Don’t throw your DEET Away!

deetfailThere’s been a lot of reporting about new research about the insect repellent DEET this week. Unfortunately, some of the media didn’t quite get it right.  Headlines like this one were common…and completely wrong.

The media coverage left a lot of people confused about DEET, and if it still worked. The results of the research were a lot more nuanced than “DEET suddenly stopped working so we are all screwed.”

Here is what the average person being bitten by mosquitoes needs to know, condensed:
DEET still works fine. It’s still one of the best insect repellents out there.  We know a way it might become less effective now, as demonstrated in the laboratory.

The un-condensed version:

DEET is one of our oldest and best insect repellents.  It’s universally acknowledged as the best repellent around, and has broad activity against several types of biting flies and ticks. This is why a problem with DEET is big news–it’s invaluable in preventing transmission of several different diseases.

Amazingly, scientists are just beginning to understand how DEET works, even though it’s been in widespread use for 50 years.  We know it stops ticks and mosquitoes from biting, but the exact mechanism of how that happens is still not clear.  Does it make us ‘invisible‘ by blocking mosquitoes from smelling? Does it smell horrible to biters? It’s still not settled science yet.

Martha Stuart says that's not a good thing

That’s important to know, since if we know how something works, we can copy it and try to make new and better controls.  There is always a concern with evolution of resistance in insects–they are commonly used to study genetics and mutations for a reason.   Insects breed fast, and they breed often–which means that small genetic changes, if they are helpful at keeping a bug alive and having sex, can spread quickly through a population.

Resistance to DEET, our most powerful and broad spectrum insect repellent, would be a very bad thing.  And so it makes sense that entomologists interested in human health would be studying how DEET works.

Evidence of genetic resistance to DEET in mosquitoes has actually been around since 1994.  In 2010, researchers found that they could increase the frequency of a gene that made mosquitoes ignore DEET to 50% in a couple of generations. That’s alarming, but that was in a laboratory-bred colony.

‘Laboratory-Bred’ is an important distinction for both that study and the recent one.  Mosquitoes in a cage have only one source of food (often the hapless graduate student that is rearing them).  They can’t fly off and look for other people or animals to bite.   It also means that their sexual choices are limited to other mozzies in the cage, so resistance can evolve more quickly that it would out in the wild where they have a wider choice of hookups.

Scientists use work in the lab to model the real world.  It helps us understand how organisms grow, change, and respond to their environment.  That  doesn’t mean that it’s a firm prediction of what will happen out in the larger world, especially with a group as diverse and wily as mosquitoes. That’s why I think headlines like the one at the top are irresponsible, and mangling the message of the research.

You can see an interview with one of the researchers here;  note she is careful to repeat that we should not discard DEET wholesale on the results of this research!

“What this work indicates is that there may potentially at some point in the future be some problems  with the repellents that we have, that we need to be aware of in advance.   Possibly we can use this information to  alter the repellent DEET to make it more effective, it may also help us in finding new repellents because we will know if [mosquitoes] are able to overcome certain things……Even though repellents are working fantastically at the moment, what this tells us is maybe how to prevent problems cropping up, and how to alter things for the future to make them more effective.” [emphasis mine]

Additional Resources:

Articles referenced in this post: 

  • Stanczyk N.M., Brookfield J.F.Y., Field L.M., Logan J.G. & Vontas J. (2013). Aedes aegypti Mosquitoes Exhibit Decreased Repellency by DEET following Previous Exposure, PLoS ONE, 8 (2) e54438. DOI:
  • Ditzen M., Pellegrino M. & Vosshall L.B. (2008). Insect Odorant Receptors Are Molecular Targets of the Insect Repellent DEET, Science, 319 (5871) 1838-1842. DOI:
  • Jaramillo Ramirez G.I., Logan J.G., Loza-Reyes E., Stashenko E., Moores G.D. & Vontas J. (2012). Repellents Inhibit P450 Enzymes in Stegomyia (Aedes) aegypti, PLoS ONE, 7 (11) e48698. DOI:
  • Rutledge L.C., Gupta R.K., Piper G.N. & Lowe C.A. Studies on the inheritance of repellent tolerances in Aedes aegypti., Journal of the American Mosquito Control Association, PMID:
  • Stanczyk N.M., Brookfield J.F.Y., Ignell R., Logan J.G. & Field L.M. (2010). Behavioral insensitivity to DEET in Aedes aegypti is a genetically determined trait residing in changes in sensillum function, Proceedings of the National Academy of Sciences, 107 (19) 8575-8580. DOI:


World Malaria Day: April 25, 2012

world malaria day logoOnce again, another year has gone by and it’s World Malaria Day.  There has actually been a lot of good news in the last few years; overall, deaths and infection have decreased.  But.

