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Insecticides as tools for mosquito vector control

Read Mark Hoppé describe what insecticides are and introduce some of the ways in which they can be used to control malaria mosquitoes.
© University of Basel
We’ve seen that insecticides have played a key role in reducing malaria and other vector-borne diseases, even helping to eradicate malaria from some countries.

The use of insecticidal interventions alone has not been responsible for the significant reduction in malaria seen since 2000. Increased access to effective drugs and diagnostic tools has also played an important part.

A large study undertaken in 2015 by Samir Bhatt and colleagues found that between 2000 and 2015, the incidence of clinical malaria cases fell by 40%. They estimated that 663 million clinical cases were averted over the same period. Their analysis found that 68% of cases averted could be attributed to the use of insecticide-treated mosquito bed nets. A further 10% were contributed by the deployment of indoor residual spraying, IRS (Bhatt, et al 2015). The use of insecticide-based interventions clearly had a significant impact on malaria prevalence during this period.

What are insecticides?

What exactly are insecticides, and how are they used in mosquito vector control? Insecticides are chemicals that prevent a mosquito from successfully completing its life cycle.

They do this either by killing the mosquito, preventing it from developing into an adult, or by reducing the number of offspring it can have. They may also interfere with the normal function of a mosquito, such that it is rapidly knocked down or intoxicated, and is therefore unable to bite someone.

Many insecticidal chemicals are found in nature, produced by plants, fungi, etc. For example, pyrethrin found in chrysanthemum daisies, or nicotine found in tobacco. Synthetic insecticides have often been inspired by their counterparts found in nature and have been developed by chemists to optimise their properties for insect pest control.

Broadly, insecticides used for vector control fall into two categories: those that target the adult mosquitoes, and those for control of their larvae.

Adulticides, those targeting the adult mosquito, generally rely on the mosquito landing on or contacting an insecticide-treated surface – either a mosquito bed net or a sprayed structure. Insecticides used on mosquito bed nets need to act relatively fast, to prevent the mosquito finding its way through a damaged net and biting the sleeper.

Long-Lasting Insecticide-Treated Nets, or LLINs, have been developed that remain insecticidal for the duration of their expected life, up to three years. Likewise, special formulations of insecticide have been developed to treat internal walls. These Indoor Residual Sprays (IRS) can provide control of mosquitoes that land on the treated surfaces, for extended periods.

Larvicides are applied to the water where the mosquito larvae live. There, they either kill the larvae directly or prevent them from successfully developing into adults.

Insecticides can also be used in space sprays or fogs to control the mosquitoes that come into contact with them. Traps and lures are being developed that attract mosquitoes to a specific location, where they come into contact with an insecticide; for example, Attractive Targeted Sugar Baits (ATSB), which lure mosquitoes to feed on a sugar solution that also contains an insecticide.

Not all insecticides can be used to control mosquitoes, either because they are not effective against them, they don’t have the right physical properties to be used effectively, or they don’t meet the stringent national and international regulatory requirements for that use pattern.

Evidence

The aim of a vector control programme is ultimately to reduce disease transmission, and so the epidemiological impact of insecticide use is a key metric. National Ministries of Health and vector control programmes therefore need to be confident that the chosen insecticide-based interventions will reduce the burden of malaria.

To support them in this, the World Health Organization’s Global Malaria Programme (WHO GMP) recommends vector control approaches where there is strong evidence that they have an impact on not only the mosquito populations, but also disease transmission. This enables malaria control programmes to make evidence-based decisions on the approach to take in their particular situation.

To provide this confidence, the Vector Control group of the WHO’s Prequalification Team (PQT-VC) undertakes comprehensive assessments of insecticidal products and their manufacturing facilities. Products that are supported by a positive evaluation of their efficacy, safety, and quality, are then added to the publicly available list of prequalified products.

Insecticide-based interventions have had a significant positive impact on malaria control, and it is likely that they will continue to play a key role going forward towards the goal of malaria elimination.

Author: Mark Hoppé

References

Bhatt, S., Weiss, D., Cameron, E. et al. The effect of malaria control on Plasmodium falciparum in Africa between 2000 and 2015. Nature 526, 207–211 (2015).

WHO Global Malaria Programme

WHO Vector Control Product Prequalification

© University of Basel
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The Resistant Mosquito: Staying Ahead of the Game in the Fight against Malaria

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