DDT as a repellent

I got an email today from Phillip Coticelli at Africa Fighting Malaria pointing to a study by Donald Roberts (PDF), showing that DDT has a repellent effect in addition to its toxicity. The key finding is that that three out of five DDT-resistant Aedes aegypti mosquitoes avoid huts sprayed with DDT. Roberts argues that this is a reason for preferring DDT to alternative pesticides such as dieldrin. A few points about this are worth making
* First, it’s good to see AFM acknowledging the fact of pesticide resistance, which primarily accounts for the abandonment of large-scale attempts to eradicate malaria-carrying mosquitoes with pesticides. The libel put out by people like Steven Milloy and AFM founder Roger Bate[1], in which it is suggested that the failure of the eradication program was due to a mythical ban on DDT imposed at the behest of environmentalists, who callously caused millions of deaths, depends critically on ignoring resistance.
* Second, although the study is new, the claim is not. Roberts has been arguing the importance of repellent and irritant effects for a long time. And while the reporting of this study suggests that these benefits are unique to DDT, other work by Roberts has found that permethrin and deltamethrin are just as effective in this respect.

How does this relate to the general debate over the use of hut spraying as a strategy to fight malaria?

First, it’s important to observe that, while DDT was never banned, it fell out of favor in the from the 1970s onwards as a result of the failure of the WHO’s attempt to eradicate malaria through spraying and of the adverse consequences of widespread use as an all-purpose pesticide, mainly in agriculture. The concerns which led to the abandonment of large-scale spraying weren’t as directly relevant to the alternative of indoor spraying, but past failures and the generally bad reputation of DDT didn’t help those advocating this strategy, notably including Roberts. In addition, the economic crises of this period made it difficult to organise the systematic programs required. International aid donors and organizations concerned with malaria focused mainly on the use of bednets impregnated with insecticides, and on the development of improved treatments.

If indoor spraying with DDT had an undeservedly bad reputation in the late 20th century, the reverse is the case now. Thanks to the campaigns of AFM and others, it is taken for granted in conservative circles that DDT is a panacea, banned by evil environmentalists. As a result, whereas donors were once chary of funding DDT-based approaches, now poor countries are being pushed to use DDT whether or not it is appropriate.

It is, in some sense inevitable, that aid will come with strings attached, and that these strings will be pulled by domestic political forces in the donor countries. If the result is more money to fight malaria, it’s better to accept the associated constraints and take the money. Still, it would be good if we could move to policy driven by evidence, rather than by rich-country politics.

fn1. Acting on behalf of the tobacco industry which was fighting WHO attempts to restrict the spread of smoking, and needed to hit them on another front.