Following on from our introduction to malaria, we know that malaria is one of the biggest causes of death in low income countries. Malaria is one of the most prevalent vector-borne diseases, with there being 229 million cases globally and 409 000 deaths in 87 malaria endemic countries in 2019 [1]. These statistics have shown a decrease in cases and deaths over the years, however they are still too high, especially for an entirely preventable disease. It is our duty to work together to eradicate malaria. Approximately 1.5 billion cases and 7.6 million deaths from malaria have been averted between the years 2000-2019, showing the profound effect that control and prevention tools have on this disease [1]. How do we create a world free of malaria when there are challenges such as insecticide and drug resistance that are ever increasing in strength?
Mosquitoes, the vector of malaria, are very resilient and adaptable, developing resistance quickly to insecticides used against them. Vector control is the main method of controlling transmission of vector-borne diseases. The aim of these measures is to reduce contact between the vector and humans. Long-Lasting Insecticidal Nets (LLINs) and Indoor Residual Spraying (IRS) are the main vector control methods used today. Both methods rely on an insecticide to kill the mosquito when it lands on either the net or a sprayed wall. The over-reliance on these two methods has led to mosquitoes developing resistance to the insecticides used [2].
The tools we have to combat malaria are not perfect, and are losing their effectiveness. To eliminate and eradicate malaria and other vector-borne diseases we need to improve vector control measures by:
Creating a toolblox of several control methods, which can be used to replace old tools if they lose effectiveness.
Improving and optimising current tools - they have worked before, they can work again.
Developing novel tools.
A selection of current and potential future vector control tools, each discussed below.
Optimizing current control tools
LLIN
A LLIN is a bed net impregnated with insecticide that you can sleep under. They prevent the mosquito biting a human when sleeping and also kill the mosquito when they come into contact with the net. They are the most widely used control tool and currently use only one type of insecticide, the pyrethroids, to which resistance is now widespread. [3]
How can it be improved?
The development of dual insecticide nets : nets impregnated with more than one type of insecticide, to increase the likelihood that the mosquito will be killed by one of the insecticides.
Longevity : Increasing the lifespan of LLINs, which requires investing in higher quality nets and distributing them regularly.
IRS
IRS involves the spraying of indoor surfaces with insecticide, targeting indoor mosquitoes that rest on surfaces after a blood meal [4]. The mosquito absorbs a lethal dose of the insecticide, which disables or kills them, preventing them transmitting the malaria parasite during their next blood meal. This control method is also used against other vectors such as sand flies which transmit leishmaniasis and triatomine bugs that transmit Chagas disease.
An IRS campaign is carried out before the rainy season, when malaria transmission is at its highest, with the aim the insecticide will persist for 4-6 months. It requires organisation, capital and a large skilled workforce. High coverage is key when conducting a spray, with at least 85% of structures in the area to be sprayed, for community protection to be achieved [4].
IRS can use several types of insecticides, making it an important control tool in managing insecticide resistance. The percentage of at risk populations protected from malaria by IRS declined from 5% in 2010 to 2% in 2019, which tells us that more needs to be done [1].
How can it be improved?
Novel Insecticides : discovering and developing new classes of insecticides for use in IRS. Some of these insecticides will hopefully have different modes of action, that the mosquito will not have yet developed resistance to.
Improving understanding of IRS : conducting research into the workings of IRS, and understanding why in some locations it may not be causing a reduction in malaria incidence. Further research into the interaction of the insecticide with the surface type and the environmental conditions, will increase our knowledge of IRS and hence how best to improve it.
Application technologies : Improving the technologies and equipment that are used to apply the insecticide to the surfaces, to achieve an even coverage of the target insecticide dose.
