Applying evolution theory to defeat malaria


If the theory works, why not use it, eh?

One of the most serious problems with the use of DDT is that it tends to drive insects to evolve defenses to pesticides very quickly.  Almost every mosquito on Earth today has alleles that allow it to digest DDT, rather than be poisoned by it.  These alleles arose shortly after DDT was put into use against mosquitoes, and by the mid-1960s had made fruitless the malaria eradication campaign worked by the World Health Organization.

Evolution can be used to the benefit of humans and the eradication of malaria, too.

Voice of America (remember that agency?) tells the story of Andrew Read, a researcher at Penn State University, who realized that the deadliest mosquitoes are old ones — malaria has to survive for about two weeks in the mosquito in its life cycle in order to be infectious to humans.  If the mosquito dies before that time, the malaria can’t be transmitted.

Read’s proposal?  He has a fungus that takes a couple of weeks to work, but which kills the mosquito once it gets going.

In other words, Read doesn’t worry about getting all the mosquitoes.  His method, if it works, will kill only the mosquitoes most likely to carry malaria.

Plus, since most of the breeding cycle of these mosquitoes will be completed, it won’t drive the mosquitoes to evolve around the problem.

“The good thing about just killing the old ones is that most mosquiotoes will have done most of their reproduction before you kill them, and that means the susceptible mosquitoes will indeed continue to breed, so you still have susceptible mosquitoes, and your insecticides then just work against the old guys, removing them, and they are the dangerous ones. So under those circumstances, you don’t get the evolution of insecticide-resistant mosquitoes.”

He and his colleagues have been testing a kind of fungus that makes mosquitoes sick over the course of several weeks. And it eventually kills the oldest and most infectious mosquitoes.

“The name of the game is not mosquito control. It’s actually malaria control,” Read explains. “So if you just remove the old ones, you still have lots of young, non-dangerous mosquitoes around, but you have controlled malaria.”

Read says this fungus is about 98 to 99 percent effective at killing old mosquitoes in the lab. Now he says he needs to test this fungal insecticide in villages areas where malaria is prevalent, to see whether fewer people get the disease, even if they’re still getting bitten by mosquitoes.

Read and his team propose a new concept of mosquito control, based on what we know about the life cycles of mosquitoes and how they evolve, rather than just looking for one more “new” pesticide to which the insects will soon become resistant.  Read’s article appears in the open-access Public Library of Science (PLoS), published April 7, 2009:  “How to Make Evolution-Proof Mosquitoes for Malaria Control.” His coauthors are Penelope A. Lynch and Matthew B. Thomas.

Summary

Insecticides are one of the cheapest, most effective, and best proven methods of controlling malaria, but mosquitoes can rapidly evolve resistance. Such evolution, first seen in the 1950s in areas of widespread DDT use, is a major challenge because attempts to comprehensively control and even eliminate malaria rely heavily on indoor house spraying and insecticide-treated bed nets. Current strategies for dealing with resistance evolution are expensive and open ended, and their sustainability has yet to be demonstrated. Here we show that if insecticides targeted old mosquitoes, and ideally old malaria-infected mosquitoes, they could provide effective malaria control while only weakly selecting for resistance. This alone would greatly enhance the useful life span of an insecticide. However, such weak selection for resistance can easily be overwhelmed if resistance is associated with fitness costs. In that case, late-life–acting insecticides would never be undermined by mosquito evolution. We discuss a number of practical ways to achieve this, including different use of existing chemical insecticides, biopesticides, and novel chemistry. Done right, a one-off investment in a single insecticide would solve the problem of mosquito resistance forever.

Among reasons you may want to bookmark that publication:  In the opening paragraphs the authors discuss how Indoor Residual Spraying drives mosquito resistance to pesticides, with citations to the most recent and most powerful studies.  This is the case against bringing back DDT in a big way.

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One Response to Applying evolution theory to defeat malaria

  1. Mike says:

    Ed, do you have a user id for the BPR3 project? This post is a great example of the purpose of the project would benefit from having the badge and the extra traffic from BPR3.

    Like

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