNs alone.

CONCLUSION

This study provides robust evidence that IRS with pirimiphos-methyl can be cost-effective in high transmission regions with high pyrethroid ITN coverage where the major vector is susceptible to pirimiphos-methyl but resistant to pyrethroids. The finding that insecticide cost is the main driver of IRS costs highlights the need to reduce the insecticide price without jeopardizing effectiveness.

TRIAL REGISTRATION

ClinicalTrials.gov identifier NCT02910934 (Registered 22 September 2016). https://clinicaltrials.gov/ct2/show/NCT02910934?term=NCT02910934&draw=2&rank=1.

Incremental impact on malaria incidence following indoor residual spraying in a highly endemic area with high standard ITN access in Mozambique: results from a cluster-randomized study

Background

Attaining the goal of reducing the global malaria burden is threatened by recent setbacks in maintaining the effectiveness of vector control interventions partly due to the emergence of pyrethroid resistant vectors. One potential strategy to address these setbacks could be combining indoor residual spraying (IRS) with non-pyrethroids and standard insecticide-treated nets (ITNs). This study aimed to provide evidence on the incremental epidemiological benefit of using third-generation IRS product in a highly endemic area with high ITN ownership.

Methods

A cluster-randomized, open-label, parallel-arms, superiority trial was conducted in the Mopeia district in Zambezia, Mozambique from 2016 to 2018. The district had received mass distribution of alphacypermethrin ITNs two years before the trial and again mid-way. 86 clusters were defined, stratified and randomized to receive or not receive IRS with pirimiphos-methyl (Actellic®300 CS). Efficacy of adding IRS was assessed through malaria incidence in a cohort of children under five followed prospectively for two years, enhanced passive surveillance at health facilities and by community health workers, and yearly cross-sectional surveys at the peak of the transmission season.

Findings

A total of 1536 children were enrolled in the cohort. Children in the IRS arm experienced 4,801 cases (incidence rate of 3,532 per 10,000 children-month at risk) versus 5,758 cases in the no-IRS arm (incidence rate of 4,297 per 10,000 children-month at risk), resulting in a crude risk reduction of 18% and an incidence risk ratio of 0.82 (95% CI 0.79–0.86, p-value < 0.001). Facility and community passive surveillance showed a malaria incidence of 278 per 10,000 person-month in the IRS group (43,974 cases over 22 months) versus 358 (95% CI 355–360) per 10,000 person-month at risk in the no-IRS group (58,030 cases over 22 months), resulting in an incidence rate ratio of 0.65 (95% CI 0.60–0.71, p < 0.001). In the 2018 survey, prevalence in children under five in the IRS arm was significantly lower than in the no-IRS arm (OR 0.54, 95% CI, 0.31–0.92, p = 0.0241).

Conclusion

In a highly endemic area with high ITN access and emerging pyrethroid resistance, adding IRS with pirimiphos-methyl resulted in significant additional protection for children under five years of age.

Trial registration: ClinicalTrials.gov identifier NCT02910934, registered 22 September 2016, https://clinicaltrials.gov/ct2/show/NCT02910934?term=NCT02910934&draw=2&rank=1.