A charge that sticks

THE widespread use in recent years of nets, insecticides and new drugs has helped to bring malaria under a measure of control—but evolution is constantly pushing back by generating resistant strains of both the parasite that causes the disease and the mosquito that spreads it. Even resistant mosquitoes, however, can take only so much chemical abuse, and Marit Farenhorst, a researcher at In2Care, a Dutch mosquito-control firm, and her colleagues think they have devised a way to dish out more of it.

Their method, as they report in the Proceedings of the National Academy of Sciences, is a version of the party trick of making a balloon stick to a wall by imbuing it with static electricity. Substituting mosquito nets and insecticide particles for walls and balloons, Dr Farenhorst believes, yields a way of delivering more, and more diverse, insecticides, and really making them stick where they are needed—on the cuticle of the target insect.

Current mosquito nets are woven from fibres impregnated throughout with an insecticide. This permits them to be washed and used for years without loss of potency. But it also means this potency is not as great as it could be, because the insecticide is released only slowly by the fibres. The impregnation, moreover, requires high temperatures, and only one class of insecticide, pyrethroids, can withstand these. In this case, therefore, natural selection has only one type of enemy to evolve around. Using static electricity, by contrast, means all of the insecticide is held on the surface of a net’s fibres. Much larger doses can thus be transferred to an insect which blunders into the net. In addition, a wide range of insecticides—and even, possibly, the spores of a fungus harmless to people but lethal to mosquitoes—can be applied to the fibres.

To make her nets electrically attractive, Dr Farenhorst coats their fibres with a proprietary substance that maintains a positive charge. This induces an equal and opposite charge in particles of powdered insecticide, holding them in place if they are scattered over the net. The positively charged substance, crucially, stays put when a net is washed. And because the particles of insecticide themselves pick up an electrostatic charge, they are more likely to stick to a mosquito that dislodges them—as experiments using fluorescent particles show (see picture).

Further experiments demonstrated that the new nets do, indeed, have the desired effect. When the team tested them on mosquitoes resistant to a pyrethroid insecticide called deltamethrin, between 63% and 100% of the insects died within 24 hours of contact with a deltamethrin-dosed version of their invention. When they repeated the process using conventionally impregnated nets, only 10% died.

The new net does have one unfortunate constraint. Contact with human bodies reduces its potency, because the static-enhanced insecticide sticks to human skin as well as insect cuticle. It cannot, therefore, be used for making bed nets.

Beds, though, are not the only places where nets are useful. Window and door screens also play an important part in keeping mosquitoes at bay. Nor is there any reason why the coating could not be sprayed onto walls, making the deployment of insecticide on these favoured resting sites of mosquitoes more effective.

One inevitable side-effect of the new net’s efficacy is that it needs regular resupplies of insecticide. But that is not a problem. In the field the researchers simply sealed a net and some powdered insecticide in a bucket and shook it up for 15 seconds. This was enough to recoat the net.

The upshot, then, is another useful weapon in the war on malaria. It will not win that war by itself. But it might bring victory closer.