Novel control tools
Attractive Targeted Sugar Bait (ATSB)
ATSBs attract both male and female mosquitoes to a sugar bait that is toxic and will kill mosquitoes. They are cheap and easy to use and can be placed both indoors and outdoors. They are a great tool for members of the community to use themselves. [5]
Gene Drive
A genetic element can be altered within a mosquito’s genome and will have biased inheritance over other elements when offspring are produced. Over time this altered gene will spread within a population across generations. The gene can be altered so that either the ability of the vector (in this case a mosquito) to transmit disease is reduced or the ability of the vector to reproduce is disrupted. [6]
Gene drives could be a great addition to the vector control toolbox, which could transform vector control as we know it. Currently they are only being used by research facilities under strict guidelines. Further research and trials will help decide on whether this technology can be used in communities in malaria-endemic regions. [6]
Microsporidia
Microsporidia are microbes which have been found to naturally occur in some mosquitoes and can be passed on to a mosquito’s offspring. Microsporidia block the transmission of the malaria parasite, Plasmodium, by preventing its life cycle development in the mosquito. Currently this is not used in malaria control but producing mosquitoes infected with microsporidia has potential for the future. [7]
These are only a handful of the vector control tools that can and could potentially be used together to combat malaria. Check out some of the resources mentioned below if you want to learn more!
David Beckham declares malaria eradication!
In partnership with Malaria Must Die, an older David Beckham provides us with hope for a future free of malaria, and reminds us that we have the ability to eradicate human diseases. A particularly powerful message for right now during the COVID-19 pandemic we are currently living through, that we are resilient and have the capacity to overcome even the hardest of times. Check out the video for the campaign below!
Other key steps to create a malaria free world
A range of vector control tools will not be the sole solution to ending malaria. Alongside improving and developing control tools, these key steps must be taken:
Commitment across governments, particularly those of wealthier countries, to pledge to invest in vector control.
Access to antimalarials, distribution of LLINs and IRS campaigns given to all populations including those in hard to reach areas.
Further development and distribution of malaria vaccines.
Surveillance systems in place in malaria endemic countries which will aid collection of accurate data on malaria transmission to help make informed decisions.
Investments to improve health care systems in low income countries. Every person should have access to a diagnosis and treatment for diseases, including malaria.
Investments into research to better understand malaria and its transmission, to enable control tools that are long-lasting to be created, and to enable robust and effective strategies to be carried out.
References
World Health Organisation, (2020). World malaria report 2020: 20 years of global progress and challenges https://www.who.int/publications/i/item/9789240015791
Ranson, H., & Lissenden, N., (2016). Insecticide Resistance in African Anopheles Mosquitoes: A Worsening Situation that Needs Urgent Action to Maintain Malaria Control. Trends in parasitology, 32(3), 187–196. https://doi.org/10.1016/j.pt.2015.11.010
World Health Organisation, (2019). Core Vector Control Methods https://www.who.int/malaria/publications/atoz/9789241550499/en/
World Health Organisation, (2015). An Operational Manual for Indoor Residual Spraying (IRS) for Malaria Transmission Control and Elimination. https://apps.who.int/iris/bitstream/handle/10665/177242/9789241508940_eng.pdf?se quence=1
Maia, M.F., Tenywa, F.C., Nelson, H. et al. (2018). Attractive toxic sugar baits for controlling mosquitoes: a qualitative study in Bagamoyo, Tanzania. Malar J 17, 22 https://doi.org/10.1186/s12936-018-2171-2
Hammond, A. M., & Galizi, R. (2017). Gene drives to fight malaria: current state and future directions. Pathogens and global health, 111(8), 412–423 https://doi.org/10.1080/20477724.2018.1438880
Herren, J.K., Mbaisi, L., Mararo, E. et al. (2020)A microsporidian impairs Plasmodium falciparum transmission in Anopheles arabiensis mosquitoes. Nat Commun 11, 2187 https://doi.org/10.1038/s41467-020-16121-y
Learn more...
History of vector control:
World Health Organisation, (2014). A global brief on vector-borne diseases. https://apps.who.int/iris/handle/10665/111008.
Wilson AL, Courtenay O, Kelly-Hope LA, Scott TW, Takken W, et al. (2020) The importance of vector control for the control and elimination of vector-borne diseases. PLOS Neglected Tropical Diseases 14(1): e0007831. https://doi.org/10.1371/journal.pntd.0007831
Organisations:
Malaria Must Die UK: A charity leading global campaigns to eradicate malaria. https://malariamustdie.com/
IVCC: The world’s only Product Development Partnership working in vector control. https://www.ivcc.com/
For further information about IRS challenges check out a blog I wrote on some research I did with the IVCC on surface type and IRS!
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