INFECTIOUS DISEASES

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Genetically Modified Fungi Kill Malaria-Causing Parasites in Mosquitoes

In a previous career, Dr. Mitchel, President of Target Health was involved in mosquito control programs and taught Tropical Medicine at Cornell Medical. His comments are that “While the following article is very important for control of malaria, there are additional options for mosquito control, which includes good water management to reduce the mosquito breeding sites.”

According to the World Health Organization (WHO), an estimated 225 million malaria cases occur worldwide annually, resulting in about 781,000 deaths. Although malaria is present in 106 countries, most cases occur in sub-Saharan Africa. Treating bed nets and indoor walls with insecticides is the main prevention strategy in developing countries, but the Anopheles mosquitoes that transmit malaria are slowly becoming resistant to these insecticides, rendering them less effective.

According to a new study published in the journal Science (2011;331:1074-1077), spraying malaria-transmitting mosquitoes with a genetically modified fungus, Metarhizium anisopliae, can kill the malaria parasite without harming the mosquito, potentially reducing malaria transmission to humans. Previous studies have found that this method nearly eliminates disease transmission when mosquitoes are sprayed soon after acquiring the malaria parasite. However, this strategy is not sustainable in the long term since if treating mosquitoes with the fungus kills them before they have a chance to reproduce and pass on their susceptibility to the spray, mosquitoes resistant to the fungus, which would reproduce normally, will soon become predominant and the spray will no longer be effective.

Because of this, the present study tried a more focused approach. Rather than developing fungi that rapidly kill the mosquito, they genetically modified M. anisopliae to block the development of the malaria parasite in the mosquito. The concept was that the genes added to the transgenic fungi would prevent the parasite from binding to the salivary glands of mosquitoes, so when a mosquito bites a human, the parasite is not transmitted.

For the evaluation of this approach, 11 days after feeding on blood infected by the malaria parasite, mosquitoes were divided into three groups and either sprayed with naturally occurring M. anisopliae fungi, genetically modified M. anisopliae fungi or not sprayed at all.

Results showed that two weeks post-treatment, the malaria parasite was found on the salivary glands of just 25% of the mosquitoes sprayed with the transgenic fungi, compared with 87% of those sprayed with the naturally occurring strain and 94% of unsprayed mosquitoes. The transgenic strain also reduced the density of parasites on the mosquitoes’ salivary glands by more than 95% compared with the unmodified strain.

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