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Which control methods work best against Aedes?

We have now looked at and heard about a variety of traditional vector control methods. Aedes mosquitoes will bloodfeed outside and during the day, meaning adults are hard to target using conventional tools like insecticide treated nets, or indoor residual spraying.

Another stage of the Aedes lifecycle is easier to target: the larvae. Aedes larval habitats are easy to identify and limited by the relatively short flying distance of adults (approximately 200m). Since Aedes aegypti are container breeders small larval habitats can be removed completely or larger sites can be treated with an appropriate larvicide.

In this article, we present an update on vector control tools from the WHO Vector Control Advisory Group (VCAG), and summarise the findings of a recent meta-analysis: Is Dengue Vector Control Deficient in Effectiveness or Evidence?: Systematic Review and Meta-analysis. Bowman, L.R., et al. PLoS Negl Trop Dis. 2016 Mar; 10(3): e0004551.

Update from VCAG

In March 2016, VCAG held an extraordinary meeting in Geneva to review five existing and potential vector control tools for use in the context of the response to the Zika virus outbreak. The tools they considered included:

  1. Mosquito control of human pathogens using Wolbachia that reduce the mosquitoes’ ability to transmit viruses;
  2. Mosquito control through genetic manipulation, using lethal genes that prevent larvae from reaching adulthood;
  3. Sterile insect technique, in which sterile males are released and produce inviable eggs when they mate with wild females;
  4. Vector traps, which attract and kill egg-laying female mosquitoes;
  5. Attractive toxic sugar baits, where mosquito attractants are combined with oral toxins that kill the target insects.

The main conclusions and recommendations of the meeting are outlined below:

  • First, well implemented vector control programmes using existing tools and strategies are effective in reducing the transmission of Aedes-borne diseases, including Zika virus. These tools should be promoted and used to control the Zika virus. They include: targeted residual spraying; space spraying; larval control; and personal protection measures.
  • Second, full-scale programmatic deployment is not currently recommended for any of the five new potential tools reviewed by VCAG. However, the VCAG recommended the carefully planned pilot deployment under operational conditions of two tools: Wolbachia-based biocontrol and transgenic mosquitoes, accompanied by rigorous independent monitoring and evaluation.
  • Finally, the VCAG concluded that more evidence is required before consideration of the pilot deployment of the three additional tools reviewed, namely the sterile insect technique, vector traps, and attractive toxic sugar baits.

Which methods are most commonly used?

The Bowman study1 evaluated the evidence for effectiveness of vector control strategies currently employed against Aedes aegypti and Aedes albopictus in the context of dengue. Since the known vectors of dengue and Zika are the same we can draw parallels between the two diseases. The study noted that clean-up campaigns, including environmental and around human habitations, were the most commonly practiced method for reducing local Aedes populations. It was found that education and outdoor fogging were the next most commonly practiced methods (Figure 1). This was followed by environmental management, where functional sites such as water containers are covered or treated with larvicide.

Fig 1. Summary of vector control tools and approaches for dengue (Aedes control). Histogram of frequency of interventions reported by 41 studies, stratified by study.1 CC BY 4.0.

Which methods are most effective?

As noted by Bowen, clean-up campaigns are often one element of a broader campaign that might also include fogging, larviciding and other methods, so it is not possible to determine precisely what impact this intervention has. Although the WHO promotes community participation in the prevention of insect-transmitted diseases, direct evidence that it can have an impact is lacking.2

It is important to understand that the frequency of a particular control method may not correspond to its effectiveness. In many instances in prevalent mosquito control literature, it is stated that outdoor fogging can be employed as a tool to convince the public that mosquito control is a top priority for local and regional governments. Unfortunately no randomised controlled trials (RCTs) have been undertaken in the last 35 years to evaluate the effectiveness of space spraying or fogging to reduce dengue transmission or dengue incidence,1 so as with the clean-up campaigns is difficult to determine how effective such control methods have been.

Encouragingly, there is more evidence to support the effectiveness of house screening. Ae. aegypti predominantly rest indoors, so screens that act as a barrier to access are expected to impact this species. Studies in both Australia and Taiwan have shown a significant protective effect through these measures when used on doors and windows.3,4 By contrast, indoor residual spraying, in which insecticides are sprayed onto the walls of houses, has rarely been used against Ae. aegypti, and the study revealed that there is no statistically significant effect of this control measure.

The use of bed nets and repellents is generally more difficult to monitor as in many areas it is down to the individual to buy and use them. Bowman found no evidence of any impact on dengue infection risk by insecticide-treated nets, mosquito traps, or mosquito repellents.

The study identifies shortfalls in the lack of real evidence concerning the effectiveness of Aedes control programs. Currently many control programs mimic those that have been used for the past half century. While these methods may have been effective at the time, the distribution of mosquitoes that vector diseases has changed, and there is growing insecticide resistance in many mosquito populations. The resurgence of diseases such as dengue, chikungunya and aptly, Zika, are challenging the security that these championed control methods once provided. The authors concluded that at present we do not have a clear understanding of which interventions actually work, and the reasons why they succeed or fail. It appears that for most vector control interventions there is a serious lack of evidence and this stems mainly from the lack of well-designed RCTs with entomological and epidemiological outcomes. This is something that must be addressed urgently to allow these interventions to be assessed properly which will then lead to more accurate recommendations.

What next?

Future species-specific control programs for Ae. aegypti and Ae albopictus will need to incorporate the ecology of the target species, whilst engaging transparently with the community so that they can contribute to mosquito control as individuals. Trials of control programs should record not just the impact on mosquito populations, but also the effectiveness of the intervention on disease transmission. Furthermore, trials should be designed appropriately as RCTs to provide the most robust evidence for evaluating the effectiveness of the intervention. This will ultimately improve the evidence base for selecting one method over another or indeed several. RCTs are logistically difficult and expensive and therefore pose a great challenge to vector control researchers, but with the correct investment and study design, these types of trials are possible.

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Preventing the Zika Virus: Understanding and Controlling the Aedes Mosquito

London School of Hygiene & Tropical Medicine

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