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The value of insecticide susceptibility

Read Mark Hoppé describe how we can maintain effective vector control by valuing mosquitoes’ susceptibility to insecticides.
© University of Basel
Ministries of Health, Malaria Control Programmes and aid agencies aren’t really buying insecticide-treated bed nets, or doses of insecticide for IRS.

What they are actually investing in is fewer mosquitoes, or a reduced interaction between disease-vectoring mosquitoes and the human population. This, in turn, reduces the burden of malaria in the targeted region, which is the ultimate goal of these agencies.

The value of insecticides

Insecticides can have great value as tools for the reduction of malaria; in the past, they may even have been thought of as ‘silver bullets’. However, the insecticidal interventions only have value if they actually control the mosquito vectors of malaria. An insecticide that has no impact on the mosquitoes, has no value for malaria control. One that has lost some of its efficacy, due to resistance development in the target mosquito population, will have also lost much of its ‘value’.

The value of susceptible mosquitoes

The value of an insecticide for malaria control is therefore directly linked to the susceptibility of the target mosquito population. In a strange way, susceptible mosquitoes are very valuable to those involved with malaria prevention.

It is often said that you don’t know the true value of something until it’s gone. This is also true in the case of insecticide-susceptible mosquitoes. But mosquitoes don’t have any value, right? I mean, we put so much effort and investment into getting rid of them, and we would all be so much happier if they were gone? Every year millions of dollars are spent on insecticide-based malaria control interventions, and as a result, every year millions of clinical cases of malaria are prevented, and many lives are not lost.

This is a great investment, not just with an economic return measured by gross domestic product (GDP), but in people’s lives. If mosquitoes susceptible to the available insecticides are replaced by insecticide-resistant ones, or even just less susceptible ones, then the ‘value’ in this investment is reduced. Something valuable has been lost; the susceptible mosquitoes. Probably hardly anybody thought about the value of the mosquitoes that could be controlled, until it was too late.

Valuing susceptibility

If we can undertake vector control in ways that minimise the selection pressure for insecticide resistance development in the target mosquito population, we can prolong the useful life of the interventions we have. This is a bit like servicing a car. If you never service a car, change the oil, replace worn parts, etc., then eventually it will become unreliable, break down, and become useless.

However, regular servicing is likely to keep it working well for many years. The time and money spent servicing the car is an investment in its long term ‘usefulness’. There is a problem with this analogy, however: if your car breaks down and can’t be repaired, and you have the money, you can buy another one, and there will be many to choose from. If an insecticide stops being useful due to insecticide resistance, there may not be an alternative one available to use, even if you have the money.

We should then perhaps be focusing more on Insecticide Susceptibility Maintenance, rather than Insecticide Resistance Management. This sounds like a bad case of semantics, but it might help us to focus on a more proactive and inclusive approach to the stewardship of these valuable tools, and the maintenance of ‘valuable’ susceptible mosquitoes.

There is only one thing better than a susceptible mosquito, and that’s not having any mosquitoes. However, whilst the mosquito vectors of malaria continue to turn up, we need to hope that they are mosquitoes that people can be protected from.

IRM as an investment

IRM is a stewardship responsibility of the commercial companies that develop and sell vector control insecticides, but it is also a stewardship duty on those who fund, design, regulate and implement vector control programmes. We will find out more about IRM in action in later steps.

But what can be done in our fictional village to support this? Mosquito breeding sites could be removed from around the village, such as long-term standing water in agricultural and gardening settings. The non-target exposure of the mosquito larvae to pollution and the agricultural use of pesticides should be minimised. Storing, using and washing insecticide-treated bed nets following the manufacturer’s recommendations will reduce the time mosquitoes may be exposed to a sub-lethal selecting dose of the insecticide.

These, and other, ‘investments’ in malaria control and insecticide susceptibility maintenance will have a short-term return by reducing mosquito numbers. They will also have a less obvious longer-term return, increasing the likelihood that future generations of mosquito can also be controlled.

Activities that both reduce mosquito numbers, and reduce the selection pressure for resistance development, will be an investment in the short- and long-term fight against malaria, and ultimately its eradication. That really is something of value.

Author: Mark Hoppé

Further Reading

World Health Organization. Technical consultation on the use of economics in insecticide resistance management for malaria vector control. Report of a virtual meeting, 14-16 September 2021.

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

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