It may seem counter-intuitive at first but letting mosquitoes grow up and breed may be part of the solution to tackling the devastating impact of malaria. A team of researchers led by Dr Stephen Gourley of the University of Surrey's Mathematics Department have used mathematical modelling to examine why conventional insecticides used against the insects that transmit the disease responsible for millions of deaths a year, can quickly become ineffective in areas of intensive use. Their answers may lead to unprecedented advances in malaria control.
Mosquitoes can become resistant to commonly used insecticides surprisingly rapidly and spraying them while young simply imposes intense selection pressure favouring resistant insects.
Biologists have suggested that a new kind of late-acting insecticide could slow this process and lead to insecticides which would remain effective over a much longer period of time. The strategy aims to exploit the fact that mosquitoes only become able to infect humans with malaria late in their lifetime, due to a relatively long latency stage. This means that the delayed action insecticide doesn't result in increased infection rates while the younger insects remain alive.
It could, however, mean a rise in the number of troublesome, but non-malarial, mosquitoes.
Dr Gourley said: "There is a trade-off between effective prevention of malaria transmission by mosquitoes and having to live with mosquito bites involving no malarial transmission."
In 2008 malaria caused almost one million deaths and remains one of the leading causes of child mortality in Africa. More of the insects in general, but fewer with the deadly disease, may be a price worth paying.
Conventional insecticides, such as DDT, kill mosquitoes as soon as they are exposed to the chemical. But while this approach works well in the short term, its indiscriminate action speeds up the evolution of the insect towards insecticide resistance.
In particular, because the insecticide acts on female mosquitoes before they lay eggs, this causes intense selection pressure towards insecticide-resistant females who then pass on this resistance to their offspring.
Dr Gourley's team used mathematical models to predict the effect of an insecticide that only acts after a time delay, once the mosquitoes have laid their eggs. Because this results in a much lessened selection pressure on resistant mosquitoes, the team discovered that resistance evolves much more slowly with this type of insecticide.
The technique could result in vastly improved malaria control in areas where resistance to current insecticides is rife amongst mosquitoes.
- S. A. Gourley, R. Liu, J. Wu. Slowing the evolution of insecticide resistance in mosquitoes: a mathematical model. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2011; DOI: 10.1098/rspa.2010.0413
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