From the Roll Back Malaria Coalition:

The theme for World Malaria Day 2012 – “Sustain Gains, Save Lives: Invest in Malaria” – marks a decisive juncture in the history of malaria control. Whether the malaria map will keep shrinking, as it has in the past decade, or be reclaimed by the malaria parasites, depends, to a great extent, on the resources that will be invested in control efforts over the next years.

Investments in malaria control have created unprecedented momentum and yielded remarkable returns in the past years. In Africa, malaria deaths have been cut by one third within the last decade; outside of Africa, 35 out of the 53 countries, affected by malaria, have reduced cases by 50% in the same time period. In countries where access to malaria control interventions has improved most significantly, overall child mortality rates have fallen by approximately 20%.

However, these gains are fragile and will be reversed unless malaria continues to be a priority for global, regional and national decision-makers and donors. Despite the current economic climate, development aid needs to continue flowing to national malaria control programs to ensure widespread population access to life-saving and cost-effective interventions.

I have written in past years about some of the really wonderful progress that has been made.  Unfortunately, we have controlled all the easy places. Now, as the coalition says in their statement, the gains are fragile.

You might have seen the news a few weeks ago that a drug resistant strain of malaria has arisen in Asia. If malaria becomes resistant to artemisinin, there are no other drugs to treat with. Much of current malaria control relies on a combined 1-2 punch of bed nets and drug treatment.  When populations are displaced due to political unrest, or when economies tank and programs are discontinued, those at risk of malaria lose access to medicine and regular housing. Which puts them even more at risk.

The warning note from the RBM Coalition statement I quoted above is repeated in a new paper that came out this week:

Cohen, J., Smith, D., Cotter, C., Ward, A., Yamey, G., Sabot, O., & Moonen, B. (2012). Malaria resurgence: a systematic review and assessment of its causes Malaria Journal, 11 (1) DOI: 10.1186/1475-2875-11-122  

The article is a major review of control efforts on multiple continents over the last 80 years. They find that the greatest issue in controlling malaria is economic, not biological:

“Considerable declines in malaria have accompanied increased funding for control since the year 2000, but historical failures to maintain gains against the disease underscore the fragility of these successes. Although malaria transmission can be suppressed by effective control measures, in the absence of active intervention malaria will return to an intrinsic equilibrium determined by factors related to ecology, efficiency of mosquito vectors, and socioeconomic characteristics….

The review identified 75 resurgence events in 61 countries, occurring from the 1930s through the 2000s. Almost all resurgence events (68/75 = 91%) were attributed at least in part to the weakening of malaria control programmes for a variety of reasons, of which resource constraints were the most common (39/68 = 57%). Over half of the events (44/75 = 59%) were attributed in part to increases in the potential for malaria transmission, while only 24/75 (32%) were attributed to vector or drug resistance. ”  [emphasis mine]

malaria figure: warNearly all of the 75 resurgence events identified through this review have been ascribed to some aspect of weakening of the malaria control programme, whether because of funding shortages, complacency following successful reductions, or disruptions caused by war or natural disaster. These results suggest that technical problems such as vector resistance appear historically to have been of secondary importance for resurgence to financial and operational factors.”

This research is important, because we need to learn from our failures of the past, not repeat them.  The places that are most at risk of malaria are also places where there is political unrest and little budget to support a malaria control program.

This is why “Sustain Gains” is the theme of World Malaria Day this year. We have made amazing progress-but we have in the past, too. Only by sustained effort–funded by everyone–can we continue to progress.  Nothing but Nets is trying to supply bed nets to the Sudan–if you can, consider donating!


As an addendum: every time I write about malaria, some pro-DDT trolls show up. From the paper:

One of DDT’s chief advantages is its low cost [59], and programmes that could no longer use it due to resistance were required to switch to more expensive insecticides, raising the cost of interventions and making them harder to sustain [75]. If, however, resistance to multiple pesticides was the primary driver of resurgence, it would have been extremely difficult to counteract, since vector control, one of the most effective tools available to malaria control programmes, would have proven useless. Instead, however, regions that made a determined effort were able to continue to make gains against malaria despite the obstacle of resistance.