It is also important to understand that while mosquitos is the target for the insecticides, they are really just the carrier of the malaria parasites, Plasmodium falciparum. Consequently, it would be beneficial if there was some way to fight the parasite while not causing the mosquito populations to evolve resistance. And there is indeed a way that could be done, because of the fact that the parasite takes some time to develop within the mosquito. Consider these observations:
- The gonotrophic cycle (female mosquitos feed, convert a blood meal into eggs*, lay the eggs, and then seek out a new blood meal) is two to four days long.
- Natural mortality (without insecticides) is so high that few females go through more than a few gonotrophic cycles before they die.
- Development of the parasite is rather slow. It takes about ten to sixteen days (two to six gonotrophic cycles) for it to develop and move to the salivary glands of the host, where it can be transmitted to humans.
most mosquitoes do not live long enough to transmit the disease.The obvious conclusion is that killing only old mosquitos would solve the problem of the parasite infecting humans, and would avoid selection for mosquito resistance against insecticides. This is exactly what Read et al. propose. They suggest a number of approaches through which only older mosquitos would suffer, and then present a numerical model that shows how such control would play out:
- Low sublethal doses of insecticides accumulating after continued exposure could result in death of older mosquitos.
- Microencapsulation could provide slow release of insecticide.
- Compounds disproportionally affecting mosquitos with senescence.
- Compounds disproportionally affecting mosquitos suffering the costs (e.g. metabolic) of parasite infection.
That would be an enormous benefit. There are between 350 and 500 million cases of malaria every year, and a resulting one to three million deaths, of which the majority is African children. It would totally rock.
Unfortunately, nothing much in science is as straightforward as that. The majority of the work done by Read et al. was numerical. Their model aimed to quantify to what extent the use of insecticides acting at later gonotrophic cycles can prevent the evolution of resistance. Figure A below shows the fraction of resistant mosquitos as a function of time for con(ventional) insecticides and different hypothetical late-life-acting (LLA) insecticides that kill mosquitos from their second through sixth gonotrophic cycles. The paper does not specify the time-units on the x-axis, so we cannot tell when the mosquitos evolve resistance. But if we assume that it takes one to two years for them to evolve resistance to conventional insecticides, it looks like it takes approximately five times longer in the case of C3, which is five to ten years. Quite an improvement, but not exactly evolution-proof.
The numbers in the second column in B, the relative fitness of mosquitos in the presence of insecticide, needs to be below 1 for the insecticide to be evolution-proof. If there was a way to lower it, it would theoretically be possible to use the same insecticide forever, and never need to develop new ones, as is the case today.
In figure 3 we see that the relative fitness of the infected mosquitos can indeed drop below 1 (the green columns). This happens when there is an additional cost on the mosquitos of being infected with the parasite, which kills an extra fraction of mosquitos every day. Further, if the insecticide affects uninfected mosquitos less than infected ones, then it also gets easier to lower the relative fitness of infected mosquitos.
Read et al. concludes by scaring us with the prospect of disaster if the evolution of resistance is ignored:
The Global Malaria Action Plan (GMAP)  has laudable ambitions of spraying 172 million houses annually, and distributing 730 million insecticide-impregnated bed nets by the year 2010. If implemented with existing insecticides, this program will impose unprecedented selection for resistance. The historical record , and theory (e.g., Figure 1) shows that the medium-term prognosis for the insecticides currently in use is inescapably poor. Transitioning to an LLA insecticide strategy could see the benefits of the massive GMAP effort sustained, and could maintain for the long term the contribution of several key vector control tools to the goal of eradication. Failure to address evolution now runs the risk of replaying history : operational disaster and a derailing of the whole malaria control agenda.Don't ignore the science, or you'll pay the price!
Read, A., Lynch, P., & Thomas, M. (2009). How to Make Evolution-Proof Insecticides for Malaria Control PLoS Biology, 7 (4) DOI: 10.1371/journal.pbio.1000058
This post has been submitted to the NESCent competition for a travel award for the ScienceOnline 2010 un-conference in Durham, NC, January 14‐17th, 2010.
* Make that fertile eggs.