PHA-Exchange> Re: more on DDT and pyrethroids

[London]

> Message: 2
> Date: Mon, 15 Dec 2003 09:52:17 +0100
> From: sunil.deepak at aifo.it
> Subject: PHA-Exchange> R:  More on DDT use
> To: aviva at netnam.vn, pha-exchange at kabissa.org
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> I had a question, which is different from the DDT debate - what is the
> substance used for insectide treated bednets? I thought it was pyrethroid or
> its derivatives. Does that mean that insectide treated bednets are less
> effective because of this resistance?
>
>

Hi
I understand that both DDT and pyrethroids are subject to problems with insecticide resistance. The resurgence in malaria cases in Southern Africa has been attributed to insect resistance to pyrethroids. My understanding is that pyrethroids are the 'broad-spectrum antibiotics' of the insecticide world, and that their use (or at least some pyrethroids) carries the same kinds of resistance problems as over-use of broad spectrum human antibiotics would incur.
This has been coupled with cross resistance arising because of pyrethroid use for agricultural pest control, which spills over into mosquito resistance.
As for DDT, the classic form of 'resistance' has not been actual mosquito resistance (toxicological) but behavioural - the mosquitos (I think A. funestus) have stopped resting on walls indoors and now do their resting outdoors, far away from the toxic effects of DDT which adheres to the sprayed surfaces indoors. So, they are free to fly in, take a blood meal, and then push off outside again, far away from the control exerted by DDT.
Resistance is a problem, but in many dimensions. It arises from constant mono-use (or overuse or unbalanced use) of biologically active agents.
As for reduced effectiveness of pyrethroid-treated bednets, I am not aware of empirical evidence, but I would imagine that would be likely, although the size of the effect may be the issue (i.e. how much does it reduce effectiveness, and is this significant from a public health point of view, rather than whether resistance affects effectiveness)