Additional vector control tools in addition to core control measures (ITNs+IRS) including larviciding, eave tubes and targeted sugar baits designed to attract and kill mosquitoes will further support IRM principles. This sugar bait, which is being developed to address the emerging threat of open‐air blood‐biting, is expected to reach pre‐listing by 2025. Other novel interventions such as vector traps, eave tubes, spatial repellents, and genetic control of mosquitoes are at various stages of development (Figure 10).Conserving vector susceptibility is a cornerstone of malaria and other vector‐borne disease control, requiring an approach that can be addressed through an integrated vector management (IVM) framework.
Figure 10. Roadmap for development of current and new vector control products
Source: IVCC (2022)
Figure 11. Trap structure (a) and attractive targeted sugar bait (b) on a wall in Zambia
Source: Attractive Targeted Sugar Bait Phase III Trial Group.
Attractive targeted sugar bait phase III trials in Kenya, Mali, and Zambia.Trials23, 640 (2022).
https://doi.org/10.1186/s13063‐022‐06555‐8
Figure 12. Mechanism and protective efficacy of spatial repellents
Source: spatialrepellents.nd.edu/about/spatial‐repellents
The development of personal tools for vector control is also of interest., These tools not only provide personal protection but can also play an important role in reducing the risk of infection for the entire community. Recent studies have demonstrated the protective efficacy of mosquito repellents for high‐risk groups..
Effective personal protection measures that prevent mosquito bites, regardless of location and time, can address current control gaps and complement existing interventions (Killeen 2014). Among these measures, topical repellents are a particularly attractive candidate, based on extensive data on their safety and efficacy in reducing mosquito bites (Alpern 2016; Nguyen 2023). Topical repellents can be easily distributed among vulnerable populations through partnerships with the private sector and local governments. As an intervention tool, repellents may be particularly useful for high‐risk groups, those with increased behavioral or occupational exposure to malaria vectors, and those unlikely to be protected by LLINs/IRS. These groups include refugees (Rowland 2001), miners (Olapeju 2021), foresters, soldiers or indigenous groups (Bevilacqua 2015), and others who play an important role in maintaining malaria transmission.
Figure 11. Mosquito repellent products to prevent mosquito‐borne diseases
They are commonly available as topical creams, sprays or gels, but can also be found in soap form that leaves a waterproof residue on the skin (Kroeger 1997; Rowland 2004). Plant‐derived oils, such as lemongrass (Cymbopogon), neem (Azidarachta indica) and eucalyptus (Eucalyptus maculate citriodon) have been used since ancient times for this purpose, alone or in combination with paraffin and similar preparations (Maia 2011).
Topical repellents have been used more frequently by travellers and expatriates working in malaria‐endemic areas and their use has been proposed as an alternative to prevent malaria in these and other groups where LLINs and IRSs are expected to be less effective (WHO 2023). However, it remains unclear whether integrating repellents as an additional vector control measure into existing control programmes in endemic areas would result in fewer cases of malaria (WHO 2023). There are important limitations that may affect the usefulness of repellents.
First, topical repellents do not kill mosquitoes; they protect by preventing mosquito bites. Because mosquitoes are not killed, they may be diverted from those who use repellents to those who do not (Maia 2013). This raises health equity implications, as access to these products may vary across different social classes.
Second, their effects are short‐lived and require repeated application, so protection is highly dependent on user compliance (Sangoro 2014). Although repellent products are generally well received by communities (Sangoro 2014), their consistent and full use has been shown to be poor, even in experimental settings where engagement is enhanced (Sluydts 2016). In some communities, many of the perceived benefits of repellents stem from non‐prescribed use, such as direct application to bed nets (Gryseels 2015). Finally, large‐scale distribution of topical repellents would add costs to already underfunded prevention programs, with incremental cost‐effectiveness ratios (ICERs) estimated to range from $212 to $832 per infection averted (Agius 2020).
Another personal protection measure is to use clothing impregnated with mosquito repellent chemicals. (Insecticide‐treated clothing‐ITC) has also been applied in many different communities.ITC can be used to address outdoor malaria transmission, particularly among mobile populations, as well as night workers such as rubber tappers, who may be outside the protection of core interventions.
One study showed that the use of ITC significantly reduced both malaria prevalence and indoor mosquito density. Malaria prevalence in the intervention group was reduced by approximately 70%. The idea of using ITC to prevent malaria was readily accepted by refugees and they considered it beneficial. No adverse effects associated with the use of ITC were observed in the participants. Therefore, this strategy should be considered for use in poor communities such as slum dwellers and other disadvantaged communities, such as street children and refugees, especially when the wave is in malaria‐prone areas. Further research on the cost‐effectiveness and sustainability of this strategy is needed.
A randomized, double‐blind, cluster‐based non‐inferiority crossover trial was conducted to determine the acceptability of ITC compared to identical untreated clothing (NTC) among migrant rubber tappers in Thanbyuzayat Township, Mon State, Myanmar. The acceptability of both clothing types was high. ITC was found to be non‐inferior to NTC on seven out of eight perception‐related measures (looks good, is durable, is easy to wear at night, reduces mosquito bites, would recommend the clothing, would buy the clothing, overall). A high proportion of respondents reported that the clothing reduced mosquito bites (ITC‐98%; NTC‐94%). The clothing was worn frequently (approximately 11 times in the previous two weeks). The most common reasons for not wearing clothing at night were that it was being washed or dried or that the participant was not working. The high level of acceptance suggests that ITC may be an appropriate strategy for personal protection for migrant rubber tappers at open‐air transmission sites in Myanmar. However, more research is needed on the feasibility and protective efficacy of ITC before wider implementation can be considered.
Thus, there are a large number of tools available for malaria vector control, some proven and tested, some being refined, and others in development. To date, some of the affordable and scalable tools endorsed by WHO and deployed by RBM have targeted indoor‐biting vector populations. These interventions are increasingly losing their effectiveness and are no longer appropriate in many settings where malaria transmission is now substantially sustained by outdoor‐biting vector populations. The spread of insecticide resistance in malaria vectors and shifts in vector composition and biting behavior due to persistent selective pressure on endophytic mosquitoes require additional control tools to specifically address such increasingly prevalent problems.
Compared to a decade ago, there are more vector control products with different modes of action available today. However, these tools vary in their suitability for different transmission environments/geographies… Although the list may seem long, there are actually very few approaches that combine cost‐effectiveness, scalability and sustainability. The recent development of synthetic mosquito attractants for upstream trapping has shown encouraging results in ongoing malaria surveillance and monitoring, but their cost is an obstacle to their scalability in rural areas.
The use of larvicides, perhaps combined with new deployment models through communities and/or technology, may be feasible in urban and semi‐urban areas. Among truly novel tools, sugar baits and insecticides may provide a cost‐effective and scalable angle of attack for outdoor‐biting malaria vector control and offer flexibility in how they can be delivered in a variety of settings. Finally, advances in genetic engineering and modeling of gene drives to suppress or replace vector populations offer new ways to target malaria vectors with rapidly changing biting behavior. It is hoped that a more diverse toolkit will facilitate increased flexibility and integration of vector control management, as well as the adoption of more responsible and sustainable use of insecticide vector control tools as in the past.
(Finish)
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