“This information was distributed to global media today. We are sharing it with you now because we hope, regardless of your predisposition, that it will add to the discussion on the risks, benefits and role of resistance in using DDT for malaria control and other public health programs.”
First, I’m extremely flattered to be contacted! Maybe my attempts to try to balance all the lies JunkScience.com is spreading are working!
(I’m not really that naive–it’s more likely that there’s no such thing as bad publicity.)
Second, I’ve never actually seen a scientific paper be used by a lobbying organization in this way. Have any of you ever seen this before? Odd.
[Edited 8/18 to add: Ugh. The press release has been picked up and run almost verbatum by Reuters and the Wall Street Journal. ]
Third, if this is all AFM has got for evidence, they are in big trouble. They will probably (unfortunately) still be successful at marketing their message.
Media likes to frame issues as a debate: if you can get two talking heads to argue, that’s great TV. The problem is, presenting both sides of an argument is silly when there is no actual lack of consensus.
Dissent is manufactured by using facts out of context, and finding a scientist that opposes the prevailing view. That lone scientist’s opinions are then given equal weight to the majority of scientists who don’t think using DDT is a good idea, except as a last resort.
So…This paper. I’m going to go through this in detail, since the pro-DDT trolls like to nit pick. Sorry for the length.
FYI, The senior author, Roberts, is on the board of Directors for AFM, and also frequently opines on the DDT topic in the LaRouche magazine 21st Century Science.
It’s a perfectly good paper in terms of its data, and the reporting of it. Some of it, though, is quite…odd. The first thing that made my antennae twitch was the inclusion of dieldrin. It’s a dangerous compound. There’s a very good reason it’s banned in the US, and on the “informed consent” list internationally.
From the paper:
Data will be presented on these chemicals alone. Based on laboratory tests, one of the three (dieldrin) was toxic but had no repellent or irritant actions. Another (alphacypermethrin) had irritant and toxic actions; but had no repellent action. The third chemical (DDT) exhibited all three actions; repellency, irritancy, and toxicity.
Ok, that explains the inclusion of dieldrin. What the authors are doing here is proper science. They are setting up a test so that they will get clear, measurable results about repellents and toxicity.
As a promotional tool, though, this is a stalking horse. The test is deliberately set up to make DDT look as good as possible. Of course DDT repellency looks good in this paper–the other compounds aren’t supposed to be repellent!
As a tool for AFM to try to argue a policy, it’s like… ok, I just can’t come up with an analogy as silly as this.
Next, some of the results actually don’t make DDT look good–in Table 3, for example, cypermethrin produced much, much better mortality results–100% mortality in some cases. Permethrin and it’s relatives are, in fact, commonly used for insecticide treated nets (ITNs).
In terms of the contact irritant properties of DDT, cypermethrin again outperforms DDT, and does it at lower concentrations.
“A significant (P<0.05) contact irritancy response to alphacypermethrin was observed at treatment concentrations of 0.25 nmoles/cm2 and higher… DDT produced significant contact irritancy responses at concentrations of 2.5 nmoles/cm2 and higher…Of the three compounds, only alphacypermethrin gave consistent high levels (72–98% range) of knockdown at all treatment concentrations after a one hour exposure. ” (emphasis mine)
In the lab tests, the score is permethrin 3, DDT 1 (they did get more directional movement in one of the laboratory tests for DDT.) I’ll be fair and point out they were working with DDT resistant mosquitoes, so the difference in concentration is to be expected.
On to the field tests: Here DDT did slightly better than cypermethrin. However, the data isn’t reported in a way that makes it easy to compare between treatments. This is something anyone who’s done field research can sympathize with. It’s really hard to do labor intensive experiments like this on successive days and have uniform conditions. This is just the nature of the field research beast.
From the methods, it seems they tested each compound in the two huts once, on one day, and paired it with a control. The unit of replication is each individual mosquito. This is common in field research, and it’s known as pseudoreplication. This means statistics can still be calculated, but they only apply to the difference between each individual treatment and its control.
There are clear differences between the individual treatments, but I have no way of knowing if that’s really due to the environmental conditions (“high between day variance“) or the treatment. DDT reduced risk from mosquitoes to 73%; alphacypermethrin gave 61% protection. It’s a difference, but again, there is no way to say if it’s a significant difference.
The part of the paper that I think is actually useful is a proposal to quantify the actions of insecticides in several different ways: toxicity, repellency, and contact irritant, and to look at all 3 in evaluating performance. That’s a good idea.
Nothing in this paper argues that DDT is a superior performer against malaria mosquitoes. All it says is that when compared with two pesticides deliberately chosen not to be repellent, it performed comparably to them, despite the presence of DDT resistance. This is interesting, but not the huge news they are making it out to be.
In fact, the 12% difference between the compounds that they observed might or might not actually be significantly different–we can’t say for sure.
Additionally, this study only looked at ONE mosquito species. As I’ve mentioned before, many different species are involved in malaria transmission, and they don’t all react the same way to DDT. In some cases, the repellent effects of DDT changed transmission patterns, so that outdoor transmission replaced indoor biting. This effectively guts the effectiveness of both DDT sprays and bed nets.
Choosing dieldrin let Roberts say in both his promo piece for AFM and his publication that it would cause rapid evolution of resistance to the pesticide, and to talk about insecticide resistance as a potential problem. The fact that they were working with mosquitoes already resistant to DDT is glossed over in the press release. Dieldrin is a very rarely used pesticide, and it’s presence in this study is primarily to make DDT look good. It’s a Straw man.
The authors discuss pesticide resistance briefly (“Existing criteria for dealing with insecticide resistance have resulted in countries abandoning DDT when vectors became resistant to the insecticide’s toxic actions”) but don’t discuss the important issue of cross resistance via kdr between mosquitoes resistant to DDT and permethrins. (More about that topic here.)
The reason it’s an important topic is that permethrins are commonly used to treat insect bed nets. You really don’t want to short circuit that, especially since an awful lot of research finds they are extremely helpful in stopping transmission.
Here’s what I really don’t understand:
Why, if insects are already resistant to DDT, and you have other compounds that perform as well or better with less risk of resistance and toxic effects for both people and environment, is anyone so determined to justify keeping DDT in use??
It just doesn’t make sense. The cost/benefit analysis comes into play, but I really am not convinced that the difference is that prohibitive.
I’m going to repeat myself:
DDT is NOT a cure-all solution for malaria. It has to be used–if it is used–carefully, with planning, evaluation, and forethought. It’s easy to understand why some folks want DDT to be a panacea–Malaria is a horrible disease, and children suffer the most. But jumping in and spraying DDT can have the potential to make things worse, not better, in the long run.
Each situation has to be evaluated individually before insecticide choices are made. And insecticides are not the only piece of the malaria puzzle. A 2005 review found that simple environmental interventions–under the control of local people–could reduce malarial transmission up to 80%. I’ve already mentioned Bed nets as another strategy.
An integrated strategy will work much better than ideology.