DDT ceased being the "go to" tool in the malaria fighter's tool box more than 50 years ago when mosquito populations started developing resistance and when better, safer tools began to come online. It's still available today, but the chemical's usefulness is extremely limited. Now, new research shows that in some circumstances spraying DDT is not only ineffective, but it may actually increase malaria transmission.
It has to do with repellency and community-level effects of insecticides.
We all think of DDT as something that kills mosquitoes (and lots of other things), but it turns out that it also has repellent properties, much like the DEET that's found in bug sprays. In fact, studies have shown that some mosquitoes that are resistant to DDT's toxic effects are still repelled by it. Africa Fighting Malaria, the most prominent DDT proponent, has argued that this repellency means that spraying homes with DDT can provide protection from malaria even in areas where mosquitoes have developed resistance. The argument seems reasonable: if you can keep a mosquito out of your home, then whether you've killed it or not, you've still kept it from biting you.
But that's only half the story. The problem is that you've just made it more likely that that mosquito will bite your neighbor instead, and you've also decreased the chances that it'll pick up a lethal dose of insecticide from your walls or your treated bed net. This "behavioral avoidance" of DDT has been cited as one of the reasons for the declining effectiveness of African anti-malaria campaigns based on DDT.
Now, a new paper published in the Journal of the Royal Society Interface models this behavior, and looks at the interaction between the two most popular anti-malaria interventions: indoor residual spraying of insecticides (IRS) and distributing insecticide-treated bednets. Importantly, the paper takes into account the on-the-ground realities of vector control campaigns, like the fact that IRS programs can never spray 100% of homes in a targeted area. Likewise, even when distributed for free, there will always be people who don't use those bednets. This is one of the reasons why bednets and IRS are often used together in the same area.
As the authors note, "The two tools are assumed to have synergistic benefits in reducing malaria transmission because they both act at multiple stages of the transmission cycle." But they predict that rather than synergism, what you actually get is antagonism, at least when DDT is the chemical used for spraying, and spraying and bednets are used in many of the same homes. In short, if you've already achieved good coverage with insecticide treated bednets in an area, going in and spraying with DDT (or pyrethroids, the other insecticide they looked at) isn't going to improve the situation and could very well make it worse.
This is because of "deflected bites." With treated bednets only, the people who have them are protected, but the people who don't have them are also protected — albeit to lesser extent — because in addition to providing a physical barrier between people and mosquitoes, the nets also kill mosquitoes, meaning there are less of them to transmit the disease. Add a repellent to the mix, and now some of those mosquitoes that might have been killed are instead repelled, and end up biting someone else — the deflected bite.
Pyrethroids, which unlike DDT do not repel mosquitoes, don't have this antagonistic effect. But the authors predict that if there's already 80% coverage with insecticidal nets, then spraying pyrethroids isn't going to improve the situation much either.
The authors do identify one situation where DDT does improve upon bednets, and that's when it's used to fill in the gaps in coverage. For example, if 80% of homes use bednets and the other 20% of homes — and only those homes — are sprayed with DDT, then a dramatic synergism is predicted with malaria transmission plummeting to zero. But as the authors point out, this is an impractical strategy. The most difficult homes to access with bednets are likely to be the hardest to reach with spraying. And many other people will desire both bednets and IRS in their homes. In practice, it is much easier to distribute these tools randomly in an area or to deliver IRS and nets to the same homes, than it is to achieve the bifurcated distribution that results in synergism. So much for "silver bullets."