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Winning the War on Malaria: Part Two

July 18, 2012

Versatile as it may be, DDT is not a silver bullet in the fight against malaria. Like any other prevention method, it has its own advantages and disadvantages.

Although authoritative groups such as the World Health Organization have ruled that the benefits of indoor DDT spraying outweigh potential health risks, there are still uncertainties associated with the chemical. Toxicology reports vary in their conclusions, but most do agree that in large enough concentrations, DDT is a possible carcinogen. Consensus has yet to be established in various studies, but some have found potential links between DDT and cancer, decreased fertility, and adverse affects on the immune system.

Another major concern is that of environmental impacts. If not properly used, DDT may contaminate the surrounding environment by air, land, or water. This can become problematic if the chemicals enter food or drinking supplies of an ecosystem. Wildlife can be negatively impacted, particularly those of the aquatic and avian variety.

An inevitable outcome of DDT spraying is the development of a more resistant mosquito population

But perhaps the greatest drawback is the ability of mosquitoes to develop DDT resistance. It is possible for the effectiveness of DDT to be diminished if it is overused: natural selection has a tendency to favor mosquitoes that develop resistance. The mosquitoes that survive the encounters with DDT are more likely to breed, leading to even more resistant generations. Already, pockets of resistance exist where DDT has lost much of its effectiveness, leading to localized increases in malaria. DDT has done much to combat malaria, but it certainly has its limits.

A new and novel method may become the next tool in malaria control: fungus. The particular fungus, scientifically known as Metarhizium anisopliae, has already seen commercial application in African agriculture to control locusts. Of particular interest is a genetically modified strain designed specifically to disrupt malarial transmission from the mosquito to human. Not only does the fungus kill mosquitoes, but it also releases proteins that inhibit the development of the malaria-causing parasite, P. falciparum. The treatment is unique in the sense that it “cures” malarial mosquitoes while simultaneously eliminating them from the existing population. As biochemist and pathologist Raymond St. Leger, developer of the M. anisopliae strain,explained:

The fungus acts like a little hypodermic syringe, and when it’s in the blood of the insect, the fungus then produces the anti-malarial protein, and within a couple of days it basically cures the mosquito of malaria.

Usage of M. anisopliae to prevent malaria is still in its infancy, but the preliminary results have been promising. A lab study conducted by the University of Maryland took mosquitoes infected with the malaria parasite and exposed them to the fungal spores. 14 days after exposure to the spores, malaria transmission was reduced by 78 ± 1%; after 17 days, the rate of reduction was 91%. By day 18, approximately 90% of the mosquitoes had been killed.

M. anisopliae tagged with a florescent gene to show the growth on a mosquito.

Field tests have replicated lab experiments with similar success. In the Tanzanian village of Lupiro, researchers made mineral oil-based applications of M. anisopliae and applied them to indoor structures in manners similar to DDT. Malarial transmission was reduced by 75-80% while reducing a mosquito’s chance of survival 39-57%. The authors concluded that the use of fungi was a viable alternative for malaria control, particularly if it is incorporated into the framework of a broader vector management strategy. Other researchers have found similar results in Kenya and Benin.

One of the advantages to M. anisopliae is its ability to retard the development of treatment-resistant mosquitoes. Unlike most insecticides, the fungus does not instantly kill the mosquitoes, but does so gradually. This allows the mosquito the opportunity to reproduce, passing the genetic vulnerability to its offspring. The other insecticides that kill instantly leave behind the resistant mosquitoes that pass the genetic resistance to their descendants.

The fungus has been judged safe for humans and the environment as well. Current evidence has not indicated that the genetically engineered strains of fungi behave any differently than wild ones. Furthermore, there have been no documented instances of M. anisopliae hurting humans or the environment. The EPA has gone so far to say that swallowing or inhaling the fungi is not at all dangerous.

Without a doubt, the use of M. anisopliae as a means to control malaria offers some unique benefits without some of the drawbacks of conventional pesticides. The development of it and other bio-pesticides show a bright future.

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