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Mathematical malaria management

19 January 2011

New research published in Proceedings of the Royal Society A could lead to unprecedented advances in malaria control.  The research uses mathematical analysis to show why conventional insecticides – used to kill the mosquitoes that transmit the deadly disease – tend to quickly lead to widespread resistance, suggesting that a new kind of late-acting insecticide could slow this progress and lead to insecticides which would remain effective over a much longer period of time.

Conventional insecticides, such as DDT, kill mosquitoes as soon as they are exposed to the chemical.  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.  Thus, insecticides can become almost useless within a very short time.

The researchers – led by Dr Stephen Gourley at the University of Surrey – modelled 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.  Furthermore, mosquitoes only become able to infect humans with malaria late in their lifetime which means that the delayed action insecticide doesn’t result in increased infection rates while the younger insects remain alive.  This technique could result in vastly improved malaria control in areas where resistance to current insecticides is rife amongst mosquitoes.

The only downside to this approach is that it focuses purely on disease control and would likely result in a much bigger population of irritating (but not malarial) mosquitoes.  However, given that malaria caused almost one million deaths in 2008 and is currently one of the leading causes of child mortality in Africa*, perhaps this is not such a great price to pay.  As the authors state, “There is...a tradeoff between effective prevention of malaria transmission by mosquitoes, and having to live with mosquito bites involving no malarial transmission.”

*Source: WHO, Malaria factsheet