Boosting bed nets

20 September 2016

Interview with

Professor Mark Rowland, The London School of Hygiene and Tropical Medicine

Insecticide-treated bednets are the most important tool in the fight against Mosquito Netmalaria, with chemicals called pyrethroids being the main pesticide used to treat them. But, frustratingly, many mosquito populations have now developed resistance to pyrethroids, so scientists are on the lookout for alternatives. Kat Arney went to see Mark Rowland, from the London School of Hygiene and Tropical Medicine, who's discovered that combining pyrethroids with a new chemical called pyriproxifen could be the solution to this problem.

Mark - we're looking at alternative insecticides to put on their nets. In fact, we want to take new insecticides and combine them with the old insecticides so we have a double strategy for controlling the mosquitos.

Kat - Kind of a double whammy?

Mark - A double whammy - yes that's the expression I was going to use. Because not all mosquitoes are resistant to pyrethroids but those that survive need to be dealt with in some way. So we're combining the standard insecticide with a new insecticide which, actually, doesn't kill them, it sterilizes them. It stops them from producing eggs that are fertile so the female, even if she succeeds in taking a blood meal, is unable to produce eggs from that blood meal and, therefore, there will be no second generation of mosquitoes.

Kat - You've got the insecticides on the nets that kills them dead if they're sensitive to it and then you've got the insecticide that sterilizes them anyway - how effective are these nets when you test them?

Mark - The way we're testing them is under household conditions in the field. We have these so-called experimental huts where we have volunteers sleeping under the nets. These huts are in in West Africa, in rice irrigation zones where there's lots of mosquito breeding and these mosquitos will enter these huts, as they would a normal house, and will attempt to feed on the people under the nets.

Kat - Oh God! Poor them!

Mark - There's a trapping system which allows the mosquitoes to come into the huts but we capture them on the way out so we can see if they succeed in feeding, and we can test them to see whether they're capable of laying eggs. And what we find is that after contacting this net with this new chemical on, the mosquitoes that survive the pyrethroid because they're highly resistant to pyrethroid, they're actually sterilised by the new chemical and, therefore, they will not produce a new generation of mosquitoes. So, over the course of time, the mosquito population in the area will decline and with no mosquitoes there's no transmission of malaria.

Kat - Many people are concerned about insecticides in the environment - how do we know that these new nets will be safe?

Mark - This insecticide is very safe; we're using very, very small quantities on the net. It's very potent but it's toxicity to mammals and humans is very low. It acts on the developing eggs in the mosquito and the structure of these eggs is very different from a mammalian system so this chemical is toxic to insects, but it's really not toxic to humans at all.

Kat - Is there a chance that mosquitoes could become resistant to this new insecticide as well?

Mark - Never say never when it comes to resistance and evolution. It's possible over the course of time the mosquitoes will develop resistance to this new chemical. Resistance to pyrethroids, the previous chemical that was used on nets, took about ten years to develop and it's been a further ten years for those mosquitoes to be prevalent in sufficient numbers to constitute a problem. So we're looking at, probably, at least ten years before there's significant resistance which undermine the effectiveness of the nets.

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